碳/碳化硅摩阻复合材料的研究进展

碳/碳化硅是近年来发展起来的一种新型高性能陶瓷基摩阻材料,具有密度低,抗氧化性好,摩擦性能高且性能稳定等一系列优点,在高速列车、飞机和重型汽车等高能载制动领域具有广泛的应用前景.反应性熔体浸渗法是制备碳/碳化硅摩阻复合材料的有效途径.从碳/碳化硅摩阻复合材料的设计出发,深入分析了反应性熔体渗透过程的热力学条件,Si-C反应体系的基本特征以及动力学规律.针对短纤维模压和三维针刺等两种典型C/SiC复合材料的制备过程,对材料的微结构特征和摩擦磨损性能进行了系统论述.同时,对红外热成像、X射线透射和工业CT等先进工程检测方法在碳/碳化硅摩阻复合材料构件上的应用进行了分析.     

三维针刺碳/碳化硅刹车材料的摩擦磨损性能

采用单向加压浸渍、加压固化和碳化制备了密度为1.20g/cm3的多孔碳纤维增强碳(carbon fiber reinforced carbon,C/C)复合材料,然后利用反应熔体浸渗法制备了密度为2.11g/cm3的低成本三维针刺碳纤维增强碳化硅(carbon fiber reinforced silicon carbide,C/SiC)刹车材料,研究了材料的微结构和摩擦磨损性能.结果表明:随刹车速度升高,刹车盘的平均磨损率逐渐升高,而平均摩擦系数先升高后降低,刹车速度为15m/s时,摩擦系数达到最大值0.57.低速刹车时摩擦系数曲线光滑、平稳上升;高速时曲线出现波动,存在"翘尾".低速刹车时磨屑为大颗粒,摩擦面上犁沟明显;高速时宏观犁沟消失,微米级磨屑弥散分布于摩擦面.     

碳／碳化硅复合材料的结构与性能

一、前言 碳/碳(C/C)复合材料仍属于碳素材料,它克服了石墨材料的某些缺点。具有强度高、抗热震性能好等优点,因而在一些工业部门得到广泛应用。随着对C/C材料性能的改进和人们对它不断深入的了解,其使用范围在不断扩大。抗氧化性能差和容易磨损是它的重要弱点。 碳化硅(SiC)是一种力学性能较好(强度、硬度)、在1000～1500℃范围内抗氧化性能好的材料,其密度较低,与碳有良好的物理化学匹配性,因此,用SiC来改进     

碳化硅／碳复合材料的特性及其应用

用涂层和其他表面改性处理方法制取的碳化硅/碳复合材料兼有碳化硅的硬度高、耐热性、抗磨损、腐蚀和碳素材料可加工性等优良 性,在滑动摩擦材料,电子元件热处理用夹具、单晶硅提拉用加热器、坩埚硅片外延生长用感受器、高温材料等方面获得了广泛应用,其应用范围不断扩大,被誉为划时代的新材料,东新电碳公司于1983年研制成功硅化石墨T1056,其各项技术性能指标已达到国外同类产品水平,目前已批量生产,为我国的科研和工业生产提供了高性能材料。     

碳/碳化硅复合材料上莫来石涂层的制备及氧化行为

以正硅酸乙酯[(C2H5)4SiO4,ethyl orthosilicate,TEOS]和硝酸铝[Al(NO3)3·9H2O]为原料,制各了莫来石(3Al2O3·2SiO2,mullite)溶胶,用浸涂法在碳磁化硅(Cf/SiC,下角f表示纤维,下同)上制备3Al2O3·2SiO2涂层(mullite coating,MC).对Cf/SiC和Cf/SiC MC进行了等温-氧化实验,并研究了两者的氧化规律.结果表明:Cf/SiC和Cf/SiC MC的氧化都可以划分为3个主要阶段:θ＜700℃;700℃＜θ1000℃;1000℃＜θ1200℃.在各个阶段,控制氧化速率的机理各有不同,Cf/SiC MC的氧化质量损失比Cf/SiC的低50%左右,表现出较好的抗氧化性能.     

刹车速度对C/C-SiC复合材料摩擦磨损性能的影响

对反应熔体渗透工艺制备的C/C-SiC复合材料,在MM-1000型摩擦磨损试验机上进行了模拟飞机制动刹车实验,重点研究了C/C-SiC复合材料在不同刹车速度下的摩擦磨损性能.研究表明:随着刹车速度的增加,C/C-SiC复合材料的摩擦系数先少许增加然后再减小,在10 m/s时达到最大值0.52.磨损率在低速时保持较低的数值,随着刹车速度的增加呈线性增加,但仍小于C/C复合材料的磨损率,表明C/C-SiC复合材料具有优良的耐磨损性能.当刹车速度超过20 m/s时,由于能载水平较高,摩擦表面出现犁沟现象并形成大量球状磨屑,摩擦系数急剧减小.     

纳米碳颗粒/碳化硅陶瓷基复合材料的氧化行为

以微米硅(Si)和纳米碳黑(Cp)粉体为主要原料,采用经机械化学法合成的碳化硅(SiC)和15%和25%的纳米碳颗粒与碳化硅(Cp-SiC)的复合粉体,并经无压烧结得到了Cp/SiC陶瓷基复合材料,分析了在不同温度条件下Cp/SiC烧结体的氧化行为。结果表明:当温度小于700℃时,Cp/SiC复合陶瓷在空气中的氧化受C—O2反应控制,致使其为均匀氧化;700℃时,氧化后的复合材料显气孔率最大,弯曲强度达极小值;大于700℃,氧化过程受O2的气相扩散控制,呈非均匀氧化;700~900℃之间,O2通过微裂纹的扩散控制着Cp/SiC的氧化过程;900~1 100℃之间,O2通过SiC缺陷的扩散控制着Cp/SiC的氧化过程,并在1 000℃时的最初的2 h内,复合材料弯曲强度增大,且达到了极大值。同时表明,纳米碳含量是影响复合材料强度及氧化行为的关键因素,添加纳米碳质量分数为15%的Cp/SiC复合陶瓷可以作为一种抗氧化性能优良的玻璃夹具材料。     

碳/碳复合材料SiC-HfSi_2抗烧蚀复合涂层

为改善碳/碳(C/C)复合材料的抗烧蚀性能,采用包埋技术在C/C复合材料表面制备了碳化硅-硅化铪(SiC-HfSi2)抗烧蚀复合涂层。采用氧乙炔火焰烧蚀试验评价了C/C复合材料样品的抗烧蚀性能。通过扫描电镜观察、能谱分析及X射线衍射分析研究了烧蚀前后C/C复合材料抗烧蚀涂层的表面和断面形貌、元素分布和相组成。结果表明:涂层C/C复合材料在烧蚀后其表面出现了丛生的氧化硅纳米线。同时,与未涂层C/C复合材料相比,SiC-HfSi2涂层使C/C复合材料的质量烧蚀率下降了85.6%。     

热解碳界面层对碳＼碳化硅复合材料拉伸性能的影响

本文研究了用化学气相渗工艺的均热法制备炭纤维增强碳化硅（Ｃ／ＳｉＣ）复合材料，其中有部分材料在沉积碳化硅之前先沉积少量热解碳，以作为界面层。对有界面层和无界面层的材料进行了拉伸试验。用金相显微镜和扫描电镜观察了材料微观结构及继口形貌。结果表明，Ｃ／ＳｉＣ材料力学性能主要取决于纤维与基体的界面。有热解碳界面层的Ｃ／ＳｉＣ材料，在拉伸断裂时出现大范围脱粘，断口类似毛刷，材料强度大，断裂功也大，有很大的     

碳化硅纤维中碳与氧含量的分析

利用Fourier变换红外光谱、原子吸收及元素分析等方法，对碳化硅纤维中的C，Si，O的含量进行测定，并以此建立了间接确定纤维中C，O含量的计算方法用日本生产的商品牌号为Nicalon的SiC纤维进行r验证，得到与文献报道相符的结果。同时对国防科技大学研制的牌号为KD-I的SiC纤维中游离碳的含量，用间接计算方法所得的结果也与元素分析的实验值基本吻合，表明该方法同样适用于KD型SiC纤维，且可信度较高。     

Role of Carbon in the Sintering of Boron-Doped Silicon Carbide

The effect of carbon on the sintering of boron-doped SiC was studied. The free carbon present in the green compact was found to react with the SiO₂ covering the surfaces of the SiC particles; however, even if no carbon was added, the surface SiO₂ reacted with the SiC itself at a slightly higher temperature. This latter reaction was associated with the onset of substantial pore growth in the shrinking green body, which, as the pores continued to grow at higher temperatures, prevented complete densification. Therefore, the reaction of the SiC with the SiO₂ may have led to the fracture of interparticle contacts, resulting in the onset of coarsening. Thus, the role of the carbon was to prevent reaction between the SiC and the surface SiO₂, by removing the SiO₂ at a temperature below that at which this reaction could occur.     

Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiC_f/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiC_f/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value ( d > d _c) and the temperature below a critical value ( T < T _g), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < d _c and may occur at T > T _g for d > d _c. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.     

Carbon Inclusions in Sintered Silicon Carbide

The carbon additions in the pressureless sintering of SiC are commonly used for the removal of SiO₂ layers on the starting powders. In practice, it is common to add more C than is necessary for stoichiometric removal to ensure a complete deoxidation. As a result, inclusions of excess free C are a general feature of the microstructure of sintered SiC. This phenomenon was studied by high-resolution Auger electron spectroscopy on ultra-high-vacuum-exposed fracture surfaces as well as by high-resolution transmission electron microscopy of B- and C-doped materials.     

反应烧结碳化硅中碳短纤维的形貌及增强作用

以双峰碳化硅粉末、碳黑、短碳纤维为原料,采用注浆成型、反应烧结法制备了力学性能优异的碳化硅复合材料。研究了硅化反应对碳纤维表面形貌及组分的影响。结果表明：硅化反应在碳纤维表面生成致密β-SiC层,反应过程伴随的体积膨胀增加了纤维表面的粗糙度。混合酸HNO3+HF腐蚀实验表明纤维表面由直径2～5μm的β-SiC晶粒构成。提出了硅化纤维的双层结构模型：外层由微米、亚微米尺度β-SiC晶粒构成,内层由Si–C基团组成的混合物组成。碳短纤维体积分数为30%时,复合材料的弯曲强度、断裂韧性分别达到最大值416 MPa、5.1 MPa?m0.5,相比单一反应烧结碳化硅陶瓷分别提高102%、78%。     

Effects of Carbon as a Sintering Aid in Silicon Carbide

Self-diffusion data are collected from the literature in an attempt to better understand the strong effects of carbon as a sintering aid in SiC. These data indicate that the presence of excess carbon, in addition to reducing the native SiO₂ layer of the SiC, probably enhances the rate-controling bulk self-diffusion rate of SiC by a factor of about 100.     

Effect of Carbon and Silicon Carbide/Carbon Interlayers on the Mechanical Behavior of Tyranno-SA-Fiber-Reinforced Silicon Carbide-Matrix Composites

Several CVI-SiC/SiC composites were fabricated and the mechanical properties were investigated using unloading–reloading tensile tests. The composites were reinforced with a new Tyranno-SA fiber (2-D, plain-woven). Various carbon and SiC/C layers were deposited as fiber/matrix interlayers by the isothermal CVI process. The Tyranno-SA/SiC composites exhibited high proportional limit stress (∼120 MPa) and relatively small strain-to-failure. The tensile stress/strain curves exhibited features corresponding to strong interfacial shear and sliding resistance, and indicated failures of all the composites before matrix-cracking saturation was achieved. Fiber/matrix debonding and relatively short fiber pullouts were observed on the fracture surfaces. The ultimate tensile strength displayed an increasing trend with increasing carbon layer thickness up to 100 nm. Further improvement of the mechanical properties of Tyranno-SA/SiC composites is expected with more suitable interlayer structures.     

Interfacial Characterization of Silicon Carbide Fiber/Lithia-Alumina-Silica Glass Matrix Composites

The development of the carbon-rich interphase in Nicalon SiC fiber/Li₂O-Al₂O₃–SiO₂ glass matrix composites was examined as a function of processing parameters with the use of high-resolution scanning electron microscopy and Auger electron spetroscopy. Specifically, hot-pressing temperatures (1000°, 1100°, and 1200°C) and times (15, 30, 60, and 240 min) were systematically varied in such a manner so as to fabricate dense composites suitable for evaluation of reaction kinetics. Carbon-rich interphase thickness, which ranged from 1400 to 5400 Å (140 to 540 nm), was observed to increase with either increasing times at constant temperature or increasing temperatures at constant time. The kinetics of formation of the carbon-rich interphase followed a diffusion-controlled model, with an activation energy of 25.4 kcal/mol.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Thermal Stability of Low-Oxygen Silicon Carbide Fiber (Hi-Nicalon) Subjected to Selected Oxidation Treatment

Low-oxygen silicon carbide fibers (Hi-Nicalon) were oxidized at temperatures from 1073 to 1773 K under an oxygen partial pressure of 0.25 atm. The strength of the unoxidized core was practically unaffected by the oxidation temperatures. The strength of the as-oxidized fibers with an SiO₂ film decreased markedly with increasing oxidation temperature. When exposed subsequently to 1773 K in argon, the fibers with a SiO₂ film of 0.3–0.5 μm thickness had the best thermal stability.