Tribological properties of coatings—expectations, performance and the design dilemma

So-called advanced coatings systems produced by the CVD and PVD technologies to enhance the tribological behaviour of engineering artifacts—especially tools and dies—have been available for a number of years and, commonly, excellent end-results are confidently quoted. Critical analysis of published performance data, however, reveals that behaviour may be erratic and irreproducible with ‘well-established simple’ coatings such as TiN even in single layer format. Promising, more novel, coating systems involving alternative compounds in, for example, multi layer format have been identified in the research literature, but their appropriateness to particular needs of industry has to be systematically established. It becomes evident that there is a major lack of understanding of the significance of the physical and mechanical properties of coatings materials in monolithic form, the corresponding values for the same materials as coatings, the reported differences in these parameters and especially their importance, or otherwise, for coated artefact performance. An additional major problem here is the paucity of published data on the large number of compounds—oxides, carbides and nitrides—which, from a materials science point of view, are candidate coating materials. As a result, a systematic approach to the design of the most suitable coating systems—‘fitness for purpose’-is inhibited. Finally, it is believed too little attention has, to date, been paid to the quality assurance dimension in coating design and performance. These matters are illustrated and discussed, and some suggestions for research put forward.