Developing small scale fracture tests for polycrystalline diamond

G. Sernicola

(2018) 1-224

Opening ceramics up to a wider range of applications, where their high hardness and high strength are required, necessitates our understanding and improving of their fracture properties. In the last three decades, such improvements have been sought through developing our understanding of toughening mechanisms, typically involving microstructure control that focuses on crack deflection and grain bridging at grain boundaries and interfaces. However, these are often difficult to engineer, as changing microstructural processing (e.g. through heat treatment, chemistry or powder processing) does not result in a one-to-one correlation with performance, since the influence of microstructure on crack path is varied and complex. Recent developments on characterisation at the micro-scale therefore present an opportunity to broaden our understanding of the role of individual factors on the bulk performance. To investigate the fracture properties of individual features (i.e. individual crystallographic planes, grain boundaries or interfaces), a testing method was developed. This approach is based on the double cantilever wedging to measure the fracture energy change during stable crack growth and was successfully applied at the micron scale inside a scanning electron microscope. Direct view of the crack growth in the sample and measurement of the energy absorbed during fracture, without use of load-displacement data, is afforded through the combination of a stable test geometry with an image based analysis strategy. In addition to these precise tests, characterisation of the role of microstructure on crack paths in polycrystalline metal-ceramic composites was carried out. The focus has been on using high angular resolution electron backscatter diffraction combined with microindentation, to correlate intragranular residual stress gradients, due to thermal expansion mismatches, to crack deflection. Fracture energy of individual crystallographic planes and interfaces was measured in both brittle and brittle/ductile systems. In addition, local residual stresses and microstructure in diamond were related to fracture path.