ON THE DESIGN OF COATING SYSTEMS - METALLURGICAL AND OTHER CONSIDERATIONS

Coatings and surface treatments are widely used in combating corrosion and wear. In particular thin hard coatings of borides, carbides, nitr ides and oxides - both singly and in combination - have been synthesis ed by physical and chemical vapour deposition (PVD and CVD) methods, c haracterised and evaluated for possible applications in manufacturing industry. Many such coatings in both single and multilayer formats are now available commercially eg. cutting tools coated with TiN, TiAlN, TiC/Al2O3/TiN, etc,. To progress further in this direction there is a need to understand more fully the fundamentals of various material str engthening mechanisms and their relation to wear resistance in coating s, as distinct from bulk monolithic materials. This paper reviews the mechanisms of wear and degradation in several manufacturing scenarios (especially metal cutting and metal forming), and considers the availa ble published correlations between wear resistance and microstructure, hardness, toughness and adhesion of coatings. Based on the knowledge of strengthening mechanisms in bulk materials, various possible ways t o strengthen and improve coatings are discussed. A main focus of the p aper is to consider and review the approach to, and rationale for, the selection of various possible coating materials - type and nature (eg . composition, and thickness) - for multilayer and composite coating s ystems.