Effect of Surface Wettability on Lubrication Performance Under Pure Sliding Condition

In industrial lubrication practice, excessive lubricating oil is often applied to minimize wear on contact pairs and extend the lifespan of machinery. However, this approach may result in problems such as elevated oil temperature and environmental pollution. Limited lubricant supply is now receiving increased recognition. The objective of limited lubricant supply is to optimize the efficiency of lubricant utilization and guarantee optimum lubrication of interacting surfaces during operation. To achieve this, different methods have been suggested, including employing surface textures, applying chemical coatings, or integrating specific physical structures. In this study, under the condition of limited oil supply, the AF (Anti-fingerprint) oleophobic coating was used to improve the oil replenishment on the surface of the friction pair, improve the lubrication characteristics and load-bearing capacity under pure sliding condition. First, AF oleophobic coating was applied on the surface of the lubrication raceway or rolling element. By using the property of high-viscosity polyisobutylene lubricating oil to easily form oil film dimples under pure sliding conditions, a homemade optical EHL film test rig was used to investigate how different surface wettability levels affect the shape of the lubricating oil film under various oil supply conditions. The treated AF surface demonstrated oleophobic properties, with the lubricant primarily distributed on the designated lubricating track, resulting in improved lubrication performance compared to the untreated surface. By analyzing the oil pool shape and film thickness under limited oil supply, it was determined that the oleophobic coating on the lubrication track surface outperforms the rolling element surface in terms of oil replenishment effect. Additionally, the oil film dimple shape transitions from a crescent shape to an elliptical shape. The dimple film thickness and the minimum film thickness increased gradually as the speed increased. The load had a significant impact on the morphology of the oil film dimple. As the load increased, abnormal accumulation easily occurred at the inlet of the contact area. The AF oil oleophobic coating enhanced the abnormal accumulation of oil film at the inlet, leading to an increase in the dimple area and film thickness. By using AFM to observe the microstructure of the original surface and AF surface, it was found that the microstructure of AF surface showed nano convex peaks, with a higher roughness compared to the original surface. The change in surface wettability and low surface energy were key factors in reducing friction. The friction measurement experiment showed that under the condition of fully or limited oil supply, the friction coefficient of both the original surface and the AF surface followed a Stribeck curve, displaying a pattern of declining and then increasing. The frictional coefficient of the original surface decreased gradually as the lubrication state was optimized, while the stirring resistance resulting from adequate oil supply increased with speed increment. The starved lubrication state caused by limited oil supply led to a rapid increase in friction coefficient with increasing speed. Under the condition of fully or limited oil supply, the AF surface showed a better effect of reducing the frictional coefficient, which was related to the surface slippage characteristics and replenishment effect.