The performances of a thermally sprayed Fe/Ni composite coating to resist fretting fatigue under rotational bending loads

Abstract Fretting fatigue is the main failure mechanism of railway axles under rotation bending loads. A coating to resist fretting fatigue consisting of a nickel bottom bonding layer and a ferrite and austenite dual-phase top layer was prepared by thermally spraying the axles. Fatigue experiments on the coated and uncoated axles were conducted using equipment simulating a pair of actually assembled axles and wheel loading conditions. The fatigue fractures of the axles and the effects of the coating resistance were investigated. The results showed that the early extending path of the fretting fatigue crack of the uncoated axle formed a half-bowl shaped region, and its area and the angle between the crack propagating the path and the loading direction decreased with the bending loads. The coated axles obtained much better fatigue properties in terms of both super long-life and high stress ranges. The laterally lamellar coating effectively protected the axles from fretting damage and surface cracks in the vertical propagation. Although the top coating exhibited severe strain hardening and fretting wear, the soft nickel bottom layer did not deform and remained bonded to the axle substrate completely after fatigue experiments. The thermally sprayed processing did not damage the axle interface apparently, and its interfacial, thin, recrystallised, fine-grain layer and rolling-pressed, plastic deformation microstructure maintained good rotation-bending fatigue resistance. As a result, the fretting fatigue strength of the coated axle was greater than 50% than that of the uncoated axles.