MICROSTRUCTURAL STUDY OF THE KAPPA-AL2O3 -]ALPHA-AL2O3 TRANSFORMATION IN CVD KAPPA-AL2O3

The microstructural variations occurring during the kappa --> alpha transformation in CVD kappa-Al2O3 coatings subjected to isothermal heat treatments have been investigated. The microstructures developed in the partly transformed kappa-Al2O3 layer (32% alpha-Al2O3) and fully transformed kappa-Al2O3 layer (100% alpha-Al2O3) were studied and compared with as-deposited kappa-Al2O3 (100% kappa-Al2O3). The occurrence of extensive cracking in the transformed alumina layer resulting from the volume contraction encountered in the kappa --> alpha transformation was confirmed. No changes could be detected in the surface morphology of the alumina coatings as a result of the kappa --> alpha transformation. The alpha-Al2O3 which formed as a result of the kappa --> alpha transformation exhibited a totally different microstructure from that of the as-deposited kappa-Al2O3. Except for the cracks, the microstructure of alpha-Al2O3 thus formed did not differ significantly from that reported for CVD alpha-Al2O3. The cracks in alpha-Al2O3 were situated mainly at the grain boundaries of as-deposited kappa-Al2O3 and formed a relatively regular crack network. The kappa --> alpha transformation appeared to be initiated at free surfaces; accordingly the phase transformation occurred preferentially along the thermal cracks which were formed in the coating during cooling from the deposition temperature.