Corrosion of Silicon Carbide in Gases and Alkaline Melts

The corrosion behavior of sintered SiC in gaseous environments and alkaline melts was investigated at 900°C. In oxidizing atmospheres such as normally exist in a gas turbine, SiC forms a dense coherent surface film of SiO₂ which is not corroded by thin layers of condensed sodium sulfate. However, under some conditions, especially when very low oxygen pressures are maintained at the SiC surface or when basic salt melts or slags containing carbonaceous material are present, rapid corrosion of the ceramic can occur. On the other hand, SiC is inert in pure N₂, H₂, or H₂-H₂S mixtures at 900°C. These different modes of behavior are discussed in the context of possible high- temperature applications of SiC ceramics.     

High-Temperature Mixed Oxidation of Nitride-Bonded Silicon Carbide in Oxidizing Gas Mixtures Containing 2% Cl2

Nitride-bonded silicon carbide ceramics have lower processing costs than many other SiC-based ceramics and adequate properties for use as high-temperature heat exchangers in oxidizing environments. Silicon nitride has much better resistance to attack by chlorine at temperatures above 900°C than silicon carbide. When nitride-bonded silicon carbide ceramics are exposed to gas mixtures containing 2% Cl₂ and small amounts of oxygen in this temperature range, the SiC is selectively chlorinated, leaving behind a porous matrix of silicon nitride. The rate of corrosion is controlled by a combination of interfacial kinetics at the surfaces of the SiC grains and transport of volatile species through the silicon nitride skeleton. In more oxidizing environments, the rate of chlorination is suppressed by the formation of a protective SiO₂ film. In highly oxidizing environments at temperatures in excess of 1200°C, the formation of volatile chloride reaction products at the interface between the SiC and the passivating SiO₂ layer causes bubbles to form in the SiO₂, which accelerates the oxidation.     

Corrosion of Mullite by Molten Salts

The interaction of molten salts of different Na₂O activities and mullite is examined with furnace and burner tests. The more-acidic molten salts form small amounts of Al₂O₃; the more-basic molten salts form various Na₂O–Al₂O₃–SiO₂ compounds. The results are interpreted using the Na₂O–Al₂O₃–SiO₂ ternary phase diagram, and some possible diffusion paths are discussed. The generally higher melting points of Na₂O–Al₂O₃–SiO₂ compounds lead to better behavior of mullite in molten salts, as compared to SiO₂-protected ceramics such as SiC. Mullite-coated SiC is discussed, and the corrosion behavior is evaluated.     

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.     

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.     

Effects of High Water-Vapor Pressure on Oxidation of Silicon Carbide at 1200°C

The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H₂O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO₂ scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO₂ layer, which remained constant in thickness with time as the crystalline SiO₂ continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO₂ volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO₂ to porous, nonprotective SiO₂.     

Active Gaseous Corrosion of Porous Silicon Carbide

In a reducing atmosphere, SiO₂ as an impurity in SiC can react to form SiO and CO gases. Likewise, oxygen in the atmosphere can also lead to the same gaseous species. Experimental data are reported on the rate of weight loss in H₂-H₂O atmospheres of porous SiC containing residual SiO₂. The amount of residual SiO₂ was varied by passive oxidation of the porous SiC prior to heating in the reducing atmosphere. The data are compared to a model of active corrosion to determine the mechanism of weight loss and to identify the gas species present in SiC with SiO₂ as an impurity.     

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.     

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.     

Grain-Boundary Viscosity of BaO-Doped SiC

Internal friction characterization of the viscosity of a residual SiO₂/BaO glass, segregated to grain boundaries of polycrystalline SiC, is presented. The anelastic relaxation peak of internal friction, arising from viscous slip along grain boundaries wetted by a glass phase, is analyzed. Two SiC polycrystals, containing SiO₂/BaO glasses with different compositions, are studied and compared with a SiC polycrystal containing only pure SiO₂. The internal friction peak is first analyzed with respect to its shift upon frequency change. This analysis allows quantitative assessment of both the intrinsic viscosity and the activation energy for viscous flow of the grain-boundary phase. Both parameters markedly decrease with increasing amounts of BaO dopant, which is consistent with data reported in the literature on SiO₂ and SiO₂/BaO bulk glasses with the same nominal composition. Analysis of the peak morphology is also attempted, considering the evolution of peak width while varying the grain-boundary glass composition. Moreover, the role of microstructural parameters, such as the distributions of grain size and grain-boundary angles, on the broadening of the internal friction peak is addressed, and a procedure is proposed that allows quantitative evaluation of the activation energy for viscous flow of intergranular glass merely from the width of the internal friction peak.     

Role of Carbon in the Sintering of Boron-Doped Silicon Carbide

The effect of carbon on the sintering of boron-doped SiC was studied. The free carbon present in the green compact was found to react with the SiO₂ covering the surfaces of the SiC particles; however, even if no carbon was added, the surface SiO₂ reacted with the SiC itself at a slightly higher temperature. This latter reaction was associated with the onset of substantial pore growth in the shrinking green body, which, as the pores continued to grow at higher temperatures, prevented complete densification. Therefore, the reaction of the SiC with the SiO₂ may have led to the fracture of interparticle contacts, resulting in the onset of coarsening. Thus, the role of the carbon was to prevent reaction between the SiC and the surface SiO₂, by removing the SiO₂ at a temperature below that at which this reaction could occur.     

Grain-Boundary Viscosity of Polycrystalline Silicon Carbides

Internal friction experiments were conducted on three SiC polycrystalline materials with different microstructural characteristics. Characterizations of grain-boundary structures were performed by high-resolution electron microscopy (HREM). Observations revealed a common glass-film structure at grain boundaries of two SiC materials, which contained different amounts of SiO₂ glass. Additional segregation of residual graphite and SiO₂ glass was found at triple pockets, whose size was strongly dependent on the amount of SiO₂ in the material. The grain boundaries of a third material, processed with B and C addition, were typically directly bonded without any residual glass phase. Internal friction data of the three SiC materials were collected up to similar/congruent2200°C. The damping curves as a function of temperature of the SiO₂-bonded materials revealed the presence of a relaxation peak, arising from grain-boundary sliding, superimposed on an exponential-like background. In the directly bonded SiC material, only the exponential background could be detected. The absence of a relaxation peak was related to the glass-free grain-boundary structure of this polycrystal, which inhibited sliding. Frequency-shift analysis of the internal friction peak in the SiO₂-containing materials enabled the determination of the intergranular film viscosity as a function of temperature.     

Paralinear Oxidation of CVD SiC in Simulated Fuel-Rich Combustion

The oxidation kinetics of CVD SiC were measured by thermogravimetric analysis (TGA) in a 4H₂·12H₂O·10CO·7CO₂·67N₂ gas mixture flowing at 0.44 cm/s at temperatures between 1300° and 1450°C in fused quartz furnace tubes at 1 atm total pressure. The SiC was oxidized to form solid SiO₂. At ≥1350°C, the SiO₂ was in turn volatilized. Volatilization kinetics were consistent with the thermodynamic predictions based on SiO formation. These two simultaneous reactions resulted in overall paralinear kinetics. A curve fitting technique was used to determine the linear and parabolic rate constants from the paralinear kinetic data. Volatilization of the protective SiO₂ scale resulted in accelerated consumption of SiC. Recession rates under conditions more representative of actual combustors were estimated from the furnace data.     

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.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

SiC Recession Caused by SiO2 Scale Volatility under Combustion Conditions: I, Experimental Results and Empirical Model

A high-pressure burner rig was developed to evaluate the response of chemical-vapor-deposited SiC material during exposure to simulated gas turbine combustor conditions. Linear weight loss and surface recession rates of SiC were observed in both fuel-lean and fuel-rich gas mixtures. This response was shown to result from SiO₂ scale volatility. Arrhenius-type temperature dependence was demonstrated. In addition, the effects of pressure and gas velocity were defined in terms of a gaseous-diffusion-controlled process for volatile reaction products (such as SiO, Si(OH)₄, and iO(OH) _x ). Accordingly, multiple linear regression was used to develop empirical recession relationships of the form exp(-Delta Q/RT ) P^xv^y for both lean and rich combustion conditions. Part II of this paper discusses the thermodynamics and gaseous-diffusion model of this recession. The empirical models discussed here enable prediction of SiC recession for any combination of T, P , and v in turbine environments. For typical combustion conditions, recession of 0.2-2 µm/h was predicted at 1200°-1400°C. Thus, long-term, high-temperature, high-velocity exposure may degrade silicon-based or SiO₂-forming material by recession in combustion gas environments.     

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 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 ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.