High temperature mechanical behaviour of UHTC coatings for thermal protection of re-entry vehicles

摘要

In this work the high temperature mechanical properties of UHTC coatings deposited by plasma spraying have been investigated; particularly the stress-strain relationship of ZrB_2 based thick films has been evaluated by means of 4-point bending tests up to 1500℃ in air. Results show that at each investigated temperature (500, 1000, 1500℃) Modulus of Rupture (MOR) values are higher than the ones obtained at room temperature; moreover at 1500℃ the UHTC coatings exhibit a marked plastic behaviour, maintaining a flexural strength 25% higher compared to RT tested samples. The coefficient of linear thermal expansion (CTE) has been evaluated up to 1500℃: obtained data are of primary importance for substrate selection, interface design and to analyze the thermo-mechanical behaviour 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 indentify the microstructural modifications induced by the combination of mechanical loads and temperature in an oxidizing environment.