Statistical modelling of the leaching of Polycrystalline Diamond Compacts (PDCs) for integration into the product design process

Abstract

This thesis investigated the optimisation of the high-pressure high-temperature (HPHT) sintering process for manufacturing polycrystalline diamond compacts (PDCs) with enhanced leachability and performance. A primary focus was the precise control of the key sintering parameters: pressure, temperature and holding time. The research addressed the challenges of accurate parameter measurement under extreme HPHT conditions by developing and validating a novel in situ pressure calibration method using a cubic press. The method, based on real-time volume change measurements of the PDC sample and the Murnaghan equation of state, gave improved accuracy and material specificity than conventional fixed-point techniques. Furthermore, the study examined the influence of thermocouple placement on temperature measurement accuracy, demonstrating the critical importance of optimal positioning for reliable temperature monitoring. The individual and combined effects of pressure, temperature and holding time on PDC leach rate were extensively analysed. Experiments showed that increased pressure significantly reduced leach rate, attributed to enhanced densification, reduced porosity and smaller cobalt pools. Similarly, higher sintering temperatures decreased leachability due to greater densification and altered pore morphology. Holding time also played a role, with longer durations promoting densification and thus reducing leach rate, particularly at lower temperatures. Statistical models, including linear and polynomial regression, were developed and validated to predict leach rate based on individual parameter variations. To further understand the complex interaction between the sintering parameters, a multiple regression model was developed, revealing a significant interaction between holding time and temperature. Dominance analysis was employed to establish a hierarchy of parameter importance, identifying pressure as the most influential factor in controlling leach rate, followed by temperature, with holding time having a lesser, yet significant, indirect effect. The developed models coupled with the insights into parameter interactions and relative importance provided a comprehensive framework for optimizing the HPHT sintering process of PDCs.