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

Formation of Structural Intermetallics by Reactive Metal Penetration of Ti and Ni Oxides and Aluminates

Alumina-aluminum composites can be prepared by reactive metal penetration (RMP) of mullite by aluminum. The process is driven by a strong negative free energy for the reaction (8 +x)Al + 3Al6Si2013 → 13Al2O3 + 6Si + xAl. Thermodynamic calculations reveal that titanium oxide, aluminum titanate, nickel oxide, and nickel aluminate all have a negative free energy of reaction with aluminum from 298 to 1800 K, indicating that it may be possible to form alumina-intermetallic composites by reactions of the type (2 +x)Al + (3/y) MOy → Al2O3 + AlxM3/y. Experiments revealed that aluminum reacts with titanium oxide, nickel oxide, and nickel aluminate, but not aluminum titanate, at 1673 K. Reaction with the stoichiometric amount of aluminum (x = 0) leads to the formation of alumina and either titanium or nickel. In some cases, reactions with excess aluminum (x \u3e 0) produce intermetallic compounds such as TiAl3 and NiAl