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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.

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The volatile species enter this orifice, undergo a free jet expan-A high-pressure sampling mass spectrometer was used to sion, and form a molecular beam. The beam passes through a detect the volatile species formed from SiO 2 at temperatures series of differentially pumped vacuum chambers and finally between 1200؇ and 1400؇C in a flowing water vapor/oxygen enters a chamber which has a pressure sufficiently low to opergas mixture at 1 bar total pressure. The primary vapor ate a quadrupole mass filter. An ionization energy of 40 eV species identified was Si(OH) 4 . The fragment ion Si(OH) 3 ؉ was used. This sampling system preserves the chemical and was observed in quantities 3 to 5 times larger than the dynamic integrity of the volatile species, even if they are conparent ion Si(OH) 4 ؉ . The Si(OH) 3 ؉ intensity was found to densable or reactive. Although absolute vapor pressures cannot have a small temperature dependence and to increase with be measured with this system, important qualitative trends can the water vapor partial pressure as expected. In addition, be observed. 9-12 SiO(OH) ؉ , believed to be a fragment of SiO(OH) 2 , wasThe SiO 2 -H 2 O reaction took place in a 22 mm ID fused observed. These mass spectral results were compared to the quartz furnace tube, typically at 1300ЊC. An oxygen stream behavior of silicon halides.with a flow rate of 233 sccm was saturated with water vapor and subsequently flowed through a disk-shaped fused quartz

The reaction between S i c and S O z has been studied in the temperature range 1400-1600 K. A Knudsen cell in conjunction with a vacuum microbalance and a hightemperature mass spectrometer was used for this study. Two systems were studied-1:l SIC (2 wt% excess carbon) and SiOz; and 1:l:l SIC, carbon, and SiOz. In both cases the excess carbon forms additional SIC within the Knudsen cell and adjusts to the direct reaction of stoichiometric SIC and SiOz to form SiO(g) and CO(g) in approximately a 3:l ratio. These results are interpreted in terms of the Sic-0 stability diagram. [Key words: silicon carbide, silica, reactions, high temperature, Knudsen study.]

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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.