Nanometer scale multilayered hard coatings

Multilayer coatings with layer thicknesses in the nanometer range have been shown to exhibit extremely high hardnesses, making them useful as abrasion-resistant coatings. Hardness values in excess of 5000 kg/mm(2) have been achieved in multilayers composed of two nitride materials, such as TiN and VN, with bilayer periods of 5-10 nm. This article begins with a review of the deposition of multilayered coatings by reactive sputtering and the characterization of these coatings by transmission electron microscopy and X-ray diffraction. The hardness enhancements observed in both isostructural and non-isostructural nanometer-scale multilayers are then reviewed. Several explanations for this enhancement in hardness have been developed in order to understand the hardening process and to develop coatings with even higher hardness. Models based on dislocation motion within layers and across layer interfaces, as well as Hall-Fetch-type models, have successfully been applied to multilayers to explain this hardness enhancement, and they are briefly outlined. (C) 1999 Elsevier Science Ltd. All rights reserved.