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Electronic mechanism of hardness enhancement in transition-metal carbonitrides

Nature volume 399, pages 132–134 (1999)Cite this article

Abstract

Transition-metal carbides and nitrides are hard materials widely used for cutting tools and wear-resistant coatings. Their hardness is not yet understood at a fundamental level. A clue may lie in the puzzling fact that transition-metal carbonitrides that have the rock-salt structure (such as TiCxN1−x) have the greatest hardness for a valence-electron concentration of about 8.4 per cell1,2,3, which suggests that the hardness may be determined more by the nature of the bonding than by the conventional microstructural features that determine the hardness of structural metals and alloys. To investigate this possibility, we have evaluated the shear modulus of various transition-metal carbides and nitrides using ab initio pseudopotential calculations. Our results show that the behaviour of these materials can be understood on a fundamental level in terms of their electronic band structure. The unusual hardness originates from a particular band of σ bonding states between the non-metal p orbitals and the metal d orbitals that strongly resists shearing strain or shape change. Filling of these states is completed at a valence-electron concentration of about 8.4, and any additional electrons would go into a higher band which is unstable against shear deformations.

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Figure 1: Correlation between the calculated shear modulus c 44 and experimental microhardness.
Figure 2: Influence of shear strain on the electronic structure of TiC.
Figure 3: Charge density of a state of TiC near the K-point of the Brillouin zone under shear strain (εxy = 0.1).

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Acknowledgements

S.-H.J. and J.I. were supported by the SNI, the SRC programme of KOSEF, and the BSRI programme of KRF. S.G.L. and M.L.C. were supported by the NSF and DOE. The support of a Korea-US Cooperative Grant by KOSEF and NSF is acknowledged. S.G.L acknowledges the hospitality of the Korea Institute for Advanced Study, where part of this Letter was written.

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Authors and Affiliations

  1. Department of Physics and Center for Theoretical Physics, Seoul National University, 151-742, Seoul, Korea

    Seung-Hoon Jhi & Jisoon Ihm

  2. Department of Physics, University of California at Berkeley, Berkeley, 947207300, California, USA

    Steven G. Louie & Marvin L. Cohen

  3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, 94720, California, USA

    Steven G. Louie & Marvin L. Cohen

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Jhi, SH., Ihm, J., Louie, S. et al. Electronic mechanism of hardness enhancement in transition-metal carbonitrides. Nature 399, 132–134 (1999). https://doi.org/10.1038/20148

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