Will crystallographic faces of a crystal keep their order in strength and friction coefficient when the contact force is reduced to nano/micro-Newton level?

Abstract

Properties of a crystal are generally anisotropic, which makes different crystallographic planes behave differently. By choosing a suitable crystallographic plane or textured polycrystalline surface, one may obtain optimum mechanical and tribological properties, e.g., the maximum strength and desired friction coefficient. Up to date, we have had sufficient knowledge about the relationship between mechanical properties and the crystallographic orientation for different crystal systems. However, when the contact force is decreased to nano/micro-Newton level, will the crystallographic faces of a crystal keep their order in strength and other properties? This article reports our recent studies on this issue using copper as a sample material and demonstrates that there are transitions in hardness and friction coefficient between different crystallographic planes of Cu. It has been demonstrated that the closely packed plane (111) is harder and has a smaller friction coefficient than the (001) plane; however, the situation is reversed when the load is reduced to nano/micro-Newton level that only results in distortion of a few atomic layers. Such changes are of particular importance to the application of crystalline materials in nanomachines or nanodevices.

Controllable Crystallographic Texture in Copper Foils Exhibiting Enhanced Mechanical and Electrical Properties by Pulse Reverse Electrodeposition

“…Hence, it has been mainly observed during this study that in PRED the usage of an additive-free electrolyte reduces the level of impurities in the deposited material, resulting in minimal stresses giving stability to mechanical and electrical properties with texture in addition to the minimization of hydrogen embrittlement. Among the copper foils that were deposited using different pulse parameters to control the texture, high hardness in (111) is probably due to the high atomic density in (111), resulting in a compact structure with low energy . The enhancement of properties (mechanical and electrical) also results from low residual stresses and a large number of coherent twin boundaries, which behave like conventional grain boundaries in blocking the dislocation motion while reducing the level of scattering of electrons.…”

“…The highly uniform orientation of the grains in (111) textured foils (texture coefficient) results in a reduction in the level of scattering of electrons leading to lower values of electrical resistivity . The (111) plane is harder than the other planes because of the difficulty in activating the {111} ⟨01I̅⟩ slip systems . Further, a predominant advantage with highly (111) textured copper is that it improves the electromigration resistance compared to those of all other textures .…”

“…Among the copper foils that were deposited using different pulse parameters to control the texture, high hardness in (111) is probably due to the high atomic density in (111), resulting in a compact structure with low energy. 93 The enhancement of properties (mechanical and electrical) also results from low residual stresses and a large number of coherent twin boundaries, which behave like conventional grain boundaries in blocking the dislocation motion while reducing the level of scattering of electrons. Similarly, in spite of a large number of grain boundaries, the electrical conductivity of the foils is high due to LAGBs in (111), ∼6.2% among the total number of grain boundaries with low energy and low mobility contributing to the lower resistivity of the foils.…”

“…80 The (111) plane is harder than the other planes because of the difficulty in activating the {111} ⟨01I̅ ⟩ slip systems. 93 Further, a predominant advantage with highly (111) textured copper is that it improves the electromigration resistance compared to those of all other textures. 15 In this study, among all textured copper foils, the highly (111) textured foil has prominent properties compared to the rest because of its stand-alone features.…”