Abstract

The partially stabilized zirconia powders used to plasma spray thermal barrier coatings typically exhibit broad particle-size distributions. There are conflicting reports in the literature about the extent of injection-induced particle-sizing effects in air plasma-sprayed materials. If significant spatial separation of finer and coarser particles in the jet occurs, then one would expect it to play an important role in determining the microstructure and properties of deposits made from powders containing a wide range of particle sizes. This paper presents the results of a study in which a commercially available zirconia powder was fractionated into fine, medium, and coarse cuts and sprayed at the same torch conditions used for the ensemble powder. Diagnostic measurements of particle surface temperature, velocity, and number-density distributions in the plume for each size-cut and for the ensemble powder are reported. Deposits produced by traversing the torch back and forth to produce a raised bead were examined metallographically to study their shape and location with respect to the torch centerline and to look at their internal microstructure. The results show that, for the torch conditions used in this study, the fine, medium, and coarse size-cuts all followed the same mean trajectory. No measurable particle segregation effects were observed. Considerable differences in coating microstructure were observed. These differences can be explained by the different particle properties measured in the plume.

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