Tribological properties of partly polished diamond coatings

Extremely low friction coefficient was achieved with "partly polished diamond coatings". Diamond coatings were deposited onto Si substrates by MWCVD with the mixture of CH₄ and H₂. Deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and Electron Spectroscopy for Chemical Analysis (ESCA). Sharp peak derived from polycrystalline diamond was observed by XRD. Whereas Raman profile of partly polished diamond coatings was close to that of ta-C. This result suggests that small diamond grains were surrounded by amorphous carbon structure in the diamond coatings. Deposited diamond coating was polished with each other. Surface roughness R_a was reduced to 0.3, 0.2 and 0.08 μm, respectively. The hardness of the polished diamond coatings investigated by Nanoindentation technique was approximately 40.8 GPa, which was relatively lower value compared with conventional as-deposited CVD diamond coatings. For the tribological properties, we examined the effect of surface roughness using flat-ended pin-on-disk apparatus and ball-on-disk apparatus with bearing ball (SUJ2) and stainless steel (SUS304). Diamond coatings were deposited onto flat-ended pin and disk, and they were polished to R_a = 0.3, 0.2 and 0.08 μm. After the 6000 cycle process extremely low friction coefficient, μ = 0.05, was achieved with the pair of R_a (flat-ended pin, disk) = R_a (0.08, 0.3) in flat-ended pin-on-disk apparatus. In order to clarify the effect of surface roughness, ball-on-disk was carried out with different surface roughness, R_a = 1.7, 0.3, 0.2 and 0.08 μm. Here as-deposited diamond coating, R_a = 1.7 μm, was used as a reference point. Friction coefficient of μ = 0.09 was obtained for both balls. After the tribological tests balls were analyzed by scanning electron microscope (SEM) and energy dispersed X-ray spectrometer (EDX).