A Thermo‐Chemo‐Mechanical Model for the Oxidation of Zirconium Diboride

A thermo-chemo-mechanical model was proposed, which couples the oxidation rate of ZrB2 between 1000°C and 1800°C with the induced mechanical stress in the oxide scale. The model includes the mechanism for the coupling effect. Due to the special porous microstructure of the oxide, the diffusivities of the oxidation reactants and products through the columnar pores dominate the oxidation kinetics. The pores in the oxide shrink under the compressive stress generated during the oxidation due to the constraint from the substrate to the lateral growth of the oxide. And the shrinkage reduces the Knudsen diffusivities of both the molecular oxygen coming inward and the liquid boria evaporating outward. Consequently, the oxidation rate is reduced, which also affects the stress state. Mechanical approaches, such as Mori–Tanaka homogenization method and laminate theory were adopted in the model to quantitatively describe the coupled effect. The evolutions of oxide thickness, pore diameter, and stresses in both the oxide and the substrate were predicted with the model, which showed that the oxidation rate can be significantly altered by the stress-diffusion coupling during isothermal oxidation, especially at higher temperatures in the range 1000°C–1800°C.