No abstract

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.