Thermal stability of silicon-doped Al2O3 PVD coatings

Crystalline alumina PVD-coatings offer high potential for different applications in which high chemical inertness, high hot hardness and high oxidation resistance is important. Especially the metastable -phase is topic of many researches because, in comparison to the stable -phase, it can be deposited by means of MSIP (Magnetron Sputter Ion Plating) at relatively low process temperatures below 650 degrees C and it is more fine-grained than -Al2O3. At high temperatures -Al2O3 transforms into -Al2O3, which limits the application temperature. But until now it is not clearly understood, up to which temperatures -Al2O3 coatings are stable and which mechanisms influence the stability. In this paper the influence of doping with silicon is investigated. DSC (differential scanning calorimetry) as well as XRD (X-Ray diffractometry) measurements show that adding 4 at% silicon leads to a high amount of amorphous phase in the as deposited coating and a decrease of hardness from 18 GPa to 10 GPa. Nevertheless, formation of -phase is retarded to temperatures above 1200 degrees C while -phase is formed at 1100 degrees C for the undoped coating. A transformation via the -phase was detected for the Si-doped coating, which is not seen for undoped coating.