COMPOSITION DEPENDENT HARDNESS OF COVALENT SOLID SOLUTIONS AND ITS ELECTRONIC STRUCTURE ORIGIN.

Materials with high hardness are technologically important for cutting and forming tools, engine components, valves, seals, gears, many types of wear resistant coatings, etc. The search for harder materials has a long history and remains one of the most active areas in materials science. Predictive tools for hardness will make this effort more efficient. In the past ten years, several hardness models based on electronic structure theory for perfect elemental materials/binary covalent and ionic compounds have been put forward. However, most of the engineering materials are alloys (solid solutions) and inevitably contain some impurities or defects such as vacancies. Theoretical predictions of the hardness of this kind of materials have rarely been touched. We have presented a hardness formula for multi-component covalent solid solutions. Here the composition dependence of the hardness of some covalent/ionic solid solutions, predicted with this formula, taking the parameters from our first-principles calculations, will be reported in comparison with experimental results. Furthermore, the electronic structure origin of the composition dependence of the hardness will be discussed. We will show that the band-filling effect proposed in literature to account for the composition dependence of the hardness of e.g. TiC_xN_1-x fails for some of the systems. Instead, we suggest that the composition dependence is controlled by the competition between the band-filling effect and the strength of electronic states hybridization. [ABSTRACT FROM AUTHOR]