Preparation And Characterization Of The High-temperature Gradient C_f/Zr?Hf?B₂-SiC Composites

With the rapid development of aerospace technology,hypersonic vehicles have become a research hotspot at home and abroad.When the aircraft enters the atmosphere,it will generate a lot of heat due to friction with the air and cause damage to the hot end parts of the aircraft.Therefore,there is an urgent need for thermal protection materials for the hot end parts of the aircraft.Existing thermal protection materials limit their wide application due to their high density,high cost,and low operating temperature.The fiberreinforced ultra-high temperature ceramic composite material has attracted attention because of its high temperature resistance,excellent room temperature and high temperature mechanical properties,and excellent resistance to oxidation and ablation.In this paper,the ZrB₂-SiC composite ceramic material was prepared by the impregnationpyrolysis method of the precursor.The effect of the number of impregnation-cracking times on the microstructure and mechanical properties of ZrB₂-SiC composite ceramic materials was studied,and the mechanism of preparing ceramic materials by lowtemperature replenishment and sintering of precursors was revealed.The capillary selfabsorption method was used to achieve the gradient introduction of ultra-high temperature ceramic slurry in the carbon fiber braided body,and the preparation of the high-temperature gradient C_f/Zr?Hf?B₂-SiC composite material was achieved through the low-temperature supply sintering of the ceramic precursor.Through microstructure analysis,oxyacetylene assessment and thermal insulation performance test,the antioxidation ablation performance was studied,and the anti-oxidation ablation mechanism of the high temperature gradient C_f/Zr?Hf?B₂-SiC composite material was revealed,which facilitated the engineering application process of the gradient carbon fiber toughened ultra-high temperature ceramic matrix composites.The test results of ZrB₂-SiC multiphase ceramics under different impregnationpyrolysis times show that: with the increase of the number of impregnation-pyrolysis,the density and density of the multiphase ceramics show an increasing trend,and the density increases from 61% to 71%.The mechanical properties test results show the increase in the number of impregnation-pyrolysis has significantly improved the mechanical properties of the multiphase ceramics.The bending strength of the sample after 10 times impregnation-pyrolysis is as high as 274 MPa,which is about 5 time higher than that of the sample after 3 times impregnation-pyrolysis?53MPa?.The compressive strength test was increased from 324 MPa for 3 times immersion-pyrolysis samples to 812 MPa for 10 times impregnation-pyrolysis samples,and the hardness test was increased from 0.61 GPa for 3 times impregnation-pyrolysis samples to 16.91 GPa for 10 times impregnation-pyrolysis samples.With the increase of the number of impregnationpyrolysis,the nano-SiC particles produced by the pyrolysising of the PCS precursor gradually wrapped the ZrB₂ particles,and the coated particles were closely bonded to achieve the low-temperature replenishment sintering of the ZrB₂-SiC multiphase ceramics.The structure of the gradient C_f/Zr?Hf?B₂-SiC composite remains intact after oxyacetylene examination,and no defects such as ablation pits,holes and cracks appear on the surface,and the HfO₂ and ZrO₂ layers generated by the surface oxidation avoid further ablation of the gradient C_f/Zr?Hf?B₂-SiC composites.The gradient C_f/HfB₂-SiC composite material sample has a mass ablation rate of-9.3×10-3g/s and a linear ablation rate of-3.3×10-5mm/s after 2500?/300 s oxyacetylene assessment.The gradient C_f/ZrB₂-SiC composite material has a mass ablation rate of-6.2×10-3g/s and a linear ablation rate of 1.4×10-3mm/s after 2000?/300 s oxyacetylene assessment.After the test,the dense ceramic layer on the surface of the sample was tightly combined with the fiber skeleton material without cracking and shedding.The dense ceramic layer on the surface prevented oxygen from entering the material and causing fiber oxidation damage.Regarding the thermal insulation performance of the gradient C_f/Zr?Hf?B₂-SiC composite,the sample surface structure of the gradient C_f/HfB₂-SiC composite material remained intact after being subjected to a butane flame gun for 1300?/1200 s,and the temperature of the back of the material is lower than 120?,showing good anti-heat insulation performance.