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.     

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.     

Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Thermal Stability of Low-Oxygen Silicon Carbide Fiber (Hi-Nicalon) Subjected to Selected Oxidation Treatment

Low-oxygen silicon carbide fibers (Hi-Nicalon) were oxidized at temperatures from 1073 to 1773 K under an oxygen partial pressure of 0.25 atm. The strength of the unoxidized core was practically unaffected by the oxidation temperatures. The strength of the as-oxidized fibers with an SiO₂ film decreased markedly with increasing oxidation temperature. When exposed subsequently to 1773 K in argon, the fibers with a SiO₂ film of 0.3–0.5 μm thickness had the best thermal stability.     

Interpretation of the Parabolic and Nonparabolic Oxidation Behavior of Silicon Oxynitride

The oxidation process of Si₂N₂O, prepared by a hot isostatic pressing technique, has been studied by the thermogravimetric method. The oxidation has been performed in oxygen for 20 h in the temperature range 1300° to 1600°C, producing oxide scales of amorphous SiO₂ and α-cristobalite. The weight gain for T 1350°C does not begin to follow a parabolic rate law, until a certain time, t ₀. The A ₀ parameter in the parabolic rate law, (Δ w / A ₀)²= K _p t + B , represents the cross section area, A , through which the oxygen diffuses; in the derivation of this law A is assumed to be constant during the experiment. If crystallization occurs during the oxidation process, A will decrease with time. A function, A ( t ), describing the time dependence, has been developed and incorporated into the parabolic rate law, yielding a new rate law, which reads Δ W/A ₀= a arctan √ bt + c √ t . This new rate law is valid in the time interval t < t ₀, whereas, for t > t ₀, the oxidation process follows the equation (Δ w/A ₀)²= K °_p t + B ₀. The relation of the latter equation to the common parabolic rate law is described. All of the oxidation curves are described by these equations. The activation energy of the oxygen diffusion (and of the oxidation ( K _p)) is found to be 245 ± 25 kJ/mol, which is consistent with literature values reported for oxygen diffusion.     

Suppression of Active Oxidation of Polycarbosilane-Derived Silicon Carbide Fibers by Preoxidation at High Oxygen Pressure

Three types of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon, and Hi-Nicalon S) with different SiO₂ film thicknesses ( b ) were subjected to exposure tests at 1773 K in an argon-oxygen gas mixture with an oxygen partial pressure of 1 Pa. The suppression effect of a SiO₂ coating on active oxidation was examined through TG, XRD analysis, SEM observation, and tensile tests. All the as-received fibers were oxidized in the active-oxidation regime. The mass gain and the SiO₂ film development showed a suppression of active oxidation at b values of ≧0.070 μm for Nicalon, ≧0.013 μm for Hi-Nicalon, and ≧0.010 μm for Hi-Nicalon S fibers. Considerable strength was retained in the SiO₂-coated fibers. For Hi-Nicalon fibers, the retained strength was 71%–90% of the strength in the as-received state (2.14–2.69 GPa).     

Paralinear Oxidation of Silicon Nitride in a Water-Vapor/Oxygen Environment

Three Si₃N₄ materials were exposed to dry oxygen flowing at 0.44 cm/s at temperatures between 1200° and 1400°C. Weight change was measured using a continuously recording microbalance. Parabolic kinetics were observed. When the same materials were exposed to a 50% H₂O–50% O₂ gas mixture flowing at 4.4 cm/s, all three types exhibited paralinear kinetics. The material was oxidized by water vapor to form solid SiO₂. The protective SiO₂ was in turn volatilized by water vapor to form primarily gaseous Si(OH)₄. Nonlinear least-squares analysis and a paralinear kinetic model were used to determine parabolic and linear rate constants from the kinetic data. Volatilization of the protective SiO₂ scale could result in accelerated consumption of Si₃N₄. Recession rates under conditions more representative of actual combustors were compared with the furnace data.     

Oxidation of BN-Coated SiC Fibers in Ceramic Matrix Composites

Thermodynamic calculations were performed to analyze the simultaneous oxidation of BN and SiC. The results show that, with limited amounts of oxygen present, the formation of SiO₂ should occur prior to the formation of B₂O₃. This agrees with experimental observations of oxidation in glassceramic matrix composites with BN-coated SiC fibers, where a solid SiO₂ reaction product containing little or no boron has been observed. The thermodynamic calculations suggest that this will occur when the amount of oxygen available is restricted. One possible explanation for this behavior is that SiO₂ formation near the external surfaces of the composite closes off cracks or pores, such that vapor phase O₂ diffusion into the composite occurs only for a limited time. This indicates that BN-coated SiC fibers will not always oxidize to form significant amounts of a lowmelting, borosilicate glass.     

Dependence of Oxidation Modes on Zirconia Content in Silicon Carbide/Zirconia/Mullite Composites

Two basic oxidation modes of silicon carbide/zirconia/mullite (SiC/ZrO₂/mullite) composites were defined based on the plotted curve of the gradient of the silica (SiO₂) layer thickness (formed on individual SiC particles) versus depth. Mode I, where oxygen diffusivity was much slower in the matrix than in the SiO₂ layer, exhibited a relatively large gradient and limited oxidation depth. Mode II, where oxygen diffusivity was much faster in the matrix than in the SiO₂ layer, displayed a relatively small gradient and an extensive oxidation depth. When the volume fraction of ZrO₂ was below a threshold limit, the composites exhibited Mode I behavior; otherwise, Mode II behavior was observed. For composites with a ZrO₂ content above the threshold limit, the formation of zircon (ZrSiO₄), as a result of the reaction between ZrO₂ and the oxidation product (i.e., SiO₂), might change the oxidation behavior from Mode II to Mode I.     

Oxidation of Silicon Nitride in Wet Air and Effect of Lutetium Disilicate Coating

Oxidation behavior of silicon nitride (Si₃N₄) was investigated in flowing air (2.45 cm/s) containing 10%–50% H₂O at a total pressure of 1.8–10 atm at 1300°–1500°C for 100 h. The oxidation of Si₃N₄ progressed with volatilization of the SiO₂ scale; it was more enhanced at a high partial pressure of H₂O rather than at high temperature. The total pressure had little effect on the oxidation. In order to avoid the oxidation, Si₃N₄ substrate was coated with lutetium disilicate (Lu₂Si₂O₇) layer through the intermediate SiO₂-rich phase. While the coating layer well suppressed the oxidation in case of small amount of water vapor, it was not sufficiently effective to suppress the oxidation when the water vapor was rich. SiO₂ volatilization was observed between the layer and substrate. The flexural strength of the coated Si₃N₄ at room temperature was somewhat increased after the oxidation in wet air, while that of the uncoated one was almost unchanged. This increase was attributable to crack healing of the substrate during the oxidation.     

High-Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO₄) on SiC was investigated using a LaPO₄-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO₄ is reduced in the presence of SiC and reacts to form La₂O₃ or La₂Si₂O₇ and SiO₂ as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO₂ film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO₄ coating. Continuous LaPO₄ coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO₄ coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO₂ oxidation product to penetrate the coating.     

Oxygen Transport in Silica at High Temperatures: Implications of Oxidation Kinetics

The apparent change in activation energy describing the parabolic rate constant for the passive oxidation of SiC is examined. New data are combined with reevaluated previous results to determine the influences of crystallinity, impurity contamination, and multiple flux mechanisms. The results suggest that the high-temperature transition from interstitial-dominant to network-dominant oxygen transport is a property of amorphous SiO₂ scales and does not exist for cristobalite. Highly crystalline scales do not show this transition. Agreement among various studies also suggests that, for high-purity SiO₂ scales, there is no difference between the rates of interstitial oxygen transport in amorphous SiO₂ and in β-cristobalite.     

Thermal Oxidation Kinetics of MoSi2-Based Powders

The oxidation process of MoSi₂ is very complex, and controversial results have been reported, especially for the early-stage oxidation before the formation of passive SiO₂ film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi₂ powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO₂ film. The oxidation kinetics of commercial MoSi₂-SiC and MoSi₂-Si₃N₄ powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X-ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO₃ phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo₅Si₃ phase formed first and was subsequently oxidized to solid SiO₂ and volatile MoO₃, resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi₂ and Mo₅Si₃ are proportional to their fractions in the system describes the experimental data well.     

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.     

Oxidation of CVD Si3N4 at 1550° to 1650°C

A thermo gravimetric study of the oxidation behavior of chemically vapor-deposited amorphous and crystalline Si₃N₄ (CVD Si₃N₄) was made in dry oxygen (0.1 MPa) at 1550° to 1650°C. The specimens were prepared under various deposition conditions using a mixture of SiCl₄, NH₃, and H₂ gases. The crystalline CVD Si₃N₄ indicated a parabolic oxidation kinetics over the whole temperature range, whereas the amorphous CVD Si₃N₄ changed from a parabolic to a linear law with increased temperature. The oxidation mechanism is discussed in terms of the activation energy for the oxidation and the microstructure of the formed oxide films.     

Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

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.     

Electrical Properties of the Ta-RuO2 Diffusion Barrier for High-Density Memory Devices

The electrical properties of a Ta layer prepared with and without RuO₂ addition were investigated. The Ta + RuO₂/TiSi₂/poly-Si/SiO₂/Si contact system exhibited lower total resistance and ohmic characteristics up to 800°C. Meanwhile, the Ta/TiSi₂/poly-Si/SiO₂/Si contact system showed higher total resistance and nonohmic behavior after annealing at 650°C, attributed to the oxidation of both Ta and TiSi₂ layers. In the former case, a Ta + RuO₂ diffusion barrier showed an amorphous Ta microstructure and embedded RuO _x nanocrystals in the as-deposited state. The conductive RuO₂ crystalline phase in the Ta + RuO₂ film was formed by reaction between the nanocrystalline RuO _x and oxygen indiffused from air during annealing. When the Ta layer was deposited with RuO₂ addition, therefore, both the electrical properties and the oxidation resistance of the Ta + RuO₂ diffusion barrier were better than those of TiN, TaN, and Ta-Si-N barriers.     

A Comparison of Oxidation Kinetics of O'-SiAlON and β-SiAlON Powders Synthesized from Bauxite

The oxidation mechanism and kinetics of β-SiAlON and O'-SiAlON powders synthesized from bauxite have been investigated using thermogravimetric analysis, X-ray diffraction (XRD), and scanning electron microscopy. Oxidation experiments were carried out in both isothermal and nonisothermal modes in oxygen atmosphere. β-SiAlON begins to oxidize at above 1000 K, and the rate increases significantly after 1200 K, forming SiO₂ and Al₂O₃ at low temperature and mullite at high temperature. By comparison, the oxidation of O'-SiAlON powder starts at about 1250 K and after 1300 K the rate increases significantly. The O'-SiAlON phase remains dominant up to 1623 K, as demonstrated by XRD analysis, and the oxidation products are SiO₂ as well as a small amount of mullite. At higher temperature, liquid phase resulting from the fusion of SiO₂ and the impurities in O'-SiAlON powder hinders the progress of oxidation, which causes O'-SiAlON to have more oxidation resistance. Diffusion is the controlling step during both the oxidation processes. Based on the experimental data, a new model for predicting the oxidation process is developed. The application of this new model to the two systems demonstrates the validity of this new model. The activation energy of the oxidation of O'-SiAlON and β-SiAlON has been calculated to be 224.1 and 175.7 kJ/mol, respectively.     

Oxidation and Volatilization of Silica Formers in Water Vapor

At high temperatures, SiC and Si₃N₄ react with water vapor to form a SiO₂ scale. SiO₂ scales also react with water vapor to form a volatile Si(OH)₄ species. These simultaneous reactions, one forming SiO₂ and the other removing SiO₂, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved. After steady state is achieved, the oxide found on the surface is a constant thickness, and recession of the underlying material occurs at a linear rate. The steady-state oxide thickness, the time to achieve steady state, and the steady-state recession rate can be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate also can be determined from parameters that describe a water-vapor-containing environment. Accordingly, maps have been developed to show these steady-state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of SiO₂ formers in water-vapor-containing environments, such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si₃N₄.