ETD Collection

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    Low pressure liquid phase sintered diamond composites
    (2018) Mkhize, Mandisa Queeneth
    Polycrystalline diamond (PCD) materials form part of tool components used in automobile, aerospace and mining applications. These components are commonly prepared using high pressure high temperature (HPHT) techniques. The importance of PCD is due to properties such as very high hardness, toughness and wear resistance at extreme conditions in a reproducible manner. However, few studies have examined the feasibility of using liquid phase sintering aids, such as the Y2O3-Al2O3-SiO2 oxide binder system to sinter PCD at low pressures using the Spark Plasma Sintering (SPS) method. In this study we aimed to produce a dense, strong, liquid phase-sintered diamond composite without undergoing the diamond phase’s solution re-precipitation stage, under a low pressure of 30 – 70 MPa. Diamond composites using monomodal and bimodal diamond feedstock powders were fabricated using yttrium alumino-silicate additives, with compositions of 40wt%Y2O3-25wt%Al2O3-35wt%SiO2 (yttria-rich) and 30.78wt%Y2O3-13.65wt%Al2O3-55.58wt%SiO2 (silica-rich) labelled as LPI and LPII, respectively. Diamond powder and the yttria alumina silica powders were mixed using the planetary ball milling technique and the ad-mixed components were heated and pressed using the SPS furnace. This showed that the silica-rich liquid phase sintering aid produced low density composites due to amorphous grain boundary and the move of the softening point to high temperatures. However, the yttria–rich additive produced bimodal diamond composites of high relative density of ~97% and hardness of ~13GPa due to faster densification rates. All the samples were measured for density using the Archimedes' method. Characterization was performed using powder X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersion Spectroscopy (EDS) and Vickers hardness measurement. Examination of fracture surfaces resulted in linking microstructural features such as intergranular cracks, crack branching and intergranular phases to the behavior of these additives under the sintering conditions used in this work. This study revealed that high densities were attainable using the yttria-rich binder under low pressures using an SPS furnace. The effect of the heating/cooling rates via the SPS were also observed to affect the microstructural behavior of the composites and consequently their properties.
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    Effects of diamond grain size on magnetic properties of the cobalt phase in PCD compacts
    (2018) Ngwekhulu, Tsholofelo Themba
    This research was aimed at a broad description of polycrystalline diamond (PCD) compacts sintering and effects of metal sintering aid infiltration into PCD. It concerned an investigation into the influence of diamond grain size on metal phase magnetic properties in PCD. In particular, a relationship between magnetic parameters and PCD microstructure has been investigated. The polycrystalline diamond (PCD) table utilized for investigation comprised diamond grains, which were non-magnetic, a metal phase (cobalt), which was the only ferromagnetic part in the material and WC which was non-magnetic and which moved into PCD during cobalt infiltration. Sintered PCD compacts having mean grain sizes of 4.5 μm, 13 μm and 25.3 μm were assessed respectively. The PCD tables without the substrate material were prepared by Electron Discharge Machining (EDM) and characterized by means of Scanning Electron Microscopy (SEM), Image Analysis (IA) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Magnetic saturation and coercive field strengths were resolved using KOERZIMAT 1.097 and KOERZIMAT CS 1.096 measuring systems. Strong correlation was seen between the metal phase content and magnetic saturation and also in cobalt mean-free path and coercivity. The analysis demonstrated that the shape and size of the specimen has no impact on the magnetic saturation parameter, and further investigation showed that increasing the grain size of the non-magnetic diamond phase decreases the number of magnetic domains of a metallic cobalt phase, but increases the coercive force. In WC-Co coercivity is high when WC grains are fine. The research has demonstrated that the diamond starting particle size has substantial impact on the metallic phase microstructure and magnetic properties in general. Additionally, the results demonstrated that PCD made with fine diamond particles has greater PCD density as a function of metal content in comparison to coarser diamond particles showing a higher volume fraction of metallic cobalt due to the fact that the former includes much smaller average number of contact points and thus much higher contact stresses, and furthermore has greater surface to volume ratio as well as a lower packing density. Characterisation results have demonstrated a strong correlation and showed that changing the grain size of the non-metallic phase has significant impact on the subsequent microstructure and material properties of the PCD.