The influence of microstructure on the fracture statistics of polycrystalline diamond and polycrystalline cubic boron nitride

Flexural strength data of a number of grades of polycrystalline diamond (PCD), and polycrystalline cubic boron nitride (PCBN) were analyzed using Weibull, normal and lognormal distributions. The role of microstructure in the failure mechanism of such materials was analyzed in terms of the chosen strength distributions. The best-fit distributions were determined using the maximum log-likelihood criteria and a comparison between the best and worst fit was conducted using the Akaike Information Criteria (AIC). Both large and small specimens were tested to investigate possible volume scaling effects for these materials. The different microstructures between the two materials was shown to have an effect on the statistical strength distributions. It was found that for PCD, in general, a lognormal distribution provided a better fit than the other distributions and no specimen size effect was observed. For PCBN a significant specimen size effect was observed and this also corresponded to the data fitting to a Weibull distribution.