Study On Tribological Behavior And Lubrication Mechanism Of Composite Systems Of Polyimide Composites/Lubricanting Oils

Polyimide（PI）has excellent self-lubricating performance,mechanical properties,endurance to temperature,corrosion resistance,as well as high creep resistance,dielectric properties,and low thermal expansion coefficient.Particularly,the self-lubricating composite of retainer with PI as the major structural material has promising applications in the fields of precision bearings and aerospace.However,neat PI matrix exhibits fair wear resistance and can hardly ensure the lubrication of mechanical equipment under harsh working conditions such as high speed,heavy-load,and frequent‘start-stop’.Therefore,it requires to modify PI with functional fillers such as reinforcing fibers（carbon fiber,aramid fiber,glass fiber,etc）and solid lubricants（polytetrafluoroethylene,graphite,Mo S₂,etc）in order to improve its mechanical properties and wear resistance and broaden its application scope.Among various fillers,nano-fillers with small size effect and surface-interface effect as well as tunable surface structure are of particular significance.It has been found that the friction and wear behavior of polymer self-lubricating composites not only depends on the molecular structure and lubricating performance of the liquid lubricants but also closely relates to their own molecular structure,filler component and working condition.Unfortunately,previous researches mainly focused on the design of polymer composites and the study of their lubrication mechanism under dry sliding condition but failed to deal with the interactions between the molecular structure of polymers and the polarity of lubricating oils,the competitive adsorption of functional fillers and lubricant base oils on rubber metal surfaces,and the tribochemical reaction mechanism.Viewing the above-mentioined perspectives,this thesis designed-prepared thermosetting polyimide（TPI）with improved adaptability to varying temperature and evaluated its friction and wear behavior under the lubrication of lubricating oils with different polarity in the presence of YS-20（a commercial PI）as the reference.Further,it prepared TPI-matrix composites with zero-dimensional silicon dioxide（SiO₂）and two-dimensional graphite-like carbon nitride（g-C₃N₄）as the reinforcing fillers,comparatively investigated the effects of the fillers and the lubricating base oils with different polarity on the friction and wear behavior of the TPI-matrix composites under continuous gradient varying sliding speed,and explored the synergistic lubrication mechanism of the functional fillers and the base oils.Moreover,the TPI-matrix self-lubricating composites were coupled with bearing steel（GCr15）to assemble the sliding pair while three kinds of commercial bearing lubricating oils（PDP38,RIPP555,and Jet II）were used as the lubricants.The tribological behavior of the solid-liquid composited lubrication systems was evaluated,the synergistic actions among various functional fillers and commercial lubricating oils were revealed,and the pairing plan for the composited lubrication system of polymer-lubricating oil was primarily explored.The main research contents and results are as follows:1.Effects of PI molecular structure and base oil polarity on tribological behavior of solid-liquid composited lubrication systemThermosetting polyimide（TPI）with desired endurance to temperature was designed and synthesized.Its mechanical and thermodynamic properties were tested with DMA universal testing machine in the presence of YS-20 as the reference.The wetting behavior of lubricating base oils with different polarity（weakly polar PAO10 as well as strongly polar PEG400（with hydrogen bonding）and disiooctyl sebacate（DIOS））on the surface of IPT was evaluated with a contact angle meter.The friction and wear behavior of PI and TPI under the lubrication of various base oils in single and multiple“start-stop”modes were tested.Results indicate that the solid-liquid composited lubrication systems consisting of TPI and three kinds of base oils exhibit excellent friction-reducing behavior,which is mainly attributed to its good wetting and adsorption abilities towards the base oils.This is also verified by corresponding molecular dynamics simulation results.Namely,the base oils are easier to undergo spontaneous adsorption on TPI surface,and their adsorbed energe tends to increase with increasing polarity of the base oils,which indicates that the spatial chemically crosslinked structure formed during the thermal moulding of TPI contributes to enhancing its adsorption capacity for the base oils,thereby improving the friction-reducing ability and wear resistance of TPI.2.Preparation of TPI self-lubricating composites filled with nanoscale SiO₂and g-C₃N₄ as well as effects of base oil polarity on the tribological behavior of the solid-liquid composited lubrication systemsTPI-matrix self-lubricating composites filled with nano-fillers such as SiO₂and g-C₃N₄were prepared by thermal moulding-sintering.The friction and wear behavior of the TPI-matrix composites（TPI/SiO₂,TPI/g-C₃N₄,etc）under the lubrication of various lubricating base oils in frequent“start-stop”mode was tested;and the effects of various functional fillers and lubricating base oils with different polarity on the tribological behavior of the solid-liquid composited lubrication systems were comprehensively investigated.Results show that the transfer film mainly composed of the polymer species is formed under the lubrication of weakly polar PAO10,while the polar base oils PEG400 and DIOS accelerates the formation of composite transfer films composed of nitrate,Si-N compound and metal oxides on the worn steel surface.Besides,conventional fillers such as short carbon fiber（SCF）and graphite（Gr）can significantly the friction-reducing ability and wear resistance of the filled composites,and the nano-fillers are advantageous over conventional ones in this respect.Specifically,SiO₂nanoparticle contributes to the formation of compact carbon-based protective film mainly consisting of TPI matrix,Si-N compound,metal complexes and oxides,thereby greatly reducing the wear rate of TPI/SCF/Gr/SiO₂ composite.Lamellar g-C₃N₄ nanoparticle functions to reduce friction via inter-layer slip and benefits the formation of the compact carbon-based protective film mainly consisting of TPI matrix,nitrate,Si-N compound,metal complexes and oxides on worn steel surface,thereby effectively improving the friction-reducing and anti-wear abilities.3.Study on the lubrication mechanism of solid-liquid composited lubrication systems of TPI self-lubricating composites-lubricating oil products.TPI/SCF/Gr/SiO₂and TPI/SCF/Gr/CN with excellent friction-reducing behavior were selected as the representatives,and their friction and wear behavior under the lubrication of various lubricating oil products was tested.The effects of the polarity of the lubricating oil products as well as the composition and microstructure of the polymer-matrix nanocomposites on the tribological behavior of the solid-liquid composited lubrication systems were investigated.Results demonstrate that the friction-reducing behavior of the solid-liquid composited lubrication systems of TPT self-lubricating composites-lubricating oil products is dominated by the antioxidation ability of the lubricating oils.The two kinds of nanocomposites can form graphitized carbon-matrix transfer films composed of metal complexes and oxides on worn steel surface.Particularly,TPI/SCF/Gr/SiO₂ nanocomposite can more easily form the graphitized carbon-matrix transfer film,thereby better repairing the worn surface of the steel ball and drastically improving the wear resistance of the nanocomposite.In terms of the tribological properties,the three kinds of lubricating oil products are ranked as Jet II>RIPP555>PDP38.