Modeling of friction and structural transformations in diamond-like carbon coatings

Transcription

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

2 2 Presentation outline 1. Friction and wear performance of DLC coatings a-c:h and ta-c 2. FE modelling of a-c:h and ta-c - stress generation under load 3. IMAGO DLC research 4. Effect of aging of DLC coatings 5. VTT modeling approach for aging and temperature effect studies of DLC 6. Future tasks

3 3 Friction and wear performance Friction coefficient in sliding NFC/NFC vacuum Rubber tyre vs wet road DLC/ MoS2/ DLC MoS2 Steel vs steel - oil Rubber tyre vs dry road Car brakes Shoe vs floor Ceramic vs ceramic Teflon vs steel Rubber vs rubber Steel vs steel Ice vs wood Gold vs gold Plastic vs plastic Grinding Wear rate (10-6 mm 3 /Nm) DLC = diamond-like carbon coating, MoS 2 = molybdenidisulfidi coating, NFC = nearly frictionless carbon coating

4 4 DLC coatings Ternary phase diagram for DLC coatings showing the balance of the sp 3 /sp 2 -ratio and the hydrogen content in the coating. Ferrari and Robertson, 2000 Friction performance of hydrogenfree and hydrogenated DLCs Ronkainen et al., 2007.

5 5 Hydrogenated a-c:h and hydrogen-free ta-c -coatings a-c:h Wear performance ta-c Coating H- content at % sp 3 bonding % Hardness GPa Young s modulus GPa a-c:h ±1 129±5 ta-c < ±18 445±57

6 6 Friction and wear performance of a-c:h in ambient air Steel pin Alumina pin 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 of a-c:h against steel and alumina Ronkainen et al. (1994)

7 7 Friction performance of ta-c Hard hydrogen-free ta-c films Friction coefficient in the range normal atmosphere High friction in dry atmospheres. Against steel Against alumina Friction coefficient, µ Dry N2 Dry synth. air Humid air ta-c Friction coefficient, µ ta-c ta-c (ta-c)+(h 2 :ta-c) (ta-c)+(h2:ta- C) (ta-c)+(h 2 :ta-c) (ta-c)+(ch 4 :ta-c) (ta-c)+(h2:ta- (ta-c)+(ch4:ta- C) C) H-content [at. % ]

8 kgh3cs.dsf /02/ Scratch test using FE: Application in fracture toughness & behavior evaluation Diamond stylus Coating pulling Interface sliding Ploughing fracture friction plastic deformation + + Numerical model: produce a finite element model reproducing the loading history for extraction of field variables

9 9 Scratch test using FE: Modeling primaries Verification carried out by checking the scratch geometry in both experiments & modeling 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 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.

10 10 DLC Coatings a-c:h ta-c Hardness 25 GPa 67 GPa Young s modulus 212 GPa 352 GPa

11 11 1 st principal stress in a-c:h and ta-c in sliding contact 0.3 µm 1.0 μm more uniform and higher stress ta-c a-c:h stress state very uniform throughout the coating higher load bearing capacity

12 12 Effect of coating thickness on stresses 0.6 µm ta-c after 1.2 mm slidig 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. Surface 0.3 µm below the surface Interface

13 13 Effect of coating thickness on stresses 0.3, 0.6 and 1 µm thick a-c:h after 1.2 mm slidig Surface Surface Surface a-c:h 0.3 µm at interface a-c:h 0.6 µm a-c:h 1 µm 0.3 µm below surface 0.3 µm below surface at interface 0.6 µm below surface

14 14 Effect of coating thickness on the stress behaviour of ta-c and a-c:h coatings ta-c a-c:h 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.

15 15 VTT MODELLING APPROACH TO WEAR CONTROL

16 16 INTEGRATED MATERIAL MODELLING FOR DEMANDING APPLICATIONS mm FEM CFE Resid. stress CFE PoD FEM μm Raman CFE,MDS Scratch test FEM nm Å MDS In-situ TEM scratch MDS Nano indent MDS AFM MDS FEM = Finite Element Method CFE = Constrain Free Energy MDS = Molecular Dynamic Simulation = model validation methods fs ps ns μs ms s h

17 17 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: Al 2 O 3 -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

18 18 Steel and Al2O3 sliding against DLC 1,00E 06 Steel pin wear against DLC ,00E 06 Al 2 O 3 pin wear rate against DLC Cr6 Pin Wear Rate, K [mm3/nm] 1,00E 07 1,00E 08 1,00E 09 1,00E 10 1,00E 11 B D O U *) 2012 Al 2 O 3 Pin Wear Rate, K [mm3/nm] 1,00E 07 1,00E 08 1,00E 09 1,00E 10 1,00E *) B D O U 1,00E 06 DLC wear rate against 100Cr ,00E 06 DLC wear rate against Al 2 O DLC (Si) Disc Wear Rate, K [mm3/nm] 1,00E 07 1,00E 08 1,00E 09 1,00E *) B D O U DLC (Si) Disc Wear Rate, K [mm3/nm] 1,00E 07 1,00E 08 1,00E 09 1,00E B D O U *) No measurable wear

19 19 13/02/2013 Friction performance of DLC coatings Al2O3/DLC(Si) 0,40 0,40 0,35 0,35 Friction Coefficient, µ Friction Coefficient, µ 100Cr6 against DLC(Si) 0,30 0,25 0,20 0,15 0,10 0,25 0,20 0,15 0,10 0,05 0,00 0,00 D O U Friction performance of a-c:h in repeated tests 2012 µ ,30 0,05 B µ 1992 B D 0,5 Friction coefficient Friction coefficient 0,3 0,2 0,1 100cr6/B DLC(Si) Time [h] 4 0,4 0,4 0,3 0,3 0,2 0,2 0,1 0,1 100Cr6/U DLC(Si) 0,0 6 Al2O3/B DLC(Si) Friction coefficient 0,5 0,4 U Friction performance of a-c:h in repeated tests 2012 Friction performance of ta-c in repeated tests ,5 O Time [h] [h] 4 Time 5 6

20 20 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. Liquid quenching 0ps ~5 fs ~100 fs ~300 fs ~1 ps sp2 & sp3 site plot

21 21 Example MD analysis results 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)

22 22 Indenting a-c:h film withdiamond Tip

23 23 Indenting ta-c films with a Diamond Tip Stress Generation

24 24 Indenting ta-c films with a Diamond Tip Stress Generation Compressive stresses are generated under the contact area (blue colour).

25 25 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.

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