Ab-initio atomic scale study of nearly frictionless surfaces

Abstract

This chapter presents a comparative analysis of the dry sliding friction between the atomically flat and commensurate surfaces of two different systems: between two diamond (001) surfaces and between two BN (001) surfaces. The interaction between bare diamond (001) - (2 × 1) surfaces is strongly attractive until there is a small spacing d ∼ 1.5 Å, and the interaction forms strong chemical bonds between two surfaces. However, the interaction turns repulsive if two surfaces are pushed under high loading force. Upon hydrogenation, hydrogen atoms donate charge to the carbon atom and become positively charged. The interaction between surfaces carrying the same type of charge becomes repulsive. This appears to be the most important component of the superlow friction. The repulsive interaction persists at any relative position of the sliding surfaces and is strong even at large distance to prevent C-H bonds from merging. Strong and stiff C-H bonds and stiff diamond crystal by itself prevent excessive energy from dissipation. The oxygenation of surfaces in the atmospheric conditions finally destroys the steady repulsive interaction. However, the interaction between two bare reconstructed BN (001) surfaces is different from that of the diamond (001) surface because of the ionic nature of the crystal. BN being an ionic crystal with electron transfers from B to N, the bare surface is already negatively charged. Under these circumstances, the strong chemical interaction is canceled by the repulsive Coulombic interaction, resulting in a weak attractive interaction. © 2007 Elsevier B.V. All rights reserved.