The Effect of Temperature on the Deformation of Diamond Surfaces

Abstract

ABSTRACTA brief review is given of recently published work on the fracture and plastic flow of natural and synthetic diamond crystals when deformed by softer impressors at elevated temperatures. Under these conditions, a brittle-ductile transition (BDT) temperature for the different types of diamond has been established.In this paper we show that below the BDT temperature, brittle ‘chatter’ cracking on {111} cleavage planes accounts for anisotropic wear provided that the mean contact pressure exceeds about 10 GPa. Above the BDT temperature, titanium diboride sliders develop contact pressures sufficient to cause extensive plastic deformation of the diamond specimens but insufficient to produce cleavage fracture. However, repeated sliding with TiB2 does lead to cumulative plastic flow preceding fatigue type fracture on {110} planes. Using cubic boron nitride and diamond sliders above the BD T temperature leads to a combination of both plastic deformation and {110} cracking. The coefficient offriction is a maximum in <100> and a minimum in <110> directions. and the measured scratch hardness, which confirms that the <100> directions are softer than <110>, is shown to be consistent with the predictions of a resolved shear stress model which was de velopedffor explaining anisotropy in all crystals.