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.
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.
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.
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.