Hardness and elastic properties of Ti(C xN 1− x), Zr(C xN 1− x) and Hf(C xN 1− x)

Here we report for the first time experimental results of the nanohardness and elastic properties (Young’s modulus, shear modulus, bulk modulus) of well-characterised complete series of bulk Ti, Zr and Hf carbonitrides, Ti(C x N 1− x ), Ti(C x N 1− x ) 0.81, Zr(C x N 1− x ) and Hf(C x N 1− x ), as a function of the carbon/nitrogen ratio measured by continuous nano-indentation test and an ultrasonic technique. A correlation between elastic constants and porosity of TiC and TiN was obtained and used to correct elastic constants for the zero-porosity state. Recently, band structure calculations for transition metal carbonitrides yielded a maximum of the shear modulus of Ti and Hf carbonitrides at a valence electron concentration (VEC) of ≈8.4 and ≈8.2, respectively. These results were used to explain the hardness maximum of carbonitrides, which was considered as a success of theoretical material design. For the stoichiometric carbonitrides we indeed found—though much weaker than predicted—the maximum at [C]/([C]+[N])≈0.6–0.8 (VEC≈8.4–8.2) of the shear modulus, but neither the nanohardness nor the microhardness show a corresponding maximum. Thus the conclusion of a correlation of hardness and shear modulus is inapplicable for this type of hard materials.