High temperature wear and friction properties of duplex surface treated bearing steels

Duplex treatments by thermo reactive diffusion (TRD) chromizing and puls plasma nitriding were carried out on AISI 52100 and 8620 bearing steels. Tribological behaviors of TRD chromized and duplex treated bearing steels were investigated against A1203 ball in ball-on-disc system at room temperature and 500 degrees C. The samples were pack chromized in a furnace at temperature of 1000 degrees C for 5 h. After chromizing, the samples were puls plasma nitrided for 5 h at 500 degrees C. The coated steels were characterized using scanning electron microscopy (SEM), energy dispersive Xray spectroscopy (EDX), X-ray diffraction (XRD), scratch and microhardness testing. Plasma nitriding of chromized steels increased the total thickness of the compound layer. The subsequent plasma nitriding increased the surface hardness to 2135 HK0.025 due to the formation of CrN and Cr2N. The surface hardness and scratch resistance of coating can be increased with duplex treatment of chromizing followed by plasma nitriding, resulting in high wear resistance. Tribological tests indicated that puls plasma nitriding process decreased the coefficient of fTiction values and wear rate of the chromized steels at room temperature and 500 degrees C. Also, examination of the wom surface of the samples showed that particularly at high temperature, the oxidized compact layer occurs and tribo-oxidation played an important role in oxidation behaviour of the steels after the duplex treatment. (c) 2007 Elsevier B.V. All rights reserved.