The growth of thin lubricating films of plant oils

The extreme conditions of high pressure and shear imposed in a lubricated sliding contact could promote tribochemical reactions and polymerization over long sliding distances. These chemical changes are likely to alter the film thickness and friction. Unsaturated plant oils inherently possess two characteristics that would favour the process of polymerization at the lubricant-surface interface – poor oxidation stability and a high affinity with metals. Boundary films formed by plant oils could possibly polymerize and thicken when they are subjected to long periods of lubricated sliding. A new tribometer, capable of simultaneously measuring film thickness, friction and wear was developed to test this hypothesis. The instrument is capable of measuring film thickness measurements down to 1 nm by employing a modified capacitance method. In all, thirteen refined and unrefined plant oils were tested. The results revealed for the first time, that boundary films of some plant oils can grow to thicknesses much greater than their adsorbed molecular heights. Some films grew as much as 25 times in thickness (unrefined canola oil), while others remained roughly unchanged (flaxseed and olive oil), or grew slightly and then collapsed during the test (safflower oil). It was initially expected that the film growth rate could be correlated to the degree of unsaturation of the oils. However, results from gas chromatography mass spectroscopy (GCMS) analysis revealed little evidence of such correlation. It follows that minor components, particularly in the unrefined oils, might have had an influence on the film growth behaviour. To further investigate this, surface-enhanced Raman spectroscopy (SERS) was used to study and compare the differences in the chemical nature of the boundary films formed by refined and unrefined canola oil. This is the first time that metallic surfaces lubricated by plant oils have been studied using this technique. The results of this work provided strong evidence that fatty acids were liberated from the triglyceride structure during sliding to form a fatty acid soap layer on the silver surface. The study also revealed that the fatty acid chains of the unrefined canola oil were more disordered and most likely in a gauche conformation, while that of the refined canola oil were tightly packed and oriented perpendicular to the surface. Using inductively-coupled plasma mass spectroscopy (ICP-MS), comparatively higher levels of phosphorus, were also found in the unrefined canola oil and unrefined macadamia oil samples, both of which had experienced greater film growth. It is believed that the phosphorus found originated from phospholipids in the unrefined oils, could have been directly or indirectly related to the film growth observed. Its role and the mechanism in which it affects film growth, however, remain to be elucidated.