High temperature tribological behaviour of lubricating emulsions

This study investigates the oil release mechanism of oil in water emulsions at temperatures well over the boiling point of emulsions. Emulsions E1 and E2 were prepared by shearing two different types of base oils in water. The tests were performed using a modified wear and friction tester with ball-on-ring configuration operated at temperatures up to 300 °C. The ring samples were made of AA 5182 aluminium alloy and were rubbing against SAE (AISI) 52100 type bearing steel ball in lubricated condition. The tests were performed in a range of loading conditions (e.g. load from 4 to 24 N and sliding speed from 0.1 to 0.25 m/s). The sliding distance for each test was dependent on the breakdown point of the emulsion, which was attained by increasing the sample temperature, and could reach to a maximum of 50 m. It was shown that for each type of emulsion there was a critical temperature above which the emulsion could not release oil to the contact surfaces. At and above this temperature, friction coefficient increased dramatically and severe material transfer to the steel ball was occurred. The critical temperature was found to be independent of load, sliding speed and type of sliding materials, but it was strongly related to the oil concentration in emulsions. It was shown that at the critical temperature, film boiling was inhibiting oil release to the contact surface, resulting in dry sliding condition. The critical temperature of an emulsion was defined as the temperature at which the vapour layer between the emulsion and the hot surfaces was so thick that it was able to separate all the components of the emulsion from the hot metal surfaces.