Effect of N2 + H2 gas mixtures in plasma nitriding on tribological properties of duplex surface treated steels

Thermo-reactive diffusion chromizing followed by pulsed plasma nitriding were carried out on AISI 52100 and 8620 bearing steels. The chromized samples were pulse-plasma nitrided for 5 h at 500 °C in various N2–H2 gas mixtures. The coated steels were characterized using scanning electron microscopy, X-ray diffraction and microhardness testing. The unlubricated wear behaviors of only chromized and duplex treated steels were investigated in ball-on-disc system tests at room temperature. X-ray diffraction patterns of the duplex treated samples containing H2 indicated the formation of dominant CrN and Cr2N nitrides as well as the formation of Cr3C2 and Cr7C3 carbides. Gas mixtures in the plasma nitriding, which was performed after chromizing, have a significant influence on the wear rate of the duplex treated steels. The wear and friction tests showed that the lowest friction coefficient and wear rates were observed for the samples duplex treated in a 50%N2 +50%H2 plasma. Conversely, the lowest wear resistance was observed on the samples duplex treated in a gas mixture of 75%N2 + 25%H2, probably due to formation of a hard and brittle layer.