Formation and properties of alumina coatings
Investigations concerning the microstructure and mechanical properties, composition and chemical bonding 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. Formation of the x-phase was observed at 472°C. At substrate temperatures <472°C evidence for the formation of the amorphous alumina phase was found. Films containing a mixture of K and θ-alumina phases was grown at 430°C. The crystalline film hardness was 22+-1 GPa, which is equivalent to values reported for alumina films deposited by Chemical Vapor Deposition (CVD). 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. Furthermore, a novel, very high rate reactive magnetron sputtering deposition process for alumina hard coatings at substrate temperature ≤250°C has been developed. Utilizing pulsed D.C. power to sputter A1+A1Ox off the target surface and partial pressure control of the reactive gas to maintain a certain partial pressure value (accuracy of better than 0.005 mTorr), fully dense, transparent alumina coatings could be produced at 76% of the metal deposition rate. The coatings have an elastic modulus of,140 GPa, a hardness of 12 GPa, a chemical composition close to stoichiometric, and a refractive index of 1.65.