MSE PhD Defense – Peter Marshall

Time: September 21st, Monday, 3-5 PM

Location: MRDC 3515 (Hightower)

Thesis Advisory Committee:

Prof. Joe Cochran (advisor) MSE

Prof. Thomas Sanders (MSE)

Prof. Arun Gokhale (MSE)

Prof. Robert Speyer (MSE)

Prof. David McDowell (ME)

Prof. Richard Neu (ME)

Title: “Development of Oxidation Resistant Molybdenum-Silicon-Boron Composites”

Abstract:

Three phase molybdenum-silicon-boron materials with a continuous metallic molybdenum matrix have a combination of high temperature strength, creep, and oxidation residence and thus the potential to substantially increase the efficiency of gas turbines. This dissertation addresses two factors pertaining to this material system: improvements to powder processing techniques and development of compositions for oxidation resistance at 1300C. Building upon previously developed powder processing with silicon and boron nitrides, which allows for a low final oxygen content and sintering of fine starting powders, adjustments were made to the firing cycle based upon dew point measurements made during hydrogen de-oxidation. Combined with hot isostatic pressing this will increase the low temperature plasticity and toughness. 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. These additions have a profound impact at 1300C, however, few compositions were found to both form the initial borosilicate scale and have long term oxidation resistance. A theory is also proposed to describe the mechanisms responsible for the oxidation resistance. This theory involves three stages associated with: generation of an initial surface borosilicate, thickening of the borosilicate layer, and slow parabolic oxidation controlled by the high silica surface scale.