Hertzian contact at coated glasses

It is shown that Hertzian contact is a useful probe for assessing the contact damage resistance of coated glasses. The stress field for such contacts is often elastic and, therefore, is more amenable to an exact analysis. This analysis can be complex but for stiff coatings, if the coating thickness is small compared to the contact size, the contact area is controlled by the elastic properties of the substrate. At a given indentation force this effect leads to an increase in the tensile stresses in the coating. For this study on commercial coated glasses, a simple analytical expression was derived to estimate this stress enhancement. From the average force to initiate fracture, an integrity ratio can be defined to assess the effect of the various coatings on the contact damage resistance. The coatings were found to have various influences on the contact damage resistance varying from degradation to improvement. For example, a thin silica like coating significantly increased strength by a reduction in flaw size, while a tin oxide coating led to an increase in the effective flaw size and a change in the failure location.