【作者】 马彦；

【导师】 陈朝辉；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2011， 博士

【摘要】 随着C_f/SiC复合材料在航空航天和军事领域日益广泛的应用,近年来对C_f/SiC复合材料的研究热度不断升温,其中先驱体浸渍裂解（PIP）法在设备要求、复杂构件近净成型等方面表现出来的优势使其受到越来越多的关注。不过,当长时间处于应用环境中时,PIP法C_f/SiC复合材料的组成、结构和性能会发生一系列的演变。为了对PIP法C_f/SiC复合材料的应用提供技术支持,本文开展了PIP法C_f/SiC复合材料组成、结构和性能的高温演变研究,重点研究了基体、纤维、界面和力学性能的高温演变规律以及C_f/SiC复合材料损伤失效机理。首先研究了聚碳硅烷（PCS）转化SiC陶瓷基体的高温演变规律,结果发现惰性气氛中1200℃时SiC陶瓷基体（SC1200）除含有硅元素和碳元素外,还含有杂元素氧（1.99wt%）和少量氢元素,其中氧元素主要以SiO₂形式存在,碳元素主要以SiC和自由碳（约为11wt%）形式存在,其结构为部分自由碳原子固溶于SiC相网络的无定型态;1500℃时开始碳热还原反应,氧含量减少;1600℃时,发生固溶于SiC相网络的自由碳沉淀析出,同时自由碳相开始从无定型态转变为乱层石墨结构;1800℃时,SiC陶瓷转变为由乱层石墨结构碳连续相包围SiC纳米晶（约7nm ）的多晶态。在研究SiC陶瓷基体结构演变的基础上,研究了PCS转化SiC本体陶瓷的热物理性能、氧化特性和烧蚀特性的高温演变规律。与SC1200相比,惰性气氛1800℃热处理SiC陶瓷（SC1800）的热膨胀系数从4.3×10^-6/℃增大到4.8×10^-6/℃,热扩散系数从0.016cm²·s^-1增大到0.086cm²·s^-1;同时由于SiC陶瓷的结晶程度增加、贫氧环境中自由碳相的高温稳定性优于SiC相以及热扩散系数变大缓解了烧蚀表面的热量累积,使得SC1800的抗烧蚀能力增加。SiC陶瓷的氧化特性研究结果表明:不同温度热处理SiC陶瓷中自由碳相均在600℃开始氧化;SC1200中由于部分自由碳原子固溶于SiC相网络,导致自由碳相少量氧化后SiC相就在731℃开始氧化,此时SiC陶瓷出现最大失重2.24wt%;而1600℃以上高温热处理后,SiC陶瓷中出现相分离,自由碳形成连续相保护SiC晶粒,因此自由碳相的氧化程度增加,SiC陶瓷失重超过7.00wt%,同时SiC相的氧化温度提高100℃以上。通过研究碳纤维结构和性能的高温演变发现,当热处理温度在1000℃以下时,碳纤维主要是去除表面胶;当热处理温度在1000¹600℃之间,随着热处理温度的升高,碳纤维进一步碳化排出灰分,石墨化程度和拉伸强度均逐渐增加,而且在1600℃热处理后呈现乱层石墨结构,拉伸强度达到最大;当热处理温度高于1600℃时,随着热处理温度的升高,碳纤维石墨化程度持续增加,拉伸强度逐渐减小,而且在1800℃热处理后,其结构为4nm左右的石墨带沿纤维轴向延伸,拉伸强度仍然较高（与碳纤维原丝相当）。研究了PIP法C_f/SiC复合材料界面结构的高温演变规律,发现在1200℃下裂解制备C_f/SiC复合材料（CSC1200）的界面相为一个C/SiO₂复合层,而且由于乱层石墨结构碳层和无定型态SiO₂层弹性特征不同,承力时裂纹在它们之间扩展,使碳纤维较好地发挥增强增韧的作用;在1800℃高温热处理后复合材料界面相转变为乱层石墨结构碳纤维呈化学结合的多晶态SiC陶瓷,同时存在较大的SiC晶粒（200nm）。在研究基体、纤维和界面的基础上,首次系统研究了不同条件高温热处理后PIP法C_f/SiC复合材料力学性能的演变规律。在Ar中不同温度下进行1h热处理时,当热处理温度为1600℃,C_f/SiC复合材料表现为低强度韧性断裂,经再次浸渍裂解PCS的后致密化工艺后,力学性能有较好的恢复,达到热处理前试样CSC1200的84%以上;当热处理温度大于1700℃,C_f/SiC复合材料表现为低强度脆性断裂,并且经后致密化工艺后,力学性能恢复较小,仍保持脆性断裂。因此,1600¹700℃是C_f/SiC复合材料高温损伤机理发生转变的重要温度范围。在1600℃Ar中对C_f/SiC复合材料进行不同时间热处理,结果表明随着热处理时间的延长,复合材料的失重率逐渐增加、力学强度逐渐下降,并在5h热处理后从韧性断裂转变为脆性断裂;比较研究了真空中1600℃不同时间热处理对C_f/SiC复合材料力学性能的影响,发现随着热处理时间的延长,复合材料的力学性能逐渐下降而失重率几乎不变,而且与Ar气氛相比,相同时间热处理复合材料的力学性能较好,热处理10h后复合材料才出现脆性断裂。对热处理后复合材料进行后致密化发现,Ar中5h内或真空中10h内热处理复合材料的力学性能都得到较好的恢复,并再次转变为韧性断裂。说明当温度为1600℃,Ar中5h或真空中10h热处理是复合材料结构和性能发生质变的临界点。系统地研究了PIP法C_f/SiC复合材料力学性能的高温损伤失效机理。研究表明,高温热处理后C_f/SiC复合材料中碳纤维主要受到化学损伤,同时其增韧作用也受热应力的影响。试样CSC1200的基体与界面相含有氧元素,在界面相内氧元素含量较高,界面相为一个C/SiO₂复合层;在惰性气氛中当温度升到1500℃以上,基体和界面相中发生碳热还原反应,在此过程中,生成的SiO气体加速了硅元素向碳纤维表面的扩散,并在碳纤维表面反应生成新界面相SiC。在1600℃Ar中,随着热处理时间延长,原界面相C/SiO₂被削弱,SiO气体迁移对碳纤维的化学损伤逐渐增加,与碳纤维呈化学结合的新界面相SiC逐渐加强;当热处理时间达到5h,碳热还原反应引起的碳纤维化学损伤和新界面相开始发挥主导作用,此时复合材料在热应力的促进作用下,从韧性断裂转变为脆性断裂,因此经低温去应力退火工艺后复合材料可以再次转变为韧性断裂;当热处理时间超过5h,碳热还原反应引起的碳纤维化学损伤和新界面相完全发挥主导作用,复合材料呈脆性断裂,且不可恢复。在1600℃真空中,由于及时排出SiO气体,碳纤维的化学损伤及新界面相的形成减缓,使得复合材料结构和性能发生质变的临界点从5h延迟到10h,这一结果证实了SiO气体在复合材料中迁移与反应的作用。SiO₂的熔点为1723℃,当热处理温度在1700℃以上,熔化的SiO₂加快了Si元素的扩散、碳纤维的化学损伤和新界面相的形成,使得复合材料在短时间内（<1h）从韧性断裂转化为脆性断裂,且不可恢复。采用PIP法在碳纤维表面制备碳涂层,新制备的C_f/SiC束丝复合材料的耐高温性能得到明显的改善。在研究SiC基体和C_f/SiC复合材料高温稳定性的基础上,研究了C_f/SiC复合材料烧蚀特性的高温演变,发现随着热处理温度的升高,C_f/SiC复合材料抗烧蚀性能逐渐提高,与基体抗烧蚀性能的演变规律相同,因为对抗环境破坏时基体发挥主要作用,对碳纤维增强体进行保护。之后,通过研究以四正丁氧基锆、二乙烯基苯、PCS三种有机物为先驱体制备锆改性C_f/SiC复合材料,发现改性后复合材料的抗烧蚀性能增强,原因是锆改性复合材料中存在耐超高温陶瓷相,具有较好的抗烧蚀性能而且烧蚀过程中能在烧蚀区形成氧化物保护层。更多还原

【Abstract】 Recently, the carbon fiber reinforced silicon carbide （C_f/SiC） composites have been widely investigated due to the popular applications in the aeronautic, astronautic and military fields. As one kind method to fabricate C_f/SiC composites, the precursor infiltration and pyrolysis （PIP） route has attracted much more attentions, due to the advantages of simple requests for equipments, near net shape for complex components, and so on. However, the compositions, microstructures and properties of C_f/SiC composites will be changed, when the C_f/SiC composites are applied in high-temperature circumstances for several hours. Consequently, in order to offer technologies for applications of the C_f/SiC composites, the paper investigated the high-temperature evolutions of the compositions, microstructures and properties of C_f/SiC composites fabricated via the PIP route. Especially, the evolutions of SiC matrix, C fiber and interphase, as well as the damage mechanisms of mechanical properties of C_f/SiC composites, were highly focused.The composition and microstructure evolutions of SiC matrix derived from polycarbosilane （PCS） in the protective atmosphere were researched. It is found that in the SiC matrix derived from PCS at 1200℃（called as SC1200）, there are a little of elementals O （1.99 wt%） and H except the Si and C; And the elemental O mainly exists as SiO₂, when the C mainly exists as both SiC and free carbon （nearly 11 wt%）; Its microstructure is amorphous including part of free carbon dissolved in the SiC continuum. At 1500℃, the concentration of elemental O decreases due to the carbothermic reductions; At 1600℃, the free carbon dissolved in the SiC continuum deposits, and the free carbon phase becomes turbostratic from amorphous; At 1800℃, the SiC matrix is nanocrystalline with the turbostratic carbon surrounding the SiC nanocrystals （about 7 nm）.Obviously, the thermal, oxidation and ablation characteristics of the SiC bulk ceramics derived from PCS are affected by the microstructures. Compared with the SC1200, the thermal expansion coefficient of the SiC ceramics after 1800℃annealing （called as SC1800） increases from 4.3×10^-6/℃to 4.8×10^-6/℃, when the thermal diffusivity becomes 0.086 cm²/s from 0.016 cm²/s; Due to the increase of crystalline and thermal diffusivity, and the better thermal stability of C phase than SiC phase in the poor oxidation atmosphere, the anti-ablation properties of the SC1800 also increases. The oxidation characteristics of the SiC ceramics are as follows: Although the free carbon phases all start to be oxidized at 600℃in the SiC ceramics annealed at various temperatures, the oxidation degrees are different. For the SC1200, the ratio of weight loss is only 2.24 wt% before the SiC phase starts to be oxidized at 731℃, because there is part of free carbon dissolved in the amorphous SiC continuum; For the SiC ceramics annealed above 1600℃, the oxidation degrees of the free carbon phases increase, resulting in the ratio of weight loss of the SiC matrix over 7.00 wt%, and the oxidation temperatures of the SiC phases are prolonged more than 100℃, because the free carbon dissolved in the SiC continuum deposits and the SiC nanocrystals are protected by the free carbon continuum.The high-temperature evolutions of C fibers under vacuum were also investigated. Below 1000℃, the fiber sizing of C fibers is eliminated; During 1000¹600℃, the ratio of weight loss, the graphite degree and the tensile strength of C fibers all gradually increase with the increasing temperature, and especially, at 1600℃, the texture of C fibers start to become the turbostratic carbon with the biggest tensile strength; Above 1600℃, the graphite degree of C fibers increases, whereas the tensile strength decreases with the increasing temperature. Especially, at 1800℃, there are nearly 4 nm wide straps of the graphite along the axes of C fibers, and the tensile strength is still high equal to the raw fiber.The microstructure evolutions of interphases in the C_f/SiC composites in the protective atmosphere were investigated. In the C_f/SiC composites fabricated at 1200℃（called as CSC1200）, the interphase is an organized multilayer （C/SiO₂）, and due to the differences of elastic characteristics between the amorphous SiO₂ layer and the turbostratic carbon layer, the matrix cracks extend between them when the composites bear loads, resulting in the good reinforcing effect of C fibers on the composites’strengths. After 1800℃annealing, the interface is the chemical bonding between the nanocrystalline SiC interphase and C fibers with the turbostratic carbon structure, and there are big SiC crystals （200 nm） on the interface.Effects of annealing conditions on the mechanical properties of C_f/SiC composites were researched systematically concerning temperature, holding time and pressure. When the annealing time is 1 h under Ar, the C_f/SiC composites annealed at 1600℃show the low strength and tough fracture behavior, and the mechanical properties of the annealed composites after the redensification rout recover well, which can reach above 84% of that of CSC1200; However, the C_f/SiC composites annealed above 1700℃show the low strength and brittle fracture behavior, and the mechanical properties of the annealed composites after the redensification rout are still poor and keep brittle. Consequently, the damage mechanism of mechanical properties of the C_f/SiC composites changes in the temperature range from 1600℃to 1700℃.When the C_f/SiC composites are annealed at 1600℃under Ar, the ratios of weight loss gradually increase and the mechanical properties decrease with the annealing time increasing. And the fracture behaviors of the C_f/SiC composites become brittle from tough, when the annealing time is longer than 5 h. Then, the effects of annealing time at 1600℃under vacuum on the C_f/SiC composites were compared. It is found that the mechanical properties gradually decrease while the ratios of weight loss hardly change with the increasing time, and the strengths of the C_f/SiC composites are better than that of the C_f/SiC composites annealed under Ar for the same annealing time. When the annealing time is longer than 10 h, the fracture behaviors of C_f/SiC composites become brittle from tough. Finally, after the redensification route, the mechanical properties of the C_f/SiC composites annealed under Ar in 5 h or annealed under vacuum in 10 h recover well, and become tough again. So the annealing times （5 h under Ar or 10 h under vacuum） are the critical points of the qualitative changes about the microstructures and properties of the C_f/SiC composites when the annealing temperature is 1600℃.The damage mechanisms of mechanical properties of the C_f/SiC composites annealed at various conditions were systematically investigated. The results show that the chemical damage is the major damage for the C fibers in the C_f/SiC composites during the high-temperature annealing, and the reinforcing effect of C fibers is affected by the thermo-mismatch stress at the same time. In the CSC1200, there is the elemental O in the matrix and interphase, even higher content in interphase due to a C/SiO₂ multilayer as the interphase. So during protective atmospheres, the carbothermic reductions happen above 1500℃. The product SiO gas accelerates the elemental Si diffusing to the C fibers, and then the new interphase SiC is formed at the surfaces of C fibers. At 1600℃under Ar, the original interphase （the C/SiO₂ multilayer） is gradully weakened with the annealing time increasing, while the chemical damage to C fibers is gradully enhanced due to the transplant of SiO gas, and the chemical bonding between the new interphase SiC and C fibers is also enhanced; When the annealing time is 5 h, the fracture behaviors of the C_f/SiC composites become brittle from tough, because the chemical damage to C fibers and the new interphase start to become dominant, and there are big thermo-mismatch stresses at the same time, so the fracture behaviors of the C_f/SiC composites recover tough after the low-temperature stress-releasing annealing; When the annealing time is above 5 h, the chemical damage to C fibers and the new interphase completely become dominant, so the fracture behaviors of the C_f/SiC composites become brittle from tough, and can not recover forever. At 1600℃under vacuum, the chemical damage to C fibers and the forming of the new interphase are decelerated by the releasing of SiO gas, so the critical point of the qualitative changes about the microstructures and properties of the C_f/SiC composites is prolonged to 10 h. Consequently, effects of the transplant and reaction of SiO gas in the C_f/SiC composites are confirmed by the result. When the annealing temperature is above 1700℃, the elemental Si diffusing, the chemical damage to C fibers and the forming of new interphase are deteriorated by the SiO₂ melt, because the melting point of SiO₂ is 1723℃. So the fracture behaviors of the C_f/SiC composites become brittle from tough in a short time （less than 1 h）, and can not recover forever.After the carbon coating on the C fibers is fabricated by PIP process, the thermal stability of the C_f/SiC minicomposites is obviously improved. Based on the completely understanding about the thermal stability of the SiC matrix and C_f/SiC composites, the high-temperature evolutions of ablation characteristics were further researched. It is found that the anti-ablation properties of the C_f/SiC composites gradully become better after the higher temperature annealing, and the result is the same with the evolutions of anti-ablation properties of the SiC bulk ceramics, because the SiC matrix plays the key role to prevent the destructions by the circumstance, and protects the C fibers. Then, the Zr-doped C_f/SiC composites were fabricated via PIP route using the precursors of Zirconium n-butoxide, Divinylbenzene and PCS. The anti-ablation properties of the Zr-doped C_f/SiC composites are better than that of the C_f/SiC composites, because there are ultra high temperature ceramics in the Zr-doped C_f/SiC composites, which have the better anti-ablation properties than SiC and can form the oxide films on the surfaces of the ablation zones.更多还原