The reproducibility of the coating properties is the result of an efficient control of the thermal spray process. Actually, it is not possible for the sprayer to guarantee the temporal reproducibility of the coating properties without online control. However, adjusting the system fluctuations with regards to the temperature, velocity and size of particle could alter some properties that depend or not on the in-flight particle characteristics. This study aims at discussing the existing correlations between the processing parameters and the coating properties established with a Neural Network methodology. It demonstrates the segmentation of the correlations by comparing the result of the merging procedure of two sub-network structures and the direct correlation from the processing parameters to the coating properties. The sub-structures are built considering respectively the in-flight particle characteristics relationship with the operating conditions and the in-flight particle characteristics relationship with the coating properties.

While twin wire arc spraying, strong dependences are usually noticed between elaboration process parameters and coating properties. Therefore, control is needed for increasing coating performances. The aim of this paper is to investigate the coating properties in relation with in-flight particle characteristics and with the impact modes. Measurements were achieved on iron particles when changing either the current intensity or the air flow rates. In-flight particle characteristics were determined by using the DPV diagnostic system 200 mm from the gun exit and for three different radial locations. The same locations were kept for the impact analysis. Then, the morphology of the splats was evaluated by its flattening and shape factors when the substrate is preheated or not. Finally, the coating properties were characterized in terms of porosity, oxide contents, microhardness and thickness. Through the determination of related interactions, a better knowledge of coating elaboration conditions will lead to improve reliability and properties of the deposits.

In thermal spraying, the knowledge of process basic input parameters and their influence on the final coating properties is crucial. The optimization and reliability of atmospherically sprayed (APS) coatings, especially highly porous thermal barrier coatings (TBC), is closely linked to process understanding. This investigation aims to elucidate the correlation between the basic plasma spraying parameters (like current, plasma gas, stand-off distance, etc.) and the deduced physical dimensions like particle temperatures and velocities and the coating mechanical properties. Within this project, a large range of parameter field was chosen as long as stable plasma working conditions permitted. The influence of preparation on the microstructure analysis and data of mechanical coating measurements are shown.

Titanium powder has been sprayed with nitrogen or Ar/H 2 d.c. plasma jets flowing in air. Particles have been collected at several distances downstream of the nozzle exit. In the first 40- 60 mm, convective movements created within the liquid droplets entrain homogeneously nitrogen and oxygen in the particle cores. Farther downstream, convection is less important and absorption of nitrogen and oxygen is controlled by diffusion from the particles surface. After solidification induced by high quenching rates (in the order of 10 K/s) due to different cooling means, particles are composed by a superficial layer which is an oxi-nitride of titanium and in their core by a solid solution α-Ti containing both nitrogen and oxygen.

Thermal spray technology became established in various parts of industry (e.g. aircraft industry, medical industry etc.). Some of the applications are highly sensitive, therefore the coating quality plays an important role. Most of the common quality control methods are based on destructive testing methods which are undesirable for economical reasons. In recent years the interest in non-destructive online measurement methods increased and still is growing. More detailed knowledge of the relationship between process parameters can help to improve the coating quality standards and to understand different phenomena in thermal spraying. This will make online process control possible and improves the acceptance of thermal spray technology. In this paper results of fundamental studies on the atmospheric plasma spraying process (APS) are presented. Al 2 O 3 and NiCr powder were sprayed as model materials. In the experiments modem on-line monitoring systems were used to investigate the entire process from gun parameters to the coating quality. The faces in this paper are set on investigations of the in-flight properties of spray particles with the particle sensor DPV 2000 and their relationship with resulting coating properties. A method will be presented to extract characteristic values (velocity and temperature) out of the spray process with DPV 2000. With this method the APS-process was monitored and process parameters were correlated with main coating properties.

Plasma sprayed MCrAlY bondcoats play a major role in thermal barrier coatings. During service, oxide forms on both sides of the bond coat and must be minimized to prevent coating failures. Along with powder chemistry, coating microstructure significantly influences oxide growth. It is known that both coating microstructure and coating strength are strongly related to plasma spraying parameters. This present work examines the effect of inflight particle properties on the adhesion strength and microstructure of NiCrAlY and NiCoCrAlY bondcoats. The relation between particle velocity and temperature and coating properties is particularly important. Relatively small changes in spray parameters such as arc current and gas flows can have a major impact on sprayed particles and consequently coating microstructure. Through online control of particle states it is expected that the quality of plasma-sprayed MCrAlY coatings can be significantly improved.

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.

Numerous factors, such as the spray parameters and non-predictable conditions, during the spraying process influence the properties of thermally sprayed coatings. However, the condition of the particles at the moment of impact on the substrate is the most crucial factor for the morphology and the mechanical properties of a coating. Thus, nowadays particle analyzing systems are employed in order to explain the relation between the process parameters and the properties of a layer. Yet, modeling of thermal spray processes is conducted disregarding particle parameters and only correlating process parameters with layer properties. This article presents a new approach on process modeling, for relating spray parameters to particle-inflight conditions. The modeling of the particle properties in relation to the process parameters shall allow conducting targeted adjustments during the running process, to optimize particle properties. This method will enable to influence coating properties during the spraying process, eliminating the influence of unpredictable environmental or process-related disturbances. In a series of experiments, spray beam properties were measured in an HVOF thermal spray process with agglomerated and sintered WC-Co powder. Spray parameters were correlated to the particle-in-flight conditions, which were measured by utilizing two particle analyzing systems.

Demands on functional coatings with high dimensional accuracy and high surface quality has led to increasing interest in processing of very fine powder grades in a particle size range < 25 µm. Fine powders are not only showing a distinct potential for application of thin and dimensionally accurate coatings, but are also very promising for the production of dense and homogeneous coatings with improved mechanical properties. The large specific surface of fine powders is allowing for relatively low thermal energy levels that are introduced into the process. Nevertheless this also requires a very sensitive temperature control, to prevent overheating of the particles. The reduction of the thermal energy level is resulting in significant advantages particularly for the usability of the HVOF process for coating of inner diameters. Within this work in-flight particle properties of ultrafine carbide powders were analyzed. The studied HVOF process allows the adjustment of a broad parameter range by utilization of a hydrogen stabilized liquid fuel combustion process. A conventional straight nozzle type as well as a curved nozzle for internal spraying was studied. For a further assessment of the potential of ultrafine carbide powders also spray trials with a plasma spraying system have been made.

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coating properties. At the same time innovations in the field of feedstock materials are supporting this trend. The combination of modern thermal spraying systems and new material concepts has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni basis (e.g. NiCrBSi) or metal matrix composites (MMC, e.g. WC/Co or Cr 3 C 2 /NiCr). The work described in this paper focuses on that combination and intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems (kerosene driven HVOF system K2, 3- cathodes APS system TriplexPro-200/-210) using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry (LDA). Additionally, resulting coatings are analyzed metallographically with regard to their properties and a correlation with the particle in-flight properties is given. The experimental work is accompanied by computational fluid dynamics (CFD) simulations of spray jet and particle velocities, leading to a comprehensive analysis and characterization of the achievable particle properties with state-of-the-art HVOF and APS systems.

In the course of this investigation, thermal spraying with different fuel and shroud gas combinations was investigated in terms of its effect on the in-flight particle properties (temperature, velocity) and on the final coating properties (coating thickness, porosity, oxygen content and corrosion behaviour). Independent on the shroud gas, the particle in-flight temperature and velocity were highest when using ethylene as fuel gas and lowest when using propane. Methylene resulted in intermediate properties. The change in the shroud gas from air to nitrogen generally resulted in lower in-flight particle temperatures and also lower velocity. The coating properties in terms of porosity and oxygen content directly correlated to the particle in-flight properties. With decreasing velocity and increasing temperature, the porosity and the oxygen content increased, respectively. The corrosion behaviour of the nickel coatings was studied in 0.5 M sulfuric acid media by means of potentiodynamic polarization curves. Good corrosion properties were observed when methylene and air served as fuel gas and shroud gas, respectively.

The influence of spray parameters on particle with the improved plasma gun was studied by using SpryWatch-2i System. The results show that the particle velocity increases and exceeds the velocity of sound when using a new designed nozzle (C2 nozzle) during the plasma spraying. Under a certain condition, the particle velocity varies as the spray distance rise firstly, decline then, as the primary gas pressure increases, the particle velocity increases, the particle temperature drops firstly then increase, the velocity increase a little. But as the steadily increase of the flow rate, the influence on the particle temperature drops, the influence of arc voltage on particle velocity and temperature is slightly. Arc current influent little on particle temperature, but the particle velocity increase in a range.

Previous studies have shown that gas shrouding is an effective means for controlling oxidation during HVOF spraying. In this present work, the authors attach a gas shroud to an oxyfuel torch with a longer barrel to further investigate the correlation between the state of HVOF sprayed particles and the density and oxygen content of the resulting layers. It is shown that with gas shielding, extended barrel length, and optimized spraying parameters, it is possible to accelerate powder particles to a velocity of over 750 m/sec with maintaining a high molten fraction, thereby producing very dense (zero porosity) stainless steel layers with oxygen contents less than 0.2% by weight. Paper includes a German-language abstract.

Thermal spraying technology still suffers from a lack of reproducibility due to uncontrollable factors during the process. Current methods of process control by means of observing process parameters like gas- and powder flow are insufficient to guarantee a constant quality of coatings, while a direct analysis of the deposited layer is time- consuming and can only be conducted after the process. Furthermore, recently developed mathematical models which correlate process parameters to coating properties are not applicable for all materials. As the particles’ behavior during the process affects the coating properties, a direct process control by the observation of the particles seems expedient. This method is applicable on running processes and thus avoids defective production. In this study, HVOF spraying experiments were conducted. The in-flight particles’ behavior was investigated using an optical diagnostic system, while coating properties were analysed by metallographical methods.

The present study aims to elaborate the particle in-flight behavior during plasma spraying and its significance in determining the microstructure and mechanical properties of plasma sprayed yttria partially stabilized zirconia (YPSZ) based thermal barrier coatings (TBCs). The as-sprayed YPSZ coatings were characterized in terms of the defects (such as porosity, unmelted particles, cracks and micro-cracks), hardness, elastic modulus and fracture toughness. The results showed that the total defects percentage, porosity, unmelted particles and crack content were found to decrease significantly with the improvement of temperature of in-flight particles. The mechanical properties were associated with the microstructure of these coatings, such as total defects, porosity, unmelted particles and cracks. It was confirmed that the mechanical properties, including hardness, elastic modulus and fracture toughness, notably enhanced with the total defects, porosity, unmelted particles and cracks decreased. The SAPS (supersonic atmospheric plasma spraying) coatings sprayed at 3401 } 3.76 °C and 482 ± 2.18 m/s and a spraying distance of 100 mm possessed the lowest microstructural defects percentage and the most favorable mechanical properties among the 15 coatings.

The aim of this study was to compare different processes for a spraying NiCrBSi powder in order to understand how the coating properties are influenced by the particle characteristics. For that, three processes were tested: atmospheric plasma, HVOF and flame spraying. The plasma torch was a classical F4 gun from Sulzer-Metco, the HVOF gun was the CDS system manufactured by Sulzer-Metco and the flame spray gun was a Castolyn 8000 system. For each spraying process, the particles were analyzed using the DPV 2000 (Technar Ltd) at the distance corresponding to the coating build up. Particles velocity and temperature data were then correlated to the coating quality. Results indicate that the choice of the spraying technique induces important modifications in the coating properties in terms of adherence, porosity level, hardness and Young modulus. Changes in particle velocity and temperature are the key parameters determining the coating properties.

This work investigates the effect of atmospheric plasma spraying (APS) parameters on in-flight particle properties, splat morphology, and coating microstructure for conventional and nano-size YSZ powders. Particle temperature and velocity were measured using a dual-slit velocimeter and individual splats and coating microstructures were examined in a scanning electron microscope. The results show that total porosity increases with decreasing arc current and increasing stand-off distance and that conventional powder coatings have higher total porosity at higher arc currents than coatings made from nanopowder. The effect of substrate temperature on splat formation was also assessed. Splat flattening and circularity increase with increasing substrate temperature, particularly for nanopowders.

The effects of the plasma spray process parameters on the microstructure and properties of coatings have been recognized for a long time. It is now clearly proved that the quality and the properties of the deposits are strongly dependent on the in-flight particle characteristics as well as on the splat morphology. The aim of this study was to examine in detail: firstly the interaction between an Al 2 O 3 /13%TiO 2 powder and the spraying process, secondly the splat formation in order to understand the morphology of the deposits. A good way to reach a better understanding of the particle/ plasma interaction is to determine the particle parameters at impact. The particles parameters: velocity, temperature and diameter, prior to their impact on the substrate were measured by the DPV-2000 diagnostic system. The effect of particle velocity, temperature and size on the splat morphology and deposits properties for different plasma conditions were examined. The splat morphology was characterized using the Sommerfeld dimensionless number, K; the value of which is directly related to the impact mode. These analyses were correlated to microhardness, roughness and to the tribological properties of the coatings.

A computational fluid dynamics model for understanding the HVAF process and the influence of the process parameters on the particle flight properties is investigated. Achieving this objective involves a novel approach to modeling the HVAF process with pressure inlet boundary conditions and integration of the mixing chamber. The study comprises the prediction of the flow fields described by a set of equations consisting of continuity, momentum, energy, and species transport. These equations are then solved with realizable k-ε turbulence model, a two-step chemistry model and eddy dissipation model to simulate the combustion reaction. Consequently, the interaction between the CoNiCrAlY alloy particles and the flow is modeled using a Lagrangian approach considering the forces acting on the particles and the heat transfer. The results show that the combustion chamber pressure is mainly affected by the compressed air and propane parameters. Furthermore, the flight behavior of the smaller particles is significantly influenced by the gas flow, while the larger particles tend to maintain their momentum and energy. Through the simulation model, an in-depth process understanding of the HVAF process can be achieved. More importantly, the model can be used as a tool for efficient process development.

Complexity in dynamics and mechanism of supersonic flame formation and effects of processing variables has made the understanding of interaction of particles and flame a difficult task. Lack of such understanding limits the possibilities of controlling the process to obtain desired in-flight particles temperature and velocity and consequent particles state. This problem is even more pronounced in TS systems with no dedicated decoupled temperature and velocity controlled regime. Different approaches based on total volume flow, back pressure and fuel to oxygen ratio have been examined to address the robustness of each approach to control the temperature and velocity. WC-CoCr material was used employing DJ-2600 torch. A guideline to control the in-flight particles temperature and velocity based on process variables is provided. A process map was developed to establish a correlation between process, in-flight particles state, microstructure, properties and performance.