Effect of rare-earth dopant (Sm) concentration on total hemispherical emissivity and ablation resistance of ZrB2/SiC coatings

The temperature of leading edges during hypersonic flight can be reduced by maximizing the emissivity of their surface. In this study, the concentration of a known emissivity modifier (Sm) has been varied (3, 5, and 8 mol% Sm in ZrB2/SiC coatings) and the total hemispherical emissivity and ablation resistance of the coatings has been evaluated. Maximum emissivity was observed to occur for the 5 mol% Sm ZrB2/SiC coating, with a value of 0.9 measured at 1600 °C. All the ablated Sm-doped coatings have an oxide scale consisting of m-ZrO2 and c1-Sm0.2Zr0.9O1.9, and/or Sm2Zr2O7 depending on the Sm concentration. Large amounts of porosity were observed in the oxide scale of the ablated 8 mol% Sm ZrB2/SiC coating. This was attributed to viscosity reduction of the B2O3 and SiO2 glassy phases by the Sm3+ and their subsequent vaporization. The 5 mol% Sm-doped ZrB2/SiC demonstrated the optimal emissivity properties and dense oxide scale formation.