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

Two kinds of pulsed d.c. plasma assisted chemical vapour deposition (PACVD) processes (unipolar and bipolar excitation) were studied to deposit crysta lline Al2O3 coatings on steel or WC/Co cemented carbide inserts using alumi nium trichloride (AlCl3), nitrous oxide (N2O) or oxygen (O-2) and hydrogen/ argon gas mixtures. At substrate temperatures as low as 650-700 degrees C a nd a gas mixture ratio of 1 for N2O/AlCl3, coatings with different contents of alpha-Al2O3 and gamma-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 b een determined qualitatively by X-ray diffraction. The dark grey or transpa rent coatings have a rough, less well crystallized surface in comparison to surfaces known from the thermal CVD process. The coating thickness differe d significantly between the centre and the edges of the samples. The crysta llite sizes were estimated to be 9 and 20 nm for the alpha-Al2O3 and even s maller for gamma-Al2O3 containing coatings. By increasing the substrate tem perature to 800 degrees C, the content of alpha-Al2O3 in the coatings was i ncreased. Lower substrate temperatures or conditions with a deficiency of o xygen in the gas mixture lead to the formation of aluminium or aluminium ni tride. Crystalline alpha-Al2O3 or very smooth, transparent gamma-Al2O3 were obtain ed at 700 degrees C dependent on the gas mixture ratio using O-2/AlCl3 as p recursors and bipolar pulsed d.c. plasma excitation. A clear trend becomes apparent for the formation of gamma-A(2)O(3) 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 t o be in the range 20-24 GPa. The wear properties of some smooth, transparent gamma-Al2O3 PACVD coatings were evaluated in a machine cutting test using ball bearing steel and compa red with thermal CVD alumina coatings. It could be concluded that the bipol ar pulsed PACVD-Al2O3 coatings showed very similar wear properties to therm al CVD coatings. (C) 1999 Elsevier Science S.A. All rights reserved.