Tribological Property and Corrosion of Imidazolium-Based Ionic Liquid-Lubricated Steel Sliding Against Anodic Oxide Film on Al-Li Alloy in the Presence of Bubbles Produced by Synergistic Action of Electric Field and Friction

Abstract

The tribological property and corrosion of a steel ball sliding against anodic oxide film (AOF) on Al-Li alloy lubricated by an ionic liquid (imidazolium hexafluorophosphate, LP108) are investigated without and with electric field. Upon sliding, no bubbles are observed without electric field (0 V) and with electric field at 1 V, while bubbles are produced continually with electric field at applied voltages of 5 V and 10 V. Higher voltage produces more bubbles. It is found that bubbles accelerate the worn-out of the AOF in sliding and corrosion of the steel ball after sliding. In the absence of bubbles, friction coefficient lower than 0.1 and long wear lifetime suggest that LP108 is a good lubricant at both 0 V and 1 V. In the presence of bubbles, however, AOF is rapidly worn out, i.e., 3.3±0.2 min at 5 V and 1.6±0.3 min at 10 V. Because the bubbles are produced continually at 5 V and 10 V, there is a rapid transition of the lubricating regime from a flooded state (boundary lubrication regime) to a bubble-dominated state (dry sliding regime). By turning off the electric field after one-minute sliding (no more newly born bubbles), the effective lubrication by LP108 can be preserved. Once the tribo-test is ended, the collapse of the bubbles occurs rapidly and hence the corrosion of the steel ball is greatly accelerated when compared to that in the absence of bubbles. In addition to bubble collapse, the bubbles are responsible for the accelerated corrosion of the steel ball because they are the reactive species of electrochemical decomposition of LP108 by synergistic action of electric field and friction, which is supported by experimental evidence. In other words, neither sliding friction without electric field nor electric field without sliding friction up to 10 V produces bubbles. A friction-activated mechanism is proposed to explain the synergistic action of electric field and friction. In short, sliding friction produces wear of the steel ball and this initiates friction activation of the worn surface of the steel ball. Consequently, the generation of bubbles occurs.