Reducing the temperature of a C/C composite heat shield for solar probe missions with an optically selective semi-transparent pyrolytic boron nitride (pBN) coating

Two solar missions are designed to plunge into the solar corona, and respectively get as close as 8.5 and 3 Rs (solar radii) from the Sun’s surface. Heat shields will protect the payloads within their umbra from the thermal radiation and solar winds. We report the high temperature behavior of a pBN coating on a C/C composite. We established thanks to spectral emissivity measurements at high temperature that covering the C/C composite with pBN leads to a strong decrease of the ratio α/ε of the solar absorptivity to the total emissivity. The α/ε ratios calculated from the spectral emissivity measurements are correlated to the ones obtained from integrated emissivity measurements. A resulting surface temperature reduction of 200 K is estimated at 8.5 solar radii between uncoated C/C and pBN coated C/C. In the semi-transparent region of the spectral emissivity of pBN, located in the solar spectrum, the substrate C/C keeps a strong contribution in the spectral emissivity of the assembly pBN + C/C. The level of this contribution is function of the pBN coating thickness. Finally, material characterizations performed before and after treatment at 2200 K showed that the pBN coating has a good stability at high temperature.