| Hypersonic vehicles require sharp-featured nose tips and wing leading edges to reduce aerodynamic drag. However, the geometry of these edges increases heat transfer to the surface. The thermal energy can be reduced via increasing the radiation heat transfer away from the surface. In this study, an emissivity modifier was incorporated into the ZrB2/SiC system to improve its surface radiation heat transfer rate. In one approach, Sm2O 3 or Tm2O3 were mechanical blended with ZrB 2/20vol%SiC. In the second approach, a Sm[NO3]3/ethanol solution was chemically infiltrated into the spray dried ZrB2/20vol% SiC powders. The coatings were fabricated using shrouded air plasma spray. X-ray diffraction and mass spectroscopy indicated approximately 52-56% of the rare earth elements were incorporated into the final coating. Total hemispherical emissivity results show that samarium-doped ZrB2/SiC coating has superior performance compared to ZrB2/SiC up to 1200°C. An oxyacetylene torch was utilized to evaluate the coatings under high heat flux conditions for hold times of 30 and 60 s. The resulting phases and microstructures were evaluated as a function of rare-earth type, modification approach, and ablation time. A brittle m-ZrO2 scale was observed in the ZrB2/SiC-only coating after ablative tests; during cooling this scale detached from the unreacted coating. In contrast, rare-earth modified coatings formed a protective oxide scale consisting primarily of Sm0.2Zr0.8O1.9. Finally, three samarium-dopant concentrations were investigated to determine the optimal high temperature total hemispherical emissivity and ablation resistance material ombinations. At 1500°C, total hemispherical emissivity of the 5 mol.% Sm-doped ZrB 2/SiC coatings (epsilon=0.87) is 20% higher compared to ZrB2/SiC coatings (epsilon=0.69). The main constituent of the oxide scale are the Sm 0.2Zr0.8O1.9 and Sm2Zr2O 7, which has thermal stability up to 2500°C, offering additional oxidation protection. |
