Humidity Dependence of Tribological Behavior of DLC Film Se Jun Park *#, Kwang-Ryeol Lee *, Seung-Cheol Lee * and Dae-Hong Ko # * Korea Institute Science and Technology # Yonsei University

Tribological Properties of Hard Coating Materials DLC WC TiN CrN TiCN Wear rateFriction coefficient Relative value

Dependence of Tribological Behavior of DLC Films on Humidity It is important to understand the tribological behavior of DLC films in humid environment R. Gilmore et al Surf. Coat. Technol , (2000), 437

Previous Works Surface smoothness of DLC film by chemisorbed oxygen The increase of real contact area The prevention of the growth of the carbonaceous transfer film by the water vapor C. Donnet et al., Tribo. Letters, 4 (1998) 259 B. Marchon et al, IEEE Trans. Magn. 26 (1990) 2670

Tribology Process of DLC Film DLC Counterface Wear Reaction with environment Reaction with environment Material transfer Transfer layer F FNFN Friction Coefficient = Tribo-chemical reaction

Purposes of Present Work To characterize the tribological behavior of DLC film with the humidity change in various tribo-system. DLC against steel ball DLC against DLC coated ball To find the reason for the humidity dependence of frictional behavior of DLC film in view of tribo-chemical reaction.

Deposition Condition RF PACVD(13.56 MHz) Precursor Gas : C 6 H 6 Deposition Pressure : 1.33 Pa Bias Voltage : V b Substrate : P-type (100) Si-wafer Film Thickness : 1 Residual Stress : 0.9 ± 0.02 Gpa Hardness : 10.5 ± 0.5 GPa

Tribo-test Condition Ball-on-disc type tribometer isolated by chamber Ball : AISI steel ball DLC coated steel ball Normal Load : 4 N Rotation Speed : 220 rpm Temperature : Room temperature Environmental Gas : Ambient atmosphere (Relative humidity : 0 – 90 %) Film Hygrometer Normal Load Loadcell Humidity controller Rotary Pump

Friction Coefficient with Humidity Change Using Steel ball

Images of Transfer Layer on Scar RH 0 % 100 RH 50 % 100 RH 90 % 100

Morphology of Debris on Track RH 0 % RH 50 %RH 90 % Relative humidity Size of debris Friction coefficient

Chemical Composition of Debris

Kim et al, Surf. Coat. Techno. 112 (1999) 204

Chemical Composition of Debris Wear of steel ball Friction coefficient Fe concentration in debris

Friction Behavior with Humidity RH : 50 % RH : 0 % RH : 90 % Steel DLC Steel DLC C-O-Fe C-O Steel DLC Fe-O low oxidation of ball high oxidation of ball Friction coefficient Relative Humidity Formation of Fe-rich debris Increase of the debris size

Motivations Steel DLC If the formation of Fe-rich debris was suppressed ?

Friction Coefficient with Humidity Change

Chemical Composition of Debris

Micrographs of Debris on Track with Humidity Change 0 %50 %90 % 3 3 3

Friction Coefficient with Humidity Change

Raman Spectra of Transfer Layer Steel ball DLC coated ball

Schematic Diagram of Friction Behavior in high humidity Increase of debris size Formation of Fe-rich Debris Formation of graphitic layer highly sensitive to humidity Degradation of lubricant properties of DLC film C-O-Fe Formation of C-O Debris The diamond-like bonding structure of the transfer layer Steel DLC Steel DLC

The Role of Fe in Debris Steel Fe-rich debris Steel Fe-rich debris RH : 90 % RH : 0 % Formation of graphitic layer highly sensitive to humidity Degradation of lubricant properties of DLC film Abrupt change of humidity If the Fe in debris degraded the lubricant properties of DLC film The friction coefficient slowly dropped though the humidity abruptly drop

Friction Coefficient with Humidity The effect of Fe in debris on the friction behavior The formation of the graphitic transfer layer highly sensitive to humidity` The immediate drop of friction coefficient RH : 90 %RH : 0 %

Conclusions Humidity dependence of the friction behavior of DLC film is not an inherent property of the DLC film The graphitic layer itself was very sensitive to humidity in case of using steel ball Fe element in debris enhanced the graphitization of the transfer layer Tribological behavior of DLC film was highly affected by the reaction between the counterface and humidity Especially using steel ball, Fe-rich debris formed by the reaction of steel ball and humidity affected the bond structure of the transfer layer