Hardness and elastic properties of Ti(CxN1-x), Zr(CxN1-x) and Hf(CxN1-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(CxN1-x), Ti(CxN1-x)(0.81), Zr(CxN1-x) and Hf(CxN1-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 approximate to 8.4 and approximate to 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])approximate to 0.6-0.8 (VEC approximate to 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. (C) 2000 published by Elsevier Science S.A.