Preparation,Microstructure And Performance Evolution Of SiAl(Zr)OC Ceramics

Polymer-derived SiOC ceramics（PDC-SiOC） have received more and more attention in recent years due to its excellent mechanical properties,creep and thermal shock resistance.However,the carbothermal reduction reaction will occur at high-temperature,resulting in changing the structure and composition of the SiOC ceramics,making them loose and porous,and finally affecting the performance of the material.Therefore,in order to improve the thermal stability of SiOC ceramics,introducing foreign elements（Al,Zr）to form Si（M）OC（M=Al,Zr）ceramics in this these.The microstructural evolution of Si（M）OC（M=Al,Zr）and its thermal stability under inert and air atmospheres were investigated in detail.In addition,C_f/SiZrOC ceramic matrix composites were also prepared and their properties were systematically investigated.Firstly,SiAlOC ceramics were prepared by introducing the heterogeneous element Al into the SiOC ceramics in this thesis,the evolution patterns of different Al/Si ratios at different cracking temperatures and different atmospheres were examined in detail.The experimental results show that the phase composition of SiAlOC ceramics is mullite phase and Si O₂ phase at the cracking temperature of 1000-1500℃.Compared with pure SiOC ceramics,there is no formation of Si C phase in SiAlOC ceramics.The formed mullite phase increased the thermal stability of SiOC ceramics and the temperature at which their carbothermal reduction reactions take place,and with the Al/Si ratio increasing,the ceramic yield decreases,the mullite phase increases and the Si O₂ phase decreases.In addition,the thermal stability of SiAlOC ceramics in air atmosphere was examined.The experimental results show that the phase composition of SiAlOC ceramics is mullite phase and Si O₂ phase after 50min heat treatment in 1400℃air atmosphere,and the introduction of Al element improves the thermal stability of SiOC ceramics in high temperature air atmosphere compared to pure SiOC ceramics.Secondly,SiZrOC ceramics were prepared by introducing Zr elements into SiOC ceramics in this thesis.The microstructure and compositional evolution behavior of SiZrOC ceramics at different cracking temperatures and different cracking environments were investigated in detail.The experimental results show that the introduction of Zr elements will inhibit the formation of Si C in the system and promote the breakage of Si-C bonds,thus reducing the content of SP³free carbon in the system and leading to a large number of Zr O₂ nanoparticles dispersed in the SiOC network structure,while the introduction of Zr elements will reduce the formation of Si O₂in the system.Compared to pure SiOC ceramics,the SiZrOC ceramics exhibit excellent thermal stability at high temperature under air atmosphere.Finally,C_f/SiZrOC ceramic matrix composites were prepared by the impregnation cracking（PIP）process,and the mechanical and thermophysical properties of the composites were investigated in detail for different numbers of impregnation cracking.The compressive strength of C_f/SiOC ceramic matrix composites in the Z-direction（perpendicular to the fiber layer）increased from 4.98±0.14 MPa to 7.56±0.35 MPa at 1300°C when the impregnation cracking was carried out from once to three times,and the compressive strength of C_f/SiZrOC ceramic matrix composites in the Z-direction increased from 5.23±0.15 MPa to 8.35±0.30MPa at 1300°C.In addition,the thermal conductivity of the C_f/SiZrOC ceramic matrix composites increased with the impregnation number increasing,it was from 0.325 W/m·K in the Z-direction for the once impregnated C_f/SiZrOC ceramic matrix composites to 0.489 W/m·K in the Z-direction for the thrice impregnated C_f/SiZrOC ceramic matrix composites.This is mainly due to the increase in the amount of SiZrOC ceramics in the composite and the increase in density with the impregnation number increasing,resulting in enhanced thermal conductivity of the C_f/SiZrOC ceramic matrix composite.