Toughness and toughening of hard nanocomposite coatings

Nanostructured or nanocomposite coatings are a new branch of materials that possess unique physical and mechanical properties. A nanocomposite coating comprises at least two phases: a nanocrystalline phase and an amorphous phase, or two nanocrystalline phases. The mechanical and tribological properties of nanocomposite coatings are not subjected to volume mixture rules, but depend on grain boundary effects and synergetic interactions of the composite constituents owing to the size effect [1]. It is accepted that hard, superhard, and ultrahard means the hardness is over 20, 40, and 80 GPa respectively [2]. Hard and superhard or even ultrahard nanocomposites have been a hot research topic in recent years [2-10]. Table 4.1 shows recent achievements of hardness of nanocomposite coatings. Though the value of 103 GPa for TiN/Si3N4 coating was considered too high (due to the Fischerscope used, which normally gives very high hardness values for superhard coatings [11]), the design concept is on the right track for superhard coatings. Hard coatings are used 4.1 Toughness of Nanocomposite Coatings in many applications, for example, cutting and polishing tools, molds, dies, hard disks, and other wear-resistant coatings, etc.