Process Control of Nitride-based Composite Coatings via Reactive Plasma Spray

Nitride coatings exhibit low hydrogen permeability, in addition to promising wear and corrosion properties, so a potential application of nitride coatings is Hydrogen Permeation Barriers (HPB) that prevent hydrogen ingress and mitigate hydrogen embrittlement. Thin nitride HPB coatings are generally produced using techniques like physical vapor deposition, which are difficult to scale for large surface-area production. The scalability of Atmospheric Plasma Spray (APS) implies that it can be used as an alternative approach to produce large-scale thick nitride-based coatings. However, nitrides are highly reactive at elevated temperatures and may not have a stable melting phase, so it is difficult to control their reactivity during conventional APS. Utilization of Reactive Plasma Spray (RPS) frameworks or using the spraying environment to control in-process phase transformations and develop unique microstructures, can be promising to design and synthesize higher performance HPBs. Our objective is to employ RPS and advanced microstructure characterization to explore mechanisms related to hydrogen trapping via grain boundaries and second phases to limit long-range bulk hydrogen diffusion. This work explores RPS processing using Al₂O₃ or both AlN and Al₂O₃ feedstocks with N₂ as primary gas with the aim to obtain desired phase fractions of Al₂O₃, AlN and/or metastable Al₅O₆N. A primary focus is to quantify the phase transformation kinetics that produce the deposited coatings and provide mechanistic insights into nucleation and growth of nitride-based phase transitions. Scanning electron microscopy with energy dispersive spectroscopy results show that the conversion of oxides into oxynitrides/nitrides can be controlled by the N₂ gas flow rate. Furthermore, scanning transmission electron microscopy is applied to characterize grain boundaries and phase boundaries in the coatings to assess potential hydrogen trapping sites. Lastly, this study utilizes a Devanathan-Stachurski (DS) cell to conduct hydrogen permeation tests to validate the relevance of the produced coatings and establish the processing-microstructure-property relationships.