Stress, interfacial effects and mechanical properties of nanoscale multilayered coatings

Multilayer coatings with layer thicknesses in the nanometer range have been shown to exhibit original mechanical properties: these include pronounced softening of some elastic moduli, large increase in yield strength and hardening effect. Such original properties are related to the high interface density and reduced grain sizes associated with these systems. In the first part of this article, the elastic and plastic properties of metal/metal, metal/nitride and nitride/nitride nanoscale multilayers are briefly outlined. The origin of large compressive stress commonly observed in PVD films grown with energetic particles is then presented and illustrations given for the case of epitaxial multilayers for which different stress sources exist. The second part focuses on recent experimental results obtained for three multilayered systems, Mo/Ni, TiN/Cu and ZrN/W, corresponding to different crystal structure and lattice mismatch combinations. Two issues will be addressed: the interdependence between interfacial mixing and elastic softening in Mo/Ni superlattices and the mechanical behaviour of TiN/Cu and ZrN/W multilayers combining ‘hard’ and ‘soft’ materials. These multilayers were grown epitaxially on either MgO (001) or Al2O3(112¯0) substrates by a dual ion beam sputtering technique. Low-angle and high-angle X-ray diffraction experiments as well as Transmission Electron Microscopy observations were used to characterize the microstructure and crystalline orientation, structure of interfaces and type of growth defects. Elastic properties were studied by Brillouin light scattering and hardness values determined from nanoindentation tests. For selected samples, a combined FIB-TEM technique was implemented to image the deformed nanolaminates beneath the indentor.