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Journal of thermal spray technology : JTST (2021). doi:10.1007/s11666-021-01159-1, Published by Springer, Boston, Mass.

Existing modeling techniques can determine the heat transfer within idealized spherical particles with homogenous morphology. Agglomerated particles are not homogenous and consist of multiple smaller particles which are packed together. The reduced contact area between the individual smaller particles results in a drastic reduction in the effective thermal conductivity of the agglomerate. Conversely, it can enhance the heat transfer due to the increased particle surface area and gas penetration into the agglomerate. Moreover, the momentum transfer from the plasma to the agglomerate differs from that of a homogenous spherical particle, which can significantly affect the heating dynamics of the agglomerate. All of the mentioned phenomena have been taken into account in a novel particle modeling approach by resolving the 3-D geometry of the agglomerates and the flow around it. The presented model is coupled with the particle-laden free jet model. Differences in kinematics and heating dynamics of the agglomerates have been analyzed with regard to their packing densities. The presented model was compared to a simplified approach where the agglomerates were represented by spherical particles with their mass corresponding to the agglomerates with different packing distances. The comparison proved the necessity of 3-D resolution of the particle morphology.

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 Al 2 O 3 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.

Plasma spraying is a versatile coating process, which allows processing of nearly any feedstock material. The feedstock material is injected in powder form into the plasma jet, where it is heated and accelerated towards the substrate. The impacting particles form a coating on the substrate surface by deforming and rapidly solidifying. The resulting coating microstructure strongly depends on the dynamics of the particle deformation and solidification. The simulation of the particle impact on the substrate surface allows a better understanding of these phenomena. In this work, a modified momentum source based solidification model, which is suitable for the simulation of multiple particle impacts, is developed. It represents a more robust alternative to the existing momentum source formulation and the viscosity based solidification model commonly used in the literature. The developed solidification model is implemented in a three-dimensional coating build-up simulation to acquire the coating microstructure.

A low thermal conductivity in feedstock material and high plasma temperatures generally lead to inhomogeneous heating of particles in plasma spraying. Existing modeling methods can determine heat transfer within idealized spherical particles with homogenous morphology, but in many cases, particles have an agglomerated morphology, consisting of multiple smaller particles that are packed together. The reduced contact area between the individual smaller particles results in a drastic reduction of the effective thermal conductivity of the agglomerate. On the other hand, it enhances heat transfer from the plasma gas due to the increased particle surface area and penetration of the hot plasma into the agglomerate. Moreover, the momentum transfer from the plasma to the agglomerate differs from that of a homogenous spherical particle, which can significantly affect heating dynamics. This paper presents a novel particle modeling approach that accounts for all such phenomena. Differences in kinematics and heating dynamics of the agglomerates are analyzed with regard to their packing densities.

IOP conference series / Materials science and engineering 480(1), 1-11 (2019). doi:10.1088/1757-899X/480/1/012002, Published by Institute of Physics, London [u.a.]

Air plasma spraying is a variation of thermal spraying that is used, among others, for the production of thermal barrier and wear resistant coatings. High plasma temperatures enable the processing of ceramic powder particles which have a high melting point and cannot be processed otherwise. Due to their low heat conductance, the ceramic particles are not necessarily fully melted during their flight in the free jet and prior to the impact on the substrate surface. Experimental particle temperature measurements by means of particle diagnostics systems deliver merely the surface temperature of the particles while the melting degree of the ceramic particles remains unknown. Therefore, the temperature field within spherical Al2O3 particles is numerically investigated for a commonly used particle size distribution by considering different particle sizes. The model includes a two-way coupled particle-laden free jet model and takes the latent heat of melting and evaporation into account. The effect of the particles size as well as the stand-off distance on the melting degrees of the particles in the given powder size distribution is determined.

21st Chemnitz Seminar on Materials Engineering : 6-7 March 2019, Chemnitz, Germany 21. Werkstofftechnisches Kolloquium 21st Chemnitz Seminar on Materials Engineering = 21. Werkstofftechnisches Kolloquium, WTK 2019, Chemnitz, Germany, 6 Mar 2019 - 7 Mar 2019; [Bristol] : IOP Publishing, IOP conference series / Materials science and engineering 480, 1 012003 pp. (2019). doi:10.1088/1757-899X/480/1/012003, Published by IOP Publishing, [Bristol]

IOP conference series / Materials science and engineering 181, 012013(2017). doi:10.1088/1757-899X/181/1/012013, Published by Institute of Physics, London [u.a.]

Plasma spraying is a coating process which is widely used for the application of thermal barrier coatings. High plasma jet temperatures allow the processing of ceramic material particles which characteristically exhibit low thermal conductivities. This, in turn, produces high temperature gradients inside the particles and vaporization on the particles' surface during their dwell time in the plasma jet. Thus, a single particle in the plasma-jet can exhibit 3 states of matter simultaneously: solid in the core, molten exterior and gaseous on the surface. The temperature distribution inside the particles is the foremost factor which influences the particles' behavior during their impact on the substrate surface. Experimental investigations can provide only the surface temperature of the particles, which is not a good indicator of the melting degree for ceramic particles due to the high temperature gradients. This study focuses on the determination of the temperature distributions inside the particles, during their flight in the plasma and the free jet, with the help of numerical simulations. For this purpose, a numerical model of the plasma spraying process that describes the plasma and the free jet loaded with sprayed particles is presented. The model includes three sub-models; a plasma torch sub-model, a particle-laden free jet sub-model and a powder particles sub-model. The first sub-model calculates the temperature and velocity fields of the plasma inside the plasma torch, the second sub-model the kinetics of temperature and velocity fields of the turbulent free jet generated by the plasma torch. This information is used in the third sub-model to calculate the kinetics of temperature distribution within the particles, their melting degree and mass losses due to evaporation. The third sub-model also calculates heat and mechanical impulse loses due to the particle-plasma interaction, which in turn is coupled with the free jet sub-model.