Tribochemical Interactions Between The Nitrided/Sulfurized-nanocrystallized Layers And Lubricant Additives

The piston ring works in the condition of high temperature, high pressure, high speed and lack of enough lubricant, leading to the severe wear at the sliding interface with the cylinder wall. This will shorten the life of the piston ring. Improving mechanical properties of the material by surface modification technologies and the lubrication condition are the main methods to improve the tribological properties of the pison ring. However, a single technology is insufficient to meet the requirements of specific working conditions.316L stainless steel is a typical material for the piston ring, but the low hardness and poor tribological properties of the steel can shorten the life of the component subject to wear. In this work, in order to improve the tribological properties of the steel, ultrasonic surface rolling processing(USRP), plasma-nitriding and ion sulfuration technologies were used to prepare the nanocrystallized layer, nitrided/sulfurized-nanocrystallized layers on the steel surface. The friction and wear behaviors of the modified layers were examined by a SRV tribometer under synthetic oil polyalphaolefin(PAO4) without and with 1.0wt% Mo DTC/ZDDP lubrications. SEM, 3D surface profilometer, optical microscope, XRD and microhardness tester were used to investigate the micro morphologies, structures and hardness of the layers. XPS was used to identify the valence states and contents of the elements in the tribofilms. The results are as follows:(1) The average grain size of the top surface layer of 316 L stainless steel was refined to 19 nm and the hardness was increased from 190 HV to 340 HV after USRP treatment. Mo DTC can reduce friction by forming a Mo S2-containing film on the tribological contact, while ZDDP can reduce wear by forming glass polyphosphate film. USRP sample exhibits the lowest coefficient of friction under Mo DTC lubrication and the lowest wear rate under ZDDP lubrication. This may be attributed to the higher hardness and surface reactivity of the USRP surface.(2) The hardness of both the nitrided un-USRP and USRP surfaces increased to about 1000 HV and the nitrided layers were about 20 μm thick. The tribological properties of the steel under different lubrications can be improved after nitriding. Furthermore, nitrided USRP surface can promote the decompositions of Mo DTC and ZDDP, resulting in the better synergistic effects on low friction and antiwear performances of the steel surface.(3) Under Mo DTC and ZDDP lubrications, the sulfurized USRP surface exhibited better tribological properties than the sulfurized un-USRP surface, which is attributed to the nanocrystallized layer with higher hardness supporting the soft sulfurized layer.