ETD Collection

Permanent URI for this collectionhttps://wiredspace.wits.ac.za/handle/10539/104


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2016 - 20182

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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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    Flow of a thin ribbon of molten glass on a bath of molten tin
    (2016) Sangweni, Zinhle Brighty
    The equations for the flow of a thin lm of molten glass on a bath of molten tin are extended to the case in which the viscosity of the molten glass depends on the temperature. The continuity equation for an incompressible fluid, the Navier-Stokes equation and the energy balance equation are written in the lubrication (thin fluid lm) approximation. The kinematic boundary condition and the boundary conditions for the normal and tangential stress and the normal heat flux are derived on the upper and lower surfaces of the glass ribbon. It is found for the lubrication approximation that only one equation is obtained for four unknowns which are the two horizontal velocity components, the absolute temperature difference and the thickness of the molten glass rib- bon. The remaining three equations are obtained by taking the calculation to the next order in the square of the ratio of the thickness to length of the glass ribbon. The kinematic edge condition and the edge conditions for the normal and tangential stress and the normal heat flux are derived. The four edge conditions and the boundary conditions at the inlet and outlet give the boundary conditions for the four partial differential equations. It is not the aim of the dissertation to solve the boundary value problem which has been derived, either numerically or analytically.