Influence of coating thickness on fatigue properties of co-based self-fluxing alloy sprayed steels with fusing treatment

Fatigue strength and fracture mechanism of a medium carbon steel (S35C) with gas flame thermally sprayed Co-based alloy coatings were investigated by rotating bending tests. After fusing treatment, machining was done to manufacture three kinds of specimens having 0.3 mm, 0.5 mm and 1.0 mm in coating thickness. The fatigue strength of all coated specimens was much higher than that of the substrate and that of the grit blasted ones. Especially, it was found that the fatigue strength of the specimens with 1.0 mm coating were higher remarkably compared to that of ones with 0.3 mm and 0.5 mm coatings. The result showed that at lower stress levels, the fatigue cracks were initiated inside the substrate of 0.3 mm and 0.5 mm coated specimens only (Internal failure in the substrate). At higher stress levels the fatigue cracks were initiated at the coating layer, which were originated from the porosity located in coating layer of all coated specimens (Surface failure in the coating layer). But no fracture mode transition was noticed for the case of 1.0 mm coated specimen. Discussion have been made on the cause of the fracture mode transition depending on the coating thickness on the basis of the results of the calculation of the stress at the location where the fatigue fracture was initiated.