Hardness of materials at high temperature and high pressure

International audienceIntrinsic character of correlation between hardness and thermodynamic properties of solids has been established. The proposed thermodynamic model of hardness allows one to easily estimate hardness and bulk moduli of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements or on the enthalpy at the melting point. The correctness of this approach has been illustrated by an example of the recently synthesized superhard diamond-like BC5 and orthorhombic modification of boron, γ-B28. The pressure and/or temperature dependences of hardness have been calculated for a number of hard and superhard phases, i.e. diamond, cBN, B6O, B4C, SiC, Al2O3, β-B2O3 and β-rh boron. The excellent agreement between experimental and calculated values has been observed for temperature dependences of Vickers and Knoop hardness. Besides, the model predicts that some materials can become harder than diamond already at pressures in the megabar range.