Ultralow load indentation hardness and modulus of K – and α–Al2O3 CVD coatings

Abstract

Metastable k–Al₂O₃ often occurs as islands in thin α–Al₂O₃ coatings produced by CVD. In this investigation a thick single-phased K–Al₂O₃ coating was produced in order to determine the mechanical properties of the K-phase. As a comparison, a CVD coating as well as a sintered bulk specimen of α–Al₂O₃ were studied. The mechanical behavior was probed by a depth-sensing indentation (nanoindentation) system on polished taper and cross sections at several applied loads. Hardness and elastic modulus were derived and are discussed in conjunction with the crystal structure and the microstructure of the two materials. K–Al₂O₃ shows a marked microstructural and structural anisotropy, which is also reflected in anisotropic mechanical properties. Parallel to the c-axis, which, in this study, closely coincided with the growth direction, both the hardness and the elastic modulus are significantly higher than in the perpendicular direction. α–Al₂O₃ has the same hardness and elastic modulus in the coating and in the bulk form and shows no clear anisotropy in microstructure. If the two alumina materials are compared, K–Al₂O₃ is significantly harder than α–Al₂O₃ in the growth direction (on the taper section), whereas it is significantly softer in the perpendicular direction. In either of the loading directions, the elastic modulus of K–Al₂O₃ is significantly lower than in α–Al₂O₃.