Abstract

In the spraying of functionally graded coatings, the particle ensemble delivered to the substrate varies from a relatively heavy, low-melting-point metallic particle to a significantly lighter, higher-melting-point ceramic particle. The desire is to deliver to the substrate a particle ensemble which has suitable velocity and temperature for the predictable and consistent formation of coatings with mixed particle types. The key to success is a thorough understanding of the relationship between spray gun parameters and the resulting particle condition. The gun parameters examined are powder loading (injection rate), powder mixtures, and secondary plasma gas (H2). The spray characteristics measured were particle velocity, temperature, and spray pattern. The particle temperature and velocity are both significantly influenced by the flow rate of the secondary gas (gun power). The powder feed rate was found to have a small but measurable effect on both the spray pattern and the ensemble average particle temperature. It was observed that a "tight" hot particle spray pattern, unfortunately, does not necessarily minimize the number of cold unmelted particles.

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