Properties And Failure Behaviors Of Continuous Carbon Fiber Reinforced Mullite Matrix Composites

Mullite has a promising prospect in the field of high-temperature structural and thermal protection,owing to its excellent thermal shock resistance,high-temperature resistance and oxidation resistance,and low density and coefficient of thermal expansion.However,the shortage of mechanical properties at room temperature(RT)limits the application seriously.In our previous studies,three-dimensional braided carbon fiber reinforced mullite composites with preferable comprehensive properties have been prepared through the “infiltration-drying-heat treatment” route using SiO2-rich Al2O3-SiO2 sol with a high solid content as raw materials.On this basis,mullite matrix composites reinforced by cabon fiber cloth preform with a laminationand stitching structure were fabricated in this dissertation through the “infiltration-drying-heat treatment” route using Al2O3-SiO2 sol with a 3:2 stoichiometry as raw materials.In the first place,the sintering shrinkage and the mullitization of the pressed dry gel powder were studied,and the fabrication route of C/Mullite composites was optimized preliminary.Then,the investigations on the intrinsical properties and the failure behaviors in simulative environment were emphasized.The solid content and ceramic yield ofthe sols were 30.7wt.% at 200?and 18.3wt.% at 1300?,respectively.The mullitization of Al2O3-SiO2 was nearly completed at 1300?.The pressed dry gel powders exhibited high linear shrinkage which increased with improving the temperature of heat treatment.The desirable sintering activity is in favor of the densification of mullite matrix in composites.After preliminary optimization,the fabrication route for the composites without obvious penetrating cracks and delamination was established.The as-received C/Mullite composites showed an apparent density of 2.19 g·cm-3,a total porosity of 13.4% and an open porosity of 11.3%.The room-temperature mechanical properties were investigated.The flexural strength measured by three-point-bending test(228.9 ± 11.0MPa)was a little higher than that of four-point-bending test(210.6±8.9MPa),and it was found that the flexural strength of C/Mullite composites obeyed the Weibull distribution of two parameters,i.e.m and ?.The high value of m=30.578 indicated the good uniformity of C/Mullite composites.Due to the introduction of continuous carbon fiber,C/Mullite composites presented typical non-catastrophic fracture behavior,showing a fracture toughness of 11.2MPa·m1/2,which was 4-5 times of that of monolithic mullite ceramics.In addition,the tensile strength,the compressive strength in X direction and the interlaminar shear strength of C/Mullite composites were 119.9MPa,128.2MPa and 28.1MPa,respectively.Thermophysical properties of C/Mullite composites were characterized from RT to 1400?.The coefficient of thermal expansion(CTE)increased first and then decreased,and reached a maximum of 4.83×10-6K-1 at 1000?.As a result of the further sintering of matrix,the CTE was negative at above 1300?.The specific heat capacities(Cp)increased to the maximum of 1.547J·g-1·K-1 at 1200? and remained stable at above 1200?,with the minimum of 0.756J·g-1·K-1 at RT.The thermal diffusivity decreased from 1.1mm2·s-1 at RT to 0.707 mm2·s-1 at 700? as the temperature was elevated,and remained stable at above 700?.On the contrary,the thermal conductivity was improved with increasing temperature in generaland varied from 1.859W·m-1·K-1 at RT to 2.325W·m-1·K-1 at 1200?.C/Mullite composites had good high-temperature resistance underinert atmosphere at 1200? and 1400?.After soaked for 1h,the composites showed almost no mass loss and the flexural strength retention ratios were 103.1% and 84.6%,respectively.The decrease in flexural strength at 1400? could be ascribed to the stress variation in composites due to the further sintering of matrix.However,when the temperature rose to 1600?,the flexural strength retention ratio dropped to 29.3% and the mass loss was as high as 17.2%.The grain coarsening of the matrix and the carbothermic reduction between fiber and matrix were considered to be responsible for this phenomena.C/Mullite composites showed desirable oxidation resistance at temperature ranging from 1200? to 1600?.After oxidation under static air for 30 min,the composites remained about 75% of original flexural strength.The oxidation began at the surface of the composites and propagated gradually to the inner.After oxidation in water vapor at 1000? for 30 min,the flexural strength retention ratio was 71.3%.Nevertheless,when the temperature was elevated to 1200? and 1400?,the composites were seriously corroded by water vapor,decreasing the flexural strength by more than 55%.It was found that the mass loss was directly proportional to thermal shock times approximately.After 10 times thermal shock from 1200?(soaking 10min)to RT and 1400?(soaking 10min)to RT,the C/Mullite composites only retained 43.9% and 25.4% of original flexural strength,respectively.The synergetic action of oxidation of carbon fiber and thermal stress derived from thermal mismatch between fiber and matrix was supposed to explain this failure behavior.The ablation test of C/Mullite composites was performedin oxyacetylene flame with a center temperature of 2727?.Since the center temperature was very high,it was easy for mullite to be decomposed and be blew offduring ablation.As a result,the composites showed a mass ablation rate of 0.049g·s-1 and a linear ablation rate of 0.12mm·s-1 after ablation for 30 s.The productionin this dissertation provides beneficial reference for subsequent researches on further optimization of fabrication process and improvement in comprehensive properties.