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Rotary bending fatigue tests have been conducted using DLC-coated alloy steels, SKD11, SKD61 and SNCM439, with different hardness and carbide or inclusion size in order to investigate the effect of DLC film on the fatigue behavior. DLC films with the thicknesses of 1μm and 15μm were evaluated. The effect of DLC film on fatigue strength was found to depend on substrate hardness, carbide or inclusion size and film thickness. DLC-coated SKD11 and SKD61 showed the similar fatigue behavior. In SKD11 and SKD61 with lower substrate hardness, the thin DLC film exerted little influence on the fatigue strength, but the thick DLC film enhanced the fatigue strength due to the transition of crack initiation from surface to subsurface. In SKD11 and SKD61 with higher substrate hardness, the subsurface crack initiation occurred at the higher applied stress at which cracks were generated at surface in the substrate material, resulting in the improved fatigue strength. In those materials, coarse carbides or nonmetallic inclusions invariably acted as the crack origin. In SNCM439 with the lowest substrate hardness, the fatigue strength was not affected by the thin DLC film, but was improved by the thick DLC film. With increasing substrate hardness, the subsurface crack initiation occurred at nonmetallic inclusions whose sizes were much smaller than carbides and nonmetallic inclusions in SKD11 and SKD61.

Effect of DLC Film on Fatigue Behavior in Alloy Steels with Different Hardness and Inclusion Size