Effect of Hybrid Surface Treatment Composed of Plasma Nitriding and Various DLC Coatings on Properties of Stainless Steel SUS316

The effect of hybrid surface treatment composed of plasma nitriding and various DLC (diamond-like carbon) coatings on the tribological properties, the mechanical properties and the fatigue properties of austenitic stainless steel SUS316 was comprehensively investigated. In this hybrid surface treatment, plasma nitriding was made as a pre-treatment and then the following three kinds of DLC films were coated by UBMS (unbalanced magnetron spattering) method: a regular DLC film, a silicon-doped DLC (Si-DLC) film and a titanium-doped DLC (Ti-DLC) film, respectively. As the test temperature was increased, the change to the graphite structure with high solid-lubricity was promoted, so the friction coefficients of all DLC-coated materials were reduced. The friction coefficient of the Si-DLC material was the lowest at room temperature, but it became comparable to that of the regular DLC material when the test temperature was beyond 373 K. The Ti-DLC material had a higher friction coefficient than the other two DLC-coated materials without depending on the test temperature. Since the microstructure of the substrate was not influenced by plasma nitriding and each DLC coating, there was no marked change in the mechanical properties with them. The hardened layer formed by plasma nitriding effectively suppressed the initiation of fatigue cracks from the surface and markedly improved the fatigue strength. Also in all DLC-coated materials, since the hardened layers formed under the DLC films suppressed the initiation of fatigue cracks, their fatigue strengths reached the same level as that of the plasma-nitrided material.