11 - Superlubricity of Ag nanometer-thick layers under macroscopic sliding system in UHV condition

Abstract

This chapter presents the characteristic features of the superlubricity by epitaxial Ag films on a Si (111) clean surface in an ultrahigh vacuum (UHV) environment. Because the roughness of the contact surface is nanometric in dimension, the critical thickness at which the minimum friction coefficient is obtained becomes 1–10 nm. The minimum friction coefficient is 0.007 at the thickness of 5 nm in which the morphology shows a network structure. The mechanism of superlubricity by the Ag film is different from that in the tribofilm formation theory, which was elucidated for the superlubricity by MoS2 and diamond-like carbon (DLC). The superlubricity of the Ag film is originated from the weak shear strength between highly oriented Ag (111) lamellae parallel to the sliding direction. Because the shear strength between C atoms and Ag atoms yields a much higher friction coefficient of 0.2 than that in the case of interlayer shearing between Ag (111) lamellae, the interfacial sliding between C atoms and Ag atoms is not the cause of the superlubricity. The network structure of a 5-nm Ag film contains moderate defects within, which serves to form atomically flat lamellae during the superlubric state of the Ag film. The frictional experiments of polycrystalline Ag films show that the effect of the crystallographic orientation of Ag crystallites in the film is an important parameter to determine the friction coefficient of Ag.