Development of In-situ Nanocrystalline NiCoCrAlTaY Coatings by Cold Spray on a Single-Crystal Nickel-base Superalloy for Gas Turbine Applications
Creators: name: Guo, Deliang
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2021
Subjects: MCrAlY
Bond coat
Cold spray
Nanocrystalline
Superalloy
Gas turbine

description: MCrAlY coatings are commonly applied as the bond coat in TBCs used in modern gas turbines. Cold spray (or CS), characterized by low process temperature and high particle impact velocity, has been demonstrated as a promising alternative to thermal spray processes, such as air plasma spray (APS) and high velocity oxygen fuel (HVOF), for manufacturing MCrAlY coatings. The general objective of the thesis research is to characterize CS deposition on a single-crystal nickel-base superalloy and to develop low-cost/high-performance NiCoCrAlTaY coatings using the CS technique. Several individual studies were carried out with each having a specific focus towards achieving the general research objective. CS deposition of NiCoCrAlTaY coatings using nitrogen was first examined to verify the feasibility of replacing the expensive helium gas typically used as the CS process gas. Several materials were used as the substrates, and the effects of substrate materials and surface preparation on coating microstructure and properties were investigated. Recycling of non-deposited powder particles was then explored to reduce the costs associated with the feedstock powder. A cost model that includes the economics of powder recycling was developed for the CS process, showing that the use of nitrogen and powder recycling could potentially be cost-effective for CS deposition of MCrAlY coatings. A CS process that can produce in-situ nanocrystalline NiCoCrAlTaY coatings was proposed to develop coatings with enhanced oxidation performance. This CS approach utilizes conventional commercial powders instead of pre-milled nanocrystalline powders. Detailed characterization using the scanning electron microscope (SEM), scanning transmission electron microscope (STEM), and X-ray diffraction (XRD) was carried out to investigate the microstructure of the resulting CS NiCoCrAlTaY coatings, single-crystal substrate, and their interface. Isothermal oxidation performance of the CS NiCoCrAlTaY coatings was evaluated at 1100°C for 1h to 500h. Results revealed that the nanostructure promoted the α-Al2O3 scale formation and sustained α-Al2O3 scale growth, suggesting good isothermal oxidation performance. Finally, the effects of different processing sequences on CS NiCoCrAlTaY coating characteristics and short-term isothermal oxidation performance were investigated. Specifically, CS deposition of NiCoCrAlTaY coatings was carried out on single-crystal superalloy substrates that underwent various degrees of full heat treatments prior to being coated. The remaining superalloy heat treatments required were then performed on the coated samples after the CS deposition. The microstructures of the superalloy substrates and CS NiCoCrAlTaY coatings were characterized after each heat treatment. Isothermal oxidation performance of the coated samples following different sequences was evaluated at 1100°C for 2 hours. The results suggested a promising processing sequence that could potentially further improve the oxidation performance of CS NiCoCrAlTaY coatings.