The plasticity of diamond

• Main Author: Brookes, Jill
• Published: University of Hull 1992
• Subjects: 530.412; Engineering design and manufacture
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.553775

Aspects of the crystal structure of diamond, and its associated defects, have been considered with reference to the effect such characteristics might have on its mechanical properties. Also, established resolved shear stress models, which account for anisotropy in conventional Knoop indentation hardness of all single crystals, have been reviewed. Particular attention has been given to the role of microplasticity and the nature of crack formation in the deformed zone formed beneath the indenter. It is then shown that a similar approach can be applied to the case where a cone, made from a softer material, replaces the conventional rigid indenter. By using different materials covering a range of hardness, impressions can be formed beneath which there is a controlled density and depth of dislocations. In this work, the 'soft' indenter technique has been extended to high temperatures and applied to study the plasticity of various types of natural and synthetic diamond. Consequently, the effect of temperature on the critical resolved shear stress of synthetic type Ib, and natural type Ia and type IIa has been established. Above a critical threshold temperature for the onset of plasticity, time dependent growth of the impression volume occurs whilst the mean contact pressure is decreasing. It is shown that geometrical similarity, i.e. the ratio of the impression size to dislocated volume, is maintained whilst the critical mean pressure continues to be exceeded during this process of 'impression creep'. Activation energies of about 2.9 eV and 1.2 eV were determined, from rates of volume change, for natural (both type I and II) and synthetic type Ib respectively. Whilst no significant differences were observed between 98.9% 12C (natural abundance) and 99.9% 12C (isotopically enriched) synthetic diamonds, their behaviour was most like that of a type IIb diamond. Finally, by studying type la diamonds with a nitrogen concentration ranging from 14 - 750 ppm, evidence is obtained which supports the suggestion that this element reduces the intrinsic resistance to dislocation movement and encourages the initiation of cracks in the diamond structure.