INFLUENCE OF ETCHING THE TiC UNDERLAYER WITH CH₄/AlCl₃/H₂ ON THE CVD FORMATION OF KAPPA-Al₂O₃

Abstract
Al₂O₃ coatings deposited by CVD on cemented carbide inserts precoated with TiC did not lead to kappa-Al₂O₃ formation under laboratory conditions. An "etching treatment" with CH₄/AlCl₃/H₂ gas mixtures prior to Al₂O₃ deposition did however promote kappa-Al₂O₃ growth. A pronounced dependence of the amount of kappa-Al₂O₃ on the etching time and on the CH₄ concentration during the etching procedure was observed. A decrease in the deposition temperature increased the amount of the kappa-Al₂O₃. The alpha:kappa-Al₂O₃ ratio was also influenced by the Co content in the cementea carbide inserts. Higher percentages of kappa-Al₂O₃ were observed for inserts with the lower Co concentrations than with the higher Co concentrations. However, no kappa-Al₂O₃ was deposited when CH₄ was added during Al₂O₃ deposition. Only the pure and undoped AlCl₃/CO₂/H₂ system led to the kappa-Al₂O₃ formation.