【作者】 李思维；

【导师】 冯祖德；

【作者基本信息】 厦门大学 ， 无机化学， 2008， 博士

【摘要】 航空航天事业的迅速发展迫切需要低密度、高强度、高模量、高韧性和耐高温的连续纤维增韧碳化硅陶瓷基复合材料（CMC-SiC）。然而,目前的CMC-SiC材料还不能满足高推重比航空发动机热端部件的长时间（上千小时）服役要求,必须进一步改善其抗氧化性能,发展具有自愈合功能的碳化硅陶瓷基复合材料（CMC-MS）。CMC-MS材料的抗氧化寿命主要受纤维、基体、纤维/基体界面相和涂层的成分及微结构影响,因此,必须深入研究用于CMC-MS材料的Hi-Nicalon纤维、自制含铝碳化硅纤维、自愈合基体及自愈合涂层的微结构及其随处理温度和环境的变化规律,从而指导材料设计和工艺改进。本文以CMC-MS材料为研究对象,通过微结构评价和表征,系统深入地研究了Hi-Nicalon纤维在高温模拟环境处理后的微结构演变及力学性能,评价了纤维的失效过程;探索了自制含铝碳化硅纤维中铝元素的存在位置和状态,明确了铝在纤维结构形成过程中的重要作用;揭示了CVD B-C陶瓷涂层及其热处理后的微结构特征;研究了CVD B-C陶瓷改性后的自愈合涂层和自愈合基体在环境处理后的微结构特征,解释了CVD B-C陶瓷的自愈合机制;评价了环境处理后改性CMC-MS材料的自愈合情况和微结构演变。为CMC-MS的材料设计与工艺改进提供了依据。主要研究内容与结果如下:1、对水氧耦合环境中1300℃-1600℃处理1小时的Hi-Nicalon纤维进行了残余强度测试和显微结构的表征。结果表明:纯惰性气氛中高温处理后纤维强度下降,水氧耦环境处理后,纤维残余强度先升高后降低。与惰性气氛环境相比,水氧环境使纤维发生惰性氧化,氧化后钝化层生成、气孔愈合、厚度增加、及均匀析晶对保持强度有重要作用。在纤维结构完整的情况下,晶粒粗化对强度的不利影响在各种因素的综合作用下被淹没。2、研究了自制掺杂Al元素的连续SiC纤维中Al的作用、状态和存在位置。结果包括:（1）利用TEM观察结合像模拟技术研究了Al-SiC纤维的晶体特征,并观察了立方SiC晶粒中的孪晶结构和析出六方结构。（2）Al作为烧结助剂,对改善纤维结构致密度,控制晶粒长大的作用十分明显。但其作用受交联时间、烧结温度、和晶粒尺寸等因素的影响。（3）含Al相以Al₂OC、Al₄SiC₄等形式存在于SiC晶粒或晶粒团之间。最终建立了Al-SiC纤维的结构模型。3、研究了不同工艺制备的CVD B-C陶瓷涂层微结构组成及其氧化处理后的微结构演变。结果表明:（1）B-C陶瓷涂层主要分为晶态和非晶态B-C陶瓷涂层。（2）高温处理后,非晶B-C陶瓷向晶态的B₄C相转变,且晶化程度随热处理温度升高而提高。整个晶化过程受C、B元素扩散控制,导致两B-C层界面处B₄C晶体含量最高,并沿低能面外延生长,使涂层间结合更强。（3）直接面对水氧环境时,CVD B-C层因氧化和快速挥发而失去对材料的保护。对于SiC/B-C/SiC涂层,低温时,B-C陶瓷层的氧化主要受氧化气体（O₂/H₂O）穿过表面SiC层的制备缺陷以及热处理过程中生成的裂纹控制。中温时,表层SiC层已经开始部分氧化为SiO₂,而B-C层的氧化仍受氧化气体通过缺陷氧化B-C层控制。B₂O₃和SiO₂形成硼硅酸盐玻璃（B₂O₃·χSiO₂）。高温时,表层SiC氧化反应速度加快,缺陷处B-C陶瓷层也因氧化而发生快速的体积膨胀。另外,表层SiC被大量氧化后,H₂O有机会通过SiO₂直接到达B-C层,氧化由H₂O透过SiO₂层导致B-C涂层的全面氧化所控制。解释了SiC/B-C/SiC涂层在水氧耦合环境不同温度中具有良好的抗氧化能力的微结构本质。4、研究了B-C陶瓷涂层自愈合改性和基体改性复合材料在氧化环境中处理后的微结构和性能演变,观察了微裂纹的扩散途径和材料缺陷的氧化愈合。结果表明:（1）多种相及相间界面的存在使得两种改性复合材料中的微裂纹皆存在多种扩展模式,复合材料纤维束间的结构在一维尺度上存在着连续的弱结合点。（2）热解碳（PyC）相和C纤维在高温热处理后趋于形成取向一致的共格结构,形成了强界面结合,有效促进了界面相内裂纹偏转。（3）涂层改性材料低温处理后基体缺少愈合组元,但高温处理后的多种微缺陷都靠SiO₂的生成和缓慢流动实现了有效的愈合。对大尺寸的制备缺陷,目前的材料还难于被流动层填封。（4）基体改性材料的B-C陶瓷层氧化后可使材料基体微缺陷有效愈合。在测试温度和时间内,低温下的愈合由B₂O₃的流动实现;中高温的愈合则有SiO₂和硼硅酸盐玻璃（B₂O₃·χSiO₂）参与。更多还原

【Abstract】 Continuous fiber reinforced SiC-matrix composites（CMC-SiC） are considered as the most promising thermo-structural materials due to their high thermal stability,low density,high specific strength,high specific modulus,good oxidation-ablation resistance,especially improved flaw tolerance and non-catastrophic mode of failure. However,the bad oxidation resistance of CMC-SiC in high temperature,high load and oxidation environment limits their long-term application in aero-engine.Consequently CMC-SiC with self-healing function should be developed,which can be named CMC-MS（Multilayer Self-healing Ceramic Matrix Composites）.Since the CMC-SiC was used at a high temperature under a complex environment containing O₂,H₂O and corrosives,the mechanical property will be controled by a series of thermo-physical chemistry reactions（such as surface oxidation,phase transformation and atomic diffusion） which occurred under the effects of temperature and environment.So,it is important to study the microstructural evolution of the CMC-SiC components annealed in the temperature/environment coupling conditions.In this thesis,the mechanical and structural evolution of Hi-Nicalon^TM SiC fiber annealed in O₂-Ar-H₂O atmospheres were investigated.Much of analysis and discussion will then reveal the factors related to the residual strength and microstructural evolution of the annealed fiber.Then,the location and chemical state of Al element in Al-SiC fiber were explored and the effect of Al during the structure formation process was discussed.Bisides,a CVD B-C ceramic coating used as a self-healing modification component of CMC-SiC was also annealed in pure Ar and O₂-Ar-H₂O atmospheres respectively to explore the crystallization and oxidation.Finally,two kinds of CMC-MSs were fabricated.The self-healing performance,oxidation resistance and mechanical properties of the two CMC-MSs were demonstrated.The main subjects and results are summarized as follow:1.Hi-Nicalon fibers were exposed in O₂/Ar / H₂O atmosphere for 1 hour at 1300℃, 1400℃,1500℃,1600℃,respectively.Results indicated that residual tensile strength increased with increasing temperature from 1300℃to 1500℃,then decreased after annealing in 1600℃.In conclusion,the control effects of water vapor on formation and structural evolution of the passive film were remarkble.The change of residual strength of the annealed fiber was influenced by the competition of negative and active effects. Below 1500℃,the active effects were dominated factors.Above 1600℃,the negative effects such as rapid oxidation,volatility,film-melting,and fihn/fiber reactions occurred and became dominated reactions.After these processes,fiber’s structure was evidently destroyed.2.The role,location and chemical state of Al element in Al-SiC fiber were explored. The results and conclusions were as followed:（1）The structure and orientation of the crystals and defects in Al-SiC fiber were investigated by HRTEM and structural simulation technic.（2） Al can lower the densification temperature and restrain the grain growth of SiC crystals during the sintering process,but these effects were limited by sintering temperature,oxidative crosslinking time and grain size of the SiC crystals.（3） The Al containing compound which mainly existed as the intergranular phase between SiC crystals were Al₂O₃,Al₂OC and Al₄SiC₄.Finally,a model was built to illustrate the structure of Al-SiC fiber.3.Microstructure of CVD B-C coatings deposited under various parameters was examined by electron microscopy and spectroscopy.Then,structural evolution of the coating annealed in different environments was also investigated.The results and conclusions were as followed:（1） The B-C coatings produced by CVD can be divided as crystallized coatings and amorphous coatings.（2） A two-layer B-C coating was annealed in an Ar atmosphere for 2 hour at 1600℃to 2000℃.After annealing,the amorphous boron carbide had partly transformed to B₄C crystals,and the crystallization was controlled by the diffusion of C and B.TEM showed an evident grain growth with the increase of annealing temperature.The B₄C crystals produced near the layer/layer interface mainly grew epitaxially along the（012） direction.（3） The protection effect of B-C coating could be destroyed in a short time because the rapid oxidation of BC_x and volatilization of B₂O₃,so it was unsuitable to make B-C coating as an outer coating. Thus,a SiC/B-C/SiC structure was fabricated and annealed.Results showed that there were different oxidation mechanisms under different annealing temperatures:the oxidation mechanism of low temperature annealing was controlled by the diffusion of the O₂/H₂O gas through the defects in the SiC surface coating,and then controlled by the oxidation of the B-C coating;in the middle temperature district,reaction was controlled by the local oxidation of SiC surface coating,the diffusion of the O₂/H₂O gas through the surface coating defects,and then controlled by the oxidation of the B-C coating and the partial formation of B₂O₃·xSiO₂;in the high temperature district, reaction was controlled by the rapid oxidation of SiC coating,the diffusion of the O₂/H₂O gas through the surface defects and oxidation product SiO₂,the rapid oxidation of B-C and substantive formation of B₂O₃·xSiO₂,orderly.Weight-change data was also used to explain these mechanisms,all the information indicated that the SiC/B-C/SiC coating possess excellent performance after annealing in the O₂/H₂O/Ar atmosphere under different temperatures.4.Two kinds of CMC-MSs with good properties were fabricated.One called coating-modified C_f/SiC composite,another called matrix-modified C_f/SiC composite. The antioxidation properties and self-healing funtions of each CMC-MS were examined under different temperature regions.The results and conclusion were as follows:（1） At least 4 types of the microcracks can be observed in both two composites,and the defects in CMC-MS structure had a linear distribution.（2） The PyC coating had a similar epitaxy with the carbon fiber after annealing,which indicated that the coherent structure was the key factor to form a strong bond between them.And the microcracks can hardly deflect at the fiber/PyC interface.（3） Micro-defects in the coating-modified composite could be healed by SiO₂ after high temperature oxidation.Limitted by the fluidity of SiO₂,it was difficult to heal a large-size deflect.（4） By contrast,much of the fabrication and oxidation induced deflects had been healed by the B₂O₃ fluidity in matrix-modified composite,assisted by SiO₂ and B₂O₃·xSiO₂.更多还原