Effect of hybrid surface treatment composed of nitriding and DLC coating on friction-wear properties and fatigue strength of alloy steel

Abstract

This study was conducted to comprehensively investigate the effect of hybrid surface treatment composed of nitriding and DLC (diamond-like carbon) coating on the friction-wear properties, mechanical properties and fatigue strength of JIS SCM440 alloy steel. In the hybrid surface treatment, plasma nitriding and gas nitriding were performed as pretreatments to form hardened layers with different thicknesses. DLC layers were then generated on the surfaces of both nitrided materials under the same treatment conditions. The obtained results showed that gas nitriding as a pretreatment was more effective in improving the friction-wear properties of DLC-coated alloy steel than plasma nitriding. This was because gas nitriding formed a much thicker hardened layer which could maintain the planarity of the DLC layer even if a high contact force was applied. The examined hybrid surface treatment did not affect the internal microstructure so there was no influence on the tensile strength. Moreover, gas nitriding improved the fatigue strength of DLC-coated alloy steel to a much higher level than plasma nitriding.