Microstructure and Mechanical Properties of Compact SiC/SiC Composite Fabricated with an Infiltrative Liquid Precursor

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature and fusion applications. In this study, SiC/SiC composite was fabricated by polymer impregnation and pyrolysis process with LPVCS, a liquid precursor with active Si–H and ‐CH=CH₂ groups. The cross‐link and ceramization processes of LPVCS were studied and SiC/SiC composite was fabricated with LPVCS. The porosity and mechanical properties of the SiC/SiC composite was investigated, and the results indicated that the SiC/SiC composite exhibited low porosity and superior mechanical properties owing to the compact matrix derived from LPVCS.     

A New SiC-Whisker-Reinforced Lithium Aluminosilicate Composite

The glass-ceramic matrix of the well-known lithium aluminosilicate (LAS)/SiC composite is usually formulated near the spodumene composition. We report here a new composition which is rich in alumina (78 wt%) and lean in silica (21 wt%) and lithia (1 wt%). This formulation offers a new option of converting the glass-ceramic matrix to a mullite/alumina matrix upon annealing above 1400°C, and hence better creep resistance and other high-temperature mechanical properties. Using a transient-phase processing method that we developed previously for the superplastic forming of mullite, we are able to hot-press a composite containing 30 vol% SiC whiskers at ∼1350°C to achieve full density. Flexural strength measurements up to 1400°C have confirmed the improved high-temperature strength and creep resistance over conventional LAS. The fracture toughness is also higher than that of LAS. The results suggest that the new composition may be chosen as a better candidate matrix for SiC-fiber-reinforced composites.     

Enhanced microwave‐absorbing property of precursor infiltration and pyrolysis derived SiCf/SiC composites at X band: Role of carbon‐rich interphase

Ceramic matrix composites (CMCs) can be microwave‐absorbent when endowing the composite constituents with proper dielectric properties. In this work, we report a new method to enhance the microwave‐absorbing property of CMCs by in situ fabrication of a carbon‐rich interphase at the fiber/matrix interface. This was achieved in a SiC fiber reinforced SiC matrix (SiCf/SiC) composite fabricated by precursor infiltration and pyrolysis (PIP). We found that as the PIP temperature increased from 800 to 1000°C, the microwave‐absorbing property of the SiCf/SiC composite was significantly enhanced at X band, which also surpassed those of the SiC fiber and monolithic SiC ceramic fabricated at the same temperature. The dominant mechanism was studied by decoupling the effect of individual SiC fibers, SiC matrix, and fiber/matrix interface. The results showed that the SiC fiber and SiC matrix were barely microwave‐absorbent, due to their low dielectric losses. The microwave‐absorbing mechanism was finally ascribed to the fiber/matrix interface, which was carbon‐rich, containing Si and O elements. The interphase showed a conductivity that was superior to that of the fiber and the matrix, and mainly dominated the dielectric property of the overall composite. The results highlight the role of carbon‐rich interphase on the microwave‐absorbing property of CMCs.     

Fabrication and properties of 2D C/C–ZrB2–ZrC–SiC composites by hybrid precursor infiltration and pyrolysis

Abstract

Two dimensional C/C–ZrB₂–ZrC–SiC composites were fabricated through precursor infiltration and pyrolysis process using a mixture of polycarbosilane and ZrB₂ precursor and ZrC precursor as the impregnant. The microstructures, mechanical properties and ablation properties of the composites were investigated. The results showed that the homogeneity of the composite improved on using novel precursors that can dissolve with polycarbosilane through the formation of nanocomposite matrix. The flexural strength and fracture toughness first increased and then decreased on increasing the pyrocarbon content in the composite. Compared with the C/C–SiC composite, the ablation resistance of C/C–ZrB₂–ZrC–SiC composite was greatly enhanced. The mass loss rate and linear recession rate exposed to the plasma torch were 1?7 mg/s and 1?8 μm/s, respectively. The formation of a ZrO₂–SiO₂ glassy layer on the surface significantly contributed to the excellent ablative property of the composite.     

传统陶瓷工艺制备玻璃陶瓷/SiC晶须复合材料

针对传统陶瓷工艺难以直接用于制备玻璃陶瓷/SiC晶须复合材料问题,本文以白云鄂博尾矿基透辉石系玻璃粉和商用SiC晶须为主要原料,在石墨粉包埋条件下,采用传统陶瓷工艺成功制备出透辉石玻璃陶瓷/SiC晶须复合材料.在此基础上,研究了填加0～40wt％SiC晶须对所制备复合样品显微结构及性能的影响.结果证明了所制备复合材料样...     

Effect of oxidation treatment on KD–II SiC fiber–reinforced SiC composites

Continuous silicon carbide fiber–reinforced silicon carbide matrix (SiC_f/SiC) composites have developed into a promising candidate for structural materials for high–temperature applications in aerospace engine systems. This is due to their advantageous properties, such as low density, high hardness and strength, and excellent high temperature and oxidation resistance. In this study, SiC_f/SiC composites were fabricated via polymer infiltration and pyrolysis (PIP) with the lower–oxygen–content KD–II SiC fiber as the reinforcement; a mixture of 2,4,6,8–tetravinyl–2,4,6,8–tetramethylcyclotetrasiloxane (V4) and liquid polycarbosilane (LPCS), known as LPVCS, was used as the precursor; while pyrolytic carbon (PyC) was used as the interface. The effects of oxidation treatment at different temperatures on morphology, structure, composition, and mechanical properties of the KD–II SiC fibers, SiC matrix from LPVCS precursor conversion, and SiC_f/SiC composites were comprehensively investigated. The results revealed that the oxidation treatment greatly impacted the mechanical properties of the SiC fiber, thereby significantly influencing the mechanical properties of the SiC_f/SiC composite. After oxidation at 1300 °C for 1 h, the strength retention rates of the fiber and composite were 41% and 49%, respectively. In terms of the phase structure, oxidation treatment had little effect on the SiC fiber, while greatly influencing the SiC matrix. A weak peak corresponding to silica (SiO₂) appeared after high–temperature treatment of the fiber; however, oxidation treatment of the matrix led to the appearance of a very strong diffraction peak that corresponds to SiO₂. The analysis of the morphology and composition indicated cracking of the fiber surface after oxidation treatment, which was increasingly obvious with the increase in the oxidation treatment temperature. The elemental composition of the fiber surface changed significantly, with drastically decreased carbon element content and sharply increased oxygen element content.     

Fracture Behavior of SiC-Whisker-Reinforced Barium Aluminosilicate Glass-Ceramic Matrix Composites

Barium aluminosilicate (BAS) glass-ceramic composites reinforced with various volume percents (0, 10, 20, 30, 40 vol%) of SiC whiskers were fabricated by hot pressing. The microstructure, the whisker/matrix interface structure, the phase constitution, and the mechanical properties of the composites were systematically studied by means of SEM, TEM, and XRD techniques as well as by indentation crack microfracture and single-edge-notched-beam bend testing. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrices. The addition of active Al₂O₃ to the BAS matrix reduced the amount of SiO₂ in the matrix, forming needlelike mullite, which further improved the mechanical properties.     

High-Temperature Tensile Behavior of a Boron Nitride-Coated Silicon Carbide-Fiber Glass-Ceramic Composite

Tensile properties of a cross-ply glass-ceramic composite were investigated by conducting fracture, creep, and fatigue experiments at both room temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass-ceramic matrix reinforced with SiC fibers with a SiC/BN coating. The material exhibited retention of most tensile properties up to 1200°C. Monotonic tensile fracture tests produced ultimate strengths of 230–300 MPa with failure strains of ∼1%, and no degradation in ultimate strength was observed at 1100° and 1200°C. In creep experiments at 1100°C, nominal steady-state creep rates in the 10⁻⁹ s⁻¹ range were established after a period of transient creep. Tensile stress rupture experiments at 1100° and 1200°C lasted longer than one year at stress levels above the corresponding proportional limit stresses for those temperatures. Tensile fatigue experiments were conducted in which the maximum applied stress was slightly greater than the proportional limit stress of the matrix, and, in these experiments, the composite survived 10⁵ cycles without fracture at temperatures up to 1200°C. Microscopic damage mechanisms were investigated by TEM, and microstructural observations of tested samples were correlated with the mechanical response. The SiC/ BN fiber coatings effectively inhibited diffusion and reaction at the interface during high-temperature testing. The BN layer also provided a weak interfacial bond that resulted in damage-tolerant fracture behavior. However, oxidation of near-surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was observed when samples were dynamically loaded above the proportional limit stress, creating micro-cracks along which oxygen could diffuse into the interior of the composite.     

Correlating the Mechanical Properties of a Continuous Fiber-Reinforced Ceramic-Matrix Composite to the Sliding Resistance of the Fibers

The sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS III) glass-ceramic matrix has been correlated with the mechanical properties of the composite. Push-down measurements of the sliding resistance for individual fibers show an order of magnitude increase when the ceramic composite is annealed in air for 4 h at 800°C. Such increases are correlated with a loss of toughness, a reduction in the ultimate strength, and an increase in the matrix cracking stress of the composite, as measured using tension tests and bend tests of bulk samples.     

碳化硅纤维热处理对单向碳化硅纤维增强磷酸铝基复合材料性能的影响

用不同条件热处理的碳化硅纤维制备了单向连续碳化硅纤维增强磷酸铝基复合材料.研究了碳化硅纤维热处理的温度、时间及热处理方法对制成的复合材料性能的影响.测试了复合材料的断裂强度,相对介电常数和介电损耗.通过扫描电镜分析复合材料的微观形貌,并使用电子探针对碳化硅纤维/磷酸铝基体界面进行了微区元素分析.结果表明:碳化硅纤维热处理降低了复合材料的介电常数和介电损耗;纤维/基体界面之间未发生任何化学反应.由于热处理使纤维/基体形成了强结合界面,大大降低了复合材料的力学性能.快速热处理方式直接降低纤维的自身强度.