Ultra-hard materials - Concepts and modelling

Ultra-hard materials, such as diamond or cubic boron nitride, have exceptional mechanical and physical-chemical properties. Their use is the basis for industrial tasks such as cutting, abrasion, drilling, etc. Diamond, which in its cubic form is the hardest known natural material, is also the most widely used ultra-hard material in industry, either as a coating or directly. Given the prohibitive cost of natural diamond, its synthesis on an industrial scale is an imperative. However, cost issues are compounded by thermochemical restrictions. Indeed, its use in the cutting and machining of iron-based parts is contraindicated by its temperature instability (870 K under oxygen is a temperature actually reached by friction), leading to degradation not only of the diamond itself, but also of the part to be machined (local modification of the chemical composition by insertion of excess carbon atoms).

For multinationals such as General Electric, Sandvik, Norton US, De Beer..., the production of ultra-hard materials is worth several billion euros. Upstream research is therefore essential to better understand the links between mechanical properties, chemical bonding and crystalline structure. The ultimate aim is to optimize the use of known materials, on the one hand, and above all to be able to predict new materials with comparable mechanical properties but less embrittlement under conditions of use, on the other. For this reason, and in order to replace diamond in various applications, new ultra-hard materials were sought. The predictive aspect of the numerical tool then supports the synthesis by determining, upstream, the expected physico-chemical properties, in particular hardness.

This article, written for the materials science engineer, examines this topic. The various possible ways of synthesizing ultra-hard materials are presented, along with different approaches to the concept of hardness. Taking account of mechanical resistance to changes in volume and shape will lead us to introduce concepts well known to materials mechanics (compressibility and shear moduli, elastic constants, etc.), before moving on to present new ultra-hard materials and the theoretical framework for calculations.