Tribological Behavior of Nitrided Ductile Iron D6510 under Different Contacting Conditions

Abstract

Two different nitriding techniques, named plasma nitriding and fluidized bed nitriding, were applied for surface modification of ductile cast iron NAAMS-D6510. Two test methods (i.e., Pin-on-disc tribotest and inclined sliding wear test) were used to generate different tribological contact-loading conditions. While the pin-on-disc (POD) test had a vertical contact and relatively low stress applied on the mating surfaces, the inclined sliding wear test created a tilting contact with extremely high pressing pressure. The cross-sectional hardness and nitrogen concentration profiles along the case hardened layers were measured using a microhardness tester and energy dispersive spectroscope (EDS), respectively. Surface profilometry and scanning electron microscopy (SEM) were employed to measure and observe wear tracks after the tribological tests. The test results showed that the plasma nitrided sample was subjected to more severely abrasive/adhesive wear with numerous surface fatigue cracks, compared to the sample treated by fluidized bed nitriding, during the highly stressed inclined sliding wear test. This phenomenon was found to be attributed to their different hardness and nitrogen concentration profiles. A smaller hardness gradient between the compound layer and the case diffusion zone, and a thicker case hardened layer (provided from the fluidized bed nitriding in this study) could improve the wear resistance and the surface fatigue cracking resistance against the high contact loads. On contrast, less difference could be observed between the two different nitriding-treated samples during the POD tests. Moreover, the oxidation layer on the top of the compound layer of plasma nitrided sample may be beneficial for friction reduction under a low contact pressure condition like the POD sliding, but it would be soon removed and lost its advantage under the high contact pressure condition as for the inclined sliding case.