1 June 2001 Normal and interfacial stresses in thin-film coated optics: the case of diamond-coated zinc sulfide windows
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Optical Engineering, 40(6), (2001). doi:10.1117/1.1369145
Abstract
Optical components such as mirrors or windows consisting of a substrate and a coating made up of thin films created at elevated temperatures exhibit substantial residual stresses induced by growth strains and/or thermoelastic strains that develop during the cool-down phase. A comprehensive description of these stresses must include not only the normal stresses in the film layers and the substrate but also the interfacial shearing stresses, which may cause delamination to occur. We take advantage of recent progress in describing elastic interactions in multilayered laminates for obtaining conceptually correct formulas for the residual stresses and the substrate’s curvature of thin-film coated optics. Available analytical solutions for the normal stresses of elastically isotropic structures make no assumptions regarding layer thicknesses, but disregard the potential impact of edge effects. For circular structures such as coated optics, we show that recent work by Suhir now allows us to describe the distribution of both normal and interfacial stresses as long as the thin-film conditions are satisfied. The task of evaluating the deflection turns out to be fairly straightforward, leading to the conclusion that edge effects do not alter the bow of large compliant structures. The case of diamond-coated ZnS windows illustrates how thermal expansion mismatches can give rise to compressive film stresses of gigapascal intensity, which cause substrate deformations that are unacceptable in terms of the optical performance. Since the deflection of a multilayercoated substrate reflects the sum of the contributions (positive or negative) induced by each film, the deflection can be minimized by properly designing the film stack. For a diamond-coated ZnS window, this means that a suitable buffer must be in tension; in principle, a buffer made of calcium lanthanum sulfide, about 350 mm thick, can mitigate the bending force exerted by a 50 ?m thick diamond film and suppress the shear at the substrate/coating interface, thus enhancing the adhesion
Claude A. Klein, "Normal and interfacial stresses in thin-film coated optics: the case of diamond-coated zinc sulfide windows," Optical Engineering 40(6), (1 June 2001). http://dx.doi.org/10.1117/1.1369145
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KEYWORDS
Coating

Thin films

Zinc

Diamond

Interfaces

Multilayers

Chemical vapor deposition

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