High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen at 1823 to 1923 K

The oxidation of chemically vapor-deposited SiC in wet O₂ (water vapor partial pressure = 0.01 MPa, total pressure = 0.1 MPa) was examined using a thermogravimetric technique in the temperature range of 1823 to 1923 K. The oxidation kinetics follow a linear-parabolic relationship over the entire temperature range. The activation energies of linear and parabolic rate constants were 428 and 397 kJ · mol⁻¹, respectively. The results suggested that the rate-controlling step is a chemical reaction at an SiC/SiO₂ interface in the linear oxidation regime, and the rate-controlling step is an oxygen diffusion process through the oxide film (cristobalite) in the parabolic oxidation regime.     

Creep and Microstructural Evolution at High Temperature of Liquid-Phase-Sintered Silicon Carbide

The compressive creep characteristics at 1625°C of liquid-phase-sintered silicon carbide ceramics containing 5 and 15 wt% of crystalline Y₃Al₅O₁₂ (YAG) as the secondary phase were studied. In the two cases, strains between 10% and 15% were reached without failure. The creep behavior was characterized by a stress exponent n ≈2, and the proportion of secondary phase was related to the creep resistance of the materials. The microstructural evolution during creep consisted firstly in the re-distribution of the secondary phase, probably as a consequence of its viscous flow at the creep conditions, and secondly an extensive nucleation and growth of cavities, which was more important for the highest YAG content. The latter reflects the carbothermal reduction that the secondary phase undergoes during creep.     

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.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: II, Chemically Vapor Deposited

Chemically vapor deposited (CVD) silicon carbide was subjected to constant compressive stresses (110 to 220 MN/m²) at high temperatures (1848 to 2023 K) in order to determine the controlling steady-state creep mechanisms under these conditions. An extensive TEM study was also conducted to facilitate this determination. The strong preferred crystallographic orientation of this material causes the creep rate to be very dependent on specimen orientation. The stress exponent, n , in the equation εασⁿ was calculated to be 2.3 below 1923 K and 3.7 at 1923 K. The activation energy for steady-state creep was determined to be 175 ± 5 kJ/mol throughout the temperature range employed. At temperatures between 1673 and 1873 K, the controlling creep mechanism for CVD Sic is dislocation glide, which is believed to be controlled by the Peierls stress. Although the activation energy does not change, the increase in the stress exponent for samples deformed at 1923 K suggests that the controlling creep mechanism becomes dislocation glide/climb controlled by climb.     

High-Temperature Hardness of Chemically Vapor-Deposited Diamond

The hardness of chemically vapor-deposited diamond is examined at elevated temperatures. Diamond films of 400-μm thickness are grown on silicon substrates by plasma-enhanced chemical vapor deposition with subsequent removal of the substrate by chemical etching. Vickers hardness measurements were performed in the temperature range of 500°–950°C in an inert gas environment and under 7-N load. A 30% reduction in hardness from the room-temperature value is observed above 800°C.     

High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Nitride in a Carbon Monoxide-Carbon Dioxide Atmosphere

Oxidation behavior of chemically vapor-deposited silicon nitride (CVD-Si₃N₄) in CO─CO₂ atmospheres between 1823 and 1923 K was investigated using a thermogravimetric technique. Mass loss of Si₃N₄ (active oxidation) was observed in a region of P _CO2/P_CO < 1, while mass gain (passive oxidation) was observed at around P _CO2 P _CO= 10. In the active oxidation region below P _CO2P_CO= 10 ^–4, carbon particles were formed on the Si³N⁴ surface as an oxidation product, and the mass-loss rates were independent of P _CO2/ P _CO In the active oxidation region above P _CO2/ P _CO= 10^–4 the mass-loss rates decreased with increasing P _CO2/ P co. The critical P _CO2/ P _CO value from the active to passive oxidation was 2 orders of magnitude larger than the calculated value predicted from the Wagner model.     

High-Temperature Passive Oxidation of Chemically Vapor Deposited Silicon Carbide

The oxidation behavior of chemically vapor deposited (CVD) SiC at high temperature was investigated using a thermogravimetric technique in the temperatures range of 1823 to 1948 K. The specimens were prepared by chemical vapor deposition using SiCl₄, C₃H₈, and H₂ as source gases. The oxidation behavior of the CVD-SiC indicated "passive" oxidation and a two-step parabolic oxidation kinetics over the entire temperature range. The crystallization of the SiO₂ film formed may have caused this two-step parabolic behavior. The parabolic oxidation rate constant ( K _p) varied with the square root of the oxygen partial pressure ( P ^1/2_O2). The activation energy for the oxidation was determined to be 345 and 387 kJ · mol⁻¹. These values suggest that the diffusion process of the oxygen ion which passes through the SiO₂ film is rate-controlling.     

High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in CO─CO2 Atmosphere

Active oxidation behavior of CVD-SiC in CO─CO₂ atmospheres was investigated using a thermogravimetric technique in the temperature range between 1823 and 1923 K. The gas pressure ratio, P _CO2/ P _CO, was controlled between 10⁻⁴ and 10⁻¹ at 0.1 MPa. Active oxidation rates (mass loss rates) showed maxima at a certain value of P _CO2/ P _CO, ( P _CO2/ P _CO )^*, In a P _CO2/ P _CO region lower than the ( P _CO2/ P _CO)^* a carbon layer was formed on the SiC surface. In a P _CO2/ P _CO region higher than the ( P _CO2/ P _CO)^*, silica particles or a porous silica layer was observed on the SiC surface.     

High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in an Ar─O2 Atmosphere

Active oxidation behavior of chemically vapor-deposited silicon carbide in an Ar─O₂ atmosphere at 0.1 MPa was examined in the temperature range between 1840 and 1923 K. The transition from active oxidation (mass loss) to passive oxidation (mass gain) was observed at certain distinct oxygen partial pressures ( P _O2^t). The values of P _O2^t increased with increasing temperature and with decreasing total gas flow rates. This behavior was well explained by Wagner's model and thermodynamic calculations. Active oxidation rates ( k _a) increased with increasing O₂ partial pressures and total gas flow rates. The rate-controlling step of the active oxidation was concluded to be O₂ diffusion through the gaseous boundary layer.     

Deposition and Microstructure of Vapor-Deposited Silicon Carbide

Vapor deposition of Sic from methyltrichlorosilane in a fluidized bed and the microstructure of the deposit were studied over a range of deposition temperatures, carrier gas flow rates, and reactant fluxes. The rate-determining factor for the deposition of Sic was the rate of supply of reactant. The microstructure of vapor-deposited Sic was primarily dependent on the deposition temperature; however, carrier gas flow rate and reactant flux had a secondary influence on microstructure. At low temperatures and high carrier gas flow rates, laminar deposits containing excess silicon were produced. At higher temperatures and lower carrier gas flow rates deposits were characterized by faulted columnar grains. The grain diameter increased from about LP at 1400°C to about 15 μ at 1800°C. The grain size also increased, but less markedly, with increasing reactant flux. The deposits characterized by columnar grains were predominantly β-Sic with traces of a-SiC and excess carbon.     

Active-to-Passive Transition and Bubble Formation for High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Carbide in CO–CO2 Atmosphere

Oxidation behavior of chemically vapor-deposited SiC in CO─CO₂ atmospheres (0.1 MPa) was investigated using a thermogravimetric technique at temperatures from 1823 to 1923 K. Active or passive oxidation was observed depending on temperature and CO₂/CO partial pressure ratio ( P _co2/ P _co). The critical P _co2/ P _co value for the transition was 1O² times as large as a theoretical value calculated from the Wagner model. In the passive oxidation above 1873 K, SiO₂ bubbles were grown. The expansion and rupture of bubbles caused cyclic rapid mass gain and mass loss.     

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.     

Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments

The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H₂ with various P _H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO₂ layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO₂ layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO₂ inherent in the fibers.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: I, Reaction-Bonded

The kinetic characteristics and the controlling mechanism of steady-state creep were determined for NC–430 reaction-bonded silicon carbide which was subjected to high temperatures (1848 to 1923 K) and constant compressive stresses (110 to 220 MN/m²). Both as-received and as-crept materials were studied extensively by transmission electron'microscopy as one means of determining the controlling creep mechanism. Small variations in sample density resulted in large variations in the creep rate. The stress exponent, n in the relation εασⁿ, was found to be 5.7 and the creep activation energy 711 ± 20 kJ/mol. The controlling creep mechanism was determined to be dislocation glide/climb controlled by climb.     

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.     

Microstructural Changes in Liquid-Phase-Sintered Silicon Carbide during Creep in an Oxidizing Environment

The knowledge of the microstructural evolution during exposure to high temperatures is important to understanding the mechanisms responsible for the creep resistance of silicon carbide (SiC) ceramics. This includes not only the phase transformation of the SiC grains, but also the phase transformations of the oxynitride grain-boundary phases. For this study, a series of SiC specimens were prepared with varying molar ratios of AlN-Y₂O₃ additives. Increased creep resistance was observed in specimens with an additive system containing a 2:3 molar ratio or 60 mol% Y₂O₃. A continuous oxide layer of Y₂Si₂O₇ formed at the surface during elevated temperature testing in air. No blistering or cracking was observed in this oxide coating. Further increase of the creep resistance was achieved by a post-sintering nitrogen anneal.     

Microstructural and Microchemical Characterization of Silicon Carbide and Silicon Carbonitride Ceramic Fibers Produced from Polymer Precursors

Several continuous SiC and SiC/N-based ceramic fibers prepared from different polymer precursors have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and high-resolution electron microscopy (HREM). Methods to prepare longitudinal as well as cross-sectional thin specimens from brittle ceramic fibers were developed to facilitate HREM and EELS studies. Lattice images clearly showed nanometer-sized crystallites, as well as amorphous regions. Microchemical analysis using EELS permitted study of the form and distribution of the various chemical species within the fibers.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: III, Sintered α-Silicon Carbide

The operative and controlling mechanisms of steady-state creep in sintered α-SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B₄C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/ T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain-boundary sliding accommodated by grain-boundary self-diffusion; at high temperatures, the controlling mechanism becomes grain-boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.