In cold spray and thermal spray applications, one of the primary factors affecting coating deposition is the location where particles are injected into the gas jet. Therefore, a detailed knowledge of the gas flow distribution is required. Non-resonant laser scattering allows to spatially resolve the distribution of drift velocity and mass density within the flow, particularly at locations close to the injector. Based on laser scattering, this paper presents a new diagnostic that locally measures drift velocity, as well as a relative mass density distribution of a gas stream. Its application is mainly focused on cold gas flows, where velocity measurements in a supersonic nozzle, obtained by means of laser scattering, correlate well with theoretical calculations and particle image velocimetry (PIV) experimental results.

One of the most sensitive factors for the simulation of the gas flow outside the plasma torch is the determination of the velocity and temperature profiles at the torch outlet. It requires the solution of the flow and electric potential equations within the torch. In this work this problem is solved through numerical simulations, with two free parameters: the radius of the plasma core near to the cathode and the length of the electric arc (or alternatively the potential between cathode and anode). In order to reduce the number of the free parameters to only one, an analytical model is also developed, which solves the same equations as the numerical simulations but in a simplified way. The reduction of parameters is achieved in this simple model through the additional condition that the physical plasma corresponds to an extremal value in the entropy production. Since the results of the simplified analytical model are compared to more detailed numerical simulations, the free parameter can be adjusted. The effect of high hydrogen content on the gas flow is thus studied, showing that the velocity profile at the outlet displays a more pronounced peak, as expected experimentally.

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.

Numerical simulation of a plasma spraying process from ceramic particle injection to a coating formation was conducted by integrating particle-laden plasma flow, ceramic splat formation and coating formation models. Velocity and temperature of both plasma flow and ceramic particles under an applied RF electromagnetic field were clarified by using the first model. Radial distributions of particle impact location, velocity and temperature were obtained based on both an unsteady effect of a plasma flow and distributions of particle size and injection velocity. Nextly, splat thickness and diameter of melted ceramic particles after impact on a substrate were clarified by using the particle impact velocity and temperature. Radial distributions of splat thickness and diameter became more uniform. Finally, coating thickness distributions were evaluated by using the last model. They were strongly influenced by particle size and injection velocity distributions.

Computational modeling is used to systematically examine many of the sources of statistical variance in particle parameters during thermal plasma spraying. Using the computer program LAVA, a steady-state plasma jet typical of a commercial torch at normal operating conditions, is first developed. Then, assuming a single particle composition (ZrO2) and injection location, real world complexity (e.g., turbulent dispersion, particle size and density, injection velocity and direction, etc.) is introduced "one phenomenon at a time" to distinguish and characterize its effect and enable comparisons of separate effects. A final calculation then considers all phenomena simultaneously, to enable further comparisons. Investigating each phenomenon separately provides valuable insight into particle behavior. For the typical plasma jet and injection conditions considered, particle dispersion in the injection direction is most significantly affected by (in order of decreasing importance): particle size distribution, injection velocity distribution, turbulence, and injection direction distribution or particle density distribution. Only the distribution of injection directions and turbulence affect dispersion normal to the injection direction, and are of similar magnitude in this study. With regards to particle velocity and temperature, particle size is clearly the dominant effect.

This study investigates cold spray particle velocities achieved at different pressures and particle feed rates using particle image velocimetry (PIV). Particle dispersion and velocity evolution along the jet axis were investigated for several feedstock materials. It was found that average particle velocity decreases with increasing particulate loading. The effect is aggravated at lower pressures, but mainly depends on feedstock material, which implies more complex, volume-fraction related physics playing a role. Velocity distribution and particle dispersion were also found to be influenced by particle feed rate, depending on the material. Increased particle feed rates affect the magnitude and distribution of impact velocity and consequently the efficiency of cold spraying.

In the spraying of functionally graded coatings the particle ensemble delivered to the substrate can vary from a relatively low melting point metallic particle to a significantly higher melting point ceramic particle. At various stages in the spray process the particle ensemble can be either predominantly metallic, ceramic, or an intermediate combination. For co-injected particles the mixtures do not behave as a simple linear superposition of the spray patterns of the individual particle types. The particle temperature, velocity, size distributions, and pattern characteristics of the resulting spray fields is examined for all ceramic particle sprays (ZrO 2 ), all metallic particle sprays (NiCrAlY), and for a 1:1 mixture. The major particle-particle interaction occurs in the injector itself and results in a modified spray pattern which is different from that of either material sprayed alone. The particle velocity distributions generally exhibit a bimodal nature which is dependent on the size and density of the injected particles.

The performance (particle velocity and velocity distribution) of a typical injector, and the resulting particle spray pattern for metallic (NiCrAlY) and ceramic (ZrO 2 ) particles are examined as a function of carrier gas flow rate and the effect of varying the geometry immediately upstream of the injector. Injector performance is also examined for a 1:1 mixture of ceramic and metallic particles such as is used in the spraying of functionally graded materials. The upstream geometries tested included a 90° "tee," a 90° elbow, and a straight entrance. The elbow geometry was tested in both "up" and "down" orientation to determine the influence of gravity. The upstream geometry can alter the average particle injection velocity by 10-15% influencing both the spray pattern trajectory and width.

The present study is conducted to simulate the multi-component chemical reactions of a turbulent argon-hydrogen plasma jet flowing into the surrounding atmosphere under typical operating conditions. Based on the initial spraying process parameters, the temperature, velocity profiles and the basic chemical species at the nozzle exit are calculated, and then used for the boundary conditions of the next step, where the chemical reactions in the multicomponent plasma jet are calculated by employing the Eddy-Dissipation model and the Realizable k-ε turbulence model. As the result of the calculation, the distribution of the mole fractions of the products as well as the temperature and velocity distributions is discussed. The approach presented in this paper will be theoretically helpful to perform further analysis of the interaction between the plasma and the particles.

A typical feature of cold spray process is that a deposit can be formed without change of the original structure and compositions of spray materials. Only particles which reach the velocity higher than the critical velocity can be deposited on a substrate in cold spraying. When the spray particle impacts on the substrate at an off-normal angle, the normal component of particle impact velocity will change with the approaching angle of spray particle to substrate. In the present study, copper and titanium powders are used to deposit coating using cold spray process at different impact angles with regard to substrate. The deposition characteristics of spray materials are examined. The results show that the impact angle has a significant influence on the deposition characteristics. The relative deposition efficiency changes with the spray angle. It has been found that there is a critical impact angle at certain particle conditions below which no deposition occurs. The relation between spray angle and relative deposition efficiency can be divided into three spray angle ranges: maximum deposition angle range, transient angle range and no deposition angle range. In the transient angle range, the relative deposition efficiency increases with an increase in spray angle from zero at the critical spray angle to 100%. The transient angle range depends on the particle velocity distribution. A model is proposed to explain the relation between the spray angle and the relative deposition efficiency.

In the cold spraying process, particle velocity is commonly regarded as the key factor that influences the deposition efficiency and properties of the coating. The present paper reports on a study in which the velocity of in-flight particles was measured using a DPV-2000 system. The influences of He and N 2 gas pressure and temperature and particle morphology on the particle velocity and deposition efficiency of the coating using stainless steel 316L powders were studied. The microstructure of the coating was examined using optical microscopy. The critical velocity of stainless steel 316L powders was estimated according to the particle velocity distribution and deposition efficiency of the coating. The experiment results suggested that the gas pressure has a more significant influence on the particle velocity and deposition efficiency of the coating than the gas temperature. The particle morphology also has significant influence on the particle velocity. The critical velocity of stainless steel 316L powders was in the range of 630 to 680 m/s, and it decreased slightly with the gas temperature.

The cold spray process is a relatively new process using high velocity metallic particles for surface modifications. Metallic powder particles which are injected into a converging-diverging nozzle are accelerated to supersonic velocities. In this study two-dimensional temperature and velocity distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperatures on particle velocities and temperatures inside and outside of the nozzle are also investigated. It is found that the acceleration of the gas velocity takes place in the area of the nozzle throat and it increases and reaches maximum value at the nozzle exit. Due to compression shocks in the area after the nozzle, the gas jet properties show irregular shape and these result in the existence of the maximum particle velocity by the change of particle size at a given gas pressure and temperature.

Arc spraying is an economical method for applying metallic layers due to its high spray rates and uniform melting of spray particles. The main disadvantage is the difficulty in achieving sufficient particle velocity to ensure good layer adhesion. This study investigates the influence of nozzle geometry, arc power, and gas pressure on the size and velocity of particles in an arc spray jet. The experiments were conducted using particle image velocimetry (PIV) to measure the spatial and velocity distribution of particles in flight. For X45Cr13 steel, particle velocities were found to be between 85 and 95 m/s at a gas volume flow of around 1 m 3 /min. Velocities of up to 150 m/s were ultimately achieved, but at the expense of higher atomizer gas consumption. Paper text in German.

Basalt, an abundant and inexpensive natural raw material, is a glass-ceramic with good abrasion-wear resistance and chemical stability. Traditionally cast-shaped into flag stones, pipe linings and even fibrous composites, basalt can be processed by thermal spraying, potentially yielding highly dense coatings with few defects. Such overlays can seal base materials for wear applications in corrosive environments. Basalt coatings are produced by a number of common thermal spray techniques, including water-stabilized plasma spraying (WSP), high-velocity oxy-fuel (HVOF) and conventional air plasma spraying (APS). In-flight particle temperature and velocities are monitored with a particle diagnostic system (DPV 2000). Using different feedstock size cuts, the attainable ranges of particle states are delineated. Spray parameters are selected for each of the processes, based on deposition efficiency and porosity criteria. For typical conditions, particle velocities vary from 100 m/sec for WSP to 800 m/sec with HVOF. The microstructure and composition of the coatings are evaluated by scanning electron microscopy (SEM) and EDS-SEM. Crystal phase analysis is performed by X-ray diffraction (XRD). Abrasion resistance (ASTM G-65) and hardness (Vickers) of the as-sprayed coatings are compared. The microstructures and tribological properties are related to the particle size, temperature and velocity distributions, which are distinctly different for each process.

This paper aims to achieve improved control over the spray pattern in a wire arc torch by means of constructive measures to influence the flow dynamics. It focuses on evaluating different fluid dynamic designs of the spray torches with respect to the droplet trajectories and velocity distributions and the deposit distribution. First, the paper describes the approaches for novel nozzle designs including the addition of shrouds and the torch evaluation diagnostics, and then presents the results for four different nozzle arrangements. It is observed that the nozzle design and the placement of the wire tips within the nozzle have the greatest effect on the divergence of the spray pattern. Paper includes a German-language abstract.

Cold spraying has emerged as a promising technique for the repair of metallic components. For controlled material deposition, manipulation of the cold spray gun is performed by industrial robots. Such robot-guided cold spraying provides flexible control and automation of the entire process. To enable effective and material-efficient material deposition at specified repair locations, this work proposes a method for automated planning of cold spray paths and trajectories. The method begins with the extraction of the volume to be filled by comparing the nominal and actual component. To produce the extracted volume, it is divided into suitable adaptively curved layers and converted into point clouds for path planning. The cold spray path is then converted into a trajectory by adding a suitable velocity distribution for the spray velocity to produce the required locally varying layer thickness. To validate the suitability of the path and trajectory, a simulation of the material deposition is performed. In addition, the implementation of the entire method is demonstrated by exemplary use cases. The results demonstrate that the proposed method enables successful automated path and trajectory planning, both contributing to the overall goal of automated repair of damaged components by cold spray.

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 of metals and metallic alloys performed in controlled atmosphere or soft vacuum results in coatings with a low oxidation level and excellent thermomechanical properties. Unfortunately, the spraying cost is drastically increased by one or two orders of magnitude compared to air plasma spraying (APS). Thus the minimisation of oxidation during APS is a key issue for the development of such coatings. Oxygen concentrations sucked into plasma jets have been measured by an enthalpy probe linked to a mass spectrometer. This technique allows to determine simultaneously plasma composition, temperature and velocity distributions within the plasma plume. Results have been compared to those obtained with a two-dimensional turbulent flow model. The obtained results have shown that surrounding air entrainment is reduced when using adequate Ar/H2/He mixtures which viscosity is higher than that of Ar/H, mixtures, limiting the turbulence in the jet fringes and pumping of the surrounding atmosphere.

This paper presents an innovative method for thermal spray particle diagnostics. In the demonstration, twelve CCD cameras are arranged around the spray cone created by a wire arc gun. The cameras are aligned radially with 15° offsets and are synchronized with a hardwired trigger. Images simultaneously captured from different directions contain information from which the location and trajectory of the particles can be determined. In addition to describing the setup of the camera system, the paper also develops the mathematics required for image processing and the calculation of 3D particle trajectories and velocity distributions.

In the kinetic spraying process, the critical velocity is an important criterion which determines the deposition of a feedstock particle onto the substrate. It was experimentally and numerically proven that the critical velocity is determined by the physical properties and the state of materials such as initial temperature, size and the extent of oxidation. Compared to un-oxidized feedstock, oxidized feedstock required a greater kinetic energy of the in-flight particle to break away the oxide film during impact. The oxide film formed on the surface of particle and substrate is of a relatively higher brittleness and hardness than those of general metals. Because of its physical characteristics, the oxide significantly affected the deposition behavior and critical velocity. The effects of oxidation on the critical velocity and the deposition behavior of the feedstock were investigated and evaluated by individual particle impact tests in this study. The velocity of pure Al particles was measured for a wide range of process gas conditions.