| This thesis aims at active demand of performance and service life improvement of high-speed train brake discs.The TiC-Fe homogeneous composites,gradient composites and interpenetrating phase composites were successfully prepared by pressureless sintering,hot-pressing sintering and pressureless infiltration processes.The scientific and technical problems about preparation technologies,microstructure,mechanical properties,friction and wear behavior of TiC-Fe composites with different structural styles were studied systematically,which were expected to lay the foundation of preparing surface wear-resistance layer for high-speed train brake discs.The phases and chemical compositions,interface structure between reinforcement and iron matrix were analyzed.The density,Vickers hardness,bending strength,tensile strength,interlaminar shear strength and fracture toughness of the fabricated composites and compression strength of TiC porous preforms were tested and analyzed.Besides,the effect and inner mechanism of carbonyl iron powder contentson the properties of Fe-based materials and TiC-Fe composites were analyzed.Through above tests and analyses,the optimize fabrication process was confirmed by comparing the microstructure and mechanical properties of the of the TiC-Fe composites.Furthermore,the coefficient of friction and wear rate of TiC-Fe composites fabricated by different processes were also tested and illustrated.The effect of compositions,structural styles and friction test parameters on friction performance of TiC-Fe composites was studied.The results showed that:(1)The sintering temperature for fabricating Fe-based materials and TiC-Fe composites can be reduced by adding carbonyl iron powder,and it can also promote the relative density and Vickers hardness of Fe-based materials and TiC-Fe composites.It is also revealed that there is a basic linear relation between the radial shrinkage ratio of Fe-based materials and the carbonyl iron powder content.The mechanism of adding carbonyl iron powder is that it can increase C element in Fe-based materials during sintering process.(2)The 70?90vol.%TiC-Fecomposites were successfully prepared by pressureless sintering.The most suitable carbonyl iron powder content is taking account 60%of Fe-based materials powders,and the optimum sintering temperature is 1500?.Among these TiC-Fe composites,the relative density of PS70T60C and PS80T60C samples reach 99.4?99.5%,the bending strength of PS70T20C samples peak at 496 MPa and the Vickers hardness of PS80T60C samples peak at 12.2 GPa.Adding zinc stearate during the milling process could promote the property of TiC-Fe composites,and the optimal weight fraction of zinc stearate accounts for 0.5wt.%of total composite powders.(3)The properties of Fe-based materials and 10?60vol.%TiC-Fe composites fabricated by hot-pressing sintering were effected by TiC volume fraction,carbonyl l iron powder content in Fe-based materials and sintering temperature.The optimum sintering temperature for TiC-Fe composites by hot-pressing sintering is 1270? under 25 MPa pressure.Among TiC-Fe composites,the tensile strength of HP20T60C samples peak at 634 MPa,the Vickers hardness of HP40T60C samples peak at 5.1 GPa and the fracture toughness of HP30T60C samples peak at 17.0 MPa-m1/2.The interface between TiC and Fe is thin and no impurity is found on it.(4)TiC-Fe/Fe bilayer composite consisting of a pure Fe layer and a TiC-Fe composite layer.The strong interlaminal bonding between pure Fe layer and the TiC-Fe composite layer endows the bilayer composite with synergistic enhancement.For the TiC-Fe/Fe bilayer composites,30vol.%TiC-Fe/Fe bilayer composite exhibits the largest interlaminar shear strength of 335 MPa.The 40vol.%TiC-Fe/Fe bilayer composite owns the maximum tensile strength of 588 MPa.In addition,there is a 72.3%increment of interlaminar strength for 40 vol.%TiC-Fe/Fe bilayer composite with 0.5wt.%zinc stearate compared with the 40 vol.%TiC-Fe/Fe bilayer composite without zinc stearate.(5)The TiC porous preforms were prepared by the organic template impregnation method.TiCx phases were synthesized through chemical reaction of Ti element and residual carbon generated by pyrolysis of organic matter.The lattice constant a of TiCx phase is 0.4306 nm through XRD pattern of TiC-30wt.%Ti,the x value is 0.55 through further calculation.The prism diameter of porous TiC preforms prepared with 10,20 and 30 PPI template increases linear as impregnation times.After the fifth dipping,the prism diameter of preforms reach 1.9-2.7 times of that throuth the third dipping,which reveals a well-controlled structure.(6)TiC-Fe interpenetrating phase composites were fabricated by infiltrating molten reduced iron into TiC porous preforms.The variations on TiC-Fe IPC properties are related to the compositons of TiC porous preforms.The tensile strength of the 4vol.%TiC-Fe interpenetrating phase composite with Mo addition reaches 343 MPa,which is 66.50%hither than that of the 4 vol.%TiC-Fe composite prepared by pressureless sintering.(7)The coefficient of friction and wear rate of TiC-Fe composites are influenced by normal pressure,sliding speed,TiC volumefraction,carbonyl iron powder content and zinc stearate content.With sliding speed of 3m/s and normal pressure of 0.5 MPa,90vol.%TiC-Fe composite prepared by pressureless sintering owns the lowest wear rate 3.68×10-12 mm3/N·m comparing with other TiC-Fe composites prepared by pressureless sintering.By contrast,the wear rate of 60vol.%TiC-Fe composite prepared by hot-pressing sintering is 1.87×10-12mm3/N·m,which is better than TiC-Fe composites prepared by pressureless sintering for wear-resistance property.The coefficient of friction and wear rate of TiC-Fe interpenetrating composites are influenced by normal pressure,pore size of TiC porous preforms.The interpenetrating structure could effective promote the value and stability of coefficient of friction,and the wear rate of TiC-Fe interpenetrating phasecomposites is lower than that of TiC-Fe composites prepared by pressureless sintering and hot-pressing sintering with same TiC volume content and friction test parameters.The innovations of this work:(1)The well-performing TiC-Fe homogeneouscomposites,gradient composites and interpenetrating phase composites were successfully fabricated by different sintering technologies;the optimized fabrication technologies were also obtained for different TiC-Fe composites;the performance features of different reinforcement distribution styles of TiC-Fe composites were revealed.(2)The TiC-Fe/Fe bilayer composites expressing excellent synergy effect were fabricated by hot-pressing sintering.It is mainly attributed by strong interlaminal bonding strength between TiC-Fe composite layer and pure Fe layer.(3)The adjustable porosity and aperture structure ofTiC porous preforms were prepared by organic template impregnation method.Based on above TiC porous preforms,the TiC-Fe composites with interpenetrating network were obtained by pressureless infiltration process.(4)The interpenetrating structure could effective promote the stability of coefficient of friction,and the wear rate of TiC-Fe composites with interpenetrating structure is lower than that of TiC-Fe composites with same TiC volume content fabricated by pressureless sintering and hot-pressing sintering,which is more suitable applied for wear-resistance and braking materials. |
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