Oxidation kinetics of silicon carbide-containing refractory diborides, II: Critical assessment of permeability data for model parameters

Abstract

Permeabilities of CO, H₂O, and H₂ in silica, boria, and borosilicate glasses are critical parameters in the recently revised model of oxidation of MeB₂-SiC ultra-high temperature ceramics (UHTCs). In this work, the available literature data on these parameters have been critically reviewed and evaluated. Kinetics and bubble formation during oxidation of SiC have often been discussed in the literature in relation to the relative permeation rates of O₂ and CO in silica. The analysis presented in this paper suggests that oxidation kinetics and bubble formation during the oxidation of SiC do not provide a means to quantitatively assess the permeability of CO. In combination with the semi-empirical correlations between the permeation rates, diffusivity, and solubility of other atomic and molecular gases and their atomic or molecular dimensions, the limited quantitative data on the permeability of CO in silica available in the literature were used to propose the upper and lower bounds for CO permeability and to assign tentative values to the Arrhenius parameters of its temperature dependence. No data on the permeability of CO in either boria or borosilicate glass have been found in the literature. The available literature data on the permeability of water vapor and hydrogen in silica, boria, and borosilicate glasses have been collected, compared, and critically assessed. Significant uncertainties and gaps exist in the available data on water vapor permeation rates in boria and borosilicate glasses, and no data have been found on hydrogen permeation in boria or boria-rich borosilicate glasses (above ∼14 mol% boria). Experimental measurements of CO permeability coefficients in silica, boria, and borosilicate glasses as a function of temperature and boria and water content, as well as permeability rates of water vapor in boria and borosilicate glasses, would greatly advance modeling and understanding of the environmental oxidation of this class of UHTCs and composites.