Lens surface modification strategies – damage reduction
Tribology of Silicate Glasses. Friction and Surface Damage

In the presence of a thin film of appreciable stiffness, the stress field at the surface is modified. As the film is thin, it does not contribute to load-bearing capacity, so deformations are the same as in the absence of a film. The elastic moduli of amorphous silicates are relatively low for mineral solids, while inorganic films deposited on the surface generally have higher moduli. As a result, tensile stresses in the film are amplified. In practice, if we neglect the effects of lateral contraction (determined by Poisson's ratio), we can roughly estimate the tensile stress in the film:

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