In practical applications, thermal gradient will generate on the surfaces of mechanical components due to friction, which will result in the lubricant spontaneously loss and even lubrication failure. For example, in high-speed rolling element bearings characterized by elastohydrodynamic lubrication mechanism are prone to work in starvation states, mainly because the lubricant is displaced out of the rolling track and has less chance to replenish. In this paper, a wedge-shaped surface with a function of lubricant droplet self-migration was prepared. The wedge-shape pattern was arranged on both sides of rolling track to play a role in enhancing lubricant spontaneous replenishment. To investigate the influence of the wedge-shaped surface on lubricant replenishment and hence the contribution to the fiction reduction, tests were conducted on UMT friction and wear test device. Lubrication improvement induced by the wedge-shaped surface was confirmed.
The wedge-shaped pattern surface with wettability gradient properties was prepared via chemical vapor deposition and local ultraviolet lithography. Using a contact angle measuring instrument, the migrations of oil droplets on the surface of wedge-pattern was characterized. The effects of the volume of oil droplets, wedge angle and wedge length on the migration properties of oil droplets were studied. The results showed that the oil droplets could migrate from the tip of the wedge to the end of the wedge angle. In addition, the migration distance and migration speed of the oil droplet increased with the increase in the oil droplet volume. It was found that there existed a critical value of about 5 μl, beyond which the migration distance and migration speed of the oil droplet would decrease. The wedge angle also affected the migration properties of oil droplets. The migration distance of the oil droplet increased with the increase in wedge angle, but the migration speed would decrease continuously. There was ignorable effect of length of the wedge on the migration speed of the oil droplet, but it affected the migration distance in some extend. The longer the wedge length, the longer the migration distance of the oil droplet.
The wedge-shaped surface was prepared on both sides of rolling track of the contact pairs, and the antifriction effect of the surface was observed using UMT friction and wear device. The results showed that, under the condition of adequate oil supply, the wedge-shaped surface and the hydrophobic surface had obvious anti-friction effect comparing with the original surface. However, no obvious difference was found between the wedge-pattern pattern surface and the hydrophobic surface, mainly because starvation did not occur in the contacts and track. Furthermore, the anti-friction effects of the wedge-shaped and the hydrophobic surface were observed under the condition of limited oil supply. It was found that the wedge-shaped had more effective performance in anti-friction. For the FAS modified hydrophobic surface, the friction coefficient suddenly increased to 0.195 after 1 950 s running, whilst the friction coefficient of the wedge-pattern surface could be always remained below 0.05. The tests results under the two lubricant supply conditions indicated that the oil droplets outside the track could spontaneously reflow to the track. Meanwhile, the observed replenishment process of oil droplet contributed to the explanation of the anti-friction behavior. All the results demonstrated that the wedge-shaped surface had obvious antifriction effect.