Microstructural investigations of textured CVD (Al,Ti)N/κ-Al₂O₃ wear resistant coatings

In this work, the deposition of κ-Al₂O₃ on textured (Al,Ti)N coatings using chemical vapour deposition (CVD) is explored. Two TiN/(Al,Ti)N/κ-Al₂O₃ coatings with different texture for the (Al,Ti)N layer, 〈111〉 and 〈100〉, have been investigated. The coatings were characterized using X-ray diffraction, scanning electron microscopy, transmission and scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The κ-Al₂O₃ layer deposited on the 〈111〉 textured (Al,Ti)N layer has a 〈001〉 texture, while the κ-Al₂O₃ deposited on the 〈100〉 textured (Al,Ti)N layer shows no clear texture. The difference in κ-Al₂O₃ texture is driven by the (Al,Ti)N facets available for alumina nucleation. κ-Al₂O₃ forms on {111} (Al,Ti)N facets, while γ-Al₂O₃ forms on {100} (Al,Ti)N facets. γ-Al₂O₃ growth is not stable and is subsequently overgrown by κ-Al₂O₃. The surface of the 〈100〉 textured (Al,Ti)N layer is dominated by {100} facets, while the surface of the 〈111〉 textured (Al,Ti)N layer is built up of a mixture of {100} and {111} facets. This explains the observed microstructure for the κ-Al₂O₃ layers in the two coatings. Thus, to optimize the deposition of κ-Al₂O₃ on (Al,Ti)N, the latter should exhibit {111} facets.