The friction of solids at very high speeds I. Metal on metal; II. Metal on diamond

Abstract

The friction of metal surfaces and of metals sliding on diamond at very high speeds has been studied. A ball spinning in vacuo at surface speeds up to 800 m/s (ca. 1800 miles/h) is trapped between symmetrically arranged flat plates so that the friction causes it to slow down. The deceleration is measured electronically and recorded. The results show that the sliding resistance of metals decreases as the speed increases and reaches very low values. There is evidence that frictional heating produces high surface temperatures and a softening and melting of the metal at the regions of contact. An analysis of the heat flow in a specimen shows that the area of intimate contact is very small compared with the apparent interface. Since the temperature falls very rapidly with increasing distance from the shearing zone the metal behaves like a combination of a thin film of low shear strength supported by a hard substrate. This can explain the low friction at high sliding speeds. It is also suggested that the real area of contact depends on the velocity with which plastic strains are propagated in the metal. The shearing occurs so rapidly that full plastic yielding under the normal load is not possible. Some metals disintegrate in a brittle fashion when a critical speed is exceeded. When metals slide on diamond even the hard octahedral face is abraded and polished if appropriately high sliding speeds are used. Metals with a high melting point are most effective in this polishing process and it is concluded that the wear of diamond is due to an allotropic transformation into graphite or amorphous carbon under the influence of localized frictional heating.