氟化石墨改性PFPE润滑脂极端环境摩擦学性能

Tribological Performance of Fluorinated Graphite Modified PFPE Grease in Extreme Environment

活动机构在月球等天体表面产生的摩擦热不易消散,造成机构温度大幅升高。现有流体润滑剂在高温下存在抗磨性能不佳、稠化剂易发生相变、粘结等问题。针对空间重载高温活动机构润滑需求,进一步提升流体润滑剂的高温摩擦学性能,开展氟化石墨改性全氟聚醚(PFPE)润滑脂在真空高温极端环境中的摩擦学性能研究。分别采用不同氟碳比与不同粒径的氟化石墨与聚四氟乙烯(PTFE),制备PFPE润滑脂。评价不同的氟化石墨对润滑脂流变性的影响,在25℃大气环境、25℃真空环境以及200℃真空高温环境中,开展摩擦学性能研究。结果表明:纳米氟化石墨对润滑脂在常温下的静态增稠作用最为明显,但在高剪切作用与高温环境中,其增稠作用与微米级氟化石墨的增稠作用接近。在高温下,剪切速率达到10~15 s^(−1)时,氟化石墨由于层间距的增加,将吸附更多的PFPE油分子,导致润滑脂剪切黏度增加。氟含量越高,氟化石墨对PFPE润滑脂在真空常温以及真空高温环境中降低磨损的作用越明显,主要得益于其较高的热稳定性与对PFPE油分子的吸附作用,能够降低PFPE的断链与碳化程度。而粒径的变化并未产生明显降低磨损的作用。研究结果获得了氟化石墨的结构对润滑剂高温真空摩擦学性能的影响,为空间活动机构高温润滑提供技术支撑。

Frictional heat generated by mechanisms that take service on celestial bodies such as the moon does not dissipate easily owing to the vacuum environment and the low thermal conductivity of celestial soil.Consequently,the temperature of these mechanisms increases significantly.The wear resistance of liquid lubricants at high temperatures degenerates rapidly because the oil film thins out and the oil decomposes.Polytetrafluoroethylene(PTFE)and the soap fiber thickeners of lubricants are susceptible to phase transitions and agglomeration.The wear resistance and thermal stability of lubricants must be improved for mechanisms operating on celestial bodies.The lubricating properties at high temperatures and the thermal stability of fluorinated graphite are excellent.The wear resistance of liquid lubricants for space mechanisms can be improved using fluorinated graphite.In this study,fluorinated-graphite-modified perfluoropolyether(PFPE)greases are prepared using fluorinated graphite with different fluorine-to-carbon ratios and particle sizes,PTFE powders,and D-type PFPE base oil.The thermal behaviors of the materials are characterized using thermogravimetry and differential scanning calorimetry.Electron spectroscopy and X-ray diffraction are used to determine the fluorine-to-carbon ratios and the structures of three types of fluorinated graphite.The effects of different fluorinated graphites on the rheological and tribological properties of the greases are evaluated at 25℃in atmospheric and vacuum environments,as well as at 200℃in a high-temperature vacuum environment.The results show that the decomposition temperature of the three types of fluorinated graphites are higher than 595℃,whereas that of the D-type PFPE base oil is 450℃.The fluorine-to-carbon ratios of C2FJ1002,CFT10,and CF500 fluorinated graphites are 0.92,0.88,and 1.04,respectively.Among them,the fluorine-to-carbon ratio of the nanoscale fluorinated graphite,CFT10,is the lowest.The(001)reflection of this nanofluorinated graphite is higher than the others;therefore,its(CF)_(n)is greater than those of the others.The nanoscale fluorinated graphite exhibits the most significant thickening effect on grease at room temperature under low shear owing to its larger specific surface area.However,under high-shear and high-temperature conditions,the thickening effects of the three types of fluorinated graphites are almost uniform At high temperatures,the increased interlayer spacing of fluorinated graphite results in more PFPE oil molecules being absorbed,thus resulting in an increase in the shear viscosity of the grease at a shear rate of 10-15 s^(−1).The wear-scar diameter of the grease modified by the abovementioned three types of fluorinated graphites under a 25℃vacuum environment decreases by 7.7%,11.7%,and 13.2%,respectively.The CF500 fluorinated graphite with the highest fluorine-to-carbon ratio demonstrates the best wear resistance in grease.Additionally,it exhibits a decreasing worn function under a 200℃vacuum environment.The C 1s core-level spectra of the wear scars lubricated by the PFPE grease suggest the formation of amorphous carbon on the wear scar due to the degradation of PFPE.However,the C 1s core-level spectra of the wear scars lubricated with grease,which are modified by the CF500 fluorinated graphite,do not suggest the formation of amorphous carbon.The CF500 fluorinated graphite can shield the tribological surface and mitigate the degradation of the PFPE base oil.The higher the fluorine content,the more prominent is the reduction in wear of the PFPE grease in both vacuum and high-temperature vacuum environments.This is primarily attributed to its higher thermal stability and adsorption capacity for PFPE oil molecules,which reduces the chain breakage and carbonization of PFPE.However,reducing the particle size does not significantly reduce wear.