Research On Preparation And Ablation Performance Of2D-SiO_2f/SiO₂-BN Composites

2D-SiO_2f/SiO₂-BN ceramic matrix composites （SBN series） were prepared bysol-gel methods and slurry impregnation method followed by hot-pressing using silicafiber woven cloth, silica sol, BN powder and fused silica powder as raw materials.SBN-BN series and SBN-BNS series were prepared by the introduction of the weakinterface layer on the basis of SBN series. The influence law of fabrication process andintroduction of weak interface layer to the properties including crystallization behavior、mechanical properties and ablation properties of2D-SiO_2f/SiO₂-BN ceramic matrixcomposites were studied by means of XRD and SEM. The mechanical properties of2D-SiO_2f/SiO₂-BN were measured by three point bending method and single edgenotched beam.The experimental results showed that after800℃heat treatement, the fiber inSBN series were slightly crystallized; Silica sol impregnation treatment for thecomposite improved the density and mechanical properties; the fracture mode ofcomposites which were impregnated silica sol was ductile fracture, fracture surfacewere rough and fibers were pulled out from fracture surface; Vacuum treatment madethe hanging pulp mass of fiber cloth increased by188%, but the density of compositesdecrease by0.47%. In addition, the bending strength of composites also decreased by5.97%.The crystallization temperature of2D-SiO_2f/SiO₂-BN（SBN series） was1200℃,The flexural strength, fracture toughness and elastic modulus were49.48MPa（Z）,46.69MPa（X-Y）;12.6GPa（Z）,9.5GPa（X-Y）;1.03MPa·m1/2（Z）,0.86MPa·m1/2（X-Y） at1200℃、10MPa, respectively. The best preparation process of composite is thatimpregnate silica sol for24hours, take ultrasonic treatment for1hour, dry for3hoursunder50℃（no vacuum）, then dip matrix slurry for24hours, and take ultrasonictreatment for1hour, dry for2hours under30℃, finally spread the layers withoutpressure and hot-pressing sinter under1100℃,10MPa.The composites of SBN-BN series had obvious anisotropic, the flexural strength,fracture toughness and elastic modulus of SBN-BN series hot-pressed at1100℃、10MPa were8.92MPa（Z）,22MPa（X-Y）;0.56MPa·m1/2（Z）,0.42MPa·m1/2（X-Y）;13.2GPa（Z）,19.9GPa（X-Y）, respectively. SBN-BNS series were prepared by adding asmall amount of nm-SiO₂to the weak interface layer. nm-SiO₂reduced the anisotropy of SBN-BN series, fracture toughness and elastic modulus of SBN-BNS serieshot-pressed at1100℃、10MPa were15.19MPa（Z）,15.85MPa（X-Y）;0.29MPa·m1/2（Z）,0.28MPa·m1/2（X-Y）;11.4GPa（Z）,13.1GPa（X-Y）, respectively.The results of the ablation characteristics of SBN、SBN-BN and SBN-BNS seriesshowed that the mass ablation rate and linear ablation rate of SBN series reached to theminimum which were0.0304g/s and0.0323mm/s respectively under the same ablationtime and ablation temperature, the ablation resistance of SBN series was the best; Theablative mechanisms of the2D-SiO_2f/SiO₂-BN ceramic matrix composite under theO₂-C2H2condition were synergistic effect of thermo-oxidation, thermo-physics ablation（sublimation） and mechanical erosion. The ablative mechanisms of different series indifferent parts differented obviously; The addition of nm-SiO₂played an important rolein layered denudation. The mass ablation rate and linear ablation rate of SBN-BNSseries were0.0357g/s and0.0512mm/s respectively.