| High precision bearings are widely used in high-end CNC machine tools,civil aircraft and other fields.However,the mechanical and wear failure problems of high precision bearing cage materials represented by high-performance polyimide have seriously restricted the development of China’s cutting-edge fields.In order to enhance the mechanical properties of polyimide materials and improve their poor wear resistance,molecular dynamics simulation and experimental verification were used to design polyimide lubricating materials and study their tribological micro-mechanism.Firstly,the tribological properties and micro mechanism of polyimide self-lubricating composites reinforced by different functional fillers were investigated.Three different dimensions of nano fillers,namely amino functionalized silica,carbon nanotubes and graphite carbon nitride,were selected to modify the tribological properties of polyimide.The theoretical results showed that when the filling amount of amino silicon dioxide,carbon nanotubes and graphite carbon nitride was 5 wt.% respectively,it not only had little effect on the temperature resistance of polyimide composites,but also could effectively improve the mechanical properties and anti-wear properties of polyimide matrix.By further extracting and analyzing the microscopic information such as free volume fraction,temperature distribution,radial distribution function and atomic concentration distribution,the microscopic anti-wear mechanism of polyimide reinforced by three kinds of fillers with different dimensions was revealed: After the addition of zero-dimensional amino functionalized silica,due to the strong hydrogen bond interaction between it and polyimide molecular chain,the stacking density of polyimide molecular chain was enhanced,thus improving the anti-wear performance;One-dimensional carbon nanotubes,due to their excellent thermal conductivity,reduced the temperature rise of the interface of the composite during friction,and prevented the wear of polyimide caused by high-temperature softening;After two-dimensional graphite carbon nitride was added to the polyimide matrix,the interaction between the polyimide molecular chain and the friction duality was significantly reduced,the contact probability of the surface atoms was reduced,and the transfer of the polyimide molecular chain at the interface during the friction process was reduced,thus improving the anti-wear performance.At the same time,the experimental results showed that the addition of three fillers could effectively improve the mechanical properties and anti-wear properties of polyimide matrix.Subsequently,considering that the molecular structure of polyimide has a direct impact on the composite lubrication effect between polyimide and base oil,this paper designed the molecular structure of polyimide by changing the flexibility of the main chain and the polarity of side chain groups.Through molecular dynamics simulation and experimental verification,the effect and mechanism of solid-liquid composite lubrication of polyimide with different structures and base oils with different polarity in the mixed lubrication range were explored.The results showed that the structure of polyimide molecular chain had a direct impact on the temperature resistance and mechanical properties of the matrix: the stronger the rigidity of the main chain of the molecule,or the smaller the polarity of the side chain group,the polyimide had higher temperature resistance and better mechanical properties.Research on the composite lubrication mechanism in the mixed lubrication range indicated that: For base oil,the greater the polarity of base oil,the faster the adsorption layer could be formed at the friction interface,reduced the running in time,but the greater the polarity,the stronger the interaction between base oil molecules and polyimide molecular chains,made polyimide molecular chains easier to undergo atom transfer in the friction process,led to higher friction coefficient and wear rate;For polyimide,the flexibility of the molecular main chain had no direct impact on the solid-liquid composite lubrication effect,but the stronger polarity of the side chain group could increase the interaction between the molecular chain and the base oil,helped to form a denser adsorption layer,so that the friction process was far away from the poly(imide)layer,thus improved the friction reduction and anti-wear properties.Finally,in order to further enhance the friction reducing and anti-wear properties of polyimide,the solid-liquid composite lubrication system of polyimide structure and base oil with ideal friction reducing and anti-wear properties was selected.On this basis,polyimide matrix was modified with multidimensional fillers by adding amino silica,carbon nanotubes and graphite carbon nitride.The calculation results showed that the mechanical properties of polyimide matrix were significantly improved by the addition of multidimensional fillers compared with unmodified polyimide.In addition,multi-dimensional modification enhanced the bulk density of the composite,reduced the temperature rise in the friction process,reduced the interaction between the polyimide molecular chain and the base oil,and formed a denser adsorption layer.The three fillers showed a synergistic effect,which together enhanced the antifriction and anti-wear properties of the polyimide matrix in the solid-liquid composite lubrication.At the same time,the experimental results showed that the elastic modulus of polyimide matrix was increased by about 24% by multidimensional modification,and the friction coefficient and wear rate under the solid-liquid composite lubrication system were reduced by about 23% and 82% respectively,which was significantly better than that of single filler modification or solid-liquid composite lubrication. |
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