Superelastic nitiinterlayers for improving the wear resistance of hard coatings on soft substrates

The tribological performance of hard-coatings such as chromium nitride and diamond-like carbon (DLC) depends intimately on the mechanical properties of the support structure beneath the coating. For soft substrates, poor support characteristics severely degrade hard coating performance. The present work examines the potential for sputtered thin films of superelastic NiTi to improve hard-coating performance on soft substrates by limiting asperity contact stresses and mitigating the effects of compliance and thermal expansion coefficient mismatches between base metal and coating. NiTi thin films were deposited on 6061-T6 aluminum substrates using DC magnetron sputtering. CrN and DLC hard coatings were then deposited by unbalanced reactive magnetron sputter deposition. Reversible stress-induced martensite transformations and mechanical properties of the NiTi films were investigated by temperature-controlled X-ray diffraction and nano-indentation experiments. The tribological properties of C_rN coatings, with and without superelastic NiTi interlayers, were measured by ball-on-disk wear and scratch tests at different temperatures. It is shown that reversible martensite transformations in NiTi interlayers can exert a significant influence on the tribological properties of hard coatings.