1. Residual Stress Behavior of DLC Film in Humid Environment Young-Jin Lee a),b), Tae-Young Kim a), Kwang-Ryeol Lee a), In-Sang Yang b) a)Future Technology Research Division, KIST b)Department Of Physics, Ewha Womans Univ.

2. Introduction Low friction and high wear resist Chemical inertness Various application Artificial Knee Joint Artificial Hip Joint High hardness VCR head drum Hard disk

3. Disadvantage T. Ohana at al. Diamond Rel. Mater., 13 (2004) 1500 Environment dependence of DLC film. Tribo test under aqueous condition Tribo test under dry condition S. J. Park at al. Diamond Rel. Mater., 12 (2003) 1517

4. Two Possibilities of Film Delamination Residual stress is changed by water molecules. Interfacial energy is changed by water environment. Film delamination enhanced. Possibility 2. Film delamination enhanced. Possibility 1.

5. Purpose of This Work To investigate the residual stress behavior of various DLC films in humid environment systematically. To find the reason for the humidity dependence of DLC film in the point of residual stress.

6. Stress Measurement In Situ stress measurement : kMOS (Multi-beam Optical Sensing). (k-space Associate, Inc.) Humidity control : 10 ~ 90 % in air (± 5%). step control. Temperature : room temperature(19~26°C). Measurement time : about 600sec for each step. Stress measurement resolution : maximum 4Km in radius (In this system 1MPa)

7. Synthesis of DLC Films r.f. PACVD (13.56 MHz) Precursor Gas : C 6 H 6, CH 4 Deposition Pressure : 1.33 Pa Bias Voltage : -100V ~ -800V Substrate : P-type (100) Si-wafer 525 P-type (100) Si-wafer 200 (5×50 ) Film Thickness : 500

8. Residual Stress & Film Structure C6H6C6H6 CH 4 C 6 H 6 -100V Polymeric film CH 4 –500V Graphitic film CH 4 –150V Diamond-like film

9. Polymeric Film Polymeric Film in Humid Condition R.H. 20% 90% 16MPa Immediate & reversible stress response Compressive residual stress Relative humidity

10. Diamond-like Film Diamond-like Film in Humid Condition R.H. 20% 90% No change Compressive residual stress has no relationship with humidity.

11. Graphitic Film Graphitic Film in Humid Condition R.H. 20% 90% 12MPa Immediate & reversible stress response Compressive residual stress Relative humidity

12. Summary Residual Stress of DLC film has a humidity dependence. This is related with the film structure. Especially in polymeric and graphitic film, the compressive residual stress increase as humidity increases. Diamond-like film is no relationship with humid condition.

13. Effect of Water Molecule F1F1 F1F1 > F2F2 = substrate F1F1 Case 1. Reaction at surface Case 2. Water penetration

14. Thickness Effect in Humid Condition Polymeric DLC film R.H. 20% 90% 400nm film : 27 MPa 500nm film : 16 MPa 1000nm film : 9 MPa

15. Effect of Water Molecule F1F1 F1F1 > F2F2 = substrate F1F1 Case 1. Reaction at surface Case 2. Water penetration

16. Effect of Water Molecule F1F1 F1F1 > substrate Case 1. Reaction at surface : Initial residual stress : added force : film thickness In the case of polymeric DLC film, added surface force is 9.4±1.4 Nm -1

17. Conclusion Residual stress of DLC film has a relationship with humid condition. It is related with the film structure. Especially in polymeric and graphitic DLC film, compressive residual stress increase as the humidity increases. It is possible to enhance the stability of DLC films by structure control. This is due to the reaction between water molecules and film surface. During residual stress is changed, the structure or chemical composition is not changed. Humidity dependence of residual stress could be controled by surface treatment.