A Study On Tribology Behaviors Of Nitrided And Sulfur-nitrided Surfaces Under Oil Lubrications

The anti-wear properties of steel contacting surfaces under boundary lubrications can be obviously improved by using plasma nitriding or sulfur-nitriding technology. The mechanical efficiency and durability of steel parts can also be enhanced by liquid lubrication. Thus the service lifes of AISI 4135 steels and AISI 52100 steels can be increased effectively by rational application of surface treatment and liquid lubrication. The synergistic effects of different concentrations of molybdenum dithiocarbomate (MoDTC) and zinc dialkyl- dithophpsphate (ZDDP) additives added in the lubricant and nitrided and sulfur-nitrided surfaces were studied with the aim of finding a most effective lubrication system.The synergistic effects between additives and modified layers were investigated using a small MS-T3000 ball-on-disc friction and wear tester. SEM equipped with EDS was employed to observe the morphologies of surfaces, cross-sections and wear scars. XRD was adopted to detect the surface phase structures. AES was used to detect the elements variation along depth of the nitrided and sulfur-nitrided layers and XPS was adopted to identify the valence states of boundary lubrication films.The experimental results showed that the anti-wear properties of steel surfaces have been improved remarkably under PAO4 oil lubrication when the nitriding or sulfur-nitriding technique was used, the sulfiur-nitrided surface possesses better anti-wear property than the nitrided surface. The friction coefficients of modified layers reduced also under 0.5wt% MoDTC lubrication, while no variation appeared on the surface of substrates. The three surfaces exhibited the lowest friction coefficients under 1.0wt% MoDTC lubrication. However the substrate surface possessed the highest value than other two modified surfaces, and the sulfur-nitrided layer showed the lowest value. The wear volumes of three surfaces were decreased also by lubrication with ZDDP additive. The wear volume of sulfur-nitrided surface was less than that of nitrided surface, and both modified layers presented better anti-wear properties than the substrate surface. AISI 52100 steels exibited longer wear time and less wear volume than AISI 4135 steel. The former possessed higher friction coefficient under MoDTC lubrication and similar values under ZDDP lubrication than the latter. The higher hardness of AISI 52100 steel was beneficial to support layers and boundary lubrication films strongly.