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Open Access Research Article Issue
Mechanical properties and thermal stability of carbon fiber cloth reinforced sol-derived mullite composites
Journal of Advanced Ceramics 2019, 8 (2): 218-227
Published: 13 June 2019
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For the wide application as thermal protection materials, it is very necessary for mullite ceramics to improve fracture toughness. In this paper, the laminated and stitched carbon fiber cloth preform reinforced mullite (C/mullite) composites were prepared through the route of sol impregnation and heat treatment using the Al2O3-SiO2 sol with a high solid content as raw materials. The C/mullite composites showed a flexural strength of 228.9 MPa that was comparable to that of dense monolithic mullite although the total porosity reached 13.4%. Especially, a fracture toughness of 11.2 MPa·m1/2 that was 4-5 times that of dense monolithic mullite was obtained. Strength deterioration due to the carbothermal reduction between carbon fiber and the residual SiO2 in matrix was found above 1200 ℃. A pyrolytic C (PyC) coating was deposited on carbon fibers as interfacial coating. The chemical damage to carbon fibers was obviously alleviated by the sacrifice of PyC coating. Accordingly, the C/PyC/mullite composites kept strength unchanged up to 1500 ℃, and showed much higher strength retention ratio than C/mullite composites after annealing at 1600 ℃.

Open Access Research Article Issue
Fabrication and oxidation resistance of mullite/yttrium silicate multilayer coatings on C/SiC composites
Journal of Advanced Ceramics 2017, 6 (4): 360-367
Published: 19 December 2017
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A tri-layer coating of mullite/Y2Si2O7/(70wt%Y2Si2O7+30wt%Y2SiO5) was prepared on carbon fiber reinforced silicon carbide (C/SiC) composite substrate through dip-coating route for the sake of improving oxidation resistance of C/SiC composites. An Al2O3-SiO2 sol with high solid content was selected as raw material for mullite, and a slurry of Y2O3 powder filled silicone resin was used to synthesize yttrium silicate. The microstructure, phase composition, and oxidation resistance of the coating were investigated. The as-fabricated coating shows high density and favorable bonding to C/SiC substrate. After oxidation at 1400 and 1500 ℃ for 30 min under static air, the flexural strengths of coated C/SiC composite were both increased by ~30%. The desirable thermal stability and the further densification are responsible for excellent oxidation resistance. With the additional help of compatible thermal expansion coefficients among substrate and sub-layers in coating, the coated composite retained 111.2% of original flexural strength after 12 times of thermal shock in air from 1400 ℃ to room temperature. The carbothermal reaction at 1600 ℃ between free carbon in C/SiC substrate and rich SiO2 in mullite resulted in severe frothing and desquamation of coating and obvious degradation in oxidation resistance.

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