Liquid Metal Boundary Lubrication of Sliding Electrical ContactsPublic Deposited
Liquid metal melt-lubrication of high-power and high-speed sliding electrical contacts improves electrical current collection and reduces friction. However, armature material loss may cause transition to arcing or plasma contact. Boundary lubrication of the sliding contact with low melting-point alloys can provide comparable improvement in current collection at reduced temperatures. Bismuth, tin, and indium alloys were examined at the material and small-caliber electromagnetic launcher scale for this purpose. High temperature friction measurements of bismuth-indium lubricated aluminum-on-aluminum sliding showed a sharp drop in sliding friction at the eutectic alloy melting point. Bismuth was found to be effective in replacing aluminum melt lubrication on copper rails at velocities up to 170 m/s in an augmented railgun. The launch efficiency, as measured by the effective inductance gradient, remained constant. Powder x-ray crystallography of the transfer film showed principally bismuth peaks and provided no evidence for copper-aluminide intermetallic formation. Low melting-point alloys were used to lubricate aluminum monolithic armatures sliding on aluminum cladding at velocities up to 360 m/s. Arcing and melting of the cladding occurred when the aluminum-aluminum sliding contact was not mechanical pre-loaded. This led to a decrease in the effective inductance gradient over five consecutive launches. Adding bismuth effectively suppressed arcing and reducing the motion lag due to static friction at the start of the launch. The launch efficiency of the boundary-lubricated armatures was comparable to aluminum melt-lubricated armatures over five launches. The porosity of the solidified transfer films can be explained by air entrainment, a mechanism previously not considered in high-velocity sliding electrical contacts. Air entrainment occurs in forced wetting when the three-phase contact-line velocity exceeds a critical wetting velocity. The critical wetting velocity is estimated to be less than 40 m/s for liquid bismuth at 300°C, which is exceeded nearly instantaneously at launch. These results suggest that air entrainment is one source of transfer film porosity Contrary to expectation, the lubricant transfer film did not dewet from the cladding or overlay surface even when dewetting is thermodynamically favored. Film dewetting is most likely kinetically constrained by the rapid cooling and solidification of the film.