Study On Preparation And Tribological Property Of Composite Lubricating Films Based On Polyimide

Aluminum alloys have been widely used in aerospace, military industry, automobile, electrical and electronic industries due to their low density and high specific strength. However, the defects of aluminum alloys（such as low surface hardness, high friction coefficient and poor corrosion resistance） severely limit not only its application fields but also its service life. Therefore, the surface modification of aluminum alloys is particularly important. In some practical application, machine parts are working in the severe conditions, such as high temperature, high load, strong radiation etc. Commonly used lubricating oils（mineral oils, vegetable oils and synthetic oils） and greases are hardly applicable in these conditions. While solid lubricating materials have unique superiority. Hence, it is of great significance and research value to develop some solid lubricating films to meet practical industrial requirements.Recently, polyimide（PI） has been extensively concerned due to its outstanding advantages such as excellent thermal stability and mechanical properties, good radiation and solvent resistance. What’s more, PI possesses excellent antifriction and anti-wear properties even though under the environment of high temperature, high pressure and high speed. So solid lubricating films based on polyimide were prepared by simple and efficient spin coating proceeding on the surface of aluminum alloys with polyamide acid as organic binder, MoS₂ or graphite as inorganic solid lubricants in this paper. Several composite lubricating films were obtained by changing the components and contents of inorganic solid lubricants in organic binder. The structure, physical phase and performance of synthesized lubricating films were characterized by using XRD, SEM, FTIR, TGA and nano-indentation methods. And the friction coefficients of these composite lubricating films were measured by UMT-3 and MG-2000 tribometers. The adhesion between synthesized lubricating films and aluminum alloy matrix were measured by cross-cut method according to GB/T9286-1998. The main contents include the following aspects.MoS₂/PI（PM） and Graphite/PI（PG） composite lubricating films were successfully prepared by directly dispersing MoS₂ and Graphite particles into polyamide acid solution followed by spin coating and thermal imidization, respectively. The results showed that MoS₂ and Graphite particles were well dispersed in the PI matrix, and the compatibility between inorganic filler and the organic binder was good. The lubricating films had good adhesion strength with the aluminum alloys matrix. Compared with pure PI film, PM and PG films exhibited better thermal stability, they might be used as a kind of good heat-resistant lubricating films. Then, effects of MoS₂/Graphite content on fraction coefficient for PM/PG composite films were discussed. Results showed that, the friction coefficient of PM and PG composite films could be reduced significantly, compared with the pure PI film. For PM composite films, with the increase of MoS₂ content, the friction coefficient of composite membrane decreased initially, then increased markedly. And for PG composite films, the friction coefficient of composite membrane decreased initially, then reached the minimum value and kept stable. By calculating the average wear scar diameter（AWSD） of different lubricating films, it could be found that when the MoS₂ and Graphite content were less than 20 wt%, PM and PG films exhibited good wear resistance; While, as MoS₂ and Graphite content continued to increase, the PM films maintained excellent wear resistance, but the wear resistance of PG films dropped sharply. Based on the research above, a simple discussion of the friction and wear mechanism of two kinds of thin films had been carried on finally.MoS₂/Gr/PI（PMG） composite lubricating films filled with MoS₂ and Graphite particles were prepared by the same method as above. Afterwards the thermal properties, mechanical properties and tribological performance were analyzed. Results showed that, due to the synergistic effects of MoS₂ and Graphite, PMG coatings exhibited the best thermal stability and the maximum elastic modulus. What’s more, PMG composite could exhibit the tribological characteristics of PM composite and PG composite at the same time. It not only effectively reduced the friction coefficient but also showed good wear resistance.