Ultra-High-Temperature Testing of Sintered ZrB2-based Ceramic Composites in Atmospheric Re-entry Environment

An experimental campaign has been carried out to characterize a new class of Ultra-High-Temperature Ceramic Matrix Composites for near-zero ablation Thermal Protection Systems. Small-sized specimens, with ZrB₂-based matrix and different carbon fiber architectures, were exposed to a simulated air supersonic flow generated by an arc-jet wind tunnel, achieving specific total enthalpies up to 20 MJ/kg and cold wall fully catalytic heat fluxes over 5 MW/m², in an aero-thermo-chemical environment representative of atmospheric re-entry. Ablation rates were estimated by means of mass and thickness measurements before and after testing, demonstrating an excellent performance of the developed materials. Surface temperatures were monitored by means of infrared pyrometers and a thermo-camera, and during all the tests a spontaneous temperature jump was observed, with temperatures that reached values over 2800 K at the steady state. Post-test microstructural analyses revealed the formation of a porous oxide layer with a thickness of few hundred microns, mainly consisting of zirconia, with substantial removal of both SiC and carbon fibers. Below the oxide, the bulk material was unaffected. Computational Fluid Dynamics simulations allowed rebuilding the thermo-fluid-dynamic and chemical flow field. Moreover, it was possible to propose an innovative correlation of the temperature jump with an increased catalytic activity and a dramatic reduction of the thermal conductivity of the oxide layers forming on the exposed part of the sample, which anyway had a key role in preserving the unoxidized bulk materials at reasonable temperatures.