The friction of clean diamond on diamond is high, and the shear strength at the interface is comparable with the shear strength of diamond. The low friction normally observed with diamond is due to the presence of adsorbed oxygen and other gases. Carbon and graphite have a negative tem perature coefficient of friction. There is evidence that with clean graphite surfaces there is strong adhesion at the interface, so that when sliding takes place slip and shearing occurs beneath the surface. Both physical adsorption and chemical adsorption are important. The admission of a small amount of oxygen, water vapour or other contaminant will reduce the friction. The removal of surface films which are normally present causes a large increase in the friction. This paper describes an experimental study of the frictional behaviour of diamond, graphite and of carbon which have been outgassed in vacuo.
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