Phenomenological evidence for the wear-induced graphitization model of amorphous hydrogenated carbon coatings

Amorphous hydrogenated carbon (a-C:H) coatings have been made experimentally in a modified physical vapor deposition (PVD) process using reactive plasma. One such experimental coating was used to examine the effect of changing some typical pin-on-disc parameters, such as the sliding speed and applied load, on the friction coefficient. The influence of having a coated counterpart material as opposed to an uncoated steel ball was also examined. A ‘transfer’ layer, which was created during wear testing to failure of the experimental coating; and a ‘transformed’ layer, formed at an intermediate stage in the wear process of a commercially sold diamond-like-carbon (DLC) coating were compared with graphite. Both the ‘transfer’ layer and the intermediate ‘transformed’ layer were more graphitic than the original coating, which provided further proof for the wear-induced graphitization model of Liu, Erdemir and Meletis. A combination of techniques was employed to study both the ‘transfer’ layer and the ‘transformed’ coating. Scanning electron microscopy (SEM) was used to identify the ‘transfer’ layer; nano-indentation testing yielded information on the mechanical properties; and Raman spectroscopy proved that the layers had become more graphitic after wear testing.