The first part of this overview on plasma spraying covers the general behavior of plasma jets generated by d.c. and r.f. plasma torches, including the fluid dynamics of such jets. In the second part, interactions of injected powders with the plasma are considered with emphasis on those processes which dominate heat and momentum transfer from the plasma to the powder particles. Both experimental studies as well as modeling efforts are included in this overview.

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 paper describes an integrated mathematical model of the atmospheric pressure plasma spray process and presents a comparison between computed results and experimental measurements. The model describes fluid flow and heat transfer in the plasma plume; heating, melting and vaporization of injected powder particles; deposition and freezing of the molten particles; and the transient thermal history of the substrate. The plasma plume calculations are based on the solution of the two-dimensional turbulent equations of motion. A dynamic and thermal balance permits the calculation of the particle temperatures and velocities. The thermal history of the growing coating is calculated under typical cyclic spraying conditions. The computed results are compared against existing experimental data obtained from commercial plasma spraying torches.

Recent experimental investigations of reactive spray deposition of aluminum alloys have indicated that oxides could not be detected for atomization gas oxygen contents lower than 10%. In order to elucidate this behavior, an analysis of the oxidation kinetics during reactive spray deposition based on the Mott-Cabrera theory of oxidation is proposed herein. A linear growth law is obtained that indicates that the oxide growth rate decreases with decreasing temperature or oxygen pressure. Furthermore, the oxide growth rate is found to decrease faster at low oxygen pressure with decreasing temperature as well as at low temperature with decreasing oxygen pressure. Calculations of the width of oxide stringers as a function of oxygen content and superheat temperature based on this analysis are in good agreement with the experimental observations.

The high-velocity air-fuel process (HVAF) is an emerging technology used in the thermal spray industry. The Praxair HVAF process combines air and a liquid fuel (e.g., kerosene, diesel) to generate an energy source with extremely high gas velocities. Analytical studies were conducted to investigate gas and particle dynamics in the Praxair HVAF process for coating with WC-l2Co and stainless steel powders. The mass, momentum, and energy conservation equations were first solved, using the TORCH computer program. Typical output from the model includes temperature and velocity profiles as a function of radial and axial position. The PROCESS gas/particle computer program was then used to calculate from these temperature and velocity profiles the dynamics of particles injected into the gas plume. The primary result of the gas/particle code is a description of the injected particle temperature and velocity as a function of position in the plume. A thorough understanding of the process was obtained using this modeling technique. The results of the modeling were confirmed with process diagnostics. Particle temperature measurements for the WC-Co powder system were obtained with a two-color pyrometer; particle velocity measurements were obtained using particle imaging velocimetry. The coatings produced in the study exhibit superior quality, with high-density, high-hardness, low-oxide content, and high-bond strength.

This paper is dedicated to the comparison of experimental and numerical results obtained on iron particles characteristics during plasma spraying. Each part of the spray process (particle injection, behavior of the particle in the plasma jet and coating formation) may be studied using appropriate diagnostic techniques. In the present work, on-line measurements of in-flight particle characteristics were performed using an optical diagnostic technique insuring a real time control of temperature, velocity and diameter of particles during the spray process. Besides, numerical results were obtained considering momentum and thermal exchanges between the plasma jet and particles using an Eulerian-Eulerian model. The evolution of velocity and temperature with the particle size given by in-situ measurements were then correlated to those obtained with the numerical code.

The plasma spray deposition of a zirconia thermal barrier coating (TBC) on a gas turbine component has been examined using analytical and experimental techniques. The coating thickness was simulated by the use of commercial off-line programming software. The impinging jet was modelled by means of a finite difference elliptic code using a simplified turbulence model. Powder particle velocity, temperature history and trajectory were calculated using a stochastic discrete particle model. The heat transfer and fluid flow model were then used to calculate transient coating and substrate temperatures using the finite element method. The predicted thickness, temperature and velocity of the particles and the coating temperatures were compared with these measurements and good correlations were obtained. The coating microstructure was evaluated by optical and scanning microscopy techniques. Special attention was paid to the crack structures within the top coating. Finally, the correlation between the modelled parameters and the deposit microstructure was studied.

During the formation of coating systems we find strong temperature gradients within the coating-substrate system resulting in thermally induced stresses in the coating after the spraying process. The purpose of this investigation is to develop numerical tools which provide insight into the relationship between coating parameters and the thermomechanical response of the produced composite. Transient finite element modelling is applied to calculate temperature and stress histories in thermally sprayed coatings due to the deposition process. Two different models with different levels of complexity have been developed to investigate the spraying process on a macroscopic and on a microscopic scale. Experimental investigations of the temperature history during spraying in the substrate have been carried out to determine boundary conditions for the mathematical model and to enable a verification of the obtained results.

Plasma transferred arc (PTA) is now currently used for reclamation of worn materials or to provide wear or corrosion resistant coatings welded to the base material. However, the powder injection in the molten pool created at the coated part surface is a critical parameter. In order to avoid coating reproducibility problems induced by the powder feed rate, the way to coat substrate surface with powder before the PTA treatment has been studied. As the powder cannot simply be deposited onto the substrate because of the plasma flow which would blow it off before melting it, tape casting process was used to obtain an adherent powder layer on the material surface. In this paper, tape casting of NiCu particles is described and the different organic additives used to obtain a homogeneous nickel copper film on cast iron and AG3 aluminum alloys are presented. The first results of the treatment of these films by PTA reclamation are then shown.

The present paper describes the synthesis of nanocrystalline 316-stainless steel coatings by high velocity oxy-fuel (HVOF) thermal spraying. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 100 nm. The powders were introduced into the HVOF spray to successfully produce nanocrystalline coatings. X-ray diffraction analysis and transmission electron microscopy were used to determine the average grain size of the milled powders. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the as-sprayed coatings. The properties of various coating materials were characterized by microhardness measurements performed on the polished surface of the cross section.

The detonation gun (D-Gun) coating technology which results in coatings with outstanding properties has found extensive applications especially in the aerospace and engineering industry. Though the D-Gun process has been in use for the last 30 years or so, the basic mechanisms which lead to such an outstanding combinations of properties is not yet fully understood. This has been primarily due to the fact that the coating property ultimately depends on the combination of powder particle temperature and velocity at the time of its impact on the substrate to be coated and so far measurement of these parameters has not been possible. As a result, indirect correlations between D-Gun variables like fuel gas to oxygen ratio, powder feed rate, firing frequency and working distance and coating properties have been developed but without the knowledge base of how these D-Gun variables affect the powder particle temperature and velocity. An in-flight particle diagnostic sensing system has been recently installed in our laboratory and this system allows for the simultaneous measurement of particle velocity and temperature. In view of the above, a major programme has been launched to study the effect of particle velocity and temperature on the coating properties (like coating hardness, porosity, bond strength, XRD etc.) for a wide range of coatings. The preliminary results from the above investigation, pertaining to Cr3C2-25NiCr coatings obtained using D-Gun, will be presented and discussed in this paper.

The deformation and spreading of fully molten particles impacting onto a rough surface have been investigated by numerical simulation. A numerical technique, based on finite element analysis, was developed specifically for this simulation. The Lagrangian method with an automatic remeshing technique has been used to trace accurately the free surface of the molten matter and to improve the accuracy of the computation. A friction limiting condition at the particle substrate interface was introduced to describe the effects of the substrate surface roughness. This surface characteristic significantly influences the flattening degree, the flattening time, the spreading velocity of the liquid particle and its final shape.

The transitional behavior of the splat pattern of Ni particles sprayed on a flat substrate was investigated. Based on Auger analysis and SEM observation, it was confirmed that the splashing was formed not by material flowing on the substrate surface from the impingement center to the periphery, but by jetting away from the central disk. Observations of etched splat surfaces revealed that the bottom portion of the central disk solidified rapidly after impingement, and it was also confirmed, based on the direction of the splash pattern, that the splashing was caused by stumbling due to some type of deterrent to liquid flow, such as poor wettability at the flow tip or initial rapid solidification of the splat. The drastic change of the splat pattern near the transition temperature seems to occur when the Weber number of the liquid flow coincides with some critical value.

This paper presents a one-dimensional heat transfer model which predicts the solidification and cooling of a plasma-sprayed alumina splat after the flattening process is completed. A heterogeneous nucleation process taking place on the substrate surface was assumed. The density and average size of the formed nuclei were determined from the integration of the nucleation rate calculated from the classical kinetic theory for nucleation. This rate depends on the activation energy required for nucleation which takes into account the effect of the surface via a wetting angle between the growing nucleus and the catalytic surface. This contact angle was estimated from the comparison of the computed grain density with the density observed on splat surface using an atomic force microscope. When 67% of the splat surface in contact with the substrate are covered by grains, a planar solidification front was assumed to move through the melt. The theoretical model accounted also for the selection of the crystalline phase. Calculations were performed for various substrate materials at different initial temperatures. Results are expressed in terms of nucleation temperature, nucleation rate, density and grain size distribution.

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.

Plasma sprayed deposits consist of multitude of flattened lamellar particles - 'splats' - which as basic building elements form their structure and determine the deposit properties. Therefore, knowledge of the mechanism of their formation and characteristics is important for understanding the processing property relationships. Although extensive studies have been done on splat formation, there is a lack of correlation to macroscopic deposit properties. Among factors influencing the deposit properties and performance is residual stress, originating from splat quenching and thermal mismatch between the substrate and coating. This phenomenon has been studied mostly on macroscopic level. In the present study, an attempt is made to establish a connection between these two approaches. In the focus of this study is the effect of selected processing variables on splat characteristics, deposit properties and residual stress in a single-splat layer. The processing variables of primary interest were deposition temperature and substrate material. Molybdenum as a representative material of practical interest was used throughout this study. The correlation between stresses and processing conditions is discussed with regards to microstructure and relevant coating properties.

The main purpose of this work is the development of mathematical and computer models for the integrated simulation of all stages of the atmospheric plasma spraying process (APS) with temperature dependent thermophysical and mechanical properties of the used materials and gases and experimental verification of the simulated results. The following mathematical models of APS were created: particle heating and movement in the plasma jet; coating structure formation; heat transfer and residual stresses in the coating-substrate system. The computer realization of these models enables us to model all stages of APS (integrated or separately). Databases of coating, substrate and plasma-gas substances include the temperature dependent properties. The model of APS is divided in 3 parts, which are connected by continuous data interface. Two dimensional approximation of plasma-gas velocity and temperature in the free plasma jet was used for computation of particle velocity, trajectory and temperature. This information was created with a special Graphic program module and included in database. Computer experiments for plasma spraying of Ah03 and ZrO 2 +8%Y 2 O 3 in Ar/H 2 plasma were carried out. The experimental verification of developed models with High-Velocity-Pyrometry (HVP) and Laser-Doppler-Anemometry (LDA) have shown the satisfactory precision of simulated results.

Dynamic and thermal behaviors of particles injected into an HVOF supersonic flow were modeled using the Lagrangian formulation coupled with a moving grid system which allows to treat melting and solidification problems in a particle. The particle behaviors of alumina, Tribaloy-400 and MCrAlY were examined by numerical simulations. Velocities calculated by this model were compared with experimental measurements available in publications. For investigating the particle temperatures for which experimental data are hardly available, qualitative experiments were conducted with an MCrAlY powder. It was thus observed that unmelted particles predicted by the numerical simulation were also found in the microstructure of the sprayed MCrAlY coating. From this work, it was shown that the established model provides a reasonable prediction of dynamic and thermal particle behaviors in HVOF flows.

A mathematical model of the impingement of a plasma jet on a flat structure is proposed. This model can be used to predict temperature and velocity fields in the jet and in the near substrate region, but also to estimate thermal exchanges at the surface of this substrate. Different options were tested concerning the modeling of the near wall region and results indicate that a boundary layer calculation is necessary to predict the energy flux transferred to the substrate with a good accuracy. Nevertheless, the influence of the presence of the substrate on temperature and velocity fields was found to be important only in the near substrate region, indicating that the flow fields calculated from free jet modeling are accurate over the major part of the domain.

Effect of nozzle geometry (such as throat diameter of a barrel nozzle, exit diameter and exit divergence angle of a divergent nozzle) on HVOF thermal spraying process (thermodynamical behavior of combustion gas and spray particles) was investigated by numerical simulation and experiments with Jet Kote II system. The process changes inside the nozzle as obtained by numerical simulation studies were related to the coating properties. A NiCrAlY alloy powder was used for the experimental studies. While the throat diameter of the barrel nozzle was found to have only a slight effect on the microstructure, hardness, oxygen content and deposition efficiency of the coatings, the change in divergent section length (rather than exit diameter and exit divergence angle) had a significant effect. With increase in divergent section length of the nozzle, the amount of oxide content of the NiCrAlY coatings decreased and the deposition efficiency increased significantly. Also, with increase in the exit diameter of the divergent nozzle, the gas temperature and the degree of melting of the particle decreased. On the other hand the calculated particle velocity showed a slight increase while the gas velocity increased significantly.