The role of nanocomposite coatings in surface engineering

It is well known that the grain size of a material can have a considerable effect on its mechanical properties - especially yield stress and hardness. Some years ago, this led to an increased interest in developing wear-resistant nanocomposite coatings with grain sizes in the range of several to tens of nanometers. In particular, nanostructured ceramic coatings, produced mainly by physical/chemical vapour deposition (PVD/CVD) techniques, have attracted strong research interest - with very high hardnesses claimed for such films. However, due to recent improvements in the understanding of the factors controlling wear in real tribological contacts (especially with respect to coated surfaces), it has become evident that the hardness (H) is not the dominant property influencing wear as was previously thought. In fact, the elastic modulus (E) is also very influential, and the ratio H/E gives a better indication of wear resistance than H or E alone. The realisation of the importance of this parameter has led coating developers to further refine the concept of grain-size control with appropriate material selection, to "engineer" coatings having H/E ratios which allow them to accommodate surface deformation and absorb impact loads without failure. This paper discusses the background to the H/E requirement, describing how coating and treatment technologies, especially those based on plasma-assisted vapour deposition (and emerging plasma-electrolytic) techniques, are being developed to achieve enhanced H/E ratios using a nanocomposite (or nanolayered) approach. The paper also discusses the suitability of nanocomposite coatings to provide "adaptive" characteristics in demanding applications.