Study On Preparation And Properties Of C_f/SiBCN（Zr） Compositites

With the development of new hypersonic aircraft,its flight speed is getting higher and higher,and higher requirements are put forward for the thermal stability,mechanical properties and high temperature oxidation resistance of ultra-high temperature materials,among which boride and carbon-based ultra-high temperature ceramic materials are expected to be high.However,ZrB2 or ZrC as the typical representative of ultra-high temperature ceramic materials are difficult to meet the engineering requirements because of their inherent brittleness and low reliability.The fabrication process of continuous fiber toughened ceramic matrix composites can improve the damage tolerance and reliability of the materials,which has a good prospect of engineering application.In order to avoid the fiber produced in the process of preparation of structural damage,in preparation of fiber surface coating and introduction of ceramic precursor in fiber preparation is an effective way to solve such problems,which can realize ultra-high temperature ceramic matrix composites of high toughness and high reliability,so for the research of fiber toughened ceramic matrix composites is of great significance.In this paper,carbon fiber was selected as reinforcement to study the preparation process of carbon fiber surface coating,and the synthesis process of new SiBCN(Zr)ceramic precursor was developed to prepare Cf/SiBCN(Zr)ceramic matrix composites.The mechanical properties and oxidation resistance and ablation resistance of Cf/SiBCN(Zr)ceramic matrix composites were evaluated.A new type of SiBCN ceramic precursor was prepared by using a synthesis route of a single source precursor.The curing temperature in the reaction process was determined.The structure and properties of SiBCN ceramic precursor were studied.SiBCN precursors were ceramicized by high temperature pyrolysis,and SiBCN ceramics with uniform composition and compact structure were obtained with a ceramic conversion rate of 49.19%.The pyrolysis products of SiBCN ceramic precursors at 1200℃-1600℃ were analyzed.It was found that they could still remain amorphous at 1400℃,and the main pyrolysis products were SiC and Si3N4 at 1600℃.SiBCN(Zr)precursors were synthesized by chemical modification of SiBCN precursors by zirconium dichloroeneocene(Cp2ZrCl2).The effect of different mass fraction of Zr on the chemical structure of SiBCN(Zr)precursors was studied.After pyrolysis at high temperature,it was found that the ceramic conversion of SiBCN(Zr)precursors reached 57.54%.The carbon fiber was coated with 1.8-1.9μm carbon coating by hydrothermal method using glucose as raw material.Cf/SiBCN(Zr)ceramic matrix composites were prepared by precursor impregnation pyrolysis(PIP)process using 2.5D and 3D braided carbon fiber braids as reinforcements.The microstructure and mechanical properties of the two composites were tested and analyzed.The mechanical properties test results show that in the XY direction,the two fiber braiding methods have little effect on the average fracture toughness and bending strength of the composites.In the Z direction,3D braided Cf/SiBCN(Zr)ceramic matrix composites have good mechanical properties with bending strength up to 277 MPa.The oxidation resistance of Cf/SiBCN(Zr)ceramic matrix composites was investigated using oxyacetylene flame.The test results show that the introduction of Zr element can effectively improve the oxidation resistance of the composites.After the ablation test of 1900℃/90s oxyacetylene,the linear ablation rate of Cf/SiBCN(Zr)ceramic matrix composites is 2.62×10-3cm·s-1,which is significantly lower than that of Cf/SiBCN ceramic matrix composites.