Development of tribological PVD coatings

Escola PolitécnicaUniversidade de São Paulo

roberto.souza@poli.usp.br 1

Roberto M. Souza .

Laboratório de Fenômenos de Superfície

Departamento de Engenharia Mecânica

Escola Politécnica da Universidade de São Paulo

roberto.souza@poli.usp.br

Outline

• Overview – Development of PVD coatings

– Ideas currently in use - Examples

• Thin film development at the Surface Phenomena Laboratory

– Measurement, tribological evaluation, film processing

– Example – Film residual stresses

PVD• Global value of PVD industry

– Data from 2007

– Predicted annual

growth rate for

the next 5

years:

11 % per year

[http://www.bccresearch.com/report/MFG015C.html]

Development of PVD Coatings• Book chapter: O. Salas, J. Oseguera “Megatendencia:

Ingeniería de Superficies”– Question asked to a number of specialists: “… currently,

surface engineering pushes or pulls the market?”

– Possible interpretation of the question:

What question occurs more frequently?

Developer to industry: I have developed this coating, would you test it for me?

Industry to developer: I need a coating with the following properties, would you develop one for me?

– No clear trend in the answers

Thin film development

[Donnet & Erdemir Surf. Coat. Technol. v.257, 2004]

• This presentation

– PVD

– Mostly hard coatings

• Recent developments

– Drive or driven by industrial needs?

• Development in three main areas

Examples – Materials and Structures

• Adaptive coatings– Merging multiple complementary

solid lubricants in a nanocomposite coating

– Continuous response of the contact surface to the surroundings

• Load

• Sliding speed

• Environment

• Temperature

8[Muratore & Voevodin Ann. Rev. Mat. Res. 39 (2009) 297-324]

Examples - Processing• Pulsed Magnetron Sputtering (PMS)

– Use for the deposition of highly insulating materials– Reduction of arcing events

0 50 100 150

Time, microseconds

Al2O3 films

DC reactive sputtering Pulsed reactive sputtering

[P.J. Kelly et al. Surf Coat Technol 86-87 (1996) 28-32][P.J. Kelly and D.R. Arnell, Vacuum 56 (2000) 159-172]

Side View

Top View

Hiden electrostatic quadrupole plasma mass spectrometer (EQP)Al Cr

Examples - Processing• Pulsed Magnetron Sputtering (PMS)

– Plasma monitoring during

the deposition of

Cr1-xAlxN coatings for Al die

casting applications• Ion flux

• Ion energy

[Courtesy: J.J. Moore, ACSEL, Colorado School of Mines ]

0 20 40 60 80 100 120 140 160 1800.0

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

3.0x106

Pulsing both targets synchronously Pulsing both targets asynchronous Only pulsing Al target

Ion Energy (eV)

[J. Lin et al., Surface and Coatings Technology 201 (2007) 4640]

Examples - Processing

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

38( Ar+)

40( Ar+)

42( N3

56( N4

54( Cr+)

53( Cr+)

52( Cr+)

50( Cr+)

36( Ar+)

29( N2

28( N2

26.7( Al+)

18( H2O)

14( N+)

66( CrN+)

68( CrO+)

• Pulsed Magnetron Sputtering (PMS)– Ion energy and ion flux

Examples - Processing• High Power Pulse Magnetron Sputtering – HPPMS (also known as High Power Impulse Magnetron Sputtering – HIPIMS)

– Highly ionized flux of sputtered material instead of large amount of neutrals

– Low deposition rates

• Modulated Pulse Power – MPP– Advantages of HPPMS– High deposition rates

Examples - Processing• Modulated Pulse Power – MPP

0 20 40 60 80 1000.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

0 20 40 60 80 1000.0

2.0x105

Continuous dc discharge Pulsed dc discharge MPP discharge

Ion Energy [eV]

=1.2 kW

5 mTorr

Continuous dc discharge Pulsed dc discharge MPP discharge

Ion Energy [eV]

=1.2 kW

5 mTorr

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

3.0x106

MPP Pulsed DC

Continuous DC

[Courtesy: J.J. Moore, ACSEL, Colorado School of Mines ]

Examples - Materials• Materials – Nitrides and carbonitrides

– TiN, TiCN, CrN, ZrN, TaN

– TiAlN, TiSiN, TiAlCN, CrAlN, TiCuN, TiNbN, TiHfN, TiVN, TiZrN, TiMoN

– TiAlSiCrN

– TiAlVN, TiCrAlN, ZrCrAlN, TiAlNbN

– Drive or driven by industrial needs?

roberto.souza@poli.usp.br15

Surface Phenomena Laboratory

• Tribological (wear, friction) behavior of thin films

Thin Film Characteristics

Hardness Fracture Toughness

Residual Stresses

Adhesion

Evaluation Modeling

Experimental

Analytical

Wear x characteristic

Material Processing

Residual Stresses

Adhesion

Evaluation Modeling

Experimental

Analytical

Material Processing

Residual Stresses in Thin Films• Several reviews are available in the literature. Usually:

– Effects of thin film stresses– Measurement techniques– Sources

• Sources– Classification. Different views in the literature

• Intrinsic: During deposition• Extrinsic: After the film growth step is concluded

In the contents

[J.A. Thornton, D.W. Hoffman, Thin Solid Films, 171 (1989) 5-31.]

Residual Stresses in Thin Films• Extrinsic stresses – In general,

• Intrinsic stresses – Commonly due to defects generated during deposition

• Effect of intrinsic and extrinsic stresses– Ratio between deposition and

melting temperature Td / Tm

– vs. Adatom mobility18

PVD CVD

dSubstrateTFilmTFilm

Filmthermal

Residual Stresses in Thin Films• Physical Vapor Deposition (PVD) films:

– Different techniques developed to achieve denser films, which have better tribological properties

– Effect of ion bombardment – Structure zone models

Evaporation

[B.A. Movchan, A.V. Demchishin, Phys. Met. Metallogr., 28 (1969) 83-

Close-fieldUnbalanced MS

[P.J. Kelly, R.D. Arnell, Vacuum, 56 (2000) 159-172.]

Sputtering

[J.A. Thornton, Ann. Rev. Mater. Sci.,7 (1977) 239-260.]

Residual Stresses in Thin Films• PVD films:

– Models to determine intrinsic stresses• Windischmann (1987,1992): Stresses

are directly proportional to the flux of energetic particles arriving on the substrate and to the square root of their kinetic energy.

• Davies (1993): Thermal spikes to reduce stress by causing displacement of the implanted atoms.

• Conventional MS Davis’ model Unbalanced MS Davis’ model

[Y. Pauleau, Vacuum 61, 2001, 175-181]

Residual Stresses

Adhesion

Evaluation Modeling

Experimental

Analytical

Material Processing

First set of TiN specimens

Pulsed Magnetron Sputtering

First Set of TiN Specimens• Set of specimens prepared with or without target pulsing (PMS)

– Residual stresses increased with applied bias and with PMS

-100 -50 0-12

-1-250 -200 -150 -100 -50 0

Bias voltage (V)

DC specimens: D0, D50, D100 PF P50 PG

-12 -11 -10 -9 -8 -7 -6 -5 -4 -30.05

k c (x

4 m2 /N

Residual Stress (GPa)

DC0 and DC100 PF P50 PG

[R.C. Cozza et al. Surf. Coat. Technol. 201 (2006) 4242-4246][M. Benegra et al. Thin Solid Films 494 (2006) 146-150]

• Micro-scale abrasion tests– Overall trend of wear reduction with the increase in film stresses

First Set of TiN Specimens• However, film debonding if

the film stresses were too compressive

• Confirm literature data that compressive film residual stresses may be beneficial as long as film/substrate adhesion is not impaired

Second set of TiN specimens

Gradient stresses

Residual Stresses

Adhesion

Evaluation Modeling

Experimental

Analytical

Material Processing

Second set of TiN specimens• Main idea: Prepare films grading

the residual stress level – Control of deposition substrate bias

Reduce wear

Improve adhesion

[E. Uhlman & K. Klein, Surf. Coat. Technol. 131 (2000)]

– Stress graduation obtained

based on the control of the

pressure during the deposition

• Previous works in the literature– Fischer and Oettel, Surf. Coat. Technol.

97 (1997)

Second set of TiN specimens• Idea poses two questions

– Question 1: How to measure the stress gradient• TiN films obtained in hybrid reactor after plasma nitriding

– Triode magnetron sputtering deposition– D2 substrates– M2 substrates– Substrate bias increasing, decreasing or constant

Specimen Layer Time (min) Bias (V)

1 45 -20

2 45 -40

3 45 -100

4 45 -150

5 45 -200

1 45 -200

2 45 -150

3 45 -100

4 45 -40

5 45 -20

S1 1 120 -20

S2 1 120 -40

S3 1 120 -100

S4 1 120 -150

S5 1 120 -200

Increasing bias

Decreasing bias

Constant bias

• Stress measurement – D2 substrates

Specimen Layer Time (min) Bias (V)

1 45 -20

2 45 -40

3 45 -80

4 45 -100

1 45 -100

2 45 -80

3 45 -40

4 45 -20

Increasing bias

Decreasing bias

• Stress measurement – M2 substrates

Residual Stresses

Adhesion

Evaluation Modeling

Experimental

Analytical

Material Processing

• Stress measurement – X-ray diffraction with grazing

incident angle

– Results of “single layer”

thin films: S1 to S5

– Agreement with literature

0 -50 -100 -150 -200

Bias voltage (V)

incident angle– How to measure the gradient?– Idea: H. Dolle, J. Appl. Cryst. 1979

mean value of residual stress over

a depth x

dxedxedxedxedxe

0.00 0.42 0.84 1.26 1.68 2.100.0

Depth penetration m)

I para =1.5 I para =2.5 I para =3.5 I para =4.5 I para =6 I para =8 I para =10

incident angle – D2 substrates– Use values of the single layers

to calculate value measured

with X-ray diffraction

dxedxedxedxedxe

1,2,3,4,5

0 2 4 6 8 100

a)Angle of incidence (deg)

G1D2 (experimental) G1D2 (model) G2D2 (experimental) G2D2 (model)

incident angle – M2 substrates– Use values of the single layers

to calculate value measured

with X-ray diffraction

dxedxedxedxe

1,2,3,

0 2 4 6 8 100

a)Angle of incidence (deg)

G3M2 (experimental) G3M2 (model) G4M2 (experimental) G4M2 (model)

Second set of TiN specimens• Idea poses two questions

– Question 2: How to evaluate the tribological behavior

• Pin-on-disk testing– High contact pressure– Low contact pressure

Residual Stresses

Adhesion

Evaluation Modeling

Experimental

Analytical

Material Processing

Second set of TiN specimens• Tribological behavior – M2 substrates

– Increasing bias

– Decreasing bias

– Constant bias

0 20 40 60 80 100 120 140 1600

Epaisseur (m)

Temps (minutes)

Croiss Const Decroiss

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4

– Differences in curvature: initial bias determined the curvature even if the average bias value was equal

Increasing Const. Decr.

Ensaios de Deslizamento – Pression Alta (2)

• Tribological behavior – Sliding test at high contact pressure– Pin: Steel, R = 5 mm– Track length = 10 mm– Frequence: 10 mHz (v = 0.2 mm/s)

• F x t (Pressão x t):

1 cycle– 200 à 400 N (2,8 à 3,5 GPa)– 150 à 350 N (1,7 à 3,3 GPa)

Second set of TiN specimens• Tribological behavior

– Conventional scratch test – Critical load for adhesive failure• Increasing bias: 16.2 ± 1.8 N• Decreasing bias: 3.1 ± 0.2 N

– “Non-conventional” scratch test: Steel spherical stylus (R = 5 mm),

increasing normal load from 150 to 400 N (Nominal Hertz Pmax from

1.7 to 3.4 GPa)

Constant Increasing Decreasing

Second set of TiN specimens• Tribological behavior – Sliding test at low contact pressure– Pin: Steel, R = 100 mm– Track length = 3 mm– Frequence: 100 mHz (v = 0.6 mm/s)– F x t (P. Max Hertz x t):

150 cycles

50 N (0,24 GPa – PMax Hertz)

Hole (trou)

Rupture

Second set of TiN specimens• Optical spectroscopy

– 594 sectional profiles– Average of profiles

Increasing bias

Constant bias

Decreasing bias10

Increasing

Track width

• Friction coefficient: Results similar to those in the literature for TiN – High friction coefficient values– High values from the first

cycles– All films behaved similarly

0 5 10 15 20 250,0

Cycles

Constante - trou

0 5 10 15 20 250,0

Cycles

Decroiss. - trou

0 5 10 15 20 250,0

Cycles

Croissante - trou

0 5 10 15 20 250,0

1,0 Constante - Rupture

Cycles

0 5 10 15 20 250,0

1,0 Decroiss. - Rupture

Cycles

0 5 10 15 20 250,0

1,0 Croissante - Rupture

Cycles

Second set of TiN specimens• This study has shown

– Quick formation of a third body layer at the pin surface and high friction coefficient

– Gradual oxidation of the specimen surface

– Negligible wear of the specimens– Similar behavior in all cases

Concluding Remarks• Discuss, with examples, some routes for the

development of PVD coatings

• Present an overall organization of the study of the tribological behavior of thin films – driven by the market– Emphasize necessity of accurate measurement and

the importance of the choice in tribological testing

Acknowledgements• FAPESP, CNPq, CAPES• Research groups – Actual experimental analysis

– LFS – Esc. Politécnica USP – ACSEL – Colorado School of Mines, USA– TMI – INSA-Lyon, France– IPEN– Inst. Física – USP– CNEA, Argentina

• Plus valuable discussions with many other groups

Second set of TiN specimens• Tribological behavior – Tensile testing

– In theory, provides quantitative results in terms of film fracture toughness and film/substrate adhesion

-0,01 0,00 0,01 0,02 0,03 0,040

TiN sur M2 - Jan, fev 2008

Déformation - L/Lo

* 2 m UDESC * 1 m UDESC #1 * 1 m UDESC #2 + 1.4 m Croiss. #1 + 1.4 m Croiss. #2 + 1.4 m Decroiss. #1 + 1.4 m Decroiss. #1 + 1.4 m Bias = #1 + 1.4 m Bias = #2

(*) Couche a fissuré(+) Couche n'a pas fissuré

Catastrophic failure before significant plastic deformation of the substrateFilm fracture was not observed

during the test Impossible to compare the deposition conditions

Second set of TiN specimens• Evolution of specimen surface throughout the cycles

« particles »Third body

oxidationFirst body

Second set of TiN specimens• Fraction of “particles” and oxidized area

0 20 40 60 80 100 120 140 1600

40TrouRupture

Début - Constante Fin - Constante Fin - Constante Début - Decroiss. Début - Croiss.

Cycles

Début - Constante Fin - Constante Début - Decroiss. Début - Croiss. Fin - Croiss. Fin - Croiss.

Rupture0 20 40 60 80 100 120 140 160

"Cycles

Second set of TiN specimens• FEG: Field Emmission Gun - Specimen

Increasing bias, hole

– Observation of iron oxyde on track surface: Free and agglomarated particles

Second set of TiN specimens• Pin surface

Increasing bias

Constant bias

Rupture Hole

Second set of TiN specimens• Scanning electron microscopy- Pin

Pol. Croissante - Trou

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