Tribological studies of thermally and chemically modified vegetable oils for use as environmentally friendly lubricants

Boundary lubrication in a dynamic mechanical system is primarily governed by the formation of a stable tribochemical film. Polar functional groups in the triacylglycerol molecule of vegetable oil in conjunction with oil-additive-metal interaction during the metal rubbing process can significantly improve the wear resistance and extreme-pressure lubrication. Increasing the polar functionality of vegetable oil structure has a positive impact on wear protection resulting from stronger adsorption on metal surface as well as greater lateral interaction between the ester chains. The results reveal that ester groups of triacylglycerol molecule react with phosphate groups to form a protective fatty-phosphite layer. Antimony impregnation into the metal surface following strong complexation reaction with metal sulfide results in significant improvement in extreme-pressure characteristics due to alteration in surface metallurgy. In this case, seizure is averted even when the fluid adsorbed layer and phosphite coating is removed because of antimony induced surface hardness. The difference can be observed by analyzing the metal surface using scanning electron microscope (SEM). Selected bio fluids viz. soybean oil (SBO), thermally modified soybean oil (TMSBO) and chemically modified soybean oil (CMSBO) were investigated for potential application as industrial fluids.