Utilizing bipolar pulsed PACVD for the deposition of alumina hard coatings

In the present investigation, a commercially available PACVD system suited for the deposition of titanium nitride (TiN) was adapted in order to allow alumina (Al 2O 3) deposition up to substrate temperatures of 800 °C. Besides a modification of the gas supply system, an additional heating system inserted into the reactor was needed. Bipolar voltage pulses were applied to the substrate holder whereas the grounded reactor wall served as counter electrode. In order to achieve a uniform discharge across the substrate holder, it was necessary to place an additional grounded counter electrode above the substrate holder. The alumina deposition was investigated on TiN-coated boron-doped silicon (100) substrates and austenitic steel plates placed on the substrate holder. It turned out that the coating thickness decreased strongly with the distance from the central gas inlet. The coatings were analysed by XRD for phase composition, by micro-indentation for hardness, by XPS for chemical composition and by SEM for morphology. At a temperature of 800 °C, only two crystalline modifications of alumina, the α- and the γ-phase, were observed. The γ-phase was stable up to a temperature of 1000 °C.