Abstract
Cold Spray (CS) technology is an innovative thermal spraying process employed to create dense, durable protective coatings for extreme environments. The research investigated the incorporation of iron (Fe) Metal Core Oxide Shell (MCOS) Nanoparticles (NPs) in INCONEL625 alloy matrix for CS coating. The incorporation of MCOS NPs in the alloy matrix is considered to have a good potential for self-healing behavior and radiation damage tolerance. The MCOS nano capsules were designed to heal microcracks by releasing metal from their core upon fracture of the oxide shell, enhancing resilience under stress and extreme conditions. The MCOS was prepared by controlled oxidation of iron nanoparticles at low temperatures.To understand the mechanism of MCOS formation, the research explored various oxidation kinetic models from the literature, focusing on their role in void formation within the core of NPs. Iron NPs exhibited unique oxidation behaviors at low temperatures due to their high surface area-to-volume ratio and the associated curvature effects. The effects of NP size, oxidation temperature, and duration were investigated on the formation of core-shell morphology without voids. The results revealed that particles under 20 nm size completely oxidized, while larger particles formed the desired MCOS structures. The Gibbs energy and chemical potential of NPs on curved surfaces directly influenced the oxidation kinetics. A logarithmic rate law was observed during low-temperature oxidation. Over longer oxidation times, the initial magnetite oxide phase transformed into maghemite, accompanied by the formation of Kirkendall voids at the core-shell interface. Further, complete oxidation caused particles morphology to change from circular to hexagonal shape from the transmission electron microscopic (TEM) image observations. A quality control protocol based on Raman spectroscopy was developed to predict the oxide shell structure of the MCOS particles produced at a large scale.
A high-pressure CS process was utilized to spray the composite on to stainless steel substrates once the optimum material requirements for MCOS NPs were achieved. Incorporation of MCOS nanoparticles increased the hardness of the coating. Microscopic analysis revealed the presence of microcracks, primarily in the top layer of the coating. The TEM images revealed that some of the microcracks were filled with MCOS suggesting potential for self-healing behavior. However, challenges such as achieving high CS deposition efficiency, and poor coating adhesion require further exploration. This study contributes valuable insights into the potential of CS technology for developing high-performance coatings with applications in extreme environments and concludes that understanding the low-temperature oxidation behavior of iron NPs is crucial for advancing materials science and technology, particularly in designing materials for compact self-healing under extreme conditions.