A comprehensive multi-scale study on the growth mechanisms of magnetron sputtered coatings on open-cell 3D foams

The deposition of functional coatings on open-cell foam substrates using magnetron sputtering is gaining popularity, particularly for applications like oxygen evolution reaction/hydrogen evolution reaction catalysis, batteries, and supercapacitors. While most research focuses on performance, little attention has been paid to the coating growth mechanisms or properties within the foam, which could significantly impact device performance. This work investigates the properties and growth mechanisms of TiO2 coatings inside porous foams, using experimental and modeling techniques. The structure, composition and thickness of the coating on the outermost surface of the foam are studied using focused ion beam (FIB), scanning transmission electron microscopy (STEM), energy-dispersive x-ray spectroscopy (EDS), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The experimental results reveal the formation of a dense, (quasi-)stoichiometric and crystalline coating. Numerical simulations and experiments highlight the transport of plasma particles in the foam. Interestingly, direct simulation Monte Carlo (DSMC)/particle-in-cell Monte Carlo (PICMC) models, coupled with mass-energy analyzer (MEA) experiments, demonstrate that the particle flux is reduced, but the particle energy distribution is not affected while traveling inside the foam. Using kinetic Monte Carlo thin film growth models provided by Virtual CoaterTM, the physical properties of the coating inside the foam have been modeled, and the drop in coating thickness as well as the impact of bias voltage on densification, resistivity, and optical absorption are confirmed. synchrotron x-ray diffraction (SXRD) analyses of the foam demonstrate that the same crystalline phase is obtained along the foam thickness, but it can be tailored with bias voltages. The decrease in the recorded SXRD signal with increasing depth inside the foam also suggests a drop in coating thickness. The new insights on the properties of coatings inside open-cell foams presented in this study can be used to improve future foam-based devices.