Solid-Liquid Duplex Lubrication Behavior and Mechanism of Polyimide with Different Molecular Structure

With outstanding properties such as excellent mechanical performance, good thermal stability and chemical resistance, polyimide (PI) plays a vital role in many high-tech applications such as air-engines, aerospace vehicles, microelectronics industry and precision machinery. For a long-life lubrication in harsh condition, the recent creative methods combined self-lubricating PI with different base oil have come into practical application, such as porous oil-impregnated cage and oil-containing coating. In this article, a new type of thermosetting polyimide (TPI) material was synthesized and its mechanical properties, thermal properties and wetting behavior of surface base oil (PAO10) were characterized based on the technique of dynamic mechanical analysis (DMA), universal testing machine and contact angle measuring instrument, respectively. The commercial PI (YS-20) was also explored in comparison as different molecular structures of TPI. It was clear that TPI with the special structure of chemical crosslinking poses a higher heat-resistance and poor mechanical performance. Besides, the analysis of contact angle indicated that both TPI and YS-20 had a better wettability on the surface. In addition, the tribological behavior of the two PI with PAO10 solid-liquid duplex system was investigated to focus on the effect of tribological behavior of different PI molecular structures with PAO10 and mechanism of synergistic effect was further verified theoretically. On this basis, two modes of friction with ball-disk contact were used in order to reflect the varies of application condition. The continuous rotating mode would basically represent with a single “start-stop” and intermittent reciprocating motions could be on behalf of multiple “start-stop” to some extent. Compared with dry friction conditions, both TPI and YS-20 under oil lubrication showed extremely low friction coefficient and wear rate, while the system of TPI and PAO10 was better than YS-20 with PAO10. These results showed that the solid-liquid duplex system of TPI and PAO10 had excellent performance of anti-wear, no matter in the simulation of multiple “start-stop” reciprocating motions or a single “start-stop” rotating contact. In contrast to YS-20, a further analysis demonstrated that the chemical crosslinking of TPI with a special benzene ring endows PI with a better wettability and rapid extension. Hence, we believe that the result of TPI-PAO10 with excellent friction-reducing and wear resistance might be attributed to the better wettability and physical adsorption between TPI and PAO10. Besides, the first-principles calculation about adsorption energy was also used to confirm our speculation between different PIs with PAO10 in this field. The result showed that TPI (∆E_adsorption= −32.30 kJ/mol) had a more positive adsorption with PAO10 than common structure of YS-20 (∆E_adsorption= −20.92 kJ/mol). Through molecular dynamics simulation, it further verified that the TPI with special cross-linked structure enhances the physical adsorption between TPI surface and PAO10, which would be helpful to improve the construction speed and carrying capacity of the boundary film, thereby improving the composite system of tribological behavior.