Deposition of Lubricant Layer by Electrical Discharge Machining during Finishing Process (2nd Report)

Abstract

Solid lubricants, such as molybdenum disulfide (MoS2), gold, silver or polytetrafluoroethylene (PTFE) are often used on the sliding parts exposed in the space environment. When MoS2 powder is mixed in the working oil, MoS2 layer can be deposited on stainless steel during the finishing electrical discharge machining (EDM) process. The deposition process must be clear to improve the endurance of the lubricant layer. In this paper, the deposition process is discussed with the conditions such as the discharge location and the gap distance monitored during EDM and the influence of some other machining conditions investigated through the friction test. Because these phenomena are the same as those in EDM with Si powder, the main factors to make the surface smooth should be the discharge dispersion and the expansion of the gap distance. Then the reciprocating friction test in air was carried out to investigate the influence of machining time and measured point of the deposited layer. A low friction coefficient held out at the areas near the front edge and at the center of the deposit for long lubrication duration. The biased concentration in a gap between the electrode and the specimen caused the unstable lubrication duration. Because the lubrication duration was saturated over 30 min, the deposited MoS2 was removed during deposition in the case of machining for a long time. The influence of the thermal properties of the specimen was also investigated. MoS2 could be deposited on the metals with lower melting point than MoS2.