Development of oxidation resistant molybdenum-silicon-boron composites
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The development of molybdenum - silicon - boron (Mo-Si-B) composites having a combination of high temperature strength, creep, and oxidation residence has the potential to substantially increase the efficiency of gas turbines. The refractory nature of the αMo, Mo3Si (A15), and Mo5SiB2 (T2) phases results in good strength and creep resistance up to 1300°C. At this temperature, the formation of a borosilicate surface scale from the two intermetallic phases is able to provide oxidation resistance. However, realization of these advantages has been prevented by both a high brittle to ductile transition temperature and difficulty in forming the initial surface borosilicate to provide bulk oxidation resistance. This dissertation addresses two factors pertaining to this material system: 1) improvements to powder processing techniques, and 2) development of compositions for oxidation resistance at 1300°C. The processing of Mo-Si-B composites is strongly tied to their mechanical properties by establishing the αMo matrix, limiting impurity content, and reducing silicon supersaturation. These microstructural aspects control the brittle to ductile transition temperature which has traditionally been too high for implementation of Mo-Si-B composites. The processing here built upon the previously developed powder processing with silicon and boron nitrides which allowed for a low oxygen content and sintering of fine starting powders. Adjustments were made to the firing cycle based upon dew point measurements made during the hydrogen de-oxidation stage. Under a relatively high gas flow rate, 90% of the total water generated occurred during a ramp of 2°C /min between 450 and 800°C followed by a hold of 30 minutes. The oxidation resistance of Mo-Si-B composites was studied for a wide range of compositions. Silicon to boron atomic ratios were varied from 1 to 5 and iron, nickel, cobalt, yttria, and manganese were included as minor additions. In all these compositions, the αMo volume fraction was kept over 50% to ensure the potential toughness of the composite. For the oxidized surface glass, a silica fraction of 80 to 85% was found to be necessary for the borosilicate to have a sufficiently high viscosity and low oxygen permeability for oxidation resistance at 1300°C. For the Mo-Si-B bulk composition this corresponds to a Si/B atomic ration of 2 to 2.5. Higher viscosity compositions failed due to spallation of poorly attached, high silica scales. Lower viscosity compositions failed from continuous oxidation, either through open channels or repetitive MoO3 bubble growth and popping. Additionally, around 1% manganese was necessary for initial spreading of the borosilicate at 1300°C. In conjunction with flowing air to prevent MoO3 accumulation, oxidation weight loss rates below 0.05 mg/cm2-hr were measured. Finally, a theory is proposed here to describe the mechanisms responsible for the development of oxidation resistance. This theory involves three stages associated with: 1) generation of an initial surface borosilicate, 2) thickening of the borosilicate layer, and 3) slow parabolic oxidation controlled by the high silica surface scale.