Microstructural investigation of plasma-sprayed alumina splats
- CEA/DAM, Bruyeres le Chatel (France)
- Univ. de Limoges (France)
Alumina splat formation on smooth (Ra {approximately} 0.2 {micro}m) substrates preheated to 300 C was studied because in this case very regular disk shaped splats quite adherent to the substrate were obtained. Thermal contact resistance evaluations suggested that the true contact area between the lamella and the substrate was less than the apparent area covered by the splat because of entrapped gas, surface contamination or oxide film. This resulted in good contact area through which the heat flux was released during crystal growth and poor ones for which the heat flux had to flow through the already solidified parts. This means that, emphasized by the low thickness of the splats (of the order of 1 {micro}m), the heat flux directions within the splats may be both perpendicular and/or parallel to the interface. Furthermore, very good contacts with the substrate produced very high cooling rates (up to 10{sup 9} K/s as measured for zirconia at least at the beginning of the cooling) theoretically in favor of homogeneous nucleation whereas bad contacts linked to lower cooling rates (10{sup 6}--10{sup 7} K/s) should induce columnar growth. In this paper atomic force microscopy of the splats enabled the authors to show different types of crystallization fields linked to the true splat contacts with the substrate. Different substrates nature (steel 304L, pyrex and polished {gamma} alumina coatings) and spraying angles have shown the difficulty to dissociate the spreading stage from the solidification one and differences in crystalline structures.
- OSTI ID:
- 377767
- Report Number(s):
- CONF-9509182--; ISBN 0-87170-541-9
- Country of Publication:
- United States
- Language:
- English
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