Preparation And Dry Friction Properties Of MoS₂/Mica Filled PI Composite Film

The traditional lubricating oil has been widely used in aerospace,automobile industry,engineering machinery,and other fields.However,it is unable to meet the new requirements of the modern industry’s dramatic development in prohibiting grease and maintenance-free.Therefore,the development and use of new solid lubricating materials are particularly important.Polyimide（PI）is a common self-lubricating polymer,which is very suitable for parts with high precision requirements.But as a solid lubricating material,its wear resistance is weak,which severely limits its application.Therefore,the influence of chemical structure and inorganic wear-resistant particles on the wear resistance of PI was studied in this paper,finding that the improvement on the wear resistance of PI was achieved by adjusting the above two aspects.Firstly,PI with three different chemical structures was prepared by a two-step method,namely PMDA-PI,BPDA-PI,and BTDA-PI.The most flexible BTDA-PI has the highest crystallinity and the smallest friction coefficient and wear rate.On the other hand,the most rigid PMDA-PI molecular chain has the lowest crystallinity and the highest friction coefficient and wear rate.The higher the flexibility of the molecular chain,the stronger the segments ability to move during the heating process.It is easier to arrange into an orderly and compact crystal structure,and the force between the molecular chains inevitably increase.It effectively avoids the slippage and breakage of molecular chains during friction,making it perform better in friction performance.Thus,the BTDA-PI was selected as the matrix by comprehensively analyzing the performance and cost of these three films.The layered Mica and MoS₂ particles were incorporated into the selected matrix as inorganic wear-resistant particles.Mica/PI（PMI）and MoS₂/PI（PMO）composite lubricating films were prepared by in-situ polymerization.It was found that the mechanical properties,thermal properties,crystallinity and dry friction properties of the composite films were significantly improved compared with the pure films.The smallest friction coefficient of the PMI composite film is only 0.061 at 1% Mica fillingand the longest breakdown time,which is 2.22 times that of pure PI film.In addition,the PMO composite film has the smallest friction coefficient of 0.057 when the MoS₂ filling amount is 2%,and the breakdown time is 2.74 times that of the pure film.It suggests that the unique layered structure and the crystal induced by the two kinds of inorganic particles are the fundamental reasons for the wear reduction.After comparing the performance of the two composite films,it was found that PMI composite films had better friction properties when the additive amount was less than 1%.PMO composite films had better friction performance when the inorganic particle filling amount was higher than 1%.It is due to the higher hardness and surface activity of Mica particles.When the filling amount is low,the high-hardness Mica particles are evenly dispersed in the PI matrix as a hard phase,and the PMI composite film has excellent wear resistance.When the filling amount is high,the Mica particles are close to each other.The active groups on the surface（Si-O-Si,-OH-）increase the van der Waals force between the particles,causing the particles to be redundant and even act as abrasive particles in the gap of the friction pair,which aggravates the wear and deteriorates the wear resistance performance.This results provide PI with experimental protocol and feasibility analysis in the field of solid lubrication.