| In order to reduce the independence on imported high-speed train brake pads, present thesis conducted systematical investigations on design of the brake pads with the emphasis on fabrication of matrix, friction components and lubricant components. The properties, namely, wear resistance, friction coefficient and corresponding stability, thermal conductivity, of obtained pads were systematically characterized and investigated. As results, the finally fabricated Cu-based brake pad can be used for high-speed trains operating at 380km/h, which was tested and confirmed by 1:1 bench experiments and the performance of the pad meets the requirements of UIC and CRCC.The different iron powders with different particle size and content were investigated as the matrix components of the brake pads. Consequently, the brake pad containing about 22 wt% water atomized iron powders (75μm) presents compact microstructure and best performance with high physical strength and less linear wear loss. However, the enhancement of iron concentration results in decrease in the thermal conductivity of the pads. Due to good high-temperature performance in terms of thermal conductivity, physical strength and wear resistance, Oxide (Al2O3) Dispersion Strengthened Cu (Al-ODS Cu) was applied as reinforced components for metallic matrix of brake pad. The pads containing more than 30% Al-ODS Cu show high porosity. However, the pads containing 10%-30% Al-ODS Cu present compact microstructure with low porosity, which provides highest sheering strength (25MPa), highest friction coefficient (0.256) and lowest linear wear loss (0.009 mm per time per surface). Additionally, during braking process, dense and unbroken oxide films generate and stick to the surface of samples firmly. These phenomenon confirm that the main wear mechanism is oxidative wear.The type of friction components has significant influence on friction coefficient and its stability. Basically, Al2O3 ceramic powders show low affinity to metallic matrix, which leads to poor combination between Al2O3 powders and metallic powders. Therefore, Cu-coated Al2O3 was applied to replace conventional Al2O3 powders due to its high affinity to metallic matrix, resulting in increase of friction coefficient and friction coefficient stability. Nevertheless, under high-speed operation, the friction components still drop off, causing decrease in friction coefficient. All of this are due to physical combination between Cu-coated Al2O3 and the matrix. Different carbon-content CrFe powders show certain degree of solid solubility to metallic matrix, and, consequently, friction components are not easy to drop off during braking process. Hence, CrFe powders were used as friction component in brake pads to replace Cu-coated Al2O3. As an important result, the friction coefficient of CrFe powders containing pad decrease initially with increase in rotating speed and then increase with further enhancement of rotating speed. Moreover, compared with brake pads containing Cu-coated Al2O3 powders, the friction coefficient of high-carbon CrFe contained samples is higher than that of coated Al2O3-containing samples by 12%-27%. The stability of friction coefficient is by 10%-20%. And the friction coefficient and its stability are optimum when CrFe concentration is 6 wt%.Graphite powder is an indispensable component in friction materials due to its good high-temperature lubrication ability. However, it shows low affinity to copper matrix. Therefore, with the increase of graphite content, the microstructure of brake pads gets more porous and the wear resistance increases initially and then decreases, meanwhile, the thermal conductivity and physical properties deteriorate. It can be conclude from present thesis that the overall performance of brake pads containing 11wt%～13wt% graphite is best. According to the results from theoretical analysis and computational calculation in terms of thermal conductivity of friction materials, the brake pads containing vertically orientated (vs friction surface) graphite present much higher thermal conductivity than ones with non-orientated graphite, which leads to a considerable reduction in face temperature. Based on these conclusions, present thesis fabricated firstly brake pad containing orientated graphite to solve the problem of high temperature on friction face. As results, after unification of orientation of graphite in brake pads, the microstructure of matrix become more compact, resulting in a friction surface on which flake graphite is rooted homogenously in matrix without whole flake graphite lying on the surface. Moreover, proposed brake pads present double thermal conductivity than those sample containing the graphite distributed in a random way. The unification contributes to considerable increase of 14-21% in stability of friction coefficient. And that results in decrease of 1% in friction coefficient, which still meet the requirements, and reductions of highest temperature on friction surface and linear wear loss by average 60℃ and 10%-20%, respectively.Through the design of fabrication process and establishment of proportion of raw ingredients based on the obtained results, the final brake pad was obtained. The obtained brake pad is better than imported one in terms of, hardness (HRR) (62 vs 56), sheering strength (43.2 vs 22.4MPa), thermal conductivity (122.4 vs 44.2W/m·k), (home-made brake pad vs imported one). Additionally, the properties of obtained brake pad was measured through 1:1 bench tests by China Railway Test & Certification Centre (Chang Zhou). During single braking test model, it presents good high-temperature strength and stable friction coefficient, and the highest temperature on the friction surface is 612 ℃. Moreover, under the same model, the obtained brake shows lower wear loss (0.1cm3/MJ) than that of imported one (0.335cm3/MJ) under the same operation speed (380km/h). Besides, during continuous braking test model, the fabricated brake pad shows higher friction coefficient and lower highest temperature of friction surface that corresponding properties of imported one. All of the tested properties satisfy the qualifications proposed by CRCC for braking process of CRH380A high-speed train. |
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