Investigation On The Preparation And Properties Of High Performance Zirconium-Containing Silicon Carbide Ceramic Fibers

Silicon carbide(SiC)fiber fabricated via the precursor conversion method exhibits many excellent properties,such as high strength,high modulus,high temperature resistance,excellent oxidation resistance,and corrosion resistance.Thus,it has a wide range of applications in national defense,aerospace,and civil fields.The use temperature of SiC fibers is constantly improving though the control of composition and microstructure.To date,three generations of silicon carbide fibers have been developed.In addition,small foreign elements such as zirconium,titanium,aluminum and boron are added to the silicon carbide fiber,which can have a sintering effect or inhibit the growth of crystal grains at high temperatures,and high-performance silicon carbide fibers with high temperature resistance can be obtained.Based on the above,this paper uses the oxygen-free ceramic precursor polyzirconium carbosilane synthesized in the laboratory and introduces boron in the fiber preparation process to prepare SiC-ZrC composite ceramic fibers and SiC-ZrB2 composite ceramic fibers.The morphology,structure and properties of the two composite ceramic fibers are also studied.The main research contents and conclusions are as follows:(1)SiC-ZrC nanocomposite ceramic fibers are prepared through precursor pretreatment,melt spinning,electron beam crosslinking,pyrolysis,and high-temperature sintering.The chemical composition of the SiC-ZrC composite ceramic fibers is SiC1.26O0.06Zr0.01.The silicon and zirconium elements in the fibers are evenly distributed without enrichment.C/(Si+Zr)is 1.25,and there is some amount of free carbon in the fibers.The free carbon in the fibers mainly includes two forms:a disordered layer structure and a graphite structure.The average grain size of SiC is 9.8 nm.The strength of the fibers reaches 2.7 GPa,and the modulus reaches 2.66 GPa.(2)SiC-ZrC composite ceramic fibers exhibit excellent heat resistance and oxidation resistance.After being kept at 1600 ? or 1800 ? for 1 hour,the fiber structure remains dense.After treatment at 1600?,the modulus of the fibers remains basically unchanged,the strength is 1.5 GPa,and the grain size of SiC reaches 23.5 nm;after treatment at 1800? the modulus of the fibers increases to 286 GPa,and the strength decreases to 1.1 GPa.The particle size reaches 33.9 nm.After oxidation at 1100?1400?,a dense SiO2-ZrO2 oxide layer appears on the surface of the fibers,and the dense oxide layer slows down the further oxidation of the fibers.The SiC grain size and the structure of free carbon in the fibers do not change significantly.By calculation,the oxidation activation energy of the SiC-ZrC composite ceramic fibers at 1100?1400? is 162.9?179.4 kJ/mol.(3)By changing the cross-linking method in the fiber preparation process,boron is effectively introduced and combined with zirconium in the high-temperature treatment process to produce ZrB2,thus preparing SiC-ZrB2 composite ceramic fibers.Three cross-linking methods,NI2/BCl3,BCl3/NH3,and electron beam/BCl3,are compared.The cross-linking capacity of the three cross-linking methods is following the order:NO2/BCl3>electron beam/BCl3>BCl3/NH3.(4)The SiC-ZrB2 composite ceramic fibers prepared by electron beam/BCl3 have a strength of 2.0 GPa and a modulus of 380 GPa.There is an 80 nm carbon-rich layer on the surface of the fibers,and its performance is close to that of third-generation SiC fibers.The SiC-ZrB2 composite ceramic fibers exhibits excellent high-temperature performance.After being oxidized at 1500? for 1 hour,the fiber strength is still maintained at 1.5 GPa.The creep resistance of the fibers at 1200 ?-1500 ? is better than that of Hi-Nicalon S,KD-S and Tyranno SA fibers.(5)Adding hydrogen during the pyrolysis process can effectively reduce the existence of free carbon in SiC fibers.The decarburization temperature of hydrogen is mainly between 400? and 1000?.The removal of free carbon mainly depends on the generation of H· radicals at high temperatures,and the concentration of H· radicals is influenced by temperature and the concentration of hydrogen.After 0%,25%,50%,75%and 100%hydrogen treatment,the C/Si ratios in the multiphase ceramic fibers are 1.35,1.2,1.13,1.06 and 0.92,respectively.(6)SiC-ZrB2 multiphase ceramic fibers are treated with different hydrogen concentrations.After annealing at 1400?,the fibers can maintain a dense morphology,and the fiber grain sizes obtained with different degrees of decarburization are close.After treatment at 1500?,large SiC grains appear on the surface of the fibers due to the decomposition of the SiCxOy phase and SiC1+x phase,and nanosized pores appear in the fibers treated with 75%and 100%H2.After treatment at 1600?,the average SiC grain sizes of F1?F5 are 18.3 nm,18.7 nm,24.2 nm,25.6 nm,and 34.4 nm.The presence of free carbon in the fibers can inhibit the coarsening of SiC grains.After 1800? treatment,due to the sintering effect of boron,the fibers treated with H2 concentrations of 0%,25%,50%,and 75%are denser than those treated at 1600?.(7)In the SiC-ZrB2 composite ceramic fibers,there are also two types of pyrolytic carbon:a disordered layer structure and a graphite structure.At different annealing temperatures,the ratio of the two pyrolytic carbons is different.With increasing fiber annealing temperature,the structure of pyrolytic carbon in the fibers is mostly graphite.