X-ray photoemission electron microscopy observation of wear tracks on hydrogenated amorphous carbon films after ball-on-disk tests

Hydrogenated amorphous carbon (a-C:H) films are applying sliding surface as solid lubricant because of their low frictional coefficient and high hardness. The structure of the film changes locally under non-uniform loading, and investigating changes in the distribution of carbon bonding states within the sliding region is particularly important. In this study, changes in the sp2 and sp3 bonding distribution of carbon due to sliding with SUJ2 ball were studied through the X-ray photoelectron emission microscopy (X-PEEM) analysis, which clarified how the carbon bonding structure of the a-C:H film surface changes by sliding. The a-C:H films deposited on Si from C2H2 and CH4 by CVD were evaluated by ball on disk test using SUJ2 ball. The number of emitted photoelectrons is equivalent to the amount of absorbed X-ray, and spectra equivalent to near-edge X-ray absorption fine structure (NEXAFS) spectra can be obtained when X-PEEM images are obtained while the irradiation X-ray energy continuously varies. X-rays near the CK edge at 275–330 eV was used to X-PEEM observation. Acquiring the spectra of each part of the wear track based on the contrast of these X-PEEM images, the sp2 or sp3carbon bonding distribution in the wear track was investigated. The wear track of the center of ball contact area and the side part of 40 μm from the center were observed by X-PEEM, the sp2 / (sp2 + sp3) relative ratio of the a-C:H film surface decreased at the center of the wear track. The abundance of carbon sp3 bonds on the wear track surface at the sliding center increased compared with that in other regions, and CC bonds became dominant. This change suggested that the structural changes are due to local differences in loading and heat or wear rate at different locations.