Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

微米、纳米SiC表面涂覆、改性的方法及研究现状

介绍了微米、纳米SiC表面涂覆、改性的实验方法,探讨了各种方法的特点.从微米、纳米SiC的三种不同形态入手,详细介绍了微米、纳米SiC的表面涂覆、改性的研究进展,并对微米、纳米SiC表面涂覆、改性技术给出了简要评价.     

Microscopic and Spectroscopic Characterization of Stacking‐Sequence Disordered SiC

Nanometric silicon carbide (SiC) powder (~5 nm) with a stacking‐sequence disordered structure (SD‐SiC), synthesized from elemental powders of Si and C, was investigated by microscopic and several spectroscopic methods. The structure of SD‐SiC was characterized by transmission electron microscopy (TEM), ¹³C, and ²⁹Si‐NMR, and by infrared (IR), Raman, and X‐ray photoelectron spectroscopy (XPS) methods. TEM characterizations showed relatively large deviations of the lattice parameters in the as‐synthesized SiC, indicative of the presence of stacking‐sequence disorder. IR analysis showed a weaker Si‐C bond in the SD‐SiC than in the 3C‐SiC. XPS determinations showed that C and Si in SD‐SiC are similar to those in 3C‐SiC. Broader peaks of ²⁹Si and ¹³C MAS‐NMR also indicate that the structure of SD‐SiC is different from that of 3C‐SiC. Raman spectroscopy exhibited activities for the crystalline polytypes and the amorphous of SiC but lack of them for the SD‐SiC. The inactivity of Raman spectroscopy for the SD‐SiC along with large deviation of the lattice constant and the extremely broad X‐ray diffraction peaks would indicate that SD‐SiC is a possible intermediate state between conventional polytype SiC and amorphous SiC, that is, a possible new type of SiC.     

Flexural behaviors and microstructures of SiC/SiC composites fabricated by microwave sintering assisted with heat molding process

To improve densification degree and reduce process time, microwave sintering and heat molding method were combined to prepared SiC matrix reinforced SiC (SiC/SiC) composite via polymer infiltration and pyrolysis process (PIP). The effects of heat molding pressures on the densification process, flexural behaviors and failure modes of the fabricated SiC/SiC were examined via scanning electron microscopy (SEM), computed tomography (CT) technique and mercury intrusion test. Results indicate that heat molding process promoted the densification degrees of SiC/SiC and adjusted the interphase bonding between SiC matrix and SiC fibers on the basis of rapid microwave heating. Owing to the appropriate interphase bonding, SiC/SiC composites fabricated under the heat molding pressure of 3 MPa had preferable flexural properties and failure mode.     

Thermal plasma synthesis of Si/SiC nanoparticles from silicon and activated carbon powders

In this study, Si/SiC nanocomposites were synthesized by non-transferred arc thermal plasma processing of micron-sized SiC powder. First, micron-sized SiC was synthesized by solid-state method where waste silicon (Si) and activated carbon (C) powder were used as precursor materials. The effect of Si/C mole ratio and solid-state synthesis temperature on structural and phase formation of SiC was investigated. Diffraction pattern confirmed the formation of SiC at 1300 °C. High C content was required for the synthesis of pure SiC as Si remained unreacted when Si/C mole ratio was below 1/1.5. Highly agglomerated micron-size (0.6–10 µm) SiC particles were formed after solid-state synthesis. Thermal plasma processing of solid-state synthesized micron-sized SiC resulted into the formation of uniformly dispersed (20–60 nm) Si/SiC nanoparticles. It was proposed that Si/SiC nanocomposites were formed due to partial decomposition of SiC during high temperature plasma processing. The formation of Si/SiC nanoparticles from micron-sized SiC was resulted from dissociation of grains from their grain boundary during plasma processing.     

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.     

Preparation and characterization of surface modified silicon carbide/polystyrene nanocomposites

A simple method is reported to coat silicon carbide (SiC) nanoparticles with polystyrene (PS) to improve the interfacial adhesion between polymer matrix and SiC nanoparticles. The morphology of untreated SiC nanoparticles, PS coated SiC (p‐SiC) nanoparticles, SiC/PS nanocomposites, and p‐SiC/PS nanocomposites are observed. The HRTEM image of p‐SiC shows that the thickness of PS on the surface of SiC is about 1.5–2.0 nm, which is consistent with the TGA results. With 24.7 vol % untreated SiC nanoparticles dispersed into PS matrix, the thermal conductivity (λ) of the SiC/PS composites increases by about 192%. However, when the same volume fraction of p‐SiC nanoparticles is used, the increase is about 353%. This big difference could be attributed to the promoted dispersion of the p‐SiC in the PS matrix. The measurements of glass transition (T_g), dielectric constant (ε), and tensile strength at break (σ_b) also support this explanation. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Preparation and tensile behavior of SiCf/SiC mini composites containing different types and thicknesses of interphase

SiC fiber-reinforced SiC matrix (SiC_f/SiC) composites are promising materials for high-temperature structural applications. In this study, KD-II SiC fiber bundles with a C/Si ratio of approximately 1.25 and an oxygen amount of 2.53%, were used as reinforcement. PyC interphase, PyC-SiC co-deposition interphase I and II, with different thicknesses, and SiC matrix were deposited into the SiC fiber bundles by using chemical vapor infiltration (CVI) to form SiC_f/SiC mini composites. When the thickness of the interphase is approximately 1000 nm, the ultimate tensile stress and strain of SiC_f/SiC mini composites with PyC-SiC co-deposition interphase I can reach 1120.0 MPa and 0.72%, respectively, which are significantly higher than those of SiC_f/SiC mini composites with a PyC interphase (740.0 MPa, 0.87%) and PyC-SiC co-deposition interphase II (645.0 MPa, 0.54%). The effect of thicknesses and types of interphase on tensile fracture behavior of mini composites and then the fracture mechanism are discussed in detail.     

The mechanical,thermophysical and electromagnetic properties of UD SiCf/SiC composites in different directions

Unidirectional (UD) silicon carbide (SiC) fiber-reinforced SiC matrix (UD SiC_f/SiC) composites with CVI BN interphase were fabricated by polymer infiltration-pyrolysis (PIP) process. The effects of the anisotropic distribution of SiC fibers on the mechanical properties, thermophysical properties and electromagnetic properties of UD SiC_f/SiC composites in different directions were studied. In the direction parallel to the axial direction of SiC fibers, SiC fibers bear the load and BN interphase ensures the interface debonding, so the flexural strength and the fracture toughness of the UD SiC_f/SiC composites are 813.0 ± 32.4 MPa and 26.1 ± 2.9 MPa·m^1/2, respectively. In the direction perpendicular to the axial direction of SiC fibers, SiC fibers cannot bear the load and the low interfacial bonding strengths between SiC fiber/BN interphase (F/I) and BN interphase/SiC matrix (I/M) both decrease the matrix cracking stress, so the corresponding values are 36.6 ± 6.9 MPa and 0.9 ± 0.5 MPa?m^1/2, respectively. The thermal expansion behaviors of UD SiC_f/SiC composites are similar to those of SiC fibers in the direction parallel to the axial direction of SiC fibers, and are similiar to those of SiC matrix in the direction perpendicular to the axial direction of SiC fibers. The total electromagnetic shielding effectiveness (EM SE_T) of UD SiC_f/SiC composites attains 32 dB and 29 dB when the axial direction of SiC fibers is perpendicular and parallel to the electric field direction, respectively. The difference of conductivity in different directions is the main reason causing the different SE_T. And the dominant electromagnetic interference (EMI) shielding mechanism is absorption for both studied directions.     

Microstructure and Thermal Conductivity of Silicon Carbide with Yttria and Scandia

A fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 vol% Y₂O₃–Sc₂O₃ additives. The ceramic had a bimodal microstructure consisting of large and small equiaxed SiC grains. Observation with high‐resolution transmission electron microscopy (HRTEM) showed two kinds of homophase (SiC/SiC) boundaries, that is crystallized and clean boundaries, and a fully crystallized junction phase. The thermal conductivity of the SiC ceramic was 234 W (m·K)^?1 at room temperature. The high thermal conductivity was attributed to a clean SiC lattice and good contiguity between SiC grains.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.     

Fabrication and Properties of Ti3SiC2/SiC Composites

Ti3SiC2/SiC composites were fabricated by reactive hot pressing method. Effects of hot pressing temperature, the content and particle size of SiC on phase composition, densification, mechanical properties and behavior of stress-strain of the composites were investigated. The results showed that ： （ 1 ） Hot-pressing temperature influenced the phase composition of Ti3SiC2/SiC composites. The flexural strength and fracture toughness of composites increased with hot pressing temperature. （2） It became more difficult for the composites to densify when the content of SiC in composites increased. It need be sintered at higher temperature to get denser composite. The flexural strength and fracture toughness of composites increased when the content of SiC added in composites increased. However, when the content of SiC reached 50 wt%, the flexural strength and fracture toughness of composites decreased due to high content of pore in composites. （3） When the content of SiC was same, Ti3SiC2/SiC composites were denser while the particle size of SiC added in composites is 12. 8 μm compared with the composites that the particle size of SiC added is 3 μm. The flexural strength and fracture toughness of composites increased with the increase of particle size of SiC added in composites. （4） Ti3SiC2/SiC composites were non-brittle fracture at room temperature.     

Influence of precursor concentration on the densification efficiency and properties of SiC/SiC composites

The SiC/SiC composites were manufactured by polymer precursor impregnation pyrolysis process with near stoichiometric SiC fiber 2D preform as the reinforcing phase, the mixed solution of polycarbosilane (PCS), and xylene as impregnant. The effects of PCS concentration on the densification process, microstructure, and mechanical behavior of SiC/SiC composites were investigated using mechanical property testing, scanning electron microscopy, and other characterization techniques. Results showed the porosity and flexural strength of SiC/SiC composites increased first and then decreased with the increase of PCS concentration. When the concentration of PCS was 55% and 60%, the flexural strength of SiC/SiC composites reached 565.77 and 573.02 MPa, respectively. The mechanical behavior of SiC/SiC composites presented typical pseudoplastic characteristics such as fiber pulling-out, fiber bridging, and interface layer peeling, which would meet the dual requirements of optimizing the matrix and interface structure.     

聚丙烯/石墨烯微片/碳化硅复合材料微观形态与导电导热性能研究

采用熔融共混法制备聚丙烯/石墨烯微片/碳化硅(PP/GNP/SiC)复合材料,研究了SiC用量对PP/GNP/SiC复合材料的微观形态、结晶度和导电导热性能的影响。SEM与XRD测试结果表明,SiC粒子有助于提高拉伸力场对GNP的剥离效果以及GNP在PP基体中的分散程度,随着SiC粒子用量的增加,GNP的片径尺寸和片层厚度均减小,并与SiC粒子相互搭接。导电导热分析结果表明,随着SiC粒子用量的增加,PP/GNP/SiC复合材料的电导率先升高后降低,热导率逐渐提高。SiC用量为5%时,复合材料的电导率最高;SiC用量为20%时,复合材料的热导率最高。     

Preparation and characterization of pure SiC filters with an asymmetric structure

In this work, SiC filters with an asymmetric structure were successfully constructed, consisting of solid-state-sintered SiC (S–SiC) supports and in-situ synthesized SiC membranes. The S–SiC supports were prepared by a pressureless sintering technique. Following the preparation of the supports, SiC membranes ~30 μm in thickness were formed in-situ on their surfaces through a dip-coating process combined with carbothermal reduction. Using X-ray diffraction, only the peaks of the SiC phase were detected for both the S–SiC supports and the SiC membranes, indicating the high purity of the asymmetric SiC filters. The distribution of the pore throat size in the SiC membranes was narrow with an average value of ~130 nm, suggesting a high filtration precision. The flexural strengths of the S–SiC supports at room temperature and 1000 °C were 123.6 ± 18.1 MPa and 114.0 ± 4.5 MPa, respectively, enabling the asymmetric SiC filters to withstand high temperatures and high pressures during service. Moreover, asymmetric SiC filters presented excellent acid and alkali corrosion resistance, indicating their great potential for applications in corrosive environments.     

Synthesis of a silicon carbide coating on carbon fibers by deposition of a layer of pyrolytic carbon and reacting it with silicon monoxide

A method for preparing a SiC coating on carbon fibers is presented. The SiC coating was generated from the reaction of silicon monoxide (SiO) with a pyrolytic carbon (PyC) layer deposited on the fibers. The influence of holding time on the microstructure of the SiC layer was discussed. The oxidation behaviors of the uncoated and SiC coated carbon fibers were compared. The formation mechanism of the SiC coating was evaluated. With increased reaction time, the SiC coating becomes thicker and its surface becomes rough. The oxidation resistance of the carbon fiber was improved by the SiC coating. The initial oxidation temperature of the SiC coated carbon fiber is about 200 °C higher than that of the uncoated carbon fiber. The growth of the SiC coating is mainly attributed to the indirect reactions of SiO with PyC in the SiO/SiC/C system, in which silicon is considered a critical intermediate product.     

Comparison on Microstructure and Infrared Emissivity Properties of 3D Needled Composites

Three‐dimensional (3D) needled composites, C/C, C/C‐SiC, and C/SiC, were prepared and their infrared emissivity properties in 1000–1600°C were investigated. Results showed that the infrared emissivity of all composites increased almost linearly with temperatures. In comparison with C/C, C/C‐SiC composites reduced by 10% in the total emissivity. Twenty‐two vol% SiC in C/C‐SiC composites caused a low zone in 10–14 μm, corresponding to the theoretical emissivity of SiC. For C/SiC composites prepared by CVI, 68 vol% SiC caused a “V” shape in the spectral emissivity. The high SiC content also endued the high infrared emissivity properties for C/SiC composites.     

沉积温度和时间对多孔SiC薄膜的光致发光性能的影响

为了增强碳化硅(SiC)的光致发光性能,设计了三层结构的多孔SiC薄膜,衬底是单晶硅,中间层是双通阳极氧化铝(AAO)模板,顶层是SiC薄膜。采用磁控溅射工艺在AAO模板上沉积SiC薄膜,沉积温度为100~500 ℃,溅射时间为1~30 min。研究了沉积温度和沉积时间对SiC的光致发光性能的影响。结果表明:SiC薄膜为非晶态,SiC主要沉积在AAO模板的上层骨架结构上;与未经过溅射的样品相比,当衬底温度为200 ℃,溅射时间为1 min时, SiC的荧光性能增强至14.23倍;多孔SiC薄膜的荧光主要来自2.3 eV的主峰和2.8 eV的次主峰,主峰可能来自Al₂O₃的O缺陷发光与SiC本征发光,次主峰可能来自SiO₂的O缺陷发光。磁控溅射结合双通AAO模板法可应用于多孔荧光SiC薄膜的工业化快速制备。     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Mechanical and microwave dielectric properties of SiCf/SiC composites with BN interphase prepared by dip-coating process

The BN interphase of SiC fiber-reinforced SiC matrix (SiCf/SiC) composites was fabricated by dip-coating process with boric acid and urea as precursor. The results show that the tensile strength of SiC fiber decreases about 30% after BN coating treatment, but the BN coating has little influence on the electrical resistivity of SiC fiber. Compared with the as-received SiCf/SiC composites, the SiCf/SiC composites with BN interphase exhibit a toughened fracture behavior, and the flexural strength is about 2 times that of the as-received SiCf/SiC composites. From the microstructural analysis, it can be confirmed that the BN interphase plays a key part in weakening interfacial bonding, which can improve the mechanical properties of SiCf/SiC composites remarkably. Owing to the close dielectric properties between SiC and BN, the complex permittivity of SiCf/SiC composites with and without the BN interphase is similar.