The kinetic frictional shear stress of ZnO nanowires on graphite and mica substrates

Abstract

The frictional shear stress between a nanostructure and a smooth substrate plays a crucial role in the development of nanodevices; however, it is extremely difficult to measure. In this work, the kinetic frictional shear stress of hexagonal ZnO nanowires on graphite and mica substrates was measured in an ambient atmosphere by optical microscope based nanomanipulation. Both substrates have similar surface roughness values of sub-angstrom-scale and interfacial adhesion energies with ZnO nanowires. Yet, a kinetic frictional shear stress of 0.51 MPa was obtained for the ZnO-graphite system, significantly lower than that of 5.1 MPa for the ZnO-mica system. The results demonstrate that the kinetic friction at a perfectly smooth contact interface may not be controlled by the adhesion, whilst being commonly referred to as adhesive friction or adhesion-dominated fiction. Similar to the combining equations for adhesion, we propose two empirical combining equations to estimate the frictional shear stress between two smooth surfaces using results more simply obtained from a reference surface. The validity of the equations is supported by our experimental results and recently published data for atomically smooth interface systems obtained under the similar environmental conditions