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Silicon-based ceramics and composites are prime candidates for heat engine and heat exchanger structural components. In such applications these materials are exposed to combustion gases and deposit-forming corrodents. In this paper combustion environments are defined for various applications. These environments lead to five main types of corrosive degradation: passive oxidation, deposit-induced corrosion, active oxidation, scale/substrate interactions, and scale volatility. Each of these is discussed in detail. The key issues in oxidation mechanisms of high-purity silicon carbide (Sic) and silicon nitride (Si,N4) in pure oxygen are discussed. The complicating factors due to the actual combustion environment and commercial materials are discussed. These discussions include secondary elements in the ceramics; additional oxidants, such as water and carbon dioxide (COJ; combustion environment impurities; longterm oxidation effects; and thermal cycling. Active oxidation is expected in a limited number of combustion situations, and the active-to-passive transition is discussed. At high temperatures the limiting factors are scale melting, scale volatility, and scale/substrate interactions. Depositinduced corrosion is discussed, primarily for sodium sulfate (Na,SO,), but also for vanadate and oxide-slag deposits as well. In applying ceramics in combustion environments it is essential to be aware of these corrosion routes and how they affect the performance of a component.

In combustion environments, volatilization of SiO 2 to Si-O-H(g) species is a critical issue. Available thermochemical data for Si-O-H(g) species were used in the present study to calculate boundary-layer-controlled fluxes from SiO 2 . Calculated fluxes were compared to volatilization rates of SiO 2 scales grown on SiC, which were measured in a highpressure burner rig, as reported in Part I of this paper. Calculated volatilization rates also were compared to those measured in synthetic combustion gas furnace tests. Probable vapor species were identified in both fuel-lean and fuel-rich combustion environments, based on the observed pressure, temperature, and velocity dependencies, as well as on the magnitude of the volatility rate. Water vapor was responsible for the degradation of SiO 2 in the fuel-lean environment. SiO 2 volatility in fuel-lean combustion environments was attributed primarily to the formation of Si(OH) 4 (g), with a small contribution of SiO(OH) 2 ( g). Reducing gases such as H 2 and/or CO, in combination with water vapor, contributed to the degradation of SiO 2 in the fuel-rich environment. The model to describe SiO 2 volatility in a fuel-rich combustion environment gave a less satisfactory fit to the observed results. Nevertheless, it was concluded-given the known thermochemical data-that SiO 2 volatility in a fuel-rich combustion environment is best described by the formation of SiO( g) at 1 atm total pressure and the formation of Si(OH) 4 ( g), SiO(OH) 2 ( g), and SiO(OH)( g) at higher pressures. Other Si-O-H( g) species, such as Si 2 (OH) 6 , may contribute to the volatility of SiO 2 under fuel-rich conditions; however, complete thermochemical data are unavailable at this time.

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In this paper, we report the results of equilibrium pressure measurements designed to identify the volatile species in the Cr-O-H system and to resolve some of the discrepancies in existing experimental data. In addition, ab initio calculations were performed to lend confidence to a theoretical approach for predicting the thermochemistry of chromium-containing compounds. Equilibrium pressure data for CrO2(OH)2 were measured by the transpiration technique for the reaction 0.5Cr2O3(s) + 0.75O2(g) + H2O(g) = CrO2(OH)2(g) over a temperature range of 573 to 1173 K at 1 bar total pressure. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the condensate in order to quantify the concentration of Cr-containing volatile species. The resulting experimentally measured thermodynamic functions are compared to those computed using B3LYP density functional theory and the coupled-cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)].

High‐temperature oxidation of monolithic boron nitride (BN) is examined at 900–1200°C. Hot‐pressed BN and both low‐ and high‐density chemically vapor‐deposited BN are studied. The oxidation product is B2O3(l) and the oxidation kinetics are sensitive to crystallographic orientation, porosity, and impurity levels. The B2O3 product also reacts readily with ambient water vapor in the test furnace (ppm levels) to form the volatile species HBO2(g), leading to overall paralinear kinetics. The linear rate constant extracted from these experiments agreed with that predicted from diffusion of HBO2(g) across a static boundary layer.