Electromagnetic acceleration plasma generators, which are called Magneto-Plasma-Dynamic (MPD) arcjet generators, can produce higher-velocity, higher-temperature and higher-density plasmas than those of conventional thermal plasma torches, because MPD arcjet plasma is efficiently accelerated by electromagnetic body forces in MW-class input power operation. These properties are effective for deposition of rigid coatings adhering strongly to substrate surfaces. In the present study, we newly developed an ablation type MPD arcjet generator for titanium nitride (TiN) reactive spray coatings. The coatings were deposited onto steel substrate. The phase structure and the composition of the coatings were analyzed by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), and their Vickers hardness were measured. These analyses showed that the MPD spray process could successfully form dense and uniform titanium nitride coatings. The properties of the titanium nitride coatings were highly sensitive to the titanium cathode diameter and discharge current.

In most small and medium enterprises (SMEs) that conduct plasma spraying, a one-cathode, one-anode plasma generator (OCOAPG) is used due to its cost-effectiveness. To achieve high deposition rates, the highest possible fraction of the injected powder has to be melted and accelerated toward the substrate. A sufficiently long and reproducible residence time in plasma is needed that is adequate in relation to the amount and size of the particles This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. In OCOAPG, an arc is operated between a cathode and a central tubular anode, which causes different instabilities in the effluent plasma jet. Due to the instable interaction between the plasma jet and the carrier gas jet continuously incorporating the powder, fluctuations occur resulting in reduced coating quality. Coating systems with a higher number of electrodes (and hence using several arcs) show higher stability and therefore can provide higher coating quality. However, due to their complexity and cost intensity, the investment hurdle for SMEs usually turns out to be too high. Recently, research steps to improve the plasma spraying process with OCOAPG have been undertaken by using a controllable current source to create a uniform interaction between the particle and the gas jet. As the movement of the anodic arc attachment can be actively controlled by current pulses, the plasma jet can be lengthened and shortened at periodic intervals. Furthermore, by pulsing the particle delivery synchronously to the power modulation, an improved particle penetration and consistent residence times can be achieved. First diagnostic results, including example coatings, are presented and discussed within the paper.

Due to the growing application fields, the development of plasma generators has become increasingly important in recent years. Nowadays, DC-plasmas used for a plethora of coatings on metallic surfaces, metallization of plastic materials, surface cleaning and activation, plasma welding and spacecraft propulsion systems. The use of plasma generators for surface coating has grown enormously in the field of thermal insulation and wear-resistant coatings, especially with regard to deposition of refractory oxide ceramic layers using powders such as aluminum oxide, alumina / titania, zirconia and chromium oxide. In addition, for the plasma-assisted surface treatment, generators for both wide extended planar surfaces, as well as structures that are more complex have designed and industrially established. Nonetheless, new plasma sources still developed to provide more-efficient, tailor-made solutions due to their specific characteristics. Within this paper, a brief overview of the new innovative development of DC-plasma generator “Penta” shown. Thereby the special setup, first diagnostic results, including coatings, presented and discussed. The paper concludes with an outlook for future diagnostic measurements and possible future applications for this DC-plasma torch.

The use of plasma generators for thermal surface coating has grown enormously in the field of thermal insulation and wear-resistant coatings, especially with regard to deposition of refractory oxide ceramic layers using powders such as aluminum oxide, alumina / titania, zirconia and chromium oxide. Nonetheless, innovative plasma sources are still being developed to provide more-efficient, tailor-made solutions due to their specific characteristics. A brief overview of the DC-plasma generator Mettech “Axial III” are shown, which generator are allowed to use the central injection (advantage for suspension applications). In this paper first diagnostic results characterized the physical behaviour and special plasma symmetry, which can be described by three partial plasma jets. Thereby computer tomography (CT), high speed camera investigations coupled with time-adjusted current-voltage measurements, particle parameter detections and first resulting coatings are presented and discussed. The complete paper concludes with an outlook for future diagnostic measurements and possible future applications for this DC-plasma torch.

Plasma generators are frequently used in a wide field of applications. These applications include thermal coating, plasma-welding, preprocessing of surfaces, sterilization and propulsion systems. The industrial use of plasma generators demands a high level of accuracy and reproducibility which has to be monitored throughout the process. Rather than analyzing results of e.g. a coating process (by electron microscopic) one prefers to have an online assessment of the plasma generator. The plasma generated is defined by various parameters (e.g., temperature, conductivity) which can be measured with suitable measuring systems. Determining all plasma parameters in one setup is very costly and requires a big setup. In industrial facilities it is often suitable to measure one or two physical quantities and derive the plasma parameters from these measurements. This paper focuses on the evaluation of the electric current consumed by the plasma generator and the sound emitted by the whole system. A quick analyzation of these signals - that may result in an online assessment -was accomplished by using the wavelet transform. Multiple experiments were performed to gain reliable and significant data (behavior of the arc/plasma).

In most plasma spraying SMEs a One-Cathode-One-Anode-Plasma-Generator (OCOAPG) is used due to its cost-effectiveness. To achieve high deposition rates, the highest possible fraction of the injected powder has to be melted and accelerated towards the substrate. Adequate to the amount and size of the particles, a sufficiently long and reproducible residence time in plasma is therefore needed. This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. In OCOAPG an arc is operated between a cathode and a central tubular anode, which causes different instabilities in the effluent plasma jet. Due to the instable interaction between the plasma jet and the carrier gas jet continuously incorporating the powder, fluctuations occur resulting in reduced coating quality. Coating systems with a higher amount of electrodes (and hence using several arcs) show higher stability and therefore can provide higher coating quality. However, due to their complexity and cost intensity, the investment hurdle for SMEs usually turns out to be too high. Recently, research steps to improve the plasma spraying process with OCOAPG have been undertaken by using a controllable current source to create a uniform particle gas jet interaction. As the movement of the anodic arc attachment point can be actively controlled by current pulses, the plasma jet can be lengthened and shortened at periodic intervals. Furthermore, by pulsing the particle delivery synchronously to the power modulation an improved particle penetration and consistent residence times can be achieved. First diagnostic results, including coatings, are presented and discussed within the paper.

The present paper examines a hybrid DC arc thermal plasma torch with gas-water stabilisation and its application for thermal spraying. The torch was worked out based on the water-stabilised plasma torch WSP, main features of which are high temperature of the generated plasma and high powder through-put. In the hybrid torch the cathode part is modified in the way like in gas torches, which not only provides proper stabilisation of the arc in this region and cathode protection from contact with stabilising water but also allows to vary properties of the generated plasma. Change of the secondary gas flow rate, which is usually argon, results in strong changes of the plasma gas density, while the energy balance of the torch remains almost unchanged. This in turn leads to modification of the plasma jet properties. The paper describes effect of the plasma torch parameters on behaviour of the generated plasma jet and injected powder particles. The results show how particle velocities follow changes of the plasma jet properties according to the plasma torch operation parameters. Both increase of arc current and argon flow rate provided increase of particle velocities.

This paper presents the numerical simulation of the plasma flow into a dense atmosphere. The plasma generation is performed by the simple model developed by Eichert. Two models are exposed to take in account the arc fluctuation inside the anode. They permit us to simulate plasma puffs convected into the flow. The aim of this study is to compare these two models with experiments and to determine which one is the most relevant.

In magneto-plasma-dynamic (MPD) arc jet generators, plasma is accelerated by electromagnetic body forces. Silicon nitride reactive spraying was carried out using an MPD arc jet generator with crystal silicon rods and nitrogen gas. Because higher-velocity, higher-temperature and higher-density and larger-area plasmas are produced with the MPD arc jet generator than those with conventional thermal plasma torches, nitriding of silicon can be enhanced. A dense and uniform β-Si 3 N 4 coating 30 µm thick was formed after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 kA and a substrate temperature of 700 °C. The Vickers hardness reached about 1300. Furthermore, silicon carbide and aluminum nitride sprayings were conducted with the same spraying system. Surface modification is under study with lots of chemically reactive gases. All results showed that the MPD arc jet generator had high potential for spraying and surface modification.

The process properties for DC plasma spraying are affected by the arc as the source of energy for plasma generation. For instance, the position of the anode attachment point, the arc movement and the arc formation are significantly influencing process stability, reproducibility and coating quality. Following a qualitative and objective assessment of the complete arc movement and plasma generation is leading to an improved process characterization. Therefore, a preventive arc-based analytical method for DC plasma spraying is developed. Based on a representative data volume and realized by an automatic analysis of high-velocity recordings of the arc dynamics and correspondent arc voltage measurements, the evaluations are carried out. A developed software algorithm automatically detects for all images of the video sequence (at least 3200 images) the anode center axis, the arc orientation and the position of anode attachment point on the anode surface referring to the anode center, with simultaneous compensation of aberrations. This allows an objective assessment of the complete arc movement. In further investigations, the detection limit of the developed measuring system is determined and the effects on the arc behavior and the coating process could be quantified. Thus, the developed automated analysis of the arc dynamics in the DC plasma generator corresponds to an arc-based process characterization in DC plasma spraying process with relevance to developers (e.g. new anode nozzle designs) and end users (process control).

Instabilities and fluctuations of the plasma jet can have a significant influence on the particle in-flight temperatures and velocities, thus affecting the properties of plasma sprayed coatings. Presented in this paper is a novel method for capturing the effects particles are exposed to in the plasma spraying process. High-speed camera images of a plasma jet generated by a cascaded three cathode plasma generator (TriplexPro-210) are recorded for varying operating conditions. The images are processed using the inverse Abel transform. This transformation accounts for the fact that the images represent a 2-D projection and generates correct intensity values of the plasma jet images. These images are then combined with particle tracks resulting from CFD simulations of the plasma jet to match the particles path with the recorded plasma jet. This new method allows a precise description of the plasma intensity experienced by individual particles with a high temporal resolution. The results show a high sensitivity of the method, it can even detect the influence of the plasma jet originating from the cascaded triple arc plasma generator, which is considered as rather stable, on the particles.

In this study, a ZrO 2 – 7 % Y 2 O 3 (YSZ) powder (-90 +16 µm) was nanostructured by high energy ball milling and sprayed using a modern three-cathode plasma generator TriplexPro- 210 as well as a conventional plasma generator F4MB-XL. The parameters were varied in order to investigate their influence on build-up, microstructure and properties of the thermal barrier coatings (TBC). Powders and coatings were characterized in terms of their morphology, microstructure and phase composition by means of light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray analysis (XRD). Thermo-shock behavior of TBC was evaluated using thermal cyclic tests at 1300 °C and 1150 °C. The results show that the milled powder contained nano-sized particles. TBC from the nanostructured powder by TriplexPro-210 had high porosities and numerous fine pores, leading to lower microhardness and higher thermos-shock resistance than the reference TBC.

In plasma spraying, compared to other thermal spraying process variants, only a small part of the available energy is used to build up a coating. Another peculiarity of this process is the relatively strong oxidation of the sprayed metallic particles, caused by the high temperatures and turbulent flow of the plasma jet in combination with the ambient air. A promising solution for increasing energy efficiency is a solid shroud that surrounds the plasma jet and thus prevents air entrainments from mixing with the plasma gas. The primary goal of this study is to develop a numerical model to investigate the effect of an external fixed nozzle extension on the plasma jet as a shroud. To this end, the existing simulation models of the plasma jet from the previous works of the authors were extended to model a solid nozzle extension at the outlet of a three-arc plasma generator. Furthermore, the length and diameter of the nozzle extension were parametrized to investigate their effects on the plasma temperature and the turbulence of the flow. This model can be used to optimize the geometry of the nozzle extension based on experimental measurements to adapt it to the flow conditions of the plasma jet. The results revealed that the plasma temperature could be increased using the nozzle extension, thereby raising the energy efficiency to melt the particles in plasma spraying.

We present the results of an investigation of the effects of a travelling magnetic field (TMF) on the plasma flow using a commercial program package. The argon plasma generation in the electric arc and the Lorentz force induced by the TMF are simulated with specific equations, which are derived from the magnetofluiddynamic equations using appropriate simplifications. First calculations confirm the predicted effects.

Conventional DC plasma torch designs lead to a circular cross-section of the emanating plasma jet. Consequently in surface treatment applications the plasma jet hits the substrate within a limited circular working area. Large scale workpieces therefore have to be scanned resulting in a time-consuming procedure. The innovative DC plasma torch system LARGE is characterized by the arrangement of the anode and the cathode opposite to each other on a common axis with a variable distance. The central body of the torch between the electrodes is divided into electrically insulated cascade plates. The plasma gas is injected perpendicular to the torch axis. Passing through the arc, the gas is transferred to the plasma state and leaves the torch laterally through a slit as a plasma jet with extended stripe width. The plasma torch LARGE is investigated by electrical, optical and enthalpy probe diagnostics. Shrouding the electrodes with an inert gas and feeding reactive gas mixtures as main plasma gas allow the torch to be used for plasma chemical reactions, too. Preliminary applications focus on preheating, surface modification of paper and plastic materials as well as on sterilization of nutrition packaging. The capability of plasma enhanced CVD is examined experimentally.

Plasma spraying is a key industrial coating process that exhibits intricate nonlinear interactions among process parameters. This complexity makes accurate predictions of particle properties, which greatly affect process behavior, very challenging. Specifically, particle velocities and temperatures profoundly impact coating quality and process efficiency. Conventional methods often require empirical correlations and extensive parameter tuning due to their limited ability to capture the underlying physics within this intricate system. This study introduces Physics-Informed Neural Networks (PINNs) as a solution. By seamlessly integrating known physical laws and constraints directly into the model architecture, PINNs offer the potential to learn the underlying physics of the system. For comparison, Artificial Neural Networks (ANNs) are also developed. Computational Fluid Dynamics (CFD) simulations of a plasma generator and plasma jet model provide data to train both ANN and PINN models. The study reveals an improvement in particle velocity prediction through the proposed PINN model, demonstrating its capability to handle complex relationships. However, challenges arise in predicting particle temperature, warranting further investigation. The developed models can aid in optimizing the plasma spraying process by predicting essential particle properties and guiding necessary process adjustments to enhance coating quality.

The use of gas mixtures containing hydrocarbons for plasma generation results in higher plasma enthalpy because, molecular gases must dissociate before ionization which requires larger energy input. The torch developed at CACT which operates with CO 2 +CH 4 gas mixtures was used for coating deposition with input power in a range of 30 to 45 kW. This study was focused on the effect of CO 2 +CH 4 mixtures on the particle parameters during spraying of Ni alloy powder. Results of gas composition analysis at various distances from the nozzle exit are presented. The particle in-flight conditions, coating microstructure, and deposition efficiency also were analyzed.

Thermal spray torches commonly use argon for plasma generation. Low thermal properties of argon, however, limit the thermal efficiency of the torches. Use of molecular gases, which must dissociate before ionization, requires larger energy input resulting in enthalpy increase of the plasma. In this paper, the effect of various gas compositions (Ar, Ar+H 2 , and CO 2 +CH 4 ) on the torch voltage-current characteristics, power and thermal efficiency were studied. At the same time, in-flight YSZ particle conditions were compared. The higher thermal conductivity and efficiency of CH 4 +CO 2 gas mixture produce more favorable sprayed particle conditions, in particular temperature. At a 50mm spray distance, YSZ particle temperatures were 2470°C and 2896°C for Ar+H 2 and CH 4 +CO 2 , respectively. Typical arc voltage for the torch operating in CO 2 +CH 4 was 130-180V compared to 45-60 V for Ar+H 2 . Thermal efficiency was also 20-40% higher.

This work provides a new in-situ measurement method for the analysis of the spray-spot geometry and the thermal properties of the coating. The new approach is based on infrared detection of the thermal radiation from the coating surface combined with a subsequent automated spray-spot characterization. With this method it is possible to describe the geometry, the axis-position of the torch, the powder injection properties, and the temperature distribution in of the spray-spot. Especially for the automated production in high quantity the spray-spot analysis is a useful assistance for the operator because the detector reacts very sensitive on small changes of the process conditions. With regard on important fields of application (e.g., gas turbine production) the sensor is suitable to detect drifting spray system parameters. Also, the progression of wear at the nozzle, injector and electrode can easily be estimated. In recent research the in-situ spray spot analysis is being developed further for the characterization of multipair electrode plasma generators.

Three-cathode plasma spraying is characterized by high process robustness in terms of particle in-flight properties due to high stability of plasma jet. However, during coating parts with complex geometries, process parameters such as stand-off distance and spray angle usually cannot be kept at optimized conditions. In this study, the process robustness during three-cathode plasma spraying of a novel FeCrMnBC alloy was investigated by varying stand-off distance and spray angle to simulate spray conditions for complex geometries. A three-cathode plasma generator TriplexProTM-210 was used to spray FeCrMnBC powder with a fraction of -45 +20 µm onto substrates of cast iron EN-GJL-250. The stand-off distance and the spray angle were varied from d=90 mm to d=110 mm and between ø=90° and ø=60°, respectively, while the other process parameters were kept constant. The results revealed that the reduction of spray angle caused an about 15 % decrease of coating thickness, about 40 % porosity increase and about 15% increase of bond strength. In contrast, the variation of stand-off distance changed only insignificantly the coating thickness, porosity and bond strength. The variation of stand-off distance and spray angle did not result in significant changes in the coating microhardness. The reduction of spray angle resulted in significant worsening of the corrosion protection ability of the coating system.