New strategies to understand oxidation processes of high temperature materials

Abstract

Materials oxidation limits the lifetime of components used in high temperature applications such as gas turbine engines, hypersonic vehicles, and solar thermal power. Developing more durable new materials requires detailed understanding of the oxidation behavior. Conventional methods are however not always sufficient to characterize the oxidation of materials with more complex composition and microstructure. The work in this thesis focuses on the development and application of new strategies to understand oxidation processes of high temperature materials. The first area of research involved developing a technique using solid state electrochemical cells to measure the oxygen consumption rate of materials during oxidation. The capability of zirconia based oxygen pump cells to control the oxygen partial pressure was first evaluated using an empty chamber. The technique was then validated by studying the oxidation of niobium and nickel. Improved control over the oxidation potential was achieved by employing a separate oxygen sensor cell. The second area of research focused on the oxidation behavior of refractory multi-principal element alloys. Short-term oxidation tests provided insight into the oxide scales formed on the alloys, and long-term tests characterized the oxygen consumption rate up until complete oxidation. Comparison of the results from these tests helped understand how the alloy composition affects the oxide scale formed, and how the oxide scale protects the underlying alloy against further oxidation. Finally, the third area of research describes an approach using embedded oxygen markers to characterize oxygen transport in amorphous Si(O)C produced by polymer infiltration and pyrolysis. The effect of porosity on the extent of oxygen ingress was studied. A higher porosity resulted in the oxidation of TiC oxygen markers throughout the specimen, while increasing the Si(O)C content, thereby lowering the porosity, limited oxygen transport as oxygen markers were only oxidized at the surface.