The microstructure of CVD κ-Al2O3 multilayers separated by thin intermediate TiN or TiC layers

The aim of this paper is to describe and compare the detailed microstructure of thick (8 μm) κ-Al2O3 multilayer coatings separated by thin (50 nm) intermediate layers of either TiC or TiN. The films were formed by chemical vapour deposition (CVD) and were characterised using transmission electron microscopy (TEM).The general microstructure of TiN/κ and TiC/κ is very similar. The κ-Al2O3 and TiX grains have about the same grain size and have the same preferred growth direction. No dislocations or cracks could be found in the coatings. The same types of epitaxial columns with similar orientation relationships between the grains were found. Pores were present at some of the κ-Al2O3/TiX (X = C or N) interfaces below the TiX layers. The presence of γ-Al2O3 above the TiX layer was more frequent in TiC/κ. The pore density and pore size were larger in TiN/κ. When the deposition was switched from TiX to alumina, κ-Al2O3 was often obtained directly, but in some occasions γ-Al2O3 was formed in the first 50 nm of the alumina layer. The γ-Al2O3 region was usually, but not always, heavily faulted. The amount of γ-Al2O3 varied locally in the coatings, but generally there were more γ-Al2O3 regions present in TiC/κ than in TiN/κ. The different degree of interfacial porosity was probably the most important factor for the observed difference in thermal stability of the coatings, as the pores increased the probability for the nucleation of the stable α-Al2O3.