Processing, microstructure and hardness of TiN/(Ti, Al)N multilayer coatings

Multilayer films of TiN/(Ti, Al)N were deposited on stainless steel substrates by rotating the sample continuously between TiN and (Ti, Al)N targets in a cathodic arc evaporation chamber. A complex set of microstructures has been found in which alternate layers of TiN and (Ti, Al)N are separated, at the interface, by another layered interfacial region which consists of extremely fine (4 nm) sub-layers, which result from the secondary planetary rotation of the specimen in the deposition chamber and which is thought to arise from a varying Ti/Al vapour concentration in the chamber. The hardness values of the multilayers show little influence of layer periodicity and no change from those of monolithic TiN and AlTiN films. This absence of strengthening from layer refinement is discussed with respect to the gradient of composition change at the interface and prevailing models of hardening in coherent multilayered systems.