Deposition of alumina hard coatings by bipolar pulsed PECVD

Pulsed and mid-frequency plasma sources are gaining increased attention for the deposition of ceramic hard coatings by PVD as well as plasma enhanced chemical vapor deposition. Especially bipolar techniques allow the deposition of insulating layers under stable plasma conditions. In this paper, the deposition of alumina films on the molybdenum based superalloy TZM and on steel substrates from gaseous mixtures of AlCl3, O2, H2 and Ar in a bipolar mid frequency glow discharge at temperatures from 500 up to 620 °C is reported. Special attention was paid to the correlation between deposition temperature and power density and film properties. The measurements revealed that the structure as well as the properties of the resulting coatings were significantly influenced by the plasma parameters during deposition. Increasing plasma power densities led to the deposition of films with higher crystallinity and a change in structure from amorphous Al2O3 to γ- and finally α-Al2O3. Depending on the gas composition and plasma parameters, alumina films with relatively good hardness and adhesion were deposited.