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 t otal thickness (8 mu m) but a different number of layers (8, 15 and 32 ). The transformation was studied with respect to (i) rate of transfor mation, (ii) transformation growth, (iii) morphology and (iv) the adhe sion of the different layers. The rate of transformation is highly tem perature dependent for all three multilayers and the transformation is 3-4 times faster at 1090 degrees C than at 1030 degrees C. The transf ormation is faster for 32 kappa, slower for 15 kappa and slowest for 8 kappa. There are two different types of preferential nucleations oper ating 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 transforme d Volume in the outermost layer. This is explained by the larger free surface area on the top surface than between the layers where only int erfacial 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 nucl eation sites for the transformation. For all three types of multilayer s, having different surface roughnesses, the surface morphology did no t change, except for intergranular cracking, during transformation. Th e extent of cracking within the layers was smaller than for thick sing le layers. It was also found that the adhesion decreased with increasi ng degree of transformation. The adhesion did not depend on the number of layers, but only on the alpha:kappa ratio. In fracture surface spe cimens, the innermost alumina layer showed full adherence, which indic ates that the adhesion is better for thin( approximate to 1 mu m) tran sformed alumina layers than for thick transformed layers (approximate to 4 mu m). All the other alumina layers flaked along the kappa modifi cation layers, possibly because of interfacial pores.