Modeling of friction and structural transformations in diamond-like carbon coatingsMultiscale modelling and design for engineering applicationsVTT, Espoo, 5th of February, 2013H. Ronkainen, A. Laukkanen, K. Holmberg VTT Technical Research Centre of Finland

213/02/2013

Presentation outline

1. Friction and wear performance of DLC coatingsa-C:H and ta-C

2. FE modelling of a-C:H and ta-C - stress generation under load

3. IMAGO – DLC research4. Effect of aging of DLC coatings5. VTT modeling approach for aging and temperature

effect studies of DLC6. Future tasks

313/02/2013

0.01 0.0001 1 100 DLC = diamond-like carbon coating, MoS2 = molybdenidisulfidi coating, NFC = nearly frictionless carbon coating

Fric

tion

coef

ficie

nt in

slid

ing

Wear rate

1.0

1.5

0.5

0.1 0.01

Steel vssteel

Ceramic vsceramic

Grinding

Gold vsgold

Carbrakes

Shoe vsfloor

Ice vswood

Rubbertyre vsdry road

Rubbertyre vswet road

Steel vssteel - oilMoS2/

MoS2DLC/DLC

NFC/NFC vacuum

(10-6 mm3/Nm)

Teflon vssteel

Rubber vsrubber

Plastic vsplastic

10000

Friction and wear performance

413/02/2013

DLC coatings

Ternary phase diagram for DLC coatings showing the balance of the sp3/sp2-ratio and the hydrogen content in the coating. Ferrari and Robertson, 2000

Friction performance of hydrogen-free and hydrogenated DLCs

Ronkainen et al., 2007.

513/02/2013

Hydrogenated a-C:H and hydrogen-free ta-C -coatings

ta-C

Coating H-content

at %

sp3

bonding%

Hardness

GPa

Young´s modulus

GPa

a-C:H 26–33 70 14±1 129±5

ta-C < 1 66 54±18 445±57

a-C:HWear performance

613/02/2013

The friction coefficient decreases as the load and sliding velocity are increased. Wear rate of the coating and

the counter part decrease as the load and sliding speed is increased

Friction and wear performance of a-C:H in ambient airS

teel

pin

Alu

min

a pi

n

Friction and wear of a-C:H

against steel and alumina

Ronkainen et al. (1994)

713/02/2013

Friction performance of ta-C

Hard hydrogen-free ta-C films Friction coefficient in the range 0.1 –

0.15 normal atmosphere High friction in dry atmospheres.

Against steel Against alumina

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

ta-C (ta-C)+(H2:ta-C)

(ta-C)+(H2:ta-C)

(ta-C)+(CH4:ta-C)

Fric

tion

coef

ficie

nt, µ

0

2

4

6

8

10

12

14

16

18

H-c

onte

nt [a

t. %

]

ta-C

(ta-C

)+(H

2:ta-

C)

(ta-C

)+(H

2:ta-

C)

(ta-C

)+(C

H4:t

a-C

)

Dry N2Dry synth. air

Humid air

ta-C0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Fric

tion

coef

ficie

nt, µ

813/02/2013

Scratch test using FE: Application in fracture toughness & behavior evaluation

Numerical model: produce a finite elementmodel reproducing the loading history for extraction of field variables

kgh3cs.dsf 041101

+ +

Diamond stylus

3 2 1Coating pulling

Interface sliding

Ploughing frictionfracture plastic deformation

913/02/2013

Scratch test using FE: Modeling primaries

Typically linear-elastic (or otherwise “stiff”) stylus and coating, accompanied by a substrate material undergoing plastic deformation during the scratch testing [with fairly insignificant hardening].

Model dimensions usually of the order of 10000 150-500 50-250 m.

Material properties generated using integration point input → enables simple analyses of gradient, layered, functional etc. structures.

Commercial (Abacus), open source and in-house packages used for computations, depending on the “degree of novelty” of a specific problem.

Verification carried out by checking the scratch geometry in both experiments & modeling

1013/02/2013

DLC Coatings

a-C:H ta-CHardness 25 GPa 67 GPaYoung´s modulus 212 GPa 352 GPa

1113/02/2013

0.3 µm 1.0 μm1st principal stress in a-C:H and ta-C in sliding contact

a-C

:Hta

-C

more uniform and higher stress

higher load bearing capacitystress state very uniform throughout

the coating

1213/02/2013

Effect of coating thickness on stresses0.6 µm ta-C after 1.2 mm slidig

Surface

0.3 µm below the surface Interface

Similar uniform stress through the coating With increased thickness stress

accumulation occurs on the scratch edge due to bending of the coating Higher thickness provides load-carrying

capacity.

1313/02/2013

Surface

0.3 µm below

surface

at interface

a-C:H 0.6 µm

Surface

0.3 µm below

surface

0.6 µm below

surface

a-C:H 1 µm

Surface

at interface

a-C:H 0.3 µm

Effect of coating thickness on stresses0.3, 0.6 and 1 µm thick a-C:H after 1.2 mm slidig

1413/02/2013

Effect of coating thickness on the stress behaviour of ta-C and a-C:H coatings

Characteristics of first principal stress field in ta-C and a-C:H coatings. “s” is the distance from plane of symmetry at the coating surface at the locale of maximum principal stress.

ta-C a-C:H

1513/02/2013

VTT MODELLING APPROACH TO WEAR CONTROL

1613/02/2013

INTEGRATED MATERIAL MODELLING FOR DEMANDING APPLICATIONS

Å

μm

nm

mm

hmsμsnsfs ps s

CFEFEM

MDS

ScratchtestFEM

RamanCFE,MDS

Resid.stressCFE PoD

FEM

= model validation methods

NanoindentMDS AFM

MDS

FEM = Finite Element MethodCFE = Constrain Free Energy

MDS = Molecular Dynamic Simulation In-situ

TEM scratchMDS

1713/02/2013

Effect of aging on tribological performance –Pin-on-Disc tests of ”old” DLC coatings

DLC Si wafers coated 1992: U – University of Helsinki,

Department of Physics ta-C (arc discharge)

Commercial coatings: B D O

Pin-on-Disc counter parts: Al2O3-pin: Load 5 N (0.8 GPa) 100Cr6-pin: Load: 10 (0.8 GPa)

Sliding speed: 0.1 m/s Sliding distance: 2000 m (5.5 h) Normal atmosphere: 23 C, 50 % RH

1813/02/2013

*) No measurable wear

1,00E‐10

1,00E‐09

1,00E‐08

1,00E‐07

1,00E‐06

B D O U

DLC (Si) Disc Wear R

ate, K 

[mm3/Nm]

DLC wear rate against Al2O3

1992

2012

1,00E‐10

1,00E‐09

1,00E‐08

1,00E‐07

1,00E‐06

B D O U

DLC (Si) Disc Wear R

ate, K 

[mm3/Nm]

DLC wear rate against 100Cr6

1992

2012

1,00E‐11

1,00E‐10

1,00E‐09

1,00E‐08

1,00E‐07

1,00E‐06

B D O U

Al2O

3Pin Wear R

ate, K 

[mm3/Nm]

Al2O3 pin wear rate against DLC

1992

2012

1,00E‐11

1,00E‐10

1,00E‐09

1,00E‐08

1,00E‐07

1,00E‐06

B D O U

100C

r6 Pin W

ear R

ate, K 

[mm3/Nm]

Steel pin wear against DLC

1992

2012

Steel and Al2O3 sliding against DLC

*)

*)

*)

1913/02/2013

Friction performance of DLC coatings

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

B D O U

Frictio

n Co

efficient, µ

100Cr6 against DLC(Si)

0

0,1

0,2

0,3

0,4

0,5

0 2 4 6

Frictio

n coefficient

Time [h]

100cr6/B DLC(Si)0,0

0,1

0,2

0,3

0,4

0,5

0 2 4 6

Frictio

n coefficient

Time [h]

100Cr6/U DLC(Si)

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

B D O U

Frictio

n Co

efficient, µ

Al2O3/DLC(Si)

µ 1992

µ 2012

Friction performance of a-C:H in repeated tests 2012

Friction performance of ta-C in repeated tests 2012

0

0,1

0,2

0,3

0,4

0,5

0 1 2 3 4 5 6

Frictio

n coefficient

Time [h]

Al2O3/B DLC(Si)

Friction performance of a-C:H in repeated tests 2012

2013/02/2013

Development of DLC: a-C:H Atomic Scale Model• Atomic structures of interest:

• ta-C• a-C:H (10-50 at% H)

• In all models, a pre-stage of creating an amorphous DLC structure by way of a liquid quench process• Spontaneous melting of cubic lattice• Stabilization and thermostating to final

amorphous structure • Soft Ware: LAMMPS (Sandia web)

combined wiht VTT in-house modules

• Load cases: • DLC against DLC tip• DLC against diamond tip

(nanoindentation)• DLC against DLC surface.

0 ps ~5 fs ~100 fs

~300 fs ~1 ps

Liquid quenching

sp2 & sp3 site plot

2113/02/2013

Stress distribution in a-C:H (25at% H) during indentation (blue ~ compressive, model sliced in half)

Coordination (sp2 & sp3 sites) analysis in a-C:H (25at% H) during scratch testing, displaying indications of sp2 graphite layer formation (for multiple scratches, model sliced in half)

Example MD analysis results

2213/02/2013

Indenting a-C:H film withDiamond Tip

2313/02/2013

Indenting ta-C films with a Diamond TipStress Generation

2413/02/2013

Indenting ta-C films with a Diamond TipStress Generation

Compressive stresses are generated under the contact area (blue colour).

2513/02/2013

Status of DLC research

MDS models of a-C:H and ta-C coatings developed. Characterization and evaluation of DLC coatings (a-C:H) deposited in Argon

National Laboratory (Dr. Ali Erdemir) on-going. Tribological performance of “old” DLC coatings evaluated and the

characterizatin of the coatings on-going.

Future work Aging and temperature effects of DLC coatings by MDS, CFE and FE

modeling Contact PoD: DLC vs DLC (steel) with graphitic shear Variable: T = 20,50,100,150,200,300,(400)°C + aging Validation by indentation (VTT), AFM (JyYO, VTT), TEM (SU), PoD

(VTT) at various temps, measure: μ, wear-rate combined with characterization and evaluation.

2613/02/2013

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