抄録
Rotating bending fatigue tests have been conducted at a room temperature in laboratory air using specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6Al-4V) with high velocity oxy-fuel (HVOF) sprayed coating of a cermet (WC-12%Co) and S45C with wire flame spraying (WFS) sprayed coating of a 13Cr steel (SUS420J2), and the fatigue strength and fracture mechanisms were studied. The fatigue strength evaluated by nominal stress was strongly influenced by substrate materials and the thickness of sprayed coatings. Detailed observation of crack initiation on the coating surface and fracture surface revealed that in the WC cermet-sprayed materials, small defects initiated at WC grain boundaries coalesced and then the crack grew rapidly in the coating, while in the 13Cr steel-sprayed material, many microcracks were initiated from defects on the coating and coalesced to be a main crack. Cracks were initiated in the substrate due to the stress concentration of the crack in the coating, which was modeled by finite element analysis. The fatigue strength of the sprayed materials was dominated by that of the sprayed coating. Thus, the fatigue strength could be evaluated uniquely in terms of the true stress on the coating surface.
