INFLUENCE OF CH4 AND AR ON THE MORPHOLOGIES OF AL2O3 - CVD COATINGS

The influence of CH4 and Ar addition to the AlCl3/CO2/H2 system on the morphology and grain size of Al2O3 CVD deposition was examined. The Al2O3 growth rate decreased at a CH4 concentration of 10 vol%. High concentrations of CH4 caused fine grained Al2O3 crystals but these coatings were porous and consisted of branched crystals. High Ar additions of more than 27 mol% led to the formation of more monolytic Al2O3 coatings. The simultaneous addition of CH4/Ar mixtures during deposition allowed the formation of uniform, compact and extremely fine grained Al2O3 coatings. The adhesion of the Al2O3 coatings was measured by a simple Rockwell indentation test. The grain refinement is explained by a carbon codeposition resulting from methane pyrolysis.