Study On Interface Design,preparation And Characteristics Of Mullite/Al-Si Fiber Fabric Composites

With the development of space navigation,aviation,near space and other hightechnology fields,the high-performance composites are desperately in need.During the hypersonic speed,long-time flight,hypersonic vehicles face a harsh heat environment.The temperature of the vehicles' surface is extremely high,thus the thermal protection system is the key technic related to vehicles' safety.In order to break through the restriction on application temperature of traditional SiC/SiC CMCs which used C or BN as the interphase,oxide/oxide CMCs which were being paid more and more attention by scientists in recent years,are recognized as the most potential composites to possess high temperature strength,fracture toughness,thermal shock resistance and oxidation resistance.Conventionally,fibers tend to react with the matrix at high temperature if without surface treatment to form a strong bonding which leads to brittle rupture of the composites.Interphase design is one way to solve this problem.Using mullite as the matrix,2-dimentional Al-Si fiber fabric as the reinforcement material,AlPO₄ and LaPO₄ as the interphase,respectively,the heat resistant composites were prepared by the progress of vacuum impregnation,stress recombination and pressureless sintering.The density,porosity,flexural strength,phase constitutions and micro morphology of the composites are tested and further studied.The experimental results showed that when the solid content of the matrix slurry is up to 70 wt%,the number of layers is 6 and the recombination pressure is 20 KN,the performance of the material is most modified after sintering at 1200 ? for 3 h.The flexural of the sample could reach 12.78 MPa.With the concentration of AlPO₄ solution increasing from 10 wt% to 20 wt%,the density of samples increased from 2.87 g/cm3 to 2.99 g/cm3,the porosity decreased from 27.70 % to 26.56 % and the flexural strength increased from 21.84 MPa to 27.59 MPa which is 115.9 % higher than that of the samples without an interphase.With the increase of impregnation times,the flexural strength of samples decreased.Observing from the micro morphology of ruptured samples,AlPO₄ presented a porous structure which provides diffusion paths of micro cracks in the matrix and dissipated the residual stress within the matrix at the same time,which is very helpful to the promotion of material properties.With the concentration of LaPO₄ solution increasing from 10 wt% to 20 wt%,the density of samples increased from 2.70 g/cm3 to 2.81 g/cm3,the porosity decreased from 28.66 % to 28.23 % and the flexural strength increased from 16.12 MPa to 24.38 MPa.With the increase of impregnation times,the flexural strength of samples increased.When the concentration is 20 wt% and the impregnation time is 3,the flexural strength could reach 36.21 MPa which is 183.3 % higher than that of the samples without an interphase.Observing from the micro morphology of ruptured samples,LaPO₄ presented a layered structure and formed a weak bond with both matrix and fibers,which allowed crack deflection and growing along fibers instead of harming fibers during crack growth,thus avoiding catastrophic damage and exhibiting toughening effect.