Study On In-Situ Modification Of High-performance Fiber With Nanosilica And The Properties Of Its Paper-based Friction Materials

Paper-based friction materials are widely used in oil-immersed clutches and brakes and have attracted tremendous attention of scholars at home and abroad because of its excellent properties such as stable friction coefficient,smooth fitting performance,long service life,etc.Paper-based friction materials are made of reinforced fiber,fillers and friction performance regulator with the process of wet-forming and resin impregnation.However,the poor tribological property of paper-based friction material has become a problem which restricted its wide applications.Herein,the reinforced fibers?aramid fiber and carbon fiber?have been used as a new generation of ideal reinforcement in paper-based friction material for their high strength and modulus,good chemical and erosion resistance,good thermal stability and stable friction coefficient,which play a critical role in maintaining the tribological properties of materials.However,the smooth and chemical inertness of the fibers lead to the poor interfacial bonding with resin and more interfacial defects,which cause the poor tribological properties of the materials.Therefore,looking for an effective way to improve surface properties of fiber has been an urgent problem in paper-based friction materials.Nanosilica have been always employed to modify high molecular compound or fibers for its small particle size,large specific surface area,with hydroxyl group on surface,strong adsorption of surface,high heat-resistant wearability and other excellent properties.Therefore,this study aimed at in-situ modification of fiber with nano-SiO₂ by sol-gel method for surface defects of aramid fiber and carbon fiber,so as to improve the mechanical properties and tribological properties of paper-based friction material.Firstly,the sol-gel method was used to modify aramid fiber by in-situ deposition,and the response surface methodology was utilized to explore the optimal parameters of nanosilica particle size grown on surface of aramid fiber based on quadratic polynomial regression model.The results showed the pure water had the most significant effects on nanosize of nano-SiO₂ and the amount of TEOS had a minimal impact.And the optimal parameters of nanosilica growth were 276.42 g of ethanol,43.2 g of pure water,18.75 g of ammonia and 62.5 g of TEOS,The technical proportion of ethanol,pure water,ammonia and TEOS is20:8:0.3:1?mass ratio?.Then the particle size was 298.4 nm.In this case,the smaller particle size of nano-SiO₂ could be obtained and it was well established on fiber surface with a dense and uniform coating layer.Secondly,nano-SiO₂ was used to modify the pretreated aramid fiber according to the optimal parameters above.The morphology,chemical structure,wettability and thermal stability of aramid fibers,and the mechanical properties and tribological properties of paper-based friction materials,have been conducted and analyzed.The results showed there appeared a dense and uniform coating layer of nano-SiO₂ on surface of modified aramid fiber with a connection of C-O-Si chemical bond.The wettability?contact angle?of fiber was improved after nano-SiO₂ modification,which was reduced from 88.2°to 59.1°.The thermal stability of fiber tended to be improved after modification because the maximum rate of weight loss was increased from 529.5^oC to 543^oC,but the initial thermal decomposition temperature decresed.Compared to the control samples,the tensile strength of paper-based friction materials was increased from40.5 N·m/g to 44.4 N·m/g and the interlayer bonding strength was improved from 602.9 J/m² to 1077.5 J/m²,with an increasement of 9%and 44%respectively.The stationarity of friction coefficient of materials significantly increased,and the wear rate of materials was reduced by 27%.Thus,in-situ modification of aramid fiber by nano-SiO₂ could improve the friction and wear properties of paper-based friction material.Finally,the dopamine pretreated carbon fiber was modified with nano-SiO₂,and the microstructure of fiber and macro-properties of paper-based friction materials were investigated and studied.The results showed that a dense SiO₂nanoparticles layer was deposited on carbon fiber surface with a chemical bond of C-O-Si.The crystallinity of modified carbon fiber was improved and the grain size also increased,arranging more orderly along the fiber axis and owning the higher orientation degree of fiber.The tensile strength of paper-based friction materials after modification was increased from 46.8 N·m/g to 55.6 N·m/g,and interlayer bonding strength was improved from 560.2 J/m² to 1123.3 J/m²,increasing by 19%and 100.5%,compared to the control samples.The friction coefficient stability of the material was improved and the wear rate of materials was reduced by 40.9%after modification because of the high heat-resistant and wearability of nano-SiO₂.Compared to paper-based friction materials with modifed aramid fiber,the materials with modified carbon fiber had an obvious improvement in mechanical properties and tribological properties.The self-lubricating property of carbon fiber and the modification of nano-SiO₂ could weaken peeling and embossment of asperities thereby generating fine wear abrasive particles and lower the wear rate.Therefore,the modification of carbon fiber with nano-SiO₂ could effectively alleviate the seriousness of the wear and improve the friction and wear performance.