Lubrication Enhancement by Micropillar Array Surface with Oil Capture under Limited Lubricant Supply

In the practice of lubrication, excessive lubricating oil is often proposed to be supplied to form higher film thickness and reduce the wear of contact pair in industry, which brings a series of problems such as the rise in oil temperature, resource waste and environmental pollution. With the continuous development of the concept of green lubrication, accurate and efficient lubrication with limited oil supply has attracted researchers’ attention and is becoming a new point in tribology research, which can change the problems brought by traditional oil supply approach. The purpose of limited lubrication is to improve the utilization efficiency of lubricant and ensure the lubricant supply of tribo-pairs. Inhibiting the spreading of lubricant on the surfaces of tribo-pairs or enhancing the replenishment of lubricants are critical to improve the performance of limited oil supply. People have proposed various strategies to achieve this purpose such as the use of surface texture, chemical coatings or assistant of physical structure. In this paper, a lubrication enhancement surface with oil capture capability was prepared through the formation of stepped wettability gradient, which was generated by the central oleophilic lubricating track and micropillar arrays (MA) with anti-fingerprint coating (AFC). The film thickness and shape of the oil pool on this fabricated surface under reciprocating motion were measured on a ball-on-disc test rig with limited oil supply. The influences of reciprocating frequency, width of lubricating track and applied load on the lubrication properties were investigated. Meanwhile, the lubrication performance of micro-pillar array surface without AFC modification and the surface only modified with AFC were studied. The results showed that the state of oil droplets on prepared AFC-modified micropillar array surface changes from Wenzel state to Cassie-Baxter state, thus it presented stronger oleophobic property and the lubricant oil gets mostly pinned on the lubricating track. Compared to the other two oil gathering surfaces, the micro-pillar array surface had a better lubrication performance. Through enhancing the replenishment of lubricants on both sides of lubricating track, the oil supply at the middle point of reciprocating stroke and two end points was obviously improved under the reciprocating motion (4 Hz), and the lubrication state gets improved with a higher film thickness. The measured central film thickness presented film thickness higher by up to 51.4% than the original surface. The experimental results showed that the oil replenishment was affected by the width of lubricating track. The oil collection capacity was the strongest when the central lubrication track width was close to the radius of the Hertz contact region. With the increase of the track width, the lubricant distribution on both sides of lubricant track became more dispersed, and the lubricating oil requires longer reflow time, resulting in smaller oil collection capacity of a micro-pillar array surface. The applied load presented significant influence on oil collection capacity of this new surface. Increase in load enhances mechanical separation of lubricant and deteriorates the capillary effect, resulting in the decrease of the oil collection capacity and then lower film thickness. This study provided a new approach for improving the oil replenishment under limited lubricant supply.