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

Abstract Nanoidentation responses of Al 2 O 3 coatings obtained using chemical vapour deposition on cemented carbides were studied and related to the Al 2 O 3 modification and microstructure. Five different Al 2 O 3 layers were studied: (i) κ-Al 2 O 3 , (ii) α-Al 2 O 3 , obtained through the κ → α phase transformation, (iii) as-grown α-Al 2 O 3 with (101¯2) texture, (iv) as-grown α-Al 2 O 3 with (101¯4) texture and (v) as-grown α-Al 2 O 3 with (0001) texture. All the Al 2 O 3 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-Al 2 O 3 coatings were compared with κ-Al 2 O 3 and α-Al 2 O 3 formed as a result of the κ → α phase transformation. The as-grown, textured α-Al 2 O 3 layers exhibited clearly a higher hardness and Young's modulus than the κ-Al 2 O 3 and the α-Al 2 O 3 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-Al 2 O 3 films. Among these the (101¯2) textured α-Al 2 O 3 coating exhibited the lowest hardness and modulus. Opposite to earlier reports, the present investigation clearly showed that κ-Al 2 O 3 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-Al 2 O 3 were in fact performed on transformed κ-Al 2 O 3 .