1. 1 Friction behaviour of diamond-like carbon films with varying mechanical properties The International Conference on Metallurgical Coatings and Thin Films – ICMCTF 2006 May 1-5, 2006, San Diego, California, USA May 1-5, 2006 R. Arvind Singh 1, Eui-Sung Yoon 1, Hong Joon Kim 1, Hosung Kong 1, Se-Jun Park 2 and Kwang-Ryeol Lee 2 1 Tribology Research Center, Korea Institute of Science and Technology, Seoul, Korea 2 Future Technology Research Division, Korea Institute of Science and Technology, Seoul, Korea

2. Outline Introduction (background, Objectives) Experimental Details and Specimens – – Test specimens and test conditions – – Micro-tribo tester Micro-scale friction behaviour – – Friction behaviour of uncoated Si-wafer – – Friction behaviour of DLC films – – Effect of contact area on friction at micro-scale – – Wear at micro-scale – – Summary Conclusions

3. Background (Micro/Nano Tribology) Micro Tribology Nano Tribology Information storage device Areal density Flying height Contact Information storage device Areal density Flying height Contact Flying height (nm) Year MEMS elements in relative motion Lubricating problems (Stiction) MEMS elements in relative motion Lubricating problems (Stiction) Multi gear speed reduction unit Ultimate goal: Reliable motion and durability increase

4. Topographic modification Surface roughening - Asperity contact radius Surface texturing - Shape of surface Decrease real contact area Increase distance between surfaces Chemical modification Thin film Coating - Fluorocarbon film - Diamond-like carbon film Langmuir Blodgett film Self-assembled mono-layers Convert the surface to be hydrophobic Decrease friction & wear Research Trend

5. Objectives Diamond-like carbon films with varying thickness coated on Silicon wafer Influence of Elastic modulus on friction at micro-scale (Effect of contact area on micro-scale friction property) Elastic modulus of films varies with film thickness

6. Test conditions Micro friction: Micro-tribo tester Normal load : 1500 to 4800  N Stroke : 3 mm Sliding speed : 1 mm/sec (averaging over 3 times) Temperature : 25  1  C Relative humidity : 45  5 % Test specimens & Test conditions Properties of ball and test specimens Test specimens – DLC Films with varying thickness DLC - radio frequency plasma-assisted chemical vapor deposition method (r.f.PACVD), C 6 H 6 MaterialElastic Modulus (GPa) Poisson ’ s Ratio Hardness (GPa) Soda Lime Glass680.16- DLC – 100 nm400.36-8 DLC – 500 nm850.311-12 DLC – 1000 nm880.311-12

7. Micro Tribotester Micro-tribometer 1.25  m Reciprocating Type Ball-on-flat configuration Counterface sliders - Counterface sliders - Soda Lime glass balls (Radii : 0.25 mm, 0.5 mm and 1 mm)

8. Elastic Modulus with film thickness Elastic Modulus of the DLC films increases with their film thickness Biaxial elastic modulus of very thin diamond-like carbon (DLC) films J.W. Chung, C.S. Lee, D.H. Ko, J.H. Han, K.Y. Eun and K.R. Lee, Diamond and Related Materials 10 (2001) 2069-2074

9. Micro-friction behaviour of uncoated Si-wafer Silicon Wafer Ball Size Radius : 0.5 mm Load: 3000  N Wear Debris Solid-Solid Adhesion The effect of contact area on nano/micro-scale friction E.S. Yoon, R.A. Singh, H.J. Oh and H. Kong, Wear 259 (2005) 1424-1431

10. Coefficient of friction at micro-scale Coefficient of Friction : Ratio of Friction force to Applied normal load Silicon Wafer Ball Size DLC – 100 nm Ball Size Friction increases with applied normal load and ball size

11. Coefficient of friction at micro-scale DLC-1000 nm DLC-500 nm Ball Size Friction inversely related to the ball size

12. Total Friction F = Fa + Fp Friction behaviour Adhesive component Plowing component F a =  A r  is the shear strength A r the real area of contact F p = d 3 P/12R d is the track width P the mean pressure required to displace the material R the radius of curvature of the slider

13. Effect of Contact area on micro-friction DLC – Hertzian Theory A r =  [RF n /K ] 2/3 R: Tip size K: Effective elastic modulus F n : Applied normal load DLC-100 nm: Lager contact area – Friction dominated by adhesive component DLC-500 nm & 1000 nm: Smaller contact area – Friction dominated by plowing component At all applied normal loads and ball sizes the 100 nm has higher contact area than the 500 nm and 1000 nm films due to lower elastic modulus Ball radius 0.5 mmApplied normal load 3000  N

14. DLC surfaces after sliding Radius : 0.25 mm Load: 3000  N Radius : 1 mm Load: 4800  N DLC-100 nm DLC-1000 nm

15. Summary Elastic modulus increases with film thickness Higher elastic modulus Smaller contact area Plowing component prevails Lower elastic modulus Lager contact area Adhesive component dominates

16. Conclusions 1. 1.Friction property of DLC films with the ball size varies with their film thickness. The coefficient of friction increases with the ball size in 100 nm thick film, whereas it has an inverse relation in the case of 500 nm and 1000 nm thick films. 2. 2.The contact area of the DLC films decreases with the film thickness, owing to the increase in their elastic modulus with the film thickness. 3. 3.The contact area of the DLC films influences their friction behaviour. The 100 nm thick film exhibits adhesive dominant friction, which is due to higher contact area. Thus, its friction property increases with the ball size. The 500 nm and 1000 nm thick films exhibit plowing dominant friction, due to lower contact areas (higher contact pressures). Thus, their friction property has an inverse relation with the ball size.