Prof Allan Matthews, University of Sheffield, UK.

It is well known that a material’s grain size can have a considerable effect on its mechanical properties – especially yield stress and hardness.  Some years ago, this led to increased interest in the development of (especially) ceramic-based nanocomposite coatings with grain sizes in the range of several to tens of nanometres with very high hardnesses.  However, due to recent improvements in the understanding of the factors controlling wear in real tribological contacts (especially with coated surfaces), it has become evident that hardness (H) is not the dominant property influencing wear that people previously thought it was.  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, to use it, with appropriate material selection, to “engineer” coatings having H/E ratios which allow them to (for example) accommodate surface deformation and absorb impact loads without failure. The presentation discusses the background to the H/E requirement, and discusses how deposition and treatment technologies (especially plasma-based ones) are being developed to achieve enhanced H/E ratios using nanocomposite and nanolayered coating systems. Test results and applications information are also provided.