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.

Current state‐of‐the‐art environmental barrier coatings (EBCs) for Si‐based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3·2SiO2) or mullite + BSAS ((1−x)BaO·xSrO·Al2O3·2SiO2, 0 ≤x≤ 1) layer, and a BSAS top coat. Areas of concern for long‐term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt‐infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low‐melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water‐vapor‐containing atmospheres. Both reactions can be EBC life‐limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.

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 SiO 2 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 SiO(OH) x ). Accordingly, multiple linear regression was used to develop empirical recession relationships of the form exp(−⌬Q/RT )P x v 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, longterm, high-temperature, high-velocity exposure may degrade silicon-based or SiO 2 -forming material by recession in combustion gas environments.

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͑ZCS͒ have been investigated for use as potential aeropropulsion engine materials. These materials were oxidized in water vapor ͑90%͒ using a cyclic vertical furnace at 1 atm. The total exposure time was 10 h at temperatures of 1200, 1300, and 1400°C. Chemically vapor deposited SiC was also evaluated as a baseline for comparison. Weight change, X-ray diffraction analyses, surface and cross-sectional scanning electron microscopy, and energy dispersive spectroscopy were performed. These results are compared with tests conducted in a stagnant air furnace at temperatures of 1327°C for 100 min, and with high pressure burner rig ͑HPBR͒ results at 1100 and 1300°C at 6 atm for 50 h. Low velocity water vapor does not contribute significantly to the oxidation rates of UHTCs when compared to stagnant air. The parabolic rate constants at 1300°C, range from 0.29-16.0 mg 2 /cm 4 h for HS and ZCS, respectively, with ZS results between these two values. Comparison of results for UHTCs tested in the furnace in 90% water vapor with HPBR results was difficult due to significant sample loss caused by spallation in the increased velocity of the HPBR. Total recession measurements are also reported for the two test environments.

Qualitative residual stresses in current environmental barrier coatings (EBCs) were inferred from the curvature of EBC-coated SiC wafers, and the effects of EBC stresses on the durability of EBC-coated SiC were evaluated. The magnitude of substrate curvature correlated fairly well with the EBC-SiC coefficient of thermal expansion (CTE) mismatch, EBC modulus, and thermally induced physical changes in EBC. BSAS (1ÀxBaO . xSrO . Al 2 O 3 . 2SiO 2 , 0rxr1) components in the current EBCs, i.e., Si/mullite or mullite1BSAS/BSAS or yttria-stabilized zirconia (YSZ: ZrO 2 -8 wt% Y 2 O 3 ), were the most beneficial for reducing the EBC stress in as-sprayed as well as in post-exposure EBCs. The reduced stress was attributed to the low modulus of BSAS. The addition of a YSZ top coat significantly increased the substrate curvature because of its high CTE and sintering in thermal exposures. There were clear correlations between the wafer curvature and the EBC durability. The Si/mullite120 wt% BSAS/BSAS EBC maintained excellent adherence, protecting the SiC substrate from oxidation, while the Si/mullite120 wt% BSAS/YSZ EBC suffered delamination, leading to severe oxidation of the SiC substrate, after a 100 h À13001C exposure in a high-pressure burner rig. 3483J ournal

Two commercially available additive-containing silicon nitride materials were exposed in four environments which range in severity from dry oxygen at 1 atm pressure, and low gas velocity to an actual turbine engine. Oxidation and volatilization kinetics were monitored at temperatures ranging from 1066 to 1400°C. The main purpose of this paper is to examine the surface oxide morphology resulting from the exposures. It was found that the material surface was enriched in rare earth silicate phases in combustion environments when compared to the oxides formed on materials exposed in dry oxygen. However, the in situ formation of rare earth disilicate phases offered little additional protection from the volatilization of silica observed in combustion environments .It was concluded that externally applied environmental barrier coatings are needed to protect additive-containing silicon nitride materials from volatilization reactions in combustion environments.

Three SiC/BN/SiC composite specimens reinforced with different SiC fibers (Sylramic, Sylramic‐iBN, and Hi‐Nicalon Type S) were exposed in a combustion environment. Exposures were carried out for 151 h in a fuel‐lean high pressure burner rig at 0.9 MPa total pressure, sample temperatures near 1573 K, and a gas velocity of 15 m/s. Weight loss of all three composites was observed. Extensive oxidation of SiC fibers was observed in cracked locations. A mechanism based on borosilicate enhanced oxidation coupled with volatilization of boria is described. Ramifications of this degradation mechanism are discussed for long‐term applications of SiC/BN/SiC composites in combustion environments.