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