Failure Mechanism Of EPND Oil-Based Superlubricity And The Expansion Of Its Operation Conditions

The concept of superlubricity(friction coefficient < 0.01)was proposed in the1990 s.After 30 years of development,superlubricity systems of solid,water-based and oil-based materials have been established under various working conditions.Considering the different application fields of various superlubricity systems,it is important to clarify the evolution and failure mechanism of superlubricity behavior under different working conditions for finding suitable industrial objects to involve superlubricity into practical applications.For macroscale oil-based superlubricity,previous studies on 1,3-diketone synthetic oils focused on the 1,3-diketone material itself as lubricating medium,including molecular structure,chemical reactivity,rheological properties,molecular orientation.However,the influence of external factors such as load and speed and other operating conditions on friction coefficient,wear mechanism,and running-in behavior of superlubricity materials is still unclear.Therefore,it is necessary to study the evolution of superlubricity and superlubricity failure mechanism of the 1,3-diketone system,and then propose further improvement methods for the system to expand its operation range that could achieve superlubricity.In this study,with 1,3-diketone EPND(1-(4-ethyl phenyl)nonane-1,3-dione)as lubricant on steel friction pairs,a series of ball-on-disk rotation friction tests were carried out under different working conditions(load and speed),the lubrication performance of EPND in this typical friction system was studied.The superlubricity failure mechanism of EPND under different working conditions was studied by analyzing the wear of the friction pair,the chemical change of the lubricating oil and the wear debris during the running-in process.The results show that under this lubrication system,when the load and speed reach a certain critical value,the superlubricity state cannot be achieved by running-in within the friction test time(5 h):(1)For load range(the speed is constant at 300 mm/s),the critical lower limit load is 3.5 N.In this case,due to insufficient contact of the surface asperities,the runningin process is not sufficient,and the contact surfaces cannot form well asperity level conformity to achieve superlubricity;the critical upper limit load is 64 N,at this time,the excessive friction in the running-in stage causes severe mechanical wear and produces large wear particles that hinder the realization of superlubricity;(2)For speed range(the load is constant at 16 N),the critical lower limit speed is100 mm/s.In this case,the running-in is insufficient due to the shortening of the sliding distance within the same time,the weakening of hydrodynamic effect also induces the generation of large wear debris;the critical upper limit velocity is 500 mm/s,at which time the oil migration occurs under high centrifugal force,resulting in oil starvation in the contact area.Based on the critical operation conditions and the failure mechanism of superlubricity,this work also studies the method to further expand the operation condition of the EPND to achieve superlubricity and propose a pre-running-in method under optimized operation condition(16 N and 300 mm/s).With this method,the negative effects under the original critical operation condition during running-in could be avoided and the operation conditions under superlubricity could be effectively expanded.For example,superlubricity could be achieve under 100 N load.