High Temperature Mechanical Behavior of UHTC Coatings for Thermal Protection of Re-Entry Vehicles

In this work, the high temperature mechanical properties of ultra high temperature ceramics (UHTC) coatings deposited by plasma spraying have been investigated; particularly the stress-strain relationship of ZrB2-based thick films has been evaluated by means of 4-point bending tests up to 1500 A degrees C in air. Results show that at each investigated temperature (500, 1000, and 1500 A degrees C) modulus of rupture (MOR) values are higher than the ones obtained at room temperature (RT); moreover at 1500 A degrees C the UHTC coatings exhibit a marked plastic behavior, maintaining a flexural strength 25% higher compared to RT tested samples. The coefficient of linear thermal expansion (CTE) has been evaluated up to 1500 A degrees C: obtained data are of primary importance for substrate selection, interface design and to analyze the thermo-mechanical behavior of coating-substrate coupled system. Finally, SEM-EDS analyses have been carried out on as-sprayed and tested materials in order to understand the mechanisms of reinforcement activated by high temperature exposure and to identify the microstructural modifications induced by the combination of mechanical loads and temperature in an oxidizing environment.