Novel thermodynamically stable and oxidation resistant superhard coating materials

A theoretical concept for the design of novel, nanocrystalline and thermodynamically stable materials with hardness of ≥50 GPa (about 5000 kg mm−2), elastic modulus of ≥500 GPa and a high stability against oxidation in air up to 800°C is described together with its experimental verification on several systems nc-MexN/a-Si3N4 (Me  Ti, W, V). The concept is based on avoiding the formation and multiplication of dislocations in the nanocrystalline phase, and blocking the crack propagation in a 0.3–0.5 nm thin amorphous tissue. The theoretical principles of the design of such materials and the thermodynamic criteria for the segregation of the nc- and a-phases, which is necessary for the preparation of such materials, are discussed. Several micron thick films of such materials have been prepared by plasma CVD at a rate of 0.6–1 nm s−1 from the corresponding metal halides, hydrogen, nitrogen and silane at deposition temperatures of ≤550°C. A low content of chlorine of ≤0.3 at.% assures their stability against corrosion in air. Upon microindentation up to a load of ≥100 mN the films show a remarkably high elastic recovery of about 80%. Unlike diamond, c-BN, and C3N4 these materials are thermodynamically stable and relatively easy to prepare.