Mechanical modelling of multilayer optical coatings

Modern anti-reflective or solar-control coatings are usually tailored around optical properties even though failure and damage (e.g. handling damage, scratching and other accidental contacts) will be controlled by their mechanical properties. Many of these coatings are multilayered structures. Testing all the possible combinations is both expensive and time consuming and thus there is a need for a predictive modelling approach that can be used to predict the mechanical properties of any proposed multilayer design. We have been applying a new energy-based model for the composite hardness (as a function of contact depth) of a multilayered system. Not only can this provide a fit to experimental data produced by low-load, continuously recording indentation tests (e.g. Nanoindentation) but using a few chosen (or measured) values it can also be used to predict the mechanical performance of a variety of multilayered stack systems. This paper reports the initial application of this model to very thin (<300 nm) coatings consisting of single layers, a bi-layer and a multilayer stack. Generally, the fit is very good, even at these high spatial resolutions. However, for the multilayer stack, a progressively poor fit suggests that besides plastic deformation, other energy absorbing deformation processes are occurring during indentation, such as contact-induced fracture. Reassuringly, this prediction is borne out by acoustic emission evidence even when no cracks are visible even in high resolution AFM and HRSEM images. This suggests that the model also may be able to indicate the occurrence of fracture or other energy-absorbing deformation events.