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.

Cold spray is a well-established thermal spray process for metal coatings, but it is unsuitable for depositing ceramics. However, cold spray has been used to spray a thin layer of ceramics to improve surface properties. This paper aims to find the spraying parameters to deposit alumina particles onto an aluminum substrate, investigating the retention phenomenon through computational modelling. We used the finite element analysis in the coupled Eulerian Lagrange formulation to predict the particle-substrate interaction. The Johnson-Cook plasticity model and Mie-Gruneisen equation of state were employed to describe the substrate behaviour. Alumina particles were assumed to be elastic. To assess the retention phenomenon, we varied spraying parameters such as particle speed, substrate temperature, and deposition angle. The findings showed that initial velocity and the substrate temperature facilitate penetration of the ceramic particles into the substrate; thus, the penetration increases the chance of retention into the substrate. The deposition angle affects the jet shape, and specific deposition angles cause erosion. Overall, the findings denote that certain cold spraying parameters may improve the retention of ceramic particles into metal substrates.

This study investigates the effect of substrate conditions on supersonic jet flow pattern, particle acceleration and substrate preheating process in cold spraying. Computational fluid dynamic (CFD) approach is employed in the present work to achieve this objective. The simulated results show that substrate diameter has some effects on gas flow regime and thus particle acceleration. When the substrate diameter is smaller than the nozzle exit diameter, the size of the bow shock formed in front of the substrate is rather small, which contributes to increase the particle impact velocity. With increasing the substrate diameter gradually, the flow regime becomes more and more insensitive to the substrate diameter and the pattern becomes uniform. Moreover, the current numerical work also reveals that substrate preheating process can be significantly influenced by substrate thickness. With the increment in substrate thickness, the preheating effect becomes increasingly undesirable and the substrate surface temperature presents a downward trend.

Cold Spraying is an emerging additive manufacturing method that uses a high-speed collision of micrometre sized powders capable of producing a solid-state bonding. Such a principle has led to the recent development of a coating for various surface functionalization and additive manufacturing applications. This paper is the result of an experimental study on the evolution of the deposit properties (ultimate strength, and porosity) generated by the additive growth during cold spraying. The deposit characterization shows the existence of ultimate strength gradient. For samples taken from the bottom to the top of the deposit, the ultimate strength decreases but there is no significant change in porosity value. The porosity evolutions do not allow to establish a generalized law of variation. The numerical analysis of the additive growth shows that the thermomechanical response of the stacking powder during the additive growth can decrease the bonding capacity, the thermomechanical heating (due to the plastic work) and the gradient of thermal kinetics.

Oxygen present in the High Velocity Air-Fuel (HVAF) process can react with the in-flight metallic particles and cause their oxidation. A grown brittle oxide shell on metallic microsize particles can reduce their deposition efficiency and impair the coating final deposited properties/microstructure. In the current study, the oxide growth of MCrAlY particles, where M stands for Ni and Co, during their flight in the HVAF process has been modeled using the particle tracking scheme. A comprehensive theoretical oxide layer growth background is presented and used to track the particle oxidation process. The oxidation development includes the Mott-Cabrera theory for very thin films, which is function of the particle surrounding temperature and oxygen partial pressure. The very thin film regime, applicable under a limiting thickness, is defined based on the electric field present across the growing oxide layer. As the electric field decreases with oxide thickness, the oxidation rate is determined by thermal diffusion. The obtained results provide a correlation between HVAF system design and surface oxidation phenomena while offering a clear description of different oxidation stages.

The coatings of zinc and its alloys are broadly used to prevent the rusting of substrate surfaces such as steel. Cold gas dynamic spray (CGDS) is an innovative coating technique in which the deposition of solid powder particles depends upon the kinetic energy of the particles rather than thermal energy. Therefore, application of cold spray is to provide superior rust resistance by depositing more materials, formation of passivation layer, and cathodic protection. In this study, numerical investigations on zinc micro and nano size particles in CGDS were carried out. The height of the injector, the expansion ratio and the diameter of the inlet of the de-Laval nozzle was varied systematically by optimizing the stand-off distance using the two-dimensional axisymmetric models of CGDS, to study their effects on the velocity and the distribution of the particles. Prediction of the deposition efficiency was carried out using the various critical and erosion velocity models.

In thermal spray processes, the interaction between the gas jet and the particulate feedstock can affect the coating build-up mechanisms considerably. Especially under high-kinetic and low-pressure conditions, small particles are subjected to rapid deflection and velocity changes close to the substrate. In this work, numerical studies were performed to investigate the particle trajectories in the substrate boundary layers (BL). Typical conditions for suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) were taken as a basis. Particular importance was attached to the consideration of rarefaction and compressibility effects on the drag force. The particle impact conditions in the different thermal spray processes are compared. Possible effects on the resulting coating build-up mechanisms and microstructure formation are discussed.

Dual layer electrode coating for alkaline water electrolysis was prepared by plasma spraying. For improving performance this work aims at reducing the oxide in electrode coating. Regarding the necessity of obtaining high specific area, atmospheric plasma spray was employed under protection of argon which was used as shrouding gas. Composite cathode was established on Ni-coated perforated steel sheet with crushed and gas atomized Nickel-based alloy powders. The dual-layer structure was a composite of 5 layers of NiAl at the bottom and 10 layers of NiAlMo as the top layer. Microstructure and morphology were studied by scanning electron microscope (SEM). Element content was estimated by energy dispersive spectrometer (EDS). Enthalpy probe was introduced for measuring plasma temperature and velocity as well as gas composition. Numerical calculation was carried out with same condition for better understanding the shrouding effect. The results showed moderate protection by using of arranged gas shrouding. Overall, in the dual layer region, oxygen content was decreased by 0.3%, from 3.46% to 3.15%. With gas shrouding coating exhibited similar element contents as coating sprayed by VPS. However, no obvious difference was observed in microstructure and morphology with or without gas shrouding.

Cold spray is continuously expanding for the repair of parts made of aluminum-based alloys. Beyond repair applications, the process is now expected to be exploited efficiently for the additive manufacturing of shaped parts. However, up to now, cold spray is limited to the achievement of rather simple shapes due to a lack of basic knowledge on coating build-up mechanisms to result in dimension-controlled deposition. The objective of this work is to fill that gap through an experimental and modeling study of the coating build-up in cold spray for this specific application. Experimentally, Al-based coatings were deposited for a large range of particle velocity due to the use of low-pressure, medium-pressure and high-pressure cold spray facilities. Particle velocity was monitored as a function of cold spray conditions. Two different types of Al 2024 (Aluminium 2024 Alloy) powders were tested. Coating porosity and microhardness were studied as a function of (both morphological and metallurgical) powder characteristics and spray conditions, primarily in the light of particle velocity. Various correlations could be exhibited. Finite element (FE) simulations of particle impacts were developed, including particle velocity from experimental measurements. These will be used as inputs in an in-house morphological model, the first stages of which could be established successfully.

Plasma spray technology is widely employed by industry to apply coatings on different components to protect them from corrosion, wear and high temperature environments. The gases introduced into the DC plasma torch are heated by the arc and a plasma jet exits the torch. Powders are injected into the plasma jet where they are then accelerated, heated, and melted before impacting the substrate, which is placed at some distance from the outlet of plasma spray torch. Plasma arc exhibits strong voltage fluctuations which correspond to the movement of the anode arc root attachment. Understanding the arc movement within the torch and how it affects the flow and temperature fields of the plasma jet exiting the torch is of great importance. Understanding the flow, temperature and electromagnetic fields within the DC plasma torch is extremely challenging and there is a limited number of investigations in the literature. In order to provide unique sets of surface characteristics, e.g., thermal barriers, wear and corrosion resistance, a high quality coating with appropriate combination of powder and base materials must be produced. To produce a high quality coating, powder particles should be uniformly heated and accelerated, and then deposited onto the substrate. In this paper, an unsteady 3-dimensional model of the arc movement within the plasma torch is reported. The proposed model is employed to solve electric potential and magnetic vector potential equations in addition to continuity, momentum and energy equations. The k-ε turbulence model was used to model the turbulence of the flow field inside a non-transferred DC argon plasma torch. The geometry of the torch was that of SG-100 torch (Praxair). TO study the effect of the arc length on the voltage, first a steady-state model was considered for a range of arc lengths and arc-root radii. The results of this model provided the relation between arc length and arc voltage for a set of arc root radii and given argon flow rate. Then, given voltage fluctuation profile, the unsteady, arc root attachment movement was simulated from the estimation which found from steady models. Results show that the effects of velocity and temperature fluctuations at the outlet of the torch (where the particles are injected) are not negligible and such fluctuations exceed 15% of their average values. These will in turn affect the particle heating history and will negatively impact the microstructure of the coating.

Precursor Plasma Spraying (PPS) using Radio Frequency (RF) induction plasma spray is a new process used to synthesize functional materials. RF plasma spray has the advantages of stability, cleanness, high temperature and high chemical reactivity. In this paper, a two-dimensional numerical model has been developed to investigate the induction electromagnetic (EM) field and the thermo-fluid field in a radio frequency inductively coupled plasma (RF-ICP). In flight particle interaction with the plasma jet will be investigated. The traditional micron-size powder particles, e.g. zirconia (PSZ), are injected with carrier gas such as argon. During their interaction with the RF plasma, the powder particles experience acceleration, heating, melting and evaporation and particle heat transfer is considered coupled with the thermo-fluid flow of the RF plasma. A generalized particle model is developed and applied to the precursor plasma spray process operated in a vacuum chamber. The effects of power input, standoff distance and powder size on the RF plasma and particle in flight characteristics are investigated.

The resulting thermal and mechanical properties of atmospheric plasma sprayed coatings, are strongly dependent on the particle in-flight characteristics, which in turn depend on the spray gun variables. In industrial production the spray gun variables are set to constant values and closed loop controlled. However, calibrations of the variable levels are regularly performed and variations within specified tolerance limits allowed, which cause variations in the particle in-flight characteristics. The objective of this work was to investigate how these calibration variations affect the particle in-flight characteristics and the final coating properties. The investigation was performed using three-dimensional computational fluid dynamics simulations. The process model correspond to the SM-F-100 Connex gun, spraying ZrO 2 for thermal barrier coating applications. Particle in-flight characteristics were calculated using a stochastic discrete particle model. Validation of the model was performed using the optical DPV2000 system, and fair agreement was found. Voltage, arc current, primary, secondary and carrier gas flow rates were systematically varied one factor at a time and their separate effects on the particle in-flight characteristics evaluated. The most important variables influencing the particle characteristics were current and voltage. Final simulations considering extreme cases enabled determination of the particle characteristics limiting conditions due to tolerance variations. Coating microstructure evaluations of two of these extreme cases revealed that the total porosity could vary up to 4% due to tolerance variations.

Numerical simulations of gas/particle flows of cold spray are performed for N 2 and He as a process gas respectively, to investigate the usefulness of the two material-independent combination parameters derived from the equations of particle motion and temperature. The first combination parameter is the particle-diameter multiplied by the material density, which governs the particle velocity. The second one is the squared particle-diameter multiplied by the material density and specific heat, which affects the particle temperature. In the numerical simulation, the materials of the spray particle selected are WC-12Co, Cu and Ti. The diameter of the particle is in the range of 0.1 – 30 µm. The present numerical results show that the maximum impact velocity of particle is obtained when the first combination parameter takes specific value regardless of the material type. Furthermore, it is shown that the particle diameter and its temperature corresponding to the maximum impact velocity can be graphically estimated by using the two combination parameters for any powder-materials normally used for the thermal spray.

Spattering is an unavoidable phenomenon in the selective laser melting (SLM) process, which can cause various printing defects and harmful powder recycling. Since the size of powder spattering is too small at the micron level, it is difficult to investigate the entire dynamic spattering process experimentally. The comprehensive understanding of the intricate dynamics of powder spattering during the SLM process remains incomplete. Therefore, we develop a new multiphase flow model to study the transient dynamic behaviors of the gas phase and powder spattering, which agrees well with the experimental observation result. It is the first time that the whole transient dynamic process of powder motion from starting to move induced by the vapor jet to falling to the substrate wall and stopping completely was observed. Powder spattering motion dynamics induced by metal vapor jet and argon gas flow, as a function of time, laser parameters, and location, are presented. The moving speed, total amount, and dropping distribution on the substrate of powder spattering that varies with laser parameters are quantified.

This paper investigated the role of particle injection on inflight particle behavior and its coupling effect on plasma plume in an external orthogonally injected air plasma spray system as well as effects of primary, secondary and carrier gases on in-flight particle status through both experiments and simulations. Diagnostic sensors such as In-flight Particle Pyrometer (IPP) and Spray Position Trajectory (SPT) have been used to obtain the plume characteristics and ensemble temperature, while DPV-2000 was used to measure the distributions of individual particle status such as temperature, velocity and size at the maximum particle flux point. Three-dimensional simulations have been carried out for the corresponding experimental conditions to examine the effects of in-flight particle heating on the plasma plume and in-flight characteristics at different spray distances. Both experiment and simulation results show that particle temperature and velocity will initially increase with plume angle and then decrease after reaching a maximum value for different combination of process parameters at the same plume angle. Theoretical analysis shows strong dependence of the plume angle on the velocity ratio of vertical component from the carrier gas to the horizontal one from primary and secondary gas at their respective nozzle exits. This study enables a better understanding of influence of plasma forming and stabilizing parameters on the particle in-flight characteristics.

The impact of plasma sprayed sieved Ni-5%Al particles on a titanium substrate was investigated. The particles had a narrow diameter range (-65 + 75 µm), and a speed and temperature just prior to impact of about 100m/s and 2400°C, respectively. These powder particles were sprayed on two sets of polished titanium alloy surfaces. One set was a non-oxidized surface and the other one was a previously oxidized surface at 600°C for two hours. Resulting splats were characterised experimentally by infrared pyrometry and scanning electron microscopy. The effects of the substrate’s oxide layer on the shape of the splat, also on the cooling rate and flattening speed during impact were studied and discussed. This problem is also being investigated numerically. A first step was devoted to the selection of relevant simulation parameters. Test cases to study qualitatively the effect of surface oxidation through parameters such as the contact angle and thermal contact resistance are in progress. They will be compared to the experimental results.

In this work, finite element modeling is used to investigate the influence of segmentation cracks on stress distribution and failure in thermal barrier coatings deposited by atmospheric plasma spraying. The results indicate that the presence of segmentation cracks does not improve thermal insulation, but it may be beneficial in regard to thermal shock resistance, depending on crack density, and residual stress around crack tips, depending on crack length. It may also improve strain tolerance, which is affected by crack density as well as length. A model is proposed to explain the mechanism of failure in thick TBCs exposed to thermal shock. Damage caused by thermal shock can be attributed to the propagation of segmentation cracks and the formation of horizontal cracks at the bond coat-topcoat interface.

Recently, cold spray (CS) technology has attracted extensive interest as an alternative to thermal spray methods to build a coating, which uses high kinetic energy solid particles to impact and adhere to the substrate. To date, numerous numerical studies have been carried out to investigate the deposition processes and associated mechanisms during multiple particle impact in CS. However, in the commonly used numerical techniques, the individual powder particles are often treated separately from one another, thus fail to properly consider the adhesion mechanisms during deposition. In this study, we propose a new numerical approach on base of peridynamics (PD), which incorporates interfacial interactions as a part of constitutive model to capture deformation, bonding and rebound of impacting particles in one unified framework. Two models are proposed to characterize the adhesive contacts: a) a long-range Lenard-Johns type potential that reproduce the mode I fracture energy by suitable calibrations, and b) a force - stretch relation of interface directly derived from the bulk materials mode I fracture simulations. The particle deformation behavior modeled by the peridynamic method compares well with the benchmark finite element method results, which indicates the applicability of the peridynamic model for CS simulation. Furthermore, it is shown that the adhesive contact models can accurately describe interfacial bonding between the powder particles and substrate.

Wire arc is an effective and affordable type of coating process. Detailed understanding of it helps improving design and optimization of currently available guns. In wire arc process, an arc forming between two charged wires generates enough heat to melt the metallic wires. Blow of air at high velocity over this bath of molten metal leads to atomization. The formed droplets are then accelerated towards the substrate where they impact and solidify during the coating process. The current work is a follow up to our earlier numerical study where numerical simulation of wire arc spraying using ANSYS FLUENT revealed flow circulations inside the gun can affect flow pattern and contribute to energy dissipation. In this study, an attempt has been made to validate and measure this matter using experimental procedures and to seek potential ways of improvement.

Very low pressure plasma spraying (LPPS) is the objective of numerous studies for the past years. However, numerical simulations and experiments revealed some difficulties such as the presence of successive expansions and constrictions of the plasma jet that has an influence on the deposition efficiency and on the coating structure. Optimization of the plasma gun is thus required and the use of bell-contoured De Laval nozzle extensions seems particularly promising. In this paper, new developments concerning the use of an in-house bell-contoured nozzle extension are presented and both numerical calculations and experiments were performed.