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Thermal barrier coatings (TBCs) are a simple and proven method to protect hot section components. Suspension Plasma Spray (SPS), an emerging process technology to generate TBCs, compared with traditional Atmospheric Plasma Spray APS, can deposit thinner coat-ings with finer microstructure. Operating parameters play an important role in developing certain properties of coating. In this thesis work, power level, gas flow rate, number of spray-ing strokes, spray gun's nozzle size i.e. internal diameter and suspension rate were controlled to produce coatings with different microstructures and porosity levels. According to the ex-perimental results, the power level of plasma gun play an essential role on coating micro-structure, for instance, the density of vertical cracks increased with growing the power level. The number of spraying strokes showed also an impact on coating porosity. However, due to different nozzle sizes i.e. diameter, the same coating property were controlled by different operating parameters. For coatings deposited by small and large nozzles, their coating thick-ness and roughness mainly relied on power level and gas flow rate. In contrary, it seems that the coating roughness was not influenced by the same parameters when it was deposited by medium nozzle. Also, gas flow rate do not have as big as influence on coating thickness

Ph.D., School of Process and materials Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, 2005

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The zirconia is one of the transition metal oxides with a wide band-gap, showing many good mechanical and physical properties. This study concentrates on the preparation of the polystyrene template and the characterization of the ordered mesoporous zirconia thus prepared. The polystyrene (PS) templates have been prepared by the emulsifier-free emulsion polymerization of styrene monomer (SM) in mixed solution of K2S2O8 and H2O, without any cross-linking agent. The PS spheres so prepared have diameters ranging from 50 ¡V 400 nm. These spheres are then arranged in array by sedimentation method; then the infiltration of zirconia sol solution is followed. After the sol has been transformed to the gel, calcinations under high temperature are proceeded to remove the PS template to obtain the ordered mesoporous zirconia with various pore sizes. Doping of the zirconia also is conducted by applying an AgNO3 solution to the mesoporous zirconia. Results from SEM indicate that with a fixed ratio of K2S2O8/styrene = 5.98 wt¢H, the PS size decreases with the SM concentration; PS as small as 66 nm can be obtained when SM/H2O = 0.05¢H by volume. The PS spheres forms a hexagonal close packing when using a solution of H2O/ethanol = 3¡G7 by wt during sedimentation. Shrinkage ranging from 60 ¡V 80% of the original PS size happens when the PS template is removed from the calcined zirconia. Results from XRD indicate the same tetragonal crystalline structure of zirconia after calcinations at 500oC, regardless of the different pore sizes. BET measurements show the specific surface area of the zirconia from 6.05 - 42.61m2/g. For Ag-doped zirconia under hydrogen atmosphere, it is found from EDS mapping that the AgNO3 can be reduced to the metallic silver without particles aggregation but random distribution on the pore walls or surfaces, while the tetragonal crystalline structure of zirconia remains unaltered.

An investigation has been carried out into the effects of three common oxide impurities, TiO2, A12O3, and SiO2/ on the properties and behaviour of Y-T.Z.P. These impurities are present in varying amounts in almost all commercially available Y-T.Z.P. materials, and substantial costs are incurred in removing them in the purest systems. However, the effects of these impurities, both individually and in combination have received relatively little study in the published literature, and it has not been made clear to what degree these impurities influence the properties and behaviour of the material. To carry out the investigation it has been necessary to develop a novel technique for introducing the impurities as dopants into a high purity, commercially available Y-T.Z.P., whilst retaining a high degree of chemical homogeneity in the material. The technique developed uses a variant on the alkoxide sol-gel process to coat the individual powder particles with a thin layer of dopant atoms and offers a number of advantadges over other doping techniques. The process could be exploited to solve a variety of ceramic processing problems. The results obtained from impurity doped materials showed that alumina and silica reduced the sintering temperature and promoted enhanced densification at lower sintering temperatures, whilst titania impaired the sintering at lower temperatures. Alumina additions resulted in pronounced grain growth and associated destabilisation of the tetragonal phase of zirconia, particularly for higher sintering temperatures. A factorial experiment was carried out to obtain additional, and previously unreported information. This showed that there were significant interactions occurring between all of the additives investigated some of which appeared to be beneficial. An investigation into the effect of the additives on the mechanical properties (hardness and fracture toughness) was carried out for a range of sintering temperatures. The results of these experiments suggested that the impurities had very limited direct effects on the transformation toughening mechanism, although there were differences in properties associated with the effects of the impurity additions on the microstructures of the sintered materials.