Modeling Oxidation Kinetics of SiC‐Containing Refractory Diborides

Abstract

Experimental data on the oxidation kinetics of SiC‐containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter‐current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO2 counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB2 containing varying volume fractions of SiC, and for SiC–HfB2 ultra‐high temperature ceramics (UHTCs) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory‐furnace‐based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc‐jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc‐jet environment.

Effect of surface oxidation on thermal shock resistance of ZrB 2 –SiC–ZrC ceramic at temperature difference from 800 to 1900 °C

“…The development of the layered structure has analyzed with the aid of a thermodynamic model that involved volatility diagrams for ZrB 2 and SiC [19]. When exposed to air at high temperatures of below 1800 • C, ZrB 2 reacts to form crystalline ZrO 2 and boria (B 2 O 3 ) which is a quite fluid liquid.…”

Creep of Hi-Nicalon S Ceramic Fiber Tows at 800 deg C in Air and in Silicic Acid-Saturated Steam

“…Several studies have investigated the thermal stability of SiC fibers in oxidizing environments and the effects of oxidation on SiC fibers and its relation to subcritical crack growth (delayed failure) [24,43,49,105,111,121,[123][124][125][126][142][143][144][145][146][147][148][149][150][151][152][153][154][155][156][157][158]. LaraCurzio modeled the effects of oxidation on the stress-rupture behavior of fiber bundles and presented results for Nicalon T M fibers.…”

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.…”

“…The literature suggests CO/CO 2 (g) counter-current diffusion could also be used to explain the source of O 2 (g) for further active oxidation of the SiC 26,90 . Figure 46 shows that the temperature dependence of observed depletion layer growth is minimal, suggesting that gas phase diffusion is rate limiting.…”

“…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.…”