Tailoring residual stresses in CrNx films on alumina and silicon deposited by high-power impulse magnetron sputtering

Chromium nitride films, deposited using reactive magnetron sputtering, were optimised for wear resistance. The performance was measured by scratch resistance and optimised by tailoring the residual stresses. The depositions were carried out with either direct current magnetron sputtering (DCMS) or high-power impulse magnetron sputtering (HiPIMS), and with varying substrate bias and nitrogen gas flow. With DCMS, all films remained under tensile stresses and exhibited poor performance in scratch testing. Although the tensile stresses could be reduced by increasing the nitrogen flow, compressive stresses could only be induced when employing HiPIMS. Substrate bias had a strong effect in HiPIMS in contrast to the DCMS. The effect of the substrate bias in HiPIMS can be explained by the high ionisation of the flux of film forming species. In all cases, increased nitrogen flow favoured formation of CrN over Cr2N. All films showed signs of limited adhesion, which was improved using a titanium interlayer. Cracking across the scratch could be completely avoided for films with lower tensile or compressive stresses, the latter also exhibiting the highest critical load. The results show that it is possible to increase the scratch resistance by tailoring the residual stresses, for which HiPIMS proved a very useful tool.