Microstructure and Fracture Toughness of Hot-Pressed Silicon Carbide Reinforced with Silicon Carbide Whisker

The effects of β-SiC whisker addition on the microstructural evolution and fracture toughness ( K _IC) of hot-pressed SiC were investigated. Most of the whiskers added disappeared during the densifcation process by transformation into the α-phase. The remaining whiskers acted as nuclei for grain growth, resulting in the formation of large tabular grains around the whiskers. The tabular grains around the whiskers were believed to be formed because of the extreme anisotropy of the interfacial energy between α- and β-SiC. The K _IC of the material was improved significantly by the whisker addition. The increase in the K _IC was attributed to crack bridging followed by grain pullout as a result of the formation of tabular grains in a fine matrix.     

Auger Analysis of Hot-Pressed and Sintered Silicon Carbide

Intergranular and transgranular chemistries of hot-pressed and sintered silicon carbides were investigated by Auger electron spectroscopy. Results indicated major differences in grain-boundary compositions between the two. Hot-pressed silicon carbide displayed a complex intergranular chemistry. Sintered silicon carbide displayed grain facets that were free of impurities and additives. The observed intergranular chemistries for both silicon carbides are discussed in terms of their relation to the processing methods.     

Secondary Phases in Hot-Pressed Aluminum-Boron-Carbon–Silicon Carbide

Silicon carbide hot-pressed with aluminum, boron, and carbon as sintering aids (ABC–SiC), was studied by transmission electron microscopy. Both grain-boundary films and inclusions were prevalent in this material. The present study characterized the inclusions located in triple-junctions, grain boundaries, and the interior of the SiC grains, with emphases on phases not scrutinized before. These inclusions were crystalline, in contrast to the amorphous grain-boundary films. Two dominant types of boron-free triple-junction phases containing Al(Si)-O-C-(S) and Al(Si)-O were identified, where sulfur was an unexpected contaminant, and silicon came from SiO₂ or from dissolution of SiC. Boron-containing inclusions with a composition Al-O-B-C were frequently observed inside SiC grains. Although the boron-free aluminum-rich phases wet the grain boundaries completely and are, therefore, effective sintering additives, the boron-containing Al(Si)-O-B-C did not wet the grain boundaries. The structure and chemical composition of these boron-containing intragranular inclusions were determined, and their mechanism of formation is discussed.     

Size-Scaling of Tensile Failure Stress in a Hot-Pressed Silicon Carbide

Quasi-static Weibull strength-size scaling of hot-pressed silicon carbide is described. Two surface conditions (uniaxial ground and uniaxial ground followed by grit blasting) were explored. Strength test coupons sampled effective areas from the very small (4 × 10⁻³ mm²) to the very large (4 × 10⁴ mm²). Equibiaxial flexure and Hertzian ring crack initiation were used for the strength tests, and characteristic strengths for several different specimen geometries were analyzed as a function of effective area. Characteristic strength was found to substantially increase with decreased effective area for both surface conditions. Weibull moduli of 9.4- and 11.7 well-represented strength-size scaling for the two ground conditions between an effective area range of 10⁻¹ and 4 × 10⁴ mm². Machining damage was observed to be the dominant flaw type over this range. However, for effective areas <10⁻¹ mm², the characteristic strength increased rapidly for both ground surface conditions as the effective area decreased, and one or more of the inherent assumptions behind the classical Weibull strength-size scaling were in violation in this range. The selections of a ceramic strength to account for ballistically induced tile deflection and expanding cavity modeling are considered in context with the measured strength-size scaling. The observed size-scaling is briefly discussed with reference to dynamic strength.     

Effect of Alumina Content on the Oxidation of Hot-Pressed Silicon Carbide

The oxidation behavior at 1370°C of dense SiC, hot-pressed with the aid of Al₂O₃, has been investigated as a function of Al₂O₃ content. Increasing amounts of the Al₂O₃ hot-pressing aid increased the oxidation rate. Observations of the oxide surface show that a glassy phase (indicating formation of a liquid at the oxidation temperature) containing Si, Al, Fe, and K forms over the residual Al₂O₃ in the hot-pressed material. It is suggested that the oxygen transport through an impure aluminosilicate liquid is faster than that through a pure SiO₂ scale, thus causing an increased oxidation rate.     

Endothermic Reactions between Mullite and Silicon Carbide in an Argon Plasma Environment

Reactions between SiC and mullite in an Ar plasma were investigated using a model composite in which a free-standing CVD SiC coupon was imbedded in mullite cement. After treatment in a radio frequency (RF) plasma, the Si content of the mullite in contact with SiC was found to be less than that in the starting material, and deposits were found on the walls of the plasma chamber due to the reaction of mullite with SiC as follows: Al₆Si₂O₁₃( s )+ SiC( s )= 3Al₂O₃( s )+ 3SiO( g )+ CO( g ). This reaction, which is endothermic (1405 kJ/mol at 1500 K), absorbs thermal energy and consequently prevents the rapid sintering which is observed for single-phase mullite in similar environments. As a consequence, it is suggested that RF plasma sintering probably cannot be used to densify SiC-reinforced mullite-matrix composites because of the resulting energy consumption and damage to the SiC phase.     

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.     

Phase Transformation and Thermal Conductivity of Hot-Pressed Silicon Carbide Containing Alumina and Carbon

Phase transformation and thermal conductivity of hot-pressed β-SiC with Al₂O₃ and carbon additions were studied. Densification rate was a complex function of both Al₂O₃ and carbon. Simultaneous additions of Al₂O₃ and carbon accelerated the 3C → 4H phase transformation. Carbon additions lowered the thermal conductivity of the compact as did the high-temperature hot-pressing. The 3C → 4H transformation and the thermal conductivity were deduced to be related to each other.     

Selectivity of Silicon Carbide/Stainless Steel Solid-State Reactions and Discontinuous Decomposition of Silicon Carbide

Solid-state reactions of SiC with a multicomponent system, stainless steel, have been studied at 1125°C. Four-layered reaction products consisting of modulated carbon precipitation zone, random carbon precipitation zone, four-phase mixture zone, and grain-boundary precipitation zone were formed in the reaction zone. The carbon precipitates were embedded in a matrix of complex metal silicides. In addition, extensive interfacial melting was noted. Carbon atom was found to diffuse faster than Si and selectively reacted with Cr to form Cr carbide(s) along the grain boundaries of stainless steel. No Fe carbides or Ni carbides were ever detected. Among the consituents existing in stainless steel, Ni atoms have the highest affinity for Si. An uphill diffusion of Ni toward the SiC reaction front was observed. While the diffusion of Cr and Fe toward SiC followed a downhill concentration gradient, very small amounts of Cr reached the SiC interface. The selective reactions of Si and C with Ni, Fe, and Cr are discussed on the basis of Gibbs free energy of formation of various compounds. The diffusion kinetics of C and Si atoms in selected metal/SiC reactions are discussed on the basis of their chemical affinities for respective metals. The modulation of carbon precipitation is correlated with previous results from Ni/SiC, Ni₃Al/SiC, Fe/SiC, Co/SiC, and Pt/SiC, reactions. A general model describing discontinuous decomposition of SiC is proposed to explain the origin of carbon modulation.     

Chemical Instability of Silicon Carbide in the Presence of Transition Metals

SiC has been proved chemically unstable at temperatures >1073 K (800°C) in the presence of cobalt, nickel, or iron. The instability results from reactions of cobalt, nickel, or iron with SiC to produce silicides and free carbon. A method of calculation is proposed for thermodynamic analyses of these reactions. The results show that the silicides of cobalt, nickel, and iron are thermodynamically more stable than SiC. The metal–silicon eutectic may provide kinetic condition to the reactions. Calculations also are performed for reactions of SiC with other transition metals. Instabilities of SiC in the presence of those metals are predicted.     

Oxidation of Silicon, Silicon Carbide, and Silicon Nitride in Gases Containing Oxygen and Chlorine

Chlorine contamination accelerates the oxidation of silicon-based ceramics through the formation of volatile silicon chloride or silicon oxychloride species which degrade the protective character of the SiO₂ film. Accelerated attack may occur by active corrosion or formation of bubbles in the oxide layer. Si₃N₄ is much more resistant to this attack than either silicon or SiC. This resistance may be related to the presence of a thin silicon oxynitride layer below the SiO₂ scale which forms on Si₃N₄.     

Influence of Oxygen on the Formation of Aluminum Silicon Carbide

X-ray diffraction studies have been used to follow the formation of Al₄SiC₄ from Al₄C₃ and SiC and the role played by impurity oxygen. The phase Al₂OC forms in the early stages of reaction and reacts with SiC at ≈ 1700°C to produce Al₄SiC₄ plus a small amount of an aluminosilicate liquid. This liquid dissociates at higher temperatures, the resulting evolution of gases hindering complete densification. Higher densities are obtained on hot-pressing.     

Reactions of Silicon Carbide and Silicon(IV) Oxide at Elevated Temperatures

The reaction between SiC and SiO₂ has been studied in the temperature range 1400–1600 K. A Knudsen cell in conjunction with a vacuum microbalance and a high-temperature mass spectrometer was used for this study. Two systems were studied—1:1 SiC (2 wt% excess carbon) and SiO₂; and 1:1:1 SiC, carbon, and SiO₂. In both cases the excess carbon forms additional SiC within the Knudsen cell and adjusts to the direct reaction of stoichiometric SiC and SiO₂ to form SiO( g ) and CO( g ) in approximately a 3:1 ratio. These results are interpreted in terms of the SiC-O stability diagram.     

Oxidation Kinetics of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen

The oxidation kinetics of chemically vapor-deposited SiC in dry oxygen and wet oxygen ( P _H2O= 0.1 atm) at temperatures between 1200° and 1400°C were monitored using thermogravimetric analysis. It was found that in a clean environment, 10% water vapor enhanced the oxidation kinetics of SiC only very slightly compared to rates found in dry oxygen. Oxidation kinetics were examined in terms of the Deal and Grove model for oxidation of silicon. It was found that in an environment containing even small amounts of impurities, such as high-purity Al₂O₃ reaction tubes containing 200 ppm Na, water vapor enhanced the transport of these impurities to the oxidation sample. Oxidation rates increased under these conditions presumably because of the formation of less protective sodium alumino-silicate scales.     

六铝酸钙材料的合成、性能和应用

本文对六铝酸钙材料的合成方法、使用性能和工业应用进行了详细的论述.目前,六铝酸钙的合成方法主要有反应烧结,电熔法和熔盐法.六铝酸钙耐火材料以其优越的导热性、高温体积稳定性、抗热震性和抗侵蚀性,在钢铁、炼铝、陶瓷、石化等高温行业中已成功得到应用.     

Effects of Oxygen Partial Pressure on the Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was studied at different partial pressures of oxygen. The diffusion rate constant was found to vary with the logarithm of the partial pressure of oxygen according to the theory of oxidation of thin films as proposed by Engell and Hauffe. An alternative explanation based on the change of free energy with surface coverage was also found to fit the data.     

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.     

Silicon Carbide/Calcium Aluminosilicate: A Notch-Insensitive Ceramic-Matrix Composite

Tension experiments performed on a 0/90 laminated silicon carbide/calcium aluminosilicate composite at room temperature establish that this material is notch insensitive. Multiple matrix cracking is determined to be the stress redistribution mechanism. This mechanism is found to provide a particularly efficient means for creating local inelastic strains, which eliminate stress concentrations.