Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations

Abstract

The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6• gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition.

Twin-induced template effect on the inelastic deformation of hierarchically nanotwinned copper under indentation and scratch

“…Figure 3(b) shows the friction coefficient, which is calculated as the ratio of the vertical force over the lateral force, as a function of the scratching length. From this figure, we can see that after the rapid increase in the initial scratching stage, the friction coefficient fluctuates acutely around a certain value in the later stage, which is similar to that observed in the nanoscratch of the conventional NT copper samples containing parallel TBs along the scratching direction (Zhang et al, 2013). The friction coefficient of the current sample is about 0.38, which is higher than those of conventional NT copper samples as studied in Zhang et al (2013).…”

“…The copper atoms at the bottom are fixed to avoid rigid body motion during indentation and scratch. The simulation procedure and parameters are similar to those adopted in previous studies (Kelchner et al, 1998;Sun et al, 2011;Zhang et al, 2013). The time step utilized in the MD simulations is 2 fs.…”

Nano-machining of materials: understanding the process through molecular dynamics simulation

“…The effects of crystallographic orientation have been addressed by MD through the scratching on a nano-twinned copper workpiece (containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces with different twin spacing) [38]. This work showed that the random coupling of crystallographic orientation and twin boundary orientation with respect to the scratching direction strongly influenced the competition between individual deformation mechanisms.…”

Atomistic Investigation of Anisotropic Nanoindentation Behavior of Nanotwinned Aluminum Containing Inclined Twin Boundaries

“…Brown et al showed that the plasticity in twinned Cu nanopillars greatly depends on the TBs’ orientation with respect to the loading axis [ 10 ]. Zhang et al investigated the effect of the inclination angle on the nanoscratching behavior of nanotwinned copper, and found a critical inclination angle of 26.6° for the lowest yield strength and the highest friction coefficient [ 11 ]. However, there is rather limited work focusing on the effect of the inclination angle of TBs on the mechanical properties of aluminum.…”