Frictional behavior of extremely smooth and hard solids

Frictional behavior of extremely smooth (average roughness ~1 nm) and hard (approximately 800-2200 kg mm^-2) solids in air has been investigated. The present study was conducted to obtain the minimum friction under dry sliding conditions by eliminating the friction components due to mechanical interactions such as plowing and asperity deformation. By using extremely smooth and hard materials (silicon, sapphire, SiC, quartz and glass), static friction coefficients as low as 0.06 and steady state friction coefficients between 0.09 and 0.17 could be obtained with an undetectable amount of wear. The low friction and wear behavior of smooth and hard solids in air is attributed to the low probability of asperity interaction and wear particle generation as well as to surface films. Even for such low factional interaction, plastic deformation is still the major cause of friction in crystalline systems, as suggested by the existence of dislocation etch pits on silicon surfaces along the sliding track.