Interfaces in nanostructured thin films and their influence on hardness

Nanocrystalline structures have attracted increasing interest in modern development of hard films for wear-resistant applications. In plasma-assisted vapor-deposited thin films, nanostructures can be generated during growth or post-annealing treatment. In this review, we demonstrate, using the four model-films overstoichiometric TiB2, TiN-TiB2, Ti(B)N, and Ti0.33Al0.67N, how interfaces influence the mechanical properties of ceramic thin films. For overstoichiometric TiB2 films and TiN-TiB2, a two-dimensional and three-dimensional nanostructure, respectively, is generated during growth by segregation-driven processes. Growth of less B containing Ti(B)N films and Ti0.33Al0.67N results in the formation of a supersaturated TiN phase with NaCl structure, where either the non-metallic part or the metallic part is substituted by B or Al, respectively. For these films, the nanostructure forms during post-annealing, due to the decomposition of the supersaturated phases into their stable constituents. As the hardness of a material is determined by resistance to bond distortion and, dislocation formation and motion, which themselves depend on the amount of interfaces and their constitution, there is a direct relation between hardness and nanostructure.