A Hybrid Model for the Evolution of Liquid Lubrication Aimed at Achieving Ultra-Low Friction during Running-In

Ultra-low friction operating state has attracted more and more attention from academia and industry. In the past thirty years, liquid lubrication system achieving ultra-low friction coefficient are fruitfully discovered on the ball-disk sliding experiment during running-in. Among them, it can be found many researches of liquid lubrication with ultra-low friction are carried out dependent on experiments. However, theoretical research of liquid lubrication with ultra-low friction is relatively backward, and quantitative of analysis are relatively a few. For this purpose, a hybrid model of the friction coefficient evolution was established during running-in coupling of fluid lubrication, asperity contact mechanics and Archard wear model. The time varying function of liquid viscosity, surface roughness and wear coefficient were also proposed in this model, which were a common “S” shape to describe those parameters change based on the state evolution. Subsequently, the hybrid model of liquid lubrication performance evolution during running-in was numerical programmed to simulate and analyze the influence mechanism of liquid lubricant’s viscosity and surface parameters on tribological performance. The surface profile, the film hydrodynamic pressure, contact pressure and friction coefficient can be predicted during running-in. Numerical results can be shown that: the worn surface profile changed sharply in the early period of running-in, while the contact surface became relatively flatted after running-in process. Based on the curve of asperity contact ration and film load ratio, the liquid lubrication gradually transformed from boundary/mixed lubrication to hydrodynamic film lubrication. At the same time, the friction coefficient evolved from large value to ultra-low value. Under the liquid superlubricity state, the thickness of the liquid lubrication film was sufficient to completely separate the contact surface. The friction mainly came from the internal viscous shear of the lubricant. Further, the effect of lubricant viscosity and surface roughness on the liquid lubrication performance were also investigation. liquid lubrication with achieving ultra-low friction was a result of the significant effect of hydrodynamic pressure enhanced and the contact between rough peaks weakened during running-in. Therefore, when designing liquid lubrication system with ultra-low friction, the sliding contacts surface roughness was expected to be deceased and the lubricant effective viscosity appropriately increase could form the hydrodynamic thin lubricant film during running-in process. Moreover, based on the physical and chemical properties of contact pairs, it can improve the surface interface effect and reduce the boundary friction coefficient that further are benefited for achieving the ultra-low friction liquid lubrication state. This development method can provide a numerical method to design and estimate friction behavior in the liquid lubrication during running-in process.