Investigating atomic structure of thin carbon film under mechanical stress and frictional heat generation

Thin carbon film has been widely used as a surface protective coating for many engineering and scientific devices due to its chemical and mechanical stability. When a carbon film experiences high speed sliding contact, its material properties can be degraded by mechanical contact stress and temperature rise by friction, which thus can lead to system malfunction or failure. In this study, thin amorphous carbon film applied onto a head slider surface of hard disk drive (HDD) carried out high speed sliding contact with a single asperity defect and a bulk surface of a rotating magnetic disk. From the post-experiment analysis, it was found that the contact with a single asperity defect produced a deep scratch on the head carbon film, while the bulk surface contact generated burnishing wear on the film. After the two different types of contact test, the atomic structure of head carbon film was investigated using micro-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was observed that the carbon atoms in the area of deep scratch and burnishing wear clearly showed the increasing of sp² bonds or clusters. This implies that the head carbon film under high speed sliding contact would have experienced the graphitization process (i.e., material softening) by contact stress and frictional temperature rise. Next, the change in mechanical strength of carbon film was evaluated using the nanoscratch experiment. From the measured scratch width and depth values, it could be found that the more burnished carbon film showed the wider and deeper scratch due to the softer mechanical strength, which was consistent with the micro-Raman and XPS measurements.