Microstructural design of hard coatings

Microstructural design has attracted increasing interest in modern development of hard coatings for wear-resistant applications. In plasma-assisted vapor deposited thin films, the material's microstructure can be designed during growth or post-deposition annealing treatments. In this review, we demonstrate the correlation between microstructure and mechanical as well as tribological properties of hard ceramic coatings. This is done for single-phase coatings and composition or phase modulated layers. In the latter case, the microstructure can be designed by choice of the deposition technique chosen, understanding the growth processes taking place on a film surface, either by sequential deposition of layers or by taking advantage of newly discovered self-organization processes including segregation effects of the elements. Consequently, the effects of individual microstructural features like grain size, defect density (and hence residual stress), phase arrangements in a one-, two- or three-dimensional manner on the mechanical properties are treated. Here, especially TiN-TiB2 is used as a model system to describe the development of two- and three-dimensional coating nanostructures. Due to their particular structures, such coatings can exhibit superhardness (H ⩾ 40 GPa). The microstructural changes of hard ceramic coatings during a post-deposition annealing treatment are discussed in detail. Although the significance of heat treatments to optimize properties (by a well-designed microstructure) for specific applications is recognized in bulk material science, only a few elements have been applied for hard ceramic coatings so far. Due to limited atomic assembly kinetics during the deposition process (e.g., by using a low substrate temperature), defects (point-, line-, and area-defects), supersaturated, and metastable phases can easily be obtained. For example, growth of (Ti,Al)N and Ti(B,N) films can result in the formation of a supersaturated TiN based phase. Such films undergo age hardening processes during post-deposition annealing due to the decomposition of the supersaturated phases into their stable constituents. The review clearly shows that nanostructure dependent hardness increase (compared to hardness of the bulk counterparts) sustains higher annealing temperatures than hardness increase due to an increased density of point- and/or line-defects. Tribological properties of hard thin films can be engineered by adding phases with lubricious properties at operation temperature (either room or elevated temperatures) and prevailing environment. Especially in high speed and dry cutting applications, low-friction and lubricating mechanisms of the thin film itself are required in addition to excellent mechanical properties.