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_xN_1-x), Ti(C_xN_1-x)_0.81, Zr(C_xN_1-x) and Hf(C_xN_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 maximu. Thus the conclusion of a correlation of hardness and shear modulus is inapplicable for this type of hard materials.