Performance And Tribology Research Of Fe₃Al/Si₃N₄-base Friction Materimal

In recent years, the development of highway and railway has accelerated the process of high speed and loading of transport machines, which deserves higher properties of the friction material in the brake system. Higher speed of vehicles needs stable properties of the friction material in a comparatively larger scale of speed and temperature. This thesis has for the first time in China put forward a theory that friction materials were made by developing a new matrix named Fe3A1/ Si₃N₄-base . This dissertation, based on the study and application of friction materials in the world, is a study on new friction materials from the aspects of development, optimization, assessment of integrated properties of friction materials, friction wear mechanism and coupling of thermal and strength, etc.The composite has been made successfully. The study covers areas such as material science, thermodynamics, tribology and some other disciplines and the research is creative as well as systematical.The main contents are as follows:Fe₃Al powder was prepared by milling mechanical alloying technology. Relevant studies were conducted on its formation process and mechanism, its technology process and the necessity to apply annealing treatment. The structural evolution of Fe-Al elemental during mechanical alloying process, and the shape of Fe₃Al particle have been investigated and tested by means of XRD, TEM, SEM and DSC. Depending on milling mechanical alloying technology, with the increase in milling time, Al atom can dissolve crystal lattice of Fe, and form disordered a-Fe solid solution. During annealing at 750℃,through Al atom order rearrangement and removing of APS domains, the lower B2 ordered and higher ordered structure were obtained. Making use of the technology, relatively inexpensive Fe and A1 powder were made into Fe₃Al -base to prepare Fe₃Al-base composite.Based on systematic analysis of the structure and performance of Fe₃Al/Si₃N₄ materials to prepare, Further analysis and calculation were done on the physical and chemical compatibility between them. A new composite material formula was then developed and these components were mixed. Subsequently sintering technology was optimized and sintering theory was explored. The structure and performance of friction materials prepared were studied, and bond and structure of interface were studied in detail. The composite sintered presented favorable interface in the course of components and no obvious reaction products emerged. It was found that the densification process was affected by the component of original powders, particle size and processing parameters such as sintering temperature, pressure and holding time.The optimal parameters are as follows:sintering temperature 1320℃,pressure 10-20MPa, holding time for pressure 10-30min, holding time for temperature 30-60min, with the density of 90%-95%.On the ground of design principle of friction materials, Fe₃Al/Si₃N₄ composite material was chosen as the matrix, with flake graphite as lubricants, as friction particles. The chemical compatibility between the components according to the basic principle of thermodynamics was examined. XRD,SEM,EMPA,TEM and EDS investigations indicated that the main phases in the Fe₃Al/Si₃N₄-based composite friction materials are Fe3A1,graphite, Si₃N₄,AlN,MgO,Graphite and Al2O3 , a small quantity of AlN as impurity. Graphite and Al2O3 were dispersed uniformly in the matrix. Because of its unfavorable interface bonding with other bond,Graphite decreased mechanics capacity. In the course of friction test, graphite can shift into lubricating layer with a low graphite content, composite exhibited slight adhesion abrasion;while with a high graphite content, because of much boundary limitation, mechanics capacity declined sharply. When graphite slice cracked because of cutting to form much graphite particle, wear performance is transformed to heavier particle abrasion with brittle split of materials. The wear forms of the Fe₃Al/Si₃N₄-based friction materials are mainly particle wear, oxidation wear and fatigue wear, exhibiting the micro cutting, micro furrows, dot corrosion and spalling away.The coupling of thermal and strength of ceramic friction materials is simulated with finite element method (FEM). And the temperature change of the friction face is tested on friction tester with constant speed. The comparison between the simulation result and the friction experiment with constant speed shows the consistency between the two. This proves the feasibility of finite element method in the analysis of coupling of thermal and strength, which provides theoretical and experimental foundation for the development of new friction materials and the study of wear mechanism of friction materials. The results of friction tests with constant speed show that the temperature of brake lining material surface rises with the passage of time. The higher the pressure is, the faster the temperature rises. It can be found that the temperature of friction interface calculated by FEM is higher the temperature measured in friction test. Ignoring the heat dissipation by means of cross-ventilation, radiation and temperature rising of abrasive particles induces the difference between the results of simulation and test. In order to obtain more exact results by means of simulation, wear and chemical reaction of friction material should be taken into account.To sum up, the thesis developed Fe3A1/Si₃N₄-base friction materials, and testified the feasibility of the formula. The experiment tested its mechanics capacity and probed its wear mechanism. The experiment not only explored the effects of other components its friction capacity, but also tested and simulated its friction capacity in complicated working conditions. It is testified that its capability of anti-oxidation and anti-corroding excelled one of Fe-base friction material.Braking test found that the material exhibited excellent braking performance, and possessed great potential in its development as a new friction braking material.