In situ observation of the capillary infiltration of molten silicon in a SiC/SiC composite by X-ray radiography

Capillary infiltration is an innovative fabrication method for metal and ceramic-matrix composites. SiC/SiC composites can be infiltrated by molten silicon to decrease residual porosity. Physical and chemical mechanisms involved during Liquid Silicon Infiltration (LSI) are complex to analyse. An in situ observation setup for capillary infiltration of molten silicon has been designed for synchrotron observations. The setup reproduces the extreme high temperature and high vacuum conditions used in the LSI process. It is also designed for X-ray observations in synchrotron beamlines and tomography stages. Sets of 2D X-ray absorption radiographs were acquired at high frequency during the LSI process. The study outlines the capillary infiltration mechanisms of molten silicon inside SiC/SiC composites. It proves that full saturation of the composite is not directly achieved after the rise of molten silicon. It is a two step mechanism. First, the infiltration occurs inside the intra granular porosity of the SiC powder matrix. Then, larger scale porosities such as cracks are filled. These phenomena have been discussed previously in the literature but never observed in situ.     

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC_f/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.Comparing the microstructures of SiC_f/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC_f/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC_f/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.In addition, among the SiC_f/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC_f/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.     

Evaluation of the moulding process for production of short-fibre-reinforced C/C-SiC composites

For the production of C/C-SiC brake discs via the liquid silicon infiltration method (LSI), the hot pressing process is the state of art technique for the moulding of the CFRP composites. This technique consists of several manual steps which increase production cost. The overall cost can be reduced by implementing injection moulding process.In this paper the influence of the moulding process (hot pressing, injection moulding) on the properties of semi-finished and final products during the production of short-fibre-reinforced C/C-SiC composites by means of the LSI process are examined. The starting polymer is chemically characterised. Carbon-fibre-reinforced plastic (CFRP) composites are fabricated by hot pressing, as well as injection moulding process. The CFRP composites are converted into porous C/C composites by pyrolysis. Liquid silicon is infiltrated to form dense C/C-SiC composites, which are further investigated during the course of this paper. Significant differences in properties of the composites are discussed.     

Microstructure Evolution and Reaction Mechanism of Biomorphous SiSiC Ceramics

Liquid Si infiltration (LSI) of beech wood-derived biocarbon (C_B) templates at 1550°C yields biomorphous SiSiC ceramics with the morphology of the initial biological preform. The biomorphous SiSiC ceramic consists of solidified Si in the cell lumina, polycrystalline β-SiC and residual carbon islands located at the position of former wood cell walls. The evolution of the microstructure during reactive Si melt infiltration was assessed by infiltration experiments at various times and investigated by X-ray diffraction as well as light scanning electron and transmission electron microscopy in combination with elemental analysis by energy-dispersive X-ray spectrometry. Four different stages of the reactive infiltration process could be distinguished, starting with a heterogeneous nucleation of nano-grained SiC on the pore surfaces of the C_B template by a Si vapor phase reaction below the Si melting temperature. After spontaneous Si melt infiltration, a stepwise reaction results in the simultaneous formation of a nano-grained SiC layer and a coarse-grained SiC phase on the inner pore surfaces. Further reaction proceeds slowly by diffusion of the reactants through the formed SiC layer and the microstructure evolution is dominated by dissolution and re-crystallization processes.     

SiC陶瓷基体及抗氧化涂层

介绍了5种主要SiC基体的成型方法,分别是化学气相渗透(CVI)、聚合物先驱体浸渍-裂解法(PIP)、液相硅渗透法(LSI)、反应烧结法、化学气相反应法(CVR)。阐述了各种基体的组织结构、致密效率及陶瓷基复合材料的性能,其中CVI+PIP/LSI的复合成型技术可达到优化的制备过程,提高基体的组织结构和致密化效率;C/C及C/SiC复合材料表面化学气相转换法SiC涂层及多层涂层技术是提高CMC抗氧化性能的有效途径,并已得到工程实际验证。     

Inhibition of liquid Si infiltration into carbon-carbon composites by the addition of Al to the Si slurry pre-coating: mechanism analysis

The mechanism of the inhibition of liquid Si infiltration (LSI) into a two-dimensional carbon-carbon composite (2D-C/C) by the addition of Al to the Si slurry pre-coating was investigated. It was shown by means of a vapor treatment experiment designed intentionally that the surface composition of the inner pores beneath the Si slurry pre-coating before the occurrence of LSI was pure carbon and SiC, while before the occurrence of the LSI with the Si-6 wt.%Al slurry pre-coating, the surface composition of the inner pores was Al₄C₃, SiC and a small amount of pure carbon. The formation of the SiC and the Al₄C₃ was the result of the evaporation of almost all the Al additive and a little Si during the heating. For reactive infiltrations, reactions at the vapor-liquid-solid triple line are believed to affect the final infiltration depth. Faster reactions at the triple line lead to faster infiltration velocity and hence deeper reactive infiltration. The reaction at the triple line for the LSI with the Si-6 wt.%Al slurry pre-coating was mainly between liquid Si and the surface Al₄C₃, which was probably slower than the reaction of liquid Si with the pure carbon at the triple line corresponding to the LSI with the Si slurry pre-coating. Therefore, the extent of the penetration of liquid Si during the LSI with Si-6 wt.%Al slurry pre-coating was lower than that with the Si slurry pre-coating.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

Microstructure and tribological properties of PIP-SiC modified C/C–SiC brake materials

A combination method of precursor infiltration and pyrolysis (PIP), chemical vapor infiltration (CVI) and liquid silicon infiltration (LSI) was proposed to prepare PIP-SiC modified C/C–SiC brake materials. The SiC ceramic matrix pyrolyzed by polymethysilane (PMS) homogeneously dispersed in the fiber bundles region, which improved the plough resistance of local C/C region and the wear resistance of C/C–SiC brake materials. When the braking speed rises to 28 m/s, the fluctuation range of friction coefficient was limited to 0.026. The linear wear rate of the as-prepared composites was could be ~50% less than that of C/C–SiC, when the braking speed was above 15 m/s (for instance, the wear rate of 1.02 μm/(side·cycle) at 28 m/s less than 2.02 μm/(side·cycle) of traditional C/C– SiC). The fading ratio D of CoF under wet conditions was ~11%. The results showed that introducing PIP-SiC could stabilize the braking process and effectively prolong the service life of C/C–SiC brake materials.     

Numerical Simulation of Effect of Methyltrichlorosilane Flux on Isothermal Chemical Vapor Infiltration Process of C/SiC Composites

A two-dimensional axisymmetrical mathematical model for the isothermal chemical vapor infiltration process of C/SiC composites was developed. Transport phenomena of momentum, energy, and mass in conjunction with infiltration-induced changes of preform structure were taken into account. The integrated model was implemented by the finite-element method to simulate numerically the isothermal chemical vapor infiltration (ICVI) process of C/SiC composites at different methyltrichlorosilane (MTS) fluxes. The influence of MTS flux on concentration distribution and time-dependent densification behaviors of C/SiC composites was studied in detail. Calculation results imply that MTS flux has an obvious influence on infiltration in micro-pores and little influence on infiltration in macro-pores. Increasing flux will lead to an evident acceleration for infiltration in micro-pores. Moderate flux is preferable by a combination of both a relatively high infiltration rate and a relatively low fabrication cost. This model is helpful to understand the fundamentals of the ICVI process for the fabrication of C/SiC composites.     

Properties of C/C-SiC composites produced via transfer moulding and inner siliconization

Short carbon fiber reinforced polymers (CFRP) are successfully prepared by transfer moulding technology. For this purpose, compounds on the basis of novolac/urotropin with different 6 mm chopped carbon fibers and silicon powder contents are produced utilizing a laboratory tempered sigma-blade kneader. These compounds are then shaped into 46 × 8 × 3 mm³ CFRP specimens using a transfer moulding machine. Depending on the material composition, the conversion to C/C-SiC composites is performed through liquid silicon infiltration (LSI) process or inner siliconization. First, the short fiber content is varied between 30 and 50 wt% and its influence on the process and properties of the composites is studied. Second, an investigation of the inner siliconization through the co-mixing of silicon powder (1-23 wt% in CFRPs) during the compound production as well as a comparison with the external silicon infiltration process are presented and discussed. According to the results, the best mechanical properties are achieved at a fiber content of 40 wt% in the case of the external silicon infiltration and at silicon content below 14 wt% for composites produced by the inner siliconization process.     

Preparation of low residual silicon content Si-SiC ceramics by binder jetting additive manufacturing and liquid silicon infiltration

Complex silicon carbide (SiC) ceramic components are difficult to fabricate due to their strong covalent bonds. Binder jetting (BJ) additive manufacturing has the outstanding advantages of high forming efficiency and no thermal deformation, especially suitable for printing complex structure SiC components. This study tried to obtain low silicon content silicon carbide ceramics by binder jetting followed by phenolic resin impregnation and pyrolysis (PRIP) and liquid silicon infiltration (LSI). BJ was used for the SiC green parts fabrication, and the highest compressive strength (7.7 ± 0.3 MPa) and lowest dimensional deviations (1.2–1.6 mm) were obtained with the printing layer thickness of 0.15 mm. Subsequently, PRIP treatments were introduced to increase the carbon content for the following LSI process. As the number of PRIP cycles increased, the carbon density of SiC/C preform increased and the porosity decreased. After the LSI treatment, the final Si-SiC composites processed with 2 PIRP cycles reached the highest flexural strength (257 ± 14.26 MPa) and the best wear resistance. This was attributed to the low residual silicon content (10.2 vol%) and almost no residual carbon. Furthermore, several complex structural components were fabricated using these methods. The preparation of complex components verifies the feasibility of BJ and LSI for manufacturing high-strength and high-precision SiC ceramics. Besides, this work hopes to provide technical guidance for the preparation of complex SiC composites in the future.     

Fabrication of biomorphic SiC composites using wood preforms with different structures

Biomorphic SiC composites were fabricated from wood, including high-density compressed cedar, high-density fiberboard (HDF) and low-density paulownia followed by the fabrication of a preform and liquid silicon infiltration (LSI) process. The degree of molten silicon infiltration was strongly dependent on the cell wall thickness and pore size of the carbon preform. The mechanical properties of the biomorphic SiC composites were characterized by compressive tests at room temperature, 1000 °C and 1200 °C, and the relationship between the mechanical properties and the microstructural characteristics was analyzed. The compressive strength of the biomorphic composites was found to be strongly dependent on their bulk density and decreased as the test temperature increased to 1200 °C. Strength reduction in the biomorphic SiC composites occurred due to the deformation of the remaining Si at elevated temperatures under ambient atmospheric conditions.     

空间望远镜用C/SiC镜面研制综述

综述了空间望远镜的主镜用高强度、高表面精度、低热膨胀系数的低温（约4K）用镜面的制备和检测过程.日本将Φ710mm的高强度反应烧结SiC材料已用于红外望远镜镜面.在短切炭纤维增强C/C复合材料毛坯的基础上进行液相硅渗透（LSI）而制备的C/SiC复合材料在光学镜面方面具有更大的优势.通过提高C/C复合材料毛坯中沥青基炭纤维体积分数及控制硅化速度,可有效地提高LSI-C/SiC复合材料的机械性能和表面光学精度;通过不同规格的炭纤维的混杂化,可使C/SiC复合材料热膨胀系数的各向异性降低至小于4％的差异.SiC、Si-SiC浆料涂层处理可有效地提高表面精度至2 nm rms的极高要求.     

Studying the wettability of Si and eutectic Si-Zr alloy on carbon and silicon carbide by sessile drop experiments

The contact angles of two different systems, molten silicon and a eutectic Si-8 at. pct Zr alloy and their evolution over timeon vitreous carbon and polycrystalline silicon carbide (SiC) substrates were investigated at 1500°C under vacuum, as well as in argon using the sessile drop technique. The contact angle and microstructure of the liquid droplet/solid substrate interface were studied to understand fundamental features of reactive wetting as it pertains to the infiltration process of silicon and silicon alloys into carbon or C/SiC preforms. Both pure Si and theeutectic alloy showed good wettability onvitreous carbon and SiC characterized by equilibrium contact angles between 29° and 39°. Theeutectic alloy showed a higher initial contact angle and slower spreading as compared to that of pure Si. On vitreous carbon bothsilicon and the eutecticalloy formed SiC at the interface, while no reaction was observed on the SiC substrates.     

MCMB–SiC composites; new class high-temperature structural materials for aerospace applications

Graphite–silicon carbide (G–SiC), carbon/carbon–silicon carbide (C/C–SiC) and mesocarbon microbeads–silicon carbide (MCMB–SiC) composites were produced using liquid silicon infiltration (LSI) method and their physical and mechanical properties, including density, porosity, flexural strength and ablation resistance were investigated. In comparison with G–SiC and C/C–SiC composites, MCMB–SiC composites have the highest bending strength (210 MPa) and ablation resistance (9.1%). Moreover, scanning electron microscopy (SEM) and optical microscopy (OM) are used to analyze the reacted microstructure, pore morphology and pore distribution of carbon-based matrices. As a result, SiC network reinforcement was formed in situ via a reaction between liquid silicon and carbon. The unreacted carbon and solidified silicon are two phases present in the final microstructure and are characterized by X-ray diffraction (XRD). Based on the results obtained and the low-cost processing of pitch-based materials, the MCMB–SiC composite is a promising candidate for aerospace applications.     

Effect of pyrolysis temperature on the microstructure and capillary infiltration behavior of carbon/carbon composites

Carbon fiber fabric reinforced plastics were pyrolyzed at temperatures between 900?°C and 1600?°C to convert them into carbon/carbon (C/C) composites. The effects of pyrolysis temperatures on the microstructure, mechanical properties, and especially on the capillary infiltration behavior of C/C composites, suitable for liquid silicon infiltration (LSI), were investigated. The porosity of these C/C composites shows a decreasing trend with increasing pyrolysis temperature. The established model can explains the pyrolysis mechanism and the infiltration behaviors. Within the initial stage, the capillary infiltration rate of C/C composites with the model fluid water increases rapidly. In the second stage, where thermal imaging indicates that water has reached the top area of the plates at the initial stage. Capillary infiltration rate, based on water infiltration experiments mass increase, decreases because the shrinkage of micro-delamination take place at higher pyrolysis temperature. In combination with LSI results, a model for the capillary infiltration behavior of C/C is proposed.     

Fabrication of dense SiSiC ceramics by a hybrid additive manufacturing process

In this work, we report the fabrication of Silicon infiltrated Silicon Carbide (SiSiC) components by a hybrid additive manufacturing process. Selective laser sintering of polyamide powders was used to 3D print a polymeric preform with controlled relative density, which allows manufacturing geometrically complex parts with small features. Preceramic polymer infiltration with a silicon carbide precursor followed by pyrolysis (PIP) was used to convert the preform into an amorphous SiC ceramic, and five PIP cycles were performed to increase the relative density of the part. The final densification was achieved via liquid silicon infiltration (LSI) at 1500°C, obtaining a SiSiC ceramic component without change of size and shape distortion. The crystallization of the previously generated SiC phase, with associated volume change, allowed to fully infiltrate the part leading to an almost fully dense material consisting of β-SiC and Si in the volume fraction of 45% and 55% respectively. The advantage of this approach is the possibility of manufacturing SiSiC ceramics directly from the preceramic precursor, without the need of adding ceramic powder to the infiltrating solution. This can be seen as an alternative AM approach to Binder jetting and direct ink writing for the production of templates to be further processed by silicon infiltration.     

Capillary infiltration studies of liquids into 3D-stitched C–C preforms: Part A: Internal pore characterization by solvent infiltration,mercury porosimetry,and permeability studies

Mathematical modeling of silicon infiltration in porous carbon–carbon (C–C) preforms is the key to fabricate liquid silicon infiltration based carbon–silicon carbide (C–SiC) composite components. Existing models for silicon infiltration are based on straight capillaries. For interconnected capillary systems, e.g. as in 3D-stitched C–C preforms these show large deviations when compared with experimental observations. The aim of the present study is to develop a mathematical model suitable for silicon infiltration in 3D-stitched C–C preforms. The work is being presented in two parts: A and B. This part (Part A) describes the experimental details pertaining to the fabrication of the C–C preforms and their pore structure characterization by mercury porosimetry, infiltration of solvents by capillary rise, and by permeability studies. A two-pore capillary infiltration model termed as modified Washburn equation has been proposed. It has been validated by experimental data of solvent infiltration. The same model correlates silicon infiltration observations as well (Part B).     

High-Temperature Behavior of Silicon Carbide Fibers in a Si-Al-Ca-O-N Ceramic Matrix

A Nicalon SiC fiber-reinforced Si-Al-Ca-O-N composite was fabricated by a slurry infiltration process followed by hot pressing at 1600°C. A carbon-rich interfacial layer (∼100 nm) as well as a crystalline silicon-rich layer (∼15 nm) was observed between the fiber and matrix. Based on this interfarcial phenomenology, the following behabior of SiC fibers in the matrix was proposed: fine SiC grains (diameter of ∼ 1.7 nm in as-received fibers) decomposed at fiber surfaces (SiC → C + Si), followed by silicon migration into the glasshy phase of the matrix. The glassy phase was interpreted to play a key role as a silicon consumer in fostering the formation of the carbon-rich layer. The presence of silicon implied that the oxygen activity in the matrix was low enough to avoid SiC oxidation.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.