Particle–plasma interactions: particle melting state and its impact on the phase composition and deposition efficiency in atmospheric plasma-sprayed alumina coatings

Plasma–particle interactions strongly influence the formation of atmospheric plasma-sprayed (APS) coatings . Especially for alumina, which usually undergoes a complex phase transformation of α-Al₂O₃ to metastable phases such as γ-, η- or δ-Al₂O₃ during thermal spraying, the heating and cooling behavior of the particles is of great importance.

This study therefore investigates the effect of plasma fluctuation and particle morphology on the melting behavior of aluminia and alumina-based powder particles. In particular, the impact of the degree of particle melting on the known phase transformation and the deposition efficiency is being investigated. In addition to the phase analysis of the coatings, powders collected during the spraying process were examined to evaluate their changes in the plasma. The particle morphology of the powders collected provides clear indications of their degree of melting. Moreover, both the plasma fluctuations and important particle parameters, such as particle temperature and velocity, are recorded.

The results show that the degree of particle melting in strongly linked to the plasma fluctuations, which can be significantly influenced by adjusting the process parameters. While higher α-Al₂O₃ contents in the collected powder cannot be transferred into the coating if they are attributed to non-melted particles, coatings with higher α-Al₂O₃ contents can be achieved by partially melted particles or the use of Al₂O₃-based solid solutions without negatively affecting the deposition efficiency.