The Internal Relationship Between The Macro Tribological Properties And The Micro Control Mechanism Of Polyimide Composites Under Oil Lubrication

Polymers and their composites have been widely used in tribology because of their excellent mechanical properties.Especially under the harsh boundary lubrication conditions,the polymer-metal mating pair can effectively prevent the"locking"phenomenon of the rotating shaft caused by metal bite.This technology of replacing steel with plastic is highly favored by the high-end equipment market.Thermoplastic polyimide（PI）is used as a special engineering plastic in many industrial occasions because of its excellent temperature resistance,corrosion resistance and easy processing.However,its application as a self-lubricating friction pair is limited by its defects such as large fluctuation of its own friction coefficient,high wear rate and long running-in time.Therefore,it is urgent to prepare PI self-lubricating composites with high performance and long life.In this study,a variety of functional fillers（fiber/filler）reinforced PI composites were prepared,the formulation design was optimized,and the tribological properties,thermal properties and mechanical properties were regulated to meet the requirements of being used as friction pair materials under harsh boundary lubrication condition.The macro tribological behavior of PI composites under harsh oil lubrication conditions was investigated by slide block tester.The composite materials and mating surfaces were comprehensively evaluated by advanced analysis methods.The nanostructure of the interface transfer film and the surface structure and composition of PI composite were analyzed systematically,and its growth mechanism and chemical action mechanism were revealed.The main research contents are as follows:1.Systematically investigate the tribological and mechanical properties of transition metal nickel（Ni）micro particles and carbon fiber（CF）reinforced PI or both synergistic reinforced PI composites under boundary lubrication conditions.The research and analysis results indicate that the PI composite material filled with Ni particles exhibits ultra-low friction and wear under boundary and mixed lubrication conditions.The results of FIB section and TEM characterization showed that a very thin layer of nickel oxide（NiO）glaze was formed on the steel surface,indicating that the NiO glaze inhibited the oxidation at the initial stage of sliding.At the same time,a layer of uniform C-based film was detected on the NiO glaze layer,which may be due to the deposition of C-based film caused by the dehydrogenation of oil molecules catalyzed by the NiO glaze layer.The composite transfer film formed prevents the direct contact of the friction pair,makes up for the defects under the boundary or mixed lubrication state,and greatly improves the tribological properties of the composite.This study also compared the friction and wear of5Ni/10CF/PI composite under PAO oil lubrication and 10CF/PI composite under ZDDP oil lubrication.The tribological properties of the former are better than the latter,indicating that Ni particles can be used as solid lubricant for polymer filling.2.The influence mechanism of gradient hardness submicron metal oxide ceramic particles on the mechanical properties and macro tribological properties of PI was studied,and the control mechanism of gradient hardness ceramic particles on the interface transfer film structure of friction pairs was investigated.The results show that PI filled with high hardness ceramic particles is more suitable for lean oil lubrication.PI materials with hard ceramic particles ZrO₂（8.5）showed the lowest wear rate(3.29×10^-7 mm³/N·m),while PI materials with high hardness ceramic particles Al₂O₃（9.5）showed ultra-low friction coefficient（lower than 0.01）.Compared with pure PI,the wear rate of ZrO₂/PI composite and the friction coefficient of Al₂O₃/PI composite decreased by 65.8%and 92.4%respectively.The microstructure and EDS element analysis of the worn surface of steel found that the steel and ZrO₂/PI and Al₂O₃/PI composite formed a thick transfer film on its surface after grinding.It is speculated that the transfer film formed by the friction and sintering of the high hardness ceramic particles released from the polymer matrix with the polymer fragments under the action of shear force and flash temperature has high bearing capacity,prevents the friction oxidation of the steel surface,prevents the direct contact of the friction pair,and thus reduces the friction and wear.Even under the condition of oil immersion,the tribological properties of PI composites are better than those of pure PI matrix,but their tribological properties are worse than those under the condition of oil depletion.3.The tribological properties of PI reinforced by Al-MOFs with three different structures under boundary lubrication were systematically studied.The effects of different amounts of Al-MOFs on the macroscopic tribological properties of PI composites were investigated.The results show that MIL-96-HPBC type Al-MOFs exhibit excellent macroscopic tribological properties under boundary and mixed lubrication conditions.The microstructure,mechanical test and chemical analysis showed that the three different structures of Al-MOFs were transferred to the dual surface during the friction process,while the bearing capacity of the PI composites prepared by filling the harder MIL-96-HPBC type Al-MOFs particles into the PI matrix increased significantly,and the tribological properties were the most prominent.The results show that the transfer film embedded with harder MIL-96-HPBC type Al-MOFs particles has higher bearing capacity and can prevent the direct contact of the friction pair,thus reducing the wear rate and friction coefficient of the matrix material.In this study,the internal relationship between the macroscopic tribological properties of transition metals,gradient ceramic particles,MOFs reinforced PI matrix and composites and the nanostructures of transfer films and their tribological properties were discussed.The microstructure,interfacial physicochemical properties,micro-nano mechanical properties,etc.of the transfer film on the steel dual surface were analyzed by using a variety of macro/micro characterization methods,which provided certain technical and theoretical support for the preparation of ultra-low friction and wear polymer composites suitable for boundary lubrication conditions or non-ZDDP environmental protection lubrication system.