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.