Joining of Silicon Carbide/Silicon Carbide Composites and Dense Silicon Carbide Using Combustion Reactions in the Titanium–Carbon–Nickel System

A new ceramic joining technique has been developed that utilizes an exothermic combustion reaction to simultaneously synthesize the joint interlayer material and to bond together the ceramic workpieces. The method has been used to join SiC/SiC composites and dense SiC ceramics using TiC-Ni powder mixtures that ignite below 1200°C to form a TiC-Ni joining material. Thin layers of the powder reactants were prepared by tape casting, and joining was accomplished by heating in a hot-press to ignite the combustion reaction. During this process, localized exothermic heating of the joint region resulted in chemical interaction at the interface between the TiC-Ni and the SiC ceramic that contributed to bonding. Room-temperature four-point bending strengths of joints produced by this method have exceeded 100 MPa.     

Green State Joining of Silicon Carbide Using Polycarbosilane

Green state joining of SiC was investigated using a paste consisting of polycarbosilane polymer and SiC powder. The joining process and densification were described. Initial experiments resulted in the formation of symmetrical black bands and cracks on both sides of the joint. However, with modifications in processing conditions, the cracks were eliminated and the resulting joints were indistinguishable from the matrix. The flexural strength of joined samples was measured to be 234 MPa, which was comparable to that of the control sample with similar density. As the applied pressure during joining was increased from 34 to 138 MPa, the strength of the joined samples increased from 180 to 250 MPa.     

Modified Tape Casting Method for Ceramic Joining: Application to Joining of Silicon Carbide

A simple modified tape casting procedure has been developed for application to ceramic joining when the joining materials are in powder form. The method involves preparation of a slurry from the powder, solvent, and thermoplastic binder, and then casting directly onto the joining surface using a moving doctor blade. Handling of the tape prior to joining is not necessary: therefore, binder content is minimized, plasticizers are not required, and viscosity is controlled by solvent content. The utility of this technique for producing joints with thin, uniform interlayers is demonstrated for silicon carbide materials joined with TiC + Ni and SiC + Si.     

High-Temperature Morphological Evolution of Lithographically Introduced Cavities in Silicon Carbide

Internal cavities of controlled geometry and crystallography were introduced in 6 H silicon carbide single crystals by combining lithographic methods, ion-beam etching, and solid-state diffusion bonding. The morphologic evolution of these internal cavities (negative crystals) in response to anneals of up to 128 h duration at 1900°C was examined using optical microscopy. Surface energy anisotropy and faceting had a strong influence on the geometric and kinetic characteristics of evolution. Decomposition of {12     10} cavity edges into {101     x } facets was observed after 16 h anneals, indicating that {12     10} faces are not components of the Wulff shape. The shape evolution kinetics of penny-shaped cavities were also investigated. Experimentally observed evolution rates decreased much more rapidly with those predicted by a model in which surface diffusion was assumed to be rate limiting. This suggested that the development of facets and the associated loss of ledges and terraces during the initial stages of evolution resulted in an evolution process limited by the nucleation rate of attachment/detachment sites (ledges) on the facets.     

Thermochemical Analysis of the Silicon Carbide-Alumina Reaction with Reference to Liquid-Phase Sintering of Silicon Carbide

The stabilities of different phases in the Si-Al-C-O system are calculated from thermodynamic considerations with the objective of identifying the liquid phases formed during sintering of SiC in the presence of Al₂O₃. It is shown that a liquid phase can form at the sintering temperatures by the reaction of SiC with Al₂O₃. Depending on the carbon activity, the liquid can be either of the following: Al₂O₃+ Al₄C₃, SiC + Al₄C₃, or molten aluminum. The stability of the aluminosilicate melts that can form by the reaction of Al₂O₃ with the surface silica layer on SiC powders is also evaluated. Several factors that influence liquid-phase sintering, such as the solubility of SiC in the melts and the generation of gases during sintering, are discussed. The results of the thermodynamic analysis are compared with the observed sintering behavior for SiC.     

Silicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC-monofilament-reinforced SiC or Si₃N₄ matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si₃N₄ matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si₃N₄ monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si₃N₄ matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si₃N₄ matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.     

Experimental Design Applied to Silicon Carbide Sintering

Silicon carbide is a promising structural ceramic used as abrasives and applied in metallurgical components, due to its low density, high hardness, and excellent mechanical properties. The composition and content of the additive can control liquid-phase sintering of SiC. Compositions based on the SiO₂–Al₂O₃–RE₂O₃ system (RE = rare earth) have been largely used to promote silicon carbide densification, but most studies are not systematically presented. The aim of this work is to study the effect of several oxide additives in the SiO₂–Al₂O₃–Y₂O₃ system on the densification of silicon carbide using experimental design. This technique seems to be effective in optimizing the values of maximum density with minimum weight loss.     

Multiscale Genome Modeling for Predicting the Thermal Conductivity of Silicon Carbide Ceramics

Silicon carbide (SiC) ceramics have been widely used in industry due to its high thermal conductivity. Understanding the relations between the microstructure and the thermal conductivity of SiC ceramics is critical for improving the efficiency of heat removal in heat sink applications. In this paper, a multiscale model is proposed to predict the thermal conductivity of SiC ceramics by bridging atomistic simulations and continuum model via a materials genome model. Interatomic potentials are developed using ab initio calculations to achieve more accurate molecular dynamics (MD) simulations. Interfacial thermal conductivities with various additive compositions are predicted by nonequilibrium MD simulations. A homogenized materials genome model with the calculated interfacial thermal properties is used in a continuum model to predict the effective thermal conductivity of SiC ceramics. The effects of grain size, additive compositions, and temperature are also studied. The good agreement found between prediction results and experimental measurements validates the capabilities of the proposed multiscale genome model in understanding and improving the thermal transport characteristics of SiC ceramics.     

反应烧结碳化硅研究进展

对有关反应结合碳化硅(RBSC)材料的研究进展作了综述,并对存在的问题和今后可能的发展方向提出了自己的见解,包括:进一步提高性能;降低游离硅含量,提高使用温度;提高材料的可靠性和稳定性;低成本化.     

新奇的生物材料制碳化物陶瓷工艺

概述了木材等生物材料制碳化硅及其复合材料的工艺。列举了碳化硅晶须，木质碳化硅，碳化硅反射镜，波纹蜂窝及叠层碳化硅陶瓷的制备实例。     

Oxidation of Silicon and Silicon Carbide in Ozone-Containing Atmospheres at 973 K

The oxidation behavior of a silicon wafer, chemically vapor-deposited SiC, and single-crystal SiC was investigated in an oxygen—2%–7% ozone gas mixture at 973 K. The thickness of the oxide film that formed during oxidation was measured by ellipsometry. The oxidation rates in the ozone-containing atmosphere were much higher than those in a pure oxygen atmosphere. The parabolic oxidation kinetics were observed for both silicon and SiC. The parabolic rate constants varied linearly with the ozone-gas partial pressure. Inward diffusion of atomic oxygen formed by the dissociation of ozone gas through the SiO₂ film apparently was the rate-controlling process.     

Intrinsic Reaction and Self-Diffusion Kinetics for Silicon Carbide Synthesis by Rapid Carbothermal Reduction

A one-dimensional nonisothermal model has been developed for the "rapid carbothermal reduction" synthesis of fine silicon carbide powders. Intrinsic reaction and self-diffusion kinetics are identified through simulation of the model and comparison to experimental results. The reaction rate follows a shrinking-core mechanism and is described by the relation [formula omitted] The self-diffusion coefficient for SiC in the aerosol flow reactor is described by the relation The self-diffusion coefficient for SiC in the aerosol flow reactor is described by the relation      

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.     

渗硅碳化硅材料的高温氧化

研究了全碳粉反应渗硅碳化硅(PCRBSC)材料,在1300℃静态空气中的高温氧化行为.研究结果表明:PCRBSC材料的氧化过程遵循直线-抛物线规律,其结构对高温氧化有很大的影响,特别是游离硅fsi和游离碳fc的含量对氧化影响更大,fsi含量高的PCRBSC材料单位面积氧化增重(Δm/s)明显,fc含量高的PCRBSC材料氧化后表现为先减重后增重,氧化层断口经扫描电镜观察有明显的气孔存在.     

Silicon Nitride–Silicon Carbide Nancocomposites Fabricated by Electric-Field-Assisted Sintering

Starting with Si-C-N(-O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm³) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si₃N₄ increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano-nanocomposites of silicon nitride and silicon carbide.     

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.     

Some New Perspectives on Oxidation of Silicon Carbide and Silicon Nitride

This study provides new perspectives on why the oxidation rates of silicon carbide and silicon nitride are lower than those of silicon and on the conditions under which gas bubbles can form on them. The effects on oxidation of various rate-limiting steps are evaluated by considering the partial pressure gradients of various species, such as O₂, CO, and N₂. Also calculated are the parabolic rate constants for the situations when the rates are controlled by oxygen and/or carbon monoxide (or nitrogen) diffusion. These considerations indicate that the oxidation of silicon carbide and silicon nitride should be mixed controlled, influenced both by an interface reaction and diffusion.     

碳化硅基材表面涂层方法综述

碳化硅基陶瓷是应用于高温工作环境下的理想材料，但其高温氧化影响了它的进一步应用。本文简要叙述了碳化硅材料的氧化机理，重点总结了在其表面涂层的各种方法。同时也对目前所做工作的不足提出了见解。     

Effects of Joining Pressure and Deformation on the Strength and Microstructure of Diffusion-Bonded Silicon Carbide

The effects of joining pressure and deformation on the strength and microstructure of diffusion-bonded SiC were examined using a hot-press to apply varying amounts of interfacial pressure and a close-fitting die to restrict deformation. SiC substrates were successfully diffusion bonded at 1950° and 2100°C. Joints which had uniform grain structure across the joint interface and bend strengths up to 300 MPa (44 ksi) were achieved. Pressing pressure was found to be a requirement for producing reasonable joint strength. It was also found that macroscopic deformation (>4% joint area expansion) is not necessary for effective diffusion bonding. Various methods for joining SiC are reviewed with regard to ease of processing, use limitations, and joint strength.     

Characterization of Silicon Carbide Whiskers

SiC whiskers from six manufacturers were characterized by bulk chemical techniques, X-ray photoelectron spectroscopy, X-ray diffraction, and scanning transmission electron microscopy or scanning electron microscopy. Major component (C, Si, and O) surface chemistries of the whiskers fell into four general categories: high oxygen content with oxide resembling a SiO₂, high oxygen content with oxide resembling a Si-O-C glass, and hydrocarbon. Several whiskers exhibited significant surface impurities—in particular, Fe. From a morphological viewpoint, significant differences in diameter, debris level, straightness, and types and quantities of defects were observed from one manufacturer to another.