【作者】 郑晓慧；

【导师】 肖加余； 堵永国；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2009， 博士

【摘要】 碳纤维增强碳化硅（C_f/SiC）复合材料是航空航天用热结构部件极具竞争力的候选材料,高温下易被氧化的特性是影响其应用化进程的关键因素之一。本文在研究先驱体浸渍—裂解（PIP）法制备的C_f/SiC复合材料的氧化性质和氧化机理基础上,提出理想抗氧化涂层材料的基本性能要求:高致密度、匹配的热膨胀系数、工作温区合适的粘流特性、与复合材料强结合,以此为依据选择三种用于不同温区的材料体系:MgO-Al₂O₃-SiO₂（MAS）玻璃陶瓷、BaO-Al₂O₃-SiO₂（BAS）玻璃陶瓷和硅酸钇,测试各材料的热膨胀系数、自愈合温区和烧结致密度等性能,针对各自的优缺点设计制备出五种不同的涂层结构体系。以玻璃粉体为主要原料,采用微波技术首次在C_f/SiC复合材料表面原位烧结玻璃陶瓷涂层,研究了1000～1500℃范围不同使用条件下涂层复合材料的静态氧化和热冲击氧化性能。用热重法研究了C_f/SiC复合材料在600～1500℃范围内的等温氧化特性和非等温氧化特性。结果表明,复合材料的氧化以转化率70%为界,可分为线性阶段和非线性阶段。低温区（600～800℃）,线性氧化阶段和非线性氧化阶段的表观活化能分别为135.08kJ/mol和164.95kJ/mol;中温区（900～1100℃）,复合材料的表观活化能大大降低,线性氧化阶段和非线性氧化阶段的表观活化能分别为36.64kJ/mol和58.79kJ/mol。低温区内不同温度下氧化时复合材料的最大失重率Xmax数值接近。高温区（1100～1500℃）,Xmax随着氧化温度的升高而逐渐减小;氧化后复合材料的强度保留率随氧化温度的升高而降低。采用无模式函数法计算C_f/SiC复合材料的非等温氧化动力学,结果表明Friedman-Reich-Levi法为较好的分析方法,随着氧化过程进行,氧化温度升高,活化能逐渐减小。通过熔盐热歧化反应,在C_f/SiC复合材料表面制备了厚度为6～10μm的钛金属化层。金属化层中与复合材料的结合良好,有封孔作用和阻碳作用,在1000℃具有一定的抗氧化能力。以钛金属化层为过渡层,设计并制备了适用温区为1000～1200℃的Ti/MAS1双层涂层。涂层复合材料在1000℃氧化720min后,失重率为0.95%,强度保留率为99.3%,抗氧化性能良好。在1200℃的温度下,涂层复合材料仍具有较好的长时抗氧化能力,但是在更高温度下,MAS1玻璃陶瓷的粘度过小,出现明显的流淌和气孔现象。设计并制备了在1350℃附近自愈合能力较好的BAS₂/BAS1双层涂层。作为C_f/SiC复合材料用涂层材料,BAS系玻璃陶瓷的热膨胀系数稍大,使涂层内部热应力较大,但高温烧结时BAS₂涂层元素向复合材料扩散形成的过渡层,可提高涂层与复合材料的结合强度,改善二者的相容性。BAS₂玻璃陶瓷的优化烧结温度为1050℃,该温度下烧结的涂层结晶度最高。BAS₂/BAS1涂层由内而外具有梯度渐变的热膨胀系数,且BAS1外层愈合能力较强,有利于保证涂层的致密性。涂层复合材料在1350℃经过96次热冲击960min氧化后,失重率为5.04%,表面只有少量小尺寸微裂纹出现,具有较好的抗氧化性能和一定的抗热冲击性能。在BAS₂玻璃中加入Y₂O₃,制备了BAS₂-Y₂O₃涂层。BAS₂玻璃与Y₂O₃在1327℃下反应生成高熔点、低热膨胀系数的硅酸钇,在冷却过程中BAS熔体析出六方钡长石,因此BAS₂-Y₂O₃涂层的耐温性能较好,热膨胀系数较低。微波烧结法能有效提高BAS₂-Y₂O₃涂层材料的烧结致密度,Ar-H2保护气氛下烧结可提高涂层元素的扩散能力,改善涂层与复合材料的结合。对于Si:Y为1:1的BAS₂-Y₂O₃涂层,优化烧结工艺为:烧结温度1500℃,烧结时间30min,随炉冷却。优化工艺得到的BAS₂-Y₂O₃涂层复合材料在1400℃下300min氧化后失重率为1.08%;由于涂层材料热膨胀系数的限制,涂层的抗热冲击性能一般。微波烧结制备了BAS₂-Y₂O₃/SiO2-Y₂O₃双层涂层,内层为六方钡长石-Y2SiO5复合相,外层为Y2Si2O7。内层与复合材料和外层均有较好结合。Y₂O₃与SiO2在1500℃下常规烧结时,硅酸钇合成反应难以进行;微波烧结Y₂O₃:SiO2为1:2的涂层得到Y2Si2O7。BAS₂-Y₂O₃/SiO2-Y₂O₃双层涂层复合材料在1400℃下经过15次热冲击150min氧化后失重率为1.22%。氧化实验后涂层愈加致密,没有微裂纹生成,具有良好的抗热冲击性能。以BAS₂玻璃为烧结助剂,制备了BAS₂玻璃含量梯度渐变的B-50/B-10/B-0硅酸钇涂层,使用温区为1400～1500℃。BAS₂玻璃将硅酸钇生成温度降低到1350℃,并显著促进块体烧结致密化。B-50/B-10/B-0三层涂层在烧结过程中由内而外逐步软化,使致密度提高。烧结涂层主晶相为Y2Si2O7,还有少量分布着板条状钡长石晶体颗粒的BAS相。B-50/B-10/B-0涂层在1400～1500℃下具有优异的抗热冲击性能和良好的抗氧化性能,随着氧化时间的延长,涂层复合材料失重率近似线性增大,热冲击对氧化速率影响很小。1400℃下经过11次热冲击110min氧化后,涂层复合材料的失重率仅为0.87%;1500℃下经过11次热冲击110min氧化后,涂层复合材料的失重率为2.45%,两个样品的强度保留率均大于70%。研究表明,玻璃陶瓷是复合材料抗氧化涂层的理想选材。其中,高熔点、低热膨胀系数的晶体相维持涂层的高温完整性,实现涂层与复合材料热匹配;特定温区内软化粘流的玻璃相具有较好的愈合能力,在制备过程中封填复合材料表面缺陷以提高涂层结合力,在使用过程中保证涂层的致密性。两相互相配合,使玻璃陶瓷涂层具有良好的抗氧化性能。更多还原

【Abstract】 Carbon fiber reinforced silicon carbide matrix （C_f/SiC） composite is an attractive candidate for thermal structural components. Meanwhile, it is easy to be oxidized above 400℃in oxidizing atmosphere, which is a key obstacle for its application. In this dissertation, based on the study of oxidation kinetics for C_f/SiC composite fabricated by precursor infiltration and pyrolysis （PIP） process, four basic requirements for oxidation resistant coating were proposed. They are high density, appropriate coefficient of thermal expansion （CTE）, proper viscosity at working temperatures and excellent binding with C_f/SiC composite. Three kinds of materials, which were MgO-Al₂O₃-SiO₂（MAS）glass-ceramic, BaO-Al₂O₃-SiO（2BAS）glass-ceramic and yttrium silicate, were chosen to prepare oxidation resistant coating. Some properties of them, such as CTE, self-sealing temperatures and sintering density, were characterized. Based on the properties, five effective coating systems used at different temperatures were designed and prepared. Microwave was adopted to firstly sinter coatings on the surface of C_f/SiC composite. Using glass as staring materials, glass-ceramic coatings were prepared by in situ sintering method. The performance of different coated C_f/SiC composites was tested in static state or under thermal shocks at 1000～1500℃in air.Kinetics of isothermal oxidation and non-isothermal oxidation of C_f/SiC composite was characterized by thermogravimetry at 600～1500℃. The results show that the oxditaion of C_f/SiC composite can be divided into linear stage and non-linear stage by the transition ratio of 70%. The apparent activation energies, at linear and non-linear oxidation stages at lower temperatures （600～800℃）, are 135.08kJ/mol and 164.95kJ/mol, respectively. Whereas at medium temperatures （800～1100℃）, those greatly decrease to 36.64kJ/mol and 58.79kJ/mol, respectively. At lower temperatures, the maximal weight losses （Xmax） of C_f/SiC composites are closer to each other at different temperatures, while Xmax decreases with the increasing of temperature at higher temperatures （1100～1500℃）. The residual strength of C_f/SiC composite after oxidation debases with the increasing of oxidation temperature at temperatures above 1100℃. Model-free method was used to calculate isothermal oxidation kinetic of C_f/SiC composite. The results show that Friedman-Reich-Levi is an appropriate method for C_f/SiC composite. With the progress of oxidation, the oxidation temperature increases gradually and the activation energy of C_f/SiC composite decreases sharply.By disproportionation reaction in molten salt, titanium coating with 6～10μm in thickness can be fabricated on the surface of C_f/SiC composite. The titanium coating, binding tightly with the composite, can seal the defects on the surface of the composite and prevent the diffusion of carbon. Therefore, titanium coated composite can be protected from oxidation for certain duration at 1000℃. Using titanium coating as a transition layer, Ti/MAS1 coating was designed and prepared to prevent C_f/SiC composite from being oxidized at 1000～1200℃. The weight loss of the coated sample was 0.95% after oxidation at 1000℃for 720min, and the residual strength ratio was 99.3%. The coated sample was effective for long time at 1200℃in oxidizing atmosphere, while it receded above 1200℃for the low viscosity of MAS1 glass.BAS₂/BAS1 coating with good self-sealing ability at 1350℃was designed and prepared. The CTE of BAS glass is higher than that of C_f/SiC composite. Consequently, great thermal stress is induced in BAS coating during cooling process and the compatibility between the coating and the composite is not very good. However, a graded transition layer, formed by the elements diffusion of BAS glass into the composite at high temperatures, can enhance the binding strength between the coating and the composite, and improve the compatibility between them. The crystallinity of BAS₂ glass-ceramic coating sintered at optimized sintering temperature of 1050℃is the highest. The CTE of BAS₂/BAS1 coating decreases gradually from the outer layer to the inner layer, and the outer layer has better sealing ability. As a result, the weight loss of the BAS₂/BAS1 coated sample was 5.04% and little micro-cracks appeared after oxidition at 1350℃for 960 min and thermal shocks for 96 times.By adding Y₂O₃ to BAS₂ glass, a novel BAS₂-Y₂O₃ coating was fabricated. The reaction between Y₂O₃ and BAS₂ fusion and the influences on oxidation resistant capability were studied. The results show that yttrium silicate with higher melting point and lower CTE than BAS₂ glass-ceramic is produced in BAS₂-Y₂O₃ at 1327℃, then hexacelsian crystallizes from BAS fusion during cooling process. Consequently, BAS₂-Y₂O₃ coating with lower CTE can be used at higher temperatures. The densification of BAS₂-Y₂O₃ coating can be improved by microwave sintering. The diffusion capability of coating elements is enhanced in Ar-H2 atmosphere; thereby the binding between the coating and the composite is improved. For BAS₂-Y₂O₃ coating with Si:Y =1:1, the optimized technological conditions are: sintering at 1500℃for 30min and furnace cooling. After oxidation at 1400℃for 300min, the weight loss of BAS₂-Y₂O₃ coated composite was 1.08%. Because of higher CTE of the coating materials than that of the composite, the thermal shock resistance of the coating can not meet the requirements well.BAS₂-Y₂O₃/SiO2-Y₂O₃ double layered coating was designed and prepared. After sintering, the main phases of the inner layer are hexacelsian and Y2SiO5, and the main phase of the outer layer is Y2Si2O7. Little yttrium silicate is obtained in SiO2-Y₂O₃ sintered at 1500℃by the traditional method. Meanwhile, Y2Si2O7 was produced in the coating with Y₂O₃:SiO2=1:2 sintered by microwave. After thermal shocks of 15 times and oxidation for 150min at 1400℃, the weight loss of BAS₂-Y₂O₃/SiO2-Y₂O₃ coated composite was 1.22%. The coating became denser and crack-free during thermal shocks, which indicated that the coating should have good thermal shock resistance. Using BAS₂ as sintering addition, a graded B-50/B-10/B-0 yttrium silicate coating was prepared, which was effective at 1400～1500℃. The reaction temperature of Y₂O₃ and SiO2 is decreased to 1350℃because of BAS₂ glass, and the densification is also improved. During sintering, BAS glass diffuses from the inner layer to the outer layer, and B-50/B-10/B-0 coating is softened from the inner layer to the outer layer, thereby the coating becomes denser. The main phases of the sintered coating are Y2Si2O7 and BAS glass with some hexacelsian crystals. The thermal shock and oxidation resistance of B-50/B-10/B-0 coating is excellent. With the increase of oxidation time, the weight loss of the coated sample increased in linear. Thermal shock slightly influences the coating. After thermal shocks of 11 times and oxidation for 110min at 1400℃, the weight loss of coated sample was only 0.87%, while that was 2.45% at 1500℃. The residual strength of both samples was higher than 70%.Glass-ceramic is a promising candidate for oxidation resistant coating for C_f/SiC composite. Crystals with high melting points and low CTE keep the integrity of coating at high temperatures and the thermal matching between the coating and the composite. Glass softened at given temperatures has good sealing capability. On one hand, glass can seal the defects in the surface of the composite and improve the bond between them. On the other hand, the density of the coating is ensured by the softened glass at high temperatures. These two phases act together to endow good oxidation resistance for glass-ceramic coating.更多还原