Gold surfaces repair themselves at room temperature
Micromechanical systems and electric switches are based on smallest sliding contacts. They only work without loss of energy or material, if the surfaces are very smooth and without any defects. So far, little has been understood about the underlying atomic-scale principles. In cooperation with researchers at the universities of Münster and Gießen as well as the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, scientists at the INM - Leibniz Institute for New Materials were able to show that on atomic scale gold surfaces smoothen out by themselves at room temperature. In their publication in Physical Review Letters, they reveal that this effect disappears at low temperatures.
So far, it has been assumed that perfect sliding works the better the more rigid the surface is. On the atomic scale this could mean freezing lattice vibrations in the crystal at low temperatures below -100°C; where the atoms hardly move. Against expectation, smooth sliding on gold surfaces is not quite possible at these temperatures, but, however, at room temperature. The scientists explain this phenomenon with the diffusion of the gold atoms: If they are able to move freely on the surface, the gold atoms migrate into defects on the surfaces and remove holes and bumps. The diffusion effectively stops below -100°C.
"Imagine a record player whose needle made from rubber moves over a wax plate. If the wax is hard, wax pieces will be scratched out and, after a while, the needle pushes a pile of wax, which can only be surmounted by the needle after it bends strongly", explains Roland Bennewitz, Head of the Program Division "Nanotribology". If the temperature rises, the wax melts and the needle leaves no more traces in the wax. In fact, the liquid wax removes holes and bumps at once, and the needle slides uniformly through the wax.
A similar process occurs on the gold surfaces. Although they do not melt at room temperature, the diffusion of the gold atoms is so strong that smallest asperities on the nanoscale are removed at once. The regular structure of the surface is preserved.
Experiments were performed by atomic force microscopy (AFM). A thin needle slides forth and back on the gold surface. The measured signal shows how strong the needle bends in contact. On a crystalline surface, the needle "jumps" regularly from atom group to atom group - the scientists measure a stable so-called stick-slip pattern. In the event of defects, such as the accumulated gold atoms, the needle bends stronger and the stick-slip pattern will be broken.
In their research, the scientists also employed atomistic modelling on the computer. Here, they were able to reproduce the stick-slip pattern for the scanning of the gold surface with gold and nickel needles. With a 3D simulation, they were also able to show how gold atoms accumulate at low temperatures. The accumulated gold atoms are attracted by the needle like a liquid into a capillary.
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