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Title: The solid particle erosion of polycrystalline diamond at high and low temperatures
Author: Henderson, Arthur James
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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Polycrystalline diamond (PCD) is formed by sintering together micron sized diamond grits at high temperatures and pressures in the presence of a metal catalyst, usually cobalt. Combining the hardness of diamond with the toughness of a polycrystalline microstructure, a material is produced with properties very suited to drilling applications due to its high abrasion resistance. Throughout the research presented in this thesis, solid particle erosion (SPE) has been used to simulate wear in freestanding PCD discs. The objective was to investigate how PCD responded to SPE over a range of conditions and at both ambient and high temperatures. Experiments were performed on three monomodal grades of PCD with grain sizes ranging from 2 μm to 30 μm. The erosion rates of three erodents, SiO₂, Al₂O₃ and SiC were compared and SiC was chosen as the principle erodent as higher erosion rates could be achieved. The response of erosion rate with velocity was measured for the three PCD grades and, when modelled as a power law, the spread of the exponents indicated that there were different material removal mechanisms taking place as the grain size changed. Increasing the grain size was found to decrease erosion resistance and reducing the cobalt binder content was found to significantly decrease erosion resistance. Erosion at oblique angles demonstrated that the mechanism was purely brittle, and analysis of the eroded surfaces under SEM showed that both microchipping and Hertzian cone cracks could be seen, with cone cracks significantly more prevalent in the finest grain sized material. A set of multimodal PCD compacts were sintered at a range of temperatures and pressures and their wear rates were tested using SPE. It was found that, over the range studied, increasing the sintering pressure and decreasing the sintering temperature both increased wear resistance. The effects of temperature on the wear of PCD are highly relevant due to the frictional heating that can occur during drilling. The effect of thermal history was studied by eroding PCD discs at ambient temperature after a range of vacuum heat treatments at temperatures up to 800 °C. A further test was performed comparing the effects of heat treatment in air to those in vacuum at 700 °C, finding much more severe surface degradation when the treatment was performed in air. A description of how the SPE apparatus has been adapted to allow erosion with high temperatures (up to 650 °C) in-situ is given, together with the results obtained using the apparatus. It was found that the erosion rate for all grades initially decreases with temperature, down to a minimum at around 200 °C, before increasing again at between 450 °C to 500 °C, or higher for PCD with a reduced binder phase. A further test was performed to help understand the reasons for the change in erosion rate with temperature by eroding samples that had previously been eroded at high temperatures, at ambient temperature. A modification was made to the high temperature SPE apparatus to allow erosion at temperatures below ambient temperature by cooling using liquid nitrogen. To the best of the author’s knowledge, low temperature erosion has never before been performed in this way, and the response of PCD to erosion at temperatures down to -100 °C was measured. The research yields insight into the response of PCD with temperature taking care to distinguish those thermal effects that alter the surface at temperatures above 450 °C to those that alter the bulk microstructure above 700 °C. Eroded surface fractography demonstrates the mechanisms for material removal during erosion and helps to explain the changes in erosion rates as the conditions are varied.
Supervisor: Jardine, Andrew Sponsor: Element Six Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Polycrystalline Diamond ; Erosion ; Solid Particle Erosion ; High Temperature Solid Particle Erosion ; Low Temperature Erosion ; Below Ambient Temperature Erosion ; Diamond ; Wear ; High Temperature Erosion