Perspirable skin: Thermal buckling achieved by complex functionally graded materials

A perspirable skin is a new design concept of thermal protection system to autonomously reduce the surface temperature in many applications such as reentry for space shuttle and hypersonic vehicles. A unique design features an assembly of tiles and core parts, which buckles upon heating. Potentially, a large gap can be generated through this buckling action to increase the cooling efficiency. The compressed coolant gas onboard is passed through this gap onto the surface. The coolant gas is expected to mix with the surface air eliminating the frictional heating and reducing the surface temperature. These tiles will be assembled and shrink-fitted within an opening on the existing skin (RCC). To induce the buckling action, each tile needs to have a unique CTE gradiency, which causes expansion radially and shrinkage tangentially upon heating. For our preliminary design, the core and the tiles are made of pure ZrW2O8 (zirconium tungstate) and ZrW2O8-ZrO2 (zirconia) Functionally Graded Material (FGM), respectively. ZrW2O8 has a large negative CTE value over a wide temperature range. The assembly design is modeled and verified buckling action through FEA with the best estimated thermal loading conditions. Most importantly, the fabrication process of these tiles made of complex Functionally Graded materials (FGMs) is described in this thesis. During the manufacturing process, influences of both nanopowders and oxides sintering additives on the final mechanical properties of ceramic samples are discussed. The final sintered density can be improved by introducing the nanopowders due to its large surface area, while the oxides sintering additives can also help the densification by forming liquid phase. A micromechanics model is used to predict the Young's modulus values and compare with the experimental data. It is showed that two groups of values have a very good agreement based on the relative density data. In order to make the assembly to shrink-fit into the RCC skin, samples are pre-sintered and machined into specified geometry. Finally, the homogenous assembly is made in order to run the test with the furnace.