Study of the interfacial structure and chemistry of CVD κ-Al2O3/TiC multilayer coatings

This article describes the interfacial regions in CVD grown TiC/kappa -Al2O3 multilayers. A number of microanalytical techniques were used including HREM, EDX and EELS. Occasionally, the first 50 nm of the alumina layers deposited on the intermediate TiC layers grew as a cubic alumina, heavily faulted, containing small amounts of sulphur (S), maybe as a stabiliser. The presence of slightly rounded TiC (111) facets may act as preferred nucleation sites for the cubic Al2O3 phase, with a 'cube on cube' orientation relationship. In this way the nucleation Of kappa -Al2O3 is less favourable. After some tens of nanometres the cubic phase cannot be stabilised any longer and the layer continues to grow as kappa -Al2O3. A number of observations point towards the reaction zone (RZ) being eta- and/or gamma -Al2O3. The diffraction work and the FFT analysis of the HREM images show that the RZ is an fee phase with a = 7.9 Angstrom, which matches with eta- and gamma -Al2O3. The EELS Al fine structure indicate more tetrahedral Al ions than in kappa -Al2O3, as in eta- and gamma -Al2O3. The RZ contains small amounts of S, as has been reported for gamma -Al2O3. Due to the structural similarities between eta- and gamma -Al2O3 it was not possible to determine which of these cubic phases is present in the RZ. (C) 2001 Elsevier Science Ltd. All rights reserved.