Studies on cleavage fracture and etching of diamond

Abstract

In Part I a brief historical review of the existing information on diamond is given. In Part II a short account of all the experimental techniques used in the present investigation is given. Part III deals with the optical and interferometric studies of the crystallographic traces on (111) cleavage faces of diamond octahedrons cleaved by three different methods(a) by the usual method of cleaving (b) by breaking and (c) by pushing. These studies are further extended to the traces on (111) cleavage faces obtained from (a) a regular octahedron (b) a dodecahedron and (c) a crystal of irregular habit. The occurrence of these traces is fully discussed and they have been attributed due to the deposition of some weak (111) planes during the growth of the crystal. This is confirmed by the measurements made on their displacements across large cleavage steps (Chapter 7). Part TV deals with the etching of diamond cleavage faces. A brief review of the etch methods used for crystallographic research and the existing information on the etching of diamond surfaces are given in Chapter 4. In Chapter 5 various etch patterns formed on the octahedral cleavages are described and fully discussed. It is established that etch pits are distributed in three ways (a) random small pits (b) striking linear arrangements and (c) individual isolated pits usually larger than others. The pit concentrations are found to vary from 10 per sq.mm. to over 10 per sq.mm. and some small regions are found quite free from imperfections. The etching has revealed small oriented ring cracks. Their occurrence is explained as being due to the shock of cleavage. Chapter 6 deals with the etching of oppositely matched faces. It is demonstrated that there is a remarkable degree of coincidence and matching (in mirror images) of the etch patterns on the matched faces. Nearly one to one correspondence between the centres of the isolated pits and also between the sets of complex linear patterns is established. It is observed that the cleavage ledges displace sideways due to the solution process and this produces an apparent systematic displacement of the etch patterns. These observations are fully discussed and are accounted for. In Chapter 7 by etching small cleaved out blocks a striking stratigraphy is revealed and it is proved that the individual stratigraphical sheets go right through the whole crystal. The etch strata therefore reveal for the first time the whole history of the growth of the diamond studied. The implications are briefly discussed. In Chapter 8 the formation of rectilinear and curvilinear pits is fully described and it is shown that (a) the rectilinearity or curvilinearity of etch pits depends primarily on the rate of etch (b) rectilinearity is highly sensitive to temperature. The mechanism of pit formation is discussed in some detail and the way in which linear crystal defects lead either to flat bottomed or point bottomed etch pits is also discussed. In Part five a valuable technique developed for testing the glazier's diamonds is described. The onset and progressive deterioration of the cutting edges (both polished and natural) of glazier's diamonds (and consequently of the cut) studied by this technique is fully described. It is observed that the natural cutting edges are much more wear resistant compared to the polished cutting edges. The results are briefly discussed. In Appendix A some graphitization experiments aiming at clarifying the meaning of the frosted appearance of some natural stones are described. A small diamond graphitized at 1250°C in N2 at a pressure of 3 mm. when heating in potassium nitrate fractured by cleavage in the melt. Thus a section across and through the graphitized region was exposed and subjected to etch. Prom the etch patterns on the cleavage faces the depth of the graphite layer is assessed from 40 to 50 microns. In Appendix B observations made on trigons on (111) cleavage faces of natural octahedron and (111) faces of synthetic diamonds are described. It is observed that the orientation of trigons and etch pits on (111) faces of synthetic diamond is the same as on (111) faces of natural diamonds. The results are discussed in the light of the formation of trigons.