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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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