Research On The Properties And Mechanism Of Rare-earth-modified Bamboo-fiber-reinforced Resin-based Friction Materials

Natural fiber has some characteristics,such as biodegradability,regeneration,a low density and high strength,which makes the new,reinforced resin-based friction material become a research hotspot in recent years.However,the interfacial adhesion between natural bamboo fiber and resin is poor,and the heat resistance of bamboo fiber is not good,which limits the application of natural bamboo fiber in high performance resin-based friction material.The main focus of current research in copper-free friction materials is on searching out a replacement for copper,which has been recently proved to be a hazard to human health and the environment.In this study,rare-earth-lanthanum-chloride-solution（LCS）was used for the surface treatment of bamboo fiber to improve the interfacial adhesion between bamboo fiber and resin matrix,and increased the heat resistance of bamboo fiber as well,effectively improving the mechanical and tribological properties of bamboo-fiber-reinforced resin-based friction materials.The rare earth oxide was successfully explored for the replacement of copper in friction materials,showing excellent tribological properties at elevated temperature.LCS modification was used for the surface modification of bamboo fiber.The results show that the LCS modification significantly improves the interfacial adhesion between the bamboo fiber and resin,improving the heat resistance of bamboo fiber at the same time.LCS modification on the mechanical and tribological properties of friction materials is better than that of alkali treatment.The addition of the appropriate amount of La₂O₃ to the friction materials can effectively improve the mechanical and tribological properties of the friction materials.The optimum formula of the friction materials can be shown as follow:LCS-treated bamboo fiber 7%,ceramic fiber 10%,phenolic resin14%,lanthanum oxide 14%,alumina 19%,barite 28%,rubber powder 5%,graphite 3%.Compared with the high quality brake materials at home and abroad,the friction coefficient of the optimum formula at various temperatures is higher than that of the native specimen（Honda）and imported specimen（BMW）,while the wear performance is equivalent to that of the contrast samples.The Chase testing results of the optimum formula show that the friction coefficient at low temperature is 0.563,and the friction coefficient at elevated temperature is 0.580,showing excellent heat fading resistance and recovery performance.The classification of friction coefficient is HH,which can be used for high performance brake friction materials such as racing cars and aircraft.The interfacial bonding properties between the bamboo fiber and resin matrix were characterized by nanoindentation.Fourier-transform infrared spectroscopy（FTIS）and X-ray photoelectron spectroscopy（XPS）were used to analyze the change on the surface chemical composition of bamboo fiber before and after the modification.The results show that the LCS modification can decrease the hydroxyl concentration on the surface of LCS-treated bamboo fiber.Rare earth ions La³⁺react with oxygen in the hydroxyl group at the C₂ position in the glucosylic ring of cellulose,and simultaneously,La³⁺can still be coordinated with the oxygen in the hydroxyl group in the molecular chain of the phenolic resin.The LCS-modified bamboo fiber bonds with the resin matrix to form a rare earth complex,which significantly enhances the interfacial adhesion between the bamboo fiber and resin matrix.Scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,and three-dimensional surface topography measurement were used to analyze the worn morphology of the samples.Compared with the addition of Cu in friction materials,the results show that La₂O₃ is more conducive to the formation of continuous and smooth friction and transfer films and causing the decrease in surface roughness,which is beneficial to the tribological properties of the friction materials,playing a role in stabilizing the friction coefficient.X-ray diffraction was used to make phase analysis on the worn morphology of the samples.The results show that the reaction between La₂O₃ and Al₂O₃ can form La Al O₃,and the reaction between Al₂O₃ and Ba SO₄ can produce Ba₁₈Al₁₂O₃₆ and Al₂（SO₄）₃ during the friction and wear processes.This mechanism enables the previously accumulated inorganic filler to bind together with chemical bonds,which can effectively improve the tribological properties of the friction materials at elevated temperature.