Vickers Hardness of Diamond and cBN Single Crystals: AFM Approach

Abstract

Atomic force microscopy in different operation modes (topography, derivative topography, and phase contrast) was used to obtain 3D images of Vickers indents on the surface of diamond and cBN single crystals with high spatial resolution. Confocal Raman spectroscopy and Kelvin probe force microscopy were used to study the structure of the material in the indents. It was found that Vickers indents in diamond has no sharp and clear borders. However, the phase contrast operation mode of the AFM reveals a new viscoelastic phase in the indent in diamond. Raman spectroscopy and Kelvin probe force microscopy revealed that the new phase in the indent is disordered graphite, which was formed due to the pressure-induced phase transformation in the diamond during the hardness test. The projected contact area of the graphite layer in the indent allows us to measure the Vickers hardness of type-Ib synthetic diamond. In contrast to diamond, very high plasticity was observed for 0.5 N load indents on the (001) cBN single crystal face. Radial and ring cracks were absent, the shape of the indents was close to a square, and there were linear details in the indent, which looked like slip lines. The Vickers hardness of the (111) synthetic diamond and (111) and (001) cBN single crystals were determined using the AFM images and with account for the elastic deformation of the diamond Vickers indenter during the tests.

Micromechanics of machining and wear in hard and brittle materials

“…The classical indentation pattern of Figure 1B with well‐developed radial cracks is observed in (a) soda‐lime glass and (b) single crystal silicon 42 . In (c) ultra‐hard diamond, radial cracks are still apparent, but the inner hardness impression has the appearance of discrete, near‐concentric shear fissures 43 . In (d) single crystal magnesium oxide, slip is strictly constrained to preferred crystallographic planes; and while radial cracks are still evident, they no longer extend from the impression corners.…”

“…42 In (c) ultra-hard diamond, radial cracks are still apparent, but the inner hardness impression has the appearance of discrete, near-concentric shear fissures. 43 In (d) single crystal magnesium oxide, slip is strictly constrained to preferred crystallographic planes; and while radial cracks are still evident, they…”

“…F I G U R E 3 Vickers indentations in hard, brittle solids: (A) soda-lime glass (courtesy David Marshall), (B) (111) single crystal silicon, 42 (C) (111) single crystal diamond, 43 indenters. 8,37,[45][46][47][48] Typical thresholds for brittle solids lie in the region a c < 1 µm.…”

“…Vickers indentations in hard, brittle solids: (A) soda‐lime glass (courtesy David Marshall), (B) (111) single crystal silicon, 42 (C) (111) single crystal diamond, 43 (D) (100) magnesium oxide single crystal (courtesy Bernard Hockey), (E) fine‐grain polycrystalline alumina and (F) coarse‐grain polycrystalline alumina 38 …”

Electrodischarge Drilling of Microholes in c-BN

“…This chemical compound exists in an amorphous form (a-BN) and in its basic and the most stable soft hexagonal form (h-BN), which is commonly used as a lubricant. The cubic form of boron nitride c-BN, however, is one of the hardest materials on Earth [2]. Its hardness makes it very attractive in many applications, and c-BN is used more frequently than the other forms of boron nitride.…”