A STUDY ON THE IMPROVEMENT OF THE SERVICE LIFE OF SHAFT-BUSHING TRIBOSYSTEMS BY PLASMA SULFUR NITROCARBURING PROCESS

摘要

To improve the service life of the shaft-bushing tribo system, the bushing was treated by plasma sulfur nitrocarburizing process. Before a plasma sulfur nitrocarburizing, the specimens were treated by plasma nitriding. As a result, a compound layer was formed with thickness about 20 μm. After then, by introducing a plasma sulfur nitrocarburizing process, a very fine and porous FeS layer with the grain size less than 100 nm was formed on the top of the compound layer. This fine and porous FeS layer formed on the samples led to the improved friction coefficient and resulted in the better wear resistance during the ball on disk test in dry conditions. Moreover in oil conditions, the fine and porous structure in the FeS layer could act as an oil pocket, so the wear properties would be much improved. So, it is considered that this fine and porous FeS layer could replace the PTFE and M0S2 coating that have been commercially used in busing parts. So, in this study, the endurance tests have been tired on the commercialized bushings and the plasma sulfur nitrocarburizing bushing by a rig test machine that was designed and made by our research team to estimate the durability of the plasma sulfur nitriding bushing. Although the S bushing had higher friction coefficient and resulting higher operating grease temperature, they had a longer expected service time that was determined on the point where a rapid increase of the friction coefficient occurred. This must be FeS layer on the top of the compound layer had enough endurance during the operation condition for the bushing. So it is considered that the S coating could be used for the busing instead of M coating.