Flexible hard nanocomposite coatings

Abstract

The article reports on flexible hard nanocomposite coatings prepared by magnetron sputtering. It is shown that the flexible hard nanocomposite coatings (i) represent a new class of coatings which are simultaneously hard, tough and resistant to cracking, (ii) exhibit high values of the hardness H and effective Young's modulus E* ratio H/E* ≥ 0.1, elastic recovery W_e ≥ 60%, compressive macrostress σ < 0 and dense, void-free microstructures, and (iii) are formed in the zone T of the Thornton's Structural Zone Model (SZM); here E* = E/(1 − ν²), E is the Young's modulus and ν is the Poisson's ratio. The magnetron sputtering, which is a very powerful process used in the preparation of nanocomposite coatings, is described in detail. The basic principles of the formation of the flexible hard coatings are also described in detail. It is shown that the key parameters which determine the formation of these coatings are (1) the energy E_p = E_ca + E_bi delivered to the growing coating by condensing atoms (E_ca) and bombarding ions (E_bi) (the non-equilibrium heating), (2) the substrate heating controlled by the substrate temperature T_s (the equilibrium heating) and (3) the melting temperature T_m of the coating material. The flexible hard coatings have a huge application potential. Four examples of flexible coatings are given: (1) flexible protective coatings, (2) flexible functional coatings, (3) flexible over-layer preventing cracking of brittle coating and (4) flexible multilayer coating. Also, the principle of low-temperature sputtering of flexible nanocomposite coatings is described in detail. Finally, trends for future development of these nanocomposite coatings with unique properties are given.