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APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY
OF TI6AL4V ALLOY
BHARAT PANJWANI
NATIONAL UNIVERSITY OF SINGAPORE
2011
APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY
OF TI6AL4V ALLOY
BHARAT PANJWANI (B.Tech, IIT Kanpur, India)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2011
Preamble
Preamble
This thesis is submitted for the degree of Master of Engineering in the
Department of Mechanical Engineering, National University of Singapore under
the supervision of Associate Professor Sujeet Kumar Sinha. No part of this thesis
has been submitted for any degree or diploma at any other University or
Institution. As far as the author is aware, all work in this thesis is original unless
reference is made to other work. Parts of this thesis have been published and
under review for publication as listed below:
Journal
1. B. Panjwani, N. Satyanarayana and S. K. Sinha. "Tribological
characterization of a biocompatible thin film of UHMWPE on Ti6Al4V
and the effects of PFPE as top lubricating layer", Journal of the
Mechanical Behavior of Biomedical Materials 4 (2011) 953-960. (a part
of Chapter 4)
2. B. Panjwani and S. K. Sinha. “Evaluation of tribological properties of
PFPE over-coated 3-glycidoxypropyltrimethoxy silane self-assembled
monolayer on Ti6Al4V surface”, manuscript in preparation. (a part of
Chapter 5)
Conference
1. B. Panjwani, N. Satyanarayana and S. K. Sinha, “Improving the tribology
of Ti6Al4V through a biocompatible thin UHMWPE coating”, ICMAT
2011 - International Conference on Materials for Advanced Technologies,
Singapore from 26 Jun to 1 July, 2011.
i
Acknowledgements
Acknowledgements
This is the great opportunity to acknowledge and express my thanks to
people for their support and encouragement in my postgraduate studies. First of
all, I would like to express my earnest gratitude and sincere thanks to my
supervisor Associate Professor Sujeet Kumar Sinha for providing me this
priceless opportunity to pursue my postgraduate studies. I am pleased to thank my
graduate advisor Assoc. Prof. Sujeet Kumar Sinha for his invaluable guidance,
supervision, encouragement, support and offering this great opportunity to work
with him.
I would like to express my special thanks to Dr. Nalam Satyanarayana for
his consistent help and support offered during my research work. I would also like
to thank Dr. R. Arvind Singh, Dr. Mohammed Abdul Samad and Dr. Myo Minn
for their support and valuable discussions.
I would like to say thanks to all my colleagues, Ehsan, Jonathan, Keldron,
Nam Beng, Prabakaran, Robin, Sandar, Sekar, Srinivas, Yaping, Yemei, for
stimulating research environment of mutual support and help in the team. I would
also like to thank all my friends, Amit, Archit, Chandra, Luv, Meisam, Sashi,
Srinivasa, Tapesh for their friendship and support.
I am grateful to the lab staff, Mr. Thomas Tan Bah Chee, Mr. Abdul
Khalim Bin Abdul, Mr. Ng Hong Wei, Mr. Maung Aye Thein, Mr. Juraimi Bin
Madon, Mr. Suhaimi Bin Daud, for their continuous support and assistance. Many
thanks to Mr. Juraimi Bin Madon for his technical expertise and support offered
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Acknowledgements
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in the fabrication of fixtures. I would also like to express my sincere thanks to the
ME dept office staff, Ms. Teo Lay Tin, Sharen and Ms. Thong Siew Fah, for their
support.
Finally, I want to express my gratitude and sincere thanks to my family for
their support, love and encouragement.
Table of Contents
TABLE OF CONTENTS
Page Number
Preamble
Acknowledgements
Table of Contents
Summary
List of Tables
List of Figures
List of Notations Chapter 1 Introduction
1.1 Importance of tribology
1.2 Brief history of tribology
1.3 Tribological applications
1.3.1 Industrial tribology
1.3.2 MEMS/NEMS tribology
1.3.3 Biomedical tribology
1.4 Importance of titanium and titanium alloys
1.4.1 Industrial applications
1.4.2 Consumer durables
1.4.3 Medical applications
1.4.4 MEMS applications
1.5 Titanium and titanium alloys tribology
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1 1 2 3 3 3 4 4 5 6 7 7 8
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Table of Contents
1.6 Objectives of the thesis
1.7 Methodology in the present thesis
1.8 Structure of the thesis
CHAPTER 2 Literature Review
2.1 Surface engineering and tribology
2.2 Existing tribology solutions for titanium alloys
2.2.1 Surface treatments
2. 2.1.1 Thermally sprayed coatings
2. 2.1.2 Electroplating and electroless plating systems
2. 2.1.3 Physical vapor-deposited coatings
2. 2.1.4 Surface modifications
2.2.2 Thermo-chemical processes
2.2.2.1 Nitriding
2.2.2.2 Oxidizing
2.2.3 Energy beam surface alloying
2.2.3.1 Laser gas nitriding
2.2.3.2 Electron beam alloying
2.2.4 Duplex treatments
2.3 Ti6Al4V alloy surface treatments for biomedical applications
2.3.1 Plasma nitriding
2.3.2 Bio-ceramic coatings
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Table of Contents
2.4 Thin film coatings in tribology
2.4.1 Polymer coatings in tribology
2.4.1.1 UHMWPE polymer coating tribology
2.4.2 Self-assembled monolayers coatings
2.4.2.1 Applications of SAMs coatings on titanium
2.4.2.2 Applications of SAMs coatings in MEMS tribology
2.5 Friction and wear mechanisms in polymer tribology
2.6 Solution-based coating methods for polymers
2.7 Use of PFPE as a top layer
2.8 Pretreatment methods
2.9 Biocompatibility testing
2.9.1 In-vitro testing
2.9.2 In-vivo animal testing
2.9.3 Clinical testing
CHAPTER 3 Materials and Experimental Procedures
3.1 Materials
3.2 Coatings preparation procedure
3.3 Polymer coating thickness measurement method
3.4 Contact angle measurement
3.5 Optical microscope
3.6 FE-SEM surface morphology observation
3.7 AFM surface topography measurement
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Table of Contents
3.8 FTIR-ATR analysis
3.9 XPS analysis
3.10 Cytotoxicity assessment
3.11 Tribological characterization
CHAPTER 4 Tribological Characterizations of Thin UHMWPE Film and PFPE Overcoat 4.1 Physical characterizations
4.1.1 Coating thickness measurement
4.1.2 Water contact angle results
4.1.3 SEM surface morphology
4.1.4 AFM surface morphology
4.2 Chemical characterizations
4.2.1 FTIR analysis results
4.2.2 XPS analysis results
4.3 Tribological characterization of UHMWPE coating
4.4 Investigation of underlying wear mechanism
4.5 Effect of PFPE overcoat on UHMWPE coating
4.6 Explanation of wear resistance increase by PFPE overcoat
4.7 Biocompatibility assessments
4.7.1 Cytotoxicity test results
4.8 Potential applications of coatings
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Table of Contents
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CHAPTER 5 Tribological Evaluations of Molecularly Thin GPTMS SAMs Coating with PFPE Top Layer 5.1 Physical characteristics of the coatings
5.1.1 Water contact angle results
5.1.2 AFM morphology results
5.2 Chemical characteristics of UHMWPE coating
5.2.1 XPS analysis results
5.3 Tribological characterizations
5.4 Optical microscopy of wear track and counterface surface
5.5 Biocompatibility test
5.5.1 Cytotoxicity test results
5.6 Potential applications of GPTMS/PFPE coating
CHAPTER 6 Conclusions CHAPTER 7 Future Recommendations References Appendix A Cytotoxicity Test Procedures
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Summary
Summary
Titanium and its alloys have been extensively used in many biomedical
and industrial applications due to their high specific strength with acceptable
elastic modulus, corrosion resistance and biocompatibility. However, high
coefficient of friction and low wear resistance of titanium and its alloys limit their
usage in some applications.
To improve the tribological properties of titanium and its alloys, various
surface modifications, coatings and treatments have been explored. In spite of
these developments, there is still a need to further investigate effective solutions
to improve tribological properties of titanium and its alloys.
In this thesis, application of thin organic coatings to improve tribology of
titanium and its alloys has been explored with emphasis on biomedical
applications. Ti6Al4V alloy, a commonly used titanium alloy, has been chosen as
substrate material in the studies of this thesis.
In the first study, ultra-high molecular weight polyethylene (UHMWPE)
polymer thin film (thickness of 19.6±2.0 µm) was coated onto substrate using dip-
coating method. Physical characterizations (contact angle, thickness
measurement, Field emission-scanning electron microscopy (FE-SEM)
morphology and atomic force microscopy (AFM) imaging), biocompatibility test
(cytotoxicity) and chemical characterizations (Fourier transform infrared
spectroscopy-attenuated total reflectance (FTIR-ATR) and X-ray photoelectron
spectroscopy (XPS)) were carried out for the obtained UHMWPE coating.
Tribological characterization of this coating was carried out using 4 mm diameter
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Summary
Si3N4 ball counterface in a ball-on-disk tribometer for different normal loads (0.5,
1.0, 2.0 and 4.0 N) and rotational speeds (200 and 400 rpm). This coating
exhibited low friction coefficient (0.15) and high wear life (> 96,000 cycles) for
the tested conditions. Perfluoropolyether (PFPE) overcoat on UHMWPE coating
further increased the wear resistance of coating as tested at even higher rotational
speed (1000 rpm). UHMWPE coatings (with and without PFPE overcoat) meet
the requirements of cytotoxicity test using the ISO 10993-5 elution method. Due
to their low surface energy, wear resistance and noncytotoxic nature, the thin
coatings of UHMWPE and UHMWPE/PFPE can find various applications in
biomedical implants and devices.
Despite having suitable properties for biomedical applications, higher
thickness of UHMWPE and UHMWPE/PFPE coatings may prevent their usage in
micro-electro-mechanical systems (MEMS) biomedical applications. In the
second study of this thesis, 3-glycidoxypropyltrimethoxy silane (GPTMS) self-
assembled monolayers (SAMs) with PFPE overcoat has been deposited onto
substrate. For comparison, PFPE coating has also been formed onto same
substrate. Ti6Al4V alloy specimens with PFPE overcoat and GPTMS/PFPE
composite coating showed low coefficient of friction and high wear durability as
tested at 0.2 N normal load and rotational speed of 200 rpm. The wear durability
of the obtained GPTMS/PFPE coating is much higher than that for only PFPE
coating. Obtained coatings were also characterized by contact angle measurement,
AFM imaging and XPS analysis. Formed PFPE and GPTMS/PFPE coatings are
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Summary
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biocompatible in nature. Due to the combination of hydrophobicity, low friction
coefficient, high wear resistance and noncytotoxicity, these coatings can find
usage in biomedical applications where low coating thickness may be crucial.
Molecular thickness (< 4 nm) of these coatings is particularly advantageous for
their applications in biomedical MEMS devices.
List of Tables
List of Tables
Table 4.1 Table 4.2 Table 5.1 Table 5.2 Table 5.3
Measured water contact angle values for different specimens. Summary of tribological tests on Ti6Al4V/UHMWPE specimens. Measured water contact angle values for different specimens. Measured surface roughness for different Specimens in AFM imaging. Coefficient of friction for specimens tested in the study.
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List of Figures
List of Figures
Figure 1.1 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 4.1 Figure 4.2 Figure 4.3
Research methodology followed in the research studies. Schematic of typical SAM molecule structure and attachment with substrate. General classification of the wear of polymers [Briscoe and Sinha 2002]. Schematic representations of wear mechanisms (N: normal load; V: sliding velocity). (a) Adhesive wear. (b) Abrasive wear. A schematic diagram of contact angle measurement. Experimental apparatus. (a) Dip-coating machine. (b) Clean air furnace. Optima contact angle measurement set-up. Experimental instruments. (a) Optical microscope set-up. (b) Ball-on-disk tribometer. (c) Ball-on-disk tribometer stage. Ball-on-disk tribometer schematic (R: Track radius; r: Ball radius; F: Normal load; ω: Rotational speed of the disk). Step-height measurement method. Measured water contact angle values for different specimens. (a) Ti6Al4V. (b) Ti6Al4V/O2 plasma treated. (c) Ti6Al4V/O2 plasma treated/UHMWPE. (d) Ti6Al4V/O2 plasma treated/UHMWPE/PFPE. Surface morphology of Ti6Al4V/UHMWPE surface using FESEM. (a) At lower magnification, 100x. (b) At higher magnification, 500x.
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List of Figures
Figure 4.4 Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9
AFM morphology of surfaces. (a) Polished Ti6Al4V alloy surface (scan area: 40µm×40µm, vertical scale: 1µm). (b) Ti6Al4V/UHMWPE surface (scan area: 40µm×40µm, vertical scale: 5µm). FTIR-ATR spectrum of the UHMWPE coating on Ti6Al4V substrate. XPS spectra of specimens. (a) UHMWPE coating on Ti6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V. Variation of friction coefficient as a function of the sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 3 mm, normal load: 0.5 N, spindle speed: 200 rpm). Variation of friction coefficient vs. sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 4 N, spindle speed: 400 rpm). Wear track morphology. (a) FESEM morphology of wear track for bare Ti6Al4V alloy for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after 1,000 cycles, magnification 60X. (b) FESEM morphology of wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 sliding cycles, magnification 80X. (c) AFM surface morphology inside the wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 cycles (scan area: 40µm×40µm, vertical scale: 500 nm).
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List of Figures
Figure 4.10 Figure 4.11 Figure 5.1 Figure 5.2 Figure 5.3 Figure 5.4 Figure 5.5 Figure 5.6 Figure 5.7
Wear track and counterface analysis. (a) EDX analysis of wear track for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after completion of 175,000 cycles. (b) EDX analysis of Ti6Al4V surface without polymer coating. (c) Optical image of Si3N4 ball for high normal load tribology test after completion of 175,000 cycles, 100X. (d) Optical image of Si3N4 ball after cleaning with acetone for high normal load sliding tribology test after completion of 175,000 sliding cycles, 100X. Effect of PFPE overcoat on wear life (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm). Water contact angle measurement from representative samples. (a) Ti6Al4V. (b) Ti6Al4V (after O2 plasma treatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heat treated). Water contact angle measurement from representative samples. (a) Ti6Al4V/GPTMS. (b) Ti6Al4V/GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE (heat treated). AFM imaging (scan area: 5µm×5µm, vertical scale: 100 nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) Ti6Al4V/PFPE (heat treated). (d) Ti6Al4V/GPTMS. (e) Ti6Al4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE (heat treated). Wide scan XPS spectra of Ti6Al4V/GPTMS specimens. Comparison of C1s peaks for bare Ti6Al4V/GPTMS and Ti6Al4V in C1s scan. Wear durability (number of sliding cycles before failure) of tested specimens in the study. Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE (heat treated) and (c) bare Ti6Al4V) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm).
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List of Figures
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Figure 5.8 Figure 5.9 Figure 5.10 Figure 5.11 Figure 5.12 Figure 5.13
Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b) Ti6AL4V/GPTMS/PFPE (heat treated) and (c) Ti6Al4V/GPTMS) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm). Optical micrographs of Ti6Al4V specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/GPTMS specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/PFPE specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/PFPE (heat treated) specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Optical micrographs of Ti6Al4V/GPTMS/PFPE specimen’s wear track and counterface surface after 100,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.
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List of Notations
List of Notations
ADLC: Amorphous diamond-like carbon
AFM: Atomic force microscopy
ASTM: American society for testing and materials
CVD: Chemical vapor deposition
DLC: Diamond-like carbon
EDX: Energy-dispersive x-ray spectroscopy
Epoxy SAM: 3-Glycidoxypropyltrimethoxysilane
FE-SEM: Field emission- scanning electron spectroscopy
FTIR-ATR: Fourier transform infrared spectroscopy-attenuated total reflectance
GPTMS: Glycidoxypropyltrimethoxysilane
HSS: High speed steel
ISO: International standards organization
L-B: Langmuir-Blodgett method
MEM: Minimum essential medium
MEMS: Micro-electro-mechanical systems
MPa: Mega Pascal
NEMS: Nano-electro-mechanical systems
PDMS: Polydimethylsiloxane
PE: Polyethylene
PEEK: Poly ether ether ketone
PFPE: Perfluoropolyether
PI: Polyimide
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List of Notations
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PMMA: Polymethylmethacrylate
PS: Polystyrene
PTFE: Polytetrafluoroethylene
PVD: Physical vapor deposition
RMS: Root mean square roughness
SAMs: Self-assembled monolayers
SEM/EDS: Scanning electron microscope equipped with x-ray energy dispersion spectroscopy Si3N4: Silicon nitride
TO: Thermal oxidation
UHMWPE: Ultra-high-molecular-weight polyethylene
XPS: X-ray photoelectron spectroscopy
Chapter 1: Introduction
Chapter 1
Introduction
1.1 Importance of tribology
Tribology is defined as the discipline to study the science and technology
of interacting surfaces in relative motion and of associated subjects and practices
[Jost 1966]. The word “Tribology” was originated from the Greek word “Tribos”
which means rubbing [Dowson 1979]. Tribology investigates the principles and
related practices of friction, lubrication and wear phenomena to understand the
interaction of contact surfaces in a given environment.
Friction is defined as the resistance between interacting surfaces under
relative motion. Wear can be described as the material removal phenomenon due
to interaction of surfaces in relative motion. Friction and wear are often
unavoidable phenomena in sliding and rolling surface contacts. Lubrication is the
method employed to reduce friction and wear of contact surfaces in relative
motion by interposing a material called lubricant. Lubricant can be of any
material state such as solid, liquid and gas or a combination of them.
Friction and wear play important roles in many places in natural
phenomena as well as man-made devices such as automobile, manufacturing etc.
Friction and wear are often undesirable factors in many applications and
adversely affect the performance and efficiency of systems thus scientists and
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Chapter 1: Introduction
engineers strive to come up with means to minimize friction and wear to increase
life and durability of such systems.
Tribology has grown into an important discipline for studying friction,
lubrication and wear principles in order to improve the efficiency of mechanical
systems.
1.2 Brief history of tribology
Although full appreciation of significance of tribology as an independent
discipline has been recognized only recently, human civilization had realized the
importance of friction and wear phenomena since ages. Wheel is the most
important mechanical invention of human civilizations and has been important
milestone in the journey to come up with solutions to address friction and wear
phenomenon in transportation. Known oldest wheel, discovered in Mesopotamia,
dates back to 3500 BC although archaeologists believe that it was invented around
8,000 BC. In 1880 BC, the Egyptians used sledges to transport large statues and
made use of water to lubricate sledges. Leonardo Da Vinci (1452-1519) is known
as the first person to study friction systematically as indicated by the sketches
discovered several hundred years later. Amonton (1699) stated through
experiments that friction force is directly proportional to the applied normal load
and is independent of the apparent area of contact. These two laws are known as
Amonton’s laws of friction. Charles Augustine Coulomb (1785) discovered the
third law that kinetic friction force is independent of sliding velocity. These three
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Chapter 1: Introduction
laws of friction were discovered on the basis of experimental observations and
were related to dry friction.
1.3 Tribological applications
Tribology as a discipline has grown tremendously in sync with the
scientific and technical developments in the world. Being a multidisciplinary
discipline, tribology keeps reinventing itself with developments in science and
technology. Today, tribology has found place in every aspects of everyday life.
Due to the growth of knowledge and interest in tribology for different
applications, this discipline has been further divided into different areas.
1.3.1 Industrial tribology
One of the important factors affecting the performance of the machines is
the nature of interacting surfaces. Thus friction and wear become important
considerations in the functioning of machines. Industrial tribology has found
important place in production, manufacturing, fabrication, aviation, aerospace and
marine sectors.
1.3.2 MEMS/NEMS tribology
With the development of MEMS/NEMS applications, it has been observed
that friction and wear at small length scales become limitations for efficiency and
durability of devices. At small length scales, surface forces become predominant
compared to inertial forces. Thus MEMS/NEMS devices require specialized
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Chapter 1: Introduction
solutions to address tribological limitations to reduce friction and increase wear
durability.
1.3.3 Biomedical tribology
With the development of biomedical engineering, researchers are
investigating the application of tribological principles for the improvement of
functioning of medical implants and patients’ comfort.
With continued research by tribologists, this area has grown tremendously
and has been able to make useful contributions to biomedical engineering.
Tribology has found importance in improving implants life and in reducing
patient trauma in biomedical applications.
1.4 Importance of titanium and titanium alloys
British mineralogist and chemist, William Gregor, discovered titanium
metal in 1791. A Berlin chemist, Martin Klaporth, independently isolated titanium
oxide in 1795. He named it titanium after Greek mythological name “Titans”. The
most popular titanium alloy Ti6Al4V was developed in the late 1940s in the
United States [Leyens and Peters 2003].
Titanium and its alloys are widely used in biomedical, aerospace, aviation,
marine, chemical industry, sports and leisure applications due to its high specific
strength and excellent corrosion resistance.
The ASTM defines a number of alloy standards from Grade 1 to 38
(ASTM B861 - 10 Standard Specifications for Titanium and Titanium Alloy
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Chapter 1: Introduction
Seamless Pipe). Ti6Al4V is the most commonly used titanium alloy for
biomedical and industrial applications. Its chemical composition consists of 6%
aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen and
remaining of titanium. It is significantly stronger than pure titanium while
stiffness and thermal properties are same as that of pure titanium (although
thermal conductivity is about 60% lower in Grade 5 Ti compared to that of pure
Ti). Grade 5 is heat treatable and is an excellent combination of strength,
corrosion resistance, weldability and fabricability. Grade 5 can be used upto 400
degrees Celsius temperature [Leyens and Peters 2003]. Ti6Al4V is most widely
used among titanium alloys [Donachie 2000] and is considered as the
"workhorse" of the titanium industry.
1.4.1 Industrial applications
Titanium and its alloys are used in industrial applications due to its high
specific strength, fatigue strength and creep resistance at high temperature
[Leyens and Peters 2003].
The much higher payoff for weight reduction in aircraft and spacecraft is
the driving factor for the usage of titanium and titanium alloys. In jet engine,
titanium is the second most common material after Ni-based super-alloys. It is
widely used in airframe and gas turbine engine due to the weight saving
considerations.
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Chapter 1: Introduction
Highly stressed components of helicopters such as rotor mast and head are
made from titanium alloys. In space applications, titanium alloys are used
extensively due to small payload requirement of space vehicles.
Titanium is reactive metal but is extremely corrosion resistant due to its
stable oxide layer at surfaces. Due to its corrosion resistant behavior, titanium
alloys are popular in chemical, process and power generation industries. Heat
exchangers, condensers, containers, apparatus and steam turbine blades are made
from titanium alloys. It is also used in photochemical refineries and flue gas
desulphurization plants.
Titanium alloys show excellent corrosion resistance in seawater and sour
hydrocarbons, thus they are widely used in marine and offshore applications.
Titanium alloys are used in automobile industry to improve performance at
reduced weight although their use has been limited to racing and high
performance sports car due to higher cost.
1.4.2 Consumer durables
In sports and leisure, titanium alloys are used in making golf clubs, tennis
racquets, baseball bats, pool cues, high speed cycling, scuba diving equipment,
expedition and trekking equipment [Leyens and Peters 2003]. Titanium alloys
have also found usage in architecture due to its excellent immunity to
environmental corrosion and a low coefficient of thermal expansion.
In jewellery and fashion industry, titanium alloys are gaining popularity
due to its lightweight, corrosion resistant, hypo-allergic nature and possibility of
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Chapter 1: Introduction
creating a large range of surface finishes by utilizing anodizing and heat
treatment. Besides titanium alloys are finding place in musical instruments,
optical instruments, information technology and security applications due to its
versatile properties.
1.4.3 Medical applications
Excellent compatibility with the human body makes titanium a key
material for biomedical implant materials. It is resistant to corrosion from body
fluids. Their excellent fatigue property, high specific strength and low modulus of
elasticity make it a preferred material for orthopedic devices. Bone fracture plates,
screws, nails and plates for cranial surgery are made from titanium alloys [Leyens
and Peters 2003].
Shape memory property of Nitinol (a titanium alloy) makes it suitable for
some specialized applications such as stent. Titanium has widespread usage in
dental implants due to its biocompatibility and low thermal conductivity.
1.4.4 MEMS applications
Titanium is also been proposed as a potential MEMS (Micro-electro-
mechanical systems) material for its physical and mechanical properties. Titanium
and titanium alloy MEMS can be preferably used in biomedical applications due
to its excellent biocompatibility.
As a potential MEMS material for its physical and mechanical properties
as well as biocompatibility, titanium alloy can be used in many MEMS
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Chapter 1: Introduction
applications [Aimi et al. 2004]. In MEMS applications, lubrication is required to
reduce adhesion, friction and wear to ensure the reliability and durability of
devices.
The durability and reliability of MEMS/NEMS devices are affected by
surface properties such as adhesion, friction, and wear [Bhushan 2003, 2004,
2005]. This requires the application of ultra-thin lubricant films having low
friction and low adhesion as well as high wear durability to protect the contact
surfaces in MEMS/NEMS devices.
1.5 Titanium and titanium alloys tribology
Titanium and titanium alloys have found many applications due to its high
strength-to-weight ratio, excellent corrosion resistance and biocompatibility.
Unalloyed titanium is as strong as steel but has 45% less weight. Titanium can be
alloyed with aluminium, vanadium, molybdenum and iron to produce lightweight
strong alloys to produce alloys of importance in biomedical, industrial, marine,
automotive and aerospace applications.
Application of titanium alloys in many areas is limited by its tribological
properties such as high friction coefficient, poor wear durability and low surface
hardness. Its poor tribological properties are caused by severe adhesive wear with
a strong tendency to seizure, low abrasion resistance and the lack of mechanical
stability of the oxide layer [Budinski 1991; Yildiz et al. 2009]. Titanium tribology
has found great interest among researchers due to possible application of titanium
alloys with improved tribological properties in many potential areas.
8
Chapter 1: Introduction
In industrial applications, various surface treatments such as thermo-
chemical processes, energy beam surface alloying and duplex treatments have
been proposed by researchers to address the tribological limitations of titanium
alloys [Bloyce 1998].
For biomedical applications, plasma nitriding and bio-ceramic coatings
are widely investigated solutions to improve the tribological properties of alloys
in orthopedic implants [Molinari et al. 1997; Yildiz et al. 2008; Fei et al. 2009].
1.6 Objectives of the thesis
The objective of this thesis is to evaluate some of the potential solutions
for surface modifications of titanium and titanium alloys to improve its
tribological properties.
In the first study, UHMWPE and UHMWPE/PFPE thin film coatings were
evaluated to address Ti6Al4V alloy tribological limitations. Experimental
characterizations of the physical, chemical and tribological properties of coatings
were carried out. This study also investigates the underlying mechanism for
excellent UHMWPE thin film tribological properties. In this study, following
approaches have been used:
Use of UHMWPE polymer coating to improve the tribological properties
of Ti6Al4V alloy.
Use of PFPE as a top layer to further improve the wear resistance of
UHMWPE film.
9
Chapter 1: Introduction
In the second study, GPTMS self-assembled monolayers (SAMs) coating
with PFPE overcoat was evaluated to address tribological limitations of Ti6Al4V
alloy. In this study, following approaches have been used:
Use of PFPE to improve the tribological properties of Ti6Al4V alloy.
Use of PFPE overcoat to improve the tribological properties of GPTMS
SAMs coated Ti6Al4V alloy.
1.7 Methodology in the present thesis
To achieve above objectives, UHMWPE polymer and GPTMS SAMs
coatings were deposited onto Ti6Al4V alloy substrate. Oxygen plasma treatment
was used to clean and improve adhesion properties of Ti6Al4V alloy substrate
with coatings. PFPE top layer was used to enhance the wear durability of
coatings.
Following process diagram (Fig. 1.1) represents the summary of the steps
followed in this thesis for different studies.
Figure 1.1: Research methodology followed in the research studies.
10
Chapter 1: Introduction
11
1.8 Structure of the thesis
Present thesis consists of a total of seven chapters. Literature review in the
field of titanium and titanium alloys tribology as well as thin film coatings is
presented in Chapter 2. Chapter 3 provides the detailed information of materials
and experimental characterization techniques (physical, chemical, biological,
tribological) used in this thesis. Chapter 4 presents the results and discussions for
the tribological evaluation of UHMWPE thin film coating on Ti6Al4V alloy
substrate and the effect of PFPE overcoat. Chapter 5 consists of results and
discussions for the tribological characterization of PFPE overcoated bare
Ti6Al4V alloy and GPTMS SAMs deposited Ti6Al4V alloy. Chapter 6
summarizes the conclusions drawn from studies and Chapter 7 presents the
recommendations for future studies from the work presented in this thesis.
Chapter 2: Literature Review
Chapter 2
Literature Review
2.1 Surface engineering and tribology
All solids are bound by surfaces. Surfaces act as an interface between
solid and its environment. Many of the properties of the solids are governed by
the solid’s interaction with the environment through the surfaces. Surface is the
most important part in many engineering applications since most failures such as
corrosion, fatigue and wear initiate at surfaces. Even though many solids have
desired bulk properties, they lack suitable surface properties. Surface engineering
involves modifying surface properties of the components to suit different
applications. Surface engineering techniques are used in the automotive,
aerospace, missile, power, electronic, biomedical, textile, petroleum,
petrochemical, chemical, power, steel, cement, machine tools and construction
industries. Tribological properties such friction coefficient and wear are mainly
affected by surface properties. For tribological applications, surface engineering
consists of surface modifications, surface coatings and treatment techniques.
Surface modification also consists of changing the texture of a component.
Objective of surface engineering in tribology is to reduce friction coefficient and
increase wear durability at contact interfaces in many applications.
Many materials possess required bulk properties such as strength and
toughness but do not possess suitable surface properties for tribological
applications. In these cases, modifying surface properties to reduce coefficient of
12
Chapter 2: Literature Review
friction and wear resistance by surface engineering serves the purpose [Bhushan
and Gupta 1991].
2.2 Existing tribology solutions for titanium alloys
Higher friction coefficient and low wear durability of titanium and
titanium alloys are attributed to the presence of mechanically unstable titanium
oxide film and the high surface energy leading to adhesive wear [Budinski 1991;
Yildiz et al. 2009]. In titanium and titanium alloys, surface wear occurs by
adhesive and abrasive mechanisms as well as by subsurface damage via plastic
deformation. The different processes used for the tribological improvements of
titanium alloys are surface treatments (thermally sprayed coatings, electroplating
and electroless plating systems, physical vapour-deposited coatings and surface
modifications), thermo-chemical processes (nitriding and oxidising), energy beam
surface alloying (laser gas nitriding and electron beam alloying) and duplex
treatments [Bloyce 1998].
2.2.1 Surface treatments
2.2.1.1 Thermally sprayed coatings
In thermal spraying process, solid rod or powder of metal and/or ceramic
is partially or fully melted and sprayed onto the substrate on which it re-solidifies.
No special pre-treatment is required for titanium and titanium alloys before
thermal spraying. Plasma spraying, detonation gun, high-velocity oxy-fuel and
vacuum plasma spraying are used to deposit these coatings [Bloyce 1998].
13
Chapter 2: Literature Review
Hard materials such as WC-Co, Mo and Cr-Ni are sprayed onto Ti6Al4V
to provide wear resistant coatings. 75 µm of molybdenum is sprayed onto the
stems of titanium automotive valves to prevent galling. Molybdenum coating
exhibits low coefficient of Friction, lubricant retention ability, hardness and wear
resistant properties. In aero engines and other gas turbine applications, sprayed
coatings on titanium are used to improve wear durability. Tungsten carbide-cobalt
(WC-Co) material is sprayed onto titanium alloy mid-span support faces to
provide protection against fretting wear in the low-pressure compressor. Thermal
spraying methods are more suitable for localized areas than for complete surface
of components.
2.2.1.2 Electroplating and electroless plating systems
Since passivating film of TiO2 acts as a weak interface between the
coating and titanium alloy substrates, pretreatments methods such as abrasive
blasting and copper strike are required prior to plating. Hard chrome plating,
coatings based on electroless nickel and soft metallic coatings including silver and
copper, are used on titanium alloy substrates to protect against wear. Oil seal
collars, pistons, racing car fly-wheels and bearing housings are plated with hard
chrome [Bloyce 1998].
2.2.1.3 Physical vapour-deposited coatings
Metals, alloys, compounds or metastable materials are deposited on
different substrates using physical vapour deposition (PVD) methods. TiN coating
14
Chapter 2: Literature Review
using PVD method is coated onto titanium alloy parts for racing cars and
aerospace components where strength-to-weight ratio is an important
consideration. TiN is coated onto pump parts and valve components in oil,
chemical and food industries due to its corrosion resistance property [Bloyce
1998].
Diamond-like carbon (DLC), amorphous diamond-like carbon (ADLC),
hydrogenated carbon films (a-C:H) [Kustas et al 1993] and MoS2 [Buchholtz and
Kustas 1996] coatings by PVD method are finding applications due to their low
coefficients of friction and wear durability. Applications of the most of the
developed PVD coatings is limited to low contact stress areas to avoid plastic
deformation of the substrate.
2.2.1.4 Surface modifications
Ion implantation is one of the widely used surface modification techniques
for titanium alloys [Perry 1987]. Commonly implanted species are nitrogen and
carbon for Ti6Al4V alloy. Improvement in the wear durability is caused by the
increase in surface hardness. An increase in hardness results in resistance to the
plastic deformation and thus, oxide layer can support higher stresses. Various
titanium prosthetics in biomedical applications are ion-implanted.
By anodizing titanium and titanium alloys, layers of TiO2 less than 100
nm thickness are produced on the surface to increase the wear resistance of the
alloys. Anodizing improves the tribological properties of titanium and titanium
alloys compared to untreated material but it is not able to support higher loads.
15
Chapter 2: Literature Review
Anodizing is frequently used on titanium fasteners and considered as minimum
base treatment for titanium and titanium alloys to improve tribological properties
[Bloyce 1998].
2.2.2 Thermo-chemical processes
Titanium can be thermo-chemically alloyed with interstitial elements such
as boron, carbon, nitrogen and oxygen. Phase equilibrium exists for boron and
carbon with titanium element so only a thin compound layer can be produced with
boron and carbon which means that solid solution hardening does not exist below
the thin compound layer. Thus this condition is similar to surface modification
produced by PVD coatings [Bloyce 1998]. Solid-solution-hardened diffusion
zones exist below the surface compound layers in the case of nitriding and
oxidizing so these methods are more useful considering depth-hardening criteria.
2.2.2.1 Nitriding
Nitriding of titanium and titanium alloys has been widely used effectively
as a surface treatment for protection against wear [Molinari et al. 1997].
Components treated by nitriding include racing engine components, surgical
instruments, racing car components, watchcases, precision mechanical parts and
golf club heads. Plasma ion or glow discharge nitriding have long been applied in
the surface treatment of titanium-based materials. Treatment gases such as
nitrogen, nitrogen-hydrogen mixtures, nitrogen-argon mixtures or cracked
ammonia and temperatures in the range 700-900 ºC are used in plasma nitriding.
16
Chapter 2: Literature Review
Racing car steering racks, gears and ball valves are treated using plasma nitriding
[Bloyce 1998].
2.2.2.2 Oxidising
Tribological properties of titanium and its alloys can be improved by
oxidizing. Oxygen in solution in αTi results in the significant strengthening of the
material although it affects adversely the properties such as tensile ductility,
fracture toughness and fatigue crack growth. Considering balance of the change in
the properties, oxidising is used in limited cases. Controlled oxidizing in lithium
carbonate salt baths is used for the production of titanium pistons. A treatment
based on oxidizing in air has been used to surface treat Ti22V4Al alloy valve
spring retainers. Diffusion hardening has been successfully applied in the surface
treatment of the biomedical alloy Ti-13Nb-13Zr alloy to improve tribological
properties [Mishra et al. 1994]. Ti6Al4V alloy treated by a thermal oxidation
(TO) process exhibits low coefficient of friction and low wear rates. Improvement
in properties can be attributed to the formation of oxide layer and a hardened
diffusion zone. TO is used for surface treatment of auto-engine components.
To address the tribological limitations of titanium alloys by oxidising,
different solutions such as plasma nitriding, plasma immersion ion implantation,
plasma spraying, PVD, CVD and laser surface treatments have been explored
[Bloyce 1998].
17
Chapter 2: Literature Review
2.2.3 Energy beam surface alloying
Energy beam surface alloying method changes the chemical composition
of the material in the liquid state during surface melting. Greater depth of
hardening at surface is obtained through this method compared to other surface
engineering methods.
2.2.3.1 Laser gas nitriding
Nitrogen is a widely researched alloying element for titanium and titanium
alloys. Laser gas nitriding is carried out using CO2 laser, a gas jet of blowing
nitrogen or nitrogen and argon at the melt pool. Different structures are obtained
by controlling the amount of nitrogen in the gas jet [Bloyce 1998].
2.2.3.2 Electron beam alloying
Different alloying elements can be used to increase surface hardness and
wear durability by using electron beam alloying method [Bloyce 1998]. This
process can produce different microstructures with defined properties by
controlling the amount of alloying elements. Silicon and carbon alloyed surfaces
exhibit significant improvements in tribological performance. Titanium alloyed
with silicon and carbon results in a tough hard layer, whereas titanium alloyed
with silicon and nitrogen results in a hard wear-resistant layer. Electron beam
alloying method is a line-of-sight method and is not suitable for complex
geometries.
18
Chapter 2: Literature Review
2.2.4 Duplex treatments
Combination of two processes can be used to obtain the best improvement
in tribological properties as observed in different studies. Relatively deep cases in
material surfaces can be achieved by combining surface alloying processes and
thermo-chemical treatments. Low-friction hard surfaces can be obtained using a
combination of thermo-chemical processes and PVD coating processes [Dong et
al. 1996].
2.3 Titanium alloys surface modifications for biomedical
applications
Plasma nitriding, CVD, PVD, plasma immersion ion implantation, plasma
spraying and laser surface treatments are used to improve wear and corrosion
properties of titanium and its alloys in medical implants applications [Yildiz et al.
2009].
2.3.1 Plasma nitriding
Thermo-chemical diffusion plasma nitriding process is one of the common
methods to produce hard and wear resistant nitrides on the surface of titanium
alloys. Compound and diffusion layers formed on the surface as a result of the
nitriding process increase the surface hardness, wear and corrosion resistance of
the titanium alloy medical implants.
19
Chapter 2: Literature Review
2.3.2 Bio-ceramic coatings
Different bio-ceramic coatings made of inorganic material such as TiN,
TiAlN and Al2O3 on the surface of titanium alloys exhibit anti-allergic and non-
cancerous nature as well as good corrosion resistance and excellent tribological
properties. Al2O3 coating shows lower friction coefficient (0.4), high strength,
wear resistance, chemical inertness and excellent corrosion resistance in hip–
prosthesis applications [Yildiz et al. 2009]. TiAlN deposited by PVD techniques
is used in many implant applications. TiAlN coating has good mechanical
properties, high wear resistance and biocompatibility.
2.4 Thin film coatings in tribology
Thin film surface coating of functional material is commonly used in
various fields of technology such as optical devices, electrical equipment, tools
for cutting, forming etc [Hedenquist et al. 1992; Sproul 1996; Zweibel 2000].
Depending upon the application, pure metals, compounds and ceramics are used
as coating materials. Thin surface coating technique has many advantages.
Surface properties can be tailored by coating while bulk properties of the
materials are retained. This provides capability to provide optimized properties
for the desired application. Materials that are difficult to synthesize utilizing other
methods can be used as a coating material. Since coating requires usage of a small
amount, expensive material can also be used as thin film coatings.
Thin film coatings have also become popular in tribology due to their
effectiveness and potential applications [Holmberg 1994]. Thin film coating
20
Chapter 2: Literature Review
method can be applied to improve tribological performance of materials by
applying a thin layer of a low friction coefficient and wear resistant material.
Various methods such as gaseous state and solution state as well as molten and
semi-molten state processes can be applied to deposit coatings.
2.4.1 Polymer coatings in tribology
A polymer can be defined as a molecule composed of many (poly) parts
(mer) joined together by chemical covalent bonds. If all monomer segments of
polymer are the same, it is called a homopolymer. If the monomer segments of
polymer are different, it is called a copolymer. Polyethylene is a polymer formed
from monomer ethylene (C2H4). Ethylene is a gas having a molecular weight of
28. The chemical formula for polyethylene (PE) can be written as -(C2H4)-,
where n is the degree of polymerization. Polymer thin films, when coated onto
various metallic substrates such as steel and aluminium, show excellent
tribological properties [NASA report 1980; Fusaro 1987].
Hard coatings such as metal, ceramic, nitride, carbide and oxide are used
widely to provide wear resistant layers onto components in different engineering
applications. Recently, researchers are showing interest to explore applications of
polymer coatings in different applications due to their good wear and corrosion
resistance, self lubricating characteristics, low noise emission, cost effectiveness
and impact loading performance [Demas and Polycarpou 2008; Zhang et al 2010].
Due to the higher cost and operational difficulties of PVD and CVD coating
21
Chapter 2: Literature Review
techniques, polymer coatings can provide a good alternative protecting
technology [Klein 1988].
Commonly used polymers for coatings are polytetrafluoroethylene
(PTFE), polyetheretherketone (PEEK), ultra high molecular weight polyethylene
(UHMWPE), epoxy, polydimethylsiloxane (PDMS) and polymethylmethacrylate
(PMMA). PTFE coating is used in tribological applications due to its excellent
properties such as low friction coefficient, high chemical resistance and high
temperature stability. However, PTFE coating has poor wear and abrasion
resistance which lead to failure in the machine parts [Unal et al. 2004]. PEEK is a
suitable polymer for coating due to its excellent tribological properties, chemical
resistance and high strength. Many researchers [Lu and Friedrich 1995; Friedrich
and Schlarb 2008] have investigated friction and wear behavior of PEEK based
composites.
2.4.1.1 UHMWPE polymer coating tribology
UHMWPE is a linear homopolymer. In UHMWPE, the degree of
polymerization can be upto 200,000. UHMWPE can have average molecular
weight of upto 6 million g/mol. UHMWPE molecular weight can not be measured
by conventional means and is measured using intrinsic viscosity. UHMWPE
polymer has outstanding physical and mechanical properties. It is a unique
polymer with excellent impact resistance, abrasion resistance, chemical inertness
and lubricity. UHMWPE exhibits high wear resistance compared to PMMA,
polystyrene (PS), PEEK, and PE. It has been used in many industrial applications.
22
Chapter 2: Literature Review
Bulk UHMWPE is commonly used for orthopedic applications due to its excellent
wear resistance.
Besides usage of bulk UHMWPE, there are different studies to explore the
application of UHMWPE coating for different tribological applications.
UHMWPE, as a thin film coating, has shown low coefficient of friction and wear
resistance in some studies [Satyanarayana et al. 2006; Minn and Sinha 2008;
Abdul Samad et al. 2010]. Due to its excellent wear resistance, UHMWPE
coating has the potential to provide a solution for tribological limitations of
titanium alloys in biomedical and industrial applications.
2.4.2 Self-assembled monolayers coatings
Molecularly thin films have received widespread interest due to their
ability to tailor the functionality of the constituent molecules. Molecular thin films
facilitate the investigation of intermolecular forces, molecule-substrate and
molecule-solvent interactions. These interactions affect interfacial properties such
as wettability, biocompatibility and corrosion resistance of the surfaces for
different types of materials [Mrksich 2000; Yan et al. 2000]. Chemical
transformations on molecular films can be investigated in details which can
provide new mechanistic insights as well as methods to tailor surface properties.
Such transformations allow functionalities such as the tethering of biologically
important molecules to surfaces at precisely controlled positions. These
functionalities are of the significant importance in different studies of chemical
biology and micro-array technology [Petty 2002; Pirrung 2002]. Development of
23
Chapter 2: Literature Review
molecular films by the chemisorptions between the substrate and head group of
organic molecules provides a method for making ultra-thin organic films having
controlled thickness [Azzam et al. 2002].
Ultra-thin organic molecular layers have also found application as the
lubricants for MEMS systems [Bhushan et al. 1995 (a); Komvopoulos 1996;
Rymuza 1999]. There are primarily two methods to form lubricant layers;
Langmuir–Blodgett method and Self-assembly method [Ulman 1991].
Langmuir–Blodgett (L-B) method cannot be applied for three dimensional
surfaces and is used mainly for flat surfaces such as magnetic recording media
[Ando et al. 1989; Bhushan et al. 1995 (b)] as well as L–B films have only
physically bonding with the substrate by vander Waals forces [Koinkar and
Bhushan 1996]. Compared to this, self-assembled monolayers (SAMs) have easy
preparation methods and possess excellent properties such as low thickness, stable
chemical and physical properties as well as good covalent bonding with the
substrate. Moreover, the properties of the SAMs can be tailored by changing the
type and length of the molecules, terminal group and the degree of cross-linking
within the layer. These properties make SAMs more attractive than the L–B films
[Ulman 1991].
SAMs are ordered molecular assemblies formed spontaneously by the
immersion of a substrate into a solution of the active surfactant due to adsorption
of the surfactant with affinity of its head group to substrate [Ulman 1991]. SAMs
consist of three building blocks (Fig. 2.1). A head group bonds covalently
strongly to a substrate. A tail group constitutes outer surface of the film. A body
24
Chapter 2: Literature Review
chain connects head and tail group. For strong binding of SAM molecules to the
substrate, head should contain a group that bonds chemically with the substrate.
Molecular structure and cross-linking of SAMs also affect the friction and wear
properties of coatings.
Figure 2.1: Schematic of typical SAM molecule structure and attachment with substrate.
SAMs are usually formed by solution-based method (immersing a
substrate in a SAMs solution that is reactive to the substrate surface) though vapor
based deposition methods are also used [Ulman 1991]. In general, SAM
molecules are not aligned perpendicularly to the substrate and have tilt angle with
respect to surface. Alkane-thiolates adsorbed on Au exhibit a tilt angle of 30-35o
with the surface normal [Ulman 1996].
By having SAMs with different terminal groups, the film surface can be
made hydrophilic or hydrophobic. A non-polar methyl (-CH3) or trifluoromethyl
(-CF3) terminal group are used for a hydrophobic surface film with low surface
energy. To achieve a hydrophilic film, surface terminal groups such as alcohol (-
OH) or carboxylic acid (-COOH) groups are used. The commonly used surface
25
Chapter 2: Literature Review
head groups are thiol (-SH), silane (e.g. trichlorosilane -SiCl3), and carboxyl
groups (-COOH). The commonly used substrates are gold, silver, silicon and
steel. There are many research studies of various SAMs having interaction with
different substrate. The alkyl-silane SAMs on Si [DePalma and Tillman 1989;
Ruhe et al. 1993; Srinivasan et al. 1998; Cha and Kim 2001; Ren et al. 2002;
Sung et al. 2003] and alkyl-thiol SAMs on Au [Lio et al. 1997; Bhushan and Liu
2001; Liu et al. 2001] have been widely studied.
2.4.2.1 Applications of SAMs coatings on titanium
Increasingly, titanium based substrates are gaining importance. SAMs
have been widely investigated as a versatile tool for surface modification in
various applications such as microelectronics, corrosion resistance and biosensors.
They also facilitate studies of wetting, adhesion, friction and related phenomena
[Ulman 1991].
For decades, mineral or metallic surface properties have been tuned by
SAMs [Van Alsten et al. 1999; Barriet et al. 2003; Noel et al. 2006;
Satyanarayana and Sinha 2005; Raman et al. 2006]. Material degradation can be
controlled without altering the visual aspect of the material by forming SAMs of
alkyl perfluorinated chains which are known for their chemical inertness.
The formation of SAMs on metals [Folkers et al. 1995] or metal oxide
surfaces [Gao et al. 1997] is widely employed for the fabrication of model
surfaces with highly controlled chemical properties. SAMs can be used in the
modification of metal oxide surfaces for the investigation of protein adsorption
26
Chapter 2: Literature Review
[Shumaker-Parry et al. 2000], for the study of cell behavior [Noel et al. 2006] as
well as for the fabrication of tailored sensor surfaces [Nyquist et al. 2000].
Passivation of metal surfaces, adhesion promotion, and interface corrosion
protection in metal/lacquer systems are other examples of industrial applications
of SAMs [Van Alsten et al. 1999].
Surface modification of titanium and its alloys is of great importance for
their practical applications in biomedical implants. A recent interest has emerged
for organic functionalization of the native oxide surfaces of tantalum, titanium
and related alloys due to their wide use as biocompatible materials in implants
[Viornery et al. 2002; Gawalt et al. 2003]. Covalent surface modification of
titanium dioxide is of great interest in view of its applications in medical implants,
catalysis and polymer fillers.
2.4.2.2 Applications of SAMs coatings in MEMS tribology
Self-assembled monolayers (SAMs) can reduce adhesion and stiction as
well as control friction and wear at the contact interfaces. Thus a lot of attention
has been paid to study them in tribological applications [Choo et al. 2007; Singh
and Yoon 2007].
Even though some SAMs exhibit low coefficient of friction, the wear
resistance achieved by these monomolecular layers is not sufficient to provide
high wear life to the high velocity moving MEMS components. Therefore,
researchers have realized the importance of the mobile portion as the top layer
which can provide self-repairability due to the migration of mobile molecules into
27
Chapter 2: Literature Review
the sliding contact resulting into high wear resistance [Katano et al. 2003]. This
concept has been well studied for hard disk lubrication.
2.5 Friction and wear mechanisms in polymer tribology
This section of the thesis has a brief review of polymer tribology with
emphasis on the associated friction and wear mechanisms.
In polymer tribology, friction between two sliding surfaces is primarily
affected by two terms, ploughing term and adhesion term [Briscoe and Sinha
2002].
(a) Ploughing term: This term of the friction is the result of plowing of the
asperities of the counterface into polymer besides the sub-surface
deformation involving plastic flow and fracture depending upon contact
conditions.
(b) Adhesion term: This term of the friction is caused by the adhesive
interaction of very low thickness layer of the polymer in contact with
counterface.
If hardness of one of the sliding surface is high relative to another surface,
plowing term becomes important. Adhesion term depends upon the interfacial
shear stress at the contact surfaces in the absence of hard asperities. Shear stress
(τ) depends upon the contact pressure P as given in Equation (2.1) [Bowden and
Tabor 1986].
τ = τ0 + αP (2.1)
28
Chapter 2: Literature Review
where τo is defined as the initial shear stress; α is the pressure coefficient; τ is
defined as the friction force (F) divided by real contact area (A); P is calculated as
the ratio of applied normal load (L) to real contact area (A) [Briscoe et al. 1973].
Thus Equation (2.1) can be rewritten as Equation (2.2) as given below.
F/A = τ0 + α (L/A) (2.2)
Initial shear stress (τ0) can be neglected in most conditions (especially high load
conditions) [Briscoe and Tabor 1975]. Therefore, Equation (2.2) modifies as
Equation (2.3) as follows.
F = α L (2.3)
Where α = μ (coefficient of friction) is the ratio of friction force to the applied
normal load.
Due to visco-elastic nature of polymers, their tribological behavior is more
complex than that for metals or ceramics. For polymers, friction coefficient
depends upon load, contact geometry and loading time [Bowden and Tabor 1986].
General classification of the wear of polymers is shown in Fig. 2.2 [Briscoe and
Sinha 2002].
29
Chapter 2: Literature Review
Figure 2.2: General classification of the wear of polymers [Briscoe and Sinha 2002].
In the generic scaling approach, friction is defined as interfacial and bulk
types which result into interfacial and cohesive nature of wear. In the second
approach, phenomenological methods are used to define wear by the different
prevailing mechanisms at the interface. Third approach defines wear using
material characteristics of different polymer types (Fig. 2.2).
There are different mechanisms that contribute to wear such as adhesion,
abrasion, fatigue, erosion and corrosion in sliding conditions (see Fig. 2.3 for
schematic representations of adhesive and abrasive wear mechanism). In most
polymers, wear occurs primarily by adhesive, abrasive and fatigue mechanisms
[Opondo and Bessell 1982]. In adhesive wear, isolated spots on sliding interfaces
adhere and afterwards, shear takes place in sliding at some point other than
original adhering interface. Adhesive wear is the purest and most important form
30
Chapter 2: Literature Review
of wear and it can not be eliminated fully in sliding conditions. This form leads to
material transfer from the worn part to the counterface. Load and geometry of
contact as well as surface energy of sliding surfaces affect the characteristics of
the transfered layer. The nature of formed transfer layer greatly affects friction
and wear rate of polymers. In some sliding conditions, polymer has linear
molecules and thin transfer layer protects the counterface. This condition
generates very less friction and wear rate by resulting sliding between polymer
and polymer due to easy shearing in the sliding conditions. Molecular orientation
with respect to sliding direction also affects the friction coefficient. If molecular
orientation is aligned with sliding direction, friction force reduces [Briscoe and
Sinha 2002]. Thick transfer layers in sliding conditions lead to high wear rate.
Molecule types and glass transition temperature of polymer also affect nature of
formed transfer layer.
Figure 2.3: Schematic representations of wear mechanisms (N: normal load; V: sliding velocity). (a) Adhesive wear. (b) Abrasive wear.
Abrasive wear is affected by two-body and three-body mechanisms. In
two body abrasive wear, polymer surface and counterface are involved. If
31
Chapter 2: Literature Review
generated debris or foreign particles are trapped between sliding surfaces, it
results into three-body abrasive wear. Abrasive wear depends upon bulk material
properties of polymer as investigated by many researchers [Ratner et al. 1964;
Lancaster 1969 and 1973]. Equation (2.4) describes the relationship between wear
rate and bulk material properties.
Se
LV
(2.4)
Where V = Wear Volume; = Proportionality constant; = Normal load; L =
Sliding velocity; = Hardness of the polymer; = Ultimate tensile stress; =
% Elongation to break.
S e
Equation (2.4) has been validated experimentally in different studies
[Ratner et al. 1964]. Fatigue wear occurs due to the repetitive cyclic loading in
sliding conditions and wear particles are generated in the form of delaminated
flakes by the phenomenon of the fatigue crack growth. Fatigue wear is generally
smaller than adhesive and abrasive wear but can result into severe damage for
mechanical components such as bearings.
2.6 Solution-based coating methods for polymers
Many solution-based coating techniques have been developed by making
use of the physical interaction between the deposited molecules and the substrate
[Advincula et al. 2004]. Following are some of the widely used solution-based
coating techniques:
Spin coating
32
Chapter 2: Literature Review
Dip-coating
Printing/droplet evaporation
Doctor blading
Spray coating
In the above mentioned deposition techniques, molecules are adsorbed
from solution onto substrate and solvent evaporates during the coating process.
Layers with desired thickness and homogeneity can be deposited without
significant effort by controlling deposition conditions.
In this thesis, dip-coating method was used to coat thin films of
UHMWPE on Ti6Al4V substrate as well as to overcoat PFPE. In dip-coating
process, a substrate is dipped into a liquid coating solution for the specified time
duration and is withdrawn afterwards from the solution at a controlled speed. Dip-
coating method is an excellent method for producing high-quality and uniform
coatings. Being a cost effective method, it can be easily used in laboratory set-up.
2.7 Use of PFPE as a top layer
Perfluoropolyether (PFPE) lubricants were developed in the early 1960’s
and have been used as lubricants in different applications. PFPE lubricants, a
unique class of lubricants and functional fluids, have found usage in different
applications because of their versatile nature. PFPE lubricants have following
advantageous properties:
Good lubrication properties
33
Chapter 2: Literature Review
Low volatility
Non-flammable
Chemical inertness
Low surface tension
Excellent oxidative and thermal stability
Effective in wide temperature range
Non-cytotoxic and biologically inert
Good radiation resistance
PFPE has been applied as a top layer to improve the wear durability of
high speed steel (HSS) tools [Fox-Rabinovich et al. 2002]. It was observed that
after applying PFPE as a top layer, coefficient of friction decreased and wear
resistance of tool improved. Improved properties were attributed to the top layer
of PFPE. Use of a mobile hydrocarbon-based lubricant as top layer also improved
the wear life of monolayers deposited onto Si based MEMS in a study [Eapen et
al. 2005]. PFPE overcoat also resulted in the improved tribological properties of
UHMWPE films deposited on Si substrate [Satyanarayana et al. 2006; Minn et al.
2008]. The method of using PFPE as a top layer to improve the tribological
properties of polymer and SAMs coatings has also been investigated in this thesis.
2.8 Pretreatment methods
Pretreatment methods for surface cleaning and surface energy
modification play a very important role in the adhesion of thin films coating onto
34
Chapter 2: Literature Review
a substrate. The wetting property of the substrate affects significantly the adhesion
between the film and the substrate. High surface energy of the substrate is one of
the most important factors that strongly influence the adhesion strength of the
coating [O’Brien et al. 2006]. Hydrophobic substrate surface results into low
adhesion strength of the applied coating.
Surface energy of the surface can be estimated using water contact angle
(θ), which is defined by the Young’s Equation [Wu. 1982]. The contact angle is
defined as the angle at which a liquid/vapor interface meets a solid surface. The
contact angle is a system specific property and is determined by equilibrium of
the drop under the action of interfacial energies (Fig. 2.4). Surface energy of the
interface can be calculated by water contact angle of the surface. The relation
between the contact angle (θ) and interfacial energies is defined as given below
(Equation 2.5).
γLV cosθ = γSV – γSL
(2.5)
γSV = Surface free energy of solid S.
γLV = Surface free energy of liquid L.
γSL = Interfacial free energy between solid and liquid.
θ = Contact angle.
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Chapter 2: Literature Review
Figure 2.4: A schematic diagram of contact angle measurement.
Depending upon the surface energy, surfaces can be divided into high
energy surfaces and low energy surfaces. High energy surface exhibits
hydrophilic nature while low energy surface exhibits hydrophobic nature [Wu
1982]. Different physical and chemical treatments can be used for modifying
surface energy. Higher surface energy (hydrophilic surface) of the substrate
promotes adhesion of the deposited coating.
After going through various surface treatments for titanium alloys, oxygen
plasma was chosen as pretreatment method for Ti6Al4V alloy substrate for
coatings of UHMWPE polymer and SAMs. Oxygen plasma is an environmental-
friendly surface pretreatment technique with cost effectiveness and easier
processing method. Plasma cleaning is one of the effective methods for pre-
treatment of metals to increase surface energy to improve the adhesion between
the film and the substrates. Pre-treatment generates functional groups which cause
an increase in the hydrophilic nature of the surface.
In normal conditions, surfaces exposed to the atmosphere are covered by
organic or inorganic contaminants, CO2 and hydrocarbon. These contaminants
36
Chapter 2: Literature Review
prevent good bonding of the coating. Oxygen plasma discharge subjects the
surface to very high energy bombarding electrons which break the molecular
bonds on the surface. Thus, contaminants covering the surface are removed in the
form of CO2 due to the reaction of carbon with the free oxygen radicals in the
plasma. Oxygen radicals generated in the oxygen plasma oxidize the surface
resulting in functionalized groups. “Functionalized groups making the surface
hydrophilic” is called the oxidative effect. Higher wettability assists in improved
adhesion property of the surface [Loh 1999].
Oxygen plasma treatment has been used to create pure and stoichiometric
surface oxide layer on Ti. Oxide layers generated by oxygen plasma method
exhibit reduced stiction for hydrocarbon contaminations [Aronsson et al. 1997].
Oxygen plasma treatment also hydroxylizes Ti surface [Yoshinari et al. 2006] to
facilitate covalent attachment of SAMs on surface. Gas plasma has been used in
SAMs deposition studies as pretreatment for metal oxides to attach SAMs
[Mahapatro et al. 2006]. Stability of octadecyltriethoxysilane and
octadecyltrichlorosilane SAMs deposited on mica was enhanced greatly by the
application of gas plasma pretreatment process [Kim et al. 2002, 2008].
2.9 Biocompatibility testing
Biocompatibility refers to the evaluation of the suitability of materials for
use in implantable medical devices. Biocompatibility examines the interaction
between a medical device and tissues as well as physiological systems of the
patient treated with the device. An evaluation of biocompatibility is an important
37
Chapter 2: Literature Review
and early part of the overall testing of a device for intended medical application.
The biocompatibility of a device depends on different factors. Some of the
important factors are the following:
• Physical and chemical nature of device materials.
• Types of patient tissue that will come in contact with the device.
• Duration of device exposure.
ISO 10993 “Biological Evaluation of Medical Devices” documents the
testing strategies that are acceptable in Europe and Asia. Part 1 of the ISO 10993
standard has the general guidance on selection of tests. Part 2 covers requirements
of animal welfare. Parts 3 through 19 are guidelines for specific test procedures
and guidelines on testing-related issues.
Different biocompatibility tests such as cytotoxicity, hemocompatibility,
sensitization, irritation, systemic toxicity, genotoxicity and implantation have
been advised in ISO 10993 as per the requirements of device such as nature and
duration of contact. Any medical implant needs to go through following
categories of tests before getting approval to be used in medical practice.
2.9.1 In-vitro testing
In this method, testing is carried out using cells and tissues outside the
body in an artificial environment.
38
Chapter 2: Literature Review
39
2.9.2 In-vivo animal testing
Studies are carried out for implant material in an animal model.
2.9.3 Clinical testing
Tests are carried out in human subjects to verify the safety and
effectiveness of a medical device for intended applications.
Chapter 3: Materials and Experimental Procedures
Chapter 3
Materials and Experimental Procedures
This chapter provides detailed information on the materials used in sample
preparation, coating preparation procedures and descriptions of experimental
methodologies used in physical, chemical, biological and tribological
characterizations of the formed coatings in this thesis.
3.1 Materials
Ti6Al4V-ELI Grade 5 (as per ASTM B265) specimens were used as the
substrate to form coatings. Size of the Ti6Al4V alloy specimen used for coating
deposition was 25mm×25mm (5mm thickness). Ti6Al4V alloy specimens were
procured from Titan Engineering, Singapore.
In the chapter 4 of this thesis, grinding of titanium alloy samples was done
using SiC abrasive papers using 800, 1000 and 1200 grit sizes successively. After
grinding, samples were polished using 5 μm alumina powder paste and 1 µm
diamond paste subsequently. Afterwards, samples were sonicated using acetone,
ethanol and distilled water for the duration of 10 min in each solution and dried
with N2 gas. After grinding, polishing and cleaning, surface roughness of
obtained Ti6Al4V alloy specimens was measured as 44 nm. Roughness of the
Ti6Al4V alloy specimen was measured using AFM in a scan area of
40μm×40μm. AFM roughness measurements were carried out at three different
40
Chapter 3: Materials and Experimental Procedures
locations onto the sample surface and it provided same roughness value within the
deviation of 5 nm.
UHMWPE polymer powder was procured from Ticona Engineering
Polymers, Germany through a local Singapore supplier. UHMWPE polymer
powder was of GUR X143 grade. Physical properties of the UHMWPE powder
used are as follows:
Melt flow index 190/15 = 1.8±0.5 G/10 min
Density = 0.33±0.03 g/cm3
Average particle size = 20±5 μm
Decahydronapthalin (decalin) was used as the solvent to dissolve
UHMWPE powder. PFPE (Z-dol 4000) with chemical formula (HOCH2CF2O-
(CF2CF2O)p-(CF2O)q-CF2CH2OH; ratio p/q is 2/3) was procured from Solvay
Solexis, Singapore. H-Galden (ZV60) was used as solvent for PFPE and procured
from Ausimont INC. Chemical formula of H-Galden is (HCF2O-(CF2O)p-
(CF2CF2O)q-CF2H, ratio p/q is 2/3). Ethanol, acetone, and distilled water were
used for cleaning of titanium alloy sample surfaces before any surface treatment
and coating.
In the chapter 5 of this thesis, grinding of titanium alloy samples was done
using SiC abrasive papers using 160, 320, 800 and 1000 grit sizes successively.
After grinding, samples were polished using 1 µm and 0.25 µm diamond pastes
subsequently. Afterwards, samples were sonicated using acetone, ethanol and
distilled water for the duration of 15 min in each solution and dried with N2 gas.
Resulting surface roughness (rms) of Ti6Al4V specimens (after grinding,
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Chapter 3: Materials and Experimental Procedures
polishing and cleaning) was 12 nm (measured by AFM in a scan area of
5μm×5μm). 3-Glycidoxypropyltrimethoxysilane (CH2–O–CH–CH2–O–(CH2)3–
Si–(OCH3)3) (purchased from Aldrich) was used for the preparation of SAMs
solution. Toluene was used as the solvent to form SAMs solution. A commercial
PFPE Zdol 4000 (molecular weight 4000 g/mol, with OH terminal groups at both
ends, monodispersed) was used to form an overcoating layer.
Hydrofluoropolyether solvent (H-Galden ZV) purchased from Ausimont Inc. was
used for the preparation of PFPE solution without further purification. Toluene
(99.5% anhydrous), ethanol (99.8%), acetone and distilled water were also used
for cleaning the samples.
3.2 Coatings preparation procedure
In the chapter 4 of this thesis, oxygen plasma treatment was carried out on
dried Ti6Al4V alloy specimens (after grinding, polishing and cleaning) in Harrick
Plasma Cleaner. RF power supply of 18 W was used for oxygen plasma
treatment. Titanium alloy surfaces were treated with oxygen plasma under
vacuum for the time duration of 10 min. Oxygen plasma treatment on titanium
alloy surfaces removes contaminations and helps in better adhesion of the
UHMWPE coatings. After oxygen plasma treatment, specimens were used for
dip-coating in UHMWPE solution. Dip-coating was carried out within 30 min of
oxygen plasma treatment to avoid any contamination of the surface after plasma
cleaning.
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Chapter 3: Materials and Experimental Procedures
UHMWPE powder dissolution was carried out in decalin first by heating
the polymer solution to 80 °C for 20 min followed by heating to 160 °C for 40
min. UHMWPE polymer powder solution in decalin was prepared at 3 wt%
concentration. To assist dissolution process, magnetic stirrers were used. After the
heating process, solution turns transparent from white which indicates uniform
and complete dissolution.
Dip-coating process was carried out immediately using dipping and
withdrawal speeds of 1.9 mm/s and an intermediate soaking time of 35 sec in
solution (See Fig. 3.1 (a) for dip-coating machine). After completion of dip-
coating, samples were kept in air for 1 min. Afterwards, samples were kept in
clean air furnace at 100 ◦C for a time duration of 20 hrs (See Fig. 3.1 (b) for clean
air furnace). Samples were cooled slowly to room temperature after heat
treatment. Thus obtained UHMWPE coated samples were kept in desiccator
before using it for Ti6Al4V/UHMWPE specimen characterizations.
To achieve overcoat of PFPE on UHMWPE, UHMWPE coated samples
were dip coated in PFPE solution. PFPE dip coating solution was prepared using
0.5 wt% PFPE in H-Galden solvent. Dip-coating was carried out in PFPE solution
for 1 min soaking time and using dipping as well as withdrawal speeds of 2.1
mm/s. For PFPE coating, no heat treatment was carried. These samples were
stored in desiccator till further characterizations for Ti6Al4V/UHMWPE/PFPE
specimens.
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Chapter 3: Materials and Experimental Procedures
Figure 3.1: Experimental apparatus. (a) Dip-coating machine. (b) Clean air furnace.
In the chapter 5 of this thesis, Ti6Al4V alloy specimens (after grinding,
polishing and cleaning) were oxygen plasma treated for 10 min. Plasma treated
Ti6Al4V alloy specimens were immersed into the epoxy SAM solution (using
toluene as the solvent) at a concentration of 1 vol% and left for 18 h. Afterwards,
the samples were washed with toluene and ethanol to remove any physisorbed
SAM molecules and dried with N2 gas. Samples were kept in desiccator overnight
before any further characterization for GPTMS coating. This procedure is similar
to that reported in an earlier study [Luzinov et al. 2000] except that the process
was carried out in ambient atmosphere of 25°C and a relative humidity of ~70%
in this study.
PFPE was dip-coated onto the GPTMS SAMs modified samples. Dipping
and withdrawal speeds of 2.1 mm/s were used for the dip-coating process with an
intermediate soaking time of 60 sec in solution. After dip coating, heat treatment
was carried out for samples at 150 ◦C for approximately 1 hr in clean air furnace.
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Chapter 3: Materials and Experimental Procedures
3.3 Polymer coating thickness measurement method
In the literature, polymer film thickness has been measured by using
different methods such as non-contact laser sectioning/SEM [Minn and Sinha
2008], profilometer [Satyanarayana et al. 2006], AFM [Lobo et al. 1999] and
focused ion beam/SEM [Abdul Samad et al 2010].
In this study, stylus profilometer in 2D scan mode was used to measure
thickness. Contact force of 25.5 mg and scan distance of 500 µm was used for
scanning. Step-height method was used to measure the thickness from the scanned
profile. Scratches were created in UHMWPE coating using a sharp corner of
another bare Ti6Al4V alloy sample to expose Ti6Al4V substrate. This method
made scratch only on the polymer coating as the scratching tip and the substrate
were of same materials. For measurements of average and standard deviation,
data from three samples were used. In each sample, three scratches were made
and data from three different locations on each scratch were used to measure
thickness. Method used in this study was based on the assumption that bare
Ti6Al4V sample corner will not penetrate another Ti6Al4V substrate significantly
to affect validity of measurement. To validate this assumption, scratches were
created using similar method on bare Ti6Al4V alloy samples. It was observed that
the scratch-depth created in this way on bare Ti6Al4V alloy substrate did not
exceed 0.7 µm in any of the case. This value has been taken into account in
standard deviation measurement of the coating thickness.
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Chapter 3: Materials and Experimental Procedures
3.4 Contact angle measurement
Contact angle measurement is one of the methods to evaluate surface free
energy of surfaces. Contact angle measurement is simple, inexpensive and widely
used technique for characterizing the wettability of surfaces. Contact angle is
sensitive to the conditions such as chemistry and topography of top layer. Water
(polar) is the popular liquid to measure the contact angle. Other liquids such as
formamide (polar), hexadecane (non-polar) and diiodomethane (non-polar) are
also used in contact angle measurements.
In this study, static contact angle is used to measure hydrophilic or
hydrophobic nature of the different surfaces (See Fig. 3.2 for contact angle
measurement set-up). VCA Optima Contact Angle System (AST Products, Inc.
USA) was used to measure static contact angle. Distilled water droplets with a
volume of 0.5 µl were used for the measurements. To arrive at average contact
angle and standard deviation of different surfaces, data were collected for three
different samples for five different locations for each sample.
Figure 3.2: Optima contact angle measurement set-up.
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Chapter 3: Materials and Experimental Procedures
3.5 Optical microscope
Olympus microscope was used to study counterface and wear track
conditions before and after completion of tribological tests (See Fig. 3.3 (a) for
optical microscope set-up). This microscope uses monochromatic light source and
allows observations at 50, 100, 200 and 500 magnifications. Each silicon nitride
ball, used in the tribological tests, was observed under optical microscope before
test at various magnifications to ensure cleanliness of the counterface. After
completion of each tribological test, wear track and counterface were observed to
investigate underlying wear and friction mechanisms.
Figure 3.3: Experimental instruments. (a) Optical microscope set-up. (b) Ball-on-disk tribometer. (c) Ball-on-disk tribometer stage.
3.6 FE-SEM surface morphology observation
Surface morphology of the Ti6Al4V and UHMWPE coated Ti6Al4V
surfaces was observed using Hitachi S-4300 Field Emission Scanning Electron
47
Chapter 3: Materials and Experimental Procedures
Microscopy (FESEM). Thin gold coating was carried out on polymer deposited
Ti6Al4V surfaces to make the surface conductive. JEOL, JFC-1200 Fine Coater
was used to perform gold coating. A current of 10 mA and the coating duration of
30 sec were used to deposit gold film on polymer coated surfaces.
3.7 AFM surface topography measurement
Atomic force microscopy (AFM) is a very high resolution microscopy
having resolution on the order of the fractions of a nanometer. In AFM imaging,
cantilever with a sharp tip at its end is used to scan the specimen surface. Tip
interaction with the surface causes tip to deflect due to operating forces between
the tip and the sample. Deflection of the tip is measured by a laser reflected from
the top surface of the cantilever into photodiodes. AFM operates in primarily two
modes. Contact Mode and Tapping mode [Digital Instruments MultiModeTM
Instruction Manual 1997]. In contact mode, tip scans the surface and tip deflection
is used as feedback signal. During scanning, force between the tip and the surface
is kept constant by maintaining a constant tip deflection.
In the tapping mode imaging, the tip is oscillated up and down at near its
resonance frequency while scanning the sample surface. The reflected laser beam
from the tip generates a sinusoidal signal in the photodiode array whose amplitude
provides the image of the scanned surface. As the oscillating tip is scanned over
the surface, tip interactions with the surface cause the change in the amplitude of
oscillating tip. In contact mode, scanning over the surface can cause damage or
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Chapter 3: Materials and Experimental Procedures
modify surface features. Thus tapping mode is preferred over contact mode to
measure topography of polymer coatings.
Tip geometry also affects the accuracy of measurements. Sharp tip
generates accurate representation of film while blunt tip provides erroneous
imaging results. Besides tip geometry, the spring constant and the resonance
frequency of tip are important considerations for the accuracy of imaging. A small
spring constant is effective in measuring small forces and a high resonance
frequency is recommended for making tip insensitive to vibrations and
mechanical noise.
Topographic (roughness) measurement of specimens’ surfaces was carried
out using Atomic force microscope (Dimension 3000 AFM, Digital Instruments,
USA). Images were scanned in air using a monolithic silicon tip using the tapping
mode. In measurements, set point voltage used was 1-2 V and scan rate of 0.5 Hz
was used.
3.8 FTIR-ATR analysis
FTIR-ATR (Fourier transform infrared spectroscopy-attenuated total
reflectance) is an analytical technique to identify organic and inorganic materials
by measuring the absorption of infrared radiation by the sample material versus
wavelength. Molecular components and structure of the material can be identified
by infrared absorption. When an infrared radiation is irradiated over the material,
molecules are excited by absorbed IR radiation into a higher vibrational state. The
energy difference between excited vibrational state and ground state determines
49
Chapter 3: Materials and Experimental Procedures
the wavelength of absorbed light by a particular molecule. Thus absorbed
wavelengths by the sample are characteristic of its molecular structure.
In FTIR spectroscopy, an interferometer is used to modulate the
wavelength generated from a broadband infra-red source. A detector is used to
measure the intensity of transmitted or reflected light as a function of its
wavelength. Thus detector is able to provide the interferogram (intensity of light
as a function of its wavelength). Interferogram is analyzed by computer using
Fourier transforms to obtain a single-beam infrared spectrum. The FTIR spectra
are plotted as intensity versus wave number (in cm-1). The wavenumber is the
reciprocal of the wavelength. The intensity is plotted as the percentage of light
transmittance or absorbance vs wavenumber.
Bio-Rad FTIR model 400 spectrophotometer was used to collect FTIR-
ATR spectrum in air for UHMWPE film. This spectrum was collected from 32
scans at a resolution of 4 cm−1 at four different locations. The spectra were
collected at 5 replicate points. Plasma treated bare Ti6Al4V was used for
background scan for ATR-FTIR spectrum.
3.9 XPS analysis
X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic
technique to measure chemical state, elemental composition, empirical formula
and electronic state of the elements of the material. XPS is operated in ultra high
vacuum (UHV) conditions.
50
Chapter 3: Materials and Experimental Procedures
In XPS spectra, material is irradiated with x-rays and kinetic energy and
numbers of electrons that escape from the top 1 to 10 nm of the material surface
are analyzed. X-rays are usually generated either by Mg or Al source. X-rays
cause a core electron to be emitted from the sample. The kinetic energy of thus
emitted electron is detected using an electron multiplier and is equal to the
difference between the energy of the X-ray (1253 eV for Mg, 1486 eV for Al) and
the binding energy of the electron.
In this study, XPS is used to study the chemical state of the sample
material. XPS (Thermo Fisher Scientific Theta Probe) was used to study chemical
states of the coatings. A monochromatized Al Kα X-ray source (1486.6 eV
photons) was used at a constant dwell time of 100 ms and pass energy of 40 eV. A
photoelectron take-off angle of 90o was used to obtain the core level signals. All
binding energies (BE) were referenced to the C1s hydrocarbon peak at 284.6 eV.
3.10 Cytotoxicity assessment
The cytotoxicity test was carried out by NAMSA (Northwood, OH, USA).
This test was done according to the guidelines of “International Organization for
Standardization 10993-5: Biological Evaluation of Medical Devices, Part 5: Tests
for In Vitro Cytotoxicity” (see Appendix A for details on cytotoxicity testing
methods). Before cytotoxicity testing, Ti6Al4V/UHMWPE specimens were
sterilized using ethylene oxide and degassed. Fifteen specimens were used for
each test for the cytotoxicity testing using ISO elution method-1×MEM extract
method.
51
Chapter 3: Materials and Experimental Procedures
In a single preparation, test specimens were extracted in single strength
minimum essential medium (1×MEM) at a temperature of 37 ◦C for the time
duration of 24 hours. The negative control, reagent control and positive control
were also prepared as per the requirements mentioned in testing standards.
Triplicate monolayers of L-929 mouse fibroblast were dosed with prepared
extracts. Incubation was carried out at a temperature of 37 ◦C in the presence of
5% CO2 for a time duration of 48 hours. Following the incubation, the triplicate
monolayers were microscopically (100 magnification) examined for any abnormal
cell morphology and cellular degeneration.
3.11 Tribological characterization
Tribological tests were carried out in this study using UMT-2 (Universal
Micro Tribometer, CETR, USA) (see Fig. 3.3 (b) and (c) for tribometer set-up). It
can be operated in both ball-on-disk and ball-on-plate modes. This tribometer can
apply normal loads upto 500 gm (5 N) and rotational speeds of upto 5000 rpm
(1.05 m/s at a track radius of 2 mm). In this study, tests were carried out in ball-
on-disk mode under dry conditions. Ball-on-disk mode measures dynamic friction
coefficient. Many contact geometries can be simulated using ball-on-disk mode
thus this mode was chosen to evaluate tribological properties of coatings in this
thesis. Ball-on-disk mode schematic has been shown in Fig. 3.4. A Si3N4 ball of 4
mm diameter was used as the counterface for tribological tests. Si3N4 ball had the
surface roughness of 5 nm as provided by the supplier. The ball was thoroughly
cleaned using acetone before each test. Si3N4 ball is used as the counterface since
52
Chapter 3: Materials and Experimental Procedures
53
Si3N4 material has much higher hardness when compared to Ti6Al4V alloy and is
inert towards organic species.
For tribological characterization, the spindle rotational speeds of 200 and
400 rpm were used in different tests. Track diameter of 4 mm was used for all the
tests. The spindle rotational speed of 200 rpm gives a sliding speed of 41.9 mm/s
at the used track diameter. Applied normal loads from 0.2 N to 4.0 N were used
for various tests. At least three repetitions were carried out for every tested load
and speed combination. Tribological tests were carried out in a class 100 clean
booth environment. Temperature of 25±2 °C and a relative humidity of 55±5 %
were maintained in the clean booth for tests.
The wear life for the tribological tests was taken as the number of life
cycles after which the coefficient of friction exceeded 0.3 or a visible wear
appeared on the substrate whichever occurred earlier [Miyoshi 2001]. The FE-
SEM and optical microscope were used to observe the worn track surfaces after
appropriate number of sliding cycles to infer wear mechanism.
Figure 3.4: Ball-on-disk tribometer schematic (R: Track radius; r: Ball radius; F: Normal load; ω: Rotational speed of the disk).
Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
Chapter 4
Tribological Characterizations of Thin UHMWPE Film
and PFPE Overcoat
In view of the stated objective of this thesis in Chapter 1, first study of this
thesis titled “Tribological characterizations of thin UHMWPE film and PFPE
overcoat” investigates the potential of thin UHMWPE polymer coating to address
tribological limitations of Ti6Al4V alloy. Rationale behind the selection of
UHMWPE polymer coating in this study has been explained in chapter 2.
UHMWPE polymer thin film was coated onto Ti6Al4V alloy substrate
using dip-coating method. Results of physical characterizations (contact angle,
thickness measurement, FE-SEM morphology and AFM imaging),
biocompatibility test (cytotoxicity) and chemical characterizations (FTIR-ATR
and XPS) of UHMWPE thin film have been reported in this chapter. Use of PFPE
overcoat to further improve the wear resistance of UHMWPE coating has been
evaluated at higher RPM. It was observed that the coating of PFPE onto a
Ti6Al4V substrate without any intermediate layer does not achieve high wear
durability.
Wear track and counterface conditions have been examined to infer underlying
friction and wear mechanism. Potential applications of this coating have been
suggested based upon prior literature studies and the results of this study.
54
Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
4.1 Physical characterizations 4.1.1 Coating thickness measurement
Thickness of UHMWPE coating obtained in this study was measured
using stylus profilometer by step-height method. Fig. 4.1 shows the schematic of
thickness measurement in step-height method. Measured average thickness and
standard deviation of the characterized UHMWPE coatings are 19.6 µm and 2.0
µm respectively.
Figure 4.1: Step-height measurement method. 4.1.2 Water contact angle results
For different specimens, static contact angle measurements were carried
out using VCA Optima Contact Angle System. After grinding, polishing, cleaning
and drying processes, Ti6Al4V alloy surface exhibits water contact angle of
55.1±4.3◦. This measured value matches quite closely with value observed in an
earlier study 50.0±3.1◦ [Ponsonnet et al. 2003]. Bare Ti6Al4V (after polishing,
cleaning and drying processes), when exposed to O2 plasma treatment, showed
water contact angle of 10.4±1.1◦. This low water contact angle value indicates
increased surface free energy of the surface due to plasma treatment. Increased
surface energy of bare Ti6Al4V after oxygen plasma treatment affects the
adhesion of UHMWPE coating on bare Ti6Al4V substrate positively. Surface
energy is one of the significant factors which affect the adhesion of a coating on a
55
Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
substrate [Silbernagl et al. 2009]. UHMWPE coating formed in this study
exhibited a water contact angle of 135.5±3.3◦ and UHMWPE/PFPE composite
layer showed a water contact angle of 128.5±2.9◦. High water contact values of
Ti6Al4V/UHMWPE and Ti6Al4V/UHMWPE/PFPE coatings indicate low
surface energy of the obtained coatings.
Bulk UHMWPE water contact angle values as documented in literature
are close to 90◦ [Chen et al. 2003]. Water contact angle is significantly influenced
by the surface chemistry and surface conditions. The differences in the water
contact angle of obtained UHMWPE coating in this study when compared to that
of bulk UHMWPE can arise due to different surface conditions such as roughness
[Torrisi et al. 2003].
Table 4.1 summarizes static water contact angle values for different
specimens prepared in this study. Fig. 4.2 shows the pictures of water contact
angle measurements of representative specimens from different surface
modifications.
Table 4.1: Measured water contact angle values for different specimens. Specimen Type Water contact angle (degree) Ti6Al4V 55.1±4.3 Ti6Al4V/O2 plasma treated 10.4±1.1 Ti6Al4V/O2 plasma treated/UHMWPE 135.5±3.3 Ti6Al4V/O2 plasma treated/UHMWPE/PFPE 128.5±2.9 Bulk UHMWPE [Chen et al. 2003] 90.0
56
Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
Figure 4.2: Measured water contact angle values for different specimens. (a) Ti6Al4V. (b) Ti6Al4V/O2 plasma treated. (c) Ti6Al4V/O2 plasma treated/UHMWPE. (d) Ti6Al4V/O2 plasma treated/UHMWPE/PFPE. 4.1.3 SEM surface morphology
The surface morphology of the UHMWPE film on Ti6Al4V surface,
observed under FE-SEM, has been shown in Fig. 4.3. UHMWPE film exhibits
fibrous structure as observed in the surface morphology of the film. This coating
shows polymer chains protruding to the surface containing valleys between them.
Fig. 4.3 (a) and (b) show UHMWPE coating surface morphology at 100 and 500
magnification respectively. Images indicate the presence of uniformly distributed
polymer chain density in the UHMWPE coating.
Figure 4.3: Surface morphology of Ti6Al4V/UHMWPE surface using FESEM. (a) At lower magnification, 100x. (b) At higher magnification, 500x.
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
4.1.4 AFM surface morphology
AFM morphology imaging results are shown in Fig. 4.4. AFM imaging
was done for the scan area of 40µm×40µm. This scan area is much smaller than
used for SEM imaging. AFM morphology of Ti6Al4V alloy surface after
grinding, polishing and cleaning is shown in Fig. 4.4 (a). AFM measurement of
Ti6Al4V alloy surface shows an average roughness (Ra) of 44 nm. Fig. 4.4 (b)
shows sub-micron features of the UHMWPE polymer coating formed in this
study. Fig. 4.4 (b) shows the presence of islands and valleys in the formed
UHMWPE coating. The roughness (rms) of the UHMWPE coating measured
using scan area of 40µm×40µm is 0.795 µm. The roughness (rms) of the bulk
UHMWPE measured in an earlier study was 0.332 µm [Satyanarayana et al.
2006]. Different roughness value of the formed UHMWPE coating compared to
that of the bulk UHMWPE can cause differences in the observed water contact
angle results of the coating from the bulk polymer even though both have same
chemical structure.
Figure 4.4: AFM morphology of surfaces. (a) Polished Ti6Al4V alloy surface (scan area: 40µm×40µm, vertical scale: 1µm). (b) Ti6Al4V/UHMWPE surface (scan area: 40µm×40µm, vertical scale: 5µm).
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
4.2 Chemical characterizations
4.2.1 FTIR analysis results
Chemical nature of the coating was analyzed using FTIR-ATR analysis
spectrum. Spectrum was observed to be identical for all measurements at different
locations.
Fig. 4.5 shows the FTIR spectrum of UHMWPE coating on Ti6Al4V alloy
substrate. FTIR spectrum indicates CH stretching modes at 2866 and 2939 cm−1, a
band at 1475 cm−1 corresponding to polyethylene–methylene (CH2) bending and
CH2 rocking mode at 731 cm−1. This spectrum’s characteristics are similar to the
observed UHMWPE or PE polymers spectra characteristics in earlier studies
[Elliott 1969; Kang and Nho 2001]. This FTIR analysis validated that present
coating’s chemical nature is similar to that of bulk UHMWPE.
Figure 4.5: FTIR-ATR spectrum of the UHMWPE coating on Ti6Al4V substrate.
4.2.2 XPS analysis results
XPS spectra of UHMWPE coating on Ti6Al4V alloy substrate has been
shown in Fig. 4.6 (a). Observed XPS spectra of UHMWPE film coating indicates
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
high carbon content due to the presence of polyethylene (CH2) group of
UHMWPE polymer. Absence of titanium peak in the observed XPS spectra
confirms the fact that the Ti6Al4V substrate is completely covered by the
UHMWPE polymer film and there are no pin-holes.
XPS spectra of UHMWPE/PFPE coating on Ti6Al4V substrate has been
shown in Fig. 4.6 (b). The presence of strong F1s peak in XPS spectra confirms
the presence of PFPE overcoat on UHMWPE.
Figure 4.6: XPS spectra of specimens. (a) UHMWPE coating on Ti6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V. 4.3 Tribological characterization of UHMWPE coating
Bare Ti6Al4V alloy tribological properties were characterized using Si3N4
as counterface in ball-on-disk tribometer and it exhibited a higher friction
coefficient of 0.6~0.7 as shown in Fig. 4.7. Higher friction coefficient of bare
Ti6Al4V alloy is attributed to high surface energy leading to adhesive wear and
the presence of mechanically unstable titanium oxide film [Budinski 1991; Yildiz
et al. 2009]. After coating of UHMWPE film onto the Ti6Al4V alloy surface, the
friction coefficient was reduced significantly from ~0.6 to ~0.15. Bulk UHMWPE
polymer has excellent lubrication properties and its tribological properties have
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
been widely investigated and documented in earlier research studies [Wang et al.
1995].
In the initial study, tribological tests were carried out for the normal loads
of 0.5, 1.0 and 2.0 N. Five samples were characterized for each testing conditions.
Tests were carried out for the duration of 96,000 cycles (8 hours) at 200 rpm.
Sliding test beyond 96,000 cycles was not carried out due to long duration of
testing. The UHMWPE film did not fail in any of these tests for the tested
duration of 96,000 cycles. Table 4.2 shows the summary of the tribological tests
carried out on Ti6Al4V/UHMWPE film.
Table 4.2: Summary of tribological tests on Ti6Al4V/UHMWPE specimens.
Test Condition
No.
Test Parameters (Track Diameter,
Normal Load, Spindle Speed)
Maximum Friction Coefficient during
Test Duration Tested cycles
1 6 mm, 0.5 N, 200 RPM 0.15 96,000 (not failed) 2 4 mm, 1.0 N, 200 RPM 0.15 96,000 (not failed) 3 4 mm, 2.0 N, 400 RPM 0.11 96,000 (not failed)
Fig. 4.7 shows the variation of friction coefficient vs. number of sliding
cycles for the test condition No. 1 in Table 4.2. The test specimens exceeded
96,000 cycles without failure.
For comparison, bare Ti6Al4V substrate tribological properties were also
evaluated for the same testing parameters using Si3N4 as counterface in the ball-
on-disk tribometer. Bare Ti6Al4V substrate showed a higher coefficient of
friction of 0.6~0.7 as shown in Fig. 4.7. After coating of UHMWPE polymer film
onto the Ti6Al4V alloy surface, the coefficient of friction was reduced to ~0.15 as
shown in Table 4.2 and Fig. 4.7.
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
Figure 4.7: Variation of friction coefficient as a function of the sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 3 mm, normal load: 0.5 N, spindle speed: 200 rpm).
Afterwards, four samples were tested at a high normal load of 4 N (track
radius = 2 mm, spindle speed = 400 rpm) till film failure was observed. Fig. 4.8
shows the variation of friction coefficient vs. number of cycles of revolutions for
a typical run at this high normal load test condition. Test result shows wear
durability of 187,000±8,000 cycles and a maximum coefficient of friction of 0.10
before failure of the film.
Figure 4.8: Variation of friction coefficient vs. sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 4 N, spindle speed: 400 rpm).
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
4.4 Investigation of underlying wear mechanism
To investigate the underlying wear mechanism in the above tribological
studies, wear track and counterface conditions were investigated for one of the
high load test conditions (track radius = 2 mm, normal load = 4 N, spindle speed
= 400 rpm) where test was stopped after 175,000 sliding cycles to investigate
counterface and wear track conditions even though the film had not failed.
FESEM image of the wear track of Ti6Al4V alloy (Fig. 4.9 (a)) showed
that the specimen surface, without UHMWPE coating, is extensively damaged
only after 1,000 sliding cycles and there is significant accumulation of wear
debris around the wear track. Compared to this, wear track morphology of
Ti6Al4V/UHMWPE is smooth even after 175,000 sliding cycles as seen in Fig.
4.9 (b). AFM morphology (Fig. 4.9 (c)) of the surface inside the wear track
showed an average roughness (rms) of 35 nm which is very smooth compared to
the initial roughness (0.795 µm) of the UHMWPE polymer coating. Protrusions
and valleys of the polymer coating are flattened (ironed) in the initial running-in
period of the sliding cycles of the tribology test.
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
Figure 4.9: Wear track morphology. (a) FESEM morphology of wear track for bare Ti6Al4V alloy for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after 1,000 cycles, magnification 60x. (b) FESEM morphology of wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 sliding cycles, magnification 80x. (c) AFM surface morphology inside the wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 cycles (scan area: 40µm×40µm, vertical scale: 500 nm).
EDX analysis at different locations inside the wear track confirms the
presence of polymer film proving that the coating was not removed even after
175,000 cycles of sliding (Fig. 4.10 (a)). Six locations were investigated in EDX
analysis and in all cases EDX analysis indicated high percentage of carbon (>
97%) and no Ti presence (only trace Ti peak was visible because of the detection
of the substrate). Compared to this, EDX analysis of bare Ti6Al4V alloy showed
high percentage of Ti (as seen in Fig. 4.10 (b)). Au peaks have also been observed
in the EDX analysis because of the gold coating deposited on the specimens
before FESEM/EDX analysis to make coating conductive as required in EDX
analysis. The optical image of Si3N4 ball (Fig. 4.10 (c)) at the end of tested
175,000 sliding cycles on the UHMWPE film showed that there was only a little
polymer transfer to the counterface. After cleaning the ball with acetone, no
permanent physical damage was observed on the Si3N4 ball surface (Fig. 4.10
(d)), indicating that the UHMWPE coating was able to protect the counterface as
well during tribological tests.
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
Figure 4.10: Wear track and counterface analysis. (a) EDX analysis of wear track for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after completion of 175,000 cycles. (b) EDX analysis of Ti6Al4V surface without polymer coating. (c) Optical image of Si3N4 ball for high normal load tribology test after completion of 175,000 cycles, 100x. (d) Optical image of Si3N4 ball after cleaning with acetone for high normal load sliding tribology test after completion of 175,000 sliding cycles, 100x.
Since polymer coating presence is observed on the substrate and the
counterface surfaces which suggest that after certain number of sliding cycles,
contact is between polymer/polymer which can protect the substrate and the
counterface surfaces. The phenomenon of sliding between the polymer coating on
the substrate and the transferred polymer film on the counterface can result in low
friction coefficient and high wear resistance as observed in similar studies if the
polymer transfer layer formed on counterface is thin and material transfer is not
extensive [Briscoe and Sinha 2002].
4.5 Effect of PFPE overcoat on UHMWPE coating
Tribological tests were carried out at high rotational speed (1000 rpm) for
UHMWPE/PFPE coating to evaluate the effect of PFPE coating on UHMWPE
film wear durability. For the comparison, UHMWPE coating was also tested for
65
Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
similar conditions (track radius = 2 mm, normal load = 4 N, spindle speed = 1000
rpm). Five samples were tested for each surface coating to obtain the wear life.
UHMWPE on Ti6Al4V showed a wear life of 28,000±8,000 cycles at these
sliding conditions. After PFPE coating, wear life increased to 60,000±14,000
cycles. Thus PFPE top layer improved the wear life of UHMWPE film as
evaluated at the high rotational speed testing condition. This comparison has also
been plotted in a bar chart shown in Fig. 4.11.
Figure 4.11: Effect of PFPE overcoat on wear life (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm).
4.6 Explanation of wear resistance increase by PFPE overcoat
The observed improvement in wear resistance by PFPE overcoat is similar
to the work done in earlier research studies [Satyanarayana et al. 2006; Abdul
Samad et al. 2010]. PFPE exhibits lower surface energy [Makkonen 2004],
excellent lubrication properties and thermal stability [Liu and Bhushan 2003].
These properties are beneficial for severe tribological condition for the tested
condition of 1000 rpm in this study.
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
Since UHMWPE polymer surface does not have any reactive chemical
groups to react with PFPE molecules, possibility of any chemical interaction
between UHMWPE and PFPE can be eliminated. Therefore as hypothesized in
earlier studies, PFPE molecules can be entrapped during coating into rough
features of UHMEPE film such as valleys and can result into good lubrication
properties during sliding testing due to availability of lubricant [Satyanarayana et
al. 2006; Abdul Samad et al. 2010]. PFPE spreads well on polymer surfaces due
to low surface tension.
4.7 Biocompatibility assessments
Different biocompatibility tests have been suggested by ISO 10993
guidelines for biomedical devices based upon mode of contact, application area
and duration of contact in different applications. However cytotoxicity test is
common test applicable to all implant applications and used as a preliminary test
to evaluate the feasibility of using any coating in biomedical implant applications.
This test assesses the potential cytotoxic effects of leachable extracted from test
specimens’ surfaces.
4.7.1 Cytotoxicity test results
In this study, UHMWPE and UHMWPE/PFPE coatings were tested for
cytotoxicity on Ti6Al4V alloy substrate using ISO elution method-1×MEM
(minimum essential medium) extract method. In cytotoxicity testing carried out in
this study, no cytotoxicity effects such as cell lysis were seen in any of the test
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
wells during microscopic examination in the ISO elution method-1×MEM extract.
No change in the pH value was noticed after duration of 48 hours.
Grade should be less than the grade 2 (mild reactivity) for the tested
coatings to meet the requirements of the ISO elution method-1×MEM extract.
Based upon test analysis results, it was inferred that coatings exhibited
cytotoxicity level of grade 0 (reactivity: none) according to test guidelines and
thus meet the requirements of the ISO elution method-1×MEM extract.
4.8 Potential applications of coatings
As observed in this study, the UHMWPE and UHMWPE/PFPE thin film
coatings exhibit hydrophobicity, high wear durability and noncytotoxicity.
Hydrophobic coatings surfaces are water repellent and do not need a liquid
lubrication medium to reduce friction. Due to the low friction coefficient and self-
cleaning nature of surfaces, hydrophobic coatings can find many applications in
biomedical area. For example, in coronary guide-wire application, hydrophobic
coatings improve tactile feedback thus making it easier to grip by cardiologist
during operation. In endoscopic applications, hydrophobic coatings prevent the
sticking of organic material on the surface of the device. Low surface energy of
hydrophobic coatings has been observed to be beneficial in bio-film inhibition for
some in-vivo applications [Roosjen et al. 2006]. In a metallic stent, a hydrophobic
and noncytotoxic coating on stent surface can assist in improving the
compatibility of metal with body fluids by inhibiting release of potential cytotoxic
leachables from metal surface into body environment [Pendyala et al. 2009].
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Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat
69
Similarly, wear resistant property of a biocompatible coating is advantageous in
orthopedics and bio-devices applications. In considerations of the above-
mentioned potential applications, UHMWPE and UHMWPE/PFPE thin film
coatings studies can find usage in biomedical devices.
CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
CHAPTER 5
Tribological Evaluations of Molecularly Thin GPTMS SAMs Coating with PFPE Top Layer
As investigated in the Chapter 4, a thin film of UHMWPE coating
exhibited low friction coefficient and high wear resistance. Use of PFPE as the
top layer further improved the wear resistance of the obtained thin UHMWPE
coating. Due to the combination of hydrophobicity, noncytotoxicity and excellent
tribological properties, potential applications of these coatings in biomedical
instruments were suggested.
In spite of having suitable properties for the biomedical applications,
higher thickness of UHMWPE (~19.6 µm) may prevent its usage in biomedical
MEMS applications. This chapter investigates the feasibility of using molecularly
thin GPTMS/PFPE coating (~4 nm) to address the tribological limitations of
titanium and its alloys in MEMS and in particular bio-MEMS applications. In this
study, GPTMS SAMs with PFPE overcoat has been deposited onto Ti6Al4V alloy
substrate. For comparison studies, PFPE has also been coated onto Ti6Al4V alloy
substrate. Obtained coatings have been characterized by contact angle
measurement, AFM imaging, XPS analysis and tribological tests. Wear track and
counterface surfaces have been examined to understand the underlying friction
and wear mechanism.
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
5.1 Physical characteristics of the coatings
5.1.1 Water contact angle results
After polishing, cleaning and drying processes, bare Ti6Al4V alloy
showed a water contact angle value of 73±4◦. This measured value is close to the
value of 72◦ observed in an earlier study [Wang et al. 1997]. After 10 min of
oxygen plasma treatment of Ti6Al4V alloy, water contact angle was reduced to
6±1◦. This is due to the hydrophilic nature of Ti6Al4V alloy surface after oxygen
plasma exposure indicating increased surface energy. Surface energy is one of the
factors which significantly influence the adhesion of the coating onto the substrate
[Silbernagl et al. 2009]. PFPE coating on Ti6Al4V alloy exhibited a water contact
value of 52±3◦. PFPE coating on Ti6Al4V alloy after heat treatment showed
increased water contact angle value of 110±2◦.
GPTMS coating on Ti6Al4V alloy exhibited a water contact angle value
of 60±2◦ which is in a good agreement with the value of 62◦ reported in an earlier
study [Elender et al. 1996]. PFPE overcoat onto GPTMS coated Ti6Al4V alloy
showed a water contact angle of 90±5◦. After heat treatment, water contact angle
value of Ti6Al4V/GPTMS/PFPE was increased to 112±6◦.
Table 5.1 summarizes the water contact angle values (average and
standard deviation) for different specimens types prepared in this study. Fig. 5.1
and 5.2 show the water contact angle measurement pictures from the
representative specimens prepared in this study.
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
Table 5.1: Measured water contact angle values for different specimens.
Specimen Type Water Contact Angle (°) Ti6Al4V 73±4
Ti6Al4V (after O2 plasma treatment) 6±1 Ti6Al4V/PFPE 52±3 Ti6Al4V/PFPE (heat treated) 110±2 Ti6Al4V/GPTMS 60±2 Ti6Al4V/GPTMS/PFPE 90±5 Ti6Al4V/GPTMS/PFPE (heat treated) 112±6
Figure 5.1: Water contact angle measurement from representative samples. (a) Ti6Al4V. (b) Ti6Al4V (after O2 plasma treatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heat treated).
Figure 5.2: Water contact angle measurement from representative samples. (a) Ti6Al4V/GPTMS. (b) Ti6Al4V/GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE (heat treated).
Thus, GPTMS/PFPE coating is hydrophobic in nature indicating low
surface energy of the obtained coating. Observed increase in the water contact
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
angle value after heat treatment of PFPE coatings is due to the reduction in the
available surface hydroxyl groups [Tao and Bhushan 2005]. Low surface energy
is one of the desirable properties for MEMS component coatings since high
surface energy leads to stiction resulting in the failure of components. Low
surface energy of the coatings suggests their potential applications in preventing
stiction arising due to the surface and capillary forces [Mastrangelo 1997].
5.1.2 AFM morphology results
Bare Ti6Al4V (after polishing, grinding and cleaning) showed the surface
roughness (rms) of 11.8 nm as measured from AFM imaging using the scan area
of 5µm×5µm. Scratches created in the polishing process can be observed in AFM
imaging (Fig. 5.3 (a)) results. Bare Ti6Al4V with PFPE overcoat exhibited the
surface roughness (rms) of 1.7 nm. PFPE coating on Ti6Al4V, after heat
treatment, showed the surface roughness (rms) of 1.2 nm.
GPTMS coating deposited on Ti6Al4V showed the surface roughness
(rms) of 6.8 nm. PFPE overcoat onto GPTMS coating exhibited the surface
roughness (rms) of 2.5 nm. After heat treatment, PFPE coating on GPTMS
showed the surface roughness (rms) of 5.0 nm.
Table 5.2 summarizes the measured surface roughness (RMS and Ra)
values of different specimens using 5µm×5µm scan area. Fig 5.3 shows the AFM
imaging results from different representative specimen types prepared in this
study.
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
Table 5.2: Measured surface roughness for different Specimens in AFM imaging.
Specimen Type RMS (nm) Ra (nm) Ti6Al4V 11.8 9.6
Ti6Al4V/PFPE 1.7 1.3 Ti6Al4V/PFPE (heat treated) 1.2 0.7
Ti6Al4V/GPTMS 6.8 4.9 Ti6Al4V/GPTMS/PFPE 2.5 2.1
Ti6Al4V/GPTMS/PFPE (heat treated) 5.0 3.4
Figure 5.3: AFM imaging (scan area: 5µm×5µm, vertical scale: 100 nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) Ti6Al4V/PFPE (heat treated). (d) Ti6Al4V/GPTMS. (e) Ti6Al4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE (heat treated).
Thickness of GPTMS coating obtained in an earlier study using similar
deposition method has been reported to be the order of (~1 nm) [Luzinov et al.
2000]. Thickness of PFPE coating deposited using similar procedure followed in
this study has been reported to be the order of (2~3 nm) in an earlier research
study [Eapen et al. 2002]. Since surface roughness of the used Ti6Al4V alloy
substrate is quite high (11.8 nm) in comparison with the deposited GPTMS SAMs
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
and PFPE coatings in this study, it is difficult to infer data such as thickness and
topography of deposited coatings from AFM imaging results. As observed in the
AFM imaging results (Table 5.2), GPTMS coating has reduced the surface
roughness of the substrate surface. Observed significant change in the surface
roughness also suggests the possibility of forming GPTMS as multi-layer rather
than monolayer probably due to high humidity ambient conditions [Luzinov et al.
2000]. PFPE coatings onto different specimens also resulted into the reduction of
surface roughness value significantly which can be attributed to the possibility of
linear and flexible PFPE nano-particles filling up the surface textures features
such as valleys.
5.2 Chemical characteristics of UHMWPE coating
5.2.1 XPS analysis
Wide scan XPS spectra of Ti6Al4V/GPTMS specimen showed strong
C1s, O1s and Ti2p peaks as shown in Fig. 5.4. Due to nanometer thickness (< 1
nm) of GPTMS SAMs coating, substrate is also detected in XPS spectra. Fig. 5.5
shows the comparison of C1s peak of Ti6Al4V/GPTMS and Ti6Al4V specimens.
High resolution XPS spectrum of C1s scan for Ti6Al4V/GPTMS specimen
showed two strong peaks. Peak at 284.6 eV represents (C–C) bonds and 286.4 eV
corresponds to the (C–O) bonds in the epoxy SAM molecules. These two strong
peaks are indicative of epoxy SAMs presence as seen in earlier studies [Wong and
Krull 2005; Cloarec et al. 2002]. Compared to this, Ti6Al4V showed only one
C1s peak. Observed change in the XPS core-level spectra of C1s for
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
Ti6Al4V/GPTMS specimen in comparison with the bare Ti6Al4V specimen
indicates the deposition of GPTMS SAMs coating.
Figure 5.4: Wide scan XPS spectra of Ti6Al4V/GPTMS specimens.
Figure 5.5: Comparison of C1s peaks for bare Ti6Al4V/GPTMS and Ti6Al4V in C1s scan.
5.3 Tribological characterizations
Bare Ti6Al4V alloy and surface modified Ti6Al4V alloy specimens’
tribological properties were evaluated using Si3N4 as counterface in a ball-on-
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
disk tribometer. Table 5.3 tabulates initial friction coefficient (evaluated at the
end of 600 sliding cycles) values of specimens investigated in this study. Fig. 5.6
summarizes the wear life (average and standard deviation) of prepared specimens
in this study. Bare Ti6Al4V alloy and GPTMS deposited Ti6Al4V specimens
showed wear life of less than 100 sliding cycles for tested conditions (not shown
in Fig. 5.6). Wear durability data of samples from each specimen type were
collected from minimum six different samples using three different tracks on each
sample.
Table 5.3: Coefficient of friction for specimens tested in the study.
Specimen Type Initial Coefficient of Friction Ti6AL4V 0.5~0.6 Ti6AL4V/PFPE 0.12~0.13 Ti6AL4V/PFPE (heat Treated) 0.11~0.12 Ti6AL4V/GPTMS 0.5~0.6 Ti6AL4V/GPTMS/PFPE 0.11~0.12 Ti6AL4V/GPTMS/PFPE(heat Treated) 0.12~0.13
Figure 5.6: Wear durability (number of sliding cycles before failure) of tested specimens in the study.
Fig. 5.7 shows the variation in the coefficient of friction with sliding
cycles for representative PFPE coated Ti6Al4V alloy specimens. For comparison,
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
result of the tribological characterization of bare Ti6Al4V alloy has also been
tabulated for the same testing conditions.
Bare Ti6Al4V alloy showed high coefficient of friction (0.5~0.6) as
shown in Fig. 5.7 and Table 5.3. High surface energy and mechanical instability
of oxide layer result into poor tribological properties of bare Ti6Al4V alloy
[Budinski 1991; Yildiz et al. 2009]. PFPE modified Ti6Al4V alloy (with heat
treatment as well as without heat treatment) specimens have showed lower
coefficient of friction although sample with heat treatment exhibited lower wear
durability (see Fig. 5.6 and Fig 5.7). Heat treatment increases the bonded portion
of PFPE on the substrate. Reduction in the wear durability can be attributed to the
reduction in the mobile portion of PFPE overcoat after heat treatment. Reduction
in the wear durability after heat treatment of PFPE coating has been observed in
an earlier study onto a different substrate [Miyake et al. 2006].
Figure 5.7: Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE (heat treated) and (c) bare Ti6Al4V) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm).
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
Fig. 5.8 shows the variation in the coefficient of friction with sliding
cycles from representative PFPE coated Ti6Al4V/GPTMS specimens. Similar
evaluation has also been obtained for the tribological test of Ti6Al4V/GPTMS
specimen.
Epoxy SAMs (GPTMS) modified Ti6Al4V alloy exhibited high
coefficient of friction (0.5~0.6) at the tested condition of 20 gm normal load and
200 rpm (see Table 5.3 and Fig. 5.8). Observed tribological properties of epoxy
SAMs modified substrate are similar to the observation in an earlier study
[Sidorenko et al. 2002]. PFPE overcoat onto epoxy-SAMs modified Ti6Al4V
alloy (with and without heat treatment) reduced coefficient of friction and
increased the wear durability (see Fig. 5.6 and 5.8). Epoxy-SAMs modified
Ti6Al4V alloy specimens with PFPE overcoat showed high wear durability
(90,700±13,900 sliding cycles) although heat treatment has resulted into some
reduction in the wear durability (74,700±24,000 sliding cycles).
Figure 5.8: Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b) Ti6AL4V/GPTMS/PFPE (heat treated) and (c) Ti6Al4V/GPTMS) using Si3N4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm).
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
PFPE overcoat (with and without heat treatment) on bare Ti6Al4V alloy
and GPTMS SAMs modified alloy has reduced the coefficient of friction to
0.11~0.13 for different specimens investigated in this study as observed in Table
5.3.
5.4 Optical microscopy of wear track and counterface surface
Fig. 5.9 shows the optical micrographs of the wear track and counterface
surface after completion of 5,000 cycles of the sliding test for bare Ti6Al4V
specimen. There was extensive wear debris accumulation around the track (Fig
5.9 (a)) and counterface surface of Si3N4 ball showed severe damage (Fig 5.9
(b)). Similar wear track and counterface conditions were observed for GPTMS
deposited Ti6Al4V specimens after completion of 5,000 sliding cycles (as seen in
Fig. 5.10). Fig. 5.10 (a) and Fig. 5.10 (b) show the accumulation of wear particles
near wear track and damaged counterface surface respectively.
Figure 5.9: Optical micrographs of Ti6Al4V specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.
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CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
Figure 5.10: Optical micrographs of Ti6Al4V/GPTMS specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.
Fig. 5.11 shows the optical micrograph images of the wear track and counterface
surface after completion of 10,000 sliding cycles for Ti6Al4V/PFPE specimen. As
observed in Fig. 5.11 (a), wear track shows mild impression of counterface with absence
of debris. There is a little material transfer to counterface surface. After cleaning with
acetone, counterface surface is smooth with no sign of any permanent damage. Compared
to this, wear track surface after completion of 10,000 sliding cycles using Ti6Al4V/PFPE
(heat treated) specimen shows the accumulation of debris and severe damage to
counterface surface attributed to the reduction in wear resistance after heat treatment (see
Fig. 5.12).
Figure 5.11: Optical micrographs of Ti6Al4V/PFPE specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. (c) Counterface after cleaning, magnification 100x.
81
CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
Figure 5.12: Optical micrographs of Ti6Al4V/PFPE (heat treated) specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x.
Fig. 5.13 shows the wear track and counterface surface after completion of
100,000 sliding cycles using the coating of GPTMS/PFPE onto Ti6Al4V alloy in
one of the sliding test where coating had not failed. As seen in Fig 5.13 (a), no
wear debris was observed around wear track although mild scratch had formed by
the impression of the counterface. There was a little material transfer to the
counterface of Si3N4 ball (see Fig 5.13 (b)). After cleaning with acetone,
counterface showed smooth surface with no signs of damage as observed in Fig
5.13 (c).
Figure 5.13: Optical micrographs of Ti6Al4V/GPTMS/PFPE specimen’s wear track and counterface surface after 100,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. (b) Counterface after cleaning, magnification 100x.
82
CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
As seen in different optical micrograph images, before failure of the film,
mild scratching on the wear track without the presence of wear debris and smooth
counterface surface accompanied by little material transfer were observed. After
failure of the film, wear debris accumulation along the wear track and worn
counterface surface were noticed. Different optical microscope images support the
wear durability data shown in Fig. 5.6.
PFPE coating on bare Ti6Al4V alloy as well as GPTMS SAMs deposited
Ti6Al4V alloy has resulted into lowering of friction coefficient and the increase in
wear durability. This can be attributed to lower surface energy [Makkonen 2004]
as well as flexible and mobile nature of PFPE molecules [Mate 1992] resulting
into low resistance in the shearing at the sliding interface.
PFPE overcoat on bare Ti6Al4V alloy as well as GPTMS coated Ti6Al4V
may consist of three parts as speculated in earlier studies [Satyanarayana and
Sinha 2005; Satyanaryana et al. 2007]. PFPE top layer consists of molecules
trapped in surface texture, strongly adsorbed portion and the mobile portion.
Strongly adsorbed portion results due to strong physical adsorption and hydrogen
bonding interaction as well as covalent bonding formed due to the heat treatment.
During the initial sliding cycles, mobile as well as bonded portion of PFPE will
lubricate the contact region. After squeezing of the mobile portion of PFPE,
lubrication will be supported by the bonded portion of PFPE molecules. After
complete removal of the mobile and bonded portion of PFPE, high friction and
wear will be followed leading to the failure of the coating.
83
CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
High wear resistance of Ti6Al4V/GPTMS/PFPE specimens compared
with Ti6Al4V/PFPE can be explained due to the possible interaction of PFPE
molecules with GPTMS intermediate layer. Chemical interaction of PFPE with
epoxy group has been reported in previous research studies [Ellis 1993; Elender et
al. 1996]. It has been observed in some studies that dual-lubricant layer
combination consisting of initially bonded layer having strong-polar top group
with top mobile layer having weaker polar group shows improved tribological
properties compared with the use of only any one of the monolayers [Katano et al.
2003; Satyanaryana et al. 2007].
5.5 Biocompatibility assessment
Biocompatibility of GPTMS has been evaluated in earlier research studies.
In one study, noncytotoxicity of GPTMS was validated by the assessment of
cytocompatibility of hybrids of gelatin–GPTMS and chitosan–GPTMS in an in-
vitro study [Ren et al. 2002]. In another study, biocompatibility of GPTMS SAM
was confirmed using both bacterial culture (E.coli DH5α, Gram negative bacteria)
and plant tissue culture (wheat seed) [Kumar et al. 2011].
Biocompatibility of PFPE polymer has been examined by researchers and
long-term biocompatibility of PFPE has been validated in different studies for
biomedical applications [Xie et al. 2006; Sweeney et al. 2008].
In this study, H-Gladen ZV60 was used as the solvent for PFPE. To
confirm that H-Galden has not affected the biocompatibility of PFPE, PFPE top
84
CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
layer coated onto UHMWPE film was tested for cytotoxicity using ISO elution
method-1×MEM extract method.
5.5.1 Cytotoxicity test results
In cytotoxicity testing carried out on PFPE over-coated UHMWPE film,
no cytotoxicity effects such as cell lysis were seen in any of the test wells during
the microscopic examinations of the ISO elution method- 1×MEM extract. No
change in the pH value was noticed after the duration of 48 hours.
Grade of the test results should be less than the grade 2 (mild reactivity) to
meet the requirements of the ISO elution method-1×MEM extract. Based upon
test analysis results, it was inferred that coatings exhibited cytotoxicity level of
grade 0 (reactivity: none) according to the test guidelines, and thus PFPE top layer
coating meets the requirements of the ISO elution method-1×MEM extract.
In view of the mentioned previous research studies and carried out
cytotoxicity test in this study, noncytotoxic nature of GPTMS and PFPE coatings
are indicated. It suggests the noncytotoxic nature of composite coating of
GPTMS/PFPE investigated in this study.
5.6 Potential applications of GPTMS/PFPE coating
GPTMS/PFPE coating exhibits hydrophobicity, noncytotoxicity and
excellent tribological properties (low friction and high wear durability).
Hydrophobic coatings exhibit low surface energy. Lower friction and water-
repellent nature of the hydrophobic coatings are useful in different biomedical
85
CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer
86
applications. Hydrophobic coatings on coronary guidewire assist in improving
tactile feedback to cardiologist. Low surface energy of biomedical coatings has
been found to be useful in bio-film inhibition for few in-vivo applications
[Roosjen et al. 2006]. Coatings, having low surface energy and noncytotoxic
nature, on metallic stent are beneficial in improving the biocompatibility of metal
surface with body fluids by inhibiting the release of cytotoxic extracts from the
metal surface into the body environment [Pendyala et al. 2009]. Thus, the
obtained composite coating of GPTMS/PFPE can find different applications in
biomedical devices. Due to the molecularly low thickness of this coating (< 4
nm), it can be particularly useful in biomedical MEMS applications where
thickness of the surface coating is a crucial consideration for its usage.
Chapter 6: Conclusions
Chapter 6
Conclusions
In the first study of this thesis titled: “Tribological characterizations of
thin UHMWPE film and PFPE overcoat”, a thin film of UHMWPE (thickness of
19.6±2.0 µm) was coated onto Ti6Al4V alloy substrate. Prior to polymer coating,
oxygen plasma pre-treatment of the substrate surface was carried out for cleaning
as well as better adhesion of the polymer coating to the substrate. Physical
characterizations (contact angle measurement, thickness measurement, SEM
morphology and AFM imaging), biocompatibility test (cytotoxicity assessment)
and chemical characterizations (FTIR-ATR and XPS) were also carried out for
the UHMWPE polymer thin coating. Tribological characterization of the coating
was carried out at different load conditions and rotational speeds using a ball-on-
disk tribometer against a counterface of 4 mm diameter Si3N4 ball.
From the observations and results of this research study, following
important conclusions can be drawn:
Obtained UHMWPE polymer coating has similar physical and chemical
structure as that of bulk UHMWPE polymer as validated by different
physical and chemical characterization of the coating.
Oxygen plasma treatment is an effective surface treatment method for
cleaning and good adhesion of polymer coating on Ti6Al4V alloy
substrate as indicated by increased surface energy of the substrate after
plasma treatment.
87
Chapter 6: Conclusions
Formed UHMWPE polymer coating on Ti6Al4V alloy substrate exhibits
excellent tribological properties such as low coefficient of friction (0.15)
and high wear durability (> 96,000 cycles) under tested conditions. Good
mechanical and tribological properties of bulk UHMWPE polymer also
results into useful tribological properties of polymer coating.
UHMWPE and UHMWPE/PFPE coatings are non-cytotoxic in nature as
per the test requirements of the ISO elution method-1×MEM extract.
PFPE overcoat on UHMWPE polymer coating enhances wear resistance
for the tested high speed loading condition of 1000 rpm and the normal
load of 4 N by providing additional thermal stability and lubrication
properties.
Due to the combination of low surface energy, wear resistance and
noncytotoxicity of the coating, the thin film of UHMWPE (with and
without PFPE overcoat) can find usage in various applications of
biomedical devices.
In the second study of this thesis titled: “Tribological evaluations of
molecularly thin GPTMS SAMs coating with PFPE top layer”, the potential of
molecularly thin GPTMS/PFPE coating (~4 nm) to address the tribological
limitations of titanium and titanium alloy has been evaluated. Physical
characterizations (contact angle measurement, AFM imaging), chemical
characterization (XPS) and tribological tests were carried out for obtained
coatings specimens.
88
Chapter 6: Conclusions
89
From the results and observations of this study, following important
conclusions can be drawn:
PFPE overcoat onto Ti6Al4V/GPTMS surface exhibits low surface energy
as indicated by high water contact angle of coatings.
PFPE overcoat on bare ti6Al4V alloy after oxygen plasma treatment
exhibits improved tribological properties such as low coefficient of
friction (0.11 ~ 0.13) and high wear durability compared to bare Ti6Al4V
alloy.
PFPE overcoat on GPTMS deposited Ti6Al4V alloy exhibits low friction
coefficient (0.11 ~ 0.13) and high wear resistance. Wear resistance of
PFPE overcoat on GPTMS deposited Ti6Al4V alloy is significantly higher
(90,700±13,900 sliding cycles) than PFPE overcoat without GPTMS
deposition (26,700±4,900) which can be attributed to increased bonding of
PFPE with substrate due to GPTMS intermediate layer.
PFPE overcoat also reduces surface roughness of the substrate due to
filling up of surface texture by nano-particles of PFPE.
Low coefficient of friction, high wear resistance, hydrophobicity and
biocompatible nature of the coatings are suitable for their proposed
applications in biomedical instruments. Due to molecular layer thickness
of the coatings (~4 nm), these coatings are particularly suitable for
biomedical MEMS applications.
Chapter 7: Future Recommendations
CHAPTER 7
Future Recommendations
Below are recommendations for future studies based upon the results and
understanding generated in this research work:
In this thesis, cytotoxicity tests for the UHMWPE and UHMWPE/PFPE
coatings were carried out. Cytotoxicity test is the preliminary test to assess
the feasibility of coatings for biomedical applications. To further assess
the suitability of obtained coatings in biomedical devices, additional
biocompatibility tests such as hemocompatibility, sensitization, irritation
studies, systemic toxicity, genotoxicity etc are recommended as per the
guidelines of ISO 10993 “Biological Evaluation of Medical Devices” to
address specific application requirements.
Besides in-vitro biocompatibility tests, in-vivo animal testing and clinical
testing are also recommended for future studies to assess the potential
usage of these coatings in biomedical applications.
Optimization studies of process parameters to obtain lower thickness
UHMWPE coating without deteriorating tribological properties are
recommended to broaden application areas.
Obtained composite coating of GPTMS/PFPE can be tested for different
biocompatibility tests such as hemocompatibility, sensitization, irritation,
systemic toxicity, genotoxicity etc as per the guidelines of ISO 10993
90
Chapter 7: Future Recommendations
91
“Biological Evaluation of Medical Devices” to satisfy specific application
requirements.
In-vivo animal testing and clinical testing are also recommended in future
studies for the obtained GPTMS/PFPE coating.
Optimization of PFPE (wt %) solution, dip-coating immersion time,
dipping and withdrawal speeds on Ti6Al4V alloy substrate can be carried
out to obtain PFPE coatings with improved tribological properties.
Heat treatment temperature as well as time duration optimization studies
can be carried out to investigate the potential to obtain coatings with
improved wear resistance at higher loads.
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Appendix A: Cytotoxicity Test Procedures
Appendix A
Cytotoxicity Test Procedures
In-vitro tests for cytotoxicity are carried out as per the guidelines of
“International Organization for Standardization 10993-5: Biological Evaluation of
Medical Devices, Part 5: Tests for In Vitro Cytotoxicity”. Cytotoxicity tests
evaluate the response of cells in contact with devices or their extracts. "Part 5:
Tests for In Vitro Cytotoxicity" contains details about the procedure of testing
using cell culture using direct or indirect contact with device or filter diffusion.
Extracts of devices or materials are exposed to a cell culture system such
as L929 mouse fibroblast cell line in the test. After exposure, loss of cell viability
indicates the presence of cytotoxic extracts. Extracts are samples of potentially
cytotoxic substances that can leach out from the device material into the tissue
from device during usage. Extraction medium is chosen according to the nature
and use of device as well as test method. Extracts are prepared by incubating the
device at a recommended surface area to extractant volume ratio. This ratio is
depended upon the thickness of testing device. Surface area to extractant volume
ratio is 60 cm2 per 20 ml extraction volume if the device thickness is greater than
or equal to 0.5 mm and 120 cm2 per 20 ml extraction volume if device thickness
is less than or equal to 0.5 mm.
In case surface area of device can not be determined, a weight to volume
ratio (4 g per 20 ml extraction volume) is used for extraction. Extractable
107
Appendix A: Cytotoxicity Test Procedures
108
substances should be maximized and device should be exposed to extreme
conditions without a significant degradation.
Other method of cytotoxicity testing by direct contact is the agar diffusion
or overlay assay. In this method, test device is placed directly on the mammalian
cell layer. Mammalian cell layer is protected from mechanical abrasion by using
an intermediate agar layer. Similar to other methods of cytotoxicity assessment,
the presence of cytotoxic leachables is indicated by the observed loss of cell
viability. The direct contact assay method requires the placement of device or
material onto the cell culture medium without the presence of an agar
intermediate layer. In a filter diffusion test, test device or material is placed on
one side of filter and exposed to cells grown on the opposite side of filter.
In general, cytotoxicty test involves a cell culture dish having one-half
million to one million cells. Device cytotoxicity is measured by the cell viability
after a period of exposure (approximately 24–72 hours). Cytotoxicity can be
measured by qualitative and quantitative analysis. To validate test results, positive
control materials (such as organo-tin-impregnated polyvinyl chloride material)
and negative control materials (such as USP-grade high-density polyethylene RS)
are also tested.
Following the test period, the cell layers are examined microscopically for
any abnormal cell morphology and cellular degeneration.