Experimental Study on the Effect of Adsorption of Stearic Acid on Lubrication with Limited Lubricant Supply

Rolling bearings are important machine components, and proper lubrication is necessary to stable operation and long service life of the bearings. However, when the lubricant on the raceway of the rolling bearings are squeezed to both sides of the raceway by centrifugal force and repeated rolling, it may cause lack of oil to form passive limited oil supply lubrication, thus affecting the oil film thickness and friction. The oiliness agent is a commonly used as an additive in lubricating oils, which can adsorb on the solid surfaces to form an adsorption film with low shear resistance, thus lead to low friction in boundary lubrication. On the other hand, the in-situ adsorption of oiliness agent can reduce the surface energy of a tribo-pair and weaken its wettability to lubricating oils. This change of wettability will affect the distribution of lubricating oils on the lubrication track with limited lubricant supply, such as in the form of the discontinuous droplets or stripes due to dewetting from low surface energy, which can definitely influence the oil supply of the tribo-pairs and thus the behaviours of full film lubrication. However, researchers have mostly studied the anti-friction of oiliness agents under boundary lubrication regime. There is almost no research work reported on elastohydrodynamic lubrication with limited oil supply under conditions of low surface energy by oiliness additive adsorption. Thus in this study using a custom-made optical ball-on-ring test rig the elastohydrodynamic lubrication of the ball and the raceway in rolling bearings was simulated by the steel ball and the inner surface of the glass ring. In order to obtain a high contrast interference image and anti-wear of the contacts, the inner surface of the glass ring was coated by a bi-layer film (bottom Cr+ top SiO₂) with a reflectivity of 20%. It was known that the adsorption of metal surfaces was strong. The inner surface of glass ring was modified by plasma to enhance its adsorption of stearic acid molecules. The distribution of PAO10S that was discontinuous discrete strip on the lubrication track of the surface of glass ring and steel ball was observed by camera, while the distribution of PAO10 was continuous thin oil film. The adsorption of stearic acid on the surface of steel ball was proved by Fourier transform infrared spectroscopy. PAO10 (poly α-olefin) and PAO10S (poly α-olefin+0.2% mass fraction of stearic acid ) were used as the lubricants and their film thickness and friction force of were measured with slide-roll ratios of 0 and 0.1. The results showed that as the entrainment speed increases, the oil film thickness first increased and then decreased. Corresponding to the film thickness turning point, PAO10S had a higher critical speed than PAO10. Consequently when the entrainment speed was higher than the critical speed of PAO10, the film thickness of PAO10S still increased and much higher than that of PAO10. For the slider-roll-ratio of 0.1 the friction force decreased first and then increased with increasing entrainment speed. When the oil supply quantity was smaller, the stearic acid adsorption make the overall friction coefficient decrease significantly. The effect of stearic acid became weaker with the increase of oil supply. The enhancement of the lubrication in the present study can be attributed to the reduction of the surface energy of the lubrication track by the stearic acid adsorption film. The lubricant was distributed in form of discrete strips due to dewetting, which was beneficial to the oil replenishment on the lubrication track.