Analysis methods and size effects in the indentation fracture toughness assessment of very thin oxide coatings on glass

Very thin oxide coatings (<100 nm) which are used as anti-reflection and barrier layers in low emissivity architectural glass have been studied by nanoindentation methods to determine the effect of coating thickness on fracture toughness. 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. In cases where radial cracks are formed and grow in a discontinuous manner there are excursions in the load–displacement curve which can be the basis for analysis. In cases where picture frame cracks are observed there are no such features and an alternative approach based on assessment of irreversible work of indentation is required. This paper reviews the methods for obtaining fracture toughness data for very thin coatings and assesses the existence of size effects in the mechanical response of oxide coatings with different thickness on a glass substrate. For oxide coatings in the thickness range 100 to 400 nm no size effects in fracture toughness were observed.