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

Chemically vapour deposited multilayer kappa-Al2O3 and multilayer alph a-Al2O3 coatings have been characterized by transmission electron micr oscopy. Cross-section thin foils were prepared from alumina-coated cem ented carbide cutting tool inserts. Special emphasis has been given to the examination of the alpha and kappa polymorphs in the different la yers. 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 gra ins with a large number of dislocations and pores, while the kappa-Al2 O3 layers consisted of columnar twinned grains which were dislocation and pore free. In all layers, the alpha-Al2O3 grains were randomly dis tributed, while the preferred growth direction Of kappa-Al2O3 was alon g the c axis. Observation of the growth of alpha-Al2O3 on both alpha- and kappa-bonding layers indicates that the growth of this polymorph i s favoured by a needle-shaped bonding layer. In both the alpha and kap pa multilayers, the occurrence of alpha-Al2O3 and kappa-Al2O3 grains s ide by side in the first alumina layer was directly related to the mor phology of the bonding layer. No cracks or defects could be found in t hese alpha-kappa interfaces.