Microstructure investigation of textured CVD alumina coatings

This study concerns the interfacial microstructure and texture development in three textured CVD alpha-Al2O3 coatings using X-ray diffraction, transmission Kikuchi diffraction, scanning transmission electron microscopy and energy dispersive X-ray analysis. It is well known that the performance of these types of coatings relies on the degree and type of texture. The aim of this work is to study the microstructure of three different CVD alpha-Al2O3 layers when deposited onto a Ti(C,N,O) bonding layer. The coatings were deposited onto cemented carbide/cobalt substrates (WC/Co). It was observed that grain boundary diffusion of W and Co occurred through the TiN and Ti(C,N) layers to the bonding layer/alpha-Al2O3 interface. This may disturb the alumina layer nucleation and early growth. Interfacial porosity was observed at the bonding layer/alpha-Al2O3 interface. The number of voids that were detected in the (0001) and (01 (1) over bar0)-textured coatings was similar when H2S was not used during the initial deposition step. When H2S was present during the nucleation step deposition of the alpha-Al2O3 more voids were introduced at the interface for the (0001)-textured samples. The alumina grain morphology developed from small (similar to 100 nm) equiaxed grains at the start of the alumina coating to larger (several microns) columnar grains at the top of the coating. The inner part of the alumina coatings had a more random orientation. The texture changed either: (i) gradually over several grains, or (ii) more abruptly from one grain to another; from more random orientations to the desired texture. The desired texture develops earlier when using H2S at the start for the (0001)-textured coatings, while the (01 (1) over bar0)-texture development benefits from the absence of H2S. Thus, in this study, H2S promotes (0001) texture and interfacial void formation