Macroscopic particle modeling in air plasma spraying

• Published in: Surface & coatings technology 2019-04, Vol.364, p.449-456
• Main Authors: Bobzin, K., Öte, M., Knoch, M.A., Alkhasli, I.
• Format: Article
• Language: English
• Subjects: Air plasma; Aluminum oxide; Biot number; Capacitance; Ceramic particles; Ceramic powders; Computer simulation; Coupling; Diagnostic systems; Free jets; Heat flux; Heating; Mathematical morphology; Melting degree; Melting points; Modelling; Plasma spraying; Resistance; Spatial resolution; Substrates; Surface temperature; Temperature; Temperature distribution; Temperature gradient; Thermal conductivity
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2018.07.056

High plasma temperatures in air plasma spraying enable the processing of ceramic powder particles which have high melting points and low heat conductance. Short dwelling times in the order of milliseconds, related to their high in-flight velocities, result in particles that are not necessarily fully molten upon impact on the substrate surface. Experimental particle temperature measurements by means of particle diagnostic systems deliver merely the surface temperature of the particles while the melting degree of the ceramic particles remains unknown. To investigate the temperature field and the melting degree within the thermally sprayed Al2O3 particles, a simulation method based on macroscopic particle modeling is introduced. This method is based on the coupling of a particle-laden free jet model with an external particle heating model. The advantage of the external model is the spatial resolution of the 3-D geometry, which would allow the calculation of melting degree of not just spherical particles, but also of particles of arbitrary shape and morphology. Furthermore, two strategies for the coupling of the heat flux from the free jet model with the particle heating model are analyzed. Taking the surface temperature of the particles during the coupling into account has resulted in a more precise melting degree calculation, which potentially will become more significant for materials with lower thermal conductivity. Different particle sizes and trajectories are used to compare the results of this method to those obtained based on conventional lump-capacitance assumption. It is evident that the lump-capacitance method noticeably overshoots the melting degree of the particles and can reasonably predict it only after a certain stand-off distance. Macroscopic particle modeling has shown to be an alternative to discrete particle modeling and has a potential to model particles with complex shapes. •Macroscopic model of 3-dimensional particle heating was built.•The model can be used for irregular particle shape and morphology.•Two strategies for coupling of the free jet and particle models were analyzed.•Coupling with surface temperatures is beneficial for large particles.•Lump capacitance is reasonable only after a certain stand-off distance.