Chemistry, physics and fracture mechanics in search for superhard materials, and the origin of superhardness in nc-TiN/a-Si3N4 and related nanocomposites

Superhard materials (H>40 GPa) are presently subject of intense research. One distinguishes between intrinsic superhard materials, such as c-BN and diamond and extrinsic ones, such as heterostructures and nanocomposites. After a brief discussion of material's parameters that determine its strength, we focus on the superhard nanocomposites nc-Me_nN/a-XN_m (Me=Ti, V, W, (TiAl)N,…; X=Si, B). Recent progress in the understanding of the origin of hardness enhancement in superhard nc-TiN/a-Si₃N₄ nanocomposites (“nc-” stands for “nanocrystalline, “a-” for “X-ray amorphous”)¹The term “X-ray amorphous” does not exclude the formation of a locally pseudomorph interface because even if such, 1 2 monolayer thin, interface would be perfectly grown on the TiN template it could never give rise to Bragg reflections in the three-dimensional nanocomposites with randomly oriented TiN nanocrystals. We used the formula “Si₃N₄” in order to emphasize that Si is covalently bonded to N only with Si 2p binding energy of 101.7±0.1 eV determined by XPS and being equal to that in Si₃N₄. is summarized. The optimum nanostructure with one monolayer of interfacial Si₃N₄ was confirmed by both experiments with the deposition of TiN/Si₃N₄ heterostructures by Söderberg et al., and by first principle density functional theory calculations of Hao et al. These calculations show that the de-cohesion energy of the TiN/Si₃N₄/TiN sandwich is enhanced as compared with bulk Si₃N₄. A new, non-linear constitutive materials model, that accounts for the pressure enhancement of elastic moduli and of flow stress was developed and tested by means of advanced FEM calculations. Significant deviations of the mechanical behaviour of super- and ultrahard materials from the prediction of the linear mechanics with a constant yield stress have been demonstrated.