Failure mechanisms of diamond like carbon coatings characterised by in situ SEM scratch testing

In situ SEM scratch testing of DLC and Si-doped DLC deposited on Si wafers has been conducted using sharp 1 and 5 μm radii diamond asperities, enabling the stages of deformation and wear of DLC coatings to be evaluated in real time. With increasing load, initial plastic deformation and tensile cracking in the scratch track progresses to the propagation of radial and lateral cracks, and full coating spallation. In situ SEM imaging reveals nucleation of radial cracks in the DLC coating around the front and side of the moving asperity, followed by lateral crack propagation both ahead of, and behind, the asperity contact zone. Post-mortem FIB cross sectioning reveals microcracking and lateral cracks in the silicon substrate below DLC coatings prior to coating spallation. The DLC failure mechanisms are influenced by asperity geometry, with notable DLC coating lift up/delamination events occurring during the smaller 1 μm radius asperity scratch tests. The sharper 1 μm radius asperity required ∼20% of the applied load, and higher contact pressure, to initiate spallation during scratching compared the larger 5 μm asperity, indicating that smaller radii asperities are significantly more likely to cause DLC coating spallation, although the spallations they generate were observed to be, on average, smaller