Rotating bending fatigue tests have been conducted at room temperature in air using the specimens of medium carbon steel (S45C) with sprayed coating of a cermet (WC-12%Co). The fatigue strength and fracture mechanisms were studied. The fatigue strength was enhanced by the cermet spraying and the fatigue limit of the sprayed specimens was 30-40% higher than that of the electro-polished specimens of the substrate steel. Fatigue cracks were initiated at defects between the substrate and sprayed coating. When the specimens were blasted by alumina grits before the cermet spraying, fatigue cracks were initiated at the alumina grits remained on the surface of the substrate. Therefore, fatigue strength was increased when the substrate was blasted by steel balls or the blasting was omitted. In these specimens, fatigue cracks were initiated at the debonding defects at the interface between the substrate and sprayed coating. When a crack was initiated in the sprayed coating by the increase of localized strain due to the crack initiation in the substrate, the crack growth rate in the substrate was accelerated by the stress concentration of the crack in the sprayed coating, which led to the decrease in the crack growth life of the specimen.
