| This study is based on the urgent demand of high-speed train brake pads. In view of the actual demand for high-speed train braking,’research and development on high-performance braking materials has been conducted. And experimental research was applied to the selection of raw material and the determination of fabrication methods. It is the first time to develop the powder metallurgy friction materials for braking of high-speed train with driving speed at350km/h and below. The1:1bench test carried in the ministry of railways product quality supervision and inspection center (CSR Qishuyan Institute Co., Ltd.) shows that, the performance of brake pads has reached the requirements of China Railway Test&Centification Centre (CRCC) on the properties of friction materials, with high stability coefficient of friction, brake smoothly, low damaging in the brake disc, low abrasion and environment friendly, etc. The main results can be summarized as follows:1. The friction materials were fabricated by powder metallurgy technology. In order to obtain high thermal conductivity and good toughness, copper was select as metal matrix. The effect of copper type on mechanical and friction performances was investigated. Five kinds of copper powder, including electrolytic copper powder, atomizing copper powder, aluminum oxide dispersion strengthened copper powder, chromium copper alloy powders(0.4%Cr-Cu) and Fe-Co-Cu alloy powder (Fel5%-Co10%-Cu) has been tested. Due to high sintering activity and continuity of matrix, specimen fabricated by atomizing copper powder has high temperature hardness (68.1HR15y), compressive strength (593.81MPa) and impact toughness (78.71J/cm2) and good friction performance. Atomizing copper powder was suitable for matrix components.2. Using iron powder as the main strengthening component, the effect of iron powder type on mechanical and friction performance was investigated. Four kinds of iron powder were estimated, including reduced iron powder, carbonyl iron powder, foam fiber iron powder and Fe3Al powder. The results show that, carbonyl iron powder has smaller particle size, and can be evenly dispersed in copper matrix, which improved the hardness of the copper matrix and the compressive strength. The friction coefficient of materials with carbonyl iron powder addition declined slightly as the braking speed increased. Thus, carbonyl iron powder was the suitable matrix strengthening components.3. Add the Cr, Mo, Cr-Fe alloy and colloidal graphite, can improve the high temperature strength of the metal substrate. The results show that, Cr and Cr-Fe alloy has a significant strengthening effect on copper matrix. Compared to pure copper, the Brinell hardness was increased by2.3times, the compressive strength was increased by1.4times, and the high temperature hardness was increased by1.1times. The impact toughness of Cu-Fe-(Cr-Fe) was equivalent to the value of pure copper. Pearlitic structure of iron was formed by adding colloidal graphite in the sintered body, which improved the wear resistance of sample. Therefore, chromium, Cr-Fe alloy, and colloidal graphite were chosen as the composite strengthening components.4. It is the first time for systematic study the reaction behavior between Cu, Fe, Cr and molybdenum disulfide. For Cu-Fe-MoS2composite sintered at700℃, part of molybdenum disulfide reacted with matrix metal. As sintering temperature increased to950℃, CuFeS2phase was generate in the matrix, which can produce good lubricity and wear resistance. For specimen with Cr addition, molybdenum disulfide reacted with chromium prior to Cu or Fe, and complex chromium sulfur compounds could be generated. Using Cr-Fe alloy instead of chromium, an iron rich dense layer was left during the reaction between Cr and S, which prevented the further reaction of the internal metal and molybdenum disulfide. In addition, the reaction between molybdenum disulfide and metal matrix was also affected by the sintering atmosphere. Due to the intensive reaction in pure hydrogen atmosphere, a mixture of nitrogen and hydrogen was used as the sintering atmosphere, which had a volume ratio of4:1.5. The preparation of a multistage pore structure was studied firstly. The type and size of graphite has the most significant effect on the structure of pore. Artificial graphite can significantly increase the porosity of the material with no decreasing the strength of material. Large particle size of flake graphite is easy to form large interconnected pores in materials. And the distribution of graphite in the matrix tends to be parallel to the direction of the friction surface. Therefore, the continuity of matrix improved and the strength of materials and the friction film forming ability increased. Reduce the granularity of metal powders can obtain pores with small size and high porosity. Thus, the friction coefficient of the sample increased and at the same time the wear rate reduced. Using high molding pressure-low pressure sintering method, sample obtained high friction stability and wear resistance.6. This study established the raw material and preparation technology for braking materials of high speed train. The effect of the content of raw material on mechanical and friction performance was determined. The raw materials for fabrication the friction materials consist of atomizing copper, carbonyl iron, chromium, ferrochrome, colloidal graphite, molybdenum disulfide, graphite and silicon dioxide. The composite powder mixtures were cold compacted at a pressure of600MPa and sintered in bell type furnace at950℃and a pressure of5MPa for2h under the mixture gas. The Brinell hardness, compressive strength, thermal conductivity are66.0HB,104.64MPa,17.21W/m℃(25℃) and13.89W/m℃(400℃). During the range of160-320km/h, the average coefficient of friction is0.282, and the fluctuation is less than0.003. The wear loss is1.43g. The properties of self-made material are approximately with import one and with higher coefficient of friction, stability and wear resistance.7. Self-made friction material was assembly to the brake pads in accordance with the actual structure. And the1:1bench test was carried out in the ministry of railways product quality supervision and inspection center. The results are shown here:In the brake speed range of50-380km/h, variation of the friction coefficient of was less than15%. The instantaneous friction coefficient, the average friction coefficient and static friction coefficients all fit the requirements of the testing outline. The wear of brake pad was0.10cm3/MJ, which was much lower than the value of0.35cm3/MJ given by testing outline. The highest temperature on the brake disk surface generated in the braking process was below700℃. No hot spots, scuff, plow and other injuries were observed. The braking process was environment friendly with low braking noise. According to the bench test, the self-made powder metallurgy brake pad fully meets the CRCC’s requirements of braking of high speed train with300-350km/h driving speed. |
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