THE MICROSTRUCTURE OF BONDING LAYERS FOR CVD ALUMINA COATINGS

The phase content (alpha/kappa) in CVD alumina layers can be affected by their deposition upon different intermediate layers called bonding layers. In order to examine whether the nucleation of the alumina polymorphs is governed by the microstructure of the bonding layers, the crystal structures and lattice parameters of alpha- and kappa-bonding layers have been determined. Thin foils from CVD TiC-bonding-Al2O3 coated cemented carbides have been examined by transmission electron microscopy. The kappa-bonding layer was 200 nm thick and consisted of grains in the range 20-100 nm, while the alpha-bonding layer was 400 nm thick with a grain size in the range 50-300 nm. Both the alpha-bonding/Al2O3 and kappa-bonding/Al2O3 layer interfaces were free from pores and cracks. The crystal structure of both the kappa- and the alpha-bonding layer was found to be face centred cubic. Within the accuracy of the convergent beam electron diffraction technique used in this work, the lattice parameters for the kappa- and alpha-bonding layer were found to be equal, a = 4.3 Angstrom, Both the alpha- and the kappa-bonding layer grains consisted of Ti and Al. W and Co were present in both TiC/bonding and bonding/bonding grain boundaries. However, the interfaces between the alpha-bonding layer/alpha-Al2O3 and the kappa-bonding layer/kappa-Al2O3 were free from W and Co. The crystal structures and chemical compositions of the examined bonding layers are similar, indicating that other factors, e.g. crystal facets, may be of greater importance for the nucleation of the alumina polymorphs.