Comparative characterization of alumina coatings deposited by RF, DC and pulsed reactive magnetron sputtering

In order to investigate the structure of PVD alumina coatings as a function of substrate temperature and deposition conditions, Al2O3 films have been deposited by reactive magnetron sputtering at deposition temperatures between 100 and 600 degrees C on cemented carbide and high speed steel substrates using aluminum targets, which were sputtered in an argon-oxygen plasma in RF, DC and pulsed mode, respectively. The influence of deposition temperature and sputtering conditions upon the crystal structure of the films has been investigated by X-ray diffraction, the hardness by microindentation. Furthermore, a correlation between film growth rate and oxygen partial pressure has been determined. The analyses revealed that the oxygen partial pressure during film deposition had strong influence on the resulting deposition rate and crystallinity, which is also significantly influenced by the deposition temperature. The process window for the deposition of crystalline coatings is very narrow for DC sputtering, while it is larger with RF and pulsed plasma sources. Depending on the process parameters oxygen partial pressure and plasma source, amorphous or crystalline gamma-Al2O3 films with thicknesses in the range from 1 to 8 mu m and hardness values up to 25 GPa have been deposited. Very low oxygen partial pressures led to the co-deposition of aluminum, while none of the chosen process parameters resulted in the formation of crystalline alpha-Al2O3. (C) 1999 Elsevier Science S.A. All rights reserved.