Pressure crack-figures on diamond faces I. The octahedral face

Abstract

By pressure with a diamond ball, oriented hexagonal-shaped ring cracks (pressure marks) are produced on the octahedral face of a diamond. Typical ring cracks have a side length of the order of 1/25mm and appear at stresses of the order of 1·4 x 10 11 dyn cm -2 . The development of these figures is studied, using regularly increasing loads. After primary crack formation, further increase of loading leads to the appearance of concentric multiple cracks. The permanent surface distortions accompanying these percussion marks are studied with multiple-beam interferometry. It is established that the region within the hexagonal figure is effectively at the zero undisturbed level. At the perimeter there is a discontinuity, the surface rising in a typical instance by some 300 Å. There is then a gradual smooth fall off to zero level. It is considered that the interferograms offer strong evidence for the existence of plastic flow. The volume of displaced material surrounding a typical primary crack is some 10 -9 cm 3 . The mechanism of the hexagonal crack formation is discussed in terms of directions of easy cleavage, and accompanying cracking effects within the body of the crystal are discussed.