Theses and Dissertations at Montana State University (MSU)
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Item Development of binary and ternary chromium, aluminum, carbon thin film coatings for hot corrosion prevention(Montana State University - Bozeman, College of Engineering, 2015) Aw, Lik Ming; Chairperson, Graduate Committee: Roberta AmendolaIncreasing the operating temperature of high temperature energy conversion systems is often very desirable in order to maximize their corresponding efficiencies. Materials selected for high temperature applications must therefore incorporate associated considerations such as creep strength, high temperature corrosion resistance, and thermal fatigue. Hot corrosion is a form of accelerated high temperature oxidation associated with the deposition of salt such as sodium sulfate (Na 2SO 4) on metals or their protective oxide surfaces. Severe corrosion develops when Na 2SO 4 reacts with metals to form eutectic (low-melting point) compositions on the surface of the metal at 700 °C (Type II Hot Corrosion) or when the surface has been wetted by molten Na 2SO 4 at 900 °C (Type I Hot Corrosion). This phenomenon reduces the useful life of materials such as nickel alloys used in high temperature gas turbines for aircraft and marine applications. In this study, a variety of binary and ternary thin film coatings using chromium, aluminum, and carbon as base elements were deposited onto Ni-201 alloy and investigated as a function of exposure to hot corrosion environments. The coatings were deposited using a magnetron sputtering system and were approximately 1.5 microns in thickness. Prototypical Na 2SO 4 was applied to the samples before exposing them for up to 250 hours in an air/SO 2 gas mixture at 700 °C and 900 °C, which simulates Type II and Type I Hot Corrosion, respectively. It was determined that all coatings reduced the specific mass gains (corrosion rates) of the samples when compared to that of an uncoated sample, indicating that they provided protection against hot corrosion. For Type II Hot Corrosion, binary Cr - Al and ternary Cr - Al - C thin film coatings held up after 250 hours of exposure; whereas the other two coatings had completely disintegrated. For Type I Hot Corrosion, all thin film coatings utilized in the study had completely dissolved after 50 hours of exposure. It was suggested that the two most effective coating materials, Cr -Al and Cr - Al - C coatings should be used for future testing and that their thicknesses should be increased to help provide better protection against Type I Hot Corrosion.Item Study of solid oxide fuel cell interconnects, protective coatings and advanced physical vapor deposition techniques(Montana State University - Bozeman, College of Engineering, 2007) Gannon, Paul Edward; Chairperson, Graduate Committee: Max DeibertHigh energy conversion efficiency, decreased environmentally-sensitive emissions and fuel flexibility have attracted increasing attention toward solid oxide fuel cell (SOFC) systems for stationary, transportation and portable power generation. Critical durability and cost issues, however, continue to impede wide-spread deployment. Many intermediate temperature (600-800°C) planar SOFC systems employ metallic alloy interconnect components, which physically connect individual fuel cells into electric series, facilitate gas distribution to appropriate SOFC electrode chambers (fuel/anode and oxidant[air]/cathode) and provide SOFC stack mechanical support. These demanding multifunctional requirements challenge commercially-available and inexpensive metallic alloys due to corrosion and related effects. Many ongoing investigations are aimed at enabling inexpensive metallic alloys (via bulk and/or surface modifications) as SOFC interconnects (SOFC(IC)s). In this study, two advanced physical vapor deposition (PVD) techniques: large area filtered vacuum arc deposition (LAFAD), and filtered arc plasma-assisted electron beam PVD (FA-EBPVD) were used to deposit a wide-variety of protective nanocomposite (amorphous/nanocrystalline) ceramic thin-film (<5micron) coatings on commercial and specialty stainless steels with different surface finishes. Both bare and coated steel specimens were subjected to SOFC(IC)-relevant exposures and evaluated using complimentary surface analysis techniques. Significant improvements were observed under simulated SOFC(IC) exposures with many coated specimens at ~800°C relative to uncoated specimens: stable surface morphology; low area specific resistance (ASR <100mOmega x cm 2 >1,000 hours); and, dramatically reduced Cr volatility (>30-fold). Analyses and discussions of SOFC(IC) corrosion, advanced PVD processes and protective coating behavior are intended to advance understanding and accelerate the development of durable and commercially-viable SOFC systems.