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 degrees C in atmospheric and vacuum environments, as well as at 200 degrees C in a high-temperature vacuum environment. The results show that the decomposition temperature of the three types of fluorinated graphites are higher than 595 degrees C , whereas that of the D-type PFPE base oil is 450 degrees C . 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 degrees C 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 degrees C 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.

This study aimed to test the utility of ammonium persulfate (APS) oxidised nanofibrillated cellulose (NFC) as an additive for chitosan- and alginatebased biopolymer films that could eventually be used as paper coatings for food packaging applications. Sodium alginate and chitosan were used as the base for the films. Various concentrations of APS oxidised NFC ranging from 0% to 10% were used as a reinforcing agent, resulting in six combinations of either alginate-NFC or chitosan-NFC composite films. Biofilms were tested for their mechanical properties (tensile strength and strain), grease barrier properties, air permeability, water vapour permeability, and degradation in the soil. Overall, when using the ammonium persulfate oxidation pretreatment method, the best performance of the films was estimated with the addition of 2.5% NFC.