Prof. Bob Tait
Abstract: This paper concerns the toughness measurement of polycrystalline diamond (PCD) cut from the leading edges of large PCD “buttons”, as used in rock cutting mining applications, and subsequently cut into plates 0.5 to1.0 mm thick. These were themselves cut to form miniature double torsion (DT) specimens, and polished and notched appropriately. The PCD grades comprised a range from a starter grain size of 4 microns, through 12 to 30 microns, with a fourth grade of 30/4 micron sintered composite. The DT specimens were tested in a custom made DT rig developed to locate inside the chamber of a scanning electron microscope (SEM), facilitating observation monitoring of crack growth at micron increments. The system was made to micron tolerances thus enabling controlled crack growth to occur, and the system was also instrumented to record full load and displacement behaviour. The system was very reliable and repeatable and, with the attention to dimensional detail and tolerancing, straight cracking was achieved. A tapered starter notch was introduced into the specimens and overcame the notch effect of conventional cracking. The micro-cracking (including the crack tip) could be visually followed easily in the SEM as it developed, and features such as straight cracking, branching, crack ‘jumping’ (discontinuous cracking on the surface) were all easily monitored. The fracture toughness was also measured and ranged from 8.1 to 12 MPa√m as the grain size increased, and changes in crack path readily observed. Micro-cracking developed from intergranular to more trans-granular (as grain size increased) but also exhibited more tortuosity, which was interpreted as the crack needing to exceed the activation energy to crack the bigger grains. It was possible to draw correlations of increasing toughness with starter grain size and also contiguity, and also a clear decrease in toughness with increasing cobalt content. Fracture surfaces, of slow (often intergranular) crack growth versus fast (often trans-granular) brittle fracture, could be distinguished, from the different cobalt content on the surface.
The DT technique is very powerful and stable, for this application to such brittle materials, but needs remarkable precision alignment and accuracy, but also points to methods to improve the toughness even further. The technique is reliable and reproducible, when made sufficiently accurately, and recently this work has been extended to use digital image correlation (DIC) and electron back scatter detection (EBSD) to monitor the crack paths. This has enabled and measurement of the (J integral) fracture toughness in a non-contact manner which exhibits complementary behaviour to the current results, and forms the subject of associated later papers.