全氟聚醚羧酸铵离子液体的润滑性能及润滑机理研究

设计制备了三种全氟聚醚羧酸铵离子液体，在微动振动摩擦磨损试验机上考察了其在不同温度下对钢/铜锡合金以及钢/钢摩擦副的润滑性能，并与全氟聚醚(PFPE)和全氟聚醚羧酸(PFPEC)进行了对比. 通过测量接触角表征了所制备离子液体对金属表面的润湿性，通过测试摩擦试验过程中接触电阻的变化分析了摩擦过程中摩擦膜的变化；采用扫描电镜和X射线光电子能谱仪分别对磨斑表面形貌和元素状态进行了表征. 结果表明:作为钢/铜锡合金摩擦副的润滑剂时，全氟聚醚羧酸铵离子液体在常温下的润滑性能与PFPE和PFPEC相差不大，但在高温条件下表现出更为优异的减摩抗磨性能；而作为钢/钢摩擦副的润滑剂，其在常温、高温条件下均表现出优于PFPE以及PFPEC的减摩抗磨性能. 多种物理化学表征研究表明该类离子液体优异的减摩抗磨性能归因于其在金属表面优异的吸附性能以及稳定摩擦化学反应膜的形成. 

Lubrication Performance and Mechanism of Perfluoropolyether Carboxylate Ammonium Ionic Liquids

Perfluoropolyethers (PFPE) is a kind of high performance synthetic lubricant with low volatility, high thermal and oxidation stability, good chemical inertia and insulation performance, as well as a wide range of liquid temperature and excellent viscosity-temperature characteristics. PFPE has been widely used in aerospace, military, nuclear industry, computer disk and other fields. With the rapid development of industrial technology, new requirements have been proposed for the performance of the lubricants. As a result, the common perfluoropolyethers as lubricants gradually cannot work well under the present working conditions. Moreover, it is difficult to improve the lubrication performance of perfluoropolyethers by conventional additives technology due to their poor compatibility with conventional additives. Hence, in order to improve the antifriction and antiwear properties of perfluoropolyethers, researchers focused on modifying their structures. Great efforts have been paid to improve the performance of perfluoropolyethers as lubrication film in the field of computer disk, but as a kind of important aviation lubricating oils, their lubrication performances are also in urgent need of improvement. 　In this context, in this study three kinds of perfluoropolyether carboxylate ammonium ionic liquids (PF-BA, PF-DBA and PF-TBA) were designed and synthesized by modifying the structure of perfluoropolyether carboxylic acid (PFPEC) with different amines. The structures of synthesized ionic liquids were confirmed by infrared spectroscopy. Their shear stress and viscosity at different shear rates were evaluated by a Rheometer, and the wetting ability on metal surfaces were analyzed by the contact angle. Their lubrication performance for steel/steel and steel/tin bronze friction pairs at 25 ℃ and 100 ℃ were evaluated and compared by Optimal SRV-V oscillating friction and wear tester, where PFPE and PFPEC were used as reference samples. And the variation of the tribofilm during the friction process were revealed by the change of contact resistance. The surface morphologies and element state were analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. Tribochemical reactions were investigated by comparing the change in surface morphologies and element state of wear spot. By combining the above results, the lubrication mechanism was explored. 　Results showed that all the synthesized ionic liquids exhibited higher shear stress and viscosity than PFPEC. Specifically, the ionic liquids with more N-H bonds in the cation exhibited higher viscosity and shear stress, possibly due to the formation of hydrogen bond that can increase the intermolecular interactions. The viscosity and shear stress gradually decreased with the increase of shear rate, while the viscosity of the ionic liquids with more N-H bond in the cation decreased more rapidly. On the surface of tin bronze, the three ionic liquids exhibited better wetting performance than PFPEC, but poorer than PFPE. While on the steel surface, the three ionic liquids exhibited better wetting abilities than PFPEC and PFPE. 　As lubricants for steel/tin bronze friction pairs, the friction coefficients and wear volumes of the three ionic liquids were close to those of PFPE and PFPEC at room temperature, and their lubrication performance was improved insignificantly. The synthesized ionic liquids exhibited much better anti-friction and anti-wear performance at high temperature (100 ℃) than PFPE and PFPEC. As lubricants for steel/steel friction pairs at room temperature, the friction coefficients of the three ionic liquids were similar to that of PFPEC, and even a little higher than PFPE; while the wear volumes decreased about 60%-70% in comparison with PFPE and PFPEC. At 100 ℃, compared with PFPE and PFPEC, the friction coefficients of the three ionic liquids were much more stable and lower, and the wear volumes were reduced by 70% ~ 80%. Summarily, three synthesized ionic liquids had better friction-reduction and anti-wear properties than that of PFPE and PFPEC both at room temperature and at high temperature as lubricants for steel/steel friction pair. The contact resistances reflected the variation of the tribofilm, i.e. the higher contact resistance, the thicker tribofilm and better lubricating properties. The results of tested contact resistances were consistent with the friction tests results. 　According to the SEM morphologies of the worn surface lubricated by different lubricants, both the tin bronze and steel worn surfaces exhibited abrasive wear. Among them, the worn surfaces lubricated by ionic liquids PF-TBA were the smoother, and the parallel grooves were the shallower both at room temperature and high temperature. The steel worn surfaces lubricated by the three ionic liquids were much smoother than that of PEPE and PEPEC. 　From the XPS results, it can be seen that the composition and content of the friction films on the worn surface lubricated by the three synthesized ionic liquids were different from that of PEPEC. The tribofilm formed on the steel surfaces lubricated by the ionic liquids was consisted of Fe₃O₄, Fe₂O₃ and FeF₃, the nitrogen-containing compounds and adsorbed ionic liquids. The adsorption film of the ionic liquids on the metal surface was strong and the tribofilm was thick. 　The combined results of multiple physical and chemical analysis showed that the excellent friction-reduction and anti-wear properties of synthesized ionic liquids can be attributed to their excellent adsorption properties on the metal surface and the formation of stable tribofilm.