TiO₂ coatings were prepared by High Velocity Oxy-Fuel (HVOF) spraying using agglomerated nano-TiO₂ powders with mean diameters of 30∼35 μm, of which primary particle sizes were 200nm, 30nm and 7nm. The effects of spraying conditions on microstructure, anatase phase ratio and primary particle size have been investigated and then photocatalytic property was also evaluated. The anatase phase ratio decreased drastically with increasing fuel gas pressure in the case of agglomerated 7nm powder (P₇ powder), while higher anatase ratio could be obtained in the case of agglomerated 200nm and 30nm powders (P₂₀₀ and P₃₀ powders). The particles of anatase phase grew during spraying in the case of agglomerated 7nm and 30nm powders. Especially, in the case of 7nm powders, they haw become three times larger compared to as-agglomerated powders, while they have hardly changed in the case of 200nm powders.
These coatings showed photocatalytic degradation of gaseous acetaldehyde (CH₃CHO). As a result of experiments, it was clarifid that the main factors affecting photocatalytic properties were the anatase ratio and particle size in the sprayed coating.

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The phase transformation of anatase to rutile and the growth of nano primary particle in agglomerated TiO₂ powder in heat treatment and HOVF spraying processes were investigated systematically. The 200nm, 30nm and 7nm primary particles were agglomerated by spray dry method for the feedstock powders of P₂₀₀, P₂₀ and P₇, respectively. From the heat treatment results of feedstock powders, it was found that the phase transformation temperature of P₇ was 100K and 150K lower than that of P₃₀ and P₂₀₀, respectively. The growth of primary particles of P₇ and P₃₀ happened earlier than their phase transformation, and increased drastically with increasing the heat treat temperature. Under various fuel pressure HVOF conditions, the anatase phase content was between 86% and 5% for sprayed P₇ coating, whereas it was higher than 50% for P₃₀ coating. The particle sizes of P₇ and P₃₀ grew during spraying processes.
The anatase content of collected particles was lower than that of HVOF sprayed coatings in all conditions if phase transformation happened. Although the particle size had not significant change for P₂₀₀, it grew obviously for P₇ and P₃₀. It was considered that the phase transformation and growth of primary particles were frozen for the rapid cooling effect due to the low temperature substrate. Therefore, it was concluded that the coating with high anatase ratio (approximately 100%) and primary particle size of single nano might be developed using the substrate with high thermal conductivity and large heat capacity.

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A limiting feature of the plasma spraying is the need for powders to melt and react each other during its passage through the plasma flame. In case of using the mixture of ordinary sized powders, it is quite impossible to obtain coatings with new composition. Agglomeration techniques were used to prepare the fine powders of SrCO₃ and TiO₂. SrTiO₃ and donor-doped SrTiO₃ coatings were produced by plasma spraying of the agglomerated powders. The formation of SrTiO₃ was established by X-ray diffraction analysis. Influence of La₂O₃ additions on the resistivity and the electric constant were evaluated in SrTiO₃ and donor-doped SrTiO₃ coatings.

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Nickel powder was low pressure plasma sprayed onto a steel substrate, and the influence of the thermal history of the coating on the structures and the properties of the coating were examined. The lamellae of the coating was composed of fine columnar grains when the temperature of the coating was kept low during spraying. As the temperature of the coating during spraying increases, the length of the columnar grains became longer than the thickness of the lamellae. The coating heat-treated after spraying consisted of coarse equiaxed grains and had high porosity that came from the liberation of a plasma gas dissolved in rapidly solidified solid nickel. The hardness, apparent density and the tensile strength of the coating itself were the highest on the coating prepared at a low temperature and became low after heat-treatment. The thermal conductivity measured in the direction perpendicular to the coating was the largest on the coating that consisted of long columnar grains. The cause for the formation of the long columnar grains is discussed in terms of the momentary and local melting of the coating surface that is in contact with the sprayed droplet and the grain growth in the solid state due to the large temperature gradient in the coating.

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