The plasma spraying process is controlled by various parameters that have an influence on powder particle velocities, temperatures and trajectories just before impact to the substrate. In order to fully utilize the thermal and kinetic energy of the plasma it is important to obtain information from these powder particle properties. In this work an intensified CCD camera has been used to detect in-flight particles in an atmospheric plasma spraying process. Plasma spraying was performed using fused and crushed AI2O3 powder. The powder carrier gas flow rate was varied during the spraying experiments. All the other deposition parameters were kept constant. Coatings produced using relatively new spraygun electrodes are compared with ones produced later with the same electrodes when they were worn out. The particle concentration is determined on a relative scale by the fraction of the area of a CCD camera frame covered by particle images. Further investigations necessary to clearify the relationship between the measured relative particle concentration and the true particle concentration are identified. The coatings are analyzed for wear resistance, degree of melting, deposition efficiency, hardness and porosity. The dependence of these coating properties on the relative particle concentration and the effect of electrode wear on the relative particle concentration are studied.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

Plasma spraying is a complicated process involving many partly interdependent parameters, which are in industrial spray environments difficult to optimise without laborious and time consuming experiments. In this work a non-intensified CCD camera without any external illumination is used for in-flight particle visualisation. Particle visualisation is based purely on the spontaneous light emitted by the hot particles. The motivation for this work is to outline the possibilities to develope a CCD based, low cost and rugged in-situ measurement system suitable also for industrial use. The measurement method has been tested with Plasmatechnik A3000S plasma spraying equipment using fused and crushed Al2O3 powder. Using digital image processing techniques relative particle concentrations and particle velocities have been calculated from the acquired images. These results have been correlated with wear resistance and deposition efficiency of the coatings produced with different powder feed rate and powder port adjustments. Coatings were also produced using both new and worn electrodes. The benefits and limitations of the method are discussed and the measurement results are compared against measurements made using laser sheet illumination, which can give information concerning also the colder and/or smaller particles not visible for the passive CCD system.

This paper presents the results of an emission study on plasma spraying equipment and processes. Various measurements and samples were taken outside the spraying booth, at the operator level, and in the suction ducts upstream and downstream of the filtration equipment, creating a detailed profile of the aerosol emitted by the injection of NiAl powder in the plasma jet. The results show the existence of two families of particles, one ranging in size from 0.5 to 20 µm, the other of nanometric proportions. Concentrations of the larger particles were in the range of 600 cm-3 in the booth. As for submicron particles, concentrations of up to 107 cm-3 were observed but decreased significantly at the outlet of the filter system. The aerosol samples examined were dominated by a nanometric background of aggregates made up of oxidized nickel particles. Aggregates up to 100 nm in size, consisting of finer particles in the 5-20 nm size range, were found in high concentrations upstream of the filtration system. Great vigilance is thus required to protect equipment operators, an important part of which is placing dust collectors as close as possible to spraying booths connected by short, straight pipe runs.

Numerical modeling of the complex two-phase flow of gas-solid particles in a highly compressible field, as occurs in HVOF spraying, can serve as a tool for optimizing deposition efficiency, nozzle designs, and other parameters. In this paper, the authors develop a numerical model based on Euler’s equation and use it to investigate the effect of flow regime on the velocity of particles in an oxyfuel jet. Simulations based on typical HVOF operating conditions show large variations in the flow regime in regions of high particle concentration. Unlike other modeling approaches, the Eulerian approach predicts that the flow becomes subsonic near the centerline and drag force decreases significantly as do particle velocity and deposition efficiency. The effects of flow regime are also measured experimentally, validating the result. Paper includes a German-language abstract.

The thermal spray process melts powder at very high temperatures and propels the molten material to the substrate to produce a coherent deposit. This heating produces a certain amount of vaporization of the feedstock. Upon exiting the plasma plume the fast cooling conditions lead to condensation of the vapor. An electrical low pressure impactor was used to monitor the concentration of ultra-fine particles at various radial and axial distances. Metal, namely iron powder, showed very high concentration levels which increase with distance. Ultra-fine particles from ZrO 2 -8Y 2 O 3 reached a peak concentration at 6 cm. Use of an air barrier during spraying decreases the population of ultra-fine particles facilitating the production of a stronger coating.

Because of their characteristic spray geometry, pressure-gas-atomizers can be used to create thick coatings from molten metal. Production rates of pressure-gas-atomizers are substantially higher compared to conventional Thermal Spraying (100 – 200 kg/h based on molten tin). The shape of the spray cone can be designed by the geometry of the atomizer. The mean particle sizes and velocities (molten tin and tin-copper alloys) are controlled by the gas flow. Powder products and coatings of several millimeters on steel substrates were investigated. The average density of the layer was higher than 99%.

A computational fluid dynamic (CFD) model of the cold gas dynamic spray process is presented. The gas dynamic flow field and particle trajectories within an oval shaped supersonic nozzle as well as in the immediate surroundings of the nozzle exit, before and after the impact with the target plane, are simulated. Predicted nozzle wall pressure values compare well with experiment. In addition, predicted particle velocity results at the nozzle exit are in qualitative agreement with those obtained using a side-scatter laser Doppler anemometer (LDA.) Details of the pattern of particle release into the surroundings are visualized in a convenient manner.

The present work has the purpose of comparing different thermal spraying techniques, namely axial plasma spray, standard air plasma spray and high velocity oxygen flame (HVOF), for depositing metal matrix composites, in this case chromium carbide nickel-chromium based. The quality of the coatings deposited by these three techniques has been assessed in terms of structural characteristics (porosity, oxide concentration, unmelted particles presence, etc.) and of mechanical characteristics (hardness, adhesion, etc.) as well as surface morphology. A specific efficiency test has been carried out to compare the three examined technologies. The results of the present study indicate that, against a slightly decrease in the quality of the film in terms of structural and mechanical properties, axial plasma sprayed coatings can be sprayed with a higher efficiency in comparison to the traditional technologies.

This study developed microstructure-based finite element (FE) models to investigate the behavior of cold-sprayed aluminum-alumina (Al-Al2O3) metal matrix composite (MMCs) coatings subject to indentation and quasi-static compression. Based on microstructural features (i.e., particle weight fraction, particle size, and porosity) of the MMC coatings, representative volume elements (RVEs) were generated by using Digimat software and then imported into ABAQUS/Explicit. State-of-the-art physics-based modelling approaches were incorporated into the model to account for particle cracking, interface debonding, and ductile failure of the matrix. This allowed for analysis and informing on the deformation and failure responses. The model was validated with experimental results for cold-sprayed Al-18 wt.% Al2O3, Al-34 wt.% Al2O3, and Al-46 wt.% Al2O3 metal matrix composite coatings under quasi-static compression by comparing the stress versus strain histories and observed failure mechanisms (e.g., matrix ductile failure). The results showed that the computational framework is able to capture the response of this cold-sprayed material system under compression and indentation, both qualitatively and quantitatively. The outcomes of this work have implications for extending the model to materials design and under different types of loading (e.g., erosion and fatigue).

This paper reports on the influence of the He to N 2 ratio on the properties of low pressure cold sprayed titanium coatings and on the characteristics of the generated supersonic two-phase flow. Experiments were carried out varying the He to N 2 concentration ranging from pure He to pure N 2 . Samples were characterized by their microstructural properties (i.e. microhardness and porosity). Deposition rate was evaluated and particle velocities were measured for all conditions. Deposition efficiency, coating density, and microhardness were found to be a function of particle impact velocity. Velocity data were used to validate a computational fluid dynamic model. The numerical solution of the flow inside the nozzle was obtained from the Euler equations for the various He to N 2 concentrations. Particle tracking was carried out by using the computed distribution of density, Mach number, temperature, viscosity, and a second order Runge-Kutta scheme. In addition, mean particle velocities at the exit of the nozzle were determined. Computed velocities were found to be in good agreement with measured ones. The model was then used to calculate nozzle dimensions that would maximize particle velocity. Optimized dimensions are proposed.

The monoclinic and tetragonal phase compositions and distribution in air plasma sprayed (APS) yttria-partially stabilized zirconia (YPSZ) thermal barrier coatings were studied. The coatings were produced from powders with varying phase concentrations, chemical purity and powder production processes. Both the powder and coatings were characterized using X-ray diffraction (XRD) and Raman spectroscopy. The use of environmental scanning electron microscopy (ESEM) and X-ray energy dispersive analysis (EDS) added morphological and elemental information to the study. XRD and Raman spectroscopy were shown to be powerful combined tools and shows an overall decrease in the monoclinic phase within the coatings produced from the different powders. The distribution of both the monoclinic and the tetragonal phases could be highlighted both in the coatings and the individual powder particles. This indicated changes in monoclinic concentration in the less dense areas of some of the coatings and a varying distribution across particles in some of the powders. Raman mapping over small areas also showed how phase surface distribution, on the coatings surfaces, could be assessed.

High-performance polymers such as poly(ether ether ketone) (PEEK) are appealing for a wide variety of industrial and medical applications due to their excellent mechanical properties. However, these applications are often limited by relatively low thermal stability and conductivity compared to metals. Many methods developed to metallize polymers, including vapor deposition and thermal spray processes, can lead to poor quality control, low deposition rate, and high cost. Thus, cold spray is a promising potential alternative to rapidly and inexpensively produce polymer-metal composites. In this study, we investigated the deposition characteristics of metalpolymer composite feedstock, composed of PEEK powder with varying volume fractions of copper (Cu) flake added, onto a PEEK substrate. We prepared the Cu-PEEK composite powder in varying compositions by two methods: hand-mixing the powders and cryogenically milling the powders. Scanning electron microscopy (SEM) of the feed mixtures shows that cryogenically milling the polymer and metal powders together created uniformly distributed micron-scale domains of Cu on PEEK particle surfaces, and vice versa, as well as consolidating much of the porous Cu flake. In lowpressure cold spray, the relatively large volume fractions of PEEK in the composite mixtures allowed for lower operating temperatures than those commonly used in PEEK metallization (300-500 °C). While the deposition efficiencies of each mixture were relatively similar in single-layer experiments, deposits formed after multiple passes showed significant changes in deposition efficiency and composition in PEEK-rich feedstock mixtures. SEM of deposit surfaces and cross-sections revealed multiple co-dominant mechanisms of deposition, which affect both the porosity and final composition of the deposit. Though present in all samples analyzed, the effects of cryogenic milling were more prevalent at lower Cu concentrations.

Thermal spray of Zn, Zn/Al, or Al is extensively used to make anticorrosion coatings on steel structures. Twin arc spray and wire flame spray are the two most practised processes to achieve such coatings. This paper presents measurements of particle emissions generated by these two processes. Sampling and analysis of aerosols generated by both processes have been carried out inside the exhaust ductwork using various instruments: an ELPI impactor, a CNC (Condensation Nucleus Counter), a TEOM microbalance and sampling filters allowing sampling for SEM observations. Electric arc spraying produced much more fumes of ultra fine particles than flame spraying. Aluminum spraying also produces large fume quantities compared to the Zn spraying under the same conditions. The aerosol comprised submicron particles and 95% of the numerical particle size distribution was less than 100 nm. The nanometric nature of the fume particles was confirmed by observations on the SEM. The strong dilution caused by compressed air has the effect of strongly limiting particle coagulation and, in turn, the size of the agglomerated particles. Electric arc spray has taken market share versus wire flame spray for Zn, ZnAl, or Al spraying, but this study shows that it generates much more particle emissions.

Technology of gas-thermal deposition has to be based on precise knowledge of all the physical and chemical processes wherein particles of deposited material take part, beginning from their injection into the high-temperature jet and to the moment they are completely cooled down on the surface to be covered by a coating. This paper presents some results of investigations of heterogeneous jets loaded with powder materials at a gas-flame and plasma deposition of coatings using optical methods of recording various phase parameters, including pulse interferometry with field visualization. Paper includes a German-language abstract.

A Ni-Al pseudo-alloy powder was studied from the point of view of spheroidization during spraying by a water-stabilized plasma gun. The powder particles of irregular shape were conglomerates of elemental Ni and Al, the average Al content being 9.7 %. To conserve the shape and composition of particles flying in the plasma stream, these were trapped in liquid nitrogen. Scanning electron microscopy and X-ray microanalysis were used to obtain information about particle shape and element distribution. Most plasma sprayed particles trapped in liquid nitrogen were composed of a Ni-Al alloy, where the Al content varied in a wide range. Spherical "caps" composed of Al-oxide covered partially their surfaces. It follows that on the interface between molten Ni and Al, the interaction of both components gave rise to a Ni-Al alloy. On the contrary, if Al was exposed to air, it oxidized rapidly during the flight of the particles. The X-ray diffraction lines of the metallic phase in the particles trapped in liquid nitrogen were shifted from the positions corresponding to pure Ni as observed in the feedstock powder. This, together with the line asymmetry, showed the presence of Ni-Al alloy containing varying amounts of Al. The X-ray diffraction did not find any elemental Al in the liquid nitrogen trapped powder, i.e. neither in the metallic phase nor in the "caps". This means that all Al accessible to the ambient oxygen was converted into oxide. The "caps" contained metastable γ- and δ- Al 2 O 3 . The mechanism of the "cap" formation appears to be based on the fact that after an acceleration and melting period, significant slowing down of a molten particle occurs. Due to the drag forces, the lighter Al 2 O 3 melt concentrates on the rear part of the droplet surface. The main condition, under which this mechanism holds, is the presence of two immiscible melts in the droplets and the significantly differing densities of both melts.

Plasma spraying of ceramic nano-powders suspended in a liquid carrier medium is an emerging technology, which allows the formation of thinner coatings with microstructures more refined than conventional plasma spraying. An external injection system, where the suspension enters the plasma jet radially, is installed on a F4-Sulzer Metco dc torch for the production of nanostructured Al 2 O 3 and ZrO 2 coatings. The effect of injection parameters, such as initial droplet diameter, droplet velocity and suspension flow rate is studied. The suspension droplets are continuously generated through an exchangeable micron-sized nozzle with a superimposed pulse of variable ultrasonic frequency. The heat transfer from the plasma to the liquid feed is optimized at high droplet velocity, moderate counter-current injection angle and flow rates not exceeding a threshold value, which depends on the plasma enthalpy and the latent heat of the suspension medium. A significant effect of initial droplet size (220 – 500 µm) or solid concentration (5 – 15 %) is not observed. In-flight particle states are measured for different plasma conditions, and are related to the resulting microstructures by SEM and XRD. High particle temperatures give rise to a refinement in crystallite size, while the particle velocities govern the deposition efficiencies and porosity levels. The results show that the particles follow closely the gas flow in the free stream, as well as in the stagnation boundary layer close to the substrate by virtue of their limited inertia. The prominent difference in microstructure between highly porous alumina and very dense zirconia coatings is explained in terms of particle impact velocities, which are simulated for typical operating conditions as a function of particle size and free-stream gas velocity.

A model for oxidation of molybdenum particles during plasma spray deposition is developed. The diffusion of metal an-ions or oxygen cat-ions through a thin oxidized film, chemical reactions on the surface, and diffusion of oxidant in gas phase are considered as possible rate-controlling mechanisms with controlling parameters as the temperature of the particle surface, and local oxygen concentration and flow field surrounding the particle. The deposition of molten particle and its rapid solidification and deformation is treated using a Madejski-type model, in which the mechanical energy conservation equation is solved to determine the splat deformation and one-dimensional heat conduction equation with phase change is solved to predict the solidification and temperature evolution. Calculations are performed for a single molybdenum particle sprayed under the Sulzer Metco-9MB spraying conditions. Results show that the mechanism that controls the oxidation of this droplet is the diffusion of metal/oxygen ions through a very thin oxide film. A higher substrate temperature results in a larger rate of oxidation at the splat surface, and hence, a larger oxygen content in the coating layer. Compared to the oxidation of droplet during m-flight, the oxidation during deposition is not weak and can become dominant at high substrate temperatures.

Numerical predictions and experimental observations have been correlated to improve the qualitative understanding of the degree of thermal degradation occurring during the HVOF spray deposition of Nylon-11. Particle residence time (<1 ms) in the HVOF jet was insufficient for significant decomposition of the Nylon-11 but was sufficient for noticeable discoloration (yellowing) of the particles of a powder with a mean particle size of 30 µm. Experimental observations showed this to be the case even though numerical predictions indicated that the temperature of a 30 µm diameter particle should be considerably higher than the upper degradation limit of Nylon-11. Initial thermal oxidation of Nylon-11 promotes the formation of carbon-carbon double bonds that strongly absorb in the visible spectrum even at concentrations of parts per million, resulting in discoloration of the Nylon.

A particle laden flow in an HVOF torch is analyzed using Computational Fluid Dynamics (CFD). The torch is similar to the DJ Metco torch with a converging-diverging (de Laval) nozzle, where particles are injected through the center together with nitrogen as a carrier gas. The Eulerian formulation is used for the gas flow whereas the particle motion is described by using the Lagrangian formulation. The flow turbulence is modeled via k-e model with standard wall functions. For modeling the combustion process in the torch, a multi-reaction Eddy-Dissipation Model (EDM) is employed. The computational domain comprised the torch itself and the region outside the torch where our attention is mainly focused. The computations are performed for the torch with and without the gas shroud attachment. The results showed that the presence of the shroud affected to some degree the flow and temperature fields of the main gas and the particle stream, while at the same time, significantly reducing the entrainment of ambient air into the main stream as shown by the lower oxygen concentrations. The results of the numerical computations are compared with experimental results for the same operating conditions and the agreement is found to be good.