Translated Abstract
Rapid development of aerospace technologies and military demands make higher requirements on the high temperature stability, mechanical properties, and thermal insulation performance for thermal protection materials. Carbon-bonded carbon fiber composites (CBCF) composites are potential materials for new generation of thermal protection materials, owning to their low density, high porosity, high temperature stability, excellent thermal insulation performance and relative low thermal expansion coefficient. However, easy oxidation of CBCF above 450 °C in an oxidizing atmosphere will lead to degradation of its good properties. Oxidation resistant coatings have to be applied to solve the problem by. Up to now, most studies on the anti-oxidation coatings are for dense substrate. Because CBCF has very high porosity (~80%), and big pore sizes (about sevral hundreds micrometers), there are sevral limits on the conventional coating design and preparation methods, for example, the coating materials are easily infiltrated into the porous structure during preparation, making it hard to be dense; Differences in the thermal physical properties between highly porous substrate and highly dense coating can lead to cracking and debonding of the coating. Therefore, here we use precursor infiltration and pyrolysis, slurry painting and liquid infiltration techniques to prepare SiOC and SiC-YAS (Y2O3-Al2O3-SiO2) coatings on the carbon fibers of CBCF and CBCF composite surface, respectively, reaching comprehensive protection effects.
SiOC ceramics are better oxidation resistant coating materials because SiOC overcomes the disvantages of easy oxidation at high temperatures for carbides, and poor high temperature creep resistance for oxides. But stability of the SiOC ceramics is not clear. Here we prepared SiOC ceramics through precursor pyrolysis method using self-made polysiloxanes as precursors, and invesitigaed their stability systematically on the basis of clear understanding of their microstructures. The effects of pyrolysis temperature on the macro thermodynamic stability of SiOC have been first investigated, and then the phase composition evolution of SiOC during actual thermal annealing have been analyzed. The results indicated that SiOC has no trend to decompose into the corresponding element mixtures of Si, O2 and C; but in comparison with the corresponding crystal mixtures of cristobalite, SiC and graphite, SiOC is metastable, and the decomposition temperatures are 1300°C~1400 °C.
SiOC coatings on the carbon fibers of CBCF have been prepared through precursor infiltration and pyrolysis method. The effects of pyrolysis temperature and infiltration-pyrolysis time on the coating microstructure and properties have been investigated systematically. 1) With increasing preparation temperature, SiOC and carbon fiber had carbothermal reduction reactions, and the coating gradually transformed from amorphous SiOC (1200 °C) to crystalline SiC (＞1450 °C). Roughness of the coating surface increases, and thermodynamic stability of the bonding with carbon fibers decreases. When the pyrolysis temperature is 1200 °C, the amorphous SiOC coating is uniform and dense, in a good bonding state with carbon fibers, showing good oxidation resistance at oxidation temperatures of 600~800 °C. 2) With increasing infiltration-pyrolysis time, the coating thickness increases, the composite density increases from 0.38 g·cm-3 to 0.94 g·cm-3, the composite porosity decreases from 79.82% to 52.23%; the oxidation resistance improves, and the weight loss decreases from 26.9% to 14.3% after oxidized at 800 °C for 240 min; the mechanical performance increases, and the compressive strength increases from 3.0 to 7.09 MPa in the xy direction, and 1.02 to 3.78 MPa in the z direction. But the thermal insulation performance decreases, the thermal conductivity increases from 0.448 W·m-1·K-1 to 2.297 W·m-1·K-1 in the z direction. 3) Aiming at the problem that with increasing infiltration-pyrolysis time, the oxidation resistance and mechanical performance improves but thermal insulation performance decreases, we proposed a method of using SiC nanowires to strengthen and toughen the joints of carbon fibers, realizing preliminarily to improve mechanical performance under the guarantee of relative low density of the composites (to insure good thermal insulation performance). The strengthening mechanisms of SiC nanowires on the carbon fiber joints are nanowire deboning, pulling-out and bridging, making contributions to the improvement of material strength and toughness. The effects of preparation temperature and painting time on the microstructure and oxidation resistance of the coatings have been investigated. The results have indicated that when the preparation temperature is 1250 °C and the coating thickness is 60~80 μm, the glass viscosity is appropriate, the coating is dense, and the weight loss of the sample is 24% after oxidized at 1000 °C for 220 min, and 27% after oxidized at 1200 °C for 160 min. Anti-oxidation failure of the coating is mainly attributed to mismatch of thermal expansion coefficient between CBCF、SiC and YAS glass, resulting in the formation of cracks in the coating, and the pores hard to be sealed due to escape of gases through the coating surface, leading to oxidation of carbon fibers underneath the coating, and debonding of partial coating.
Combine the carbon fiber coating in CBCF and external coatings on the composite surface, namely use the as-derived CBCF sample with carbon fibers modified as substrate, and prepare SiC-SiC/YAS coating on the sample surface. To remit the problems of crack and debonding for the SiC/YAS coating, a SiC nanowire toughened SiC/YAS coating has been prepared by using in situ reaction method and slurry method. The microstructure, phase composition and growth mechanisms of SiC nanowires, and the effects of nanowire on the coating microstructure, mechanical performance, thermal shock resistance, and oxidation resistance have been investigated. The results indicated that the nanowire layer consists of a large number of fcc-SiC nanowires, and their growth mechanism is vapor-solid. After introducing SiC nanowires in the coating, the coating fracture toughness increases from 0.23±0.03 MPa·m1/2 to 0.65±0.05 MPa·m1/2; no debonding phenomenon of the coating is observed after 10 times thermal cycles of 1200 °C↔room temperature; the sample weight loss is 11.5% after oxidation at 1200 °C for 160 min. The improvement of oxidation resistance is mainly attributed to defect sealing by viscous flow of glass, less cracks due to the nanowire debonding, pulling-out, bridging and crack deflection, and multi-protection of carbon fiber coating and composite surface coating. Weight loss of the sample is mainly due to microcracks and pores in the coating.
During the study of anti-oxidation performance of coatings on carbon fibers in CBCF, SiC sponge with elasticity and plasticity have been first obtained. On the basis of that, a novel method for preparation of SiC sponge is proposed, namely by the combination of precursor infiltration and pyrolysis, chemical vapor reactions and removement of CBCF by using CBCF as template, through in situ growth and self-assembly of SiC nanowires, a SiC sponge constructed by SiC flakes and nanowires has been obtained. The SiC sponge is of low densities (＜0.2 g·cm-3), good abilities of resilience and plastic deformation, high temperature stability (stable in air and Ar at 1000 °C), and ultralow thermal conductivity coefficient (＜0.02 W·m-1·K-1). SiC sponge is of great potential for the application in the high temperature thermal protection fields based on those excellent properties.
Translated Keyword
[Dense coating, Microstructure control, Performance optimization, Porous Si3N4 ceramics, γ-Y2Si2O7]
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