From islands to cauliflower: Hierarchical growth of treelike PS-PVD coating

Unlike conventional physical vapor deposition (PVD) coatings that typically exhibit faceted or pyramid-like surfaces, plasma spray-physical vapor deposition (PS-PVD) coatings display a distinct cauliflower-like morphology. The novel morphology of PS-PVD coating can be induced by the treelike growth mechanism proposed in our previous research. To elucidate the formation, controlling and property of the cauliflower-like surface morphology, a three-dimensional Monte Carlo simulation model was developed for vapor deposition processes. The results display a treelike microstructure with cauliflower-like surface, demonstrating excellent agreement with experimental observations. The different contributions of shading and diffusion induce the surface growth from islands to cauliflower. Hierarchical competition and in-situ sintering within the treelike structures significantly influence the hierarchical roughness of the surfaces. Wettability analyses, validated by high-temperature contact angle measurements with molten CMAS (CaO-MgO-Al₂O₃-SiO₂), indicate decreased wettability due to hierarchical roughness. A wettability model is established using Cassie-Baxter equations. During molten CMAS infiltration, while the higher coating thickness and in-situ temperature initially increase the contact angles, reverse effect occurs at the later stages. This study provides novel insights into the growth mechanism and anti-corrosion properties of PS-PVD cauliflower-like coatings, enriching the theoretical framework of PVD processes.