Low friction of metallic multilayers by formation of a shear-induced alloy

During sliding of metallic surfaces, the near surfaces undergo significant changes in terms of topography, composition and microstructure. Since friction and wear behavior of the materials are strongly influenced by sub-surface deformations, it is fundamental to investigate these effects. Therefore, the present study aims towards a better understanding of the behavior of friction depending on well-defined initial microstructures. By performing sliding experiments on Au-Ni multilayer samples under ultrahigh vacuum (UHV) conditions, we observe that the individual layer thickness of multilayer systems has a strong influence on friction behavior due to the transition in the dominant deformation mechanism near the surface. The experiments reported here provide a new route for lowering the friction force of metallic material systems in dry contact by providing more stable microstructures and alloy formation. Through ultrafine grains present in the alloy formed by mechanical mixing the number of grain boundaries strongly increases and hence, grain boundary-mediated deformation results in the low friction coefficient.

Indentation hardness as a function of the interlayer spacing of the multilayers at 8 nN applied load. An increase in the layer thickness leads to decrease in hardness of the multilayers by 58.3 ± 7 %, from 10 to 100 nm sample (without pile-up), which is in agreement with the Hall-Petch relationship. Note however that significant amount of pile-up is observed in our measurements. Hence, in order to eliminate the pileup effect and correct the hardness of multilayer samples, a simple geometrical approach is used based on the determination of the area of the triangle indent [2] . Supplementary Note S1 | Grain coarsening following the process of sliding Grain coarsening in nanocrystalline materials is driven by the excess surface free energy. In other words, grains in a material always try to minimize their surface free energies by reducing the total grain boundary energy [3] . At this point, it is worth further addressing the effect of alloying on the microstructure evolution. Alloying contributes to the more durable microstructures by refining grains as well as creating Zener pinning sites in polycrystalline surfaces. Those fine pinning sites reduce the grain boundary mobility by applying a pinning force that can eliminate the impact of driving force to trigger the mobility. In this sense, grain coarsening on the sub-surface might be prevented during mechanical alloying (fully mixing) by introducing this kind of pinning sites, also leading to the lower friction behavior as already presented in the main text. As previously mentioned, and presented in Supplementary Fig. S6a-d, TEM analysis on the 10 nm sample confirmed the formation of a more stable microstructure by grain refinement due to the mixing of Au and Ni layers near to the surface. On the other hand, grain coarsening beneath the intermixed region has been also observed. Although residual strain has already been present in the as-grown multilayers induced by the fact of lattice mismatch between Au and Ni metals (15%), as well as the orientation dependent growth rate, TEM analysis revealed that there should still exist a huge amount of strain in the material after achieving a more stable microstructure by grain refinement (this evidence can refer to the SAED analysis mentioned in the next Supplementary Note). TEM images in Supplementary Fig. S6a-d depict coarsened grains where the process takes place between not completely deformed layers close to the intermixed region, due to that a material tends to recrystallize itself in order to reduce strain. It can be also argued that deforming the hard Ni layers by shearing, it would be possible to fold (softer) Au layers in order to agglomerate them in a grain format.

Supplementary
Bending the crystallographic planes leads to observing different contrasts in a TEM image due to the fact that changes in tilt angle are highly influential on the Bragg diffraction conditions. And mostly, existent crystal defects in the sample result in bending the planes. For this reason, TEM contrast study would be pretty indicative to understand the characteristics of the defect, depending on bending [4] . Although this aspect is more beyond our scientific purpose to achieve in this article, we presented TEM images of worn 10 nm multilayer sample, including different contrasts for the grains (Supplementary Fig. S6b-c).
Depending on the grain orientation with respect to the incident electron beam, different grains show higher contrast (appear darker) or lower contrast (appear brighter) upon tilting the sample.

Supplementary Note S2 | TEM and diffraction analysis on the intermixed tribo-layer
To distinguish Au, Ni and AuNi alloy grains from each other, SAED analysis was carried out on the fullymixed region; however, diffraction analysis is extremely challenging since Au and Ni have the same crystal symmetry and space groups (Fm3m) in addition to the similar lattice constants [5] (aAu = 0.408 nm, aNi = 0.352 nm and aAuNi = 0.38 nm) and they were splitting off in their (111) Bragg diffraction conditions (dhkl,Au = 0.235 nm -1 , dhkl,Ni = 0.203 nm -1 and dhkl,AuNi = 0.22 nm -1 ) due to the high amount of strain inside the material during the measurements as shown with the inset in Supplementary Fig. S6c. Nevertheless, the calculated diffraction intensities in our measurements were varying in between 0.2180 and 0.2197 nm -1 which might be strongly correlated to the value of 0.22 nm -1 for the AuNi alloy structure.

Supplementary Note S3 | Twin boundaries and grain rotation during shearing
Since a large amount of mixing instabilities have been generated during shearing of multilayers, which has actually resulted in residual stress in the material, we were unable to properly observe the twin boundaries and grain rotation in the corresponding tribolayer. However, one would definitely expect observing twin boundaries and grain rotation during shear deformation of such a material. To prove this argument, we would like to refer to a PhD thesis's chapter by Adrien Gola (Karlsruhe Institute of Technology, 2019) [6] on the tribological loading of Cu-Au multilayers. According to his MD simulations, in addition to that vortex formation is possible between Cu and Au during shear-mixing, the boundaries of the twinning event and the rotation of grains has been observed as the shear strain increases, and this led to the creation of distinct orientations in the material system. In order to investigate the possible size effect on the rotation mechanism, he increased the size of the cell used in