The area of real contact and the shear strength of monomolecular layers of a boundary lubricant

Abstract

Natural diamonds have been heated in the temperature range of 1850 to 2000 degrees C at zero pressure and the rates at which diamond transforms to graphite measured. For {111} and {110} surfaces activation energies of 253 $\pm $ 18 and 174 $\pm $ 12 kcal mol$^{-1}$ (1159 $\pm $ 75 and 728 $\pm $ 50 kJ/mol) respectively have been obtained. Diamonds have also been heated in the temperature range of 1950 to 2200 degrees C under a pressure of 48 $\pm $ 3 kbar (4.8 $\pm $ 0.3 GPa) and an activation volume of about 10 cm$^{3}$ mol$^{-1}$ obtained for both {111} and {110} surfaces. It is proposed that the rate controlling process in the graphitization of diamond is the detachment of a single atom from the diamond surface. This is contrary to previous proposals in which the detachment of groups of atoms have been considered to be the rate-controlling process. In the present work, it is suggested that the rate-controlling step for graphitization is the detachment of a triply bonded atom from a {111} surface and of a doubly bonded atom from a {110} surface.