Estimation of Molten Content of the Spray Stream from Analysis of Experimental Particle Diagnostics

Particle melting is one of the key issues in air plasma spray processing of high temperature ceramics such as Yttria Stabilized Zirconia (YSZ). The significance of assessing, monitoring and controlling the molten content in the spray stream on achieving reproducible coating characteristics and properties is known. This study aims to estimate the molten content of the spray stream (as an ensemble) from experimental measurement of in-flight (individual) particle characteristics.

In a previous study by Streibl et al the presence of melting signature in the particle temperature distribution was observed, which has been confirmed by simulation and though independent experimental observation by Mauer et al. Based on this observation, the particle temperature distribution could be delineated into the different achievable particle states in-flight (unmolten, partially molten and completely molten) to a first approximation. This in-turn would make it possible to estimate the molten content in the spray stream. The so estimated percentage molten content (referred in this study as Spray Stream Melting Index - SSMI) has been observed to correlate well with the experimentally measured (relative) deposition efficiency for a wide range of process conditions and feedstock characteristics. The implications of estimating SSMI for other material systems and processes are also discussed.