Modeling of Oxidation Effects on Heat Transfer Behavior of ZrB₂ and ZrB₂-SiC Ceramics at High Temperature

Hypersonic vehicles need thermal protection materials such as ultrahigh temperature ceramics (UHTCs). ZrB2 and ZrB2-SiC have been proposed as candidates for such applications. Even though they have excellent oxidation resistance, high temperature exposure of ZrB2 will result in its oxidations. After oxidation in air at high temperature, it will generate new products of ZrO2, B2O3, and SiO2. The material and geometric changes from the original ZrB2 and ZrB2-SiC ceramics will affect the heat transfer behavior due to the mismatch of thermal properties between the materials. A steady-state heat transfer analysis was conducted using finite element analysis (FEA) modeling. Adaptive remeshing technique was used to improve analytical accuracy. Thermal conductivity was calculated for liquid phase of B2O3 and SiO2 based on a theoretical formulation. In the FEA modeling, all thermal properties are temperature dependant. Simulated results indicate that the heat flux concentration occurs at the pore corner. ZrB2-SiC ceramic has higher thermal resistance than ZrB2 ceramic has after oxidation

Mechanical and Thermal Properties of Hot-Pressed ZrB2-SiC Composites

ZrB2-SiC composites were hot pressed at 2473 K (2200 A degrees C) with graded amounts (5 to 20 wt pct) of SiC and the effect of the SiC addition on mechanical properties like hardness, fracture toughness, scratch and wear resistances, and thermal conductivity were studied. Addition of submicron-sized SiC particles in ZrB2 matrices enhanced mechanical properties like hardness (15.6 to 19.1 GPa at 1 kgf), fracture toughness (2 to 3.6 MPa(m)(1/2)) by second phase dispersion toughening mechanism, and also improved scratch and wear resistances. Thermal conductivity of ZrB2-SiC (5 wt pct) composite was higher 121 to 93 W/m K from 373 K to 1273 K (100 A degrees C to 1000 A degrees C)] and decreased slowly upto 1273 K (1000 A degrees C) in comparison to monolithic ZrB2 providing better resistance to thermal fluctuation of the composite and improved service life in UHTC applications. At higher loading of SiC (15 wt pct and above), increased thermal barrier at the grain boundaries probably reduced the thermal conductivity of the composite