Multilayer SiC for thermal protection system of space vehicles with decreased thermal conductivity through the thickness

Tape casting combined with layer stacking, debinding and sintering proved to be a suitable technique for processing SiC-based multilayers to be used as thermal protection system for space vehicles. In this paper two methods have been evaluated in order to decrease the thermal conductivity through the thickness of the material: the insertion of layers containing a pore forming agent able to leave residual porosity; the deposition of an external insulating coating made of yttria-partially stabilized zirconia (YPSZ). Both techniques contribute to the reduction of thermal conductivity. The insertion into the multilayered structure of porous layers with a high level of residual porosity halves the thermal conductivity but 25% decrease in bending strength is observed. The fracture surface however shows that highly porous layers activates crack deflection mechanism, increasing fracture energy. The use of an YPSZ coating allows for a less evident reduction of thermal conductivity (from 102 to 75 W/(m K) at room temperature) but good mechanical properties are maintained (340 MPa bending strength).