Nanoindentation hardness, texture and microstructure of α-Al2O3 and κ-Al2O3 coatings

Nanoidentation responses of Al2O3 coatings obtained using chemical vapour deposition on cemented carbides were studied and related to the Al2O3 modification and microstructure. Five different Al2O3 layers were studied: (i) κ-Al2O3, (ii) α-Al2O3, obtained through the κ → α phase transformation, (iii) as-grown α-Al2O3 with (101¯2) texture, (iv) as-grown α-Al2O3 with (101¯4) texture and (v) as-grown α-Al2O3 with (0001) texture. All the Al2O3 coatings were deposited to a thickness of at least 6 μm onto identical WC – 6 wt.% Co cemented carbide substrates. Nanoindentation was performed on taper-section specimens. The microstructure, phase and texture were elucidated by using transmission electron microscopy, scanning electron microscopy and X-ray diffraction. Nanoindentation responses of the as-grown, textured α-Al2O3 coatings were compared with κ-Al2O3 and α-Al2O3 formed as a result of the κ → α phase transformation. The as-grown, textured α-Al2O3 layers exhibited clearly a higher hardness and Young's modulus than the κ-Al2O3 and the α-Al2O3 layers formed as a result of the phase transformation. Only slight differences in hardness and modulus could be detected between the (101¯2), (101¯4) and (0001) textured α-Al2O3 films. Among these the (101¯2) textured α-Al2O3 coating exhibited the lowest hardness and modulus. Opposite to earlier reports, the present investigation clearly showed that κ-Al2O3 exhibited both the lowest hardness and Young's modulus as compared with the other studied coatings. It is emphasised that the earlier nanoindentation investigations that were intended to deal with α-Al2O3 were in fact performed on transformed κ-Al2O3.