Study on Fabrication of Functional Metal Surfaces by Designing Micro/nano-scale Structures with Wet Etching Method

Abstract

chemical wet etching and reaction methods were used to fabricate the hierarchical structured surface. As titanium-based mechanical parts are widely used in corrosive environments, the outermost surface of the hierarchical structure was formed with titanium dioxide crystals through an annealing process to maintain high corrosion resistance at an increased specific surface area as well. Moreover, the wet etching and heat treatment techniques allowed the formation of the target microstructure without being restricted by the size and shape of the substrate. The hierarchically structured titanium surface demonstrated excellent water wettability that further improved with the complexity of the structure. Consequently, stable heat dissipation was feasible even in a high heat-flux environment owing to the excellent water wettability, and the critical heat flux value—corresponding to the instant of breakage of the substrate—was significantly improved. In this study, the surface microstructural shapes were engineered to realize functional metal surfaces and facile fabrication methods were proposed for manufacturing the targeted shapes according to the application.; Functional surfaces can be implemented by designing their micro/nano scale structures and controlling surface wettability. These surfaces have various applications owing to their low adhesion, corrosion resistance, and air-layer retention. However, the implementation of a functional surface on a metal is challenging because of the limitations of fine-surface patterning processing for metals, unlike that of the simple manufacturing methods for polymer materials. Recent studies have suggested that functional metal surfaces with fine patterns can be achieved using microelectromechanical systems (MEMS) that work based on a deposition technique. However, these fabrication methods are limited to a certain extent, as the practical application of functional metal surfaces ought to be scalable and economically compatible. Therefore, the development of an economical fabrication method that can mass manufacture functional metal surfaces is required. Among various functional surfaces, this study performed surface fabrication for two cases: an aluminum surface for reducing ice adhesion strength and a titanium surface for improving thermal stability. For the former, an aluminum surface was developed with a low aspect ratio to reduce the ice adhesion strength. As developed super-hydrophobic surfaces exhibit significant roughness, high-aspect ratio structures will be damaged if the droplets on the surface freeze during the removal of ice. Conversely, the ice does not hold on surfaces with a low aspect ratio, thus reducing the ice adhesion strength. Thus, we developed a wet etching method that formed a low aspect ratio microstructure by utilizing grain boundaries and defects; this microstructure exhibited reduced ice adhesion after coating with a low-adhesive thin film. The surfaces fabricated in this study using the proposed method exhibited low ice-adhesion strength and unnoticeable damages in the durable tests. In addition, the wet etching method and the immersion coating method allowed the formation of the target microstructure with various shapes and sizes of the substrate. Moreover, the developed method can be applied to diverse types of aluminum, because the microstructure was formed with techniques utilizing the grain boundaries and defects. An even lower level of ice adhesion performance was achieved upon the application of the developed surface on a mesh structure, as the low-aspect ratio microstructure worked coherently with that at the millimeter scale. The second functional metal surface constituted titanium for improved thermal stability with a high specific surface area. Conventional titanium surfaces with high thermal stability can get damaged by repeated thermal expansion resulting from structural limitations and other problems. Thus, the current study aims to fabricate a titanium surface with high thermal stability based on excellent super-hydrophilicity, along with direct integration of the titanium surface structures into the substrate. The surface was designed as a hierarchical structure using both micro- and nanostructures