Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

Fabrication of Carbon/Silicon Carbide Composites by Isothermal Chemical Vapor Infiltration,Using the In Situ Whisker-Growing and Matrix-Filling Process

C/SiC composites were prepared via isothermal chemical vapor infiltration (ICVI). A novel process of in situ whisker growing and matrix filling during ICVI was devised to reduce the porosity of the C/SiC composites, by alternating the dilute-gas species. C/SiC composites with increased density were prepared successfully using this novel process, in comparison with those obtained from the conventional ICVI process. The whiskers seem to have grown into the large pores and modified the pore structure that is filled by the SiC matrix.     

Braking Behavior of C/SiC Composites Prepared by Chemical Vapor Infiltration

Carbon fiber-reinforced silicon carbide matrix composites have the potential to overcome the shortcoming of the currently used carbon/carbon friction materials in aircraft brakes. In this article, the carbon/silicon carbide (C/SiC) composites were prepared by chemical vapor infiltration method, and the brake disks with different densities and component content were finally obtained. The friction coefficient and friction stability can be significantly improved by increasing both material density and carbon content. When the density of C/SiC composite is 2.3 g/cm³, the coefficient of friction measured is 0.23, the coefficient of friction stability remains about 0.43, the liner wear rate is less than 9.3 μm/cycle, and the weight wear rate is less than 9.1 μm/cycle. The rapid increase of friction coefficient approaching the end of braking is mainly related to the increasing of surface temperature in a short time and the enhanced adhesion and abrasion of contact conjunctions and asperities. The C/SiC composites exhibited a good stability of braking against fading versus the braking number and surface temperature. The surfaces of C/SiC brake disks were covered with wear debris including the fragment of carbon fibers after the braking tests. The wear on the surfaces is significantly determined by cyclic mechanical and thermal stresses, which result in the micro-cracks in the SiC matrix, the thin flakes of the surface materials as well as the grooves.     

Fabrication of Silicon Carbide Whisker/Alumina Composite by Thermal-Gradient Chemical Vapor Infiltration

SiC( w )/Al₂O₃ composites were made from an AlCl₃-H₂-CO₂ mixture by a thermal-gradient chemical vapor infiltration (CVI) method. Al₂O₃ was deposited from the reaction of AlCl₃ and H₂O, which was produced from the oxidation of H₂ by CO₂. The densification rate was measured at various reactant compositions and total pressures. When the reaction rate or total pressure increased, the rate-controling step shifted from H₂O production to AlCl₃ diffusion, which led to premature pore closing. To obtain dense composites in a short infiltration time, the diffusion rate of AlCl₃ had to be increased by decreasing the total pressure.     

Silicon/Silicon Carbide Composites Fabricated by Infiltration of a Silicon Melt into Charcoal

Dense Si/SiC composites were fabricated via a conventional reaction-bonding process, using oak charcoal that exhibited a honeycomb structure. The silicon melt was infiltrated into the porous oak charcoal (density of ~0.6 g/cm³) while the sample was heated to 1700°C under vacuum (10^-3 torr (~0.133 Pa)), which resulted in in situ silicon-fiber/SiC composites. The reaction product had an average density of 2.8 g/cm³ and showed three-point flexural strengths of 330 MPa at room temperature and 280 MPa at 1300°C. Good oxidation resistance also was observed at temperatures up to 1300°C in flowing air. This process provided excellent shape-making capability, because the charcoal that was used as a preform was readily machinable.     

Microstructure and Mechanical Properties of Three-Dimensional Textile Hi-Nicalon SiC/SiC Composites by Chemical Vapor Infiltration

Three-dimensional textile Hi-Nicalon SiC-fiber-reinforced SiC composites were fabricated using chemical vapor infiltration. The microstructure and mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.5 g·cm⁻³ after the three-dimensional SiC perform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room temperature and 1010 MPa at 1300°C under vacuum. Above the infiltration temperature, the failure behavior of the composites became brittle because of the strong interfacial bonding and the mismatch of thermal expansion coefficients between fiber and matrix. The fracture toughness was 30.2 MPa·m^1/2. The obtained value of shear strength was 67.5 MPa. The composites exhibited excellent impact resistance, and the dynamic fracture toughness of 36.0 kJ·m⁻² was measured using Charpy impact tests.     

Nanofiber Formation in the Fabrication of Carbon/Silicon Carbide Ceramic Matrix Nanocomposites by Slurry Impregnation and Pulse Chemical Vapor Infiltration

The objectives of this work were to investigate the fabrication of carbon-fiber-reinforced SiC ceramic nanocomposites using the slurry impregnation process and the pulse chemical vapor infiltration (PCVI) process and to study the influences of processing parameters of the PCVI process on the microstructure variation of the nanocomposites. In this work, SiC nanosized powder was added to the matrix precursor (silicon powder mixed with phenolic resin), followed by the impregnation of the slurry into the preform. In the PCVI process, to densify the nanocomposites, tetramethylsilane (TMS) vapor mixed with hydrogen was used as the vapor precursor for matrix deposition. Fabrication parameters, such as reactant concentrations, pulse number, and holding time, were studied. Morphologies obtained from various processes were compared.     

Fabrication and Characterization of Three-Dimensional Carbon Fiber Reinforced Silicon Carbide and Silicon Nitride Composites

Three-dimensional (3D) carbon fiber reinforced SiC and Si₃N₄ composites have been fabricated using repeated infiltration of an organosilicon slurry under vacuum and pressure. Open porosity of the infiltrated body was reduced from 40% after the first infiltration to approximately 8% after the seventh cycle. Further reduction of open porosity to less than 3% was accomplished by hot-press densification. The maximum values of flexural strength and fracture toughness were, respectively, 260 MPa and 7.3 MPa·m^1/2for C/Si₃N₄ composites, and 185 MPa and 6 MPa·m^1/2 for C/SiC composite.     

Silicon Carbide Platelet/Silicon Carbide Composites

α-silicon carbide platelet/β-silicon carbide composites have been produced in which the individual platelets were coated with an aluminum oxide layer. Hot-pressed composites showed a fracture toughness as high as 7.2 MPa·m^1/2. The experiments indicated that the significant increase in fracture toughness is mainly the result of crack deflection and accompanying platelet pullout. The coating on the platelets also served to prevent the platelets from acting as nucleation sites for the α- to β-phase transformation, so that the advantageous microstructure remains preserved during high-temperature processing.     

Fabrication of Cf/SiC Composites by Vapor Silicon Infiltration

Carbon fiber reinforced silicon carbide matrix composites were fabricated by the vapor silicon infiltration process. The density and the open porosity of the composite infiltrated at 1973 K were 2.25 g/cm³ and ∼6%, respectively. The flexural strength of the composite at ambient conditions was 300 MPa. When the infiltration temperature decreased, the density and flexural strength of the composite also decreased. However, the resulting composite materials exhibited non-brittle fracture behavior.     

Two-Dimensional-Woven-Carbon-Fiber-Reinforced Silicon Carbide/Carbon Matrix Composites Produced by Reaction Bonding

Plane-woven-fabric carbon-fiber-reinforced SiC/C matrix composites were fabricated at 1450°C via reaction bonding and impregnation with phenolic resin. The relationship between the flexural strength and the open porosity of the composites is dependent on the heat-treatment temperature before the impregnation. The flexural strength of composites heat-treated at 1000°C (open porosity of ∼15%) was ∼300 MPa, whereas that of composites heat-treated at 1450°C (open porosity of ∼12%) was only ∼240 MPa. The heat-treatment temperatures before the impregnation step might control the interface properties between the fiber and the matrix.     

Design, Fabrication, and Characterization of Silicon Nitride Particle-Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration

Silicon nitride particle-reinforced silicon nitride matrix composites were fabricated by chemical vapor infiltration (CVI). The particle preforms with a bimodal pore size distribution were favorable for the subsequent CVI process, which included intraagglomerate pores (0.1–4 μm) and interagglomerate pores (20–300 μm). X-ray fluorescence results showed that the main elements of the composites are Si, N, and O. The composite is composed of α-Si₃N₄, amorphous Si₃N₄, amorphous SiO₂, and a small amount of β-Si₃N₄ and free silicon. The α-Si₃N₄ transformed into β-Si₃N₄ after heat treatment at 1600°C for 2 h. The flexural strength, dielectric constant, and dielectric loss of the Si₃N_4(p)/Si₃N₄ composites increased with increasing infiltration time; however, the pore ratios decreased with increasing infiltration time. The maximum value of the flexural strength was 114.07 MPa. The dielectric constant and dielectric loss of the composites were 4.47 and 4.25 × 10⁻³, respectively. The present Si₃N_4(p)/Si₃N₄ composite is a good candidate for high-temperature radomes.     

Process and Mechanical Properties of in Situ Silicon Carbide-Nanowire-Reinforced Chemical Vapor Infiltrated Silicon Carbide/Silicon Carbide Composite

A SiC nanowire/Tyranno-SA fiber-reinforced SiC/SiC composite was fabricated via simple in situ growth of SiC nanowires directly in the fibrous preform before CVI matrix densification; the purpose of the SiC nanowires was to markedly improve strength and toughness. The nanowires consisted of single-crystal β-phase SiC with a uniform ∼5 nm carbon shell; the nanowires had diameters of several tens to one hundred nanometers. The volume fraction of the nanowires in the fabricated composite was ∼5%. However, the composite did not show significant increase in strength and toughness, likely because of strong bonding between the nanowires and the matrix caused by the very thin carbon coating on the nanowires. Little debonding and pullout of SiC nanowires from the matrix were observed at the fracture surfaces of the composite.     

Synthesis of One-Dimensional Nanostructured Silicon Carbide by Chemical Vapor Deposition

Nanorods of the wide-bandgap semiconductor silicon carbide belong to a promising group of one-dimensional materials with potential applications extending from reinforcement of composites to applications as building blocks that can be logically assembled into appropriate two- (and three-) dimensional architectures, permitting researchers to exploit their unusual electronic, optical, and other properties. Specific to the most common silicon carbide polytypes are a low intrinsic carrier concentration, an exceptionally high breakdown electric field, high thermal conductivity, high-temperature stability, and resistance to an aggressive environment. This should permit one to develop even submicron-level SiC-based devices operating under high-temperature, high-power, and/or high-radiation conditions, under which conventional semiconductors cannot function. Detailed control of the conditions favorable for the nucleation and growth processes of nanorods of a given SiC polytype is necessary because the electrical and optical properties of each SiC polytype are very different. Therefore, a systematic investigation of factors that primarily influence the morphology and polytype of a vapor-phase-grown SiC has been made in the present work. These factors were the temperature, the flow rates of the gaseous precursors, and the Si/C molar ratio in the gas phase. In order to investigate the role of these factors, the “cold gas-hot substrate” chemical vapor deposition (CVD) method has been applied, because it permits them to be closely controlled in a wide range. While in the overwhelming majority of previous investigations nanorods of the 3C SiC polytype have been grown, the present work delineates conditions that are favorable for the growth of single-phase 2H, 3C, 15R, and 6H SiC nanorods, respectively.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Pampuch, Gorny, Stobierski.This article was submitted by the authors in English.     

Chemical Vapor Infiltration of TiB2-Matrix Composites

The efficiency of the Hall–Heroult electrolytic reduction of aluminum can be substantially improved by the use of a TiB₂ cathode. The use of TiB₂ components, however, has been hampered by the brittle nature of the material and the grain boundary attack of sintering-aid phases by molten aluminum. In the current work, TiB₂ is toughened through the use of reinforcing fibers, with chemical vapor infiltration used to produce the TiB₂ matrix. In early efforts it was observed that the formation of TiB₂ from chloride precursors at fabrication temperatures below 900–1000°C may have allowed the retention of destructive levels of chlorine. At higher fabrication temperatures (>1000°C), using appropriate infiltration conditions as determined from the use of a process model, TiB₂/THORNEL P-25 fiber composites have been fabricated in 20 h. The improved composite material has been demonstrated to be stable in molten aluminum in short-duration (24 h) tests.     

Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiC_f/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiC_f/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value ( d > d _c) and the temperature below a critical value ( T < T _g), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < d _c and may occur at T > T _g for d > d _c. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.     

碳含量对反应熔渗法制备SiC晶须增强碳化硅陶瓷性能的影响研究

以SiC晶须作为增强体,通过酚醛树脂高温碳化裂解获得碳包覆的SiC晶须,与纳米碳化硅粉体、炭黑混合均匀形成复合陶瓷乙醇浆料.经过干燥、造粒、成型和排胶后获得SiCw-C-SiC素坯,利用反应熔渗法制备高体积分数的SiC晶须增强SiC陶瓷基复合材料.研究了碳黑含量对复合材料力学性能与显微结构的影响.通过扫描电镜照片显示,碳包覆的SiC晶须经高温反应熔渗后仍保持表面的竹节状形貌,且晶须与碳化硅基体间形成适中的界面结合强度,材料断口处有明显的晶须拔出;当炭黑含量为15wt％时,抗弯强度和断裂韧性达到最高值分别为315 MPa和4.85 MPa·m1/2,比未加晶须的SiC陶瓷抗弯强度提高了25％,断裂韧性提高了15％;当炭黑含量为20wt％时,复合材料中残留部分未反应的炭黑,制约其力学性能的提高.     

Continuous Fabrication of Silicon Carbide Fiber Tows by Chemical Vapor Deposition

The feasibility of preparing small-diameter, high-strength, thermally stable silicon carbide fiber tows by the continuous chemical vapor deposition (CVD) of SiC onto carbon fiber tows was experimentally evaluated. Calculations of bending stresses and stresses caused by thermal expansion mismatch between the substrate and coating were used to evaluate the influence of coating thickness and substrate fiber diameter and type. Statistically designed and analyzed processing studies quantitatively showed the influence of key CVD process variables (temperature, pressure, and flow rates of CH₃SiCl₃ and H₂) on fiber attributes such as coating thickness and uniformity, surface roughness, percent agglomeration, and strength. Emphasis was given to conceiving and evaluating various fiber spreading devices in order to enhance coating uniformity and to minimize filament agglomeration within a tow. Uniform coatings and fiber tensile strengths as high as 4 GPa were achieved.     

Characterization of the Microstructure of Three-Dimensional-Needled Carbon/Silicon Carbide Composites

Three-dimensional-needled, carbon-fiber-reinforced silicon carbide matrix composites (C/SiC) were prepared by a chemical vapor infiltration and reactive melt infiltration method. It was found that two kinds of SiC existed in the C/SiC composites, that is, micro-β-SiC grains within the range of 5–15 μm and nano-β-SiC grains with a size of about 100 nm. The interface of C/SiC and the distribution of SiC showed evidence for the reaction mechanism of the reactive melt infiltration process.     

Growth Mechanism of Silicon Carbide Films by Chemical Vapor Deposition below 1273

SiC films were prepared from the reaction of Si₂H₆ with C₂H₄ or C₂H₂ at relatively low temperatures ranging from 873 K to 1273 K by low-pressure chemical vapor deposition. The deposition rate profiles determined by gravimetry and the alloy composition (carbon content, x, for Si_1-xC_x) profiles determined by X-ray photoemission spectroscopy in the reactor were mainly investigated. The results revealed that the carbon content, x , was a function of the temperature, ratio of partial pressures of source gases, and source gas species (C₂H₄, C₂H₂). From these results we deduced that SiC formation occurred by two major competing reaction processes: (1) the silicon deposition processes, SiH₂ Si (wall) and Si₂H₆ Si (wall), and (2) the carbon deposition process, C₂H₄+ SiH₂ vinylsilane or C₂H₂+ SiH₂ ethynylsilane.