Modelling the deformation behaviour of multilayer coatings

In tribological contacts involving relatively thin vapour-deposited coatings, it is the substrate which provides the main load support. The coating must therefore be able to deflect along with the substrate without cracking if it is to function satisfactorily. Although the potential benefits of multilayer coatings in such contacts have been well documented, their deformation behaviour has not been properly understood. Since it is this response to deformation that provides their benefit (compared to single-layer coatings), we have undertaken studies into this aspect, both by simulation modelling and finite element analysis. These studies conclude that, for multilayer coatings having repeated hard/soft (or low/high modulus) layers, the main shear deformation takes place in the low modulus (soft) layers. This ensures that the harder (more brittle) layers effectively slide over each other, and do not experience high bending stresses. This helps to explain why such multilayers can deflect with the substrate under load without cracking, and therefore survive in heavily loaded tribological contacts.