Published: Jan 1991
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Theoretical investigations of stresses associated with thin-film coatings usually focus on evaluating the normal stresses in the film layers and the thick substrate. They do not address interfacial stresses, which are responsible for such effects as film delamination and substrate bowing. Thermally-induced shearing stresses in CVD-diamond coated windows made of optically transmitting materials are known to be of concern because of the lattice-expansion mismatch and the enormous elastic modulus of diamond. The purpose of this paper is to reformulate Suhir's analytical method for assessing interfacial stresses in multilayered structures [J. Appl. Mech. 55. 143 (1988)] and to apply relevant formulas to the problem of mitigating the impact of thermoelastic stresses in diamond-coated ZnS windows. Specifically, it is demonstrated that the critical shear and the central deflection are both governed by the first-interface longitudinal force per unit length, FO = Σ(hiσi), where hi and σi refer to the film thickness and the normal stress in each layer, which suggests that the presence of a “buffer” in tension may result in major improvements. Issues relating to the biaxial elastic modulus of diamond are discussed in Appendix, where it is shown that state-of-the-art diamond films behave as expected, based on single-crystal elastic constants.
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