不同界面SiC/SiC复合材料的断裂行为研究

碳化硅纤维增强碳化硅复合材料(SiC/SiC)是极具前景的高温结构材料。通过先驱体浸渍裂解(PIP)工艺分别制备了PyC界面和CNTs界面SiC/SiC复合材料, 对两种SiC/SiC复合材料的整体力学性能以及界面剪切强度等进行了测试表征, 并对材料中裂纹的产生与扩展进行了原位观测。结果表明, 两种界面SiC/SiC复合材料弯曲强度相近, 但PyC界面SiC/SiC复合材料的断裂韧性约为CNTs界面SiC/SiC复合材料的两倍。在PyC界面SiC/SiC复合材料中, 裂纹沿纤维-基体界面扩展, PyC涂层能够偏转或阻止裂纹, 材料呈现伪塑性断裂特征; 而在CNTs界面SiC/SiC复合材料中, 裂纹在扩展路径上遇到界面并不偏转, 初始裂纹最终发展为主裂纹, 材料呈现脆性断裂模式。     

界面相对3D-SiC/SiC复合材料静态力学性能及内耗特征的影响

采用先驱体浸渍裂解工艺制备无界面、SiC、PyC和PyC/SiC等界面相SiC/SiC复合材料, 研究了SiC/SiC复合材料的微观结构及静态力学性能, 并通过强迫振动法系统分析了界面相对复合材料内耗行为的影响。研究结果表明, 引入界面相有效改善了复合材料的微观结构及力学性能, 并降低了复合材料的内耗。其中, PyC/SiC复相界面中亚层SiC限制了PyC界面相与纤维的结合及塑性形变, 提高了复合材料的力学性能; 同时, 界面相对SiC/SiC复合材料内耗行为有显著影响, 材料内耗水平与界面剪切强度成反比。对比50和350 ℃时的材料内耗变化率发现, 随界面剪切强度增大, 材料内耗呈降低的趋势, 且含有PyC的PyC/SiC界面复合材料具有较低的内耗变化率, 说明PyC/SiC复相界面的SiC/SiC复合材料更适于高温振动环境。     

Mechanical behavior of SiCf/SiC composites with alternating PyC/SiC multilayer interphases

In order to tailor the fiber–matrix interface of continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites for improved fracture toughness, alternating pyrolytic carbon/silicon carbide (PyC/SiC) multilayer coatings were applied to the KD-I SiC fibers using chemical vapor deposition (CVD) method. Three dimensional (3D) KD-I SiC_f/SiC composites reinforced by these coated fibers were fabricated using a precursor infiltration and pyrolysis (PIP) process. The interfacial characteristics were determined by the fiber push-out test and microstructural examination using scanning electron microscopy (SEM). The effect of interface coatings on composite mechanical properties was evaluated by single-edge notched beam (SENB) test and three-point bending test. The results indicate that the PyC/SiC multilayer coatings led to an optimum interfacial bonding between fibers and matrix and greatly improved the fracture toughness of the composites.     

Design,Fabrication, Characterization and Simulation of PIP-SiC/SiC Composites

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature and fusion applications. Polymer impregnation and pyrolysis (PIP) is a conventional technique for fabricating SiC/SiC composites. In this research, KD-1 SiC fibers were employed as reinforcements, a series of coatings such as pyrocarbon (PyC), SiC and carbon nanotubes (CNTs) were synthesized as interphases, PCS and LPVCS were used as precursors and SiC/SiC composites were prepared via the PIP method. The mechanical properties of the SiC/SiC composites were characterized. Relationship between the interphase shear strength and the fracture toughness of the composites was established. X-ray tomographic scans of the SiC/SiC composites were performed and the closed porosities of the composites were calculated. The compatibility of the SiC/SiC composites with liquid LiPb at 800 °C and 1000 °C was investigated. High-resolution synchrotron X-ray tomography was applied to the SiC/SiC composite and digital volume correlation was employed for Hertzian indentation testing of the SiC/SiC composite. A Cellular Automata integrated with Finite Elements (CAFE) method was developed to account for the effect of microstructure on the fracture behavior of the SiC/SiC composite.     

PyC/SiC界面相对PIP法制备3D HTA C/SiC复合材料性能的影响

利用三维编织炭纤维预制件通过先驱体浸渍裂解法制备C／SiC复合材料。研究了热解碳（PyC）／SiC界面相对复合材料的微观结构和力学性能的影响。弯曲性能通过三点弯曲法测试，复合材料的断口和抛光面通过扫描电镜观察。结果表明：通过等温化学气相沉积法在纤维表面沉积PyC／SiC界面相以后，复合材料的三点抗弯强度从46MPa提高到247MPa。沉积界面的复合材料断口有明显的纤维拔出现象，纤维与基体之间的结合强度适当，起到了增韧作用；而未沉积界面相复合材料的断口光滑、平整，几乎没有纤维拔出，纤维在热解过程中受到严重的化学损伤，性能下降严重，材料表现为典型的脆性断裂。     

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.     

纤维表面涂层对SiC(f)/SiC复相陶瓷力学性能和界面结构的影响

报道了化学气相浸渍（CVI）工艺制备的SiC（f）/SiC复相陶瓷中纤维表面涂层对复合材料力学性能和显微结构的影响。SEM观察表明：C或B N表面涂层改变了SiC（f）/SiC复相陶瓷中纤维与基体间的强界面结合,使断裂过程中的界面解离和纤维拔出大大增加,与此同时材料的断裂韧性和断裂功明显提高。说明C或BN纤维表面涂层能够大大地改善SiC（f）/SiC复相陶瓷的脆性断裂行为模式。高分辨电镜的观察证实在CVI过程初期,纤维表面首先发生石墨界面相的沉积,该界面相具有明显的层状晶格条纹,而纤维表面C涂层为无定型态。     

预制体结构及界面对三维SiC/SiC复合材料拉伸性能的影响

为研究预制体结构及界面对三维编织SiC/SiC复合材料拉伸性能的影响,采用先驱体浸渍裂解法（PIP）分别制备了三维四向和三维五向SiC/SiC复合材料,并引入热解炭/碳化硅（PyC/SiC）复合界面层,进行拉伸性能测试和断口形貌观察。结果表明,三维五向SiC/SiC复合材料拉伸性能优于三维四向SiC/SiC复合材料,三维五向SiC/SiC复合材料的拉伸强度、模量和断裂应变分别是三维四向SiC/SiC复合材料的1.22倍、1.25倍、1.43倍,且比三维四向SiC/SiC复合材料具有更好的强度可靠性。这是由于三维五向SiC/SiC复合材料增加了受力方向的纤维含量,限制了纤维在外力作用下的转动和变形,起到定型和稳固作用。添加PyC/SiC复合界面层,三维五向SiC/SiC复合材料的拉伸强度、模量及断裂应变分别提高了21.7%、15.0%和11.0%。界面的存在可以保护纤维,调节纤维与基体之间的热应力,受力时诱使裂纹偏转和分叉,消耗能量,提高三维五向SiC/SiC复合材料的拉伸性能。      

Mechanical properties of 3D KD-I SiCf/SiC composites with engineered fibre-matrix interfaces

Three-dimensional (3D) silicon carbide (SiC) matrix composites reinforced with KD-I SiC fibres were fabricated by precursor impregnation and pyrolysis (PIP) process. The fibre-matrix interfaces were tailored by pre-coating the as-received KD-I SiC fibres with PyC layers of different thicknesses or a layer of SiC. Interfacial characteristics and their effects on the composite mechanical properties were evaluated. The results indicate that the composite reinforced with as-received fibre possessed an interfacial shear strength of 72.1 MPa while the composite reinforced with SiC layer coated fibres had a much higher interfacial shear strength of 135.2 MPa. However, both composites showed inferior flexural strength and fracture toughness. With optimised PyC coating thickness, the interface coating led to much improved mechanical properties, i.e. a flexural strength of 420.6 MPa was achieved when the interlayer thickness is 0.1 μm, and a fracture toughness of 23.1 MPa m^1/2 was obtained for the interlayer thickness of 0.53 μm. In addition, the composites prepared by the PIP process exhibited superior mechanical properties over the composites prepared by the chemical vapour infiltration and vapour silicon infiltration (CVI-VSI) process.     

Effect of Temperature on the Synthesis of SiC Coating on Carbon Fibers by the Reaction of SiO with the Deposited Pyrolytic Carbon Layer

The influence of reaction temperature on the preparation of SiC coating on carbon fibers by the reaction of silicon monoxide with the deposited pyrolytic carbon (PyC) layer has been discussed. With rising reaction temperature, the thickness of SiC layer increases and the SiC grain is coarsening. The apparent activation energy for the synthesis of SiC layer is about 103.3 kJ/mol. The oxidation resistance of carbon fiber can be improved by the SiC/PyC layers significantly. The initial oxidation temperature of the SiC/PyC coated carbon fiber is about 300°C higher than that of the uncoated carbon fiber. The oxidation of the SiC/PyC coated carbon fiber is owing to the diffusion of oxygen through the cracks generated by the mismatch of thermal expansion.     

CVD-SiC界面改性涂层对气相渗硅制备C_f/SiC复合材料力学性能的影响

在气相渗硅制备C_f/SiC复合材料时,界面改性涂层非常重要。良好的界面改性涂层一方面起到保护碳纤维不受Si反应侵蚀的作用,另一方面起到调节纤维和基体界面结合状况。通过在C纤维表面制备CVD-SiC涂层来进行界面改性,研究CVD-SiC界面改性涂层对GSI C_f/SiC复合材料力学性能和断裂特征的影响,并分析其影响机制。结果表明:无CVD-SiC涂层改性的C_f/SiC复合材料力学性能较差,呈现脆性断裂特征,其强度、模量和断裂韧度分别为87.6MPa,56.9GPa,2.1MPa·m1/2。随着CVD-SiC涂层厚度的增加,C_f/SiC复合材料的弯曲强度、模量和断裂韧度呈现先升高后降低的趋势,CVD-SiC涂层厚度为1.1μm的C_f/SiC复合材料的力学性能最好,其弯曲强度、模量和断裂韧度分别为231.7MPa,87.3GPa,7.3MPa·m1/2。厚度适中的CVD-SiC界面改性涂层的作用机理主要体现在载荷传递、"阻挡"Si的侵蚀、"调节"界面结合状态3个方面。     

Cf/ZrC-ZrB2-SiC-C超高温陶瓷复合材料的显微结构表征

利用X-射线衍射、扫描电子显微镜和透射电子显微镜对Cf/ZrC-ZrB2-SiC-C超高温陶瓷复合材料的相组成、纤维/热解碳层的界面特征和超高温陶瓷基体的显微结构特征进行了表征。在碳纤维表面有一层厚度为2～3μm石墨化程度较高的热解碳界面层,该界面层可以避免采用PIP工艺制备超高温陶瓷基体时可能对碳纤维造成的损伤。热解碳层与碳纤维之间为弱机械结合,其界面间分布着20～30 nm的ZrC纳米颗粒。Cf/ZrC-ZrB2-SiC-C超高温陶瓷复合材料基体主要由ZrC,ZrB2,SiC和石墨相(Cg)组成。基体中石墨的(002)面沿着ZrC,ZrB2或SiC的表面生长。在石墨与ZrB2和石墨与SiC的界面没有观察到取向关系,界面处既没有反应层也没有非晶相存在。在石墨与ZrC之间存在ZrC(111)∥Cg(002),ZrC[110]∥Cg[010]的取向关系。ZrB2和SiC之间也没有界面反应和非晶层存在。     

Oxidation behavior of 3D Hi–Nicalon/SiC composite

Oxidation behavior of a three dimensional (3D) Hi–Nicalon/SiC composite with CVD SiC coating was investigated in the simulated air using a thermogravimetric analysis (TGA) device. Below 1100 °C, the oxidation kinetics was controlled by gas diffusion through the defects in the SiC matrix and coating and resulted in the consumption of PyC interphase. The residual flexural strength did have not a remarkable fluctuation and the relationship between the residual strength to temperature and weight change to temperature of the 3D Hi–Nicalon/PyC/SiC composite indicated the same regularity. Above 1200 °C, the oxidation kinetics was controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating and matrix. And the residual flexural strength of the composites was governed by the strength degradation of the Hi–Nicalon fiber. After oxidation, the fracture displacement in flexural tests increased with the weight loss increasing and the fracture mode showed a non-brittle pattern.     

SiCf/SiC复合材料界面相研究进展

分析了连续纤维增强陶瓷基复合材料(CFRCMCs)中界面相类型以及各界面相在CFRCMCs中的作用,综述了热解碳(PyC)、氮化硼(BN)、难熔氧化物以及复合界面相在SiCf/SiC复合材料中的应用现状,最后展望了SiCf/SiC复合材料界面相的发展方向。     

Localized mechanical property assessment of SiC/SiC composite materials

SiC fiber-reinforced SiC matrix composites (SiC/SiC) are under consideration as a structural material for a range of nuclear applications. While these materials have been studied for decades, recently new small scale materials testing techniques have emerged which can be used to characterize SiC/SiC materials from a new perspective. In this work cross section nanoindentation was performed on SiC/SiC composites revealing that both the hardness and Young’s modulus was substantially lower in the fiber compared to the matrix despite both being SiC. Using scanning electron microscopy it was observed that the grain growth of the matrix during formation was radially out from the fiber with a changing grain structure as a function of radius from the fiber center. Focused ion beam machining was used to manufacture micro-cantilever samples and evaluate the fracture toughness and fracture strength in the matrix as a function of grain orientation in the matrix. Additionally microstructural characterization techniques like Raman spectroscopy, X-ray diffraction, and microtomography were used to evaluate differences in the matrix and fibers of the composite.     

疲劳氧化环境中3D SiCf/SiC复合材料损伤演变

研究了疲劳氧化环境中3D SiC_f/SiC复合材料损伤演变,并对该环境中SiC_f/SiC复合材料的失效机制进行了分析。结果表明,疲劳氧化环境中3D SiC_f/SiC复合材料的损伤主要为：基体开裂;热解碳界面相脱粘、氧化及取向性排列;纤维断裂、氧化及其结构的改变。这些损伤使氧化性气体通过基体开裂形成的裂纹不断氧化复合材料内部;界面相脱粘和取向性排列使纤维更容易拔出;纤维的氧化、纤维中无定形碳的增多以及SiC晶粒的长大导致纤维强度降低。     

Designing the fiber volume ratio in SiC fiber-reinforced SiC ceramic composites under Hertzian stress

Finite element method (FEM) analysis and experimental studies are undertaken on the design of the fiber volume ratio in silicon carbide (SiC) fiber-reinforced SiC composites under indentation contact stresses. Boron nitride (BN)/Pyrocarbon (PyC) are selected as the coating materials for the SiC fiber. Various SiC matrix/coating/fiber/coating/matrix structures are modeled by introducing a woven fiber layer in the SiC matrix. Especially, this study attempts to find the optimum fiber volume ratio in SiC fiber-reinforced SiC ceramics under Hertzian stress. The analysis is performed by changing the fiber type, fiber volume ratio, coating material, number of coating layers, and stacking sequence of the coating layers. The variation in the stress for composites in relation to the fiber volume ratio in the contact axial or radial direction is also analyzed. The same structures are fabricated experimentally by a hot process, and the mechanical behaviors regarding the load–displacement are evaluated using the Hertzian indentation method. Various SiC matrix/coating/fiber/coating/matrix structures are fabricated, and mechanical characterization is performed by changing the coating layer, according to the introduction (or omission) of the coating layer, and the number of woven fiber mats. The results show that the damage mode changes from Hertzian stress to flexural stress as the fiber volume ratio increases in composites because of the decreased matrix volume fraction, which intensifies the radial crack damage. The result significantly indicates that the optimum fiber volume ratio in SiC fiber-reinforced SiC ceramics should be designed for inhibiting the flexural stress.     

SiC／Al复合材料断裂机制的声发射研究

用ＳｉＣ单纤维Ａｌ基模型复合材料使断裂源单一化，测定这些断裂源的声发射特征，成功地用声发射特征区分知识复合材料中的纤维断裂，纤维－Ａｌ基体间的界面断裂、基体断裂，结合金相观察，分析了这些断裂产生不同声发射特征的原因。     

Influences of Fe-impurity on the production process of SiC fiber reinforced Al for electric cables

As electrical power demands increase every year, the need becomes stronger for light weight electric cables which have high transmission capacity, high thermal resistance and low sag. We have developed a SiC fiber reinforced aluminum electrical cable to meet this need. Mechanical properties of the SiC/Al composite conductor are very susceptible to iron impurity which becomes mixed in the Al matrix during manufacture of the composite conductor. In this work, we studied the effects of Fe impurity in Al on fracture behavior of the composite conductor. A preformed wire was prepared by dipping a bundle of 1500 pieces of SiC fiber (Si: 63.7, C: 35.8, O: 12.3 mass %) into molten Al in which 0.36 mass % Fe and 0.5 mass % Ti were mixed. The Ti was added to improve the wetting property. Test samples were prepared by bundling seven preformed wires together. A tensile test was carried out for the composite conductor, and pull-out behavior of SiC fiber at the fracture surface was observed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Pull-out of SiC fiber was observed at the fracture surface of the composite conductor using Fe-free Al. However, pull-out of SiC fiber was not observed at the fracture surface of the composite conductor using Fe-containing Al since Al was combined inseparably with the SiC and Fe. The fracture origin of the Fe-containing sample was a precipitated Fe-compound at the SiC/Al interface. Tensile strength of the Fe-containing sample was a half of that of the Fe-free sample. We propose the following the precipitation mechanism for the Fe compound. In manufacturing of the preformed wire, molten Al solidifies from the surface to the SiC/Al interface because of the low thermal conductivity of the SiC fiber. In the cooling process, Fe-free Ti-compound precipitates in the molten Al by a peritectic reaction. This leads to a higher concentration of Fe in the molten Al near the interface, and finally, FeAl₃ compound precipitates at the SiC/Al interface.     

碳化硅纳米线增韧碳化硅纤维/碳化硅基体损伤行为研究

通过在碳化硅纤维表面原位生长纳米线得到具有多级增强结构的碳化硅复合材料, 对复合材料引入纳米线后的微观结构、弯曲强度以及损伤的变化过程进行了研究。研究结果表明, 相较于原始的碳化硅纤维增强碳化硅复合材料, 碳化硅纳米线可以明显提高基体沉积效率并改善材料的弯曲力学性能。从声发射技术和维氏硬度压痕测试结果可以看出, 纳米线通过抑制微裂纹的产生和在微裂纹之间发生桥联来抑制早期损伤的发展。此外, 在纳米线表面沉积一层氮化硼界面相, 纳米线与基体之间的结合力变弱, 复合材料对微裂纹的抑制和偏转得到进一步增强, 弯曲性能大幅提升。