Railgun Tribology—Chemical Reactions Between Contacts

Many electromagnetic launcher experiments have been conducted with armature-rail pairs made of dissimilar alloys. When the armature is made of an aluminum alloy, the eventual formation of aluminide intermetallics is likely at the sliding contact interfaces with nonaluminum rails. Because most launcher bores are freely accessed by laboratory air, other reactions-oxidation, nitridation, and water vapor-may also occur during the reactive phase of the transfer of aluminum from the armature to the rail surface. The structure and composition of the thin layer that adheres to the rail contact surfaces is important, because it separates the new armature contact in a subsequent launch from the original rail contact surfaces and, thus, affects current transfer and the contact lubrication process. The chemistry and the morphology of the reaction layer on the rails will vary along the length of the launcher. In the armature start-up and low-velocity regions of the rails, the adhering rail deposit shows evidence of liquid-metal-enhanced, high-temperature reactions. Experimental data confirming the presence of these reaction products are presented for the case of the very low-speed and highly energetic propulsion of a copper armature between aluminum alloy rails. A copper aluminide intermetallic-CuAl₂-is formed, as predicted from the binary phase diagram. Intermetallics have properties that will degrade the multishot load-carrying capability of rail conductors