Developing small scale fracture tests for polycrystalline diamond
Name: Giorgio Sernicola
Sponsor: Element Six
Polycrystalline diamond composites (PCD) were first sintered in the 1970s following research efforts focused on producing new, more durable materials to use as cutting tools. These composites are characterised by a complex microstructure composed of two stark different phases, hard and brittle diamond grains and a network of softer and ductile cobalt. Given the high volume fraction of brittle phase, life of these tools is dominated by their fracture behaviour and catastrophic failures that still represent the major issue for their application.
In the last three decades, improvements of the fracture properties of brittle materials have been sought through the development of new insights on toughening mechanisms, typically involving microstructure control that focuses on crack deflection 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), we developed an innovative testing method. This approach is based on the double cantilever wedging to measure the fracture energy evolution with crack during stable growth and was successfully applied at the micron scale inside a SEM. Direct view of the crack growth in our 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 tests, we have targeted characterisation at the role of microstructure on crack paths in polycrystalline diamond. Our focus has been on using high angular resolution EBSD combined with microindentation, to correlate intra-granular residual stresses gradients, due to thermal expansion mismatches, to crack deflection. It was found that the crack can follow the grain boundaries if grains are small but tends to deviate along (111) in coarse grains, yet stress gradients disrupt homogeneity of individual grains and are able to deflect the crack. Exploitation of these novel techniques allows us to gather new insights on the mechanical properties of advanced ceramics that can usher in a new way of engineering the microstructure to obtain tougher ceramics.