Study On Electrical Wear And Arc Erosion Properties Of Cu-based Self-lubricating Composites

Sliding electrical contact materials are used to conduct current between stationary part and moving part of devices. Performance of sliding electrical contact materials is greatly affected by the combined effects of mechanical and electrical loading. It is desirable for sliding electrical contact materials to possess as little electrical and thermal resistance as possible, meanwhile excellent anti-friction and wear-resistance properties are also required. Cu-graphite composites are the most widely used sliding electrical contact materials due to their superior combined properties of high mechanical strength, excellent electrical and thermal conductivities and self-lubricating property. However due to the rapid development of motor and aerospace industries, the conventional Cu-graphite composites gradually cannot meet the requirements.Cu-based self-lubricating composites containing graphite at weight fractions in the range of 30%,25%,20%,15%, corresponding to WS2 at weight fractions in the range of 0%,5%,10%,15%, were fabricated by hot-pressed method. The effect of content on the electrical friction and wear behaviours of the Cu-graphite-WS2 composites was investigated using a block-on-ring tribometer rubbing against Cu-5wt%Ag alloy ring. The results demonstrated that 800 ℃ was the optimum sintering temperature for Cu-graphite-WS2 dual-lubricant composites to obtain the best comprehensive properties of mechanical strength and lubrication performance. Contact voltage drop of the composites increased with the increasing mass proportion of WS2 to graphite. Cu-20wt%graphite-10wt%WS2 composite showed the best wear resistance due to the excellent synergetic lubricating effect of graphite and WS2. Reasonable addition of WS2 into Cu-graphite composite can remarkably improve the wear resistance without much rise of electrical energy loss.The tribological behaviours of Cu-graphite-WS2 composite with and without current were investigated in both air and vacuum, the effect of WS2 nanotubes was also analyzed. The results showed that the friction coefficient and wear rate of the composite with current were higher than those without current in both air and vacuum due to the adverse effects of electrical current damaged the lubricating films and intensified the adhesion between the frictional surfaces. Because of the synergetic lubricating action of oxides of Cu, WS2 and graphite, the composite showed low friction coefficient and wear rate in air; while in vacuum, only WS2 can provide lubrication at the tribo-interfaces and serious adhesion and plough occurred, resulting in the high friction and wear. The low contact voltage drop in vacuum was attributed to the high content of copper in the lubricating films and extremely high tribo-interface temperature which caused the real electrical conduction area increased. Adding small amount of WS2 nantubes into composite to replace partial WS2 powder can improve wear resistance and decrease contact voltage drop.The effects of current polarity, current density and sliding velocity on the electrical sliding wear behaviours of Cu-based self-lubricating composites were investigated. The results indicated that the positive brush suffered a higher wear rate than the negative brush in air since the directional movement of ions activated oxidation at the positive brush surface while inhibited oxidation at the negative brush surface. Besides the regular wear, the combined effect of molten metal bridge erosion and arc erosion caused the positive brush to lose extra materials and the negative brush to gain extra materials, which made the positive brush to show a higher wear rate in vacuum. The contact voltage drop of the positive brush was lower than that of the negative brush in air, but contrarily, it showed a higher contact voltage drop in vacuum. The contact resistance decreased but the wear rate increased as current density increased from 5A/cm2 to 15A/cm2 because the thermal effect of electrical current made the materials at "a-spots" soften or even melt and decreased the integrity of lubricating films. Due to the effect of the adhesive gaseous molecule films at the tribo-interfaces, with the rise of sliding velocity from 5m/s to 15m/s, the contact resistance increased and the wear rate reached the minimum at the sliding velocity of 10 m/s.Arc erosion properties of Cu-30vol%gaphite, Cu-30vol%WS2 and Cu-30vol% MoS2 composites were investigated. The results demonstrated that graphite cannot be melted and mainly lost in the form of oxidation during the arc discharge process because of its extremely high melting point. In contrast, WS2 and MoS2 were melted and reacted with Cu matrix due to the high temperature caused by arc discharge. Therefore, the arc erosion resistance of Cu-30vol%gaphite composite was better than those of Cu-30vol%WS2 and Cu-30vol%MoS2 composites. The main arc erosion mechanisms of Cu-based self-lubricating composites were found to be oxidation, melting, ejecting, internal chemical reaction and fatigue peeling.