Kiel University
Kiel, Germany

How metals can be used depends particularly on the characteristics of their surfaces. A research team at Kiel University has discovered how to change the surface properties without affecting the mechanical stability of metals or changing the metal characteristics themselves. This fundamentally new method is based on using an electro-chemical etching process, in which the uppermost layer of a metal is roughened on a micrometer scale in a tightly controlled manner. Through this new “nanoscale-sculpturing” process, metals such as aluminum, titanium, or zinc can permanently be joined with nearly all other materials. The process can enable the metals to become water repellent. It can also improve their biocompatibility, which can make medical implants safer.

A strip of aluminum – the surface of which has been treated with an electrochemical etching process – is permanently bonded with thermoplastic by heating. [Credit: Julia Siekmann / Kiel University]

“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 Dr. Jürgen Carstensen, co-author of the publication. In the process, the surface of a metal is converted into a semiconductor, which can be chemically etched and thereby specifically modified as desired. “As such, we have developed a process which — unlike other etching processes — does not damage the metals and does not affect their stability,” said Rainer Adelung, head of the Functional Nanomaterials team at the Institute for Materials Science.

How it Works

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 a targeted etching. The top surface layer is roughened by the etching process, creating a three-dimensional surface structure. This changes the properties of the surface, but not of the metal as a whole. This is because the etching is only 10–20 μm deep, a layer as thin as a quarter of the diameter of human hair.

The change due to etching is visible to the naked eye: the treated surface becomes matte. “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 Dr. Mark-Daniel Gerngross.

Through the etching process, a 3D structure with tiny hooks is created. If a bonding polymer is then applied between two treated metals, the surfaces interlock with each other in all directions like a three-dimensional 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 publication.

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. In addition, exposing the puzzle connections to extreme heat and moisture to simulate weather conditions did not affect their stability. Carstensen noted, “Our connections are extremely robust and weather resistant.” A beneficial side-effect of the process is that the etching makes the surfaces of metal water repellent. The resulting hook structure functions like a closely interlocked 3D labyrinth, without holes that can be penetrated by water. The metals therefore possess 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.

“Because the nanoscale-sculpturing process creates a 3D surface structure, which can be purely physically bonded without chemicals, the targeted etching can also remove harmful particles from the surface, which is of particularly great interest in medical technology,” said Gerngross.

Titanium is often used for medical implants. To mechanically fix the titanium in place, small quantities of aluminum are added. However, 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 upper-most layer on a micrometer scale, the stability of the whole implant remains unaffected,” explained Carstensen.

The team’s results have been published in the journal Nanoscale Horizons of the Royal Society of Chemistry. The researchers have so far applied for four patents for the process.