Abstract

Rotating bending fatigue tests have been conducted at room temperature in laboratory air using specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6AI-4V) with HVOF sprayed coating of a cermet (WC-12%Co) and S45C with WFS sprayed coating of a 13Cr steel (SUS420J2). Plane bending fatigue tests were also conducted at stress ratios, R, of -1, -0.5 and 0 for S45C with WC-12%Co coating. The fatigue strength and fracture mechanisms were studied. The fatigue strength evaluated by nominal stress was strongly influenced by substrate materials, R and the thickness of sprayed coatings. Detailed observation of crack initiation on the coating surface and fracture surface revealed that a crack was initiated in the coating and then cracks were initiated in the substrate due to the stress concentration of the crack in the coating. The fatigue strength of the sprayed materials was dominated by that of the sprayed coating. Therefore, the fatigue strength could be evaluated uniquely in terms of the true stress on the coating surface. The influence of compressive residual stress of the sprayed coatings on fatigue strength was discussed based on the fatigue mechanisms at different stress ratios.

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