Salt Based Etching of Metals and Alloys for Fabricating Superhydrophobic and Superoleophobic Surfaces

Abstract

A process to etch hierarchical, re-entrant texture into the surface of metals and their alloys using salt-based etching solutions. The process imbues superhydrophobic, oleophobic or superoleophobic, omniphobic or superomniphobic properties by further imparting a low surface energy coating onto the etched surfaces by chemical functionalization by low surface energy hydrophobilizing compounds.

Description

FIELD OF THE INVENTION[0001]The present disclosure relates to a process for surface modification of metals by developing strategies to etch surfaces of metals and alloys, and chemically modifying the etched surface to impart low surface energy properties and features, such as water and oil repellency. The current disclosure exhibits surface modification of metals through chemical approaches using different compositions and concentrations of metallic and / or non-metallic salts in aqueous, polar or non-polar solvents in the presence or absence of heat, radiation, light or electrical energy. The chemically modified metal and alloy substrates are subsequently modified with various chemical approaches that generate surface roughness and lower surface energy on the substrates, thereby enabling superhydrophobicity and oleophobicity or superoleophobicity, and often omniphobicity or superomniphobicity. The chemical etching of metals and alloys by ionic salts, such as organic or inorganic salt...

AI Technical Summary

Benefits of technology

[0036]The invention, which overcomes the problems mentioned above, provides an etching solution for aluminum, iron, zinc, copper and titanium-based metals and their alloys to develop re-entrant texture, reduces non-uniform etching of targets to be etched, and has a homogeneous etching rate. In addition, it provides an etching solution that requires minimal safety, handling, storage and is easy to dispose. The invention also addresses a process that imparts a thin layer of a low surface energy coating on the etched metals or alloys by chemical functionalization of the surface hydroxyl and oxide groups by hydrophobilization approaches such as, silanization, phosphatization or alkylation of the metal surfaces.

[0038]As described above, the method for fabricating a hydrophobic or oleophobic coating on to the etched surface of metals and alloys, according to the exemplary embodiment of the present invention, has an advantage with: (a) safety and handling of the etchants, (b) ease of use and time conservation, (c) reusability of the etchant solution, (d) fabrication of granular nano- or micro-structures on the top layer of the metal or alloy surface, (e) a single or double step chemical functionalization process for hydrophobilization of the etched metal or alloy surface with silanes, phosphates, amines and esters, (f) low fabrication cost due to simplicity of overall process and absence of using any high-priced equipment.

Problems solved by technology

The addition of fluorodecyl POSS molecules to different polymers leads to a rapid decrease in the overall surface energy of the synthesized blends.

Because of the difficulty in achieving superhydrophobicity and superoleophobicity, most of these surfaces have been realized with specific, sometimes non-scalable and expensive fabrication processes.

Dilute acids can cause skin irritation on repeated or prolonged contact.

It may cause severe kidney damage, central nervous system effects and even death if absorbed in large amounts.

Nitric acid gives off various nitrogen oxide gases, including nitrogen dioxide that is a strong lung irritant and can cause emphysema.

In contact with chlorine-based cleaning products, highly toxic nerve gas (nitrogen mustard gas) can even be created.

Hydrofluoric acid is highly toxic and can cause severe, deep burns which require medical attention.

Another problem is that metals that are prone to corrosion, such as Al and Cu wirings, in the devices are also etched.

Mixing hydrochloric acid with potassium chlorate, for etching metals, produces highly toxic chlorine gas.

Storage and use are very dangerous, special precautions are required especially when mixing.

However, these have low titanium-based metal etching rates, while decomposition of hydrogen peroxide is rapid, making stable etching impossible.

With such etching solutions, though the etching rate is improved, intense foaming of the etching solution causes attachment of bubbles onto the substrate surface, and etching does not proceed on the bubble-attached parts.

Another problem is the low etching rate due to foaming and decomposition of the hydrogen peroxide water.

Such mixtures are used with adjustment to a designated pH with ammonia, but even slight differences in pH alter the etching rate and foaming condition, and can be problematic from the viewpoint of stability of the etching conditions.

Further, the metal salt etching approach does not liberate any toxic gas unlike the acid etching process.

Lowering surface energy with low surface energy materials can reduce wettability of metal or alloy surfaces, but achieving superhydrophobicity and superoleophobicity by relying only on lowered surface energy is not feasible (NPL 2-3).

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