Influence of humidity on the tribological performance of unmodified soybean and sunflower oils

In general, vegetable oils exhibit superior lubrication properties but lack the thermal stability of petroleum base stocks. However, vegetable oils could make an ideal candidate as a base stock for lubrication applications involving high humidity levels, such as marine and offshore applications. This study focuses on the friction and wear rate of unmodified soybean and sunflower oils in comparison with an unformulated mineral oil at various levels of relative humidity, ranging from 10% to 98% RH. It was observed that the vegetable oils retain their friction and wear reducing capabilities much better than the mineral oil at high humidity levels. This was attributed to their inherent ability to react with the metallic contacting surfaces and form multilayers of soap films. Furthermore, the soybean oil provided a superior level of wear resistance when compared with the sunflower oil at extremely high levels of relative humidity due to its lower viscosity and differences in chemical compositions. Copyright (C) 2011 John Wiley & Sons, Ltd.