This image illustrates how the ‘nanoscale-sculpturing’ process roughens the upper layer of metals such as aluminum, creating a 3D structure with tiny hooks. Image: Melike Baytekin-Gerngroß.
This image illustrates how the ‘nanoscale-sculpturing’ process roughens the upper layer of metals such as aluminum, creating a 3D structure with tiny hooks. Image: Melike Baytekin-Gerngroß.

How metals can be used depends on the characteristics of their surfaces. A research team at Kiel University’s Institute for Materials Science in Germany has now developed a way to change these surface characteristics without affecting the mechanical stability of the metals or changing the metal characteristics themselves. This fundamentally new method utilizes an electro-chemical etching process to roughen the uppermost layer of a metal on a micrometer scale in a tightly-controlled manner.

Through this ‘nanoscale-sculpturing’ process, metals such as aluminum, titanium or zinc can permanently be joined with nearly all other materials, and can also be made water-repellent or more biocompatible. This process, which is described in a paper in Nanoscale Horizons, could thus have a broad range of applications, from metalwork in industry right through to safer implants in medical technology.

“We have now applied a technology to metals that was previously only known from semiconductors. To use this process in such a way is completely new,” said Jürgen Carstensen, co-author of the paper.

“As such, we have developed a process which – unlike other etching processes – does not damage the metals, and does not affect their stability,” explained Rainer Adelung, head of the Functional Nanomaterials team at the Institute for Materials Science. “In this way, we can permanently connect metals which could previously not be directly joined, such as copper and aluminum.”

The surfaces of metals consist of many different crystals and grains, some of which are less chemically stable than others. These unstable particles can be specifically removed from the surface of a metal by targeted etching, which roughens the top surface layer of the metal to create a three-dimensional (3D) surface structure. This changes the properties of the surface, but not of the metal as a whole. This is because the etching is only 10–20µm deep, leading the research team to name the process ‘nanoscale-sculpturing’.

The change due to etching is visible to the naked eye: the treated surface becomes matt. “If, for example, we treat a metal with sandpaper, we also achieve a noticeable change in appearance, but this is only two-dimensional and does not change the characteristics of the surface,” explained Mark-Daniel Gerngroß, another co-author of the paper.

The etching process produces a 3D structure with tiny hooks. If a bonding polymer is then applied between two treated metals, their surfaces inter-lock with each other in all directions like a 3D puzzle. “These 3D puzzle connections are practically unbreakable. In our experiments, it was usually the metal or polymer that broke, but not the connection itself,” said Melike Baytekin-Gerngroß, lead author of the paper.

Even a thin layer of fat – such as that left by a fingerprint on a surface – does not affect the connection. “In our tests, we even smeared gearbox oil on metal surfaces. The connection still held,” explained Baytekin-Gerngroß. Laborious cleaning of surfaces, such as applied to ships' hulls before they can be painted, could thus be rendered unnecessary.

In addition, the research team exposed the puzzle connections to extreme heat and moisture, in order to simulate weather conditions; this also did not affect their stability. “Our connections are extremely robust and weather-resistant,” said Carstensen.

A beneficial side-effect of the process is that the etching makes the metal surfaces water-repellent. The resulting hook structure functions like a closely-interlocked 3D labyrinth, without holes that can be penetrated by water, giving the metals a kind of built-in corrosion protection. “We actually don't know this kind of behavior from metals like aluminum. A lotus effect with pure metals – i.e. without applying a water-repellent coating – that is new,” said Adelung.

“The range of potential applications is extremely broad, from metalworking industries such as ship-building or aviation, to printing technology and fire protection, right through to medical applications,” said Gerngroß. Not only can the ‘nanoscale-sculpturing’ process create a 3D surface structure that can be physically bonded without chemicals, but it can also remove harmful particles from the surface, which could be of particular interest for medical technology.

Titanium is often used for medical implants. To mechanically fix the titanium in place, small quantities of aluminum are added, but the aluminum can trigger undesirable side-effects in the body. “With our process, we can remove aluminum particles from the surface layer, and thereby obtain a significantly purer surface, which is much more tolerable for the human body. Because we only etch the uppermost layer on a micrometer scale, the stability of the whole implant remains unaffected,” explained Carstensen.

The researchers have so far applied for four patents for the process, and industry has already shown substantial interest in potential applications. “And our specialist colleagues in materials sciences have also reacted enthusiastically to our discoveries,” said a delighted Adelung.

This story is adapted from material from Kiel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.