Twinning Behavior in Uphill Nano-Tribological Process

Twinning behavior in the contact region between two tips is for the first time observed in a nano-tribological process. An Au (gold) polycrystalline tip, formed in TEM system, slides uphill on a W (tungsten) tip, mechanically inducing an Au polycrystalline transition layer. Not only does the tensile stress gradient initialize the twin formation but also the direction of the first sliding step determines the crystalline orientation of the first primary twin due to the lowest surface energy of FCC Au {111} planes. The crystalline orientation of the second primary twin is aligned with the first primary twin by coherent twin boundary regardless of the sliding direction and stress distribution. A triple boundary junction, between the Au tip, Au transition layer and vacuum is a favorable configuration probably due to the lowest energy of the system. On the other hand, defective crystalline phase is formed in the compressive region. It is proposed that crystalline rotation and active disclination dipoles are mainly responsible for the movement of twin boundary during the Au tip sliding process.