Abstract
The effects of oxidation on heat transfer and mechanical behavior of ZrB2-SiC ceramics at high temperature are modeled using a micromechanics based finite element model. The model recognizes that when exposed to high temperature in air ZrB2-SiC oxidizes into ZrO2, SiO2, and SiC-depleted ZrB2 layer. A steady-state heat transfer analysis was conducted at first and that is followed by a thermal stress analysis. A “global-local modeling” technique is used combining finite element with infinite element for thermal stress analysis. A theoretical formulation is developed for calculating the thermal conductivity of liquid phase SiO2. All other temperature dependent thermal and mechanical properties were obtained from published literature. Thermal stress concentrations occur near the pore due to the geometric discontinuity and material properties mismatch between the ceramic matrix and the new products. The predicted results indicate the development of thermal stresses in the SiO2 and ZrO2 layers and high residual stresses in the SiC-depleted ZrB2 layer.
Highlights
Ultrahigh temperature ceramics (UHTCs) such as zirconium diboride and hafnium diboride (ZrB2 and HfB2) have been proposed for thermal protection of hypersonic aerospace vehicles, which may be exposed to temperatures above 1500∘C in oxidizing environments
In the thermal stress analysis, the layout of infinite elements and finite elements, as well as the displacement constraints for the stress analysis shown in Figure 8, are used
Thermal conductivity was calculated for the liquid phase of SiO2 based on a theoretical formulation
Summary
Ultrahigh temperature ceramics (UHTCs) such as zirconium diboride and hafnium diboride (ZrB2 and HfB2) have been proposed for thermal protection of hypersonic aerospace vehicles, which may be exposed to temperatures above 1500∘C in oxidizing environments. These materials are chemically and physically stable above 1600∘C and have melting points above 3000∘C [1]. Above 1100∘C, SiC (s) oxidizes by reaction to form SiO2 (l) which has a lower volatility and a higher melting point and viscosity compared with B2O3 (l) [3,4,5]. The oxide scales that form on ZrB2-SiC consist of an outer layer of SiO2, a middle layer of porous ZrO2, sometimes filled with SiO2, and a layer of SiC-depleted ZrB2-SiC at around 1500∘C
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