MICROSTRUCTURAL INVESTIGATION OF THE KAPPA-AL2O3-]ALPHA-AL2O3 TRANSFORMATION IN MULTILAYER COATINGS OF CHEMICALLY VAPOR-DEPOSITED KAPPA-AL2O3

The kappa --> alpha transformation in different multilayer coatings of kappa-Al2O3 has been investigated. Air the multilayers had the same total thickness (8 mu m) but a different number of layers (8, 15 and 32). The transformation was studied with respect to (i) rate of transformation, (ii) transformation growth, (iii) morphology and (iv) the adhesion of the different layers. The rate of transformation is highly temperature dependent for all three multilayers and the transformation is 3-4 times faster at 1090 degrees C than at 1030 degrees C. The transformation is faster for 32 kappa, slower for 15 kappa and slowest for 8 kappa. There are two different types of preferential nucleations operating simultaneously on the top surface, both along the transgranular cooling cracks and as islands not in contact with cracks. The coatings transformed in all layers simultaneously, with the largest transformed Volume in the outermost layer. This is explained by the larger free surface area on the top surface than between the layers where only interfacial pores exist. The alpha islands on the top surface are larger and have a larger spacing between them than the a islands at the kappa-Al2O3-kappa modification layer interfaces. This is attributed to the closely spaced small interfacial pores, which act as preferential nucleation sites for the transformation. For all three types of multilayers, having different surface roughnesses, the surface morphology did not change, except for intergranular cracking, during transformation. The extent of cracking within the layers was smaller than for thick single layers. It was also found that the adhesion decreased with increasing degree of transformation. The adhesion did not depend on the number of layers, but only on the alpha:kappa ratio. In fracture surface specimens, the innermost alumina layer showed full adherence, which indicates that the adhesion is better for thin( approximate to 1 mu m) transformed alumina layers than for thick transformed layers (approximate to 4 mu m). All the other alumina layers flaked along the kappa modification layers, possibly because of interfacial pores.