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In thermal spraying, molten YSZ (Yttria Stabilized Zirconia) drop impact is crucial to the understanding of formation of splats and hence coatings. However, existing studies hardly cast attention to the possible rebounding dynamics when successive molten YSZ drops impinge onto newly solidified splats whose temperature is still high so that freezing may be delayed. This work uses a newly developed numerical model based on the conservative level set method to probe into the rebounding dynamics of YSZ drops in practical thermal spraying conditions. The free-energy-based surface tension model in the phase field method was introduced to avoid the computing of curvature, which is however necessary in the continuum surface tension model. The projection method was invoked to solve the Navier–Stokes equations. The model was validated against an experiment of drop impact with rebounding, showing reasonable agreement. Moreover, the model was applied to both dense and hollow YSZ drop impact, with the details of fluid flow being analyzed. The maximum spread factor was found in agreement with existing scaling laws. [ABSTRACT FROM AUTHOR]

Air entrapment in drop impact onto solid surfaces has riveted the attention of the scientific community since it could drastically influence heat rate and splat porosity in additive manufacturing or thermal spraying processes. Due to the limitations of experimental methods, numerical methods have been gaining popularity in probing into air entrapment dynamics. To that end, this paper developed a paralleled phase field model. The model couples the Navier–Stokes equations with the Cahn–Hilliard equation to capture the liquid–gas interface, and adopts the liquid fraction, defined all over the computational domain, to distinguish between solid and liquid. The model is discretized using a finite difference method on a half-staggered grid. The model was first compared with experimental data, achieving reasonable agreement. Afterward, it was applied to a couple of cases. The air entrapment process was captured entirely and the interaction between solidification and air entrapment was also given. It shows that under the parameters studied here air entrapment may occur twice, one in the impacting center in the form of a bubble and the other in the form of a bubble ring. Besides, the effect of phase field mobility on air entrapment was examined as well. [ABSTRACT FROM AUTHOR]

Modeling drop impact in 3D in thermal spraying conditions poses a great challenge, since it requires first of all an accurate interface tracking model and an efficient numerical scheme to solve the governing equations. This paper thus put forward an integrated model, in the framework of the conservative level set equation, to simulate drop impact in real thermal spraying conditions. The model couples the conservative level set equation with the Navier–Stokes equation and is discretized using the finite difference method on a half-staggered grid. The model was validated against experimental data, showing reasonable agreement. Then it was applied to real impact in thermal spraying conditions. Nickel drop impact with solidification was simulated first. Subsequently, effects of contact angle, thermal contact resistance, and substrate materials, were examined. It shows that lower contact angle does not lead to larger contact area, since solidification commences faster around the contact line, thereby pinning its movement, and that thermal contact resistance exerts a huge impact on final splat shape. The smaller the contact resistance, the more violent the splash. [ABSTRACT FROM AUTHOR]

In plasma spraying, copious heterogeneous nucleation starts when a molten ceramic droplet spreads on a cold surface under rapid cooling. Some nuclei will survive and grow, eventually forming a splat of grains of distinct crystalline orientations. This paper aims to predict the dynamic process of yttria-stabilized zirconia (YSZ) droplet impact with solidification microstructure formation under various plasma spray conditions. A diffuse interface model was developed to track the evolving liquid–gas and solid–liquid interfaces. Continuously dense YSZ droplet impacts with different impacting angles were conducted, along with a hollow droplet impact. Results reveal that competitive growth among crystals is limited in the planar solidification, and that columnar structure dominates all the tests performed owing to a large thermodynamic driving force, and that given the rapid spreading of YSZ droplets along a solid surface, solidification may be safely assumed to take place mostly after spreading. Besides, typical crystal growth velocities are around 1 m/s, and local equilibrium can be assumed in the bulk. [ABSTRACT FROM AUTHOR]

• The solution of the Cahn-Hilliard equation to track moving boundaries. • A heat source term added in energy equation to trace solidification profile. • Thermal contact resistance studied to find its effect on maximum spread factor. • A supersonic impact of ceramic particles examined. A modified phase-field three-dimensional model has been developed to simulate the spreading of an impacting droplet undergoing solidification. The model is based on the numerical solution of the Cahn-Hilliard equation coupled with the Navier-Stokes equations for fluid flow and the energy balance equation for heat transfer. The solidification profile is tracked by treating the latent heat as a source term in the energy equation, which is modified to work with the Cahn-Hilliard equation. To verify the model, five cases were tested and matched well with experiments. Also, the effect of thermal contact resistance on the maximum spread factor of a solidifying droplet is discussed. One case was taken from practical thermal spraying conditions where a solidifying ceramic droplet spreads on a cold surface at a supersonic impact velocity; computed results are consistent with available measurements. [ABSTRACT FROM AUTHOR]

• Sequential droplet impact with grain growth and impingement is modelled. • Typical velocities of crystal growth are within 2 m/s. • Solidification time of a single splat on a stainless steel substrate is about 0.5 μs. • The undercooling in crystal growth ranges approximately from 150 K to 500 K. A coupled CFD and diffuse interface model was developed to predict the dynamic process of solidification microstructure formation of sequential yttria-stabilized zirconia (YSZ) droplet impact under supersonic plasma spraying. The numerical model relies on the explicit finite difference solution of the Navier-Stokes and energy balance equations, coupled with the Cahn-Hilliard equation, to track liquid-gas interface, and of a phase field model for solidification microstructure formation involving polycrystalline growth to trace solid-liquid interface. Extensive simulations, differing in the status of the first droplet and the impacting parameters of the second one, were carried out. The results reveal that solidification microstructure still follows a columnar pattern if the second droplet lands on the first that is just starting spreading, and that the growth direction around the contact region in the second splat will be obviously skewed by fluid flow if the first has been solidified, owing to earlier and easier nucleation on the top surface of the first splat, thus confirming the epitaxial growth across the splat-splat interface. Computed results are also compared with thermal spray experiments, with gratifying agreement obtained. [ABSTRACT FROM AUTHOR]

• The solution of the Cahn-Hilliard equation to track moving liquid-gas boundaries. • A phase field model for polycrystalline growth used to track solid-liquid interface and grain boundaries. • Supersonic YSZ droplet impact considered. • Convection effect on solidification microstructure investigated. A comprehensive numerical model has been developed, which is capable of representing the impact and spreading of a high-velocity yttria-stabilized zirconia (YSZ) molten droplet with microstructure formation in plasma thermal spraying processes. The numerical model entails the explicit finite difference solution of the Navier-Stokes equations and the energy balance equations, coupled with the Cahn-Hilliard equation to track the liquid-gas two phase interface and with a phase field model for solidification microstructure formation involving the polycrystalline growth. Numerical model procedures are given. The model, after being checked for mesh-independence, was applied to study the spreading and solidification of a high-temperature high-velocity YSZ droplet impinging upon a preheated substrate surface under the conditions typical of supersonic plasma thermal spraying processes. Extensive numerical simulations were carried out and results reveal that the solidification microstructure formed in the spreading droplet consists primarily of columnar grains. Computed results are also compared with thermal spray experiments, and gratifying agreement is obtained. [ABSTRACT FROM AUTHOR]