Microstructural investigations of CVD TiN/ κ-Al 2O 3 multilayer coatings on cemented carbides

Abstract

Multilayer coatings of TiN and κ-Al 2O 3 have been examined by x-ray diffractometry, scanning electron microscopy and analytical transmission electron microscopy. The general microstructure of the coatings, such as grain size, grain shape, twinning, dislocations, pores and interfaces was investigated and, based on these findings, conclusions about the detailed structure of κ-Al 2O 3 were made. A model is presented for the domain formation in CVD κ-Al 2O 3. The height of the columnar, twinned κ-grains was of the order of the κ-layer thickness. The preferred growth direction for κ-Al 2O 3, was along the c-axis. The grain morphology is very different for TiN deposited on to cemented carbide as compared to TiN deposited on to κ-Al 2O 3; columnar grains with an aspect ratio of about 5 and equiaxed grains, respectively. No pores or dislocations could be found within the TiN and κ-Al 2O 3 layers. However, κ-Al 2O 3/TiN interfaces below the TiN-layers exhibit a large number of pores. The deposition rate of TiN was two-three times higher for TiN deposited on cemented carbide than for TiN deposited on to κ-Al 2O 3. Epitaxy was frequently found both for TiN on κ-Al 2O 3 and for κ-Al 2O 3 on TiN. Three twin-related κ-domains grew epitaxially on two twin-related TiN domains which grew epitaxially on three twin-related κ-domains. The orientation relationships could be described as: [Display omitted] The observed twinning of TiN on κ-Al 2O 3 indicates that the (001)-planes in κ-Al 2O 3 are close-packed in an ..ABAC.. stacking sequence. There are two types of geometrical relationships of neighbouring domains in CVD κ-Al 2O 3: (i) twinrelated domains due to different positions of the aluminium ions within the oxygen-ion sub lattice and; (ii) translation-related domains due to different stacking sequences of the oxygen-ion planes.