MICROSTRUCTURAL INVESTIGATION OF CVD ALPHA-AL2O3/KAPPA-AL2O3 MULTILAYER COATINGS

Chemically vapour deposited multilayer kappa-Al2O3 and multilayer alpha-Al2O3 coatings have been characterized by transmission electron microscopy. Cross-section thin foils were prepared from alumina-coated cemented carbide cutting tool inserts. Special emphasis has been given to the examination of the alpha and kappa polymorphs in the different layers. The thickness of each alumina layer was 1 mum. Grain refinement was obtained by the renucleation of Al2O3 on each modification layer. The microstructure of the alpha-Al2O3 layers consisted of equiaxed grains with a large number of dislocations and pores, while the kappa-Al2O3 layers consisted of columnar twinned grains which were dislocation and pore free. In all layers, the alpha-Al2O3 grains were randomly distributed, while the preferred growth direction Of kappa-Al2O3 was along the c axis. Observation of the growth of alpha-Al2O3 on both alpha- and kappa-bonding layers indicates that the growth of this polymorph is favoured by a needle-shaped bonding layer. In both the alpha and kappa multilayers, the occurrence of alpha-Al2O3 and kappa-Al2O3 grains side by side in the first alumina layer was directly related to the morphology of the bonding layer. No cracks or defects could be found in these alpha-kappa interfaces.