The Preparation And Tribological Performances Of Fe₃Al Based Composites Reinforced By Carbon Nanotubes And Rear Earth

Using tribological principle to reduce kinetic energy is one of the most important braking methods for vehicles and other equipment.The friction material is the key material for braking,and its mechanical properties,thermal conductivity and tribological behavior are very important to the safety of vehicles and moving components.In order to meet the requirements of high-speed and heavy-load development of vehicles and other equipment,the increasingly comprehensive performance of friction material was demanded.The research of middle-and high-end brake devices started late,and especially the basic research on the application of high-performance friction materials is relatively weak in our country.Therefore,the development of high-performance and low-cost friction materials has important scientific significance and application prospects.The special lattice structure of Fe3Al intermetallics shows high strength,high hardness,high wear resistance and other macroscopical characteristics.The resistance of oxidation,rust and failure under high temperature makes it completely match the requirements of friction materials.In this paper,a new type of Fe3Al-based composite friction material reinforced by carbon nanotubes and rare earth was prepared by adding Al2O3,CNTs,La,Cu and MoS2 as additives in the matrix of Fe3Al.Microstructure,chemical composition and phase structure of the composites were analyzed and characterized by means of field emission transmission electron microscopy(FE-TEM),field emission scanning electron microscopy(FE-SEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),energy dispersive spectrometer(EDS).Friction and wear tester was used.The tribological properties of the composite were studied,and the tribological properties were studied by means of multifunctional material surface comprehensive performance tester(CFT-I).The main innovative research contents are as follows:The Fe3AI intermetallic compound powders were prepared by mechanical alloying(MA),and the effects of milling medium(stainless steel and agate ball),milling time(5-30 hours),annealing process and initial particle size of raw materials on the formation of products and the mechanism were systematically analyzed.The results show that after milling with agate ball for 30 hours and then annealed in vacuum at 750? for 2 hours,the Fe3Al intermetallic compound can be prepared by the powders of Fe and A1 with the initial particle size of 1000 mesh.The density of milling medium has notable influences on the milling process and the order that phases formation,while annealing after milling is quite important to the formation of Fe3Al,the regulation of non-equilibrium phase diffusion and the elimination of stress and dislocation.Fe3Al-Cu-Al2O3-MoS2 composite friction materials were prepared by pre-pressing sintering.Influences of Cu,Al2O3 and MoS2 content on friction and wear properties were investigated.The results show that the comprehensive properties of the composites are better when the pre-pressing pressure is 200 KN,the sintering temperature is 1150? the proportion of composite components is 3% Cu,3%Al2O3 and 2% MoS2..The main functions of Cu,Al2O3 and MoS2 are to improve the interfacial bonding strength,enhance the wear resistance and adjust the friction stability,respectively.The wear mechanism is adhesive wear and fatigue wear under the experimental conditions,By introducing multi-walled carbon nanotubes(CNTs) and rare earth(La) to improve the material system,the problems of low density(plenty pores),low microhardness,insufficient bonding force between powder and easy delamination of in friction could be solved.It is notable that CNTs with large aspect ratio(132,000,000:1) can be used as fiber materials to strengthen the bonding strength of composites,while La can help improving the wettability between the components.The results show that the friction materials prepared by adding 0.5% CNTs and 0.7%La not only have good mechanical and tribological properties,but also have good performance in resistance on high temperature oxidation,thermal shock and salt solution corrosion.The wear mechanism under high load gradually changes from the coexistence of abrasive wear and fatigue wear to fatigue wear singly.For solving the problems of oxidation in sintering,loose bonding,air expansion in voids and even cracking,hot-pressing sintering(HPS) in vacuum was used.The analyzation demonstrated that the sintering pressures can improve the density of matrix,and the thermal conductivity,mechanical properties and wear resistance of HPS composites are significantly higher than those of commercial brake pads.The results mentioned above can be attributed to the HPS composites which possess compact internal structure,good bonding strength,thermal conductivity and fibre reinforcement of CNTs,and enhanced wettability by La.The composites exhibit stable coefficient of friction and well resistance of wear at medium and high temperature,and the main wear mechanisms are abrasive wear and slightly oxidative wear.The composites also have good wear resistance in different sliding speeds and applied loads at room temperature.Further observation of wear morphology shows that CNTs can significantly reduce the fatigue wear of the composites by reducing friction vibration,stress accumulation and the lubrication of debris.