High temperature scratch testing of hard PVD coatings deposited on surface treated tool steel.

Scratch testing is one of the most usual characterisation techniques for systems with thin hard coatings. Scratch tests are performed at run at room temperature, due to the risk of degradation of the indenter diamond tip at high temperature and added test complexity. However, results obtained at room temperature may differ from the final system performance in high temperature applications, due to factors such as substrate thermal softening. In this work, a simple and a robust test for the mechanical characterisation of hard PVD coatings at high temperature and open atmosphere has been developed, based on a modification of the scratch test where the diamond tip is replaced by a WC indenter. The test has been used to evaluate the effect of temperature up to 500 °C on commercial PVD AlCrN coating deposited on tool steel subject to different surface treatments. Results show a decrease in the failure load Lc2 with increasing temperature, and a very noticeable effect of substrate nitriding.