Environmental Dependence of Tribological Behavior of DLC Films Se-Jun Park and Kwang-Ryeol Lee Future Technology Research Division Korea Institute of Science and Technology AEPSE 2003, Jeju,

Properties of Solid Carbon PropertyDiamondDLCGraphite Density (g/cm 3 ) – Atomic Number Density (Mole/cm 3 ) – Hardness (Kgf/mm 2 ) <500 Friction Coeff – 0.2 Refractive Index – – 1.8 TransparencyUV-VIS-IRVIS-IROpaque Resistivity (  cm) > – 0.4

Tribological Properties of Hard Coatings DLC WC TiN CrN TiCN Wear RateFriction Coefficient Relative value

Applications of DLC Film

Problems of DLC Films Thermal Instability –Degradation at High Temperature (400 – 600 o C) High Residual Compressive Stress –Max. 10 GPa Poor Adhesion –Stable Chemical Bonds –Especially on Ferrous Materials Environmental Dependence of the Tribological Properties

Humidity Dependence of the Tribological Behavior R. Gilmore et al Surf. Coat. Technol , (2000), 437

Tribochemical Reaction The environmental dependence strongly implies that the tribochemical reaction between the test environment, the film and the counterface materials are significant. DLC

Purposes of The Present Work To investigate systematically the friction behavior of DLC film in various test environments including relative humidities. To find the reason for the humidity dependence of the tribological behavior of DLC film in the point of tribochemical reaction. What happens in Si incorporated DLC films?

Film Deposition Condition RF PACVD(13.56 MHz) Precursor Gas : C 6 H 6, C 6 H 6 + SiH 4, Deposition Pressure : 1.33 Pa Bias Voltage : V b Substrate : P-type (100) Si-wafer Film thickness : 1 ㎛ Si concentration : 2 at.%

Friction Test Ball : AISI steel ball Normal Load : 4 N Sliding Speed : 20 cm/s Temperature : Room temperature Environmental Gas : Ambient atmosphere (relative humidity : 0 – 90 %) High pure oxygen

Humidity Dependence of Friction Pure DLC Si-DLC

Scar Surface with Humidity Variation RH 0 %RH 50 %RH 90 % 250 ㎛ 100 ㎛

Raman Spectra of the Transfer Layer

Chemical Composition of Debris

Wear Rate of Track and Ball Track Ball

(e) 250 ㎛ (a) 250 ㎛ (c) 2 ㎛ (b) 2 ㎛ (d) 2 ㎛ (f) 250 ㎛ (e) RH=0%RH=50%RH=90%

a-C:H Fe Fe-O Fe Fe-O Fe Fe-O

Debris Composition & Friction M. G. Kim et al., Surf. Coat. Tech. 112, 204 (1999).

Initial Run-in Behavior M.-G. Kim et al, MRS Proc. Vol. 498, 177 (1998).

In Dry Oxygen Environment

(a) 2 ㎛ (b) 2 ㎛

in Dry Oxygen Environment a-C:H Al 2 O 3

The Environmental Dependence The increased friction coefficient in humid environment is closely related with the increased Fe concentration in the debris due to the enhanced surface oxidation of the steel ball. The humidity dependence is not an inherent property of the DLC films. 1.The Fe rich debris itself degrades the lubricating property of the DLC film. 2.The Fe rich debris enhance the agglomeration of small debris into larger one that requires larger energy dissipation to be deformed during sliding 1.The Fe rich debris itself degrades the lubricating property of the DLC film. 2.The Fe rich debris enhance the agglomeration of small debris into larger one that requires larger energy dissipation to be deformed during sliding

Chemical Composition of Debris with Humidity Change Si-DLC DLC Wear Rate of the Ball

0 %50 % 90 % 3 ㎛ Si-DLC DLC

Raman Spectra of the Transfer Layer Si-DLCDLC

Scar Surface with Humidity Variation 100 ㎛ RH 0 %RH 50 %RH 90 % 250 ㎛

Friction in Dry Oxygen Environment

3 ㎛ DLC Si-DLC 3 ㎛

Wear Rate of Ball and Track in O 2 Env.

Raman Spectra of the Transfer Layer

The Environmental Dependence Fe rich debris formed by oxidation of the steel ball increased the friction coefficient in humid environment. Less dependent on the test environment –Bond structure of the debris varied with relative humidity. Diamond-like structure in humid environment suppress the agglomeration of the debris.

Conclusions Humidity dependence of the friction behavior of DLC film is not an inherent property of the DLC film. Humidity dependence should be understood in terms of the tribochemical reaction of the tribo-system. Two major factors were suggested. –Fe concentration in the debris : Conterface Materials –Debris agglomeration : Chemical bond of the debris

Acknowledgement Discussion with Dr. H-S. Kong, Dr. E-S. Yoon & Dr. J-K. Kim. Financial Support –Center for Nanostructured Materials Technology –Center of Advanced Plasma Surface Engineering –J&L Tech. Co., Ltd.