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Aluminium oxide coatings are commonly used in cutting tool applications due to a combination of high hardness, chemical resistance and high thermal stability. The two phases that are typically found in industrial tool coatings are chemical-vapour deposited (CVD) α-alumina and κ-alumina. Recent developments have also shown promising properties for physical vapour deposited (PVD) cubic γ-alumina coatings several cutting operations such as in steel turning. A possible limitation for these coatings is, however, the phase transformations that occur at actual cutting temperatures. In this study we present the in-situ monitoring of the phase stability through high temperature x-ray diffraction of κ-alumina tool coatings as well as a first-time study on phase stability for newly developed γ–alumina coatings. The results from this study performed in-situ in vacuum indicate that the transformation temperature for κ-alumina to α-alumina occurs at temperatures similar to those previously reported using ex-situ conditions and argon atmosphere. In the case of γ-alumina, the transformation occurred at significantly higher temperatures than those typically reported for bulk γ-alumina but lower than those reported for Cr doped γ-alumina. Scanning electron microscopy showed that a crack network exists in both κ-alumina and γ-alumina after annealing, but the crack network is much less developed in the case of γ-alumina. Mismatches in the thermal expansion between the film and substrate during cooling to room temperature and in the molar volumes of the phases that form during annealing cause strains in the material and the subsequent crack network. Further, a secondary phase transformation has been observed on the surface of the γ-alumina coatings during annealing. The growth conditions of this phase will be discussed based on a combined electron microscopy and cathodo-luminiscence study. |