In recent years, there have been advances in techniques for improving sliding performance by using coated films possessing superior tribological properties to reduce failure of mechanical elements. Those techniques are often used under severe conditions, such as elastohydrodynamic lubrication (EHL). In this paper, to devise the optimum design of a coated film with a gradient layer, the stresses generated in such a coated film are analysed by using a three-dimensional numerical analysis method, and the strength of the film is evaluated numerically. As a result, it is shown that when the ratio of the modulus of elasticity of the coated film, Ec, to that of the substrate, Es, is 0.50, it is preferable to select the gradient layer in which the variation of modulus of elasticity decreases gradually approaching from the substrate to the coated film in the depth direction. Furthermore, it is shown that in the case of Ec/Es=2.00, it is preferable to select the gradient layer varying linearly the modulus of elasticity into the gradient layer.
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