Phase formation and mechanical properties of alumina coatings prepared at substrate temperatures less than 500°C by ionized and conventional sputtering

Investigations concerning the microstructure and mechanical properties of alumina coatings have been performed. Alumina coatings have been deposited by both ionized reactive magnetron sputtering (IMS) and conventional reactive magnetron sputtering (CMS) in an argon/ oxygen discharge onto stainless-steel coated silicon substrates. X-Ray diffraction (XRD) was used for the phase analysis, and nanoindentation was used to evaluate the mechanical properties. Substrate temperature during deposition was <500°C, which is the technologically interesting temperature range to coat temperature sensitive substrates such as tool steels. The phase transformation temperatures from amorphous to κ-alumina containing films was found to be between 430°C and 472°C, and the transition temperature was lower for higher ion currents. A film containing a mixture of κ- and θ-alumina phases was grown at 430°C. The crystalline film hardness was 22 ± 1 GPa. Films grown at the same temperature by conventional magnetron sputtering were X-ray amorphous, and the hardness was found to be strong function of the substrate temperature.