Preparation And Mechanical Properties Of Mullite With Bird’s Beak Composite Structure

With the rapid development of hypersonic speed and aviation and aerospace technology,the aircraft is served in the harsh aerodynamic environment of high-speed flight,and the requirements for thermal insulation materials for ultra-high temperatures,aerodynamic loads and air flow erosion are gradually increasing.Mullite ceramic has the advantages of good high temperature stability,thermal shock resistance,high temperature strength,low linear expansion coefficient and high melting point.Therefore,porous mullite ceramics have a good application prospect in the field of thermal protection of hypersonic vehicles and aerospace vehicles.However,porous mullite ceramics with high porosity have low strength and are difficult to withstand high aerodynamic loads and the erosion and erosion of airflow.Therefore,the research and preparation of high-strength porous mullite ceramics has obvious engineering significance.In this study,a mullite whisker frame was used to seal the surface to obtain a porous mullite ceramic with a bird’s beak structure.The preparation process and mechanical properties were studied.The study showed that:The Al₂O₃,Al F₃ and SiO₂ systems are sintered in a closed crucible by gas-solid reaction to prepare a high porosity and high strength mullite whisker frame.Within the sintering temperature range of 1500℃～1650℃,adjust the sintering process parameters to improve the whisker frame Mechanical properties.Finally,the comprehensive performance of the mullite whisker frame prepared at a sintering temperature of 1500℃and holding for 5 hours reached the best.The growth of the sintered neck between the rod whiskers was fully developed,and the ratio of length to diameter of the whiskers was large,with a high porosity of 65.2%.In this case,the compressive strength reaches a maximum value of 51MPa.The porous mullite whisker frame is used as the substrate,and the method of slurry spray infiltration is used.After sintering,the mullite material with dense surface and porous inner beak structure can be prepared;the number of infiltration can be controlled to control the surface of the dense layer Thickness;when the number of infiltration increases,the thickness of the surface dense layer increases from 10μm to 150μm.At the same time,the porosity of the composite structure decreases from 65.2%to 57.8%,and the bending strength of the composite structure gradually increases from 36 MPa to 62 At the same time,the compressive strength has also increased from 51 MPa to 89 MPa;removing the effect of the overall porosity drop on the strength of the sample,the additional increase in strength stems from the formed composite structure.By impregnating the surface of the whisker frame with nano-scale spherical silica suspension,a mullite ceramic with a dense beak and a beak structure with a porous frame can be prepared;adjusting the particle size of the nano-spherical SiO₂ can control the entry of spherical SiO₂ particles The amount of matrix in the matrix frame to prepare a composite structure of dense layers with different thicknesses.When the particle size of SiO₂ decreases from 450nm to 220nm,the thickness of the dense layer increases from 10μm to 60μm,and the porosity decreases from the original 65.2%45.9%;350nm spherical SiO₂ is used as the material for the sealing pore layer,the bending strength is increased by 2.1 times,and the porosity is only decreased by 8.4%.