The formation and role of interfaces in superhard nc-MenN/a-Si3N4 nanocomposites

The early finding of Veprek and Reiprich that the maximum hardness in the nc-TiN/Si3N4 nanocomposites is achieved at about one monolayer of the interfacial Si3N4 has been confirmed for a number of different nc-MenN/XxNm systems (Me = Ti, W, V, (TiAl)N,…; X = Si, B). More recently, this has been confirmed experimentally for TiN–Si3N4 heterostructures and by first principle density functional theory calculations. We present a consistent understanding of the formation of the nanocomposites with one monolayer Si3N4 interface by spinodal phase segregation. This interface is energetically stabilized as compared to bulk Si3N4, which results in an enhanced bond strength and corresponding cohesion energy. Such an enhancement appears to be of a general validity in nano-sized solids with well-ordered interfaces. The paper concludes a brief summary of the recent progress of the understanding of the mechanical properties of these nanocomposites.