Evidence for structural changes of amorphous carbon coatings on silicon carbide during tribological tests

During tribological testing of hydrogenated amorphous carbon coatings (a-C:M) on silicon carbide ceramics, a friction coefficient of 0.06 was observed under dry sliding conditions, which is thought to represent a lower boundary value for the given experimental setups. Based on chemical and thermal analyses of the coating material, a structural model of the as-deposited amorphous coating was modified taking simplified quantum chemical and statistical arguments into consideration. The tribological behavior of the coating could be rationalized by this modified structure model: Tribological loading leads to a partial release of internal stresses stored in the coating. This process results in an increase of the local mobility of atoms, which can be seen as the driving force for initial structural rearrangements within the coating. Proof for the occurrence of such rearrangements has been provided by electron energy-loss spectroscopy (EELS) measurements, monitoring the local sp(3)/sp(2) hybridization ratio, of the coating before and after tribological tests.