Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

Microstructure, mechanical properties and reaction mechanism of KD-1 SiCf/SiC composites fabricated by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) KD-1 silicon carbide fiber reinforced silicon carbide matrix (KD-1 SiC_f/SiC) composites were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and mechanical properties of the resulting KD-1 SiC_f/SiC composites were studied. The results show that the resulting SiC_f/SiC composites have high bulk density and low open porosity (<6%). The mechanical properties of the resulting SiC_f/SiC composites firstly increase and then decrease with decreasing the open porosity of the SiC_f/C composites. The KD-1 SiC fibers were not severely deformed and adhered to the matrix with a weak interface during the VSI process. As a result, the composites exhibit non-catastrophic failure behavior. Additionally, the diffusion mechanism for the VSI process was also investigated in our work.     

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.     

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。     

Improvement of SiCf/SiC density by slurry infiltration and tape stacking

SiC fiber-reinforced SiC–matrix ceramic composites (SiC_f/SiC) were fabricated by vacuum infiltration of a SiC slurry into Tyranno™-SA grade-3 fabrics coated with a 200 nm-thick pyrolytic carbon (PyC) layer followed by hot pressing using a transient eutectic-phase. The density of the composite was improved using a special infiltration apparatus with a pressure gradient and alternating tape insertion between fabrics. Their overall properties were compared with those of monolithic SiC and composite containing chopped fibers. Although the density of the composites decreased with increasing fiber fraction, SiC_f/SiC containing 50 vol.% fibers had a density of 3.13 g/cm³, which is the highest reported thus far. The composites containing continuous fibers had a maximum flexural strength of 607 MPa and a step increase in the stress–displacement behavior during the three-point bending test due to fiber reinforcement, which was not observed in the monolith.     

化学气相浸渗2D Cf/(SiC-C)复合材料的微观结构与强韧性

采用等温等压化学气相浸渗法(ICVI)制备了二维碳纤维增韧碳化硅碳二元基复合材料(2D C_f/(SiC-C)).利用扫描电镜(SEM)和背散射电子成像(BSE)研究了其基体的微观结构, 并与二维碳纤维增韧碳化硅陶瓷基复合材料(2D C_f/SiC)比较了室温力学性能和断口形貌.结果表明：2D C_f/(SiC-C)复合材料的基体是由SiC与热解碳(PyC)组成的多层结构, PyC基体层分布均匀而连续, 且与SiC基体层结合紧密.纤维束内部PyC基体层较厚的2D C_f/(SiC-C)复合材料具有较高的强韧性, 其拉伸强度、断裂应变、断裂韧性和断裂功分别比2D C_f/SiC复合材料的提高了3%、142%、22%和58%.SiC与PyC组成的多层基体使2D C_f/(SiC-C)复合材料的纤维在拔出过程中发生了两次集中拔出, 且第一次集中拔出的纤维对复合材料的强韧性起主要作用.     

预氧化Cf/SiC陶瓷基复合材料及其构件的抗疲劳特性研究

采用在线销钉集成技术实现了二维C_f/SiC复杂构件的近尺寸成型,并考察预氧化C_f/SiC销钉集成构件的高周疲劳寿命及破坏模式。实验结果表明:C_f/SiC构件在不同激振加速度条件下均表现为由销钉断裂所引起的整体分层破坏,层板连接处为C_f/SiC构件的振动疲劳薄弱部位。通过ANSYS振动应力分析和微观组织分析可以推论出,疲劳试验时,裂纹容易沿着层板间的基体扩展,在基体开裂失效后,全部应力施加于销钉处,最终在疲劳应力作用下销钉发生断裂,导致构件整体分层破坏。     

Interlaminar shear strength of SiC matrix composites reinforced by continuous fibers at 900 °C in air

To reveal the shear properties of SiC matrix composites, interlaminar shear strength (ILSS) of three kinds of silicon carbide matrix composites was investigated by compression of the double notched shear specimen (DNS) at 900 °C in air. The investigated composites included a woven plain carbon fiber reinforced silicon carbide composite (2D-C/SiC), a two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composite (2.5D-C/SiC) and a woven plain silicon carbon fiber reinforced silicon carbide composite (2D-SiC/SiC). A scanning electron microscope was employed to observe the microstructure and fracture morphologies. It can be found that the fiber type and reinforcement architecture have significant impacts on the ILSS of the SiC matrix composites. Great anisotropy of ILSS can be found for 2.5D-C/SiC because of the different fracture resistance of the warp fibers. Larger ILSS can be obtained when the specimens was loaded along the weft direction. In addition, the SiC fibers could enhance the ILSS, compared with carbon fibers. The improvement is attributed to the higher oxidation resistance of SiC fibers and the similar thermal expansion coefficients between the matrix and the fibers.     

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.     

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Interfacial control of uni-directional SiC_f/SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC_f/SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC_f/SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC_f/SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC_f/SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC_f/SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m². In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC_f/SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC_f/SiC composites and to obtain high-performance SiC_f/SiC composites.     

Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)_n composites with a density of 1.43 g/cm³ are 204.4 MPa and 3028 kJ/m³ respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm³. The enhanced mechanical properties of C/(PyC–SiC)_n composites are attributed to the presence of multilayered (PyC–SiC)_n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)_n composite is a promising structural material with low density and high flexural strength and toughness.     

Fabrication of toughened Cf/SiC whisker composites and their mechanical properties

In this paper, dense short carbon fiber reinforced silicon carbide matrix composites had been fabricated by hot-pressed (HP) sintering using Al₂O₃ and La₂O₃ as sintering additives. The results showed that the combination of Al₂O₃ and La₂O₃ system was effective to promote densification of short cut carbon fiber reinforced silicon carbide composites (Cf/SiC). The whisker structure of silicon carbide was formed during the annealed treatment at 2023 K for 1 h. However, it was noted that this structure was not observed in the as-received HP material. The mechanism of forming whisker structure was not clear, but this kind of whisker structure was helpful to improve mechanical properties. The combination of grain bridging, crack deflection and whisker debonding would improve the fracture toughness of the Cf/SiC composites.     

Effect of additive content on transient liquid phase sintering in SiC nanopowder infiltrated SiCf/SiC composites

SiC nanopowder infiltrated SiC_f/SiC composites with a high fiber volume fraction above 50 vol.% were prepared at a relatively low fabrication temperature of 1800 °C by transient liquid phase sintering using Al₂O₃-Y₂O₃-SiO₂ additives. The effects of additive content with 6-18 wt.% were investigated, based on densification, microstructure, mechanical properties and fracture behaviors of the composites. The results showed that the densification and mechanical properties of the composites were greatly improved with the additive content. Microstructural observation indicated that the infiltration of SiC nanopowder inside fiber-bundles were enchanced with the increase of additive content due to the effectively widen space by the reaction between pyrocarbon (PyC) interface and the additives especially with the addition of SiO₂. It proven that the enchanced matrix-PyC interface bonding by the high densification inside fiber-bundles played a key role in the improved mechanical properties and fracture behaviors of the composites.     

The effect on the tensile properties of PIP-processed SiC/SiC composite of a chemical vapor-infiltrated SiC layer overlaid on the pyrocarbon interface layer

A chemical vapor-infiltrated (CVI) SiC layer is often deposited on the pyrocarbon (PyC) fiber–matrix interface layer in SiC fiber-reinforced SiC matrix (SiC/SiC) composites. It is normally applied to protect the PyC layer from reacting with molten Si or sintering aids during manufacturing, and to guard against the effects of high temperature, oxidation and moisture during use. In this study, we investigated the effect of this SiC layer on the tensile properties of a composite. Tensile tests of our composite samples showed the SiC layer to have no noticeable effects on its ultimate load or fracture strain, whereas it decreased the load-to-strain ratio and proportional limit. The test results were analyzed by carrying out element tests on filaments and fiber bundle samples, fracture mirror analysis of pullout fibers, and finite element analysis (FEA) of residual thermal stress around the interface.     

Fatigue properties and fatigue fracture mechanisms of SiC whiskers or SiC particulate-reinforced aluminium composites

Fatigue properties and fracture mechanisms were examined for three commercially fabricated aluminium matrix composites containing SiC whiskers (SiC_w) and SiC particles (SiC_p) using a rotating bending test. The fatigue strengths were over 60% higher for SiC_w/A2024 composites than that for the unreinforced rolled material, while for the SiC_p/A357 composites, fatigue strengths were also higher than that for the unreinforced reference material. For the SiC_p/A356 composites at a volume fraction of 20%, the fatigue strength was slightly higher than that of the unreinforced material. Fractography revealed that the Mode I fatigue crack was initiated by the Stage I mechanism for the SiC_w/A2024 and SiC_p/A357 composites, while for the SiC_p/A356 composite, the fatigue crack initiated at the voids situated beneath the specimen surfaces. On the other hand, the fatigue crack propagated to the whisker/matrix interface following the formation of dimple patterns or the formation of striation patterns for SiC_w/A2024 composites, while for the SiC_p/A356 and SiC_p/A357 composites the fatigue crack propagated in the matrix near the crack origin and striation patterns were found. Near final failure, dimple patterns, initiated at silicon carbide particles, were frequently observed. Mode I fatigue crack initiation and propagation models were proposed for discontinuous fibre-reinforced aluminium composites. It is suggested that the silicon carbide whiskers or particles would have a very significant effect on the fatigue crack initiation and crack propagation near the fatigue limit.     

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.     

Corrosion behavior of SiC p /6061 Al metal matrix composites in chloride solutions

The corrosion behavior of silicon carbide particulates-aluminum metal matrix composites was studied in chloride solution by means of electrochemical techniques, scanning electron microscope (SEM), transmission electron microscope (TEM) and optical microscope. The materials under investigation were compocasting processed 6061 Al reinforced with increasing amounts of SiC particulates. Electrochemical tests such as potentiostatic polarization were done in 0.1 kmol·m^–3 NaCl solutions that were aerated and deaerated to observe overall corrosion behavior. In addition, pit morphology was observed after immersion tests. It was seen that the pitting potentials did not vary greatly or show definite trends in relation to the amounts of SiC _p reinforcement. However, the degree of corrosion increased with increasing SiC _p content; probably mainly due to galvanic couple. No intermetallics layer was found at the SiC _p /Al interface. Based on pitting potentials of Al-Si alloys, a pitting process around SiC particulate was proposed.Abbreviations SiC _p (silicon carbide particulates) - SiC _f (silicon carbide fibers) - SiC _w (silicon carbide whiskers) - E_pit (pitting potential) - E_prot (protection potential) - E _corr (corrosion potential) - i _galv (galvanic current density) - E _galv (galvanic potential)     

连续SiC纤维增强钛基复合材料横向高温变形机理研究

针对连续SiC纤维增强钛基复合材料(SiC_f/Ti)成形的技术难题,利用沿垂直纤维方向基体具有大变形的能力,可以采用超塑成形/扩散连接技术(SPF/DB)成形出复合材料空心构件。在不同工艺参数条件下,测试了SiC_f/Ti复合材料的横向高温变形规律,并分析了变形温度、应变速率、纤维含量等工艺参数的影响规律,对不同参数条件下拉伸试件的微观组织和断口形貌进行了对比,分析了复合材料的高温变形机制。     

Processing and mechanical properties of short fibers/Si3N4p reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Dense short silicon carbide (SiC_sf) and carbon fibers (C_sf) reinforced BaAl₂Si₂O₈ (BAS) glass-ceramic composites with silicon nitride were fabricated by hot-pressing technique. The phase characterization, microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and three-point bending tests. The results showed that short silicon carbide and carbon fibers disperse homogeneously in BAS matrix, and had good chemical compatibility with the glass-ceramic matrix. The addition of Si₃N₄ could successfully eliminate the microcracks in the BAS matrix induced by the thermal mismatch between the fiber and matrix. Both the added short fibers could effectively reinforce the BAS glass-ceramic by the associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects.     

Properties of carbon nano-tubes–Cf/SiC composite by precursor infiltration and pyrolysis process

Carbon nanotubes (CNTs) were introduced into the precursor infiltration and pyrolysis (PIP) carbon fiber reinforced silicon carbide matrix (C_f/SiC) composite via the infiltration slurry. The weight fraction of CNTs in the composite was 0.765‰. The fiber–matrix interface coating was prepared through chemical vapor deposition (CVD) process using methyltrichlorosilane (MTS). Effects of the CNTs on mechanical and thermal properties of the composite were evaluated by three-point bending test, single-edge notched beam (SENB) test, and laser flash method. Attributed to the introduction of the small quantity of CNTs, flexural strength and fracture toughness of the C_f/SiC composite both increased by 25%, and thermal conductivity at room temperature increased by 30%.