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2123 |
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Wednesday, May 05, 2010, Sophia Room 11:40 AM Aerospace Industry |
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High temperature mechanical behavior of UHTC coatings for thermal protection of re-entry vehicles |
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Mario Tului / Centro Sviluppo Materiali S.p.A., Italy Giovanni Pulci* / Rome University ?la Sapienza?, ICMA Dept., Rome, Italy, Italy Jacopo Tirillò/ Rome University ?la Sapienza?, ICMA Dept., Rome, Italy, Italy Francesco Marra/ Rome University ?la Sapienza?, ICMA Dept., Rome, Italy, Italy Stefano Lionetti/ Centro Sviluppo Materiali S.p.A., Italy Teodoro Valente/ Rome University ?la Sapienza?, ICMA Dept., Rome, Italy, Italy |
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In the last years the Ultra High Temperature Ceramics (UHTC) have been extensively investigated as innovative thermal protection systems of aerospace vehicles as well as for other applications, where oxidation and/or erosion resistance at high temperature (up to 2000 °C) is required. The use of massive ceramics is strongly limited by the intrinsic brittleness of these materials and by the difficulty in manufacturing large ceramic components. A possible solution is the use of thick UHTC coatings deposited on tough and heat resistant substrates, to guarantee a good reliability of the hot structure; nevertheless it is necessary an in-depth knowledge of UHTC coatings mechanical properties at high temperature, to foresee the behaviour of the film subjected to structural loads and to design and optimize the coating-substrate coupling and interface. In this work, the high temperature mechanical properties of 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 °C in air. Results show that at each investigated temperature (500, 1000, 1500 °C) Modulus of Rupture (MOR) and Young?s Modulus are higher than data obtained at room temperature; moreover at 1500°C the UHTC coatings exhibit a marked pseudo-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 °C: 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. |