Thermal stability of nanostructured superhard coatings: A review

It is important to evaluate the thermal stability of hard coating because at high working temperatures the mechanical and tribological properties are deteriorated. The temperature operating on the cutting tool tip during work may reach temperatures as high as 1000 °C. Environmental considerations limiting the use of lubricants and coolant liquids, increase the necessity of finding coatings that can function at such high temperature. Coatings can be differentiated by their hardness, H, into three main categories: hard with H < 40 GPa; superhard with H > 40 GPa; and ultra-hard coatings with H > 80 GPa. There are two main reasons in the high hardness coatings: either high compressive stresses or nano-scale structure. The application of high biaxial compressive stress acts as a driving force for recovery, i.e. the higher the compressive stress, the lower is the thermal activation energy needed to initiate recovery. High biaxial compressive stress increases superhardness, but reduces the coating thermal stability. Dislocations increase the micro-scale compressive stress inside the coating and consequently, enhance recovery. In nano-scale coatings, the small nanometric scale grain size restricted grain growth and boundaries sliding, and therefore the thermal stability is enhanced. This study treats the thermal stability of several types of superhard materials, i.e. nanocomposite coatings and those consisting of a hard transition-metal nitride and a soft metal. It focuses on formation mechanisms, materials and phase composition.