Abstract

The demand for parts and the continuous need to reduce manufacturing costs have promoted a constant growth in steel powder metallurgy. However, these materials have a certain porosity and are more subject to wear. As a result, several techniques have been developed to improve the tribological behavior of sintered steels, such as surface treatments like plasma nitriding. This work evaluated the synergistic tribological effect between the plasma nitriding treatment and the impregnation of the residual pores of sintered steel (Fe + 0.5 wt% C) with graphite under dry and low-viscosity (4.8 cSt) oil-lubricated conditions. Under dry conditions, nitrided steel exhibited a high average coefficient of friction (COF) of similar to 0.36, which was reduced to 0.13 with graphite impregnation. The combination of nitriding and impregnation further reduced the COF to similar to 0.09, demonstrating a synergistic effect. The wear-rate decreased significantly from similar to 10(-3) mm(3)/N m for nitrided samples to similar to 10(-6) mm(3)/N m for nitrided and impregnated samples. In oil-lubricated tests, the COF of nitrided steel was reduced from 0.13 to 0.09 when graphite was introduced, with wear-rates dropping by an order of magnitude from similar to 10(-5) mm(3)/N m to similar to 10(-6) mm(3)/N m. The results suggested a synergistic positive effect of the solid lubricant impregnation process on nitrided sintered steels, especially due to the formation of a homogeneous carbon-rich tribolayer on the tribopair surfaces.