Deposition of α-Al2O3 hard coatings by reactive magnetron sputtering

Crystalline α-Al2O3 thin films have been deposited using reactive magnetron sputtering at the deposition rate of 0.5–0.7 μm/h above 700 °C in a commercial production scale PVD system. In the experiments, using Si wafers and CrN-coated cemented carbides as substrates, an oxidization pre-treatment was done, and then deposition of Al2O3 films was carried out by varying the process temperature and the substrate bias voltage. An investigation of the correlation between characteristics of these films and deposition conditions was done. XRD study revealed that the under-layer of the Al2O3 films and the substrate temperature for deposition remarkably influenced the film structure. Although only γ-Al2O3 was observed in the films on the Si wafer, Al2O3 film mostly consisting of α-phase was obtained on the CrN under-layer at 700 °C, and pure α-Al2O3 film was deposited at 750 °C without applying bias voltage. TEM observation demonstrated that the α-Cr2O3 interlayer formed by the oxidization pre-treatment of the CrN film played an important role in forming α-Al2O3. The hardness of the α-Al2O3 film measured by nanoindentation was 23 GPa, and was enhanced to 27 GPa by applying bias.