Preparation And Properties Of C_f-SiBCN Composites

With the continuous development of the aerospace field,the demand for thermal protective materials with lightweight,high strength,high temperature resistance,and oxidation resistance for hypersonic aircraft has increased sharply.Currently,traditional thermal protection materials mainly include refractory metals and their alloys,ultra-high temperature ceramic materials,and carbon/carbon composites.Due to their respective shortcomings,these types of materials cannot be widely used in the aerospace field.Ultra-high temperature ceramic matrix composites not only have overcome the inherent brittleness of ceramics,but also have excellent oxidation resistance and ablation resistance on the other hand,becoming the research focus of thermal protection material researchers.SiBCN amorphous ceramics have excellent high-temperature properties and special microstructure,making them excellent candidates in the field of thermal protection.In this thesis,using polyborosilazane precursors with different boron content,self-crosslinking technology was developed to successfully obtain polyborosilazane precursors with higher ceramic yield.Combined with the precursor impregnation cracking technology,after 8impregnation cracking cycles,C_f-SiBCN composites with excellent oxidation resistance and ablation resistance were ultimately obtained.The main research contents of this paper are as follows:（1）Using solid phase and liquid phase polyborosilazane precursors,combined with self-crosslinking technology,a new mixed precursor was obtained,and the ceramic yield of the precursor was increased by 10%.SiBCN ceramics with excellent properties were obtained by pyrolysis of the precursor at 1200℃.SiBCN ceramics could still maintain their amorphous morphology at 1600℃for 2 h.The internal Si C nanocrystals were difficult to grow under the wrapping of BN（C）amorphous phase,indicating that SiBCN ceramics had excellent high-temperature properties.（2）C_f-SiBCN composites were successfully prepared using a hybrid precursor and needle punched carbon fiber braid,combined with precursor impregnation pyrolysis technology.After eight immersion cracking cycles,the density of the composite reached 1.7293 g/cm³,and the open porosity was only 7.23%.The mechanical properties of the composite were tested.The compressive strength of the composite in the x/y and z directions was 253 MPa and 281 MPa,and the flexural strength was 246 MPa.The composite exhibited a"pseudoplastic fracture"mode.Significant fiber debonding,fiber pullout,and fiber fracture phenomena were observed at the fracture surface.（3）The antioxidant performance of the composite material was tested by static oxidation experiments and oxygen acetylene flame ablation experiments.After 30 min of oxidation at1200℃-1500℃,the weight loss rate of the composite material remained almost unchanged.SiBCN ceramics were able to maintain some amorphous morphology.After 30 min of oxidation at 1500℃,a protective layer of Si O₂ was formed on the surface of the material,which played a protective role in the composite material.After holding at 1500℃for 30-120 min,the weight loss rate of the composite material gradually decreases with time,and the Si O₂ protective layer on the surface becomes smoother,preventing further oxidation of the carbon fibers inside the composite material,indicating that the composite material has excellent antioxidant performance.After ablating the composite material at 1800℃for 900 s under an oxyacetylene flame,the mass ablation rate is only 1.137×10^-3 g/s,with a linear ablation rate of only 2.13×10^-3 mm/s,a smooth Si O₂ protective layer is formed on the surface of the material,indicating excellent ablation resistance.