Effects on Surface, Structural and Mechanical …prr.hec.gov.pk/jspui/bitstream/123456789/7581/1/Full...Surface, Structural and Mechanical Properties of Brass by Laser and Ion beam

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Effects on Surface, Structural and Mechanical Properties of

Brass by Laser and Ion beam irradiation

Submitted to

GC University Lahore

In partial fulfillment of the

Requirements for the award of degree of

Doctorate of Philosophy

In

PHYSICS

by

Shahbaz Ahmad

REG. No. 25-GCU-PHD-PHY-2009

Centre for Advanced Studies in Physics (CASP)

Government College University (GCU) LAHORE

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So Exalted be God, the True King! And do not hasten with the Reading (Qur'an)

before its revelation is accomplished to thee, and say, my Lord, increase me in

knowledge.

(114)TA HA (20)

3

Effects on Surface, Structural and Mechanical Properties of

Brass by Laser and Ion beam irradiation

Submitted to GC University Lahore

In partial fulfillment of the requirements

for the award of degree of

Doctorate of Philosophy

In

PHYSICS

by

Shahbaz Ahmad

Reg. No.25-GCU-PHD-PHY-2009

Centre for Advanced Studies in Physics (CASP)

Government College University (GCU) LAHORE

4

DEDICATED

To

Greatest teacher of world

MUHAMMAD ( ), with whose Holy

existence and by virtue of whose knowledge spread discernment,

wisdom and light of knowledge in the world and the journey of

knowledge and intellectual activities was launched.

Abstract

Modifications to the surface, structural and mechanical properties of brass have been

investigated by using three different kinds of radiation sources. The first radiation source was

laser, second one was laser induced plasma ions and the third one was Pelletron accelerator.

Brass targets were exposed to various laser pulses ranging from 1200 to 3000 of excimer laser

(248 nm, 20 ns, 120mJ and 30 Hz) at constant fluence of 6.4 J/cm2 in oxygen atmosphere (100

Torr). In order to explore the ion induced modification in properties of brass, ions were

generated by two different ion sources. The laser induced plasma was employed as a first ion

source for the generation of Ni, Si and C ions. Excimer laser (248 nm, 20 ns, 120mJ and 30 Hz)

was used for the generation of Ni, Si and C plasma. In order to estimate ion flux and energies,

Thomson parabola technique was employed. By using this technique, magnetic field of strength

80 mT was applied on the plasma plume to give appropriate trajectory to generated ions. These

ions were detected by solid state nuclear track detector (CR39). In response to stepwise increase

in number of laser pulses from 3000 to 12000, the Ni ion flux varies from 60 × 1013 to 84 × 1016

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ions/cm2 with constant energy of 138 KeV. Similarly Si ion flux varies from 45 × 1012 to 75 ×

1015 ions/cm2 with constant energy of 289 KeV. The flux of C ions flux changes from 32 × 1011

to 72 × 1014 ions/cm2 with constant energy of 678 KeV. The second source of ion generation is

Pelletron accelerator. Brass targets were bombarded by Ni and C ions of energy 2MeV for

various ions flux ranging from 56×1012 to 26×1013 ions/cm2. Scanning Electron Microscope

(SEM) and X-Ray Diffractometer (XRD) were used to analyze the surface morphology and

crystallographic structure of irradiated brass respectively. Universal Testing Machine (UTM) and

Vickers Hardness Tester (VHT) were employed to explore Yield Stress (YS), Ultimate Tensile

Strength (UTS) and mirohardness of ion irradiated brass targets. SEM analysis reveals the

formation of micro/nano sized cavities, bumps, cones and wave-like ridges with non-uniform

shape and density distribution after laser irradiations. Whereas, ion irradiation causes the

formation and growth of nano/micro sized cavities, pores, pits, voids and cracks for lower and

moderate ion flux (in all cases). At maximum ion flux the granular morphology (in case of brass

irradiated by laser induced Ni and Si ions) and dendritic morphology (in the case of brass

irradiated by laser induced plasma and Pelletron accelerator C ions) are observed.

XRD analysis reveals that no new phases are identified in case of laser irradiated brass. However

new phases of CuZnNi (200), CuSi (311) and ZnC (0012) are identified in the brass substrate

after laser induced Ni, Si and C ions irradiation respectively. Whereas, no new phases are formed

in case of Ni and C ion irradiation obtained by Pelletron accelerator. The variation in peak

intensity, crystallite size, dislocation line density and induced stresses along with angular shifting

is observed in all cases of laser and ion irradiations. Significant variations in mechanical

properties of brass are observed after laser and ion irradiations. The changes in mechanical

properties of an irradiated brass are well correlated with surface and crystallographical

modifications and are attributed to generation, augmentation, recombination and annihilation of

the ion induced defects. The laser and ion induced surface, structural and mechanical

modifications of brass are significantly influenced by nature, energy and flux of radiations.

DECLARATION

I, Mr. Shahbaz Ahmad, Reg. No.25-GCU-PHD-PHY-2009, student of Prof. Dr. Shazia Bashir

in the subject of Physics, hereby declare that the matter printed in the thesis titled “Effects on

Surface, Structural and Mechanical Properties of Brass by Laser and Ion beam

Irradiation” is my own work and has not been printed, published and submitted as thesis in any

University, Research Institution etc. in Pakistan or abroad.

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Dated: ____________ Signature of the student:

_________________

Shahbaz Ahmad (25-GCU-PHD-PHY-2009)

RESEARCH COMPLETION CERTIFICATE

It is certified that the research work contained in this thesis titled “Effects on Surface,

Structural and Mechanical Properties of Brass by Laser and Ion beam Irradiation” has

been carried out and completed by Mr. Shahbaz Ahmad, Reg.No.25-GCU-PHD-PHY-2009

under my supervision.

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Prof. Dr. Shazia Bashir

Supervisor Centre for Advanced Studies in Physics

GC University, Lahore, Pakistan

Dated: _______________

Submitted Through

___________________

Prof. Dr. Riaz Ahmad Director _________________________ Centre for Advanced Studies in Physics (CASP)

GC University, Lahore, Pakistan Controller of Examinations GC University Lahore

ACKNOWLEDGEMENT

All the worship, praises, acclamation and thanks are to ALLAH Almighty alone, the most

gracious, merciful, sympathetic and compassionate, Who is intact source of wisdom and

knowledge to mankind, Who bestowed me strength and ability to complete this work

effectively. Whenever I call upon Him, HE always listens. I offer my sincerest and humblest

words of gratitude and thanks to PROPHET MUHAMMAD ( ),

the ideal, most Perfect and Exalted, Who eternally, is a symbol of guidance and source of

knowledge for humanity as a whole.

http://thesaurus.com/browse/worship

http://thesaurus.com/browse/sympathetic

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If there were dreams to sell, merry and sad to tell and crier rings the bell, what would you buy; I

will say “University Charming Days”. Actually it is impossible but shows my blind love to this

institution which is homeland of knowledge, wisdom and intellectuality. I love my alma-mater

with the soul of my heart, because it is just like the lap of my mother, I am proud of being

student of this university.

First of all, I would honestly thank to my supervisor Prof. Dr. Shazia Bashir, for her consistent

guidance and support throughout my research. Her encouraging and sympathetic attitude

facilitated me in improving my abilities not only in the field of physics and writing research

papers but in other aspects of life. I am awfully thankful to have implausible experience of

working with her.

I acknowledge Prof. Dr. Riaz Ahmad, Director of CASP, G.C.U Lahore, for his continuous support

both scientifically and professionally during my Ph.D research.

I also feel greatest pleasure in expressing humble and sincere gratitude to Prof. Dr. Muhammad

Shahid Rafique, Chairman Department of Physics UET. His valuable suggestions will always

serve as a beacon of light throughout the course of my life. I extend deep emotions of

appreciation, gratitude and indebtedness for his valuable guidance.

I would like to acknowledge all of my lab fellows, especially Dr. Umm-i-Kalsoom, Khaliq Muhmood,

Asma Hayat and Mahreen Akram for helping me during my research work.

I never forget my well-wishers, especially I extend heartiest thanks, to Dr. Tousif Hussain, Dr. Uzma

Ikhlaq and Sajaad Ahmad for their assistance in characterization of the material during my studies.

Special thanks of Hassan Maqbool and Furrakh for helping me to conduct all my experimental work.

I am especially thankful to my beloved friends Dr. Muddassar Zafar (Assistant Professor, University of

the Gujrat), Dr. Kaleem Iqbal (Veterinary Officer, Faisalabad), Dr. Nisar Ali (Assistant Professor, Garrison

University Lahore), Dr. Daniel Yousaf (Associate Professor FC College Lahore), Dr. Muhammad Ramzan

(Assistant Professor, Government College University, Faisalabad) and Muhammad Azhr Ashrif for their

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sincere friendship and memorable period of life I spent with them. They colored simple moments of my

life and made these really pleasurable.

No acknowledgement could ever adequately express my feelings to my affectionate and adorable father

M. Shafiq and lovable, caring sister Shama Kishwer and brothers Abdul Qadoos, Dr. Usman, Mahmood

Akhtar and Aftab Shafiq are true dense of my humblest obligations and my words will fail to give them

enough credit. I am proud of my parents who provided me all what I needed and remembered me in

their prayers. I am greatly thankful to my loving parents, thanks to all. My beloved nephews

Muhammad Umer, Muhammad Hassan and Muhammad who made me laugh during tense moments.

Shahbaz Ahmad

TABLE OF CONTENTS

Declaration i

Research completion certificate ii

Acknowledgement iii

List of figures viii

List of tables xvi

List of publications xvii

Abstract

Sr. No. Page No.

CHAPTER 1

1 Introduction 1

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1.1 Motivations and goals 2

1.2 Laser matter interaction 7

1.2.1 Mechanisms of energy absorption in metals 7

1.2.2 The equation of heat energy 9

1.2.3 Theoretical explanation of heat conduction 10

1.2.4 Surface melting 13

1.2.5 Laser induced plasma ions of metals 13

1.2.6 Laser induced plasma ions of semiconductors 13

1.2.7 Laser induced plasma ions of insulators 13

1.3 Effects of laser irradiation on surface, structural and mechanical properties of 14

metals

1.4 Ion matter interaction 15

1.4.1 Stages of cascade growth 17

1.4.1.1 Collisional stage or displacement cascades 17

1.4.1.2 Thermal spike 17

1.4.1.3 Quenching stage 17

1.4.1.4 Annealing stage 18

1.5 Ion irradiation effects for surface, structural and mechanical properties of metals and their

alloys 19

1.6 Laser induced plasma as an ion source 19

1.7 Pelletron linear accelerator as an ion source 21

1.7 Selection of Brass as a target 21

CHAPTER 2

Literature Survey 24

CHAPTER 3

Experimental Setup and characterization techniques 31

3.1 Sample preparation 32

3.1.1 Cutting, grinding and polishing of samples 32

3.1.2 Annealing 32

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3.2 KrF Excimer laser system 32

3.3 Experimental details for exposure of samples by Excimer laser 32

(a) Fabrication of the target holder for laser irradiation 32

(b) Experimentation for exposure of brass targets by Excimer laser 33

3.4 Experimental details for exposure of sample by laser induced plasma ions 35

(a) Laser induced plasma ions emissions and detections by Solid State Nuclear

Track Detectors (SSTDs) 35

(b) Experimentation for exposure of brass target by laser induced plasma ion 37

3.5 Experimental details for exposure of sample by Pelletron linear accelerator 39

(a) Description of Pelletron accelerator 39

(b) Experimentation for exposure of brass target by Pelletron accelerator 40

3.6 Characterization techniques 40

3.6.1 Scanning Electron Microscope (SEM) 41

3.6.2 X-Ray Diffraction (XRD) 42

3.6.3 Universal Testing Machine (UTM) 42

3.6.4 Vickers hardness tester 42

CHAPTER 4

Results and Discussion 45

Part (A)

Laser-irradiation

4.1 Effects of laser irradiation on the surface, structural and mechanical properties of

Brass 48

4.1.1 SEM analysis 48

4.1.2 XRD analysis 50

4.1.3 Tensile testing 53

4.1.4 Microhardness 54

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Part (B)

Laser induced plasma ions irradiation

4.2 Energy and flux measurement of laser induced plasma ions 57

4.3 Effects of laser induced Ni plasma ions irradiation on the surface, structural and

mechanical properties of brass 63





4.4 Effects of laser induced Si plasma ions irradiation on the surface, structural and

mechanical properties of brass 74





4.5 Effects of laser induced C plasma ions irradiation on the surface, structural and

mechanical properties of brass. 85





Part (C)

Accelerator-ions irradiation

4.6 Effects of accelerator Ni ions irradiation on the surface, structural and mechanical properties

of brass. 96





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4.7 Effects of accelerator C ions irradiation on the surface, structural and mechanical properties

of brass. 106





Conclusions 116

References 123

List of Figures

Figure: 1.1: Dynamics of photon absorption, electronic, vibrational excitation and relaxation

processes during laser matter interaction. The time scales relevant to stress confinement and

thermal confinement are also shown. --------------------------------------------------------------------- 8

Figure: 1.2: The illustration of various phenomena involved in ion material interaction. ------- 16

Figure 1.3: The representation of the formation of primary knock out atoms by elastic collisions.

---------------------------------- 16

Figure 1.4: The various processes involved in ion matter interaction for generation and evolution

of lattice defects. ------------------------------------------------------------------------------------------ 18

Figure 1.5: Multi stages of cascade growth during ion matter interaction. ---------------------- 18

Figure 3.1: An inside view of self fabricated target holder for exposure of brass to laser pulses

with scanning arrangement. ------------------------------------------------------------------------------- 33

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Figure 3.2: The schematic diagram of experimental setup for exposure of brass target by laser

irradiation at constant laser fluence of 6.4 J/cm2 under oxygen environments at various pulses. ---

---------------------------------------------------------------------------------------------------------------- 34

Figure 3.3: The photographic view of experimental setup for exposure of brass target by laser

irradiation. --------------------------------------------------------------------------------------------------- 34

Figure 3.4: The schematic of experimental setup for exposure of CR39 by laser induced plasma

ions. ---------------------------------------------------------------------------------------------------------- 36

Figure 3.5: The graphical representation of the Thomson parabola technique. ------------------ 37

Figure 3.6: The schematic of experimental setup for exposure of brass substrate by laser induced

plasma ions. ------------------------------------------------------------------------------------------------- 38

Figure 3.7: The photographic view of Pelletron linear accelerator. -------------------------------- 39

Figure 3.8: The photographic view of Scanning Electron Microscope (SEM). ------------------- 41

Figure 3.9: The photographic view of Universal Testing Machine (UTM). ----------------------- 43

Figure 3.10: The photographic view of Vickers Hardness Tester (VHT). ------------------------- 43

Figure 4.1: SEM micrographs illustrating the surface morphology of (a) unirradiated and

excimer laser irradiated brass target under O2 ambient environment at pressure of 100 Torr at

constant fluence of 6.4 J/cm2 for various laser pulses of (b) 1200, (c) 1800, (d) 2400 and (e)

3000. --------------------------------------------------------------------------------------------------------- 49

Figure 4.2: XRD data of laser irradiated brass target surface under O2 ambient environment at

constant fluence of 6.4 J / cm2 for various laser pulses of 1200, 1800, 2400 and 3000. (a) XRD

diffractrographs, (b) The crystallite size Vs laser pulses, (c) The dislocation density Vs laser

pulses and (d) stresses Vs laser pulses. -------------------------------------------------------------------51

Figure 4.3: Universal Testing Machine (UTM) data for laser irradiated brass target under O2

ambient environment at constant fluence of 6.4 J / cm2 for various laser pulses of 1200, 1800,

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2400 and 3000. (a) Tensile stress- tensile strain curves, (b) The yield stress Vs laser pulses, (c)

The ultimate tensile strength Vs laser pulses. --------------------------------------------------------- 54

Figure 4.4: The variations of the hardness of laser irradiated brass under O2 ambient

environment at constant fluence of 6.4 J/cm2 for various numbers of pulses of 1200, 1800, 2400

and 3000. ---------------------------------------------------------------------------------------------------- 55

Figure 4.5: Optical micrographs of tracks on the surface of CR39 used as a solid state nuclear

track detector for detection as well as measurement of energy and flux of laser induced Ni

plasma ions generated at a laser fluences of 5.1 J/cm2 for various number of laser pulses of (a)

3000, (b) 6000, (c) 9000 and (d) 12000. ---------------------------------------------------------------- 57


track detector for detection as well as measurement of energy and flux of laser induced Si plasma

ions generated at a laser fluences of 5.1 J/cm2 for various number of laser pulses of (a) 3000, (b)

6000, (c) 9000 and (d) 12000 -----------------------------------------------------------------------------58


track detector for detection as well as measurement of energy and flux of laser induced C plasma

ions generated at a laser fluences of 5.1 J/cm2 for various number of laser pulses of (a) 3000, (b)

6000, (c) 9000 and (d) 12000. --------------------------------------------------------------------------- 59

Figure 4.8: The graphical representation of increase in flux of laser induced plasma ions of Ni,

Si and C with increasing number of laser pulses. ----------------------------------------------------- 60

Figure 4.9: SEM micrographs illuminating the surface morphology of (a) unirradiated and laser

induced Ni plasma ions irradiated brass target at constant energy of 138 KeV under vacuum

condition for various ion flux of (b) 60 × 1013 ions/cm2, (c) 35 × 1014 ions/cm2 (d), 70 × 1015

ions/cm2 and (e) 84 × 1016 ions/cm2. -------------------------------------------------------------------- 64

Figure 4.10: XRD data of laser induced Ni plasma ions irradiated brass target surface for various

ion flux of 60 × 1013 ions/cm2, 35 × 1014 ions/cm2, 70 × 1015 ions/cm2 and 84 × 1016 ions/cm2 at

constant energy of 138 KeV. (a) XRD diffractrographs, (b) The crystallite size Vs Ion flux, (c)

The dislocation density Vs Ion flux and (d) Stresses Vs Ion flux. ----------------------------------- 67

16

Figure 4.11: Universal Testing Machine (UTM) data of laser induced Ni plasma ions irradiated

brass for various ion flux of 60 × 1013 ions/cm2, 35 × 1014 ions/cm2, 70 × 1015 ions/cm2 and 84 ×

1016 ions/cm2 at constant energy of 138 KeV. (a) Tensile stress- tensile strain curves, (b) The

yield stress Vs ion flux (c) The ultimate tensile strength Vs ion flux. ------------------------------ 70

Figure 4.12: The variations in the hardness of laser induced Ni plasma ions irradiated brass for

various ion flux of 60 × 1013 ions/cm2, 35 × 1014 ions/cm2, 70 × 1015 ions/cm2 and 84 × 1016

ions/cm2 at constant energy of 138 KeV. --------------------------------------------------------------- 72

Figure 4.13: SEM micrographs representing the surface morphology of (a) unirradiated and

laser induced Si plasma ions irradiated brass target at constant energy of 289 KeV for various ion

flux of (b) 45 × 1012 ions/cm2, (c) 25 × 1013 ions/cm2 (d), 56 × 1014 ions/cm2 and (e) 75 × 1015

ions/cm2.------------------------------------------------------------------------------------------------------ 75

Figure 4.14: XRD data of laser induced Si plasma ions irradiated brass target at constant energy

of 289 KeV for various ion flux of 45 × 1012 ions/cm2, 25 × 1013 ions/cm2, 56 × 1014 ions/cm2

and 75 × 1015 ions/cm2. (a) XRD diffractrographs, (b) The crystallite size Vs Ion flux, (c) The

dislocation density Vs Ion flux and (d) Stresses Vs Ion flux. ---------------------------------------- 78

Figure 4.15: UTM data of laser induced Si plasma ions irradiated brass at constant energy of

289 KeV for various ion flux of 45 × 1012 ions/cm2, 55 × 1013 ions/cm2, 65 × 1014 ions/cm2 and

75 × 1015 ions/cm2. (a) Tensile stress- tensile strain curves, (b) The yield stress Vs ion flux (c)

The ultimate tensile stress Vs ion flux. ------------------------------------------------------------------ 82

Figure 4.16: The variations in the hardness of laser induced Si plasma ions irradiated brass for


ions/cm2 at constant energy of 289 KeV. --------------------------------------------------------------- 84

Figure 4.17: SEM micrographs illustrating the surface morphology of (a) unirradiated and laser

induced C plasma ions irradiated brass target at constant energy of 678 KeV for various ion flux

of (b) 32 × 1011 ions/cm2, (c) 90 × 1012 ions/cm2 (d), 64 × 1013 ions/cm2 and (e) 72 × 1014

ions/cm2. ---------------------------------------------------------------------------------------------------- 86

Figure 4.18: XRD data of laser induced C plasma ions irradiated brass target at constant energy


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and 72 × 1014 ions/cm2. (a) XRD diffractrographs, (b) The crystallite size Vs Ion flux, (c) The

dislocation density Vs Ion flux and (d) Stresses Vs Ion flux. --------------------------------------- 89

Figure 4.19: Universal Testing Machine (UTM) data of laser induced C plasma ions irradiated

brass surface with constant energy of 678 KeV under vacuum environment at various ion flux of

32 × 1011 ions/cm2, 90 × 1012 ions/cm2, 64 × 1013 ions/cm2 and 72 × 1014 ions/cm2. (a) Tensile

stress- tensile strain curves, (b) The yield stress Vs ion flux (c) The ultimate tensile strength Vs

ion flux. -------------------------------------------------------------------------------------------------------92

Figure 4.20: The variations in the hardness of laser induced C plasma ions irradiated brass

surface for various ion flux of 32 × 1011 ions/cm2, 90 × 1012 ions/cm2, 64 × 1013 ions/cm2 and 72

× 1014 ions/cm2 at constant energy of 678 KeV.-------------------------------------------------------- 94

Figure 4.21: SEM micrographs representing the micro surface structuring of (a) unirradiated and

accelerator Ni ions irradiated brass target at constant energy of 2 MeV for various ion flux of (b)

56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2 (d), 15 × 1013 ions/cm2 and (e) 26 × 1013 ions/cm2. ----

------------------------------------------------------------------------------------------------------------------96

Figure 4.22: SEM micrographs showing the surface structuring of (a) unirradiated and

accelerator Ni ions irradiated brass target at constant energy of 2 MeV for various ion flux of (b)

56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2 (d), 15 × 1013 ions/cm2 and (e) 26 × 1013 ions/cm2. ----

----------------------------------------------------------------------------------------------------------------- 97

Figure 4.23: XRD data of accelerator Ni ions irradiated brass target surface for various ion flux

of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013 ions/cm2 at constant

energy of 2 MeV. (a) XRD diffractrographs, (b) The crystallite size Vs Ion flux, (c) The

dislocation density Vs Ion flux and (d) Stresses Vs Ion flux. --------------------------------------- 100

Figure 4.24: Universal Testing Machine (UTM) data of 2 MeV accelerator Ni ions irradiated

brass for various ion flux of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2 and 26 ×

1013 ions/cm2. (a) Tensile stress- tensile strain curves, (b) The yield stress Vs ion flux (c) The

ultimate tensile strength Vs ion flux. ------------------------------------------------------------------- 103

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Figure 4.25: The variations in the hardness of accelerator Ni ions irradiated brass for various ion

flux of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013 ions/cm2 at

constant energy of 2 MeV. ------------------------------------------------------------------------------ 104


accelerator C ions irradiated brass target at constant energy of 2 MeV for various ion flux of (b)

56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2 (d), 15 × 1013 ions/cm2 and (e) 26 × 1013 ions/cm2. ---

--------------------------------------------.----------------------------------------------------------------- 107

Figure 4.27: XRD data of accelerator C ions irradiated brass target at constant energy of 2 MeV

for various ion flux of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013

ions/cm2. (a) XRD diffractrographs, (b) The crystallite size Vs Ion flux, (c) The dislocation

density Vs Ion flux and (d) Stresses Vs Ion flux. -------------------------------------------------- 110

Figure 4.28: Universal Testing Machine (UTM) data of accelerator C ions irradiated brass

surface at constant energy of 2 MeV for various ion flux of 56 × 1012 ions/cm2, 12 × 1013

ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013 ions/cm2. (a) Tensile stress- tensile strain curves (b)

The yield stress Vs ion flux and (c) The ultimate tensile strength Vs ion flux. ----------------- 112

Figure 4.29: The variations in the hardness of accelerator C ions irradiated brass surface for


ions/cm2 at constant energy of 2 MeV. ---------------------------------------------------- 114

Figure 5.1: Flow chart of laser/ion matter interaction processes. -------------------------------- 123

Figure 5.2: Three radiation sources used for irradiation of brass at various energies and flux. ----

- --------------------------------------------------------------------------------------------------------------124

Figure 5.3: The comparison between laser and ion beam irradiation effects on surface structural

and mechanical properties of brass. ------------------------------------------------------------------ 127

List of Tables

Table 1.1: Different types of brass ---------------------------------------------------------------------- 22

Table 3.1: The evaluated energy and flux of laser induced plasma ions of Ni, Si and C. --------38

19

Table 4.1:The comparison of three radiation sources i.e. (1) Laser photons from Excimer laser

(248nm, 150 mJ, 6.4 J.cm-2, 20 ns, 20 Hz) (2) laser –induced plasma ions of Ni, Si

and C generated with Excimer laser (248nm, 120mJ, 5.1 J.cm-2, 20 ns, 20 Hz) under

UHV condition (3) Ni and C ions of MeV energy generated by Pelletron Accelerator

and their irradiation effects on structural and mechanical properties of brass.-----------

-------------------------------------------------------------------------------------------------- 116

List of publications

Included in the thesis

1. Shahbaz Ahmad, Shazia Bashir, M. Shahid Rafique, and Daniel Yousaf. “Modifications in surface,

structural and mechanical properties of brass using laser induced Ni plasma as an ion source”

Aip Advances 6, 035013 (2016)

2. Shahbaz Ahmad, Shazia Bashir, M. Shahid Rafique, Daniel Yousaf & Riaz Ahmad. “The

generation, detection and measurement of laser-induced carbon plasma ions and their

implantation effects on brass substrate” Radiation Effects & Defects In Solids, 1-18,

2016

3. Shahbaz Ahmad, Shazia Bashir, Nisar Ali, Umm-i-Kalsoom, Daniel Yousaf, Faizan-ul-Haq, Athar

Naeem, Riaz Ahmad, M. Khlaeeq-ur-Rahman, “Effect of ion irradiation on the surface, structural

and mechanical properties of brass” Nuclear Instruments and Methods in Physics Research B

325 (2014) 5–10

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4. Shahbaz Ahmad, Shazia Bashir, Daniel Yousaf, Nisar Ali, Tousif Hussain “Surface Analysis

Correlated with Structural and Mechanical Properties of Laser Irradiated Brass” Materials

Sciences and Applications, 2015, 6, 23-32

Not included in the thesis

1. Shahzaib Khurshid, Shazia Bashir, Nisar Ali, Umm-i-Kalsoom, Shahbaz Ahmad, Mahreen Akram,

Daniel Yousaf, Khushboo “Pulsed Laser ablation of Ni under ambient environment of vacuum

and N2 for various fluences” Journal of Quantum Electronics (2014) (In press)

2. Nisar Ali, Shazia Bashir, Umm-i-Kalsoom, Daniel Yousaf, Shahbaz Ahmad “Effect of laser pulses

on the surface and structural modification of ablated titanium in a liquid confined environment”

Radiation Effects and Defects in Solids 170 (2) 2015, 121-129.

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Chapter 1

INTRODUCTION

1.1 Motivations and goals

The surface, structural and mechanical properties (yield stress, tensile strength and hardness) of

metals and their alloys can be significantly improved by employing a variety of techniques such

as ion beam modification, laser assisted ablation or deposition, plasma processing [1-4]. Laser

irradiation of materials is an emerging technique for its enormous applications e.g. nanoparticle

generation, micromachining, materials processing, and nano structuring. Laser surface

engineering is an efficient and reliable technique for material processing applications. Reduced

heat affected zone, control over the laser fluence, speed, versatility, non-contact nature,

contamination free and environmental friendly ablation process are the advantages of laser

processing method over other techniques [2]. Bashir et al. [5] have investigated the effect of

laser pulses and ambient environment (dry or wet) on the laser ablation performance of brass.

22

Their results proposed that ambient environment plays a significant role in surface and structural

modification of brass. Kazakevich et al. [6] prepared brass nanoparticles by brass target ablation

in ethanol using copper-vapor laser radiation at various laser fluences. They found that the

surface morphology of the irradiated brass target strongly influence the properties of laser-

generated nanoparticles. Tam et al. [7] modified the brass surface using a 2 kW CW Nd-YAG

laser. For modified layers, the hardness was increased from 110 HV to the range of 152– 256HV.

Laser induced plasma as ion source is highly useful for material processing. It can improve

electrical, optical, structural and mechanical properties of materials [8-12]. These ions can be

implanted at different depths that modify the physical, chemical, structural and mechanical

properties of implanted surface of different metals and their alloys. Ahmad et al. [13] irradiated

Al target with 500 shot of Nd:YAG laser beam for generation of high energy plasma plume of Al

ions in vacuum. The mechanical properties of irradiated metal are decreased due to thermal

sputtering. Liu et al. [14] performed a experiment in which steel target was irradiated by He- ions

with 500KeV energy and 6.07x1017 ion/cm2 fluence of irradiated ions. The authors found that

size of grain at the penetration depth is modified after ion irradiation. Pelletier et al. [15] studied

the effect of nitrogen ion fluence on mechanical properties of 316 L stainless steel. The hardness

linearly increases from 5.25 GPa to 7.5 GPa by increasing ion fluence.

The interaction of pulsed nanosecond laser to solid targets generates plasma which favors ion

acceleration along the normal to the target surface. Laser induced plasma has an ability to

produce ions with a broad spectrum of charges (even higher than 50+) and K.E from KeV to

MeV range [16]. The thermal interactions, the adiabatic expansion in vacuum and columbic

interactions are responsible for primary ion acceleration [13]. Laser induced plasma as ion source

has many advantages as compared to conventional ion implanters. The first advantage of laser

induced plasma as ion source is the greater penetration range of implantation by reason of higher

charge and energy states. The second one is that any solid material can be ionized by this source.

The third advantage is that the energy and flux of laser induced plasma ions can be increased by

applying external electric and magnetic field. The emission of plasma species can be controlled

by laser parameter (intensity, wavelength, focal spot size, pulse duration and energy) and

irradiated target properties (physical, chemical, structural and mechanical properties as well as

work function) [17, 18]. Latif et al. [19] used the laser produced Cu ions as a source of

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irradiation to explore the variations in surface morphology and crystallography of irradiated

silver target. Rosinski et al. [20] used laser induced ion stream as a source of ion implantation.

The application of external magnetic field changes the plasma dynamics, results in the variation

of density, velocity and temperature of plasma species [21]. These ions generated by plasma

after magnetic field employment can be detected by Solid State Nuclear Track Detection

(SSNTD) technique. By using Thomson parabola technique the energy and flux of ions can be

analyzed [22]. Krasa et al. [18] determined the energy spectra of ions Pb, Ag, Sn, and Au

released from laser produced plasma by the use of Thomson parabola technique. Rafique et al

[22]. used Thomson parabola technique for detection and measurement of ions and energy of Cu

ions from laser produced plasma plume. Dirnberger et al. [23] and Rafique et al. [24] studied the

effect of external magnetic field on the shape of laser produced plasma plume.

Pelletron accelerator is also used as an ion source. The interaction of ions with surface of various

materials is scientifically important for mass analysis (PIXE, Rutherford back scattering),

material processing and ion implantation. This field has vast ranging industrial and technical

applications e.g. film growth, semiconductor industry and hardening. Afzal et al. [25]

investigated mechanical properties of proton irradiated nitinol target. The nitinol target is

irradiated by Pelletron accelerator with constant energy of 2 MeV at room temperature. The

mechanical properties of nitinol are increased after proton irradiations. When high energetic ions

interact with material, a highly disordered region is formed on the target surface in which the

mean kinetic energy of displaced host lattice atoms is up to several eV.

This dissertation deals with the investigation of surface structural and mechanical properties of

brass after irradiation with laser and ions. In first set of experiment, the brass target is irradiated

by nanosecond excimer laser. In second sets of experiment, laser induced plasma ions of Ni, Si

and C ions were employed to irradiate the brass targets under ultrahigh vacuum condition. Basic

aim is generation and detection of ions of conducting (Ni), semiconducting (Si) and insulating

(c) materials. Laser induced plasma is designed and fabricated as an ions source. The energy and

flux of these ions are estimated. Laser generated ions of Ni, Si and C were produced under the

same conditions i.e. laser fluence, number of laser pulses, angle of detection, target to substrate

distance and magnetic field strength. But the flux and range of energies of generated ions of

various materials are different and are strongly dependent upon their mass, nature, atomic

number, optical and thermal properties. In third set, Ni and C ions generated by Pelletron

24

accelerator were used for irradiation of brass with 2 MeV energy and flux variation from 56 ×

1012 ions/cm2 to 26 × 1013 ions/cm2.

According to our best knowledge no work is reported in which a comprehensive study is carried

out in which comparison of three different irradiation sources i.e. laser , laser induced plasma

ions and accelerator ions is presented. The main aim of present dissertation also includes a

detailed comparison of irradiation sources which are better for improving surface, structural and

mechanical behavior of metallic alloys. The alloys with improved properties can be used in

industry, aerospace and also in different scientific and technical fields.

The surface morphology of irradiated brass targets is characterized by Scanning Electron

Microscopy (SEM) analysis. In response to laser irradiation various microstructures like ripples,

cavities, cones and craters are observed on the surface of brass. Whereas, irradiation with laser

induced plasma ions is responsible for the formation of grains, cavities, pores, pits, voids and

dendritic structures. In case of irradiation with Pelletron accelerator produced ions various

features like cavities, craters, dendritic structures and cracks are observed. Surface structures of

the brass after laser and ion irradiation are examined by SEM analysis. Irradiation of brass with

three different irradiation sources also affects their crystallinity and dislocation line density. The

crystalline structures of irradiated brass target are evaluated by X-Ray diffraction technique

(XRD). The irradiation induced residual stresses and heat conductions are responsible for

variations in microstructures of irradiated brass. Universal Testing Machine (UTM) and Vickers

Hardness (VH) tester confirm modifications in mechanical properties of laser and ion irradiated

brass. Depending upon the laser parameters (number of shots, intensity, wavelength, focal spot

size and pulse duration) as well as parameter of incident ions (energy, flux, mass and atomic

radius), brass shows different surface, structural and mechanical behavior when exposed to laser

and different ions. The changes in mechanical properties of irradiated brass are well correlated

with surface and crystallographical modifications and are attributed to generation, augmentation,

recombination and annihilation of the ion induced defects. The substantial quantity of work is

reported on ion irradiations of metals and their alloys by many researchers [13, 22, 26, 27].

Brass was selected as target material for explorations by laser and ion beam irradiations due to

large scale applications in industry. It is widely used in making valves, pipe connectors and

automotive parts due to its outstanding mechanical properties, such as excellent corrosion

resistance, good formability and appropriate tensile strength and hardness. The surface, structural

25

and mechanical properties (yield stress, tensile strength and hardness) of brass can be further

improved by laser and ion beam irradiation.

We used laser induced Ni (conductor), Si (semiconductor) and C (insulator) ions of different

mass and nature for irradiation to explore the surface, structural and mechanical properties of

brass target. These ions of different radii have different effects on crystallite size, dislocations

line density and residual stresses of the brass. In case of laser induced plasma ions of Ni and Si

irradiated brass, we found that variations in surface morphology are similar, whereas the impact

of laser induced C plasma ions on the surface morphology of brass was different. Therefore, in

case of Pelletron accelerator we preferred to investigate the variations in properties of brass by

Ni and C ions irradiations.

First chapter of dissertation deals with the basic and fundamental principles of mechanisms

involved in laser and ion matter interactions. This chapter explains the laser and ion irradiation

affects on the surface, structural and mechanical properties of brass.

In the second chapter of dissertation a brief review related to the laser and ion irradiation effects

on surface, structural and mechanical properties of metals and their alloys and specifically on

brass reported by several research groups has been reported.

Third chapter deals with the experimental setups and their details for both Excimer laser and ion

beam irradiations. The detection and measurement of ions generated by laser induced plasma by

using Thomson parabola technique and its experimental details are also described briefly.

Furthermore motivations and working principles of different characterization techniques utilized

for investigation of irradiated brass are also explained in this chapter.

Fourth chapter describes the experimental results associated with the surface, structural and

mechanical properties of brass after laser and ion irradiations. This chapter comprises of three

parts. Part A deals with the laser irradiation effects on brass. Part B deals with the laser induced

plasma ions of Ni, Si and C ion irradiation effects on brass. It is consists of four parts. The first

part deals with evaluation of laser induced plasma ions energy and flux. Then results regarding

these laser induced plasma (Ni, Si and C) ions irradiations effects on brass targets are discussed.

Part C deals with the accelerator ions irradiation effects on brass. Micro/nano surface structuring

of irradiated brass and physical processes responsible for the formation of different kinds of

features of surface structures are discussed. A strong correlation of XRD analysis and

mechanical properties with surface morphological variations is also established in this chapter.

26

Conclusions are presented in chapter 5.

1.2 Laser matter interaction

When high intensity laser pulses in the form of electromagnetic radiations interact with the

material, they are either absorbed, transmitted or partially reflected depending upon the nature of

material [28, 29].

An atomic nucleus cannot be vibrated by a force induced by an electric field as being very small.

Therefore, inverse bremsstrahlung effect represents the processing of photons which are

absorbed by electrons. The vibrations of electron can either be restrained by the lattice phonons

(the bonding energy within a liquid or solid structure) or reradiate in all directions. These lattice

phonons are responsible to create the structural vibrations which are transmitted through the

structure by processes based upon the normal diffusion because the atoms of the structure are

linked with each other. Heat is generated due to these vibrations observed in the structures. The

intensity of the vibrations is enhanced by the absorption of the sufficient energy leading to

27

stretch molecular bonding so far that it cannot further exhibit mechanical strength and therefore,

melting of the material takes place [30]. The evaporation of the material takes place due to

loosing of the bonds caused by increase of strong molecular vibrations due to continuous

heating. The radiation can be still absorbed by the vapor however only slight radiation can be

absorbed as it will be equipped with only bound electrons [28].

The evaporation followed by the boiling from the surface is observed due to exceeding of

deposited thermal energy as compared to the latent heat of vaporization [31]. The plasma is

generated by the vaporized target surface induced by laser beams. When the pressure is raised

above the ambient range, the plasma is transformed to totally ionized vapor plasma as soon as it

gains elevated temperature [32]. The shock waves are generated due expansion of this plasma,

leading to exertion of mechanical shocks on the target material. The fatigue life of most metals

and their mechanical properties can be improved by employing these laser generated shock

waves within the confining medium. When target surface is irradiated by laser beam, a number

of processes take place such as photochemical, photothermal, photophysical and

photomechanical [33].

1.2.1 Mechanisms of energy absorption in metals

A material’s conductivity and dielectric function can help to derive the absorption coefficient

and this absorption coefficient can be further employed to find out the light penetration depth.

However, the nature of material decides the specific mechanisms of occurrence of absorption.

Generally, photons couples into states of vibrations in the substances being dependent on the

energy of the photon or into the available electronic states.

28

Figure1.1: Dynamics of photon absorption, electronic, vibrational excitation and relaxation

processes during laser matter interaction. The time scales relevant to stress

confinement and thermal confinement are also shown [34].

Inverse bremsstrahlung mechanism is responsible for the domination of optical absorption in

metals by the free electrons [35]. Collisions lead to the subsequent transfer of energy to lattice

phonons. The plasma frequency is an important parameter that relates the electron density of a

metal Ne to its optical characteristics. As electrons in the metal are directed towards the electric

field of the light therefore, absorptance and reflectivity for frequencies of light below the

frequency of plasma are high. Yet, absorptance and reflectivity are decreased gradually above the

plasma frequency, as the electrons are unable to response fast enough in order to be screened

[36]. Moreover, the states of vibration are concerned with impurities, defects or surface

mechanism like plasmons, polaritons and diffused electron scattering [37].

In semiconductors and insulators, the absorption of laser light is expected to occur through

resonant excitations like within bands (interband transitions) or transitions of valence band

electrons to the conduction band (interband transitions) [38]. The lattice phonons transfer energy

through these excited electronic states. Photons cannot be absorbed if their energy is below the

material’s band gap with an exception that there is multi-photon absorption or there might be

29

other impurities or defect states capable to couple. These energies generally respond to

frequencies of light below the spectrum ranging from visible to infrared for semiconductors and

below the vacuum ultraviolet for insulators. Yet, in some cases, under near-infrared region, there

is possibility of resonant coupling to optical phonons exhibiting high frequency [39].

The specific mechanisms and substances within the materials decide about the time required for

the transfer of energy by the excited electronic states to phonons and their thermalization. For

majority of the metals, the time of thermalization is in the order of 10-12 to 10-10 second.

However, the time of thermalization might exceed up to 10-6 s in non metals (semiconductor and

insulators) because of their significant enhanced variation in the mechanisms of absorption [38].

The details of electronic states excited transiently can never be significant if the excitation rate

induced by laser is low as compared to rate of thermalization. Rather, the direct transformation

of absorbed laser energy into heat can be considered. In such mechanisms called photothermal or

pyrolytic, a pure thermal way is applied to understand the response of the material. As an

example, photothermal mechanisms are responsible for the mechanisms observed in metals by

laser interaction. When the rate of thermalization is less than the rate of excitation induced by

laser, intermediary states are built up by large excitations. The photo-decomposition (breakage of

bonds) is due to these excitation energies. This kind of modification observed in non thermal

material is generally called photolytic (photochemical) processing [40].

1.2.2 The equation of heat energy

Elevated temperatures lead to observation of material response under photothermal processing.

Therefore, it has been crucial to make sure the heat flow inside the material. The heat equation

explains about the spatial and the temporal generation of the temperature field inside the

material. The Fourier’s law of heat conduction and law of conservation of energy help to derive

the heat equation, which states that negative of the temperature gradient are proportional to the

local heat flux. When a laser beam is used to fix the coordinate system, the heat equation can be

described as [38].

ρ (x, T) cp (x, T) vs ∇ T (x, T) - ∇ [k (x, T) ∇ T (x, T)] + ρ (x, T) cp(x, T) Ə T (x, T)

Ət

= 𝑄 (x, T) (1.1)

Where

ρ = mass density

30

cp = specific heat

k = thermal conductivity

vs = speed of substrate comparative to the source of heat.

Q = heat energy

The left hand side of above equation describes development of temperature in the surface by heat

conduction.

1.2.3 Theoretical explanation of heat conduction

Electromagnetic energy can be generated by laser. Excitation energy can be produced from this

electromagnetic energy by the interaction of laser with a metal [41]. The vaporization and

melting of the target metals is due to the energy transformed to the lattice via interaction of the

electrons with phonons. The overall atomic motion of the target atoms associated with

decreasing of stresses induced by the laser, can be enhanced by thermal energy [42].

Phonons (lattice vibrations) are among the main factors leading to the conduction of heat through

solid materials. Only lattice vibrations are observed being contributing in non magnetic materials

(semiconductor and insulator). But, electrons are considered as major contributors for metals

[43]. Conduction electrons are actually the free electrons moving in the metal. Conclusively, it

might be stated that electrons of each metal atoms are generally considered as free electron gas

occupying the majority of metal volume. The theory of thermal conductivity of a substance can

be expressed as following [44].

Q = kdT

dx (1.2)

In this equation, Q is defined as energy per unit area per unit time, while K denotes thermal

conductivity. Regarding metallic thermal conductivity, electrons play significant role for the

thermal energy conduction.

When the laser energy is absorbed by the electron of target surface, then the thermal diffusion

length can be noted in parameters of laser fluence by using following equation [44].

𝐿𝑒 =2.2486

𝑇𝑒 √

𝐼𝑜𝜏𝑝(1−𝑅)

𝛾𝛼 (1.3)

Where

Le = Thermal diffusion length of target electrons in material after laser irradiation.

31

Te = Temperature of surface electrons after laser irradiation.

R = Reflectivity of target material.

τp = Duration of laser pulse

Io = Initial intensity of incident laser pulse on target surface.

α = Absorption coefficient

𝛄 = Summerfield constant which describes the density of electrons at Fermi energy state.

Deposition of heat density on the target surface by laser fluence (F) can be calculated by

following formula [44].

𝐿𝑒 =2.2486

𝑇𝑒 √

𝐹(1−𝑅)

𝛾𝛼 (1.4)

In above equation F = Io τp

Therefore,

𝐿𝑒 =2.2486

𝑇𝑒 √

H

𝛾𝛼 (1.5)

Where H = F (1-R) is called deposition of heat density on the target surface by laser fluence (F).

Electromagnetic energy of laser beam is absorbed by target surface which can produce electric

field within the normal lattice system of irradiated target. The intensity of this laser induced

electric filed (E) can be evaluated by following relation [45].

𝐸 = √2𝐼𝑜

cnεo (1.6)

n = number of electrons

c = speed of electromagnetic radiations

εo = permittivity in vacuum

Consequently target electrons are accelerated under the action of laser induced electric force

from the irradiated zone with some velocity. The velocity of accelerated electrons can be

calculated by following equation [44].

𝑣 = 𝜏𝑝𝑒𝐸

𝑚𝑒 (1.7)

Where

v = velocity of accelerated electron inside the target surface.

32

E = laser induced electric field

e = electric charge of electron

me = mass of accelerated electrons

Hence

Le = S[J

Te√τp] (1.8)

Where

S= Shahbaz constant which depends on the physical properties of material [45].

J = electron current density

S can be calculated by following equation

𝑆 = √3.18 cεonme

2(1−R)

𝛾𝛼𝑛𝑜2𝑒4 (1.9)

The relation of laser pulse duration time τp with Le yields a significant result from the equation

1.8. Minus half power τp is proportional to Le indicating the dominance of heat conduction

generated in metals after being struck by a laser pulse for short duration of time. Just a time of

pico seconds is required by the electrons for transferring the absorbed energy through scattering

from lattice or coupling. Therefore, theory of conduction of heat in a specific direction is

dominant for a laser pulse being generated for less than a pico second duration of time. The

diffusion of heat continues after laser pulsing into the metal by phonons and electrons if the

duration of pulse is less than sub picoseconds [35]. But conduction of heat energy is only due to

the electrons during the pulse of that range. Pondromotive force can accelerate the electrons

towards the target when higher intensities of the order of 1019 W/cm2 are employed. This kind of

separation of charge leads to generation of electric field inside the target. Hence, these forces are

also supposed to play a significant role in heat energy transport in many directions, also for nano

seconds in addition to τp of the order of sub-picoseconds. Heat is observed to be affecting the

zone of 2 µm and 30–40 µm, when results of various experiments were compiled about

competitive study of nanosecond and femtosecond pulse interaction respectively.

1.2.4 Surface melting

33

The generation of transient pools of melted materials on the surface takes place due to fluences

above the melting threshold. Very high atomic solubilities and nobilities are supported by the

melted material rather than the solid phase, which lead to the enhanced homogenization of the

material. The quick dissipation of heat through a more cool surrounding material can generate

elevated self quenching rates along with process of solidification front velocities up to many m/s

[30]. The frozen in supersaturated solutes and defects in addition to meta-stable material phases

can also be due to quenching [46]. Recrystallization of larger grains as compared to the original

material can be achieved by decreasing the rates of resolidification. The dynamics of

recrystallization can be controlled by using the shaped beam profiles [47].

1.2.5 Laser induced plasma ions of metals

Very high excitations are generated in metal targets after irradiation with laser pulses. The skin

depth of metals is about 60-100 nm. The temperature of target enhances rapidly due to the

conversion of the excitation energy into heat energy. During the laser metal interaction the

temperature of subsurface is higher than the surface temperature. As a result it is able to diffuse

heat to the surroundings [48]. The distribution of an uneven pressure is due to non-uniform

distribution of temperature. Due to which the subsurface regime has higher pressure than at the

surface [13]. The point where the melt zone is produced during the laser irradiation, the ablation

takes place. The metal threshold intensities lie in the range of approximately 2 × 107 to 6 × 108

W/cm2 [48].

1.2.6 Laser induced plasma ions of semiconductors

When the surface of semiconductor material is irradiated by laser beam, energy of laser photons

is absorbed by surface atoms. The absorbed energy converts into heat energy which causes to

increase the temperature of surface and subsurface of target. The number of charge carriers

increases by increasing temperature. Consequently an extensive distortion and disordered zone

are formed by high energy particles irradiation. By reason of electronic excitation, electron hole

plasma is produced. The electronic mechanism is activated when the ablation threshold is very

much high. Therefore metallization of the semi conductor occurs.

1.2.7 Laser induced plasma ions of insulators

In the case of insulator material the valence electrons have greater ionization potential or band

gap than energy of laser photon. The valence electrons cannot absorb laser energy at low

intensities of laser beam (long laser pulse). Yet a few numbers of free electrons that present in

34

the insulator material due to the impurities are called seed electrons. These electrons impact with

valence electrons and lattice atoms (inverse bremsstrahlung) as result of absorbed laser energy.

The kinetic energy of accelerated seed electrons is in surplus amount of ionization potential of

valence electron. Thus ionization produces because these seed electrons impact with valence

electrons. After laser interaction with insulator material, many free electrons are produced

therefore preliminary phase take place. Ionization is produced due to the two simultaneous

methods, multiple photons absorption and the avalanche ionization by the collision of electron.

The comparative contribution of the both processes is dependent upon parameters of incident

laser beam (pulse duration, laser beam intensity, beam energy, wavelength) and atomic number

of target sample [49].

1.3 Effects of laser irradiation on surface, structural and mechanical

properties of metals.

The laser-matter interaction is the powerful tool, not only for the scientific point of view but also

in technological point of view. [1,2]. Laser matter interaction has vast ranging applications in

material processing e.g. laser cutting, drilling, surface treatment, patterning, thin film deposition,

creation of microstructures, production of micro and nano-electronic devices, magneto

hydrodynamic generators and ion implantation [50-57].

Modification of material by laser beam requires the sufficient excitation and movement of

electrons, ions, atoms and molecules of target surface. In order to achieve atomic motion, in

general three conditions must be met. First, threshold intensity is required to start the process.

Second, vibrational energy and electronic excitation which are directly generated by the

absorption of photons or by multi-phonon cascades. Third, energy absorbed by the target surface

must be sufficient to start and sustain the breaking of chemical bonds between the surface atoms.

The energy of surface electrons is increased by deposition of energy from incident laser photons

on the target surface. At that time this absorbed energy is transferred to the target normal lattice

systems. Therefore the absorbed energy is converted rapidly into heat energy by collisions of

electrons with lattice atoms. The local heat source is generated in subsurface of irradiated target

surface and is transferred throughout the crystal by thermal shock waves and conduction. As a

result, the surface temperature (Ts) is changed and following situations can take place.

35

(1) If surface temperature (Ts) is less than melting temperature (Tm) then the irradiated material

remains in the state of solid. But it can experience solid state transformations. For laser

hardening of the material this regime is useful.

(2) If vaporization temperature (Tv) is greater than melting temperature (Tv ˃ Ts ˃ Tm) then the

irradiated target surface is melted without appreciable vaporization. This regime is related to

liquid phase laser treatments.

(3) If vapor temperature is less than surface temperature (Tv ˂ Ts) then the irradiated surface is

vaporized partially. This regime is utilized in laser welding and laser drilling.

The energy deposition rate is very fast in laser solid interactions and results not only in the form

of rapid melting, vaporization and resolidification in surface and subsurface of irradiated target

but is also responsible for the production of strong compressive and tensile stresses in normal

lattice sites system of target. The values of laser induced stresses are predominantly in stress

confinement regime [58-60].

The temperature of irradiated surface increases as more and more photons are immersed.

Consequently various surface features namely, cavities, craters, bumps, cones, ripples, hillocks

and dendritic structures are produced. These features generate many defects in the normal lattice

sites system after laser irradiation.

When the time required for mechanical expansion (relaxation) of the heated zone of laser

irradiated target is greater than laser heating time which is defined by laser pulse duration then

the relaxation of laser induced stresses take place simultaneously with melting, vaporizations and

resolidification of heated zone of irradiated target surface [61]. Therefore high density of crystal

defects namely, points defects, interstitials, vacancies, dislocations and grain boundaries are

generated. These crystal defects play vital role in modification of structural, chemical and

mechanical properties of target [25, 55, 62].

1.4 Ion matter interaction

Surface, structural and mechanical properties of materials can be modified by interaction of

energetic ions with solid surface. Figure 1.2 depicts that during ion material interaction several

phenomena such as ion induced scattering, sputtering and implantation take place. The collisions

between incident ions and host atoms are responsible for slowing down ions due to transference

of energy to the host atoms. The necessary energy required to displace a lattice atom from their

equilibrium lattice site is called displacement energy (Ed). During the elastic interaction the

36

energy transferred to the lattice host atom is denoted by EL. If in the elastic collision between

incident ions and lattice atoms, the energy shifted to lattice host atom EL is less than Ed, then

struck atom does not displace from its normal lattice position and perform vibrations with large

amplitude [63].

Figure 1.2: The illustration of various phenomena involved in ion material interaction.

The vibrational energy is shared with nearby lattice atoms very quickly. Therefore energy

transferred to the lattice atoms by ions appears as a localized heat source. The temperature of the

impacted zone increases up to thousand degree celsius [13].

Figure 1.3: The representation of the formation of primary knock out atoms by elastic collisions.

Consequently, a large distortion is produced in normal lattice site after elastic collisions. On the

other hand if transferred energy is greater than displacement energy, the host atoms are able to

37

remove from their normal site. The vacancy is produced by the displaced atom in normal lattice

site as shown in figure 1.3.The displaced atoms are occupied as an interstitial site in the normal

lattice site. This process is responsible for generations of primary knock on atoms (PKAs). The

vacancy interstitial defect is referred to as a Frenkel pair (Frenkel defects). The displacement

energy is an intrinsic property of material and depends on binding energy of material [64].

1.4.1 Stages of cascade growth

There are four stages of cascade growth during ion-matter interaction

1.4.1.1 Collisional

1.4.1.2 Thermal spike

1.4.1.3 Quenching

1.4.1.4 Annealing

1.4.1.1 Collisional stage or displacement cascades

In the collisional period, displacement collisions which are initiated by primary recoil atoms

persist until knock out atoms contain deficient energy to produce further displacements. At the

end of this period (1ps) a damage zone is formed which consists of large number of energetic

dislodged atoms. Nevertheless, stable lattice defects have not enough time to be generated [65].

1.4.1.2 Thermal spike

When the creation of collisional cascades come to an end, then the energy of dislodged atoms

will be shared quickly between nearby lattice atoms. Finally this energy appears in the form of

lattice vibration or localized source of heat. This stage of lattice heating is called thermal spike.

The time required for the development of thermal spike is 0.1 ps [66].

1.4.1.3 Quenching stage

In thermal spike zone the temperature of the lattice system is high enough (greater than melting

point) so that material seems like liquefied material. After a time of 10 ps molten region returns

back to the condensed phase because the energy of atoms in thermal spike zone is transferred to

nearby atoms. This stage is called quenching stage. The quenching stage can take time of many

ps and forms stable lattice defects which appear either as point defects or as defect clusters.

However, for this period the total number of defects is significantly smaller than number of

defects that are generated in displacement spike (collisional stage).

38

Figure1.4: The various processes involved in ion matter interaction for generation and evolution

of lattice defects.

1.4.1.4 Annealing stage

In the annealing period auxiliary recombination and interaction of points or clusters defects take

place by thermally stimulated diffusion of mobile lattice defects.

Figure 1.5: Multi stages of cascade growth during ion matter interaction [67].

39

According to the definition of annealing process, annealing stage exists until all mobile lattice

defects are removed from the cascade region or another cascade is produced surrounded by it.

Therefore the time scale prolongs from nanoseconds to weeks/months/year which depends on the

annealing temperature and condition of irradiation [67].

1.5 Ion irradiation effects for surface, structural and mechanical properties

of metals and their alloys

Ion irradiation of metallic alloys is important not only in research fields but also has many other

applications in engineering, medicine and industry [68]. Ion irradiations are extensively utilized

to change the properties of metals and their alloys and enhance their performance according to

applications. The investigation on surface, structural and mechanical modification of metals and

their alloys by ion irradiation along with the exploration of fundamental principles of ion matter

interactions is persistently growing [13, 69]. The surface modification in many cases is

characterized by cracks, voids, grains, craters, cavities, pits and dendrites. The modification in

surface structure of target material by ion irradiation depends on the parameters of incident ion

(energy, mass and flux) and properties of target material (physical, chemical, structural and

mechanical properties). [70]. The shape and size of ion induced surface features can be changed

by increasing energy and flux of incident ions. The variations in the ion induced surface features

after ion material interaction are attributed to a large number of defects i.e. induced stresses,

Frenkel pair defects, diffusion of ions and other heterogeneities. The non uniform energy

absorption, cascade collisions, lattice relaxations, shock waves and inelastic thermal spikes are

responsible to generate these defects [71, 72]. Modifications in microstructure and mechanical

properties (yield stress, ultimate tensile strength and hardness) of the target are due to these ions

induced defects in lattice sites of irradiated material.

1.6 Laser induced plasma as an ion source

When laser beam energy is absorbed by the target surface, the conversion of laser energy into

electrical excitation, thermal energy, chemical energy and mechanical energy takes place in few

picoseconds which is responsible for evaporation, ablation and plasma formation [73]. The

generation of plasma in this way is called Laser Induced Plasma (LIP) [74, 75]. The various

factors on which the nature and properties of laser induced plasma depend upon are wavelength,

number of shots, pulse duration, angle of incidence of beam with respect to target surface, energy

deposited on the target surface, position of target surface with respect to focal position,

40

properties of target material, nature and pressure of ambient gas [57]. The region of induced

plasma with high concentration of species (ions, neutral particles, electrons, electromagnetic

radiation etc) is called Knudsen region. In this region temperature of plasma species is very high

due to mutual interactions and collisions of species with each other. The thermal interactions, the

adiabatic expansion in vacuum and coulombic interactions are responsible for primary ion

acceleration [13]. The laser induced plasma is a useful source for generation of ions. The energy

and flux of these ions are dependent on various parameters of laser (energy, wavelength and

pulse duration), material (atomic number, atomic mass and work function) and environmental

conditions. The energy and flux of laser induced plasma ions can be estimated by using various

diagnostic techniques e.g. Solid State Nuclear Track Detection Technique (SSNTD), Faraday

Cup (FC), Time of Flight Mass Spectrometer (TOFMS), Langmuir probe (LB) and Laser

Induced Breakdown Spectroscopy (LIBS).

The laser induced plasma is an important ion source which has many important applications in

the field of science and technology i.e. ionography, surface modification of target material

(metal, ceramic and plastics) and for the nanocrystal semiconductors generation[8, 12, 76, 77] .

The ions ejected from the laser induced plasma plume have angular distribution in the form of

cone [73]. Different techniques for examples Thomson parabolic technique, (Solid State Nuclear

Track Detectors (CR 39)), Faraday cup, plasma imaging by CCD can be used to study the

angular distribution of laser induced plasma ion with respect to energy and flux [16]. The ion

energy can be increased up to required value by two significant methods which operate on

temperature of plasma and drift velocity separately. The temperature of plasma and drift velocity

is increased by inverse bremsstrahlung and presence of high electric field inside the plasma

plume itself respectively. Further these ions are deflected with the help of externally applied

strong magnetic fields. Therefore a device fabrication which produces high energy and flux ion

beam is called laser induced ion accelerator. The control on the dynamic properties of transient

and energetic plasma can be improved by using electric field as well as magnetic field of

appropriate strength. During the expansion of plasma in the presence of external magnetic field,

several interesting processes namely alteration of plasma energy, conversion of plasma species

velocities, plasma ions acceleration and plume confinement are initiated. The velocity of

energetic ions can be controlled with the help of magnetic field. It has been reported that

significant variations are produced in velocity, density and temperature of plasma species by

41

applying the magnetic field [21, 78]. These accelerated ions can change the surface, structural,

chemical, electrical and mechanical properties of target material after treatment [18, 79].

1.7 Pelletron linear accelerator as an ion source

The interaction of different materials with beams of high energy ions generated by accelerators is

significantly important for the material processing. The bombardment of high energies ions on

target materials leads to the generation of different physical processes. For studying these

phenomena, new accelerators had been developed. As modification was necessary to expedite

the progress in research of accelerators therefore, some limitations are observed in their

applications.

Pelletron Accelerator has been considered among the types of electrostatic particle accelerator. It

is employed for the acceleration and production of ion beams of about all elements. The stability

of high terminal voltage is its important characteristic. The ion beam is accelerated in two stages.

The desired negative ions generated through the source are injected into the accelerating tube

provided with its positive terminal at high voltage, in the first stage. The beam of negative ions is

accelerated in the direction of terminal supplied with high positive voltage.

The exchange of charge takes place in a stripping canal meanwhile the beam of negative ion is

converted into beam of positive ion. This entire process takes place in the centre of the

accelerating tube. The beam of positive ion is accelerated in the direction of the ground terminal

at the other end of the accelerating tube, in the second stage. The desired charged particles with

the required energies are provided by a second mass selecting magnet [1].

1.7 Selection of Brass as a target

Brass is an alloy which is prepared by predominantly of copper and zinc. The main component in

brass alloy is copper which changes by weight from 55% to 95%. This variation of copper

amount in brass alloy depends on the type of brass and its application. Zinc is a second

component of brass which changes by weight between 5 % to 40 % according to class of brass.

The elasticity and ductility of brass is dependent on the amount of zinc. The smaller amount of

Zn makes the brass soften but with excellent corrosion resistance [80]. Therefore, it is generally

classified as a copper alloy. Brass alloy has superior feasibility, drawability, and excellent

properties on plating. Among the copper zinc alloy brass demonstrates exceptionally good

electrical and conductivity along with high elastic modulus at a moderate strength level than

copper. The brass colour changes from dark reddish brown to a light silver yellow which

42

depends on the Zn amount present in alloying. Brass has superior combination of corrosion

resistance, hardness, ultimate tensile strength and formability. These valuable properties of brass

have made it one of the frequently commonly used materials in foreseeable future.

Table 1.1: Different types of brass

UNS Common Name Colour

C11000 ETP Soft-pink

C21000 95/5 Gilding metal Red-brown

C22000 90/10 Gilding metal Bronze-gold

C23000 85/15 Gilding Tan-gold

C26000 70/30 Brass Green- gold

Brass was irradiated by laser and ions beams to investigate the variations in surface morphology,

structural modifications and enhancement in mechanical properties.

43

Chapter 2

LITERATURE SURVEY

44

1.1 LITERATURE SURVEY

Laser and ion induced imperfections in crystal lattice of metals and their alloys and their

association with the improvement in surface, structural and mechanical characteristics has been

reported by several research groups. A brief review is presented below.

Tam et al. [7] described the laser surface modification of brass (Cu- 38%Zn) with different

elements (Ni–10%Cr–7%Al–5%Mo–5%Fe–1%B) via 2 kW CW Nd: YAD laser. The laser

power density was 70 W/mm2 and optimized scanning speed from 15 to 35 mm/s. The hardness

of unirradiated target was 110 HV which is increased up to 256 HV after laser irradiation.

Bashir et al [5]. Exposed the brass surface by Nd:YAG laser (532 nm, 150 mJ, 10 Hz) in

different ambient environment (deionized water, methanol and air) with number of pulse

variations from 1000 to 4000. SEM, AFM and Attenuated Total Reflection (ATR) techniques

were used to investigate the surface morphology and chemical composition of irradiated brass

target. It is reported that wet ambient environment is superior tool for nanostructuring of brass

target because dry ablation is responsible for ejection of material from the target surface on

larger scale.

Zhao et al. [81] developed a relationship between efficiency of removing oxide layer and

defocusing distance. Naturally oxidized 2 mm and 16 mm brass rings surface were cleaned

through three dimensional dynamically focused laser galvanometer scanning system. The surface

morphology, element contents and roughness of surface were studied with the help of SEM, EDS

and AFM respectively. Results revealed that the surface of irradiated brass target became smooth

after laser cleaning.

Wang et al. [82] described a new method in order to increase the mechanical properties

(hardness, wear resistance and fatigue) of metals and their alloy by laser shock processing (LSP).

They used different laser pulse intensities for LSP to irradiate the brass target. The results reveal

that no remarkable variations in microstructure are observed whereas microhardness, roughness

and wear resistance are increased by enhancement in laser intensity.

Daniel et al. [83] irradiated the Al-Cu-Mg alloy by various fluences (3.8 J/cm2, 4.3 J/cm2,

4.7J/cm2, 5.1 J/cm2 and 5.5 J/cm2) of KrF laser in vacuum condition. The variations in surface,

structural and mechanical properties have been studied with the help of various scientific

techniques such as SEM, XRD and UTM respectively. The micro sized craters as well as ripples

growth information was collected from SEM image. From XRD pattern anomalous behavior is

45

observed in peak intensity with increasing fluence. The variation in mechanical properties (YS,

UTS and hardness) of irradiated alloy reveals the anomalous behavior with increasing laser

fluence.

Mahmood et al. [84] presented a variation in mechanical properties of 4N polycrystalline

titanium after laser irradiation. In this work, target surface were irradiated by Nd: YAG laser

(532 nm, 370mJ and 10 Hz) with shot ranging from 100 to 400. They found that hardness of

target surface is increased up to 400 numbers of shot whereas YS and UTS is decreased from

100 to 300 shots and is finally increased at 400 laser shots.

Impact of single and double shot laser shock processing (LSP) on residual stresses and

mechanical properties of 6061-T6 aluminum alloy was explored by Irizalp et al. [85]. The

significant improvement in surface compressive residual stresses, mirohardness and tensile

properties of aluminum alloy after single and double Laser Shock Processing (LSP) technique

has been observed. The uniform elongation of unirradiated target was 3.2 % which is increased

to 5.8 % after LSP. While the tensile strength of irradiated target was 50% higher than

unirradiated target.

Effect of KrF excimer laser on hardness of the AgxPd1-x alloy (x = 0.40, 0.50 and 0.60) was

investigated by Butt et al. [62]. Each alloy was irradiated by stepwise increase in laser shot (100,

200, 300 and 400) after heat treatment at 800Co. Authors observed an improvement in

dislocation line density by increasing number of laser shots. As a result the surface hardness of

alloy is improved after laser irradiation.

Milovanovic et al. [86] presented the effect of 222nm KrCl and 308 nm XeCl excimer lasers on

the surface of Ti6Al4V titanium alloy. The target surface was irradiated by single pulse and

multi pulses with various fluences of laser beam. At high fluence of 7.2 J/cm2, an extensive

damage was produced by laser material interaction which appears in the form of craters, wave

like features along with droplets and cracks.

Huang et al. [87] studied the variation in surface morphological features of brass after irradiation

by Nd:YAG laser (1064nm, 30nm, 10Hz) pulses. The surface before and after laser irradiation

was characterized by Scanning Electron Microscope (SEM) and size of periodic ripples is

measured by optical and atomic force microscope. Whereas, number density and temperature of

brass plasma during laser irradiation was found by Langmuir probe system. The results reveal

46

that diameter of heat affected area is decreased while laser induced ripple periodicity is increased

from 0.93 µm to 1.03 µm with increasing laser beam fluences.

Rosinski et al. [20] irradiated the surface of different targets of Cu, Ag and Ta with highly

energetic iodine laser beam for the generation of ion beam and used these ions for implantations.

The laser induced plasma ions were probed by utilization of ion diagnostics techniques such as

ion collectors, electrostatic ion energy analyzer and found the depth of these ions into substrate

by using Rutherford backscattering spectrometry. The substrates (Al, Ti and C) were positioned

at the distance of 20 cm from the target surface for laser induced plasma ion implantation. The

maximum charge state of 25+, 40+ and 57+ for Cu, Ag and Ta respectively were found by

authors. These highly charged ions also attained high energy of about 1, 2.5, and 10 MeV for Cu,

Ag and Ta respectively. They also reported that the penetration depth of these induced ions into

different substrate totally depend upon ion energy, kind of ion nature and chemical composition

of substrate.

Ahmad et al. [13] irradiated Al target with 500 shot of Nd:YAG laser beam for generation of

high energy plasma plume of Al ions in vacuum. From the plasma plume, Al ions were

accelerated by applying highly external voltage (0 to 10 KeV). In order to measure the energy of

laser induced plasma ions, Thomson parabola technique was employed and ions were detected

by solid state nuclear track detectors (CR-39). For ion implantation, Al substrate was exposed to

these Al ions. The exposed Al substrate was examined by using Digital Optical Microscope

(DOM) and Atomic Force Microscope (AFM). The authors reported that the energy of these ions

were 627, 651, 676, 702 and 730 KeV corresponding to applied acceleration voltage of 2, 4, 6, 8

and 10 KeV respectively. Furthermore the mechanical properties of irradiated metal decreases

due to thermal sputtering.

Laska et al. [88] produced 300 MeV Au ions with up to 58+ charge state by employing iodine

laser beam. The flux of these induced ions was dependent on frequency of laser irradiation (1ω

and 3ω), detection angle (0o, 10 o and 30o) and distance between ion collector detector and

irradiated target. The authors reported that a massive amount of Au ions was generated by 1ω

laser frequency than 3ω and the majority of fast ions were detected at 10o position with respect to

normal of the target.

Torrisi et al. [89] used 1064 nm Nd:YAG laser beam to irradiate the metallic target surface (Al,

Cu and Ta) under ultra high vacuum condition. Ion collectors were arranged with respect to time

47

of flight technique for detection of ions along the normal direction of the target surface.

Measurements were performed with and without the application of a 0.1 T induced magnetic

field. The authors presented that magnetic field arrangement traps electron in front of surface of

target along the axial direction. The ions were accelerated by electric field within the trap. This

electric field is not only used for focusing of ion beam on the target surface but also improving

the ion energy.

Rafique et al. [90] irradiated the copper target to Nd: YAG laser (1.064 µm, 10mJ and 1.1MW)

induced Cu ions. The authors used Faraday cup configuration and Thomson parabola technique

for detection and measurement of charge states and energy of Cu ions from induced plasma

plume respectively. For energy measurement the authors made collimated beam from ion beam

through pinhole preparation. Afterward collimated beam was passed through uniform induced

magnetic field (0.4 T). PM-355 solid state nuclear track detector was utilized for measurement

of ion flux and ion tracks. They reported that massive amount of singly positive charged Cu ion

were present in ion core from induced plasma plume. Furthermore they found that the energy of

induced ions lie in the range from 9 KeV to 50 KeV.

Rahman et al. [91] investigated the effect of uniform magnetic field on the temperature and

density of electrons produced by laser material interaction. Pure silver (4N) targets were

irradiated at 45o with repect to the normal surface by Q- Switched Nd: YAG laser system. For

the measurement of temperature and density of electron self fabricated Langmuir probe was

used. This probe was placed at different position in nonexistence and existence of uniform

magnetic field (1.2 T). Authors observed that in the presence of external strong and uniform field

significant reduction in temperature, energy and density of laser induced electrons were obtained.

Peng et al. [92] experimentally observed that the microhardness depends on the solidification

processing parameters temperature gradient, growth rate and primary dendrite arm spacing. They

developed a simple relationship between solidification processing parameters and microhardness.

Authors found that values of primary dendrite arm spacing are reduced by increasing growth rate

values whereas under constant temperature gradient, microhardness values enhance with growth

rate.

Wang et al. [93] studied the surface, structural and mechanical properties of irradiated Zr-45Ti-

3V alloy by 84 MeV Carbon ion with a dose ranging from 4x1015 ions/cm2 to 12x1015 ions/cm2.

X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron

48

Microscopy (TEM) and tensile testing were used to investigate the growth of microstructure,

surface morphology and modification in mechanical properties of target alloy after ion

bombardment. The authors observed from XRD pattern that no new phase was formed whereas,

the variation in peak intensity and position depends upon ion irradiation dose. The significant

increased in ultimate tensile strength from 693MPa to 826 MPa and a decrease in percentage

elongation by increasing ion dose was observed. These changes in the mechanical properties of

irradiated alloy are attributed to ion induced defects in normal lattice site.

Sun et al. [94] studied the variations in microstructure and mechanical properties of irradiated

Fe-14Cr-16Ni alloys after He ion bombardment. They pointed out that interaction between

induced barriers i.e. point defects, precipitates, pits and voids play an important role in ion

irradiation hardening and embrittlement phenomenon. For dislocation activation, more stress is

required on slip plane because of pinning effect of the induced barrier.

Singh et al. [95] developed a relationship between neutron fluence and mechanical properties

such as yield stress and tensile properties of stainless steel and titanium metal targets. They

observed that yield stress and tensile strength are directly proportional to the fluence.

Zuo et al. [96] irradiated the Ti-6Al-4V alloy target by hydrogen and nitrogen ion beams. The

surface and microstructure of irradiated target alloy was evaluated by SEM, TEM and XRD. The

results summarized that after H2 ions irradiation, large degree of decline is produced in tensile

and fatigue properties of target material than N2 ions. The fine grains generation and new

induced phase of TiH are the main reasons to induced brittleness when the alloy is irradiated by

H2 ions. Whereas in case of N2 ions irradiation, the mechanical properties of irradiated alloy is

reduced by single effect of induced fine grains.

Rafique et al. [97] investigated the mechanical properties of zirconium after irradiations by

neutrons at constant temperature (423K) with various neutron fluence of 1.22x1020, 4.87x1020

and 9.70x1020 n/m2. The effect of neutron irradiation on the target material is estimated by

employing different characterization processes such as Universal Tensile Machine (UTM),

Vicker hardness tester and XRD. The result indicate that the Yield Stress (YS), Ultimate

Tensile Strength (UTS) and micro hardness slightly reduce after neutron material interaction

whereas percentage elongation increases slightly by increasing neutron fluence. These deviations

in mechanical properties are due to the production and mutual annihilation of small number of

induced defects in irradiated Zr.

49

Budzynski et al. [98] investigated effect of 250 MeV Kr ions on wear resistance and micro

hardness of Ti-6Al-4V alloys with various ion fluences. They found that at lowest fluence the

wear resistance of irradiated target in vacuum and argon environment is improved as compared

to unirradiated target. While wear resistance is drastically decreased and microhardness is

increased to 25% at high fluence.

Pelletier et al. [15] studied the effect of nitrogen ion fluence on mechanical properties of 316 L

stainless steel. The target surface was irradiated by various fluence ranging from 5 × 1016

ion/cm2 to 1018 ion/ cm2 at 1 MeV fixed ion energy. The hardness is measured before and after

ion implantation by nano indenter XP (MTS Corp.) and is reported that hardness linearly

increases from 5.25 GPa to 7.5 GPa by increasing ion fluence.

Liu et al. [99] irradiated the Ni3Al base alloy with intense beam of ion by use of acceleration

voltage (250 kV), current density (100-200 A/cm2 ) and pulse duration (60ns). They estimated

that the formation of radiation induced craters is the major factor to increase the surface

roughness of irradiated surface. As a result the variation is produced in mechanical properties of

alloy.

Iqbal et al. [100] studied the surface, structural and mechanical properties of bulk amorphous

alloys [(Zr0.66Cu0.18Ni0.09Al0.08)98 M2 (M = Er and Gd)] after bombardment of argon ions. They

characterized unirradiated and irradiated targets by SEM, XRD and Vicker’s hardness tester.

Significant improvement in mechanical properties such as hardness and elastic modulus was

observed after ion beam irradiation.

Liu et al. [14] performed an experiment in which steel target was irradiated by He- ions with

500KeV energy and 6.07x1017 ion/cm2 fluence. The scanned beam size of 17x17 mm2 and

average beam current of 6.50µA were used during experiment. The effect of He-ions on the

surface morphology of steel target was evaluated by TEM. The authors found that size of grain at

the penetration depth is modified after ion irradiation. During the ion irradiation, significant

reductions in grain boundary by the atomic diffusion of atoms are attributed to ion induced

thermal spike.

50

Chapter 3

EXPRIMENTAL SETUP

AND

CHARACTERIZATION

TECHNIQUES

51

EXPRIMENTAL SETUP

AND CHARACTERIZATION TECHNIQUES

This chapter comprises of following five parts.

Part A: Sample preparation

Part B: Experimental details for laser irradiation

Part C: Experimental details for exposure of brass target by laser induced plasma ions

Part D: Experimental details for exposure of brass target by Pelletron accelerator

Part E: Characterization techniques

52

3.1 Sample preparation

The polycrystalline brass (70 % Cu, 30 % Zn) in the form of cylindrical rod was selected as a

target material. In order to prepare the brass targets for laser and ion irradiation following two

steps were involved.

3.1.1 Cutting, grinding and polishing

3.1.2 Annealing

3.1.1 Cutting, grinding and polishing of samples

Rectangular shaped brass targets with dimensions of 45mm ×6mm × 3mm were cut from as

received brass rod by using computerized numerical control wire cutting machine. After wire

cutting the target surface were extremely rough. The surface of these targets was grinded and

mechanically polished with Silicon Carbide (SiC) papers of different progressive grades ranging

from 500 to 3000 micrometer.

3.1.2 Annealing

The prepared samples were enclosed in the Pyrex tubes and vacuum of 10-6 Torr was created by

using rotary pump followed by diffusion pump. Then the vacuum-sealed tubes containing

specimens were annealed at 773 K for 1 hr under vacuum 10-6 Torr in high temperature furnace

(model Nabertherm-LHT-02/18, Germany). The aim of this process is to relieve the brass targets

from internal stresses and defects. After annealing, the specimens were ultrasonically cleaned for

30 minutes with acetone.

3.2 KrF Excimer laser system

Excimer laser KrF consists of 90% He or Ar, below 10% Kr and about 0.5 % Fl gas. The

excimer laser system (GAM LASER Inc. USA model EX200) was employed as a source of laser

irradiation. Its pulse energy varies from 70mJ to 150 mJ, pulse duration is 18 ns, wavelength 248

nm and repetition rate varies from 1 Hz to 120 Hz. It is controlled by EXLASER software. Both

laser energy and repetition rate is controlled through software. The pulse energy of laser beam is

measured by power energy meter (1-800-383-0814 U.S.A).

3.3 Experimental details for exposure of samples by Excimer laser

(a) Fabrication of the target holder for laser irradiation

In order to expose the whole surface of brass target of length 45 mm uniformly by laser pulses,

there are two ways of scanning. In the first way, the target is fixed and laser beam is scanned

uniformly. The second way for exposure is that the laser beam is fixed and the target is scanned

53

with some constant scanning speed. For the present experiments the second technique was

employed in which the laser beam was kept stationary and the target was placed inside the target

holder which was able to scan the target uniformly. For this purpose a rectangular shaped target

holder made up of Perspex was designed and fabricated to place the brass target inside the

vacuum chamber. This target holder is connected to the stepping motor which is controlled by

variable DC voltage outside the chamber. The purpose of this target holder is to scan the target

horizontally during laser irradiation.

Figure 3.1: An inside view of self fabricated target holder for exposure of brass to laser pulses

with scanning arrangement.

(b) Experimentation for exposure of brass targets by Excimer laser

KrF Excimer laser (EX GAM USA 200) with the 248 nm wavelength, 20 ns pulse duration, 150

mJ maximum pulse energy was employed to expose brass by laser pulses. After grinding,

polishing and annealing, rectangular shaped brass targets (45× 6× 3 mm3) were mounted on a

sample holder attached to DC motor. The mounted brass targets were placed in UHV chamber.

The chamber was evacuated to a base pressure of 10-9 Torr with the help of rotary and turbo

molecular pump. Then, pure oxygen gas (5N) at a pressure of 100 Torr was filled in the chamber.

The purpose of this gas is to enhance the irradiation effects of laser on the target surface by

54

restricting free expansion of plasma species and confining plasma. A laser beam was focused

through a focusing lens of focal length 50 cm and was incident at angle of 90o with respect to

target surface. The targets were scanned with the help of DC motor. The area of 40 mm x 2 mm

was exposed to laser pulses by a scanning speed of 0.55mm/s.

Figure 3.2: The schematic diagram of experimental setup for exposure of brass target by laser

irradiation at constant laser fluence of 6.4 J/cm2 under oxygen environments at

various pulses.

Four brass targets were exposed to various pulses of 1200, 1800, 2400 and 3000 at a constant

laser fluence of 6.4 J/cm2 under oxygen environments.

Figure 3.3: The photographic view of experimental setup for exposure of brass target by laser

irradiation.

55

3.4 Experimental details for exposure of sample by laser induced plasma ions

(a) Laser induced plasma ions emissions and detections by Solid State Nuclear Track

Detectors (SSTDs)

Nanosecond Excimer laser (EX GAM USA 200) with wavelength of 248 nm, pulse duration of

20 ns and energy of 120 mJ with constant fluence of 5.1 J / cm2 was employed as a source of

irradiation to produce plasma of Ni, Si and C.

In order to provide appropriate trajectory to the plasma ions, Thomson parabola scattering

technique was used. The magnetic field of strength 80 mT was applied with the help of two

permanent magnets. These magnets were placed in Teflon housing and separated at a distance of

60 mm.

In the first set of experiment, the detection and measurement of ions flux and energy is estimated

by using Solid State Nuclear Track Detection Technique (SSNTDT). CR-39 was used as Solid

State Nuclear Track Detector. The 3N pure Ni, Si and C circular shaped discs of diameter 20

mm and thickness of 10 mm were grinded, polished and ultrasonically cleaned. These targets

were mounted on aluminum target holder turn by turn. In order to avoid non uniform heating and

ablation of target, an arrangement was made to rotate the target constantly during experiment

with the help of DC motor. Four CR 39 detectors with dimension of 10 × 20× 1 mm3 were

selected and mounted on substrate holder at angle of 5o with respect to the normal of the target at

a distance of 25 mm from the target surface. The schematic of experimental setup is shown in

figure 3.4. The chamber was evacuated to a base pressure of 10-9 Torr by using rotary pump and

turbo molecular pump. After passing focusing lens of focal length 50 cm the laser beam was

incident on target surface at the angle of 45o with respect to normal. Each target was exposed

with various number of laser pulses 3000, 6000, 9000 and 12000 at constant fluence of 5.1 J/cm2.

This fluence corresponds to laser pulse energy of 120 mJ and focus spot size of 0.0235 cm2.

After the laser matter interaction, laser induced plasma plume was generated consisting of

electrons, ions and neutral species etc. Plasma ions after passing through magnetic field travel on

parabolic path according to their mass, charge and energy state and are irradiated on the detector

surface.

Figure 3.4 depicts two streaks of ions. One is the central non-deflected ion beam which consists

of neutrals and second one is the deflected beam of ions which consists of singly charged ions.

56

Tracks are developed at the CR39 surface by the interaction of ions and detector surface. The

irradiated CR39 detectors were etched in solution of NaOH (6 N) at 75Co for 8 hours.

Figure 3.4: The schematic of experimental setup for exposure of CR39 by laser induced plasma

ions.

Finally the etched detectors surface was observed by using optical microscope (Motic DMB

Series). Figure 3.5 is a graphical representation of the Thomson parabola technique. For

measurement of the energy of plasma ions, the following equation was used [22].

E =e2B2R2

2 M J (3.1)

In this equation e (electron charge) = 1.6x10-19 C, B (magnetic field strength) = 80 mT, M is

mass of plasma ions in Kg.

R is radius of curvature of ions in the presence of magnetic field and is estimated by using

following equation [22].

R = r[X + √X2 + 1] (3.2)

Where, r (radius of magnetic field) = 10 mm. X is determined by simple equation [22].

X = Y

d (3.3)

Where, Y is the distance between detector (CR-39) and central point of magnet which is equal to

30 mm and d is the displacement of ion track on the surface of CR-39 found by digital optical

microscope which is equal to 95 µm.

57

Figure 3.5: The graphical representation of the Thomson parabola technique.

From this equation the estimated average energy and flux of Ni, Si and C ions are shown in table

3.1. In order to calculate the flux of these ions (ions/cm2), number of ions tracks on each detector

was counted in specific area of 50000 µm2. This area was also measured with the help of optical

microscope. All parameters for the generation of ions were kept constant i.e. laser fluence,

detection angle, distance of substrate (brass) from the target and environmental condition (UHV

≈ 10-9 Torr). Therefore the energy of ions for one kind of material is constant, but the variation in

flux is different due to variation of number of laser pulses as shown in table 3.1. Similarly the

energy and flux of ions generated by Ni, Si and C plasma is different. This is due to difference in

atomic mass, thermal conductivity, optical penetration depth, specific heat capacity, melting

point, atomic number, valence shell radius, binding energy and sublimation energy as well as

threshold fluence values of irradiated targets (Ni, Si and C) [101]

(b) Experimentation for exposure of brass target by laser induced plasma ions

In the second experiment, irradiations of brass substrates were performed by laser induced

plasma ions of Ni, Si and C. For this purpose target of brass material were selected. The grinded,

polished, annealed and ultrasonically cleaned brass substrates (45× 6× 3 mm3) were replaced by

CR-39 and were exposed to plasma ions of Ni, Si and C under UHV (10-9 Torr) at various flux

by using laser pulses of 3000, 6000, 9000 and 12000 is shown in figure 3.6.

58

Table 3.1: The evaluated energy and flux of laser induced plasma ions of Ni, Si and C.

Sr.

No

Target

Name

Mass (Kg) Laser Pulses Ion energy

(KeV)

Ion flux (ions/cm2)

1

Ni

9.74 × 10-26

3000 138 60 × 1013

2 6000 138 35 × 1014

3 9000 138 70 × 1015

4 12000 138 84 × 1016

5

Si

4.66 × 10-26

3000 289 45 × 1012

6 6000 289 25 × 1013

7 9000 289 56 × 1014

8 12000 289 75 × 1015

9

C

1.99 × 10-26

3000 678 32 × 1011

10 6000 678 90 × 1012

11 9000 678 64 × 1013

12 12000 678 72 × 1014

Figure 3.6: The schematic of experimental setup for exposure of brass substrate by laser induced

plasma ions.

59

3.5 Experimental details for exposure of sample by Pelletron linear

accelerator

(a) Description of Pelletron accelerator

Material and medicine science have been improved by energetic ion beams from different types

of linear accelerators. The ion beams are used for modification of surface of solid target as well

as for structural and mechanical behaviour. In 1932 the first particle accelerator was designed in

order to split the atom. This objective was achieved by Cockcroft Walton particle accelerator

when Lithium atom was separated by 400KeV protons. At the same year electronic generator

invented by Van de Graaff which proved superior tool for searching the nucleus.

One kind of the electrostatic particle accelerator is Pelletron accelerator which is similar to the

Van de Graaff generator. The S-series and U-series of accelerators are manufactured by National

Electrostatics Corporations (NEC) USA. The S-series and U-series accelerators have potential

up to 5 MeV and 25 MeV respectively. We performed our experiments on S- series Pelletron

accelerator (6SDH-2) that is installed at Centre for Advanced Studies in Physics (CASP) Govt.

College University Lahore. 6SDH-2 is model of Pelletron accelerator.

With the association of these ion sources, linear accelerator has ability to produce ions of any

element from periodic table.

Figure 3.7: The photographic view of Pelletron linear accelerator.

60

(b) Experimentation for exposure of brass target by Pelletron accelerator

The singly charged Ni and C ions beam of energy 2MeV with beam size of 2 × 2 cm2 obtained

by Pelletron linear accelerator (6SDH-2 NEC USA) was used to perform the ion beam treatment

for brass target. After grinding, polishing and annealing brass target (45 mm x 6 mm x 3 mm)

were ultrasonically cleaned with acetone and mounted on sample holder. All the samples have

the same composition and have been prepared under same conditions. The grinding, polishing,

annealing and cleaning (ultrasonically) for all samples have been performed in a similar way.

Therefore it is expected that the mechanical behavior of all samples will be same before

irradiation. However, as a reference mechanical properties of one of those samples have been

investigated without ion irradiation. Secondly it is entirely impossible to investigate mechanical

properties for all virgin samples. Because after tensile testing, the sample is completely broken

into two parts. Therefore the tensile testing is the last one measurement of irradiated samples.

Similarly after microhardness testing indentations are imprinted on the sample and mechanical,

structural as well as surface properties of the sample do not remain same as it were before

microhardness testing.

Due to above two mentioned reasons the two set of experiments were designed: One for Tensile

Testing and one for Microhardness testing.

The four targets were exposed to ions of energy 2MeV for various flux of 56×1012, 12×1013,

15×1013 and 26×1013 ions/cm2 at room temperature under Ultra High Vacuum (UHV) conditions

(~10-9 Torr).

3.6 Characterization techniques

After laser and ion irradiation, brass targets are investigated in detail for surface morphology,

crystallographic structures analysis and mechanical behavior following characterization

techniques are utilized

3.6.1 Scanning Electron Microscope (SEM)

3.6.2 X-Ray Diffraction (XRD)

3.6.3 Universal Tensile Machine (UTM)

3.6.4 Vickers Hardness Tester (VHT)

61

3.6.1 Scanning Electron Microscope (SEM)

Scanning electron microscope is more tremendously useful investigative instrument. The surface

of target under investigation is scanned with highly energetic and focused electron beam.

Incident electrons are known primary electrons which cause the emission of secondary electrons

from irradiated target surface. The detector or grid collect these secondary electrons and give

information in the form of surface features of target. The brightness of the image depends upon

the number of secondary electrons collected by detector. The emissions of backscattered

electrons from the irradiated target surface are also produced by primary electrons. These

backscattered electrons have large amount of energy than secondary electrons and also have a

definite direction.

SEM images are useful in exploring the surface morphological growth, surface microstructure

and surface variation in the form of voids, cavities, craters, cone formation, ripples, surface grain

formation, surface pellets and growth of nano structuring. Scanning electron microscope (JEOL-

JSM-6480) system is employed to explore the changes in surface morphology of laser and ion

irradiated brass. The magnification of this system is 15 to 300,000 at 25kV with a resolution of

3nm.

Figure 3.8: The photographic view of Scanning Electron Microscope (SEM).

62

3.6.2 X-Ray Diffraction (XRD)

PANalytical X‟ Pert Pro X-Ray Diffraction System is used for the structural analysis of

unirradiated and irradiated brass target surface. This system is working in line mode and operates

on X-rays CuKα radiation with 1.54 Ǻ wavelength. The range of current of filament tube is 40

mA to 55 mA and range of accelerated voltage from 45 KV to 60 KV. The scanning ranges of X-

ray diffractometer are from 5o to 120o and step size entails in our experiment work is 0.1 with a

time of one second per step at 0.15osec-1 scanning speed.

3.6.3 Universal Testing Machine (UTM)

Tensile testing is the vital method to attain the information about mechanical properties of

material as like elastic and plastic deformation, stress strain behavior Yield Stress (YS), Ultimate

Tensile Strength (UTS), ductility, toughness and stiffness. The tensile test determines the

resistance of a material to a static or slowly applied force. The strain rate in a tensile testing is

very low from 10-4 to 10-12 sec-1. The unirradiated and irradiated targets were deformed by

universal tensile testing machine (AG-1 Shimadzu) installed at Center for Advanced Studies in

Physics, Govt. College University (GCU), Lahore. This machine is controlled by Trapezium

software and performs different tests like tensile, compression and bending test. Figure 3.9

depicts the image of apparatus utilized for tensile testing. The cross head speed, gauge length and

diameter of machine are adjusted before carrying the deformation test. Later on, the target was

loaded between two jaws. These jaws attached with pull rods, one of these rod is connected with

the base and other with cross head of the machine. In our experiment the gauge length of target

and cross head speed are 30 mm and 1 mm/min respectively. The experiment was carried out at

room temperature and stress strain curve was attained by Trapezium software. The yield stress,

ultimate tensile strength and percentage elongation were measured for unirradiated and irradiated

target from stress stain curves.

3.6.4 Vickers hardness tester

Hardness depending upon the context can represent resistance to scratching or indentation and a

qualitative measure of the strength of the material. In general in hardness measurements the load

applied is 2N. Finally the targets were subjected to deformation for hardness conducted using a

pyramid-shaped diamond indenter (Zwick/Roell ZHU-5030) at a 100 g load applied for 15

second.

63

Figure 3.9: The photographic view of Universal Testing Machine (UTM).

Figure 3.10: The photographic view of Vickers Hardness Tester (VHT).

64

Chapter 4

RESULT AND DISCUSSION

65

RESULT AND DISCUSSION

This chapter comprises of following three parts

Part A

Effects of laser irradiation on the surface, structural and mechanical properties of brass.

(a) SEM analysis

(b) XRD analysis

(c) Tensile testing

(d) Microhardness

Part B


(1) Energy and flux measurement of laser induced plasma ions of Ni, Si and C

(2) Effects of laser induced Ni plasma ions irradiation on the surface, structural and

mechanical properties of brass

(e) SEM analysis

(f) XRD analysis

(g) Tensile testing

(h) Microhardness

(3) Effects of laser induced Si plasma ions irradiation on the surface, structural and


(i) SEM analysis

(j) XRD analysis

(k) Tensile testing

(l) Microhardness

(4) Effects of laser induced C plasma ions irradiation on the surface, structural and


(m) SEM analysis

(n) XRD analysis

(o) Tensile testing

(p) Microhardness

Part C

Accelerator ions irradiation

(5) Effects of accelerator Ni ions irradiation on the surface, structural and mechanical

properties of brass

(q) SEM analysis

(r) XRD analysis

(s) Tensile testing

(t) Microhardness

66

(6) Effects of accelerator C ions irradiation on the surface, structural and mechanical

properties of brass

(u) SEM analysis

(v) XRD analysis

(w) Tensile testing

(x) Microhardness

67

Part (A)

Laser irradiation

68

4.1 Effects of laser irradiation on the surface, structural and mechanical

properties of brass

The effects of laser irradiation on the surface, structural and mechanical properties of brass has

been discussed in this section

4.1.1 SEM analysis

Figure 4.1(a) shows the SEM micrograph of unirradiated surface of brass. SEM micrographs of

Figure 4.1(b-e) reveal the modified surface of brass after irradiation at fluence of 6.4 J/cm2 in an

oxygen environment for various numbers of overlapping laser pulses of (b) 1200 (c) 1800, (d)

2400 and (e) 3000. Figure 4.1(b) shows that various features with non-uniform shape and density

distribution are observed after exposing the surface with the laser pulses of 1200. It includes (i)

micro-sized cavities surrounded by uplifted edges, (ii) bumps, (iii) cones and (iv) wave-like

ridges. When the number of pulses is increased to 1800, a decrease in the number density of

cavities bumps and wave like ridges is observed (figure 4.1c) whereas, the size of cavities

increases significantly. An appearance of multiple ablative layers and ripples at the edges of

cavities is also seen in the inset of figure 4.1(c). With further increase in laser pulses up to 2400,

the change in the surface morphology of brass is observed in micrograph of figure 4.1 (d). The

appearance of wave like ridges and cones becomes more pronounced, whereas density of the

bumps and cavities reduces significantly (figure 4.1d). For the maximum number of pulses i.e.

3000, the density of the ridges and bumps decreases whereas the density of cavities significantly

increases as shown in figure 4.1(e). The cracks are also generated for this number of pulses. The

cavity- formation as seen in figure 4.1 (b-e) is due to mass removal by the laser induced heating,

thermal desorption, melting and explosive boiling of the irradiated surface [102, 103]. When the

laser beam intensity exceeds a certain value than the ablation threshold, the target material

begins to evaporate, resulting into the formation of plasma. When the pressure of plasma is

higher than the surrounding pressure, the liquefied material is expelled explosively from the

irradiated surface due to the violent recoil pressure and it becomes a possible cause for the

formation of cavities, bumps and wave like ridges [104].

Cumulative laser pulse-irradiation and surface vaporization resistant impurities are responsible

for the cone formation. The direction of these cones is forward peaked towards the incident laser

radiation [105]. The interaction of laser pulses causes hydrodynamic instabilities at liquid- solid

interface and also plays a significant role for the formation of wave like ridges and cavities as

69

seen in figure 4.1(b-e) [106]. During laser matter interaction, surface plasmons are formed. The

excitation of these plasmons induce an enhancement of local fields in the surface layer and

causes the formation of ripples in surrounding of the cavities [107].

Figure 4.1: SEM micrographs illustrating the surface morphology of (a) unirradiated and excimer

laser irradiated brass target under O2 ambient environment at pressure of 100 Torr at

constant fluence of 6.4 J/cm2 for various laser pulses of (b) 1200, (c) 1800, (d) 2400

and (e) 3000.

70

A decrease in number density of cavities is observed in figure 4.1(d) with increasing number of

laser pulses up to the range of 2400. A decrease in number of cavities can be due to the reason

that shock liquidized material completely or partially refills the cavities produced by initial

melting [108]. Features with non-uniform shape and size distribution are generated at the laser

irradiated brass due to surface defects, inclusions, small pits, contaminants, oxides, laser induce

thermal stresses and other heterogeneities. The growth of laser induced defects depends on

higher sensitivity of laser absorption at that particular sites of material and therefore, causes the

non-uniform laser energy absorption inside the material [109]. The cracks in figure 4.1(e) are due

to thermal stress cracking induced due to multiple pulse-irradiation , which possibly gives rise to

breakage of atomic bonds at the irradiated surface [110]. The molten material is ejected from the

irradiated surface and then cools on a relatively colder target area. As a result, the surface and

subsurface layers exhibit laser induced temperature gradients and thermal stresses [108].

The oxygen at a pressure of 100 Torr has the ability to restrict free expansion of plasma and offer

sufficient confinement effects. This results into enhanced ablation efficiency due to more energy

coupling to the brass target [5].

4.1.2 XRD analysis

XRD technique is employed to attain phase identification, the variation in crystallinity,

dislocation densities and residual stresses in the target material. Figure 4.2 (a) shows the XRD

patterns of unirradiated and laser irradiated brass targets. Figure 4.2 (b) shows the variation in

crystallite size with increasing number of laser pulses. Figures 4.2 (c) and 4.2 (d) show plots the

variation of dislocation line density and stresses respectively as a function of laser pulses.

Unirradiated brass shows the presence of CuZn (320), CuZn (111), CuZn (332), CuZn (600),

CuZn (210) and CuZn (102) planes reflection at angles of 41.7o, 42.6o, 48.59o, 62.46o, 71.62o,

and 78.03o [111] respectively. It is also observed that d-spacing, peak intensity and the values of

full width at half maximum (FWHM) are changed after laser irradiation whereas no change in

the phase is observed. The absorption of non-uniform conduction of the energy by atoms,

recrystallization, non-uniform thermal stresses, and scattering effects may cause variations in the

peak intensity after the laser beam interaction with the brass target [69] .

The crystallite size is evaluated for the reflection plane of brass (111) by using Sherrer’s formula

[108].

Crystallite Size (D) =0.9 λ

FWHM cosθ (4.1)

71

Where D is crystallite size, λ is the wavelength of X-rays (1.542 Å), FWHM is full width at half

maximum, and θ is the angle of diffraction. FWHM and θ are measured in radian.

Figure 4.2: XRD data of laser irradiated brass target surface under O2 ambient environment at

constant fluence of 6.4 J / cm2 for various laser pulses of 1200, 1800, 2400 and 3000.

(a) XRD diffractrographs, (b) The crystallite size Vs laser pulses, (c) The dislocation

density Vs laser pulses and (d) stresses Vs laser pulses.

72

The dislocation line density and the residual strain variations are evaluated by using following

relation [108].

Dislocation Density = 1/ (Crystallite Size) 2 (4.2)

Strain (ε) = d−do

do (4.3)

Where d is the observed and d0 is the standard plane spacing and ε is the induced strain.

The laser induced stresses “σ” are calculated by using the relation given below [108].

Stress (σ) = εE (4.4)

Where E is the Young’s modulus, and for brass its value is 102 GPa [69] .

The difference in the peak intensity is related to the variation in the crystallite size and laser

induced strain on the target surface after laser irradiation. The change in d-spacing causes

variation in residual stresses and lattice distortion. The differences in inter atomic distances,

thermal expansion coefficients, heating and cooling conditions between the surface layers and

interstitial diffusion can be a possible cause to produce lattice distortions in lattice planes [112].

The peak intensity for the plane of brass (111) (Figure 4.2 a) increases with increasing number of

laser pulses up to 1800. This increase in peak intensity is ascribed to the crystal growth and

atomic diffusion of host material across the grain boundaries after laser ablation [113]. Further

increase in number of pulses from 2400 to 3000, the reduction in peak intensity of plane is

observed. The main reason for this decrease is recrystallization phenomenon due to laser induced

melting and resolidification. After laser irradiation large sized grains disintegrate into smaller

sized which result an attenuation in peak intensity of (111) plane as shown in figure 4.2 (b). The

crystallite size deceases with increasing in number of pulses up to a maximum value of 3000 and

is shown in figure 4.2 (b). Laser energy deposition to the target surface generates lattice strains

by interstitial diffusion of host atoms. As a result large numbers of laser induced defect are

produced which cause to reduce the crystallite size. Therefore, these induced defects act as a

barrier in dislocation motion [114]. Figure 4.2 (c) depicts, an increasing trend in dislocation line

density with increasing number of laser pulses, and it represents the increase in the concentration

of imperfections in lattice sites. The increase in the dislocation line density in the crystal lattice is

due to enhanced quantification of work hardening [115] . The energy absorbed during laser

material interaction is utilized not only to increase the mobility of existing dislocation defects but

also to generate a greater number of new dislocation defects by vibration or distortion [115] .

73

The variation in laser induced stresses corresponding to various number of overlapping laser

pulses is displayed in figure 4.2 (d). When high energy laser beam is incident on the solid target,

lattice distortions and shock waves are generated. Consequently compressive and tensile stresses

are produced in the irradiated brass. The high rates of heating and cooling also generate

tremendous temperature gradients. This is associated with the local heating, and the plasma-

dynamic flows. This is main reason for the production of residual stresses in the surface layer.

The compressive stresses decrease and tensile stresses increase with increasing number of pulses

[116].

4.1.3 Tensile testing

Fig. 4.3 (a) depicts stress–strain curves of (1) unirradiated and laser irradiated brass at a fluence

of 6.4 J/cm2 under an oxygen environment for various number of overlapping laser pulses of (2)

1200 (3) 1800, (4) 2400 and (5) 3000. The variations in the Yield Stress (YS) and the Ultimate

Tensile Stress (UTS) with increasing number of laser pulses are shown in figures 4.3 (b) and (c),

respectively. Both the YS as well as UTS exhibit monotonic increase with increasing number of

pulses. The laser induced defects, stresses and strain fields are possible reason for increasing

behavior of YS and UTS with increasing number of pulses. These variations in the value of

mechanical properties (YS and UTS) of brass are possibly attributed to the alterations in

microstructure and dislocation density which is produced by laser beam-material interaction. As

strengthening mechanism is directly proportional to the number of laser induced defects [117].

The impact of high energy photons of laser radiation creates initial displacement damage in the

form of primary knock out atoms (PKAs) which displace nearby atoms from their equilibrium

position. The total numbers of displaced atoms in a cascade depends upon number of pulses,

absorbed radiation energy, temperature and nature of target materials [118]. The generation of

vacancies and interstitials, and/or their clusters, act as barriers to the glide dislocations. As the

incident number of laser pulses increases, the defect density also increases. The increased

number of defects acts as strong pinning points for movement of dislocation plane. Hence,

dislocation movement reduces which causes improvement of the mechanical properties [55]. For

that reason greater mechanical stresses will be necessary to start the plastic deformation [119].

74

Figure 4.3: Universal Testing Machine (UTM) data for laser irradiated brass target under O2

ambient environment at constant fluence of 6.4 J / cm2 for various laser pulses of

1200, 1800, 2400 and 3000. (a) Tensile stress- tensile strain curves, (b) The yield

stress Vs laser pulses, (c) The ultimate tensile strength Vs laser pulses.

4.1.4 Microhardness

Figure 4.4 shows the variation in the microhardness with increasing number of laser pulses. This

graph also shows a monotonic increases in the microhardness of laser irradiated brass with

75

increasing number of pulses. The changes in micro hardness are caused by the lattice disorder,

associated with variation in crystal structure and laser induced thermal tensile stresses produced

in the material.

Figure 4.4: The variations of the hardness of laser irradiated brass under O2 ambient environment

at constant fluence of 6.4 J/cm2 for various numbers of pulses of 1200, 1800, 2400

and 3000.

The alteration in microhardness can also be correlated with the change in crystalline size and

dislocation line density as has been calculated by XRD analysis and is shown in figures 4.2 (b)

and (c) respectively. By decreasing the grain size and increasing the dislocation density the

microhardness increases [120]. This is according to Hall-Petch effect [121]

σ = σo + kd-1/2 (4.5)

In this expression d is the crystallite size, σo and k is constant parameters for specific material.

Above equation demonstrates the hardness and yield strength dependence on crystallite size for

irradiated target.

76

Part (B)


77

4.2 Energy and flux measurement of laser induced plasma ions

Optical micrographs of figures 4.5, 4.6 and 4.7 represent the tracks formation of Ni, Si and C

ions on the surface of solid state nuclear track detector CR39 respectively after etching in 6N-

NaOH solution at 75 Co for 8 hours. These ions are generated by laser induced (Ni, Si and C)

plasma at a constant fluence of 5.1 J/cm2 for various number of laser pulses (a) 3000, (b) 6000,

(c) 9000 and (d) 12000. Thomson parabola technique was used to measure the energy and flux of

ions generated by laser induced plasma.


track detector for detection as well as measurement of energy and flux of laser

induced Ni plasma ions generated at a laser fluences of 5.1 J/cm2 for various number

of laser pulses of (a) 3000, (b) 6000, (c) 9000 and (d) 12000.

The average energy of Ni ions is found 138 KeV with flux variation from 60 × 1013 to 84 × 1016

ions/cm2. Similarly average energy of Si ions is 289 KeV with flux variation from 45 × 1012 to

78

75 × 1015 ions/cm2 and 678 KeV average energy for C ions with flux variation from 32 × 1011 to

72 × 1014 ions/cm2 by increasing the number of laser pulses from 3000 to 12000.

The ions ejected from the plasma create tracks of conical shapes with various diameters on the

detectors surfaces (CR39). It is observed from figure 4.5(a-b) to 4.7 (a-b) that the density of

tracks is increased by increasing laser pulses along with uniform distribution on irradiated

detectors. The increased in densities of tracks are best evidence of larger emissions of ions from

the irradiated target surface (Ni, Si and C) by increasing the number of laser pulses up to 12000.



induced Si plasma ions generated at a laser fluences of 5.1 J/cm2 for various number


In fact when energy of laser beam is absorbed by target atoms, photon energy is converted into

electronic excitation, chemical and thermal mechanical energy. The evaporation, excitation,

ablation and plasma formation occur due to these conversions. The energy and flux of ions

79

generated by plasma depend on the laser parameters (pulse duration, number of laser pulses,

pulse energy, wavelength, focusing conduction) as well as on the properties of irradiated material

(bonds strength, chemical composition, work function, melting point etc) [122].



induced C plasma ions generated at a laser fluences of 5.1 J/cm2 for various number


During laser matter interaction high density and high temperature plasma is generated due to

multi-photon ionization and inverse bremsstrahlung processes. Hot electrons gain maximum

energy by inverse bremsstrahlung. Therefore these electrons are moved in backward direction

with high speed [123]. Figure 4.8 depicts the graphical representation of increase in flux of laser

induced plasma ions with an increase in number of laser pulses. These ions of plasma get

80

accelerated by Coulumbic acceleration, thermal interactions and gain energy in MeV range.

When plasma expands, its density, thermal pressure and temperature are decreased. Plasma

particle pressure or magnetic pressure (β) plays a significant role in the presence of external

magnetic field. When plasma plume is produced, the particle pressure is maximum because

temperature and density of plasma species is very high. At this stage the value of β is maximum

which represents that the plume lies in diamagnetic limits.

Figure 4.8: The graphical representation of increase in flux of laser induced plasma ions of Ni, Si

and C with increasing number of laser pulses.

According to the Magneto-hydrodynamical (MHD) equation [78].

β = 8πnkTe

B2 (4.6)

Where β = size of the diamagnetic effect or particle pressure

n = number of electrons

k = Boltzmann constant

Te = electron temperature

The effect of magnetic field on the plasma expansion is given by following equation [78]

V2

V1= (1 −

1

β)2 (4.7)

V1 = plasma expansion velocity in the absence of magnetic field and

81

V2 = plasma expansion velocity in the presence of magnetic field.

If the value of β = 1 then plume expansion will be stopped. Consequently plume confinement is

not formed. But when the value of β is greater or less than 1, then the plasma species cannot

move freely in the direction of perpendicular to the target surface. However the plasma species

move under the action of applied magnetic field and attain appropriate trajectory path according

to their mass and charge. As a result plume confinement occurs [124].

There are extensive collisions of plasma species because of confinement of plasma through the

magnetic field. As a result more plasma constituents are formed in the confinement region [21].

The density variations in plasma species is produced by the reason of magnetic field lines

oscillation by the effect of intrinsic plasma pressure and restoring force is attributable to

magnetic field lines . In the presence of magnetic field the ions in the plasma corresponding to

their mass and charge are affected by Lorentz force. Therefore the expansion of plasma plume

and diffusion of plasma species are decelerated by the applied magnetic field. According to

Rafique et al. [24] the lifetime of plasma plume is increased in the presence of magnetic field.

Khaleeq et al. [91] reported that the density of laser induced electrons is increased in the

presence of magnetic field.

During experiments all the parameters namely laser pulses, laser fluence, detection angle,

distance of brass substrate from the target surface and environmental conditions were kept

constant for the generation of laser induced plasma ions. But it is observed that the energy of

laser induced C plasma ions is significantly higher as compared to Ni and Si. It is attributed to

insulating nature of C material which has high resistance to thermal shocks as compared to Ni

and Si materials. Heat is conducted in target lattice systems by both free electrons and lattice

vibration (phonons). The thermal conductivity is related to each of these procedures. Due to

high thermal shock resistance and small number of free electrons (conducting electrons) the

thermal conductivity of C is smaller than Ni and Si. Therefore, a smaller amount of heat is

conducted within the C crystal lattice before removing from the irradiated surface. Another

reason for higher energy of C ions than Ni and Si is the low atomic mass of C. According to the

equation 3.1, ion energy is inversely proportional to the atomic mass. The atomic mass of C (12)

is smaller than atomic mass of Ni (58) and Si (28.01).

The graph of figure 4.8 represents that the ion flux increases by increasing number of laser

pulses. This is attributable to more energy deposition as well as incubation effect [125]. The flux

82

of Ni ions is higher than Si and C ions. It is because the radius of valence shell increases by

increasing atomic number. As a result the melting point, sublimation energy and binding energy

are decreased with increasing atomic radius [101]. This is responsible for reducing threshold

fluence of the irradiated target and consequently flux of ions increases.

83

4.3 Effects of laser induced Ni plasma ions irradiation on the surface, structural and


The effects of laser induced Ni plasma ions irradiation on the surface, structural and mechanical

properties of brass has been discussed in this section.

4.3.1 SEM analysis

SEM images of figure 4.9 reveal surface morphology of (a) unirradiated and Ni ion irradiated

brass substrate at various ion flux of (b) 60 × 1013 ions/cm2, (c) 35 × 1014 ions/cm2 (d), 70 × 1015

ions/cm2 and (e) 84 × 1016 ions/cm2 with constant energy of 138 KeV under UHV condition. It is

noted that upon Ni ions irradiation of brass with lowest flux of 60 × 1013 ions/cm2 a few

micro/nano sized irregular shaped cavities, pores and pits with random density distribution are

formed (4.9 b). When the ion flux increase up to 35× 1014 ions/cm2, an enhancement in melting,

evaporation and ejection of material occurs on the irradiated substrate surface which is

responsible for the production of cavities, pores and crack formation as is observed in figure 4.9

(c). The high rate of melting and abrupt quenching is responsible for the generation of cracks

[126]. Figure 4.9 (d) illustrates that increase in ion flux up to 70 × 1015 ions/cm2 leads to more

efficient sputtering of the target material. The size and density of cavities decrease whereas size

of pores increases by increasing ion flux. When the flux of laser induced Ni plasma ions is

further increased up to maximum value of 84 × 1016 ions/cm2, a significant variation in surface

morphology is observed in figure 4.9 (e). The incoherent shaped structures which are randomly

distributed all over the surface have been transformed into granular morphology with distinct

grain boundaries. Similar kind of structures by ion implantation on different metals have been

reported previously by Mallick et al. [126], Kaoumi et al. [64] and Liu et al.[14]. The formation

of micro/nano sized cavities is illustrated in Figure 4.9 (b–d) are credited to thermal sputtering on

the basis of ions induced heating, vaporization, thermal desorption, intense melting and

explosive boiling of the brass surface. In metals, almost all of the absorbed energy from

projectile ions converts into heat energy. As a result suddenly temperature of irradiated zone

rises to some thousand degrees celsius [127]. When the energy of incident ions is greater than the

binding energy of irradiated material, it leads to ionization and results in thermal sputtering. The

grain morphology is explained on the basis of thermal spike model [14, 64]. The ion induced

thermal spikes and their size play a central role in grain morphological process. At increased ion

flux, temperature of surface and impacts of thermal spikes are also increased significantly. As a

result migration, growth and diffusion of grain boundaries take place.

84

Figure 4.9: SEM micrographs illuminating the surface morphology of (a) unirradiated and laser

induced Ni plasma ions irradiated brass target at constant energy of 138 KeV under

vacuum condition for various ion flux of (b) 60 × 1013 ions/cm2, (c) 35 × 1014

ions/cm2 (d), 70 × 1015 ions/cm2 and (e) 84 × 1016 ions/cm2.

The migration of grain boundaries is a recrystallization phenomenon which takes place during

ion irradiation in order to minimize the surface energy and stored strain field [14]. Therefore, the

85

mobility of the primary knock on atoms (PKA) and implanted ions increases and easily crosses

the intrinsic stain field and causes reorientation of lattice leading to small and smooth grains.

The period of thermal spike only exists for few picoseconds before being quenched to ambient

temperature [64] . These diffused ions and atoms are produced strong extrinsic strain field by

abrupt quenching due to which significant changes in mechanical properties of irradiated

substrate take place. The extrinsic strain field could have acted as effective obstacles to the glide

of these defects as a result mechanical properties of irradiated substrate are changed. During the

recrystallization procedure the diffusion of Ni ions into the lattice site of the substrate causes

wider and distinct grain boundaries [128]. When energetic ions are bombarded on the substrate

surface, they strike with host atoms and transfer large amount of energy to the irradiated zone

[129]. The combined effects of energy, temperature and pressures spikes are responsible for

production of thermal sputtering, thus, producing damages in surface.

The change in surface morphology is dependent on the nature and mass of projectile ion, ion

energy, ion flux and material composition [70]. This damage can be produced by two types of

collisions between projectile ions and host atom. These two type phenomena are (i) nuclear

collision (elastic collision) and (ii) electronic collision (inelastic collision). Electronic collision

becomes more prevailing if the energy of projectile ion is equal or greater than 1MeV. But if the

ion energy lies between 100 KeV and 1 MeV then mixing of both nuclear and electronic

collision take place [130]. The diminution in cavities size is due to refilling by liquefied and

explosive melting of material. The alteration in the cavities size and enhancement in thermal

sputtering are attributed to a large number of defects i.e. induced stresses, Frenkel pair defects,

diffusion of ions and other heterogeneities. The main cause for the generation of these defects are

non uniform energy absorption, thermal spike and displacement cascades into surface and

subsurface of irradiated substrate [72].

The comparison of laser irradiated and laser induced Ni plasma ion irradiated brass of figures

(4.1 and 4.9) gives a considerable distinction in the brass surface. In the case of laser irradiation,

micro-sized cavities, bumps, cones and wave like ridges are formed on the brass surface. When

brass surface is irradiated by maximum numbers of laser pulses (3000), the density of cavities

increases while density of the ridges and bumps reduces as well as cracks are also formed. In the

case of laser induced Ni plasma ion irradiated brass, micro/nano sized irregular shaped cavities,

86

pores and pits like features are formed. While at maximum Ni flux, granular morphology is

observed.

4.3.2 XRD analysis

XRD technique is used to identify phase, crystallite behavior, dislocation line densities and

induced stresses in the irradiated substrate brass. Figure 4.10 (a) shows the XRD patterns of

unirradiated (e) and Ni ion irradiated brass at various ion flux (f) 60 × 1013 ions/cm2 (g) 35 × 1014

ions/cm2 (h) 70 × 1015 ions/cm2 and (i) 84 × 1016 ions/cm2. The XRD patterns of unirradiated

brass demonstrates the presence of CuZn (320), CuZn (111), CuZn (332), CuZn (600), CuZn

(210) and CuZn (102) plane reflection at angles of 41.7o, 42.6o, 48.59o, 62.46o, 71.62o, and

78.03o respectively. Irradiated brass by laser induced Ni plasma ions depicts the presence of

CuZn (002), CuZnNi (200), CuZn (410), CuZn (511), CuZn (631) and CuZn (721) reflection

planes at corresponding angles at 42.2o, 43.62o, 49o, 62.9o, 72.3o and 78.9o respectively. A new

phase of CuZnNi (200) is observed after Ni ion irradiation in figure 4.10 (a). The development of

new phase is attributed to high heating, induced explosive melting and implantation of incident

Ni ions into the irradiated substrate by ion irradiation. As a result of ion irradiation, energy

deposition, heating, melting, rapid cooling and resolidification phenomena are also responsible

for the formation of new phases. After ion material interaction, host atoms of irradiated substrate

experience a temporary/ permanent reposition/relocation from their normal lattice position due to

accumulation of vacancies and interstitials defects and generate a new phase. The ion induced

radiation damage and the incorporation of a certain amount of Ni atoms in the brass (CuZn)

matrix are main reason of peak shifting [131]. The incident Ni ions lose their energy due to the

collision with host atoms. Because of its collisions incident ions are scattered in all possible

directions. Consequently, stuck atoms inside the irradiated zone get recoiled. These recoiled

atoms again collide with nearby atoms, transfer their energy and produce a displacement

cascade. As a result ion irradiation generates modification of crystal structural of irradiated

substrate which gives an increase in the sputtering and implantation processes [132]. The effect

of ion flux on the peak intensity, d-spacing (d), crystallite size (D), micro strain (ε) and

dislocation line density (δ) has been probed from the XRD patterns given in figure 4.10 (a). The

crystallite size (D), dislocation line density (δ), residual strains (ε) and Stresses (σ) are evaluated

for the reflection plane of brass (200) by using equations 4.1, 4.2, 4.3 and 4.4 respectively .

87

Figures 4.10 (b), (c) and (d) depict the variation in the crystallite size, dislocation line density

and stresses with increasing ion flux.

After ion irradiation the peak intensity, crystallite size and dislocation density of brass represent

an anomalous behavior increasing ion flux. The non uniform thermal stresses induced by non

uniform energy absorption by atoms and various scattering effects also play vital rule for these

variations [69].

Figure 4.10: XRD data of laser induced Ni plasma ions irradiated brass target surface for various

ion flux of 60 × 1013 ions/cm2, 35 × 1014 ions/cm2, 70 × 1015 ions/cm2 and 84 ×

1016 ions/cm2 at constant energy of 138 KeV. (a) XRD diffractrographs, (b) The

crystallite size Vs Ion flux, (c) The dislocation density Vs Ion flux and (d) Stresses

Vs Ion flux.

88

From figure 4.10 (a) it is observed that the peak intensity of irradiated brass for the phase of

CuZnNi increases with increasing flux of incident ions up to 70 × 1015 ions/cm2. This

enhancement in peak intensity is attributed to the atomic diffusion of Ni ions across the grain

boundaries and crystal growth after ion material interaction [69]. A significant enhancement in

crystallite size up to ion flux of 70 × 1015 ions/cm2 is observed in figure 4.10 (b). The probable

reason for this sort of structure variation induced by ion irradiation can be explained by

deposition of ion energy on the irradiated surface. When the surface of brass is irradiated with Ni

induced ions, it is observed that imparted energy of the incident ions to the host atoms releases

the strain energy in the surface and subsurface of substrate in proportion to thermal spike and as

a results crystalline size increase. Another reason for these variations in crystallite size is that the

smaller grains merge together and make large sized grains due to thermal annealing [116].

During the ion material interaction, two competing phenomena take place. One is production of

interstitial and vacancies and other is their mutual annihilation [97]. The rate of annihilation is

dominant than the production of vacancies due to induced thermal stresses and thermal

expansion. Therefore, the thermal stresses and annihilation of defects play a vital role in

increasing the crystallite size (figure 4.10 b) [133]. Further increase in ion flux up to maximum

value of 84 × 1016 ions/cm2, a decrease in peak intensity and crystallite size is observed (figures

4.10 a (i) & 4.10 b). The main reason for diminution of peak intensity and crystalline size is

rapid melting, quenching and recrystallization process which take place during resolidification

after ion material interaction [134]. The large amount of energy, pressure, displacement and

thermal spikes are responsible to destroy the crystallinity and produce the large numbers of

defects and dislocation. Therefore, large sized grains break up into smaller sized grains and

destroy the crystallinity which causes of decrease in peak intensity and crystallite size [116].

Figure 4.10 (c) exhibits the decreasing trend in dislocation line density up to ion flux of value 70

× 1015 ions/cm2, which depicts the vacancy migration and annihilation in lattice sites. Thermal

site and recovery phenomena play a fundamental role in the reduction of dislocation line density

and increment in crystallite size. Whereas, at the maximum value of incident ion flux (84 × 1016

ions/cm2), a significant variation in dislocation line density is related to increased concentration

of lattice imperfections. Actually the dislocation line density is a kind of imperfections i.e.

produced during ion metal interaction. The increment in the numbers of dislocation line in a

crystal lattice is a quantification of work hardening. The change in ion induced stresses with

89

various flux of incident ions is also revealed in figure 4.10 (d). Ion induced thermal spikes,

displacement cascades and lattice defects produced by Ni ion incorporation into the lattice site

are reason of induced stress variation. In general, the compressive stresses are produced due to

Ni ion irradiation. This is justified by atomic radius of implanted Ni (1.23 Ǻ) which is smaller

than the atomic radius of host atoms of Cu (1.27 Ǻ) and Zn (1.33 Ǻ) [93]. Therefore anisotropic

contraction is produced in the normal lattice site of target.

The comparison of X-Ray diffraction of laser irradiated and laser induced Ni plasma ion

irradiated brass of figures (4.2 and 4.10) gives a substantial difference in the XRD patterns of

brass substrate. In XRD patterns of laser irradiated brass no new peaks and phases are identified

whereas XRD patterns of Ni ion irradiated brass reveals the presence of a new phase of CuZnNi.

Both tensile and compressive stresses are generated in case of laser irradiated brass whereas, in

case of laser induced Ni plasma ions irradiation only compressive stresses are generated.

4.3.3 Tensile Testing

Fig. 4.11 (a) depicts stress–strain curves of (1) unirradiated and Ni ions irradiated brass with

various flux of (2) 60 × 1013 ions/cm2 (3) 35 × 1014 ions/cm2 (4) 70 × 1015 ions/cm2 (5) 84 × 1016

ions/cm2. The alteration in Yield Stress (YS) and Ultimate Tensile Strength (UTS) as a function

of ion flux is graphically represented in figure 4.11 (b) and 4.11 (c) respectively. From these

graphs it is observed that reduction in YS and UTS with increase in flux of ions up to 80 × 1015

ions/cm2. By further increases in ion flux up to maximum value of 90 × 1016 ions/cm2, causes an

increase in YS and UTS. The deviation in ion induced effects, defects and stresses are

responsible for alteration in YS and UTS. These variations in YS and UTS are due to

modification in microstructural and dislocation line densities after ion metal interactions. The

decline in YS and UTS of irradiated substrate in contrast with unirradiated brass surface are

associated with the variation of crystallite size, dislocation line densities and ion induced

stresses. The increase in crystallite size and decline in dislocation line densities also induce a low

level of stresses in the irradiated substrate. During the thermalisation stage, there is the

development of an induced thermal spike in the ion induced displacement cascades in the lattice

site of irradiated material. Where the energy of PKA is distributed between many others nearby

atoms, causing to produce a local source of heat in the lattice site of irradiated substrate due to

which temperature of irradiated zone is increased significantly. This temperature in the cascade

and subcascade core is greater than melting temperature of irradiated material for at least few

90

picoseconds. Therefore the thermal energy is activating the migration and mutual annihilation of

ion induced interstitial and vacancies defects which causes the reduction in density of these

defects [135]. This recovery process continuous up to 10-8 second [67]. This process also

depends on the temperature, chemical composition of irradiated material and prior microstructure

of irradiated surface.

Figure 4.11: Universal Testing Machine (UTM) data of laser induced Ni plasma ions irradiated

brass for various ion flux of 60 × 1013 ions/cm2, 35 × 1014 ions/cm2, 70 × 1015

ions/cm2 and 84 × 1016 ions/cm2 at constant energy of 138 KeV. (a) Tensile stress-

tensile strain curves, (b) The yield stress Vs ion flux (c) The ultimate tensile

strength Vs ion flux.

91

The ion irradiation induced defects (interstitial and vacancies defects) were absorbed by gliding

dislocations for the duration of the plastic deformation. Therefore, these defects could not act as

effective barriers for smooth movement of dislocations [97]. At the value of ion flux up to 70 ×

1015 ions/cm2, the rate of mutual annihilation of defects is greater than the rate of generation of

vacancies and interstitial defects [136]. Therefore the mechanical properties of the brass

substrate are decreased after ion irradiation up to ion flux of 70 × 1015 ions/cm2. For the period of

the collision between incident Ni ions and host atoms (Cu and Zn) of substrate lattice the energy

transfers from ions to host atom in 10-17 second [67]. The amount of energy required to remove

the lattice host atom from equilibrium position is called displacement energy (Ed) and energy

transferred to the lattice atom is denoted by EL. By increasing the flux of incident ions up to

maximum value of 84 × 1016 ions/cm2, the collectively ion energy deposition increases so the

value of EL is also increased. Therefore an effective induced displacement cascades and

subcascades are produced. This procedure remains continuous until the energy of each implanted

ion and displaced atoms are smaller than displacement energy. The distribution of ion induced

vacancies, interstitial atoms and other types of lattice defects are generated in the region around

the primary knock-on atoms (PKA) and ion-induced tracks are formed. The total number of

Frenkel defects (Nd) produced by one PKA can be estimated as the ratio between the energy of

primary knock-on atom (EPKA) and the displacement energy, Nd = 0.5EPKA/Ed [63]. The increase

in mechanical properties of substrate for phase CuZnNi at maximum value of incident ions flux

of 84 × 1016 ions/cm2 is due to existence of large concentration of Ni ion in the lattice site which

produces significant distortion in normal lattice site. As a result distribution of ion induced

vacancies, interstitial defects, precipitates and other types of lattice distortions are produced in

the irradiated zone. The generation of these defects depends on ion fluence, ion energy,

temperature and nature of target materials [1]. The number of Frenkel pairs (vacancies and

interstitial defects) per cascade can be calculated by relation [137] NF = 0.8 Ei/ 2ED . In this

equation Ei is incident energy and ED is displacement energy. Therefore, both YS and UTS are

increased and achieve their maximum value at maximum flux due to the introduction of

additional obstacles by induced defects to dislocation glides. The dislocations movement in a

glide plane are pinned by induced defects which serve as barriers, impeding their propagation in

different glide planes and thus cause strengthening of the material associated with decrease of its

ductility [138].

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4.3.4 Microhardness

Figure 4.12 depicts the variation in the microhardness value of Ni ion irradiated brass for flux

ranging from 60 × 1013 ions/cm2 to 84 × 1016 ions/cm2. The radiation-induced hardening

decreases up to ion flux value 70 × 1015 ions/cm2 whereas at maximum ion flux of 84 × 1016

ions/cm2, an increase in microhardness has been observed. The variations in microhardness are

credited to lattice distortion, grain growth ion induced defects, their annihilation, precipitate

dispersions, crystal structuring and compressive stresses generated in normal lattice site of

irradiated target substrate due to ion induced thermal, energy and displacement spikes [139].

Variations in microhardness are also correlated with the difference in dislocation density,

crystallite size and ion induced stresses (evaluated by XRD). The change in microhardness is

also associated with the alteration in crystal size and dislocation density as has been calculated

by XRD analysis shown in figures 4.10 (b) & (d).

Figure 4.12: The variations in the hardness of laser induced Ni plasma ions irradiated brass for

various ion flux of 60 × 1013 ions/cm2, 35 × 1014 ions/cm2, 70 × 1015 ions/cm2 and

84 × 1016 ions/cm2 at constant energy of 138 KeV.

The comparison of mechanical properties of brass after laser and laser induced Ni plasma ions

irradiations reveals that with increasing number of laser pulses and dose of photons the YS, UTS

and mirohardness are increased which is attributed to accumulation and enhancement of photon

93

assisted defects. However with increase in ion flux up to a value of 70 × 1015 ions/cm2 a

decrease in YS, UTS and microhardness is observed which is related to annihilations and

recombination of defects. These processes are responsible for softening of brass as compared to

unirradiated target. Whereas at the maximum ion flux (84 × 1016 ions/cm2) the enhancement in

all kinds of mechanical properties is observed which is related to augmentation generation and

growth of defects which is not compensated by diffusion and recombination processes.

94

4.4 Effects of laser induced Si plasma ions irradiation on the surface, structural and


The effects of laser induced Si plasma ions irradiation on the surface, structural and mechanical

properties of brass has been discussed in following section.

4.4.1 SEM analysis

SEM micrographs of figure 4.13 reveal surface morphology of (a) unirradiated and Si ion

irradiated brass at various ion flux of (b) 45 × 1012 ions/cm2, (c) 25 × 1013 ions/cm2 (d), 56 × 1014

ions/cm2 and (e) 75 × 1015 ions/cm2 at constant energy of 289 KeV under UHV condition. It is

noted that upon Si ions irradiation of brass with lowest flux of 45 × 1012 ions/cm2, micro/nano

sized irregular shaped pores with random density distribution are formed (4.13b). Both the

density and size of these pores and cavities are increased by increasing the ion flux up to 25 ×

1013 ions/cm2 and is observed in figure 4.13 (c). When the flux is increased to 56 × 1014

ions/cm2, the highly disturbed surface morphology is observed in the form of microexplosion,

pores and cavities in figure 4.13 (d). A significantly different surface morphology of brass

(CuZn) is observed in figure 4.13 (e). After Si irradiation at maximum ion flux 75 × 1015

ions/cm2, the cavities and pores are completely diminished and highly organized granular

morphology with distinct grain boundaries are observed. Few of these grains are hexagonal and

some are irregularly shaped in their appearance.

Ion induced micro explosive melting, boiling, evaporation as well as thermal and pressure spike

are responsible for the formation of micro/nano sized cavities and pores on the brass surface. The

enhancement of both thermal sputtering and redeposition take place simultaneously. However

the probability of redeposition and refilling is larger than thermal sputtering at increased Ni ion

flux [140]. The deposition of energy within the absorbing lattice of material increases with

increasing ion flux. It is attributable to an enhancement of accumulated and collective effects of

ions [69]. The ion induced stresses, shocks waves, elastic rebounds and lattice distortions are

produced in well defined and localized regions and considered as one of major cause of such

kinds of features [69]. The change in surface morphology depends on many factors namely the

type of incident ion, ion energy, ion flux as well as chemical, physical, mechanical and structural

properties of irradiated brass [1]. The increase in size of these features is attributed to

coalescence process. By increasing the ion flux, the coalescence process becomes more

dominant. The decrease in size of pores is attributed to refilling by shock liquefied and intense

melting of material. The variations in the size of cavities and pores are attributed to a large

95

number of defects namely induced stresses, Frenkel pairs, diffusion of ions and other

heterogeneities.

Figure 4.13: SEM micrographs representing the surface morphology of (a) unirradiated and laser

induced Si plasma ions irradiated brass target at constant energy of 289 KeV for

various ion flux of (b) 45 × 1012 ions/cm2, (c) 25 × 1013 ions/cm2 (d), 56 × 1014

ions/cm2 and (e) 75 × 1015 ions/cm2.

The non uniform energy absorption, pressure spike, induced thermal stresses, voids and

displacement cascades phenomena play a vital role for generation of these defects into surface

and subsurface of irradiated substrate [141]. A small number of results have been reported in

which grains growth is explained on the basis of thermal spike models [64, 126]. The formation

96

of granular structures by ion material interaction is explained on the basis of thermal spike model

by few research groups [14, 64]. Primary knocked on atoms (PKA) are dislodged from their

equilibrium lattice sites after ion material interactions. These PKA have some kinetic energy in

the range of several eV [67]. But their energy also depends on the energy of incident ions and

chemical properties of material. The energy of PKAs is dissipated within the crystal lattice by

displacement cascades and subcascades. As a result local temperature of lattice system is

increased. Finally local thermal spikes are formed in surface and subsurface of irradiated brass.

The size of these induced thermal spikes will be larger or smaller depending on the incident ion

energy, PKAs energy, ion flux and properties of target material (especially thermal

conductivity). The thermal spikes are produced in the surrounding area of grain boundary.

Therefore surface energy, stored strain field and strength of grain boundary of the target are

reduced because the temperature of thermal spike zone is greater than melting temperature of

brass [64]. Consequently the implanted Si ions and thermally activated atoms of brass (within the

thermal spike zone) are jumped across the intrinsic strain field and grain boundary easily. The

local driving forces are responsible for jumping of the large number of host atoms in the

direction of reducing grain boundary curvature. As a result the intrinsic grain boundary migrates

in the opposite direction [64]. In the thermal spike zone, thermal effects are dominated for grain

growth process. But in the subsurface of brass, a thermal zone is formed, where the combined

influences of thermal conduction and implantation take place to improve the rate of grain

growth. After ion material interaction, there are some regimes which are produced in surface and

subsurface of irradiated brass having very low temperatures. In these regimes (which is also

called nonthermal zone), an implantation process is responsible for the dominant grain growth

process [142].

The short lived thermal spikes exist only for few picoseconds before being quenched to ambient

temperature [64] . These migrated ions and atoms fabricate strong extrinsic strain field by abrupt

quenching due to which significant variations are produced in mechanical properties of

implanted brass. These extrinsic strain fields could have acted as effective obstacles to glide of

these defects and as a result mechanical properties of irradiated substrate are improved. During

the recrystallization procedure, distinct grain boundaries are formed due to the diffusion of Si

ions into the lattice site of the brass [128]. The combined effects of energy, temperature and

pressure spikes are responsible for the producing damages in surface. The radiation damage in

97

the crystal lattice depends on many intrinsic properties of irradiated material (preliminary

microstructure, chemical composition and crystal structure) and extrinsic parameters (irradiation

characteristic, irradiation temperature, stress and irradiation time) [67].

The comparison of surface morphology of laser induced Ni and Si ions irradiated brass (figures

(4.9 and 4.13) shows remarkable similarity at lower ion flux. However for higher flux the larger

grains (figure 4.9 e) with disturbed surface are obtained in case of Ni ions irradiation as

compared to Si ion irradiation (figure 4.13). This is attributed greater ion flux i.e. 84 × 1016

ions/cm2 of Ni ions as compared to the Si ion flux i.e. 75 × 1015 ions/cm2. The other factor for

more energy deposition by Ni ions is its high atomic number (28) as compared to Si (14). In case

of laser induced Si plasma ion irradiation, smaller radius of Si ions (1.17 Å) as compared to Ni

(1.23 Å) can be considered as possible cause for uniformly distributed and equiaxed granular

morphology (figures 4.9 e and 4.13 e)

4.4.2 XRD analysis

Figure 4.14 (a) illustrates the XRD patterns of (e) unirradiated and Si ion irradiated brass for

constant energy of 289 KeV at various ion flux of (f) 45 × 1012 ions/cm2 (g) 25 × 1013 ions/cm2

(h) 56 × 1014 ions/cm2 and (i) 75 × 1015 ions/cm2. The XRD patterns of unirradiated brass reveal

the presence of CuZn (320), CuZn (111), CuZn (332), CuZn (600), CuZn (210) and CuZn (102)

plane reflection at angles of 41.7o, 42.6o, 48.59o, 62.46o, 71.62o, and 78.03o respectively. After Si

ion irradiation new phase of CuSi (311) is formed at angle of 520 (figure 4.14 a). It proves that Si

atoms are implanted and generated CuSi phase (311) in the form of interstitial atoms. When the

brass surface is irradiated with energetic Si ions, these ions penetrate into surface and deposit

their energy rapidly in the vicinity of ion tracks and are responsible for chain processing of

cascades and subcascades of atomic collisions. Consequently an extensive disorder is produced

in well defined and ordered matrix of brass throughout the cascade volume. The whole

procedure, energy dissipation and anisotropic atom movements take place in picoseconds. So the

lattice displaced atoms cannot find energy as well as time to attain their equilibrium lattice site

again. As a result a new phase of CuSi (311) is formed [143]. The atomic collision cascade is

characterized by its length and time which depends on the incident ion energy, ion flux and

thermal properties of target [72]. After ion material interaction, high thermal gradient surface

tension, explosive melting, abrupt cooling, recrystallization process and ion implantation are

responsible for the growth of (311) new phase of CuSi [70].

98

Figure 4.14: XRD data of laser induced Si plasma ions irradiated brass target at constant energy

of 289 KeV for various ion flux of 45 × 1012 ions/cm2, 25 × 1013 ions/cm2, 56 ×

1014 ions/cm2 and 75 × 1015 ions/cm2. (a) XRD diffractrographs, (b) The crystallite

size Vs Ion flux, (c) The dislocation density Vs Ion flux and (d) Stresses Vs Ion

flux.

The amount of implanted Si ions into brass matrix increases by increasing ion flux which is

responsible to increase the temperature of lattice and sublattice. The influence of flux on the peak

intensity, d-spacing, FWHM, crystallite size (D), microstrain and dislocation line density (δ) has

been investigated from the XRD patterns (figure 4.14 a).

99

By means of equations 4.1, 4.2, 4.3 and 4.4, the crystallite size (D), the dislocation line density

(δ) and ion induced stresses are calculated with the reference to the peak of brass (111) lattice

plane. Figure 4.14 (b), (c) and (d) depict the variation in the crystallite size, dislocation line

density and stresses respectively with increasing ion flux.

An anomalous variation in peak intensity, crystallite size and dislocation density of brass are

achieved after Si ions irradiations. The main reason of this anomalous trend is non uniform

thermal stresses and anisotropic thermal shock wave distribution produced by non uniform

absorption of energy by lattice and sublattice of host atoms (Cu and Zn). From XRD patterns of

brass (figure 4.14 a) it is observed that the peak intensity of irradiated brass plane (111) increases

with increasing flux of incident ions upto 25 × 1013 ions/cm2. This enhancement in peak intensity

is attributed to the atomic diffusion of Si ions across the grain boundaries and crystal growth

after ion material interaction. Further increase in ion flux up to maximum value of 75 × 1015

ions/cm2, a notable reduction in peaks intensity and an increase in peaks width is observed.

The presence of defects (dislocation loops, vacancies and interstitial defects) in the lattice site of

brass causes certain amount of amorphization in host matrix. Consequently decrease in peak

intensity and peak broadening are formed [116]. A momentous improvement in crystallite size

from 35 nm to 44 nm by increasing ion flux from 45 × 1012 ions/cm2 to 25 × 1013 ions/cm2 is

observed in figure 4.14 (b). The plausible reason for this sort of induced structure variation can

be expressed in terms of enhanced energy deposition due to collective behavior of Si ion with

increasing ion flux. When Si ion is implanted in the brass surface, it is observed that deposited

energy of the incident ions to the host atoms releases the strain energy in the surface and

subsurface of brass and is proportional to thermal spike. Therefore boundaries between the grains

were broken and form large grains by merging of smaller grains. Thus the length of grain

boundaries in the target surface is enhanced. The recovery phenomena are responsible for grain

size variations [27]. But significant reduction in crystallite size is observed with further increase

in ion flux. The crystallite size is reduced from 28 nm to 15 nm with increasing flux from 56 ×

1014 ions/cm2 to 75 × 1015 ions/cm2 (figure 4.14 b). The fundamental reasons for reduction of

peak intensity and crystalline size are abrupt melting, quenching and recrystallization procedures

which take place during resolidification after ion material interaction [134]. The induced

pressure, displacement and thermal spike zones within the crystal are formed. These zones are

the responsible to destroy the crystallinity and generate the large numbers of defects and

100

dislocation. Therefore, large sized grains break up into smaller sized grains and destroy the

crystallinity which is responsible for decrease in peak intensity and crystalline size. Therefore,

length of grain boundaries is reduced [116]. Figure 4.14(c) demonstrates decreasing trend in

dislocation line density from 8 × 1014 m-2 to 5 × 1014 m-2 by increasing ion flux from 45 × 1012

ions/cm2 to 25 × 1013 ions/cm2. The collective energy deposition by Si ions in the crystal lattice

of brass is appeared in the form of localized heat source. This heat source is helpful to remove

the intrinsic strains between the lattice atoms and mobility of Frenkel pair (vacancies and

interstitial defects) is increased. As a result the density of these defects is reduced by mutual

annihilation. This will be possible if the rate of mutual annihilation process is greater than rate of

generation of defects at these flux. Whereas, by increasing in the value of ion flux from 56 ×

1014 ions/cm2 to 75 × 1015 ions/cm2, dislocation line density is increased from 13 × 1014 m-2 to 44

× 1014 m-2, due to the enhancement of the concentration of imperfections in lattice sites of brass

crystal. Actually energy absorbed by lattice atoms during ion material interaction is utilized not

only to increase the movement of prevailing dislocation imperfections but also creates a large

number of new dislocation imperfections in the lattice and sublattice of brass due to extensive

lattice vibrations and distortions. For this flux the rate of defects generation process is greater

than rate of annihilation process [115]. When the Si ions hit the lattice atoms of target surface

and displace them from the normal lattice sites. Well defined and well ordered lattice planes of

brass are demolished and as a result imperfections are enhanced which are responsible for

enhancement of defects densities. This is another possible mechanism for improvement in

dislocation imperfections. When energetic Si ions are bombarded on the brass surface, lattice

distortion and thermal shock waves are produced due to implantation. For that reason, tensile and

compressive residual stresses are generated as shown in figure 4.14 (d). The tremendous

temperature gradients are formed due to high rates of heating and cooling. This is vital reason for

the production of residual stresses in lattices system of brass [116].

The comparison of X-Ray diffraction of brass after irradiation by laser induced plasma ion of Ni

and Si (figures 4.10 and 4.14) show significant crystallographical changes in brass. In case of Ni

ions irradiation a new phase of CuZnNi is identified. Whereas, in case of Si ion irradiation a new

phase of CuSi is formed. An anomalous variations in peak intensity, crystallite size and

dislocation density of brass are achieved after both Ni and Si ions irradiations. However

significant reductions in crystallite size and noticeable increments in ion induced dislocation line

101

density with increasing ion flux are observed in the case of Si ion irradiation. This enhancement

in dislocation density is attributed to the generations of enhanced defects after Si ions irradiation.


Figure 4.15 (a) depicts stress–strain curves of (1) unirradiated and Si ion irradiated brass with

various ion flux of (2) 45 × 1012 ions/cm2 (3) 25 × 1013 ions/cm2 (4) 56 × 1014 ions/cm2 (5) 75 ×

1015 ions/cm2. The influence of step wise increasing ion flux on Yield Stress (YS) and Ultimate

Tensile Strength (UTS) of brass is graphically represented in figure 4.15 (b) and (c) respectively.

From these graphs (b & c) it is observed that YS is decreased from 150 to 144 MPa and UTS is

decreased from 363 to 357 MPa by increasing flux of incident Si ions up to 25 × 1013 ions/cm2.

YS is increased from 155 MPA to 180 MPa and UTS from 373 MPa to 396 MPa by further

increasing in ion flux from 56 × 1014 ions/cm2 to maximum value of 75 × 1015 ions/cm2. It is also

observed from figure 4.15 (a), that the total elongation is decreased with increasing concentration

of implanted Si ions in the brass matrix. It is best evidence for radiation hardening [144]. The

deviation in ion induced effect, defects and stresses are responsible for alteration in YS and UTS.

These variations in YS and UTS are due to modification in microstructural and dislocation line

densities after ion metal interaction. The decline in YS and UTS of irradiated brass in contrast

with unirradiated target are related with the variation of crystalline size, dislocation line densities

and ion induced stresses. The increase in crystallite size and decline in dislocation line densities

also induces a low level of stresses in the irradiated substrate. During ion irradiation, there is the

expansion of an induced thermal spike from the collision cascades core in the lattices and

sublattices site of brass by dissipation of PKA energy among many others nearby atoms lattice.

Therefore the energy of implanted ion is appeared in the form of thermal energy. This thermal

energy is responsible for the increasing in mobility of points and dislocation defects and reducing

the strain energy by the mutual annihilation of defects. The rate of defects generation is smaller

than rate of defects annihilation upto 25 × 1013 ions/cm2 ion flux. The defects (interstitial and

vacancies defects) produced by ion irradiation are absorbed by gliding dislocations for the

duration of the plastic deformation. Therefore, these defects could not act as effective barriers to

the dislocation movement [145]. This recovery process continuous up to 10-8 second but depends

on the temperature, chemical composition of irradiated material and prior microstructure of

irradiated surface [67].

102

Figure 4.15: UTM data of laser induced Si plasma ions irradiated brass at constant energy of 289

KeV for various ion flux of 45 × 1012 ions/cm2, 55 × 1013 ions/cm2, 65 × 1014

ions/cm2 and 75 × 1015 ions/cm2. (a) Tensile stress- tensile strain curves, (b) The

yield stress Vs ion flux (c) The ultimate tensile stress Vs ion flux.

The area of induced displacement cascades and subcascades are increased by increasing ion flux

up to 75 × 1015 ions/cm2. The ion induced vacancies, interstitial defects, precipitates and other

types of lattice distortions are distributed in the crystal lattice. As a result the high density of

PKAs and Frenkel pairs are formed. The total number of Frenkel pairs (Nd) generated by one

103

PKA can be predictable with the help of simple equation Nd = 0.5EPKA/Ed [63]. In this equation,

EPKA is primary knock- on atom energy and Ed is displacement energy.

The large concentration of Si ions into matrix brass is increased by increasing flux which are

responsible for the significant improvement in the mechanical properties of brass. The

generations of these defects depend on ion fluence, ion energy, irradiation exposure time,

temperature and nature of target materials [1]. The generation of defects is more pronounced

than annihilation process upto ion flux of 75 × 1015 ions/cm2. The number of Frenkel pairs

(vacancies and interstitial defects) per cascade can be calculated by relation [137] NF = 0.8 Ei/

2ED . In this equation Ei is incident energy and ED is displacement energy. After irradiation of

brass target by maximum flux, a strong extrinsic strain fields are produced due to grouping of ion

induced defect clusters near the dislocation line. The mobility of dislocation lines in the slip

plane are pinned by defects. Because ion induced defects (vacancies, interstitial defects,

precipitates) act as barrier in the path of dislocation motion and obstructing their reproduction in

different slip plane. Hence dislocation mobility condenses which causes improvement of the

mechanical properties. For that reason greater mechanical stresses will be required to start the

slip process [119].

4.4.4 Microhardness

Figure 4.16 illustrates the variations in the values of microhardness of laser induced Si plasma

ion irradiated brass for various ion flux ranging from 45 × 1012 ions/cm2 to 75 × 1015 ions/cm2.

The radiation-induced hardening initially decreases and then increases with the increasing the

flux of Si ions. Similar results are reported by Yutian et al. [146]. The ion induced displacement,

thermal and energy spikes are responsible to generate lattice distortion, stresses, precipitate

dispersions, Frenkel Pairs agglomerations and their annihilation in well defined and ordered

lattice system of brass after ions irradiation [139]. Due to ion material interaction an extensive

variation is produced in dislocation density, crystallite size and residual stresses. These variations

play a vital role in the modification of hardness. According to the Hell Petch equation 4.5, the

hardness and yield strength dependence on crystallite size for irradiated target.

104

Figure 4.16: The variations in the hardness of laser induced Si plasma ions irradiated brass for


75 × 1015 ions/cm2 at constant energy of 289 KeV.

The comparison of figures 4.14 (b) and 4.16 shows that Hall petch criteria is fulfilled, because by

increasing Si ion flux from 45 × 1012 ions/cm2 to 25 × 1013 ions/cm2 the crystallite size increases

and respectively hardness decreases. For increasing Si ion from 56 × 1012 ions/cm2 to 75 × 1015

ions/cm2 a decrease in crystallite size and the corresponding increase in hardness is observed.

By comparing the tensile testing results of Ni and Si ion irradiated brass, it is revealed that both

Yield Strength (YS) and Ultimate Tensile Strength (UTS) after ion irradiations show anomalous

behavior. This anomalous behavior is attributed to generation, recombination, and annihilation of

the ion induced defects. An anomalous trend is also observed for microhardness in both cases of

Ni and Si ions irradiation. However, more pronounced radiation-induced hardening is obtained in

the case of Si ions irradiation. It is due to more energy of Si ions as compared Ni ions.

105

4.5 Effects of laser induced C plasma ions irradiation on the surface, structural and


This part of dissertation is associated with the effect of laser induced C plasma ions irradiation

effects on the surface, structural and mechanical properties of brass.

4.5.1 SEM analysis

SEM micrographs of figure 4.17 reveal surface morphology of (a) unirradiated and laser induced

C plasma ion irradiated brass substrate for various ions flux of (b) 32 × 1011 ions/cm2, (c) 90 ×

1012 ions/cm2 (d), 64 × 1013 ions/cm2 and (e) 72 × 1014 ions/cm2 at constant energy 678 KeV.

Figure 4.17 (b) depicts the formation of various features including micro / nano sized cavities,

pores and pits with irregular shapes and non uniform density distributions at lowest carbon flux

of 32 × 1011 ions/cm2. When the ion flux is increased from 32 × 1011 ions/cm2 to 90 × 1012

ions/cm2 a significant reduction in the size of cavities, pores and pits is observed in figure 4.17(c)

whereas number density of pores and pits is increased. By further increase in ion flux up to 64 ×

1013 ions/cm2, a significant different surface morphology is observed in figure 4.17 (d) as

compared to figures 4.17 (b) and 4.17 (c). The diffusive dendritic structures are grown along

with appearance of cavities, pores and pits for this ion flux. At the maximum ion flux (72 × 1014

ions/cm2) the dendritic structures become more distinct, regular shaped and compact whereas,

cavities, pores and pits are completely vanished for this flux as shown in figure 4.17 (e). The

dendritic morphology is observed due to solute-diffusion and uncontrolled heat flow [147]. The

formation of micro / nano sized cavities, pores and pits (figure 4.17 b-d) are due to thermal

sputtering [132]. The incident absorbed energy from the incident carbon ions appear in the form

of localized source of heat. Therefore the temperature of irradiated zone avalanche is increased

up to thousand degree celsius [127]. This heat source is responsible for the generation of lattice

distortion, intense melting and explosive boiling of the irradiated surface. Therefore a significant

pressure is built up inside the melted zone and if the melted zone is closed to the surface, an

induced pressure never exists inside the surface and outflow of material occurs from the hot

melted core spike by microexplosion as a result surface cavities are formed [132]. As the brass is

a good conductor of heat, therefore, these localized heated volumes expand rapidly and

consequently the rise in temperature takes place in the peripheries of ion irradiated zone. After

energy absorption of incidents ions, atomic vibration of irradiated surface atoms increases across

the critical point of atomic vibration and as a result the bonds between the atoms are broken

106

which lead to thermal sputtering. Firstly the size of cavities is reduced and finally they are

completely vanished at maximum ion flux because of refilling by liquefied material. The

variations in the surface morphology from porosity to dendritic formation are explained on the

basis of ion induced thermal spike.

Figure 4.17: SEM micrographs illustrating the surface morphology of (a) unirradiated and laser

induced C plasma ions irradiated brass target at constant energy of 678 KeV for

various ion flux of (b) 32 × 1011 ions/cm2, (c) 90 × 1012 ions/cm2 (d), 64 × 1013

ions/cm2 and (e) 72 × 1014 ions/cm2.

107

The host atoms are displaced from their normal lattice sites after ion material interaction. The

displaced atoms are called primary knock-on atoms (PKAs). Consequently highly distorted core

is produced in which host atoms have energy up to several eV. This core is called displacement

spike where the PKAs and implanted ions subsequently disperse their energy via displacement

cascades and subcascades. As a result local thermal spikes are formed [64]. Therefore, the

temperature within the spike is increased significantly above the melting temperature of material.

The melting temperature of C is 3527 Co which is significantly higher than melting temperature

of brass contents i.e. Cu and Zn (1085 and 419.73 Co respectively). In the preliminary stages of

solidification of molten zone of irradiated surface by carbon irradiation, high melting

temperature of C (3527 Co) as compared to low melting point of Cu (1085 Co) and Zn (419.73

Co) does not allow brass (Cu and Zn) to freeze. The Cu and Zn (low melting atoms of brass) tend

to retard growth as they cannot diffuse swiftly away from the solid liquid interfaces.

Nevertheless the liquid temperature is higher, farther away from the interface, that is, towards the

center of the molten metal. Thus portions of the interface protrude or grow rapidly into the

liquid, resulting in columns that grow from the initially solidified material, forming a cellular or

columnar structure. For even lower temperature gradients at the interface, branches can grow

from the main columns, resulting in a dendritic structures [148]. The implanted carbon has low

solubility in brass solid solution because the melting point of carbon is very high as compared to

brass. The dendritic structures are grown rapidly in the same direction in which heat flows. Thus

hydrodynamic Kelvin-Helmholtz instabilities occur during growth of dendritic structures [149].

Voelskow et al. [150] also reported that low solubility of carbon in silicon solid liquid and its

effect on dendritic growth. During dendritic growth , latent heat of fusion is transferred into the

supercooled liquid and increases the temperatrure of liquid towards the freezing point.

Subsequently secondary and tertiary dendrite arms are also grown on the primary trunks to speed

up the development of the latent heat of fusion [92, 151]. Dendritic development continues untill

the supercooled liquid can not achieve the freezing temperature. Because the energy barrier is

very high at low supercoolings for nucleus fomation and the nucleation rate is very low. When

supercooling tempeature raises, the rate of nucleus formation is increased. When the nucleus is

grown it will continue its growth process. Such growth depends on the kinetics of the atoms

attachment to interface, capillarity, solidification condition and diffusion of heat and mass [151-

153]. Then solidificatons of remainging liquid causes planar growth. The solidification time

108

influences the dendrites size. In general the size of dendrite is characterized by measuring the

distance between the secondary denderitc arm. This distance is called Secondary Denderitic Arm

Spacing (SDAS) which is reduced when solid solution is rapidly quenched. SDAS plays a vital

role in the modification of the mechanical properties of irradiated target [92, 151]. The more

efficient conduction of the latent heat of fusion to supercooled liquid produces finer and more

extensive dendritci network. The anisotropic growth is responsible for the dendritic growth by

non equilibrium conditions through diffusion limited agglomeration phenomena [154]. The

parameters which are very important for the dendritic growth are velocity of the dendritic tip

which shows nonlinear behavior for undercooling of the melted material. The morphological

stability of the growing pattern is dependent on the negative thermal gradients in the supercooled

liquid which produces driving force of the morphological instability and thermodynamic

stabilizing force due to the surface tension [153, 155].

Figure 4.17 (b-e) depicts that the noticeable damage is produced on the target surface with

generation of large number of defects including induced stresses, diffusion of ions, Frenkel Pairs

(vacancy and interstitial defects) and other heterogeneities. The thermal spike, collisional

cascades and non uniform energy absorption into surface and subsurface of brass are main reason

for these defects.

By comparing the SEM results of Ni, Si and C ion irradiated brass, we found for the ion flux

ranging from 32 × 1011 ions/cm2 to 70 × 1015 ions/cm2 that cavities, pore, and pits have been

observed in all kind of induced ions. At the maximum flux of ions Ni and Si granular

morphology is observed whereas, in the case of C ion irradiation dendritic structures are formed.

These variations are attributed to hydrodynamic Kelvin-Helmholtz instabilities and low

solubility of implanted C ion in the brass crystal.

4.5.2 XRD analysis

Figure 4.18(a) depicts XRD patterns of unirradiated (e) and C ion irradiated brass at constant

energy of 678 KeV for various ion flux of (f) 32 × 1011 ions/cm2, (g) 90 × 1012 ions/cm2 (h), 64 ×

1013 ions/cm2 and (i) 72 × 1014 ions/cm2. The XRD patterns of unirradiated brass exhibit the

occurrence of CuZn (222), CuZn (320), CuZn (111), CuZn (332), CuZn (600), CuZn (210) and

CuZn (102) planes reflection at angles of 35.3o, 41.7o, 42.6o, 48.59o, 62.46o, 71.62o and 78.03o

respectively [156]. After C ion irradiation, a new phase of ZnC (0012) at an angle 36.3o is also

observed.

109

The energy deposition, high heating, induced explosive melting and recrystallization play a vital

role in the formation of new phase of brass. By the projectile ions, the energy is deposited on the

target electrons which is transferred to the host lattice. As a result local temperature of irradiated

target surface is increased.

Figure 4.18: XRD data of laser induced C plasma ions irradiated brass target at constant energy

of 678 KeV for various ion flux of 32 × 1011 ions/cm2, 90 × 1012 ions/cm2, 64 ×

1013 ions/cm2 and 72 × 1014 ions/cm2. (a) XRD diffractrographs, (b) The crystallite

size Vs Ion flux, (c) The dislocation density Vs Ion flux and (d) Stresses Vs Ion

flux.

110

According to thermal spike model, the time scales of lattice vibration are much greater than the

required time for the energy transference from the projectile ions to the excited electrons. The

energy shift from electron to host lattice take place in time 10-17 second and this time also

depends on the strength of electron-phonon coupling [157]. It clues to a momentary raise of

temperature within irradiated zone along ion penetrating path. Therefore induced local heat is

transferred to the lattice and sublattice in the form of energy by electron-phonon coupling. The

prior crystal structure and crystalline nature of irradiated brass is destroyed due to increase in

temperature by electron- phonon coupling. As a result a new phase is formed [158]. The amount

of implanted C ions into brass matrix increases by increasing ion flux which is responsible to

increase the temperature of lattice and sublattice. The effect of ion flux on the crystallite size (D),

dislocation line density (δ) and stresses have been investigated from the XRD patterns (figure

4.15 a) and are evaluated for the reflection plane of brass (111) by using equations 4.1,4.2,4.3

and 4.4 [69].

From figure 4.14 (a) it is observed that the peak intensity of irradiated target for the (111) plane

decreases monotonically by increasing ion flux up to maximum value of 56 × 1014 ions/cm2.

While the FWHM increases significantly by increasing ion flux. The variations in peak intensity

are attributed to the generation of defects in the normal CuZn matrix. At the same time the

broadening of the diffraction peak (111) is best evidence for the defect formation after ion

irradiation. The XRD patterns of C irradiated brass demonstrate step wise reduction in peak

intensity. This is attributable to the deficiency in long range order and as a result an extensive

disorder zone is formed by the ion irradiation. The area of disorder regions is increased by

increasing flux of implanted carbon ion in surface and subsurface [126]. The number of

generated defects (∆ρ) depends upon time (t) of exposure and flux (Ф) of ions by following

relation [159].

∆ρ = Ф × t (4.6)

Figure 4.18 (b) reveals the variation in the crystallite size of C irradiated brass at constant energy


and 72 × 1014 ions/cm2. A significant decrease in crystallite size with increasing the flux of C

ions is observed. The rapid melting, quenching and recrystallization phenomena are the main

cause for reduction in peak intensity and crystallite size [134]. The large amount of energy,

pressure, displacement and thermal spike are responsible to generate two parallel phenomena

111

simultaneously duration ions irradiation. First phenomenon is generation of interstitial and

vacancies defects, agglomeration of these defects and then the formation of dislocation loop.

Second one is annihilation of these induced defects at probable sinks. By increasing flux the rate

of generation of defects is greater than rate of annihilation because the large sized grains break

up into smaller sized grains [116]. As a result a significant reduction in peak intensity and

crystallite size occurs (figures 4.18 a & b). Figure 4.18 (c) depicts increasing trend in dislocation

line density by increasing ion flux up to maximum value of 72 × 1014 ions/cm2. It is due to the

large scale enhancement in the concentration of lattice imperfections after ion irradiation. The

augmentation in dislocation line density in the brass matrix is a quantification of irradiation

hardening. Figure 4.18 (d) shows the graphical representation in residual stresses of C ion

irradiated brass by increasing flux. The diffraction peak shift (111) towards the higher angle

corresponds to compressive stresses [133]. By increasing the value of flux from 32 × 1011

ions/cm2 to 72 × 1014 ions/cm2, compressive stresses become more pronounced (figure 4.18 d).

After ions target interaction, ion induced thermal spike, displacement cascades and lattice defects

are produced which are responsible for the generation of compressive stresses in lattice systems.

Because, atomic radius of implanted C (0.91 Ǻ) is smaller than the atomic radius of host atoms

of Cu (1.27 Ǻ) and Zn (1.33 Ǻ) [93]. The discrimination in interplaner distances, thermal and

displacement spike, thermal expansion coefficients, heating and abrupt cooling conditions

between the surface layers and interstitial diffusion of host atoms are the possible roots to create

lattice distortions and lattice stresses in lattice planes of brass after ion material interaction.

By the comparing the results of XRD, we found that in the case of laser induced plasma ions of

Ni and Si ion irradiations, peak intensity, crystallite size and dislocation line density reveal

anomalous behavior but in the case of C ion irradiation, monotonically decreasing trend of peak

intensity and crystallite size is observed. Whereas, monotonic increase in dislocation line density

is revealed. Decreasing trends in peak intensity and crystalline size are attributed to rapid

melting, quenching and recrystallization phenomena. While an increasing trend in dislocation

line density is due to large scale increment in the concentration of lattice imperfections.


In order to study the effect of ion irradiation on mechanical properties of brass, tensile testing is

performed. Figure 4.19 (a) depicts stress–strain curves of (1) unirradiated and irradiated brass for

various C ion flux of (2) 32 × 1011 ions/cm2 (3) 90 × 1012 ions/cm2 (4) 64 × 1013 ions/cm2 (5) 72

112

× 1014 ions/cm2 at constant energy of 678 KeV. Figure 4.19 (a) confirms the brass target goes

through plastic deformation until fracture deformation. With increasing ion flux, the mechanical

strength shows a monotonic propensity of hardening upon implantation. The modification in

Yield Stress (YS) and Ultimate Tensile Strength (UTS) as a function of ion flux are graphically

represented in figure 4.19 (b) and (c) respectively.

Figure 4.19: Universal Testing Machine (UTM) data of laser induced C plasma ions irradiated

brass surface with constant energy of 678 KeV under vacuum environment at


72 × 1014 ions/cm2. (a) Tensile stress- tensile strain curves, (b) The yield stress Vs

ion flux (c) The ultimate tensile strength Vs ion flux.

113

Both YS and UTS of the brass target are increased monotonically with increasing ion flux from

32 × 1011 ions/cm2 to 72 × 1014 ions/cm2. The improvement in YS and UTS of irradiated brass

are correlated with the generation of the dislocation line density, grain size and the

microstructural deformation. As a result the crystallite size is decreased and dislocation line

density is increased. This is responsible for generations of high level of induced stresses in

normal lattice sites of the target after ion irradiation. The maximum transferred energy to the host

atom lattice is directly proportional to the incident ion flux and inversely proportional to the

atomic mass of the target. The collectively ion energy deposition on the target surface is

increased by increasing flux up to 72× 1014 ions/cm2. If the energy of projectile ion energy is

greater than the displacement energy then phenomenon of dislodging host atoms from normal

lattice site takes place by transfer of energy from incident energetic ions. The recoiled lattice

hosts atoms (Cu and Zn) are moved within the crystal interact elastically with other nearby host

atoms and further displace the atoms from their normal lattice sites. This process persists until

the energy of each implanted ion and displaced atoms are smaller than displacement energy.

Atomic displacement cascades are produced by the host atoms and implanted ions with the end

result that large number of vacant lattice sites and an equal number of displaced atoms wedged

into the interstitial sites of the lattices are generated [1].

Increasing flux of C ion in the solid solution of brass introduces lattice strains on the vicinity

host lattice atoms, because of smaller atomic radius of C (0.91 Å) atom as compared to Cu (1.27

Å) and Zn (1.33 Å). The induced lattice strain field interactions between dislocations and these

implanted ions occur, and consequently, dislocation movement is restricted. As a result

resistance of the implanted brass against the slip process is increased and greater applied stress is

required to first initiate and then to continue plastic deformation for the material target [145].

4.5.4 Microhardness

Figure 4.20 depicts the modifications in the microhardness value of ion irradiated brass for

various ion flux ranging from 32 × 1011 ions/cm2 to 72 × 1014 ions/cm2. The radiation-induced

hardening increases by increasing ion flux up to 72 × 1014 ions/cm2. After ion irradiation, lattice

distortion, precipitate dispersions, structural modifications and compressive stresses are

generated in lattice sites system of brass substrate which are attributed to thermal, energy and

displacement spikes [139]. By increasing ion flux, reduction in crystallite size and an

enhancement in dislocation line densities take place.

114

Figure 4.20: The variations in the hardness of laser induced C plasma ions irradiated brass

surface for various ion flux of 32 × 1011 ions/cm2, 90 × 1012 ions/cm2, 64 × 1013

ions/cm2 and 72 × 1014 ions/cm2 at constant energy of 678 KeV.

The dislocation strengthening and grains refinement are main reason for enhancement in

hardness after ion matter interaction [160].

The comparison of laser induced Ni, Si and ion irradiated brass of figures (4.11, 4.12, 4.15, 4.16,

4.19 and 4.20 respectively) gives a considerable distinction in the mechanical properties of brass.

An anomalous trend is observed in mechanical properties (YS, UTS and microhardness) of brass

after Ni and Si ion irradiations. Whereas for C ions irradiation, an increasing trend in mechanical

properties is observed. The greater rate of generations and augmentations of ion induced defects

as compared the rate of annihilations can be considered possible cause for this increasing trend.

This augmentation and generation is due to higher energy of C ions (678 KeV) as compared to

Ni (138 KeV) and Si (289 KeV). The smaller atomic radius of C (0.91 Å) as compared Ni (1.23

Å) and Si (1.17 Å) can be considered another possible cause for more effective penetration and

consequently is responsible for enhancement of defects.

115

Part (C)

Accelerator ions irradiation

116

4.6 Effects of accelerator Ni ions irradiation on the surface, structural and mechanical

properties of brass

The effects of accelerator Ni ions irradiation on the surface, structural and mechanical properties

of brass has been discussed in this following section.

4.6.1 SEM analysis

SEM micrographs of figures 4.21 depict (a) unirradiated and Ni ions irradiated brass targets for

various flux of (b) 56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2, (d) 15 × 1013 ions/cm2 and (e) 26 ×

1013 ions/cm2 at constant energy of 2 MeV.

Figure 4.21: SEM micrographs representing the micro surface structuring of (a) unirradiated and

accelerator Ni ions irradiated brass target at constant energy of 2 MeV for various

ion flux of (b) 56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2 (d), 15 × 1013 ions/cm2 and

(e) 26 × 1013 ions/cm2.

117

The magnified view of theses micrographs is exhibited in SEM micrographs of figure 4.22 for

various Ni ions flux of (b) 56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2, (d) 15 × 1013 ions/cm2 and

(e) 26 × 1013 ions/cm2.

Figure 4.22: SEM micrographs showing the surface structuring of (a) unirradiated and

accelerator Ni ions irradiated brass target at constant energy of 2 MeV for various

ion flux of (b) 56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2 (d), 15 × 1013 ions/cm2

and (e) 26 × 1013 ions/cm2.

118

For the lowest flux of 56 × 1012 ions/cm2, diffused and irregular shaped nano craters with

random density distribution are observed as revealed in figure 4.22(b). Increase in flux of Ni ions

from 12 × 1013 ions/cm2 to 26x1013 ions/cm2, nano sized circular and elliptical shaped craters are

observed in figure 4.22 (c-e). The density of nano craters increases by increasing the ion flux,

whereas their average diameter decreases from 210 nm to 130 nm. There is competitive

phenomenon between thermal sputtering and redeposition by increasing ion flux. With increase

in flux the probability of redeposition and refilling is larger than thermal sputtering, causing the

reduced diameter of craters with increasing ion flux. The surface morphology of brass is

changed after ion induced melting, thermal and pressure spike, thermal sputtering, evaporation

and deposition. These phenomena are responsible for the formation of nano meter sized craters

[140]. The deposition of energy within the absorbing lattice of material increases by increasing

the Ni ion flux and is attributable to enhancement of accumulated and collective effects of ions.

The ion induced stresses, shocks waves, elastic rebounds and ion induced stresses are produced

in well defined and localized regions and responsible for such kinds of features. When the

energetic ions are incident on the target surface, they lead to sputtering of surface host atoms.

Some incident ions are also scattered in different directions after collisions. If the energy of

incident ion is greater than the binding energy of lattice atom then it breaks the chemical bonds

between the host lattice atoms of the irradiated surface. The energy and momentum of incident

ion are completely transferred to the host atom of the irradiated surface after elastic collision.

After receiving this energy and momentum, the host atom are dislocated from their normal lattice

position. Therefore Primary Knock on Atoms (PKAs) phenomenon is activated. These PKAs

further share their momentum and energy with nearby host atoms which cause the generation of

displacement spikes and thermal spike. The violent interaction is produced between host atoms

in the central core of thermal spike [67]. Therefore, a drastic compression (up to GPa level) of

the material atoms and significant increase of temperature up to a few thousand degrees is

developed in the impact zone and subsurface. The induced pressure spike and heat spike lead to

the material melting around the impact zone. The melted zone is produced close to the surface,

where GPa pressure builds up due to the compression of material atoms which cannot be

contained in surface and subsurface. Hence, the host atoms of irradiated surface can be pushed

towards the surface by viscous flow. Consequently, surface craters are formed due to micro

explosions [161]. The formation of surface crater is dependent on the intrinsic properties of

119

target and energy of incident ions. More favorable conditions for the production of surface crater

after ion irradiation are lower material density, low melting temperature of target material, low

binding energy and smaller atomic displacements. The shape of the craters is dependent on the

incident angle. Typically round in shape craters are formed at normal incidence. On the other

hand when the angle is increased or decreased with respect to normal incidence it leads to

produce alteration in shape of craters. At the oblique impact (60o angle) the shape of crater is

usually elliptical [162]. Our targets were exposed at angle of 90o with respect to its surface. This

angle was kept constant for irradiation. But a change in shape of nano craters is observed which

is attributable to multiple scattering events. The increase in diameter of craters is attributed to

coalescence process. By increasing the ion flux the coalescence process is dominant however the

reduction in craters diameters is attributed to refilling by shock liquefied and intense melting of

material. The change in diameter of surface craters is due to ion induced imperfections in the

lattice site of target surface such as vacancy, interstitial defects, small pits, dislocation loops and

other heterogeneities. The non uniform energy absorption, displacement spike, thermal spike,

pressure spike and recrystallization are responsible for these imperfections. [69].

Comparison of laser induced (Ni, Si, and C) ions and Pelletron accelerator Ni ions irradiation

represents significant difference in surface morphology of irradiated brass target. During laser

induced (Ni, Si, and C) ions irradiation, initially cavities, pores and pits are observed but finally

granular morphology (in case of Ni and Si irradiated brass) and dendritic structure are formed (in

case of laser induced C plasma ions). Whereas accelerator Ni ions irradiation of brass generates

nanosized craters. The flux of accelerator ions is smaller as compared to laser induced plasma

ions.

4.6.2 XRD analysis

XRD spectra of unirradiated and Ni ion irradiated brass target sample are shown in figure 4.23

(a). The unirradiated brass target shows the existence of CuZn (320), CuZn (111), CuZn (332),

CuZn (600), CuZn (210) and CuZn (102) planes reflection of brass at angles of 41.7o, 42.6o,

48.59o, 62.46o, 71.62o and 78.03o respectively [156]. It is found that the intensity of CuZn (111)

peak increases by increasing ion flux (figure 4.23 a). No new phase is formed after the ion

irradiation. The close examination reveals that an increase in peak intensity of plane (111) and a

decrease in peak width is attributed to reduction in density of generated defects by annihilation

process after ion material interaction [116]. The grains of the material consist of large number of

120

crystallites which have dissimilarity orientation. A crystallite may be composed of multiple

domains. X-ray line profile analysis have been employed to indirectly find out the crystallite

size, dislocation line density and ion induced stresses in lattice planes [163] .

Figure 4.23: XRD data of accelerator Ni ions irradiated brass target surface for various ion flux

of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013

ions/cm2 at constant energy of 2 MeV. (a) XRD diffractrographs, (b) The

crystallite size Vs Ion flux, (c) The dislocation density Vs Ion flux and (d) Stresses

Vs Ion flux.

From figure 4.23 (a) it is observed that the peak intensities of irradiated target surface for the

plane of (111) increase monotonically by increasing ion flux up to a maximum value of 26 × 1013

ions/cm2. This improvement in peak intensity is due to the atomic diffusion of host atoms of

121

lattice site across the grain boundaries. Therefore, new alignment of host atom is formed after

ion irradiation and this process is called crystal growth. Figure 4.23 (b) shows graphically that a

noticeable increment in crystallite size occurs from 22 nm to 35nm with an increase in ion flux

from 56 × 1012 ions/cm2 to maximum value of 26 × 1013 ions/cm2. Ion irradiation and thermal

annealing process lead to reduced strain field by more texturing. The probable justification for

this kind of structure variation induced by thermal annealing and ion irradiation can be justified

on the following assumption that the collective deposition of energy of total irradiated ions is

more effective than single ion bombardment on the surface of brass target [133]. When the brass

surface is irradiated with Ni ions, the number of multiple collisions of incident ions with host

atoms of target increases by increasing ion flux. Therefore, the energy deposition by the

bombarded ions on the target surface is usually appeared as thermal heating in the surface and

subsurface of material. Very large amount of ion energy is transferred to the material and is

responsible for the development of very high temperature zone. Ion irradiation results in the form

of local annealing of the material surface under the effect of transitory heating by electron and

phonon coupling and is called thermal spikes [133]. This energy leads to minimize the strain

field between the grains of irradiated target. Therefore, the small grains emerge into large grain.

As a result further improvement occurs in crystallite quality of material upon irradiation. The ion

irradiation activates the annihilation process of defects which is one of the major reasons for the

enhancement in the crystallite size [164] . The effect of ion flux on the dislocation line density

and residual stress is illustrated in figures 4. 23 (c) and (d) respectively for predominant peak of

(111) plane. From figure 4.23 (a) it is notable that diffraction peak of the plane (111) shifts to

smaller angle which shows that lattice constant parameters (d-spacing and FWHM) are changed

upon irradiation. Figure 4.23 (d) depicts that the ion induced residual stresses in irradiated brass

material are tensile stresses in nature. With increasing Ni ion flux, both the dislocation line

density and residual stresses are reduced. This behavior can be explained on the base of two

competing phenomena which are produced simultaneously during ion material interaction. The

first phenomenon is production of vacancies and agglomeration of induced vacancies which are

finally collapsed into dislocation loop. The other phenomenon is the transference of high amount

of energy to the target surface via ion bombardment which leads to generation of high

temperature zone on the surface and subsurface. As a result annihilation process is activated.

Although more vacancies are produced by increasing ion flux but at the same time annihilation

122

rate of these induced vacancies also increase. Therefore sink volume increases by increasing ion

flux [165].

The comparison of XRD data of brass after irradiation by laser induced plasma (Ni, Si, and C)

ions and Pelletron accelerator Ni ions provides significant similarities and differences. In case of

laser induced plasma (Ni, Si, and C) ions irradiation, new peak is formed where as no new peak

is observed in case of accelerator Ni ions irradiated brass. During laser induced plasma (Ni, Si,

and C) ions irradiations, peak intensity, crystallite size and dislocation line density reveals

anomalous behavior whereas in case of brass target irradiated by Pelletron accelerator Ni ions,

peak intensity and crystallite size is increased whereas, dislocation line density is decreased

monotonically. This is attributed to the fact that smaller grains are emerged together and form

large sized grains after accelerator Ni ion irradiations. Consequently, ion induced defects are

decreased by mutual annihilations therefore dislocation line density is decreased.


The stress strain curves of unirradiated and irradiated brass targets for various flux are depicted

in figure 4.24 (a). The mechanical properties of irradiated brass such as Yield Stress (YS) and

Ultimate Tensile Strength (UTS) decreases monotonically by increasing ion flux and is

graphically represented in figure 4.24 (b) and (c) respectively. The change in YS and UTS of

brass target after ion material interaction is related with the improvement in crystalline size

(figure 4.23 b), a reduction in dislocation line density (figure 4.23 c) and ion induced thermal

annealing. The reduction in YS and UTS is also explainable on the basis of thermal spike which

is produced by cascade collisions after ion irradiation. The main cause of radiation damage are

elastic interactions between incident ions and host atoms of irradiated target lattice, as a result

these host atoms displace from their normal stable lattice position. When the value of transferred

energy (Em) to the host atoms due to elastic collision is greater than displacement energy (Ed)

then the atomic displacement produces. The displacement energy is intrinsic property of the

material, the range of its value 20-40eV [63, 166]. But Broeders and Konobevev [166] described

that the displacement energy value can change in wider range from 10 eV to 600 eV. The

maximum energy transferred to the host atom is directory proportional to ion fluence as well as

Ni ion energy and inversely proportional to the atomic mass of the irradiated target material. In

this regards, by increasing number of impacts and consequential radiation damage deeply

depends on the target lattice and cohesive forces of material.

123

Figure 4.24: Universal Testing Machine (UTM) data of 2 MeV accelerator Ni ions irradiated

brass for various ion flux of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013

ions/cm2 and 26 × 1013 ions/cm2. (a) Tensile stress- tensile strain curves, (b) The

yield stress Vs ion flux (c) The ultimate tensile strength Vs ion flux.

The fast moving knock on atoms (PKA) is produced by Ni ions host atom elastic interaction. The

energy of PKA atoms (EPKA) can be easily defined by the difference between Ed and Em . The

displaced PKA will colloid with other host atoms elastically and produce displacement spike.

There is production of thermal spike in the displacement spike just after the ballistic phase.

124

Where the energy of the PKAs is distributed among many atoms to the central core of disorder.

As a result temperature of surface and subsurface is increased. The induced temperature during

collision cascade is higher than melting temperature of irradiated material for at least 10 -12

second. Therefore self and ion induced vacancies and interstitial atoms have higher mobility

which leads to the thermal annihilation of vacancies and interstitial defects. The average number

of Frenkel pair (vacancies and interstitial defects) decrease with increase in the size of thermal

spikes [135].

4.6.4 Microhardness

Figure 4.25 reveals decreasing trend in microhardness with increasing Ni ion flux from 56 × 1012

ions/cm2 to 26 × 1013 ions/cm2. Due to ion induced thermal spike, energy spike and displacement

spike, lattice distortion is produced after irradiation. This causes production of Frenkel pairs

defects and their annihilation in the normal lattice site of crystal. Therefore decrease in

mirohardness of irradiated target is observed. The reduction in hardness after ion irradiation is

directly related to crystallite size and dislocation line density as shown in figures 4.23 (b) and (c)

respectively.

Figure 4.25: The variations in the hardness of accelerator Ni ions irradiated brass for various ion

flux of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013

ions/cm2 at constant energy of 2 MeV.

The increase in crystallite size is main reason to decrease the hardness. During ion metal

interactions, the energy absorbed is used to increase the mobility of existing and ion induced

dislocation defects. As a result the rate of annihilation is dominant than generation of vacancies

125

and interstitial defects [115]. Another vital reason for the reduction in hardness of target surface

after ion solid interaction is ion induced thermal sputtering [13].

The comparison of irradiated brass target by laser induced (Ni, Si and C) ions and Pelletron

accelerator Ni ions gives considerable differences in the mechanical properties of brass. An

anomalous trend is observed in mechanical properties (YS, UTS and microhardness) of brass

after laser induced (Ni, Si and C) ion irradiations. Whereas in case of Pelletron Ni ion irradiation,

mechanical properties (YS, UTS and microhardness) of brass target are decreased monotonically

with increasing ion flux due to significant reductions in ion induced lattice imperfections by

recombination and mutual annihilations.

126

4.7 Effects of accelerator C ions irradiation on the surface, structural and mechanical

properties of brass

The effects of accelerator C ions irradiation on the surface, structural and mechanical properties

of brass has been discussed in this section.

4.7.1 SEM analysis

SEM micrographs of figure 4.26 reveal surface morphology of (a) unirradiated and 2MeV carbon

ion irradiated brass samples for various flux of (b) 56×1012 ions/cm2, (c) 12×1013 ions/cm2, (d)

15×1013 ions/cm2 and (e) 26×1013 ions/cm2 at constant energy of 2 MeV. It is observed that upon

irradiation of brass with lowest ion flux of 56×1012 ions/cm2, irregular and randomly distributed

morphology is revealed in figure 4.26(b). With further increase in the value of ion flux up to

12×1013 ions/cm2, intense melting, evaporation and ejection of material take place at the

irradiated surface which is responsible for the generation of craters (figure 4.26 c). For this ion

flux the incoherent structures are distributed all over the target surface. Both density and size of

craters increase significantly. The initial increasing trend in size and density of craters with

increasing flux is due to more ion energy deposition into the localized area (figure 4.26 b-c). By

increasing ion flux, the energy deposition within the absorbing material increases due to

collective effect of ions rather than the energy of single ion impact. Therefore the increasing ion

flux tends to destroy well-defined lattice structures of the crystalline material more effectively

and lattice imperfections increase significantly as compared to lower flux. In metals, almost all

of the absorbed energy from ions appears as heat. It is the kinetic energy deposition that

generally manifests itself as thermal heating of the material. As the ion flux increases, the overall

number of multiple collisions of bombarding ions with target atoms increases, and consequently

the number of knock-on atoms and displacement cascades increases which is responsible for the

generation of enhanced defects and localized melting in the irradiated brass. This localized

melting results into enhancement of the size and density of micrometer scale craters [167]. For

further increase in flux up to 15×1013 ions/cm2, the significant difference is produced in the

surface morphology and is revealed in figure 4.26 (d). Randomly distributed incoherent shaped

structures have been transformed into dendritic structures, cracks, and some spherical/triangular

craters. Abrupt cooling and resolidification cause the formation of dendritic structures and

cracks. At the maximum flux of 26×1013 ions/cm2 the surface modification of brass is illustrated

in figure 4.26 (e). It displays the nano/micro sized craters and voids along with the appearance of

pits. Such kind of pits and voids are not observed for lower flux. If the energy of the irradiated

127

ion is greater than the binding energy or threshold displacement energy of the target then the

irradiated ion produces distortion, thermal shocks, elastic rebounds and stresses (tensile or

compressive) in lattice plane [10].


accelerator C ions irradiated brass target at constant energy of 2 MeV for various

ion flux of (b) 56 × 1012 ions/cm2, (c) 12 × 1013 ions/cm2 (d), 15 × 1013 ions/cm2 and

(e) 26 × 1013 ions/cm2.

128

The crater formation as observed in figure 4.26 (b-d) may be attributed to thermal shocks on the

basis of ion induced heating, melting and explosive boiling of the target. The molten material can

be explosively expelled from the target surface after ion irradiation because of violent recoil

pressure. When the molten material pressure goes beyond the surrounding pressure it results in

crater formation. The reduction in craters size is attributable to material melting. It is due to the

fact that the phase of relaxation is less effective and liquidized material refills the craters during

resolidification. The molten metal is expelled from irradiated surface and rapidly cools down to

relatively colder target area. Therefore irradiated surface and interior irradiated surface layer

reveals ion induced residual stresses (tensile or compressive) [168]. The cavities and cracks

originated at an ion flux of 12×1013 ions/ cm2 and 15×1013 ions/ cm2 as observed in figures 4.26

(c-d) are attributed to the thermal shock induced residual stresses. Due to thermal shocks,

thermal stresses enhance with increasing ion flux and cause to weakening of atomic bonds at the

irradiated surface. The ion induced thermal shocks initiated cracks are effectively stopped at the

maximum value of ion flux of 26×1013 ions/ cm2 as shown in figure 4.26 (e). At this flux, ion

induced compressive stresses are pronounced than tensile stresses. The thermal shock-induced

compressive stresses act as a barrier in the propagation of cracks. The elimination of crack at

maximum ion flux is attributed to relaxation of ion induced tensile stress [169]. The maximum

ion flux of carbon produces large number of vacancies. The recombination of these vacancies

cause the formation of the voids as has been observed in figure 4.26 (e) [170]. It reveals random

distribution of irregular shaped craters with the size ranging from 100 nm to microns.

Comparison of laser induced (Ni, Si, and C) ions and Pelletron accelerator (Ni and C) ions

irradiation represents significant dissimilarities in surface morphology of irradiated brass target.

During laser induced (Ni, Si, and C) ions irradiation, initially cavities, pores and pits are

observed at lower values of ion flux but granular morphology (in case of Ni and Si irradiated

brass) and dendritic structure (in case of laser induced C plasma ion irradiated brass) are formed

at maximum ion flux. Whereas when brass target is irradiated by Pelletron accelerator Ni ions

nanosized craters are observed. But in case of C ion irradiated brass target, spherical/triangular

craters, cracks and dendritic structure are formed. This is attributed to abrupt cooling and

resolidification after C ion irradiations. The energy and flux of accelerator ions is lower than

laser induced plasma ions.

129

4.7.2 XRD analysis

Figure 4.27 (a) shows the XRD patterns of unirradiated and ion irradiated brass targets.

Unirradiated sample shows the presence of CuZn (320), CuZn (111), CuZn (332), CuZn (600),

CuZn (210) and CuZn (102) plane reflection at 41.7o, 42.6o, 48.59o, 62.46o, 71.62o and 78.03o

respectively. No significant change in crystallographical structure is observed after irradiation.

Graph of figure 4.27 (b) reveals the variation in the average FWHM for all diffraction lines

which are identified in XRD patterns as a function of ion flux. This graph illustrates that the

FWHM initially decreases and then increases with increasing the ion flux.

The average crystallite size (D), Dislocation line Density (δ), induced strain (ε) and induced

stresses (σ) are evaluated by using equations (4.1), (4.2), (4.3) and (4.4) respectively [171]

The variation in peak intensity is observed which is attributable to scattering effects,

recrystallization, thermal stresses and non uniform conduction of energy absorbed by atoms [20].

The XRD patterns show no new phases which indicate that thermal stresses are not significant to

create new reflection planes [172]. An increase in average peak intensity along with decrease in

FWHM is observed with increase in ion flux from 56×1012 ions/cm2 to 12×1013 ions/cm2.

The increase in peak intensity and decrease in FWHM with increasing ion flux is due to diffusion

of host atoms across the grain boundaries and crystal growth (figure 4.27 a and b) [173]. Further

increase in ion flux up to 26×1013 ions/cm2, causes a decrease in average peak intensity and

increase in the FWHM. As ion interacts with target surface, the collective energy deposition

effect of ions produces lattice strain by rapidly sharing ion impact energy to nearby lattice atoms.

Rapid melting, cooling, resolidification and recrystallization phenomena occur due to highly

energetic ions radiation matter interaction. As a result a large number of defects are generated

which act as a barrier in dislocation motion. It causes a decrease in the peak intensity [116, 173].

Figure 4.27 (c) demonstrates that the dislocation line density is reduced by increasing flux from

56 × 1012 ions/cm2 to 12 × 1013 ions/cm2 due to thermal annealing and recovery processes. While

dislocation line density is improved by further increasing ion flux up to 26 × 1013 ions/cm2 which

is attributed to significantly enhancement in concentration of lattice imperfections in normal

lattice sites of brass after ion material interaction. This enhancement in dislocation line density

is best evidence of irradiation hardening. Figure 4.27 (d) represents the variation in residual

stresses with increasing ion flux. For ion flux of 56×1012 ions/cm2 less pronounced compressive

stresses are dominant. Peak shift towards higher angular position represents compressive stresses

130

and shift towards lower angular position illustrates the presence of tensile stresses [116]. Figure

4.27 (a) represents the peak shifting towards higher angular position for ion flux of 56×1012

ions/cm2 and confirms the presence of compressive residual stresses. An increase in ion flux up

to 12×1013 ions/cm2 , the relaxation in compressive with pronounced tensile stresses is observed.

Peak shift towards lower angular position is attributable to the production of tensile stresses

[116].

Figure 4.27: XRD data of accelerator C ions irradiated brass target at constant energy of 2 MeV

for various ion flux of 56 × 1012 ions/cm2, 12 × 1013 ions/cm2, 15 × 1013 ions/cm2

and 26 × 1013 ions/cm2. (a) XRD diffractrographs, (b) The crystallite size Vs Ion

flux, (c) The dislocation density Vs Ion flux and (d) Stresses Vs Ion flux.

131

For further increase in ion flux up to 26×1013 ions/cm2, the transformation of tensile stresses into

pronounced compressive stresses is observed. This can be due to annealing effect after increasing

ion flux. Atomic diffusions of host atoms into interstitial sites are attributed to production of

defects which consists of a variety of interstitial-vacancy pairs. This is main reason for the

production of compressive stresses. These results are well correlated with change in the surface

morphology. Increase in density of craters is observed with increase in ion flux from 56×1012

ions/cm2 to 12×1013 ions/cm2 (figure 4.26 (b-c)) which is evident for the presence of highly

tensile stresses. Further increase in ion flux up to 26×1013 ions/cm2 shows presence of voids and

pits (figure 4.26 e) which represent the presence of compressive stresses because ion induced

compressive stresses act as an obstacle in the propagation of crack [169].

Comparison of XRD data of brass after irradiation by laser induced (Ni, Si, and C) ions and

accelerator (Ni and C) ions, significant dissimilarities in peak intensity, crystallite size,

dislocation line density and ion induced stresses are observed. In case of laser induced (Ni, Si,

and C) ions irradiation, new phase is identified in each case, whereas no new phase is identified

in case of accelerator ion irradiations. During laser induced (Ni, Si, and C) ions irradiations,

peak intensity, crystallite size and dislocation line density reveal anomalous behavior. But in

case of brass target is irradiated by Pelletron accelerator Ni ions, peak intensity and crystallite

size is increased monotonically whereas dislocation line density is decreased monotonically.

While in case of Pelletron C ion irradiation peak intensity, crystallite size and dislocation line

density represent anomalous behavior.


Figure 4.28 (a) represents the stress- strain curve of brass after carbon ion irradiation for various

flux. The variations of YS and UTS as a function of ion flux are shown in Figures 4.28 (b) and

4.28 (c) respectively. These graphs show reduction in YS and UTS with increase in ions flux

from 56×1012 ions/cm2 to 12×1013 ions/cm2. Whereas, further increase in ion flux up to value of

26×1013 ions/cm2 causes an increase in stress, YS and UTS. The variation in ion induced defects

and stresses is responsible for the variation in YS and UTS [25]. The decrease in both the YS and

UTS at low carbon ion flux is attributed to reduction in radiation induced effective defects

concentration. As a result pinning effect is reduced for dislocation motion. A similar anomalous

trend in the behavior of microstructural defects and mechanical properties in proton irradiated

Zr-1.0% Nb-1.0 % Sn-0.1 % Fe have been reported by Mukherjee et al. [174]. The initial

132

decrease in mechanical properties with increasing ion dose is associated to mutual annihilation of

generated defects (vacancies and self interstitial atoms. With the further increase of ion flux the

density of generated defects increases and dominates the losses of mutual annihilation. As a

result an anomalous behavior in mechanical properties of irradiated alloy after ion irradiation

will be observed [174-177]. Similar results have also been reported by Byun et al. [177] in which

the flux dependence of mechanical properties in tantalum and tantalum alloys has been observed.

Strengthening mechanism on irradiated material depends upon the number of ion induced

defects [178].

Figure 4.28: Universal Testing Machine (UTM) data of accelerator C ions irradiated brass

surface at constant energy of 2 MeV for various ion flux of 56 × 1012 ions/cm2, 12 × 1013

ions/cm2, 15 × 1013 ions/cm2 and 26 × 1013 ions/cm2. (a) Tensile stress- tensile strain curves (b)

The yield stress Vs ion flux and (c) The ultimate tensile strength Vs ion flux.

133

Decrease in YS and UTS with increasing ions flux from 56×1012 ions/cm2 to 12×1013 ions/cm2 is

attributable to annealing of initially produced defects. Defects migration, annihilation and/or

recombination of vacancies and/or point defects cause the reduction in the number of defects.

This results a decrease in the YS and UTS with increasing ion flux [94]. When carbon ions

transfer their energy during collisions with the normal lattice atoms of the target surface, a

sufficient amount of energy is transferred from the incident ions to escape an atom from their

mean lattice site. This primary escape atom has tendency to displace neighboring atoms. In this

way vacancies and/or interstitial or their clusters are produced by the cascades of collision

among the incident ions and lattice atoms of target surface. These defects behave as obstacles

and effectively block the slip [138]. According to dispersed barrier hardening model [179], YS

increases by increasing ion flux. This is attributed to ion induced defects such as clusters, loops,

voids and precipitations. The change in yield stress is directly proportional to the square roots of

product of defect density and size. Hence dislocation mobility reduces which causes

enhancement of the mechanical strength (YS and UTS). Therefore greater mechanical stress will

be required to start the plastic deformation [172]. Dislocation motion faces the barrier due to

increased in YS and UTS and percentage elongation decreases with increasing ions flux.

4.7.4 Microhardness

It is observed from graph of figure 4.29 that value of micro hardness first decreases with increase

in ion flux from 56×1012 ions/cm2 to 12×1013 ions/cm2 and then increases by increasing ions flux

up to 26×1013 ions/cm2. The variation in the hardness can be directly related to crystallite size.

The decrease in hardness is attributable to increased crystallite size and vice versa [180]. The

increase in microhardness is due to increase in dislocation density and defect clusters which

increases with increasing ion flux. The barrier force increases against the dislocation movement

due to reduction in crystallite size and consequently hardness also increases. The factors which

are responsible for the variation of microhardness are lattice defects, modifications in the size

distribution of grains and crystal structure [181].

The comparison of irradiated brass target by laser induced (Ni, Si and C) ions and Pelletron

accelerator (Ni and C) ions irradiations show similarities and dissimilarities. An anomalous trend

is observed in mechanical properties (YS, UTS and microhardness) of brass after laser induced

(Ni, Si and C) ion irradiations.

134

Figure 4.29: The variations in the hardness of accelerator C ions irradiated brass surface for


26 × 1013 ions/cm2 at constant energy of 2 MeV.

Whereas, in case of accelerator Ni ion irradiation, mechanical properties (YS, UTS and

microhardness) of brass target are decreased monotonically while a anomalous trend in

mechanical properties (YS, UTS and microhardness) is observed in case of C ion irradiation. The

difference is due to difference in energy and flux of ions.

The real comparison of all three sources (laser irradiation, laser induced plasma ions of Ni, Si

and C irradiation, accelerator ions irradiation) is difficult due to difference in their nature, energy

and flux. However, the more pronounced surface modifications is generated by laser photons as

compared to both kinds of ions (laser induced plasma ions and accelerator ions). The comparison

of surface modification also reveals that more significant changes in the surface topographies of

brass are observed by ions of KeV energy as compared ions of MeV energy. This is attributed to

smaller penetration depth of KeV ions as compared to MeV ions. The penetration depth of Ni, Si

and C ions produced by laser induced plasma are 45 nm, 190 nm and 690 nm respectively

depending upon nature, atomic mass and energy of corresponding ions whereas, the penetration

depth of Ni and C ions generated by Pelletron accelerator is 785 nm and 1440 nm respectively. It

implies that ions generated by accelerator are more penetrating and cause pronounced changes in

bulk of the material as compared to its surface. All results are summaries in a table 4.1.

135

The mechanical properties of brass are improved with laser and ion treatment. But maximum

improvement is observed for KeV ions at maximum flux. It implies that both energy and flux of

ions are most important controlling factor for controlling the mechanical properties of brass.

XRD analysis also reveals that with laser and accelerator ions irradiation, no new phases are

identified. Whereas, in case of laser induced plasma ions of Ni, Si and C new phases of CuZnNi,

CuSi and ZnC are generated. The formation of new phases is also attributed to maximum ion

matter interaction due to enhanced doses which leads to increased implantation of ions in brass

matrix. The ions flux ranging from 32 x 1011 ions/cm2 to 84 x 1016 ions/cm2 with keV energy can

be considered an optimum for ion implantation of these ions. If the energy of ions exceeds from

certain limit, then stopping power reduces and more energy of MeV ions is utilized for nuclear

stopping as compared to electronic stopping. Due to smaller contribution of electronic stopping

less pronounced changes in structural as well as mechanical properties of brass are observed.

Modifications to the surface, structural and mechanical properties of brass have been

investigated by using three different kinds of radiation sources i.e (i) Excimer laser, (ii) laser

induced plasma ions (Ni, Si & C) of KeV energy, (iii) Pelletron accelerator ions ( Ni & C) of

MeV energy.

When laser radiation employed to the target surface, a part of it is absorbed by the conduction

electrons through mechanisms such as inverse bremsstrahlung and multi-photon ionization up to

a a few nanometers thick and incident photons energy is rapidly converted into heat energy and

shock waves through collisions between the electrons and the lattice atoms [182]. This thermal

energy and laser induced shock waves are responsible for modification in surface, structural and

mechanical properties of irradiated brass target. The conduction of heat into the target forms a

thin molten layer below the surface. Laser induced shock waves are also responsible to improve

the mechanical properties such as Yield Stress (YS), Ultimate Tensile Strength (UTS) and

hardness. When deposited thermal energy on the surface exceeds the latent heat of vaporization,

it results in the boiling and evaporation from the surface. According to our results laser material

processing is attractive technique for surface morphology modification as compared to

crystallographical and mechanical modifications.

136

Table 4.1: The comparison of three radiation sources i.e. (1) Laser photons from Excimer laser

(248nm, 150 mJ, 6.4 J.cm-2, 20 ns, 20 Hz) (2) laser –induced plasma ions of Ni, Si and C

generated with Excimer laser (248nm, 120mJ, 5.1 J.cm-2, 20 ns, 20 Hz) under UHV condition (3)

Ni and C ions of MeV energy generated by Pelletron Accelerator and their irradiation effects on

structural and mechanical properties of brass.

Sample Energy

(KeV)

Laser

pulses/Ion

Flux

(ions/cm2)

Crystallite

Size

(nm)

Dislocation

line density

(m-2)

Yield

Stress

(MPa)

Ultimate

Tensile

Strength

(MPa)

Hardness

Untreated 25 14 x 1014 160 380 106

Laser

irradiated

brass

150 mJ

1200 23 19 x 1014 164 383 110

1800 22 21 x 1014 167 385 112

2400 20 25 x 1014 172 388 115

3000 18 31 x 1014 177 391 118

Ni

138

60 x 1013 36 7 x 1014 149 360 90

35 x 1014 47 4 x 1014 142 349 83

70 x 1015 58 3 x 1014 131 344 77

84 x 1016 16 39 x 1014 179 397 122

Si

289

45 x 1012 35 8 x 1014 150 363 91

25 x 1013 44 5 x 1014 144 357 86

56 x 1014 28 12 x 1014 155 375 102

75 x 1015 15 44 x 1014 180 400 126

C

678

32 x 1011 34 9 x 1014 151 371 93

90 x 1012 29 11 x 1014 153 373 99

64 x 1013 27 13 x 1014 159 379 104

72 x 1014 14 52 x 1014 187 406 133

Ni

2 x 103

56 x 1012 30 10 x 1014 152 372 97

12 x 1013 37 7 x 1014 148 359 89

15 x 1013 39 6 x 1014 145 351 87

26 x 1013 48 4 x 1014 139 335 80

C

2 x 103

56 x 1012 21 23 x 1014 168 386 114

12 x 1013 24 17 x 1014 163 382 108

15 x 1013 19 28 x 1014 172 389 117

26 x 1013 17 35 x 1014 178 393 120

137

When the brass target is irradiated by laser induced plasma ions of Ni, Si and C, the absorbed

energy from the incident ions appear in the form of localized source of heat. Therefore the

temperature of irradiated zone avalanche is increased up to thousand degree celsius [127]. This

heat source is responsible for the generation of lattice distortion, intense melting and explosive

boiling of the irradiated surface. Therefore a significant pressure is built up inside the melted

zone. If the melted zone is closed to the surface, an induced pressure never exists inside the

surface and outflow of material occurs from the hot melted core spike by microexplosion as a

result surface morphology of irradiated brass substrate is modified.

When the brass surface is irradiated with laser induced plasma ions (Ni, Si, and C) , these ions

penetrate into surface and deposit their energy rapidly in the vicinity of ion tracks and are

responsible for chain processing of cascades and subcascades of atomic collisions. Consequently

an extensive disorder is produced in a well defined and ordered matrix of brass throughout the

cascade volume. The ion penetration, i.e. the implantation depth in the substrates, depends on the

ion energy, ion flux, charge state and mass.

After ion material interaction, host atoms of irradiated substrate experience a temporary/

permanent reposition/relocation from their normal lattice position due to accumulation of

vacancies and interstitials defects and generate a new phase. The incident ions (Ni, Si and C) lose

their energy due to the collision with host atoms. Because of its collisions incident ions are

scattered in all possible directions. Consequently, stuck atoms inside the irradiated zone get

recoiled. These recoiled atoms again collide with nearby atoms, transfer their energy and

produce a displacement cascade, thermal and pressure spike. As a result ion irradiation generates

modification of crystal structural of irradiated substrate which gives an increase in the sputtering

and implantation processes [132].

After energy absorption of incidents ions, atomic vibration of irradiated surface atoms increases

across the critical point of atomic vibration and as a result the bonds between the atoms are

broken. Therefore host atoms are displaced from normal lattice positions and produce vacancy

and interstitial defects which are referred to as a Frenkel pairs in normal lattice systems. By

increasing the flux of incident ions up to maximum values, the collectively ion energy deposition

on the brass substrate increases. Therefore an effective induced displacement cascades and

subcascades are produced. This procedure remains continuous until the energy of each implanted

138

ions and displaced atoms are smaller than the displacement energy. The distribution of ion

induced vacancies, interstitial atoms and other types of lattice defects are generated in the region

around the primary knock-on atoms (PKA) and ion-induced tracks are formed. The generations

of these defects depend on ion fluence, ion energy, irradiation exposure time, temperature and

nature of target materials [1]. After irradiation of brass substrate by maximum flux, a strong

extrinsic strain fields are produced due to grouping of ion induced defect clusters near the

dislocation line. The mobility of dislocation lines in the slip plane are pinned by defects.

Because ion induced defects (vacancies, interstitial defects, precipitates) act as barrier in the path

of dislocation motion and obstructing their reproduction in different slip plane. Hence dislocation

mobility condenses which causes improvement of the mechanical properties. For that reason

greater mechanical stresses will be required to start the slip process [119]. So laser induced

plasma ions has great capacity to produce significant modification in bulk properties as

compared to surface modification.

139

Conclusions

140

Conclusions

During laser/ion matter interaction several phenomena such as photothermal, photophysical,

photochemical, displacement, thermal and energy spike etc take place which are represented in

the form of flow chart of figure 5.1.

Modification to the surface, structural and mechanical properties of brass have been investigated

by using three different kinds of radiation sources as shown in flow chart of figure 5.2.

it appears very hard to really assess the different irradiation modes because the important

parameters are not quite matchable for laser irradiation, laser induced plasma ions irradiations

and accelerator ions irradiations.

Excimer laser (KrF) is pulsed gas laser that normally emit electromagnetic radiation in

ultraviolet range. The laser ablation takes place by applying several number of overlapping laser

pulses to generate plasma. When material is ablated with multiple laser shots, the initial pulses

are responsible to generate primary defects and to enhance the surface roughness and optical

absorption of the material. As a result, material becomes more and more absorptive to incoming

following pulses of UV irradiation and consequently ablation efficiency increases. The effect of

these pulses is to weaken the mechanical forces gradually within the material as well as to

increase the energy absorption due to photomechanical, photochemical and photothermal

processes. For metals and their alloys, incubation effect is ascribed to the accumulation of laser-

induced defects and structural changes of the material and plastic deformation of the surface [1].

By increasing the number of laser pulses, energy absorption increases nonlinearly whereas

ablation threshold of the irradiated material reduces significantly. Consequently huge increases

in ion flux are observed with increasing number of pulses. If during laser-matter interaction with

nickel, silicon and graphite, the collisional sputtering also takes place in addition to thermal

sputtering, then target atoms get enough energy through the collisions and can cause generation

of further defects. Recoil target atoms displace other target atoms in the crystal and create dense

agglomeration of defects. Such disordered regions are referred to as defect clusters. Defect

clusters have high local defect density that has more critical effect on the optical properties of Ni,

Si and C than point defects. At these point defects, agglomerates and cluster sites the absorption

of electric field of incoming laser radiation increases nonlinearly and is therefore responsible for

nonlinear emission of ion flux from laser generated plasma.

141

The main aim of present dissertation includes a detailed comparison of three different irradiation

sources (laser, laser induced ions and accelerator ion irradiations) which is better for improving

surface, structural and mechanical behavior of metallic alloys. The alloys with improve

properties can be used in industry, aerospace and also in different scientific and technical fields.

Irradiation damage is a process in which primary knock-on atoms (PKAs) induced by

bombardment of energetic with the substrate material come to rest in the crystal lattice as an

interstitial, which involves the creation of point defects and defect clusters. Ions impinging on

substrate surface transfers its kinetic energy into both electronic and lattice excitations. The

excited electrons collide with lattice atoms within the energy relaxation time of about 10 -12

second. Ion energy is converted quickly into atomic motion, thermal and mechanical energy.

Heat evolution in the target depends on beam parameters such as energy density, pulse duration,

ion mass and energy, as well as on thermal properties of the irradiated material. The irradiation

effects are the result of the evolution of these defects, aggregation or dissolution and mutually

annihilation of these ions induced defects, causing changes in physical and mechanical properties

of irradiated substrate.

SEM results are concluded as follows.

After laser treatment laser-induced micro-sized cavities, bumps, cones, cracks and wave like

ridges with non-uniform shape and density distribution are formed. With increasing number of

laser pulses waves like ridges become less pronounced and more diffusive whereas, cracks

become dominant. The initial decreasing and then increasing trend in size and density of cavities

with increasing number of laser pulses has been observed.

Laser induced plasma has been used successfully as an ion source for Ni, Si and C ion irradiation

of brass target. The energy and flux of these ions are analyzed by Thomson parabola technique.

The average estimated energy of Ni ion is 138 KeV and flux of ions varies from 60 × 1013

ions/cm2 to 84 × 1016 ions/cm2 with the variation of laser pulses from 3000 to 12000 at constant

fluence of 5.1 J/cm2. Results suggest that ion flux plays a vital role in surface, structural and

mechanical properties of brass substrate. SEM analysis of laser induced Ni plasma ion irradiated

brass reveals the appearance of nano/micro sized cavities pores and cracks up to the flux of 80 ×

1015 ions/cm2 ion flux. At maximum ion flux (90 × 1016 ions/cm2) irregular sized and unequiaxed

granular morphology has been observed whereas cavities, pores and cracks are completely

vanished at this flux.

142

The average estimated energy of Si ion is 289 KeV and flux of ions changes from 45 × 1012

ions/cm2 to 75 × 1015 ions/cm2 with the variation of laser pulses from 3000 to 12000 at constant

fluence of 5.1 J / cm2. Surface modifications of brass are observed in the form of micro/nano

sized irregular shaped cavities and pores upon Si ion irradiations of brass with lowest flux of 45

× 1012 ions/cm2. The size and density of cavities are decreased whereas, density of pores

increases by increasing ions flux up to 56× 1014 ions/cm2. At maximum ion flux of 75 × 1015

ions/cm2 the cavities and pores are completely diminished and highly organized grains with

distinct grain boundaries are observed. Few of the grains are hexagonal and some are irregular

shaped in their appearance.

The average estimated energy of C ions is 678 KeV and flux of ions varies from 32 × 1011

ions/cm2 to 72 × 1014 ions/cm2 with variations of laser pulses from 3000 to 12000 at constant

fluence of 5.1 J/cm2. SEM analysis of laser induced C plasma ions irradiated brass surface

reveals the formation of micro / nano sized cavities, pores and pits with irregular shapes and non

uniform density distributions at lowest carbon flux of 32 × 1011 ions/cm2. By increasing ions flux

up to 90 × 1012 ions/cm2, significant reduction in the size of cavities, pores and pits is observed

while number density of pores and pits is increased. By further increase in ion flux from 90 ×

1012 ions/cm2 to 64 × 1013 ions/cm2, considerable different surface morphology is observed as

compared to lowest ions flux. The diffusive dendritic structures are grown along with appearance

of cavities, pores and pits at this ion flux. At the maximum ion flux of 56 × 1014 ions/cm2, the

dendritic structures become more distinct, regular shaped and compact whereas, cavities, pores

and pits are completely vanished at this flux.

SEM analysis of Ni ions irradiated brass by Pelletron accelerator with constant energy of 2 MeV

reveals the formation of nano sized circular and elliptical shaped craters on surface of brass. The

average diameter of nano-craters is decreased whereas their density increases with increasing ion

flux from 56 × 1012 ions/cm2 to 26 × 1013 ions/cm2.

SEM analysis of brass after irradiation of C ions by Pelletron accelerator with constant energy of

2 MeV reveals the appearance of irregular and randomly distributed structures at lowest ions flux

of 56 × 1012 ions/cm2. With increasing ions flux up to 15 × 1013 ions/cm2 incoherent shaped

structures have been transformed into dendritic structures. At the maximum ion flux of 26 × 1013

ions/cm2, nano/micro sized craters and voids along with the appearance of pits were formed.

143

Figure 5.1: Flow chart of laser/ion matter interaction processes.

144

Figure 5.2: Three radiation sources used for irradiation of brass at various energies and flux.

The structural modifications of brass after laser and ion irradiations are revealed by XRD and are

concluded as following.

From XRD results of laser irradiated brass with a fluence of 6.4 J/cm2 in an oxygen environment

for various number of laser pulses ranging from 1200 to 3000, no new phases were observed.

Significant variations in peak intensity, crystallite size and dislocation line density of brass are

achieved after laser irradiation. The conversion of compressive residual stresses into tensile

stresses with increasing number of pulses has also been observed. It is also revealed that the

thermal stresses induced by laser irradiation were not enough to introduce new phases in target.

145

XRD patterns of laser induced Ni plasma ion irradiated brass depicts that a new phase of CuZnNi

(200) is generated upon ion irradiations. Whereas the anomalous trend in peak intensity,

crystallite size, dislocation line density and induced stresses is observed which is explained on

the basis of thermal spike model.

From XRD results of laser induced Si plasma ion irradiated brass, a new phase of CuSi (311) is

identified. An anomalous variations in peak intensity, crystallite size and dislocation line density

of brass are achieved after irradiations. The main reason for anomalous trend is non uniform

thermal stresses and anisotropic thermal shock wave distribution produced by non uniform

absorption of energy by lattices and sublattice of host atoms.

XRD patterns of brass after laser induced C plasma ion irradiation depicts, a new phase of ZnC

(0012). The crystallite size is decreased monotonically by increasing ions flux while dislocation

line density is increased monotonically.

XRD analysis of accelerator Ni ion irradiated brass exhibits that no new phases are formed but

variations in peak intensity, crystalline size, dislocation line density and residual stresses are

observed which are due to ions induced defects and effects.

XRD patterns of accelerator C ion irradiated brass shows that ion-induced stresses are generated

in the irradiated brass but new phases are not identified. The conversion of compressive residual

stresses into tensile stresses with increasing ions flux is observed due to high and low angle

shifting. In addition an anomalous variations are also observed in peak intensity, crystallite size

and dislocation line density after C ions irradiations.

In order to correlate surface and structural modifications of brass with its mechanical properties,

tensile testing and microhardness analyses were performed.

After laser-irradiation of brass the increasing trend in mechanical properties (YS, UTS and

hardness) with increasing number of pulses is found. The modifications in mechanical properties

after laser irradiations are attributed to generation of large concentrations of imperfections in the

lattice sites of brass.

In case of laser induced Ni plasma ion irradiation, the reduction in mechanical properties (YS,

UTS and hardness) with increase in flux of ions up to 70 × 1015 ions/cm2is observed. Further

increase in ion flux up to maximum value of 90 × 1016 ions/cm2 causes an increase in YS, UTS

and hardness. The deviation in ion induced defects and stresses are responsible for alteration in

146

YS, UTS and hardness. These variations in mechanical properties are due to modification in

microstructural and dislocation line densities after ion metal interactions.

In case of laser induced Si plasma ion radiated brass substrate the mechanical properties (YS,

UTS and hardness) are improved up to maximum ions flux of 75 × 1015 ions/cm2. The reduction

in grain size, crystallite size and an increase in dislocation line densities play a vital role for

improvement in mechanical properties after Si ion irradiations.

A significant enhancement in mechanical properties of irradiated brass occurs by laser induced C

plasma ions irradiation as compared Ni and Si ions due to the formation of dendritic

morphology. The Secondary Denderitic Arm Spacing (SDAS) are responsible for enhancement

of mechanical proepreties in case of laser induced C plasma ions irradiation.

When the brass target is irradiated by various flux of accelerator Ni ions, the mechanical

properties such as YS, UTS and hardness are reduced monotonically by increasing ion flux and it

is attributed to an annihilation of ion induced imperfections by thermal spikes.

The initial decreasing and then increasing trend in mechanical properties (YS, UTS and

hardness) with increasing ion flux is observed when the brass target is irradiated by C ions of

Pelletron accelerator.

The results of SEM, XRD, tensile testing and microhardness of brass after laser and ion beam

irradiation are summarized in the form of flow chart of figure 5.3.

The changes in mechanical properties of laser and ion irradiated brass targets are well correlated

with surface and crystallographical modifications.

147

Figure 5.3: The comparison between laser and ion beam irradiation effects on surface structural

and mechanical properties of brass.

148

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