Reactive sputter deposition of polycrystalline nitride and oxide superlattice coatings

Polycrystalline nitride superlattice films are now being reactively sputter deposited onto polycrystalline substrate materials with the same high hardness enhancement as found for their corresponding single crystal superlattice films. TiN/NbN and TiN/VN polycrystalline superlattice films have hardnesses above 50 GPa, when the superlattice period is in the range of 4-8 nm, and this hardness enhancement depends strongly on control of the process parameters. Superlattice systems such as TiN/Cr2N show pseudomorphic growth where the fcc TiN acts as template for the normally hexagonal Cr2N and forces it into an fcc structure. The primary hardening mechanism for the nitride superlattice films, according to the model of Chu and Barnett, is restriction of dislocation motion either across an interface or within a layer, and experimental data for both the single crystal and polycrystalline nitride superlattices supports the model. Work on superlattice films is now being extended to include oxide systems with the desire to produce hard, tough oxide superlattice films for optical and high temperature applications. Oxide films can now be reactively sputter deposited easily at high rates with pulsed d.c. power and automatic feedback control of the reactive gas, and the ZrO2/Y2O3 and Al2O3/ZrO2 systems are currently being developed.