Indentation Hardness of Diamond

Abstract

RECENTLY, S. Tolansky and V. R. Howes¹ published some data on indentation compression strength with a diamond ball (0.39 mm. radius) on differently orientated diamond faces. They obtained values for the pressure which caused cracking on different diamond faces ranging from 1.2 to 1.8 × 10¹¹ dyne-cm.⁻² (1,200–1,800 kgm./mm.²). By comparing these values with those quoted in the literature it appears that they are extremely low : F. Auerbach², absolute hardness of diamond 2,200 kgm./mm.²; Knoop, hardness³ of diamond 6,500–8,500 kgm./mm.²; Russian tests with triangular pyramids⁴ and my own tests⁵ with the same type of indenter indicated hardnesses greater than 14,000 kgm./mm.². Recent tests with double cone indenters (radius 2 mm., included angle 154°) showed that diamond surfaces could be loaded to well over 3,000 kgm./mm.² without showing any form of cracks even when investigated with the electron microscope (by courtesy of Mr. M. Seal, Cambridge).