Influence of Si- and W- doping on micro-scale reciprocating wear and impact performance of DLC coatings on hardened steel

Reciprocating micro-scale sliding tests and micro-scale repetitive impact tests were performed with diamond probes on un-doped, Si-doped and W-doped diamond-like carbon (DLC) coatings on hardened steel with a nanomechanical test instrument. Analytical modelling showed that differences in coating behaviour during sliding contact could be interpreted by differences in the stress distribution that develops. The softer W-doped DLC exhibited the lowest wear resistance in reciprocating sliding. The deformation in the wear track under the test conditions (R = 25 µm, P ≤ 500 mN, total sliding distance = 1 m) was largely controlled by plastic deformation and hence hardness, since micro-scale fatigue wear was only a small contributor. The relationship between friction and wear was more complex, due to the changing influence of surface topography, asperity ploughing and wear with increasing reciprocating sliding cycles. The Si-doped DLC showed the lowest resistance to repetitive impact. The hardest and highest H3/E2 coating, un-doped DLC, was also susceptible to fracture throughout the load range. Although the W-doped DLC was the softest coating studied and had low wear resistance in reciprocating sliding, it was significantly more damage tolerant to repetitive impacting than the other coatings despite its low hardness and low wear resistance in reciprocating tests.