Influence of oil polarity and material combination on the tribological response of greases formulated with biodegradable oils and bentonite and highly dispersed silica acid

Different biodegradable lubricating greases formulated with esters of fatty acids, as base oils, and bentonite and highly dispersed silica acid, as thickener agents, were tribologically investigated in a nanotribometer and compared with polyalpha olefin greases with equal thickeners. Material combinations of steel ball on steel disc and sapphire ball on steel disc were used with different normal loads. Several friction and wear effects were found depending on the thickener and the base oil. The influence of grease components is also different in both material combinations evaluated. On the one hand, the base oil exerts a much higher impact on friction and wear in grease systems thickened with highly dispersed silica acid than in those thickened with bentonite. On the other hand, the latter reacts more sensitively to a change in material combination. Results were discussed and explained on the basis of polarity influences of the base oils and solid surfaces. Copyright © 2012 John Wiley & Sons, Ltd.