Submitted in fulfillment of the requirements for the Degree of Master of Technology: Chemical Engineering, Durban University of Technology. Durban. South Africa, 2015. / Cobalt powder is a grey metallic powder that is produced by the thermal decomposition and reduction of a cobalt compound. The challenge faced by Shu Powders Africa was that sub-micron cobalt powder had never been produced in a two-step furnace by any manufacturer in the cobalt powder industry. Hence there was no prior information to guide this type of processing. Therefore this research set out to investigate the production of sub-micron cobalt powder through a two-step furnace to determine the optimum parameters for this process. For the company to remain competitive, it was imperative to begin producing sub-micron cobalt powder. Sub-micron cobalt powder is much more valuable and profitable to produce. The second production line would be operational due to the production of sub-micron cobalt powder hence creating job opportunities for the local community. Sub-micron cobalt powder shares the same chemical composition and physical characteristics as cobalt powder. The only differences are particle size (0.60 - 0.90 µm), oxygen content (0.30 - 0.80%) and the microscopic structure which is the particle size distribution d90 (7 - 10 µm). The approach taken was to understand the variables that had a large effect on the powder. The effects needed to be established by determining how it impacted on the quality of the powder which is pertinent to making sub-micron cobalt powder. Due to the experience in producing cobalt powder, variables that had a large effect on normal cobalt powder production were assumed to be the same variables that would impact the production of sub-micron cobalt powder. Some of these effects were also confirmed by literature. A strategy of statistical design of experiments was used to evaluate the conditions for sub-micron cobalt powder production. Design of experiments assisted in planning the experimental design matrices for both experiments. For the furnace experimentation a 24 factor design was selected. For the jet mill experimentation a 23 factor design was selected. Response surface methodology was used to determine optimum ranges of the variables at various process conditions. The central composite rotatable design laid out the design in which the variables interacted with one another at different process conditions. Evaluation of results was based on the generated model. Models such as the 3D surface model, cubic model and the contour model were generated to graphically illustrate the effects that the variables have on the response. Analysis of furnace data indicated that the optimal response was achieved at a temperature range (445 - 460)°C, hydrogen gas range (225 - 250) Nm3/h, belt speed (80 - 90) mm/min, and carbon dioxide gas range (80 - 90) Nm3/h. Analysis of the jet mill experimental data indicated that the optimal response particle size distribution, was achieved at a classifier speed range of (5500 - 6000) rpm, AFG grinding bin range (30 - 35) kgs and grinding gas pressure of (4.0 - 4.5) bar. The study confirms the efficiency of a two-step furnace to produce sub-micron cobalt powder at high volumes. The advantage of the two-step furnace was the increased throughput of 2.3-2.7 tons/day whilst in industry furnace throughputs are 1.3-1.6 tons/day. This represented a 60% increase in productivity over conventional furnaces. The response surface methodology also proved to be a suitable technique for process optimization.

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The aim of this study was to investigate a zirconia-matrix reinforced with metal powder (chromium, iron and stainless steel (AISI 316)) including processing, characterisation, and measurements of their properties (mechanical, thermal and electrical). Zirconia stabilised with 5.4 wt% Y₂0₃ (3 mol%) as the matrix was first studied and followed by an investigation of the effects of metal reinforcement on zirconia-matrix composites. Monolithic zirconia was pressureless sintered in air and argon to observe the effect of sintering atmosphere, while the composites were pressureless sintered in argon to avoid oxidation. Sintering was carried out at various temperatures for 1 hour and 1450°C was chosen to get almost fully dense samples. The density of the fired samples was measured using a mercury balance method and the densification behaviour was analysed using TMA (Thermo-mechanical Analysis). The TMA was also used to measure the coefficient of thermal expansion. In addition, thermal analysis using DTA and TGA was performed to observe reactions and phase transformations. Moreover, optical microscopy and SEM were used to observe the microstructures, XRD was used for phase identification, and mechanical properties including Vickers hardness, fracture toughness and bending strength were measured. The effect of thermal expansion mismatch on thermal stresses was also analysed and discussed. Finally, thermal diffusivity at room temperature and as a function of temperature was measured using a laser flash method, and to complete the study, electrical conductivity at room temperature was also measured. The investigation of monolithic zirconia showed that there was no significant effect of air and argon atmosphere during sintering on density, densification behaviour, microstructures, and properties (mechanical and thermal). Furthermore, the results were in good agreement with that reported by previous researchers. However, the presence of metal in the composites influenced the sintering behaviour and the densification process depends on the metal stability, reactivity, impurity, particle size, and volume fraction. Iron reacted with yttria (zirconia stabiliser), melted and reduced the densification temperature of monolithic zirconia, while chromium and AISI 316 did not significantly affect the densification temperature and did not react with either zirconia or yttria. AISI 316 melted during fabrication. Moreover, all of the metal reinforcements reduced the final shrinkage of monolithic zirconia. In terms of properties, the composites showed an increase in fracture toughness, and a reduction in Vickers hardness and strength with increasing reinforcement content. In addition, the thermal diffusivity of the composites showed an increase with reinforcement content for the zirconia/chromium and zirconia/iron composites, but not for the zirconia/AISI 316 composites due to intrinsic mircocracking. Furthermore, all the composites became electrically conductive with 20 vol% or more of reinforcement. It has been concluded that of those composites the zirconia/chromium system may be considered as having the best combination of properties and although further development is needed for such composites to be used in real applications in structural engineering, the materials may be developed based on these findings. In addition, these findings may be used in development of ceramic/metal joining as composite interlayers are frequently used.

Chong, Kam Cheong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references. / Text in English; abstract also in Chinese. / Chong, Kam Cheong. / ACKNOWLEDGMENT --- p.i / ABSTRACT --- p.ii / 摘要 --- p.iv / Table of contents --- p.v / Chapter 1 --- Introduction / Chapter 1.1 --- Metal Matrix Composites / Chapter 1.1.1 --- Background --- p.1-1 / Chapter 1.1.2 --- Some metallic matrix materials --- p.1-2 / Chapter 1.1.2.1 --- Aluminum alloys --- p.1-2 / Chapter 1.1.2.2 --- Titanium alloys --- p.1-3 / Chapter 1.1.3 --- Different kinds of reinforcements --- p.1-3 / Chapter 1.2 --- Conventional fabrication Methods --- p.1-5 / Chapter 1.2.1 --- Primary liquid phase processing --- p.1-5 / Chapter 1.2.1.1 --- Squeeze casting --- p.1-5 / Chapter 1.2.1.2 --- Spray deposition --- p.1-5 / Chapter 1.2.1.3 --- Slurry casting --- p.1-5 / Chapter 1.2.1.4 --- In Situ processing --- p.1-6 / Chapter 1.2.2 --- Primary solid state processing --- p.1-6 / Chapter 1.2.2.1 --- Physical vapour deposition (PVD) --- p.1-6 / Chapter 1.2.2.2 --- Powder blending and sintering --- p.1-7 / Figures for chapter 1 --- p.1-9 / Tables for chapter 1 --- p.1-14 / References --- p.1-15 / Chapter 2 --- Powder metallurgy --- p.2-1 / Chapter 2.1 --- Introduction --- p.2-1 / Chapter 2.2 --- Fabrication of metal matrix-particulate composites --- p.2-2 / Chapter 2.3 --- Our motivation --- p.2-4 / Figures for chapter 2 --- p.2-5 / References --- p.2-7 / Chapter 3 --- Effects of sintering in processing of metal matrix composites --- p.3-1 / Chapter 3.1 --- Introduction of sintering processing --- p.3-1 / Chapter 3.1.1 --- Solid state sintering --- p.3-2 / Chapter 3.1.2 --- Liquid state sintering --- p.3-5 / Chapter 3.1.3 --- Sintering in metal matrix composites(reactive sintering) --- p.3-7 / Figures for chapter 3 --- p.3-11 / Reference --- p.3-14 / Chapter 4 --- Experiments --- p.4-1 / Chapter 4.1 --- Introduction --- p.4-1 / Chapter 4.2 --- Methodology --- p.4-3 / Chapter 4.2.1 --- High temperature furnace experiment --- p.4.3 / Chapter 4.2.2 --- Arc-melting furnace experiment --- p.4-4 / Chapter 4.3 --- Sample preparations --- p.4-4 / Chapter 4.3.1 --- Sample requirements --- p.4-4 / Chapter 4.3.2 --- Sample milling --- p.4-6 / Chapter 4.3.3 --- Cold pressing --- p.4-6 / Chapter 4.3.4 --- Annealing conditions for high-temperature furnace --- p.4-7 / Chapter 4.3.4.1 --- Different sintering temperatures --- p.4-7 / Chapter 4.3.4.2 --- Different sintering duration --- p.4-8 / Chapter 4.3.5 --- Sample conditions in arc-melting furnace --- p.4-8 / Chapter 4.4 --- Instrumentation --- p.4-10 / Chapter 4.4.1 --- Arc-melting furnace --- p.4-10 / Chapter 4.4.2 --- Vickers hardness tester --- p.4-11 / Chapter 4.4.3 --- X-Ray powder diffractometer (XPD) --- p.4-13 / Chapter 4.4.4 --- Scanning electron microscopy & energy dispersive x-ray analysis --- p.4-15 / References --- p.4-18 / Chapter 5 --- Results / Chapter 5.1 --- High-temperature furnace --- p.5-1 / Chapter 5.1.1 --- XPD results --- p.5-1 / Chapter 5.1.2 --- Different sintering temperatures in 10 weight percent of Cr203 - A1 samples with 1 hour sintering time --- p.5-2 / Chapter 5.1.3 --- Different sintering temperatures in 15 weight percent of Cr203 一 A1 samples with 1 hour sintering time --- p.5-6 / Chapter 5.1.4 --- Different sintering temperatures in 20 weight percent of Cr203 ´ؤ A1 samples with 1 hour sintering time --- p.5-10 / Chapter 5.1.5 --- Different sintering temperatures in 30 weight percent of Cr203 ´ؤ A1 samples with 1 hour sintering time --- p.5-13 / Chapter 5.1.6 --- Different sintering time for 10 weight percent of Cr203 ´ؤ A1 samples at 1100°C sintering temperature --- p.5-19 / Chapter 5.1.7 --- Different sintering time for 15 weight percent of Cr203 ´ؤ A1 samples at 1100°C sintering temperature --- p.5-21 / Chapter 5.2 --- Arc-melting furnace --- p.5-24 / Chapter 5.2.1 --- XPD results --- p.5-24 / Chapter 5.2.2 --- Samples that were melted in arc-melting furnace --- p.5-25 / Chapter 5.2.3 --- Powder samples that were melted in arc-melting furnace --- p.5-28 / Figures for chapter 5 --- p.5-30 / References --- p.5-55 / Chapter 6 --- Discussions --- p.6-1 / Chapter 6.1 --- Chemical reactions --- p.6-1 / Chapter 6.2 --- Sintering --- p.6-6 / Chapter 6.2.1 --- Conditions for having larger Al13Cr2 intermetallic compound --- p.5-7 / Chapter 6.3 --- Vickers hardness results --- p.6-10 / Chapter 6.4 --- Comparisons between the two furnace results --- p.6-12 / Chapter 6.4.1 --- Cooling rates --- p.6-12 / Chapter 6.4.2 --- Volume fraction of all the intermetallic compounds --- p.6-14 / Chapter 6.4.3 --- Pore sizes --- p.6-15 / Chapter 6.4.4 --- Vickers hardness --- p.6-16 / References --- p.6-17 / Chapter 7 --- Conclusions and suggestions for further studies --- p.7-1 / BIBLIOGRAPHY