Browsing by Author "Bissett, H."

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    Design and fabrication of a chemical vapour deposition system with special reference to ZrC layer growth characteristics
    (Journal of the Southern African Institute of Mining and Metallurgy, 2017) Biira, S.; Crouse, P.L.; Bissett, H.; Hlatshwayo, T.T.; Laar, J.H. van; Malherbe, J.B.
    The overall aim of this research project was to design and construct an inhouse, thermal chemical vapour deposition (CVD) reactor system, operating at atmospheric pressure. Radio frequency (RF) induction heating was used as the energy source, with a vertical-flow design, using thermally stable materials. The steps in the design and construction of this CVD system are described in detail. The growth conditions at different substrate temperatures, gas flow ratios and substrate-gas inlet gaps were assessed as part of the project. The growth rate of ZrC layers increases with increasing substrate temperature. The microstructure properties of the ZrC layers such as lattice parameters and orientation of crystal planes were all found to be dependent on deposition temperature. The increase in free carbon in the as-deposited coatings as the temperature increased was found to be a stumbling block for obtaining stoichiometric ZrC coatings. The surface morphology of the as-deposited ZrC layers also depends on the deposition parameters.
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    Effect of temperature and CH4/ZrCl4 molar ratio on ZrC layers deposited in a vertical-wall CVD system
    (Proceedings of SAIP, 2016) Biira, S.; Alawad, B. A. B; Bissett, H.; Nel, J. T.; Hlatshwayo, T. T.; Crouse, P.L.; Malherbe, J. B.
    The synthesis of ZrC layers was performed in an in-house developed, vertical-wall chemical vapour deposition (CVD) system operating at atmospheric pressure. Zirconium tetrachloride and methane were used as zirconium and carbon sources respectively, with an excess of hydrogen as reducing agent. Argon was used to carry the vaporised ZrCl4 at 300 °C to the reaction chamber. The deposition of ZrC was carried out on graphite substrates at temperatures in the range of 1200 °C –1600 °C. The molar ratio of CH4/ZrCl4 was varied from 6.04 to 24.44. Response surface methodology was applied to optimise the process parameters for the deposition of ZrC. A central composite design was used to investigate the effects of temperature and molar ratio of CH4/ZrCl4 on the average crystallite size. Quadratic statistical models for crystallite size was established. Scanning electron microscopy (SEM) images show that the coatings became more uniform with increased particle agglomeration as temperature increased.
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    Influence of the substrate gas-inlet gap on the growth rate, morphology and microstructure of zirconium carbide films grown by chemical vapour deposition
    (Ceramics International, 2017) Biira, S.; Alawad, B.A.B.; Bissett, H.; Nel, J.T.; Ntsoane, T.P.; Hlatshwayo, T.T.; Crouse, P.L.; Malherbe, J.B.
    The influence of the gap between the gas inlet and the substrate in an in-house developed thermal chemical vapour deposition (CVD) reactor, on the growth rate, surface morphology, phase composition and microstructure of deposited ZrC films was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The ZrC films were grown on high density graphite substrates at different substrate-inlet gaps, viz. 70 mm, 90 mm, 120 mm, 145 mm and 170 mm, at substrate temperatures of 1200 °C and 1400 °C. The growth rate of ZrC films prepared at 1400 °C was observed to be higher than at 1200 °C., and was found to decrease with increase in substrate-inlet gap at both temperatures. The boundary layer thickness increased with an increase in substrate-inlet gap. The diffusion coefficients of the reactants were found to be 0.176 cm2/s and 0.200 cm2/s for the ZrC films deposited at 1200 °C and 1400 °C respectively. A model illustrating the diffusion of source materials through the boundary layer to the reacting surface was also given. The XRD results of ZrC films showed that at both 1200 °C and 1400 °C the (111) plane was the less preferred orientation, while (200) and (220) were the preferred planes. The degree of preferred orientation of ZrC films was found to decrease with increasing substrate-inlet gap. SEM results indicated that as the substrate-inlet gap increased from 70 mm to 170 mm for 1400 °C, the films became more uniform with increased particle agglomeration. The cauliflower like clusters of particles grew larger in size and covered the whole surface. By contrast, at 1200 °C the surface crystallites had complex facets that decreased in size as the substrate-inlet gap increased from 70 mm to 170 mm.
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    Investigating the thermal stability of the chemical vapour deposited zirconium carbide layers
    (Journal of Alloys and Compounds, 2020) Biira, Saphina; Thabethe, T.T.; Hlatshwayo, T.T.; Bissett, H.; Ntsoane, T.; Malherbe, J.B.
    The effect of thermal treatment on zirconium carbide (ZrC) layers deposited by chemical vapour deposition process was investigated using X-ray diffraction (XRD), Raman spectroscopy, nanoindention and scanning electron microscopy (SEM). The ZrC layers deposited at 1400 C (composed of 96% ZrC and 4% C) were annealed at 1500, 1600, 1700 and 1800 C for 2 h under high vacuum of 2.6 10 7 mbar. After annealing, the lattice constant and the average crystallite sizes were found to increase whereas the lattice strain and dislocation density decreased. The preferred orientation of the as-deposited layers was (220); it changed to (200) when annealed at 1500 C and 1600 C. At annealing temperature of 1700 C and 1800 C, the preferred orientation was (220) just like for the as-deposited ZrC layers. From Raman spectroscopy analysis, the ID/IG ratio reduced from 0.694 to 0.414 with annealing temperature indicating an improvement in crystallinity level and a decrease in the defects in the carbon material in the ZrC layers. The hardness of the layers was found to decrease slightly with annealing temperature from 26.4 ± 0.6 GPa to 21.3 ± 0.5 GPa. Some voids initially present in the as-deposited ZrC layers closed up and particles increased in size with annealing temperature.
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    Optimisation of the synthesis of ZrC coatings in a radio frequency induction-heating chemical vapour deposition system using response surface methodology
    (Thin Solid Films, 2017) Biira, S.; Crouse, P. L.; Bissett, H.; Alawada, B. A. B; Hlatshwayo, T. T.; Nel, J. T.; Malherbe, J. B.
    A chemical vapour deposition process using radio frequency induction heating operating at atmospheric pressure was developed for the deposition of ZrC coatings. The precursors utilised in this process were zirconium tetrachloride and methane as zirconium and carbon sources respectively, in an excess of hydrogen. Additionally, a stream of argon was used to, first, remove oxygen from the reactor and then to sweep the vapourised ZrCl4 at 300 °C to the reaction chamber. The ZrC coatings were deposited on graphite substrates at substrate temperatures in the range of 1200 °C–1600 °C. The molar ratio of CH4/ZrCl4 was varied from 6.04 to 24.44. Before the start of the deposition process, thermodynamic feasibility analysis for the growth of ZrC at atmospheric pressure was also carried out. Response surface methodology was applied to optimise the process parameters for the deposition of ZrC coatings. A central composite design was used to investigate the effects of temperature and molar ratio of CH4/ZrCl4 on the growth rate, atomic ratio of C/Zr and crystallite size of ZrC coatings. Quadratic statistical models for growth rate and crystallite size were established. The atomic ratio of C/Zr followed a linear trend. It was found that an increase in substrate temperature and CH4/ZrCl4 ratio resulted in increased growth rate of ZrC coatings. The carbon content (and concomitantly the atomic ratio of C/Zr) in the deposited coatings increased with temperature and molar ratio of CH4/ZrCl4. The substrate temperature of 1353.3 °C and the CH4/ZrCl4 molar ratio of 10.41were determined as the optimal condition for growing near-stoichiometry ZrC coatings. The values were 1.03, 6.05 μm/h and 29.8 nm for C/Zr atomic percentage ratio, growth rate and average crystallite size respectively
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    The role of ZrCl4 partial pressure on the growth characteristics of chemical vapour deposited ZrC layers
    (Ceramics International, 2017) Biira, S.; Crouse, P.L.; Bissett, H.; Hlatshwayo, T.T.; Njoroge, E.G.; Nel, J.T.; Ntsoane, T.P.; Malherbe, J.B.
    ZrC layers were deposited in a chemical vapour deposition (CVD) reactor on graphite substrates using a ZrCl4- Ar-CH4-H2 precursor mixture. The deposition was conducted at different ZrCl4 partial pressures at a constant substrate temperature of 1400 °C for 2 h at atmospheric pressure. The deposited ZrC layers were characterised using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The effect of ZrCl4 partial pressure on the growth rate, microstructure and surface morphology of the deposited layers was studied. The ZrCl4 partial pressure was manipulated by changing the flow rate of the argon carrier gas through the sublimation chamber. The boundary layer thickness decreased as ZrCl4 partial pressures increased due increased argon flows. The increased ZrCl4 partial pressure increased the growth rate of ZrC layers linearly. It was found that the transport process of the source materials was laminar and forced convection flow. The flow process of source materials through the boundary layer to the reacting surface was also illustrated using a model. The average crystallite size increased with ZrCl4 partial pressures, whereas the lattice parameter, lattice strain and dislocation density decreased as ZrCl4 partial pressure increased. The surface morphology of the asdeposited ZrC layers varied with the ZrCl4 partial pressure. The size of crystals grew larger and the cavities surrounding them decreased in number and size as the ZrCl4 partial pressure increased.