Etching of Silicon Carbide Materials at Elevated Temperatures in a Nitrogen-Based Gas

Two sintered SiC-based materials were heat-treated for 150 h at 1300°C in a nitrogen-based gas (1.2% H₂, 0.6% CO) at a total pressure of 130 Pa. Sintered SiC samples were also preoxidized and then exposed to this gas under the same conditions to evaluate the protective nature of an SiO₂ scale. In this atmosphere, SiO gas and cyanogens are predicted to form, rather than SiO₂. Experimental studies confirmed that etching of sintered SiC occurs. Preoxidation does not provide protection from etching, because of the rapid removal of SiO₂ by H₂ as H₂O and SiO.     

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.     

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.     

Observations of Accelerated Silicon Carbide Recession by Oxidation at High Water-Vapor Pressures

A study of the exposure of SiC at 1200°C and high water-vapor pressures (1.5 atm) has shown SiC recession rates that exceed what is predicted based on parabolic oxidation at water-vapor pressures of less than or equal to ∼1 atm. After exposure to these conditions, distinct silica-scale structures are observed; thick, porous, nonprotective cristobalite scales form above a thin, dense silica layer. The porous cristobalite thickens with exposure time, while the thickness of the underlying dense layer remains constant. These observations suggest a moving-boundary phenomenon that is controlled by the rapid conversion of dense vitreous silica to a porous, nonprotective crystalline SiO₂.     

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.     

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.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

Mechanochemical Polishing of Silicon Carbide Single Crystal with Chromium(III) Oxide Abrasive

In order to clarify the mechanism of mechanochemical polishing of SiC with Cr₂O₃ abrasive, 6H-wurtzite single-crystal specimens were dry-polished. A significant anisotropic polishing rate difference was found between Si(0001) and C(000 1 ) surfaces. The C(000 1 ) surface was removed 10 times as fast. Polished surfaces were observed from cross-sectional and plan-view directions by high-voltage TEM. There was no trace of mechanical effects such as residual strain or scratches. The polishing debris was analyzed by X-ray diffraction, high-resolution TEM, and analytical TEM. No crystalline phases were identified from X-ray diffraction patterns except for Cr₂O₃, while it was found from TEM observation that a large amount of an amorphous phase consisting of Si, C, and O was contained in the debris. These results indicated that the surface of SiC was removed mechanochemically by the aid of a catalytic oxidation effect of Cr₂O₃.     

Volatility Diagrams for Silica, Silicon Nitride, and Silicon Carbide and Their Application to High-Temperature Decomposition and Oxidation

Volatility diagrams—isothermal plots showing the partial pressures of two gaseous species in equilibrium with the several condensed phases possible in a system—are discussed for the Si-O and Si-N systems, and extended to the Si-N-O and Si-C-O systems, in which the important ceramic constituents SiO₂, Si₃N₄, Si₂N₂O, and SiC appear as stable phases. Their use in understanding the passiveactive oxidation transitions for Si, Si₃N₄, and SiC are demonstrated.     

Heat-Resistant Silicon Carbide with Aluminum Nitride and Erbium Oxide

Fully dense SiC ceramics with high strength at high temperature were obtained by hot-pressing and subsequent annealing under pressure, with AlN and Er₂O₃ as sintering additives. The ceramics had a self-reinforced microstructure consisting of elongated SiC grains and a grain-boundary glassy phase. The strength of these ceramics was ∼550 MPa at 1600°C, and the fracture toughness was ∼6 MPa·m^1/2 at room temperature. The beneficial effect of the new additive composition on high-temperature strength might be attributable to the introduction of aluminum from the liquid composition into the SiC lattice, resulting in a refractive grain-boundary glassy phase.     

Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

Whisker Toughening: A Comparison Between Aluminum Oxide and Silicon Nitride Toughened with Silicon Carbide

Two whisker-toughened materials have been studied, with the objective of identifying the mechanisms that provide the major contribution to toughness. It is concluded that, for composites with randomly oriented whiskers, bending failure of the whiskers obviates pullout, whereupon the major toughening mechanisms are the fracture energy consumed in creating the debonded interface and the stored strain energy in the whiskers, at failure, which is dissipated as acoustic waves. The toughening potential is thus limited. High toughness requires extensive pullout and, hence, aligned whiskers with low fracture energy interfaces.     

Phase Relationship between 3C- and 6H-Silicon Carbide at High Pressure and High Temperature

The phase relationship between 3C- and 6H-SiC is investigated in the pressure range 2.5–6.5 GPa and the temperature range 400°–2500°C, by analyzing recovered samples, using X-ray diffractometry and Raman-scattering techniques. The phase transition from 3C- to 6H-SiC occurs at 2200°C and 2.5 GPa. In the pressure range >4.5 GPa, 6H-SiC transforms to 3C-SiC at 2500°C, via an intermediate state, as indicated by broadening peaks in the X-ray diffraction profile. Thermodynamically, 3C-SiC appears to be the low-temperature stable form, and the temperature of transition to 6H-SiC, which is stable at high temperature, appears to increase with pressure.     

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.     

Active Corrosion of Sintered α-Silicon Carbide in Oxygen–Chlorine Gases at Elevated Temperatures

The active corrosion of sintered α-silicon carbide from heat exchanger tubes in the temperature range 900° to 1100°C in gas mixtures containing 2% Cl₂ by volume with additions of O₂ or H₂ has been investigated by thermogravimetric analysis and subsequent examination of the corrosion products. The presence of a small amount of oxygen accelerated rapid active corrosion in chlorine-containing gas mixtures, but the corrosion was suppressed by an active-to-passive transition when the concentration of oxygen in the gas mixture was too high. Low rates of attack were observed in the environments containing H₂ even when the chlorine potential was high. The concentration of oxygen necessary to produce the active-to-passive transition was found to vary from one material to another and may be related to the amount of excess carbon in the sintered silicon carbide.     

Strengths of Ceramic Fibers at Elevated Temperatures

Tensile strengths were measured and Young's moduli were estimated for two SiC-based and three oxide ceramic fibers for temperatures from 25° to 1400°C. The SiC-based fibers were stronger but less stiff than the oxide fibers at room temperature and retained more of both strength and stiffness to high temperatures. High-temperature strengths of the SiC-based fibers were limited by internal void formation and oxidation; those of the oxide fibers were limited by softening of an intergranular glassy phase.     

Silica to Silicon: Key Carbothermic Reactions and Kinetics

The primary carbothermic reactions for the reduction of silica to produce silicon were defined and the reaction kinetics were determined. Most possible reactions between silicon oxide and carbon or carbon compounds were studied by a series of thermogravimetric analyses at temperatures up to 2000°C. Four key sequential reactions occur with SiC and SiO as intermediate reactants; two reactions involve SiO₂ and two involve SiO. Reaction rate versus temperature, activation energy, and preexponential factors were determined for each of six reactions involving SiO₂ or SiO. These kinetic studies show that SiO, when combined with either carbon or Sic, reacts in the gaseous state, and the sublimation of SiO is not the rate-limiting reaction for forming silicon.     

Steam Oxidation Resistance of Silicon Carbide Castables at Elevated Temperatures

Silicon carbide castables of different SiC contents (86％ and 71％,by mass) were prepared using white fused corundum,silicon carbide particles and fines,activated...     

Deformation of Alumina/Titanium Carbide Composite at Elevated Temperatures

The deformation behavior of an Al₂O₃/30 wt% TIC composite in uniaxial tension was evaluated under vacuum over the temperature range of 1300° to 1550°C. The Al₂0₃/TiC composite exhibited the maximum elongation of 66% at an initial strain rate of 1.19 X l0^-4 s^-1 at 1550°C. The stress exponent calculated from peak stresses of true stress-true strain curves at 1500^OC was 3.8, which was in good agreement with that obtained by changing the crosshead speed during the tension test. The apparent activation energy at 20 MPa was 853 kJ/mol. In addition the deformation of the Al₂O³/TiC composite in uniaxial tension at elevated temperature was accompanied by cavitation.