Growth model for high-Al containing CVD TiAlN coatings on cemented carbides using intermediate layers of TiN

This work concerns high Al-containing Ti_xAl_1-xN coatings prepared using low pressure-chemical vapour deposition (LP-CVD). The coatings were examined using electron microscopy techniques, such as scanning transmission electron microscopy (STEM), energy dispersive X-ray analysis (EDX) and transmission Kikuchi diffraction (TKD). An intermediate TiN-layer with a 〈211〉 texture consisting of twinned, needle-shaped grains influences the subsequent growth of the TiAlN layer. The TiAlN grains were columnar with a texture of 〈211〉. As the grains grow along 〈111〉, with {001} facets, this led to a tilted pyramid surface morphology. The grains developed an internal periodic epitaxial nanolamella structure. The thicknesses were 2 nm for the low (x = 0.6) and 6 nm for the high (x = 0.9) Al-containing lamellae. The TiAlN layer growth could be described by a “two-wing” model, where two TiAlN grains with a twin-related orientation grow on a twinned TiN grain, where the two TiAlN grains gradually switch sides, making the appearance of two wings of columnar grains. In general, this work shows that it should be possible to control the growth of TiAlN layers by controlling the texture and morphology of an intermediate layer, such as TiN.