Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB₂+SiC and Oxygen Transport Mechanisms

Abstract

Refractory diboride with silicon carbide additive has a unique oxide scale microstructure with two condensed oxide phases (solid+liquid), and demonstrates oxidation resistance superior to either monolithic diboride or silicon carbide. We rationalize that this is because the silica‐rich liquid phase can retreat outward to remove the high SiO gas volatility region, while still holding onto the zirconia skeleton mechanically by capillary forces, to form a “solid pillars, liquid roof ” scale architecture and maintain barrier function. Basic assessment of the oxygen carriers in the borosilicate liquid in oxygen‐rich condition is performed using first‐principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature TC∼1500°C, the dominant oxygen carriers should be network defects, such as peroxyl linkage or oxygen‐deficient centers, instead of molecular O2* as in the Deal–Grove model. These network defects will lead to sublinear dependence of the oxidation rate with external oxygen partial pressure. The present work suggests that there could be significant room in improving the high‐temperature oxidation resistance by refining the oxide scale microstructure as well as controlling the glass chemistry.

Molecular Modeling of High-Temperature Oxidation of Refractory Borides

“…For our initial studies, a cubic supercell containing 14 Si0 2 + 7 B203 formula units is used, with total 77 atoms. The average density is 2.3 g/cm 3 . A typical liquid structure at T = 2500'C, after further equilibration by ab initio MD, clearly has a framework structure with no dangling bonds (all Si are 4-fold coordinated to 0, and all B are 3-fold coordinated to 0).…”

Oxidation Behavior of Zirconium Diboride Silicone Carbide Based Materials at Ultra-High Temperatures

“…Further study of bubble formation is required to confirm this proposed mechanism for a wider range of materials and oxidation conditions. The continued presence of C between the ZrO 2 grains as discussed in Chapter 5, instead of the borosilicate glass reported in the literature 19,22,26,27 , must also be reconciled with this mechanism. 2.…”

“…liquid roof" scale architecture described by Li et al The ZrO 2 skeleton supports against bubble breakouts and external shear flow 22 . Some bubbles still form in the borosilicate glass and burst, as CO(g) is formed at the reaction interface at significant pressures by Reactions 1.3 or 1.6 and move to escape the material.…”

“…It is important to note that the oxidation of the two phase ZrB 2 -SiC mixture offers additional protection to both phases relative to oxidation of the phases individually 19,22 . The improvement relative to ZrB 2 has been explained by the more sluggish oxygen diffusion through the SiO 2 -rich layer than through B 2 O 3 , while the improvement over pure SiC at temperatures above the melting point of SiO 2 is explained by the "solid pillars,…”

“…Bubble formation due to generation of gaseous oxidation products was assumed as the most likely source of oxidation variability, based off the microstructure and oxide products discussed in the literature 19,22,26,36 and assuming that the activity of all solids and liquids is unity, the equilibrium partial pressure of oxygen at the ZrB 2 /ZrO 2 interface was calculated to be 1.9x10 -16 atm. Using the Equilibrium module, inputting ZrB 2 -SiC, and holding the activity of oxygen at the equilibrium value of P O2 , the partial pressure of other gasses formed were calculated.…”