Fatigue strength and fracture mechanism of different post-fused thermal spray-coated steels with a Co-based self-fluxing alloy coating

The characteristics and adhesive strength of thermal-sprayed coatings at the interface between the coating and substrate are affected by the post-fusing treatment process. We investigated the effect of the adhesive strength on the fatigue strength and fracture mechanism of post-fused specimens. Rotating bending fatigue tests were conducted on specimens with a Co-based self-fluxing alloy coating on a medium carbon steel substrate. The post-fusing treatment was performed using a vacuum furnace, an electric furnace, or an induction heating system. A diffusion layer formed at the interface of the specimens treated in either furnace at 1100 °C for 4 h. These conditions produced a strong adhesive strength; the fatigue strength of these specimens also increased remarkably compared to the substrate-only specimens. However, the specimens treated with an induction heating system at 1100 °C for 120 s had a much lower adhesive strength since a diffusion layer was not formed, leading to delamination of the entire coating from the substrate during the first stage of the fracture process. The fatigue strength of these specimens was almost equal to that of the substrate-only specimens. Thus, thermal-sprayed coatings treated in vacuum and electric furnaces were more effective due to the formation of a diffusion layer, which contributed to improving the fatigue strength of the steel specimens. The expected maximum pore size of the coatings treated in vacuum and electric furnaces were estimated using statistics of extreme value.