The influence of nitrogen on the plasticity of diamond
The aim of this work has been to use the soft impressor technique to investigate the plastic deformation of single crystal diamond and in particular to determine the effect that single substitutional nitrogen has on plasticity. Traditionally hardness tests in the form of Vickers or Knoop rigid indenters have been used to investigate the mechanical properties of materials which cannot be fabricated into tensile or three point bend test specimens. The high stress concentrations created by these types of test introduce a large degree of brittle failure in ultra-hard, covalently bonded materials. The soft impressor technique, on the other hand, allows large pressures to be applied without large stress concentrations. The result is that plastic deformation can be more readily induced into super hard materials such as diamond. This work has shown that not only can diamond be readily plastically deformed but that traces of nitrogen impurities within the lattice have a significant effect on the conditions necessary to produce dislocations. For this work, several different soft impressors were used to produce a range of pressures in the temperature range 800° to 1400°C. A selection of synthetic (HPHT) diamonds with various nitrogen concentrations were impressed and compared with impressions placed in natural type IIa specimens containing no nitrogen but heavily dislocated. Numerous analytical techniques were used to determine the level of deformation produced and gain a better understanding of the effect of nitrogen related defects. The first two chapters of this thesis review, first plasticity and then diamond, with reference to those properties/characteristics relevant to this topic. The third chapter discusses the principle of the soft impressor technique and the methodologies used. In the fourth chapter, models by which single crystal diamond plastically deforms are introduced, together with results that have extended the brittle-ductile transition schematic produced by Brookes, EJ. (1992). Results on the effect of dwell time and the phenomenon of impression creep are also presented. The fifth chapter identifies the predominant defects associated with substitutional nitrogen in HPHT diamond and presents profiles of impressions for diamonds with different 'grown-in' defect levels. The results are discussed and conclusions are made, in conjunction with suggestions for further work in chapter 6.