Fatigue Behavior of Plasma-Assisted Duplex Surface-Treated Steels

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Cantilever-type rotating-bending fatigue tests in air were conducted using smooth specimens of alloy tool steel, JIS SKD61, with three kinds of surface treatment ; namely, TIN coating by the PVD method, plasma-assisted ion-nitriding, and duplex treatment with plasma-nitriding followed by TiN coating. Fatigue strength of the surface-treated specimens was improved as compared with untreated specimens. The increase in the fatigue strength of the specimen by surface treatment was due to the transition of the fatigue-crack initiation site from the surface to the subsurface by an effect of the modified surface layer becoming barrier to the formation of surface crack nuclei beneath the TiN coating film. The difference in fatigue strength between the duplex surface-treated specimen and the plasma-nitriding one, for which the crack initiation site was subsurface, is discussed through the effect of the TiN coating film on subsurface crack growth behavior. The fracture surface was reconstructed by topographical data and a fracture process computationally simulated with fracture surface topography analysis. It was found that thc TiN coating film acts as a barrier to formation of a plastic zone at the front of a subsurface crack and retards crack growth toward the surface.