3D nanoscale-sculptured surfaces consist nearly entirely of stable native oxide covered grains and thus resemble most stable mechanical and chemical surfaces/interfaces in addition to the nearly optimal topology for interlocking structures to other materials. Conventional chemical surface treatments remove defects being specific to the surface (e.g. saw damage, mechanical damage after sand blasting, mechanical polishing, or even damages induced by ion implantation), etch out (selectively) grain boundaries, form metal oxide layers on the surface, thus having mostly an intrinsically 2D-character.

In strong contrast to nearly all relevant technical surface treatments on metals and semiconductors, the sculpturing approach utilizes the intrinsic features of the surface-near grain structure on the nanoscale. The (electro-)chemistry is tuned to selectively etch out entire or at least large parts of grains on the nanolevel in a coordinated manner introducing an intrinsic micro 3D-character into the resulting surfaces. Deep cavities with undercuts allowing for mechanical interlocking are thus an intrinsic feature of sculpturing. Due to the 3D-character the preserved grains, plains, and facets, i.e., the bulk structure is extremely stable, since, e.g., no grain boundaries are widened weakening the surface microstructure.

—Baytekin-Gerngroß et al.

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 to 20 micrometers deep—a layer as thin as a quarter of the diameter of human hair. The research team has therefore named the process “nanoscale-sculpturing”.

To use this process in such a way is completely new, said Dr. Jürgen Carstensen, co-author of the publication.

As such, we have developed a process which—unlike other etching processes—does not damage the metals, and does not affect their stability. In this way, we can permanently connect metals which could previously not be directly joined, such as copper and aluminium.

—Professor Rainer Adelung, head of the Functional Nanomaterials team at the Institute for Materials Science

The change due to etching is visible to the naked eye: the treated surface becomes matt. 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 inter-lock with each other in all directions like a three-dimensional puzzle.

Top. The targeted etching process of “nanoscale-sculpturing” roughens the upper layer of metal (here aluminium, 20 µm = 0.02 mm), thereby creating a 3D-structure with tiny hooks. A surface treated with this process can inter-lock like a three-dimensional puzzle with the surfaces of almost all other materials, forming unbreakable bonds. With this method, it is even possible to create bonds between aluminium and copper. Photo/Copyright: Melike Baytekin-Gerngroß. Bottom: The roughened surface structure of zinc in 10,000x magnification (2 µm = 0.002 mm). Photo/Copyright: Melike Baytekin-Gerngroß. Click to enlarge.

These 3-D puzzle connections are practically unbreakable. In our experiments, it was usually the metal or polymer that broke, but not the connection itself.

—Melike Baytekin-Gerngroß

Even a thin layer of fat, such as that left by a fingerprint on a surface, does not affect the connection. The researchers even smeared gearbox oil on metal surfaces, and found that the connection still held, said Baytekin-Gerngroß. Laborious cleaning of surfaces, such as the pre-treatment of ships’ hulls before they can be painted, could thus be rendered unnecessary.

Extreme heat and moisture also did not affect the joins. 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 which can be penetrated by water. The metals therefore possess a kind of built-in corrosion protection.

We actually don’t know this kind of behaviour from metals like aluminium. A lotus effect with pure metals, i.e. without applying a water-repellent coating, that is new.

—Professor Adelung

Because the nanoscale-sculpturing process not only 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.

The researchers have so far applied for four patents for the process. Businesses have already shown substantial interest in the potential applications.

Resources

• M. Baytekin-Gerngroß, M. D. Gerngroß, J. Carstensen and R. Adelung (2016) “Making metal surfaces strong, resistant, and multifunctional by nanoscale-sculpturing” Nanoscale Horiz. doi: 10.1039/C6NH00140H