The abrasion of diamond

In most abrasion processes an abrasive which is harder than the material to be worked is employed. With diamond this has not been possible and the material itself is normally used. This process is unusual in that there is a very large directional variation in the resistance to abrasion of diamonds abraded with diamond powder. The wear rates for different crystallographic directions vary by a factor of 500 or more. An attempt has been made to determine whether the abrasion of diamond by diamond is primarily mechanical or thermal, i. e. whether it occurs by fragmentation of the diamond on a microscopic scale, or by burning or an allotropic change caused by the local high temperatures produced by friction. The experimental work described is in two sections: track formation on diamond, and the friction of diamond. The work on track formation, carried out at low sliding speeds with a rounded diamond slider and a plane diamond surface, was done in an attempt to reproduce on a macroscopic scale the damage occurring on a microscopic scale during abrasion. It was found that, according to the conditions of sliding, tracks of three main types can be produced, and that on an individual diamond face there is no variation of the ease of track formation with sliding direction. This last result contrasts with the large directional variation in the ease of abrasion, and indicates that high sliding speeds introduce some effect which causes the anisotropy in abrasion resistance. A thermally activated chemical effect is thought to be the most likely cause of this difference in behaviour at high and low sliding speeds, and the following explanation of the anisotropy in high-speed wear rate is suggested. The work on friction shows that there is a variation with sliding direction of the coefficient of friction between diamond and diamond. Directions in which the friction can be high are directions of easy abrasion. The local surface temperatures during abrasion will depend on the coefficient of friction and will be greater when the friction is high. These directions would thus be expected on a thermal theory to be those of easy abrasion. Furthermore, a small anisotropy in friction would account for a large anisotropy in abrasion resistance.