Preparation And Study Of Friction And Wear Mechanism Of Fe-Al Intermetallic Base Friction Material

With the rapid development of automobile industry, especially the trend towards higher speed and heavier load for the automobiles, there is an increasingly high standard for friction materials. Careful research on friction materials has been done around the word for many years and the present system include materials mainly consist of polymer matrix, powder metallurgy friction materials and C/C friction materials. In order to overcome the defects of the traditional materials, correlative literature both domestic and overseas was consulted and analyzed. With its structure of much bond, lower cost, much lower density, better thermal conductivity and what' s more important, its performance of resistance to high temperature, oxidation and corrosion brought by its unique structure, Fe₃Al intermetallics was chosen as the basic materials. This thesis has for the first time in China put forward a theory that friction materials were made by developing a new matrix named Fe₃Al-base. 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 following has been systematically and emphatically investigated. First, 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. Second, based on systematic analysis of the structure and performance of materials to prepare, appropriate friction components and lubrication components were optimized as the added ingredient. Further analysis and calculation were done on thephysical and chemical compatibility between them. A new friction material formula was then developed and these components were mixed. Subsequently sintering technology was optimized and sintering theory was explored. Third, the structure and performance of friction materials prepared were studied, and bond and structure of interface were studied in detail. Forth, mechanics performance of friction materials prepared was tested, and on the condition of different testing condition and changing content of components, friction capability and wear mechanism of material were studied. Fifth, comparative experiments on friction capacity of FeaAl and Fe-base friction materials were carried out and their wear mechanism was studied. Studies was also done on their capacity of resistance to oxidation and corrosion and the mechanism of resistance to oxidation of FesAl was discussed. Sixth, its simulating braking test was made, and its braking characteristic and friction wear capability were studied too.The major conclusions drawn from the research are as follows. 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°C .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 Al powder were made into FeijAl-base to prepare Fe3Al-base composite.During the experiment, the raw material components, such as Cu powder, AI2O3, graphite, MoS2 etc, manifested good physical and chemical compatibility. Therefore, they can be mixed altogether and sintered. The result proved that Cu powder played an important role in strengthening sintering capacity. Having taken into account most factors of sintering process, an optimized Fe3Al base friction material technology of sintering composite came into being: sintering pressure of l(M5MPa, sinteringtemperature of 10504100°C and holding time 30min .The investigations into the bond component and the structure of interface of Fe3Al-base friction materials revealed the following. 1. Fe3Al-base composite sintered consist of a majority of B2 and a few DO, structure form. 2. In the process of sintering composite, many A12O3 particles were produced. Furthermore, Cu functions as mucilage glue and felted components together. Cu dissociated with other components. 3. The composite sintered presented favorable interface in the course of components and no obvious reaction products emerged. The integration of graphite with other components in the interface was weaker and therefore it was common that micro-crackle came into being here and extended. As result of it, the material was destroyed. However in the experiment, graphite atom diffused into base in the interface and this can improve feebleness bonding strength between graphite and components.The size of Fe^Al particles affected friction capability of composite, but too small particles produced much crystal particle boundary, and more defects, which went against the improvement of friction capacity.Depending on various reinforcing mechanism, such as solid solution, precipitation hardening, etc, Cu can reinforce Fe3Al base friction materials in the course of friction experiment. Because of physical and chemical action, Cu shifted from dissociating state to the surface and formed friction layer, which changed wear mechanism and improved friction capacity. The addition of Cu not only changed the composite wear mechanism, but also transformed the composite from particle abrasion brought by brittleness rupture to a mixed abrasion of particle abrasion brought by plasticity fluxion and slight adhesion abrasion. A high Cu content made the composite exhibit heavy adhesion abrasion while appropriate Cu content can reduce wear rate on the condition of high-speed sliding test.Because of its unfavorable interface bonding with other bond,Graphite decreased mechanics capacity. In the course of friction test, graphite mixed a little Cu 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. Due to better compatibility between MoS2 and mental particles, appropriate MoS2 content improved impact strength of composite. With too much MoS2, conglomeration of MoS2 declined mechanics capacity and friction wear performance, and to be specific, it resulted in severe adhesion abrasion. The research also proved that appropriate hole density can detain worn scraps and decrease wear rate. In the experiment, the friction coefficient of Fe3Al-based friction materials is 0. 5 or so and the average wear rate is 0.45-0. 1X10"5 mm'NV.Fe3Al-base friction materials manifested excellent resistance to heat impact and high temperature oxidation. In a relatively wide range of temperature, it had no micro crackle and no oxidation layer warping. Compared with Fe-based friction, Fe3Al-base friction materials exhibited excellent resistance to oxidation between 800°C and 1000°C. This could be attributed to the formation and protection of dense and continuous A12O3 film on the surface Fe3Al-base particulate. For FesAl-base friction materials, in the process of friction test, oxidation phenomena mainly happened on the surface of composite, and it became more and more unconspicuous from outside to inside. The main reason for the great enhancement of its resistance to oxidation lies in the formation, accumulation and film formation particles during the oxidation.The simulation brake test showed that Fe3Al-base friction braking sample braked with good stability, relatively invariable frictioncoefficient and lower wear rate. Friction test manifested it had no obvious friction coefficient decline due to temperature variation. It was also found that, in the process of friction, a layer came into being that consisted of graphite and Cu, the same as the result of M-2000 test. Braking test also proved that wear mechanism of braking sample of composite was particle abrasion, oxidation abrasion and adhesion abrasion, and that coincided with the preceding tests, too.Braking test showed that Fe^l-base friction materials had no conglutination and scrape with the counterpart and the composite can avoid the adhesion to cast iron material.To sum up, the thesis developed Fe3Al-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.