Vegetable oils are increasingly used as lubricant base oils, because of sustainability issues and regulations. In present study coconut, palm, sunflower and castor oils were used to represent different levels of unsaturation, where as castor oil was selected to represent influence of hydroxyl group. The effect of oxidation on the boundary lubrication properties was evaluated by subjecting the oils to accelerated ageing at 333,353 and 373 k according to AOCS method The experiments were carried out by using of four ball tester, increase in observed wear scar diameter with temperature was more with oil having higher percentage of unsaturated fatty acids. At the same time it was also observed that increase in wear scar diameter with temperature was more with castor oil compared to other oil samples. The poor boundary lubrication properties observed with aged oil samples was possibly due to the deterioration of the triglyceride structure.
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