Properties of Boron Nitride/Hyperbranched Polysiloxane-Polyimide Bonded Solid Lubricant Coating

Polyimide (PI), as one of most promising high-performance and multi-functional polymeric materials, has been widely applied in the field of aerospace and aviation. However, as the lubricating materials, PI has been greatly limited by high wear and high friction coefficient, especially in some extreme working conditions. In this work, the amino-terminated hyperbranched polysiloxane (HBPSi-NH₂), which had the characteristics of flexible segment and low surface energy, was incorporated into the main chain of polyamic acid (PAA) molecular through the one-step approach to obtain coating binder material (HBPSi-PAA). Then, hexagonal boron nitride nanosheets coated with polydopamine (PDA@exf-h-BN), of excellent lubricity, wear resistance and heat resistance were used as a solid lubricant. HBPSi-PAA was sprayed on the surface of tinplate by spraying method. After curing, PDA@exf-h-BN/HBPSi-PI bonded solid lubricating coating was obtained. And then, the MMUD-1B friction tester and the SDTA 85 thermal weight loss analyzer were used to study the tribological and thermal properties of the coating, and the friction and wear mechanism was discussed. The results showed that the friction coefficient of the PI coating showed a trend of first decreasing and then increasing with the HBPSi-NH₂ loading, while the wear resistance life showed a trend of first increasing and then decreasing. When 10.0% (mass fraction) HBPSi-NH₂ was introduced into the PI coating (the coating was noted as 10.0% HBPSi-PI), the friction coefficient of 10.0% HBPSi-PI coating was as low as 0.24 and the wear resistance life reached 60 min. When a certain amount of PDA@exf-h-BN was introduced into the 10.0% HBPSi-PI coating, the friction coefficient and wear life of the PDA@exf-h-BN/HBPSi-PI bonded solid lubricating coating were greatly improved compared to the HBPSi-PI coating. As the increase of PDA@exf-h-BN, the friction coefficient of the PDA@exf-h-BN/HBPSi-PI bonded solid lubricating coating gradually decreased, and the wear resistance life also showed a trend of first decreasing and then increasing. Among them, when the addition of PDA@exf-h-BN was 12% (mass fraction), the friction coefficient of 12% PDA@exf-h-BN/10.0% HBPSi-PI bonded solid lubricating coating was stabilized at 0.12, the wear life reached 60 min. Meanwhile, the initial decomposition temperature increased from 319.8 ℃ to 545.6 ℃, the residual mass reached 74.6%. It not only showed excellent tribological properties, but also showed excellent thermal stability. This was mainly attributed to the synergistic friction reduction and wear resistance of the “soft-hard” particles between HBPSi-NH₂ and PDA@exf-h-BN, and the good thermal stability they provide to the coating. This research provided the possibility to develop high-performance, multi-functional bonded solid lubricating coatings in aerospace, chemical industry, microelectronics and other high-tech fields. It also laid a corresponding theoretical foundation for the research of the novel hyperbranched polysiloxane containing Si-O-C structure in the field of friction.