Tribological behaviour of nitrided and nitrocarburized carbon steel used to produce engine parts

The purpose of this study is to select a proper surface treatment to enhance wear resistance of engine camshafts. The camshaft is a relevant part of a diesel engine which works under torsion, fatigue and wear efforts. They are usually manufactured by casting, forging or machining from forged bar of low alloy steels, and in most cases, the machined surfaces are quenched and tempered by induction heating. After that, in many cases, to withstand the efforts imposed on the active surfaces and improve tribology and fatigue properties, the industry used for decades, thermochemical technologies such as salt bath or gaseous nitriding and nitrocarburizing processes.

This paper studied the effects of plasma nitriding and plasma nitrocarburizing, on the tribological behaviour of the steel SAE 1045HM3 proposed to produce camshafts. After the plasma treatments, the change in surface roughness was measured; the modified layers were studied by X-ray techniques and its thickness by optical microscopy. The diffusion zone was evaluated by Vickers microhardness determinations. Tribology tests were performed by pin-on-disc configuration using WC ball as a counterpart.

Results show that plasma nitrided samples present the best tribological behaviour compared with the nitrocarburized ones; also, the influence of the roughness produced by the thermochemical processes appears to be important.

Although both the plasma treatments have been applied for many years, and also reported separately in the scientific literature, there was no information comparing these two treatments for carbon steels, and also, there is not much about tribology in lubricated conditions of nitrided and nitrocarburized carbon steels. In fact, it is not proved that the porosity of the nitrocarburized layer is beneficial for wear resistance in lubricated conditions. In this paper, it was proved that at least in the tested conditions, it is not.

Gas or plasma nitrocarburizing is usually recommended for this kind of applications, although the modified layer is porous. This paper attempts to prove that nitriding could be better than nitrocarburizing, even with a thinner white layer.