Indentation hardness of two types of diamond in the temperature range 1500°C to 1850°C

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Abstract
Diamond octahedra have been mutually indented, the point of contact being two edges at right angles to one another. Type I and type II diamonds were used and the temperature of indentation has been in the range 1500°C to 1850°C. The rate of indentation was measured. With an applied load of 16·4 Kg the deformation was fully plastic for type II diamonds at 1700°C and above but for type I diamonds the plastic region was at 1750°C and above. The results have been shown to be consistent with theories of high-temperature indentation experiments when the diamonds deform plastically with no associated cracking at the indentations. A creep relation has been derived for diamond with Q = 10·7eV for type II diamonds and 14 eV for type I diamonds (ϵ=strain rate, S=imposed tensile stress in a tensile creep experiment, Q=activation energy for the process). The mechanical behaviour of diamond has been compared with that of germanium and silicon and seems to give an effective melting temperature of diamond of about 3000°C at atmospheric pressure. This temperature has also been suggested in recent theoretical work.

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