Deposition of hard crystalline Al2O3 coatings by pulsed d.c. PACVD

Abstract

Two kinds of pulsed d.c. plasma assisted chemical vapour deposition (PACVD) processes (unipolar and bipolar excitation) were studied to deposit crystalline Al2O3 coatings on steel or WC/Co cemented carbide inserts using aluminium trichloride (AlCl3), nitrous oxide (N2O) or oxygen (O2) and hydrogen/argon gas mixtures. At substrate temperatures as low as 650–700°C and a gas mixture ratio of 1 for N2O/AlCl3, coatings with different contents of α-Al2O3 and γ-Al2O3 besides amorphous alumina could be obtained under unipolar plasma excitation conditions. The ratio of the crystalline phases depends on plasma power density and substrate temperature, and has been determined qualitatively by X-ray diffraction. The dark grey or transparent coatings have a rough, less well crystallized surface in comparison to surfaces known from the thermal CVD process. The coating thickness differed significantly between the centre and the edges of the samples. The crystallite sizes were estimated to be 9 and 20 nm for the α-Al2O3 and even smaller for γ-Al2O3 containing coatings. By increasing the substrate temperature to 800°C, the content of α-Al2O3 in the coatings was increased. Lower substrate temperatures or conditions with a deficiency of oxygen in the gas mixture lead to the formation of aluminium or aluminium nitride.Crystalline α-Al2O3 or very smooth, transparent γ-Al2O3 were obtained at 700°C dependent on the gas mixture ratio using O2/AlCl3 as precursors and bipolar pulsed d.c. plasma excitation. A clear trend becomes apparent for the formation of γ-A2O3 and amorphous alumina films if the substrate temperature is lowered and the O/AlCl3 gas ratio is changed appropriately. The hardness of the smooth, transparent coatings are found to be in the range 20–24 GPa.The wear properties of some smooth, transparent γ-Al2O3 PACVD coatings were evaluated in a machine cutting test using ball bearing steel and compared with thermal CVD alumina coatings. It could be concluded that the bipolar pulsed PACVD-Al2O3 coatings showed very similar wear properties to thermal CVD coatings.