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Sliding on gold

​The absence of friction (a.k.a. superglide) certainly sounds appealing to all skiers. Some boundaries can indeed have this property. At L_Sim team we have simulated the behavior of such a system, and the experimental observation was made in collaboration with the Lawrence Berkeley Laboratory in California. We thus confirmed experimentally a theoretical prediction made in 2000.

Published on 5 October 2010
Friction is an important property that governs much of the world that surrounds us. The laws of friction between two surfaces are well known at our macroscopic scale. However, how these laws of the macroscopic world apply at the atomic scale has only been studied in the last few years, the motivation coming in particular from the micro- and nano- technologies, due to the important role of surfaces at these scales. An unexpected result is that some particular boundaries – called incommensurate boundaries – can have a vanishing friction coefficient, thus leading to “superglide”. This was demonstrated theoretically for a simple one-dimensional system, but the reality of such behavior for real materials was not universally accepted. Our results show that this is, indeed, the case. We also confirm experimentally our particular theoretical prediction made in 2000: that this property can take place at the core of a material, in this case gold, when the incommensurate boundary is a particular type of grain boundary.

For this purpose, we have taken advantage of a new experimental technique developed at the “National Center for Electron Microscopy” (NCEM) of the Lawrence Berkeley Laboratory, in California. Compression tests are done on pillars with diameters of a few hundred nanometers, using a tip whose position is fixed and for which the exerted force is measured. The displacements of the tip, as well as the deformations of the pillars, are observed in a transmission electron microscope (TEM). In order to prepare the experimental protocol of our nano-compression experiments, numerical simulations at a scale 1/10 were performed at L_Sim team. We have taken into account the atomic structure of our samples: the behavior of each atom is calculated by using a molecular dynamics method. These calculations have forecasted that by positioning the boundary at 45° with respect to the pillar axis, a sliding of one grain with respect to the other would occur and give a reconstruction of the crystalline structure only at the pillar surface, at the front and at the back of the sliding. These reconstructions are edge defects, which we could calculate in what way they oppose the sliding property of the boundary itself.

In order to make 200 nm diameter pillars, a gold layer of this thickness is deposited at NCEM on a germanium substrate. The result consists of crystalline grains with only two possible orientations, at 90° from each other, as wanted. However, the orientation of the grain boundaries varies such that a dozen such pillars had to be fabricated to obtain the right configurations.

Figure 1: Landscape of the sample seen by electron microscopy (NCEM) after the fabrication of the pillars by FIB.
The foreground corresponds to the germanium substrate. In the background are five gold pillars (Ø=200nm, h=1µm) as well as some remains of the thin gold layer. Among these pillars, only one was found to have the correct boundary for the study.

Pillars are then etched with a focused ion beam (FIB) throughout the thickness of the gold layer (Figure 1). They are then placed in the microscope equipped with the mechanical probe. The pillars having the right boundary (which is characterized by the electron diffraction performed on each grain) have their grains tilted as we modeled (Figures 2 and 3). Those which consist of a single grain, or have boundaries that are not adapted, are crushed by the tip. We could link the evolution of the force applied on the top of the pillars to the type and size of the surface reconstruction. This work led us to conclude that, at the core of a material, it is possible to have this surprising sliding property. This opens the way for more general studies on grain boundary behaviors, with a protocol based on a combination of techniques, with our work on this peculiar boundary serving as a reference case.

Figure 2 (left): Sliding of a grain with respect to another during the compression of a polycrystalline pillar in a TEM (see corresponding simulation in the figure 1).
Figure 3 (right): Gold bicrystal seen in TEM (a) before and (b) after compression. (c) Numerical model after compression. Note the asymmetry of reconstructions at the boundary edges and good agreement between experiment and calculations.

Incommensurate boundary
A boundary between two periodical structures is incommensurate when the two structures do not share any common periodicity along the boundary. In this case, the environment at one location of the boundary will not be exactly reproduced elsewhere. The boundary represented here (left) is a grain boundary separating two identical crystals that are twisted by 90° with respect to each other. Because of the cubic structure of these crystals, a periodicity d in one crystal faces a periodicity √2 d in the other crystal. As √2 is an irrational number, the alignment of atomic columns never repeats itself exactly. The figure on the right shows the boundary after sliding. It is not perturbed: the same atomic environments can be found before and after the sliding, just somewhere else along the boundary.

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