Finely structured ceramic coatings can be obtained by solution precursor plasma spraying. The final structure of the coating highly depends on the droplet size and velocity distribution at the injection, the evolution of the spray in the jet, and droplet breakup and collision within the spray. This paper describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in the process. O’Rourke’s droplet collision model is used to take into account of the influence of droplet collision. The influence of droplet breakup is also considered by implementing TAB and Wave droplet breakup models into the plasma jet model. The effects of droplet collisions and breakup on the droplet size, velocity, and temperature distribution of the solution spray are investigated. The results indicate that droplet breakup and collision play an important role in determining the final particle size and velocity distributions on the substrate.

Suspension plasma spraying (SPS) is increasingly studied to produce finely structured coatings with dense and columnar microstructures for promising thermal barrier coatings especially in aerospace application. However, this process involves many parameters and complex phenomena with large spans of time and space scales in many physical mechanisms, like droplet break-up, liquid droplet evaporation, and various physical phenomena occurring within the suspension droplet, making it difficult to master. Especially, understanding the interactions of liquid drop submitted to plasma with the submicronic suspended particles is essential for material process optimization and control. For SPS understanding, a meaningful modelling of suspension treatment requires a prior analysis of these physical mechanisms and their characteristic times. This study details the different phenomena, their significance and characteristic timescales as well as the selection of the main governing forces acting between the different continuous and discrete phases (plasma, liquid, submicronic particles). We explore associated mechanisms: droplet breakup, carrier liquid evaporation, convective mixing and submicronic particle diffusion within the droplets. These mechanisms involve mass and heat transfer, that should condition particle agglomeration morphology before melting.

Solution droplets injected into a plasma jet experience a sequence of thermal, physical, and chemical processes. These include droplet breakup and collisions, solvent vaporization, solute precipitation and pyrolysis, formation of the product particles, sintering, and perhaps melting. Depending on plasma conditions, solution concentration, and the properties of the solvent and solute, different particle morphologies are produced. In this paper, a heat and mass transfer model for vaporizing solution droplets was used to investigate the influence of solvent type, initial salt content, and concentration. Temperature and composition dependent thermo-physical properties were used. Temperature and concentration distributions and variations of precursor droplets (cerous nitrate and zirconia acetate in water and ethanol) were predicted.

The aim of this work is to obtain a better understanding of high-velocity suspension flame spraying though detailed modeling and analysis. A numerical model was developed and is used to analyze the effects of process parameters including droplet size, injection velocity, secondary droplet breakup, solution evaporation and combustion, mass flow rate, flame temperature, and the location of the injection point. The results show that initial injection parameters play an important role in controlling the process and should be specified to minimize cooling effects and promote droplet evaporation inside the combustion chamber. It was also found that the ideal location for the injection point is inside the chamber, which in practice, can be achieved by slightly modifying gun geometry.

Solution precursor plasma spraying is used to produce finely structured ceramic coatings with nano- and submicron features. The solution is injected into a plasma jet as a liquid stream or gas atomized droplets that break up into a finer mist. The deposition process and coating properties are extremely sensitive to torch operating conditions, injection modes, and substrate temperatures. This study employs numerical methods to investigate the size distribution of injected droplets for liquid stream and gas blast injection. Droplet or particle size, temperature, and substrate position is predicted for different injection modes.

In this work, numerical models are developed and used to simulate magneto-hydrodynamic fields inside a dc plasma torch during suspension plasma spraying and their influence on arc attachment. A Reynolds stress model is used to simulate turbulent plasma flow and a discrete phase model simulates the effects of arc fluctuation on suspension droplets in the plasma jet. Submicron yttria-stabilized zirconia particles, suspended in ethanol, are modeled as multicomponent droplets and the KHRT model is used to simulate their breakup. The results show that particles are significantly affected by plasma arc fluctuations and that fine particles near the centerline of the torch are hotter and experience better penetration into the plasma jet.

The use of liquid precursors in plasma spraying makes it possible to produce coatings with more refined microstructures than in conventional plasma spraying. Depending on the injection device, the liquid feedstock is injected into the plasma jet in the form of liquid jet or droplets. The instabilities on the liquid-gas interface cause the mechanical break-up of liquids into drops that are subjected to further break-up until the droplets reach a stable state or evaporate. The process break-up may strongly influence the size, trajectories and, therefore, treatment of the droplets in the plasma medium. This study deals with the experimental observation of liquid break-up under plasma spray conditions when using a conventional DC plasma torch with radial injection by means of a pneumatic injection system that can deliver either liquid stream or blobs.

Various models exist of the 2D impact of a molten thermal spray particle onto a flat solid surface. Such models, however, cannot be used to examine 3D effects such as the asymmetric splashing and breakup which are common under thermal spray conditions. The focus of the present work is on such effects. A 3D model of droplet impact has been developed which predicts splashing and the subsequent formation of small satellite droplets. The model is a 3D version of RIPPLE (LA12007- MS), an Eulerian fixed-grid finite volume code utilizing a volume tracking algorithm to track the droplet free surface. Simulations are presented of the impact and splashing of a molten tin droplet, and the results compared with photographs. A simple model, based on Rayleigh-Taylor instability theory, yields an estimate of the number of satellite droplets which form during impact. Finally, a simulation of droplet impact under thermal spray conditions demonstrates breakup, although in the form of a corona which separates from the bulk of the fluid.

In this study, a 3D two-way coupled Eulerian-Lagrangian approach is used to model the plasma jet and droplet-particle trajectory, velocity, and temperature achievable by suspension plasma spraying. A Reynolds stress model is used to account for turbulence and the effect of the substrate on the flow field and a Kelvin-Helmholtz Rayleigh-Taylor breakup model is used to predict the secondary breakup of the suspension. The focus of this work is on particle behavior near the substrate. Flat substrates placed at stand-off distances ranging from 40 to 60 mm are modeled to provide detailed information on particle impact behavior.

Study of the injection of liquid sprays into thermal plasmas has a number of important applications. Examples include Thermal Plasma Chemical Vapor Deposition (TPCVD), hazardous waste destruction and, ICP atomic emission or mass spectrometry in analytical chemistry. The spray is comprised of thousands of single liquid droplets. To calculate the mass, momentum and energy exchange between the gas and the spray, one must account for a distribution of droplet sizes, velocities, and temperatures. For some cases, droplet collision and breakup also are important. This paper developed a model for the transport and evaporation of liquid droplets sprayed into an radio frequency inductively coupled plasma (rf-ICP). The model considers the evolution of spray distribution function as the liquid spray travels through the discharge. Coupling calculations between plasma gas and water spray is performed for a typical rf-ICP torch. This model is capable of predicting the thermal and dynamic behavior of the liquid spray and its effects on the plasma gas for different rf-ICP operating conditions.

There is widespread industrial interest in wire arc spraying as it has proven to be a relatively inexpensive alternative providing high quality coatings in many applications. Modeling and experimental studies have been conducted with different nozzle geometries to study the influence of fluid dynamics on the behavior of the particles which form the coating. Schlieren optical diagnostics and image analysis are used to study flow patterns obtained with different nozzle geometries. Supplementary measurements are made with static and total pressure probes. A particle breakup model is used to predict the trajectory and breakup of molten particles in the jet. This investigation indicates that nozzle designs that minimize or eliminate shock formation before the atomizing gas reaches the wire tips result in lower coating porosity and a more uniform particle size distribution.

This study compares two methods for modeling the breakup of droplets during suspension plasma spraying. One is based on Taylor analogy breakup, the other on Kelvin-Helmholtz Rayleigh Taylor breakup. A three-dimensional model with two-way coupling is used to simulate flow within the plasma plume and interactions between suspension droplets, and a Reynolds stress model is used to simulate gas field turbulence. After breakup and vaporization, the solid suspended particles are tracked through the domain to determine the characteristics of coating particles. The numerical results are validated against experiments using high-speed imaging.

Solution precursor plasma spraying has been used to deposit ceramic coatings with submicron/nanocrystalline structures. Previous studies revealed that the deposition mechanism in the solution precursor plasma spraying differs from that in the conventional plasma spraying. To increase the understanding of the deposition mechanism in the solution precursor plasma spraying, a numerical model is used to predict the particle conditions on the substrate. Five types of particle conditions, melted particles; small sintered particles; dry agglomerates; wet agglomerates; and wet droplet are assumed based on the computed temperature distribution of the particles. An analysis of the deposition mechanism in the solution precursor plasma spraying is performed. Experiment results s are also collected to verify the numerical prediction and the analysis of the deposition mechanisms.

Obtaining a uniform coating on curved mechanical parts such as gas turbine blades is one of the industrial challenges in suspension plasma spraying. Through a three dimensional numerical analysis, this study is aimed at providing a better understanding of the effect of substrate curvature on in-flight particle temperature, velocity and trajectory. The high temperature and high velocity plasma flow is simulated inside the plasma torch using a uniform volumetric heat source in the energy equation. In addition, yttria stabilized zirconia (YSZ) suspension is molded as a multicomponent droplet while catastrophic breakup regime is considered for simulating the secondary break-up when the suspension interacts with the plasma flow. A two-way coupled Eulerian-Lagrangian approach along with a stochastic discrete model was used to track the particle trajectory. Particle size distribution in the vicinity of the substrate at different stand-off distances has been investigated. The results show that sub-micron particles may obtain higher velocity and temperature compared to the larger particles. However, due to the small Stokes number associated with sub-micron particles, they are more sensitive to the change of the gas flow streamlines in the vicinity of a curved substrate

Within the High Velocity Oxygen Fuel Process (HVOF-Process) various fuels can be used to provide the needed thermal and kinetic energy such as ethene, propane, methane or kerosene. Modelling the combustion in a HVOF-System poses a challenge concerning chemical kinetics of the kerosene reaction process. In this work a reduced reaction mechanism and a model describing chemical reactions as well as governing fluid dynamics are presented to simulate kerosene driven HVOF-Process. The kerosene combustion process within a HVOF-System usually takes place above temperatures of 2000 K, where some species dissociate. Therefore, accruing species have to be included in the reaction mechanism. The combustion process is described with a reduced reaction mechanism. The reaction rate is described by a finite rate model in form of Arrhenius. The gas flow is considered as a first phase and the kerosene droplets injected into the combustion chamber become a second phase. Afterwards simulation results are presented and discussed.

An initial stage of the current work including a preliminary comparative numerical analysis of molten polymer and metal droplets upon impact on a cold steel substrate was presented. A commercially available Volume-of-Fluid [VoF] code was used to model particle deformation and cooling of molten nylon-11 and zinc droplets on impact with a steel substrate. Comparison between polymer and metal splatting was chosen in order to better understand how large fundamental differences between the materials affected their spreading behavior under similar thermal spray conditions. It was found that the inertia is more strongly balanced by the viscous flow resistance in molten polymers while high surface tension of molten metals may lead to particle breakup onto rivulets and satellite during later stage of particle deformation. Spreading ratios of nylon-11 and zinc droplets were 0.53 and 0.34, respectively, owing to the zinc droplets being almost twice the size of the nylon. Zinc splats less than 5 ìm in thickness spread fully and solidified in less than 1.5 µs. Over the same time interval, 17 µm thick nylon-11 splats were also fully developed, however, only a thin boundary layer [<2 µm] was solidified owing to a significantly lower thermal conductivity.

The significance of metal atomization in the wire arc spray process is directly related to the final coating quality produced. Since the early observations of the melting behavior of the wire tips by Steffens, relatively little has been done to further the understanding of the mechanisms involved. The primary atomization of the molten wire tips show existence of sheets and extrusions on both electrodes which are strongly dependent on the system operating parameters. High-speed imaging has been used in this study, for classification of sheet, membrane and extrusion formations as a function of atomizing gas pressure, voltage and current settings. The breakup of metal structures formed on the electrodes is further classified in a manner consistent with established classifications for the break-up of other liquids, e.g. water or fuel. Quantitative descriptions of metal sheet lengths and breakup times are presented. The improved understanding of the metal breakup mechanisms in the wire arc spray process may provide a basis for (a) modification of existing computational codes for prediction of particle sizes and trajectories, and (b) for modifications to torch designs for providing more uniform particle fluxes.

Due to the low running costs, high spray rates and efficiency wire arc spraying has become one of the most important thermal spray technology, especially as a tool for coating large areas with high deposition rates. The main applications are corrosion and wear protection of large structures, such as coating with zinc for anodic corrosion protection or with high-alloyed chromium steel for longer surface service time. Disadvantages of the arc spraying process are on the one hand that only electrically conductive materials can be processed and on the other hand the higher oxygen content within the coating compared to other thermal spray processes such as HVOF or APS. Innovative and flexible power supply systems were developed for arc welding technologies in the last decade. Today power supply devices with new features for arc wire spraying technologies are present at the market, too. Particle flow conditions as well as coating properties such as morphology and oxide content can be optimized by changing the current generator characteristic and modulating the energy input for example by pulsing the spray voltage up to 500 Hz. As a consequence of the power modulation the oxidation of the particles can be reduced by a lower heat input based on lower average spray voltage. Investigations in order to influence the droplet behaviour and the coating microstructure using dynamic current generators were performed for zinc, aluminium and copper. The effect of different modulation frequencies, pulse lengths and pulse voltages are presented.

In this study, a downstream injection cold spray nozzle is modeled numerically under various loadings. Instead of micron-sized particles, liquid feedstock as a carrier of nanoparticle suspension is fed into the nozzle through a port located 6 mm downstream of the nozzle throat at the diverging section. Water is used as the liquid carrier with a droplet size distribution of 5-100 µm and liquid-to-gas ratio ranging from 5 to 15%. The radial injection of droplets is simulated by Lagrangian particle tracking which includes the effects of heating and evaporation. The effect of the feedstock on downstream flow is analyzed and the optimum solid-to-liquid fraction in the suspension is determined.

Plasma spraying using liquid precursors makes it possible to produce finely-structured coatings with a broad range of microstructures and properties. Nonetheless, issues with coating reproducibility and control of deposition efficiency continue to be a concern. With conventional dc plasma torches that inject liquid feedstock transversely into the plasma stream, coating quality depends on transient interactions between the liquid and plasma jet. Numerical models may assist in understanding these interactions provided they are able to predict droplet fragmentation, which determines the trajectories of droplets and their behavior in the plasma flow. Although various models for droplet fragmentation have been proposed in the literature, they include parameters and constants that need to be validated for plasma spraying conditions. This study simulates liquid material injection and break-up in the plasma jet using an enhanced Taylor analogy break-up (TAB) model. Model constants are adapted to plasma spray conditions by observation of liquid behavior in the plasma flow, which is accomplished by means of a shadowgraph system using pulsed backlight illumination.