| Research Abstract |
The principal findings can be summarized as follows:
1.To date, many modeling efforts related to the deformation and solidification processes of a droplet impacting on the substrate under plasma spraying conditions have been reported. However, no modeling effort has dealt with the super-cooling effects on the deformation and solidification processes, though much evidence of super-cooling effects has been reported. In this research, we could show the first results derived from our recent modeling efforts for the case of Al_2O_3 droplets, which clearly show the strong effects of the super-cooling conditions on the deformation and solidification processes. For example, we successfully predicted a significant decrease in deformation degree -defined as the ratio of droplet to splat diameters -to less than 2.0, and also a faster solidification front velocity of up to 5 m/s. Although the small deformation degree is caused by the larger value of viscosity under super-cooled conditions, the rapi
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d solidification is eventually caused by the super-cooling. The model did not predict a dendritic growth, but it clearly suggested that plasma sprayed particles are not all but may be actually in super-cooled state, and any modeling efforts should include the super-cooling effects.
2.We developed an in-situ measurement system for precise 1:1 correlation between splat morphology and temperature history during impinging on the substrate under plasma spraying conditions with sampling the events successively under controlled atmosphere. In order to reveal the effectiveness of the system, deformation and solidification process of yttria-stabilized zirconia powder sprayed by hybrid plasma was measured successively at a horizontally moving substrate heated up to 700 K by a radiation-plate. The temperature was measured by 700 and 1000 nm emissions from a droplet, and the droplet size was evaluated ex-situ precisely based on the three dimensional morphology of the splat. The system made it possible to sample about 10 single particles successively within 10 second, and correlate precisely the relations between the size, temperature and impacting velocity of each droplet, and deformation-solidification process and morphology of the splats. The most striking finding is that a part of the droplets may be actually in super-cooled condition before impinging, but a drastic difference was not found in deformation process. In addition, as secondary results, we could evaluate the viscosity of YSZ and the thermal resistance between the splat and the substrate.
From the above, it can be seen that considerable progress was made towards the overall goal of this project, and the results obtained so far clearly demonstrate the feasibility of this approach. Less
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