TRIBOLOGICAL PROPERTIES OF PLASMA AND PULSE PLASMA-NITRIDED AISI-4140STEEL

Plasma nitriding is usually used for ferrous materials to improve thei r surface properties. Knowledge of the properties of thin surface laye rs is essential for designing engineering components with optimal wear performance. In our study, we investigated the microstructural, mecha nical and tribological properties of plasma- and pulse plasma-nitrided AISI 4140 steel in comparison to hardened steel. The influence of nit riding case depth as well as the presence of a compound layer on its t ribological behaviour was also examined. Plasma and pulse plasma nitri ding were carried out using commercial nitriding processes. Nitrided s amples were fully characterised, using metallographic, SEM microscopic , microhardness and profilometric techniques, before and after wear te sting. Wear tests were performed on a pin-on-disc wear testing machine in which nitrided pins were mated to hardened ball bearing steel disc s. The wear tests were carried out under dry conditions where hardened samples were used as a reference. The resulting wear loss as well as the coefficient of friction was monitored as a function of load and te st time. Several microscopic techniques were used to analyse the worn surfaces and wear debris in order to determine the dominant friction a nd wear characteristics. Results showed improved tribological properti es of AISI 4140 steel after plasma and pulse plasma nitriding compared to hardening. However, the compound layer should be removed from the surface by mechanical means or by decreasing the amount of nitrogen in the nitriding atmosphere, to avoid impairment of the tribological pro perties by fracture of the hard and brittle compound layer followed by the formation of hard abrasive particles. (C) 1998 Elsevier Science S .A. All rights reserved.