Size effects in the mechanical response of nanoscale multilayer coatings on glass

Very thin (100-400 nm) oxide coatings which are used as antireflection and barrier layers in low emissivity architectural glass have been studied by nanoindentation methods to determine the effect of coating thickness on mechanical response. Whereas plasticity (hardness) is relatively easy to assess the elastic response of the coating is underestimated as the thickness is reduced. Thus, although some changes are observed with thickness, there is no size effect in elasticity. Size effects in plasticity are only observed for the crystalline layers such as ZnO whereas the other oxides deposited in this study were amorphous and show no size effect. Traditional microindentation-derived methods to determine the fracture toughness are unsuitable for assessing very thin coatings (< 500 nm) and alternative energy-based models are required depending on what features are visible in indentation load-displacement curves. For oxide coatings no size effects in fracture toughness were observed, however there are process-induced variations in residual stress which will affect the apparent fracture and adhesion strengths of the coatings. Failure by delamination during scratching depends on the magnitude of the applied stress which in turn depends on the thickness of coating layers deposited. This is because the friction tractions are controlled by the surface roughness which increases with coating thickness.