Combustion synthesis of mechanically activated molybdenum borosilicides for ultrahigh-temperature applications

The desire to improve the efficiency of power generation gas-turbines has led to a relentless quest for new, ultrahigh-temperature structural materials to replace the current nickel-based superalloys. These materials have reached the maximum allowable operating temperature determined by the melting temperature of these alloys, which is about 1150 °C. These materials could be replaced by molybdenum silicides and borosilicides based on Mo5SiB 2 (T2) phase due to their high melting point and mechanical properties. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at elevated temperatures. One novel approach to improve these properties is the addition of secondary phases such as TiC and TiB2. To fabricate these materials, a combustion route has been explored as an alternative. In the present work, molybdenum borosilicides based on T2 phase have been obtained using mechanically activated self-propagating high-temperature synthesis (MASHS). MASHS, however, was unable to deliver products without unwanted phases. For this reason, the so-called “chemical oven” technique has been implemented. This technique improved the quality of the products and it was possible to obtained denser and stronger Mo5SiB 2–TiC, Mo5SiB2–TiB2, and α-Mo–Mo 5SiB2–Mo3Si materials. Oxidation and mechanical tests were performed on these materials. Among them, Mo5SiB2–TiB 2 material exhibits the best oxidation resistance at temperatures up to 1500 °C. At room temperature, Mo5SiB2-TiB 2 materials had the highest elastic modulus and hardness among the materials tested. These characteristics were 135 GPa and 2.40 GPa, respectively