Electric arc dynamics in plasma torches have an influence of the plasma jet stability and could impact coating properties. Depending on the plasma torch design, arc voltage fluctuations may vary from a hundred to only a few percent of the mean arc voltage. A cascaded-anode plasma torch particularly leads to very low arc voltage fluctuation, due to the presence of neutrodes limiting the amplitude of arc fluctuations. However, the electric arc dynamics and electrode erosion process are still poorly understood in this kind of plasma torches. The aim of this work is to refine the knowledge on the influence of the nozzle’s diameter on electric arc dynamics and on zirconia-yttria coating properties. Two plasma-forming gas compositions were studied, connecting several analysis methods (end-on imaging, arc current and arc voltage time monitoring, plasma light fluctuations at nozzle exit, thermal balance evaluation and in-flight particles assessment) to coatings characterization (porosity and hardness measurements). Reducing the nozzle’s diameter from 9 to 6.5 mm results in higher arc voltage fluctuations and lower thermal efficiency of the plasma torch. This is due to the warm plasma gas being more evenly distributed in the anode nozzle. After testing, surveys of the 6.5-mm-diameter nozzle show significant longitudinal wear, which may highlight an axial movement of the electric arc on the anode’s surface, leading to high arc voltage fluctuations. Finally, the particle velocity is higher in a 6.5-mm-diameter nozzle, resulting in a lower particle temperature and a higher coating porosity.

The demand for utilizing the Axial III Plus plasma spray system has prompted the numerical modeling of its arc plasma torch, integral to creating a digital twin of the suspension plasma spray process. The Axial III Plus plasma torch is a highly efficient and reproducible tool with a unique three-torch exit jet arrangement that allows the axial injection of solid/liquid feedstock, not possible with a single cathode/anode–plasma torch setup. In this study, we employ the local thermodynamic equilibrium approximation of the magnetohydrodynamic (MHD) model to simulate plasma flow inside the single gun plasma torch of Axial III, considering electrode–plasma interactions. Describing electric arc dynamics during restrike proves intricate; thus, a restrike model is used relying on cutoff criteria based on a threshold value Eb of the predicted radial electric field at the electric arc fringes. The model successfully replicates typical electric arc behavior and saw-toothed voltage profiles during restrike, notably capturing the characteristics of the Axial III anode’s unique and complicated design variations in electric arc motion and its corresponding arc voltage profile. Analysis extends to studying variations in Eb, which directly influence mean electric arc length, arc voltage, and mean arc spot time, potentially impacting energy generation and losses in the torch. These findings provide a valuable foundation for future simulations of this design, especially with swirl gas injection and ternary gas mixtures.

In a DC plasma spray torch, the plasma-forming gas is the most intensively heated and accelerated at the cathode arc attachment due to the very high electric current density at this location. A proper prediction of the cathode arc attachment is, therefore, essential for understanding the plasma jet formation and cathode operation. However, numerical studies of the cathode arc attachment mostly deal with transferred arcs or conventional plasma torches with tapered cathodes. In this study, a 3D time-dependent two-temperature model of electric arc combined with a cathode sheath model is applied to the commercial cascaded-anode plasma torch SinplexPro fitted with a wide single cathode. The model is used to investigate the effect of the cathode sheath model and bidirectional cathode-plasma coupling on the predicted cathode arc attachment and plasma flow. The model of the plasma-cathode interface takes into account the non-equilibrium space-charge sheath to establish the thermal and electric current balance at the interface. The radial profiles of cathode sheath parameters (voltage drop, electron temperature at the interface, Schottky reduction in the work function) were computed on the surface of the cathode tip and used at the cathode-plasma interface in the model of plasma torch operation. The latter is developed in the open-source CFD software Code_Saturne. It makes it possible to calculate the plasma flow fields inside and outside the plasma torch as well as the enthalpy and electromagnetic fields in the gas phase and electrodes. This study shows that the inclusion of the cathode sheath model in the two-temperature MHD model results in a higher constriction of the cathode arc attachment, more plausible cathode surface temperature distribution, more reliable prediction of the torch voltage and cooling loss, and more consistent thermal balance in the torch.

An atmospheric microwave plasma argon was used for the inactivation of bacteria E. coli.The employed device, called Axial Injection Torch (or TIA for Torche à Injection Axiale), consisted of a microwave power source, a waveguide and a gas supply system.Using this argon plasma source, we studied the effects of the exposure time, the exposure distance, the input power, and the gas flow rate on the reduction rate of Escherichia coli cells.The first part of the study was carried out with a static sample exposed to the plasma and then in the second part the sample was set in motion relative to the plasma jet.A log reduction number of E. coli of 4 (10 -4 CFU/mL) was obtained with UV and active species, for UV only a log of 1 (10 -1 CFU/mL) was obtained.

In plasma spraying process, the plasma jet stability depends on the electric arc movement inside the plasma torch. For conventional plasma torches with rod cathode and a large anodic surface, arc roots are free to move along the anode wall, leading to high plasma jet fluctuations. To overcome these instabilities, cascaded-anode plasma torches have been developed, reducing considerably the electric arc movement and thus the plasma jet fluctuations. Unfortunately, a small number of studies reports on the behaviour of theses torches including the arc instabilities and on electrode erosion process as well. In this work, several diagnostics have been set up to further understand the cascaded-anode plasma torch behaviour under different sets of operating conditions. Influence of plasma forming gas composition on the electric arc dynamic has been studied through anodic imaging. The issue of the intense plasma radiation masking is discussed since the light emitted by the arc column interferes with anodic arc roots observations. Multiple constricted arc roots have been observed and a preferential site for anodic arc root has been identified for all experimental conditions. The addition of a secondary plasma forming gas promotes a higher number of reattachments. The influence of the electric arc movement on plasma jet stability is also studied by following the plasma jet brightness fluctuations. The low voltage fluctuation frequency is also observed on brightness spectra, which means that these fluctuations could have an impact on in-flight particle treatment.

This study presents an analysis of a three-dimensional unsteady two-temperature simulation of atmospheric pressure direct current electric arc inside a commercial cascaded-anode plasma spray torch; it coupled the arc model with the torch electrodes and used an open-source computational fluid dynamics software (code_saturne). The previously published models of plasma spray torch either deal with conventional plasma torches or assume local thermodynamic equilibrium in cascaded-anode plasma torches. The paper presents the computation of the two-temperature argon plasma properties, compares two enthalpy formulations that differ in association of the ionization part of enthalpy and finally demonstrates the influence of the radiation heat loss data by comparingthe results for two different literature sources. It is the first to compare different enthalpy formulations in the context of plasma torch and discuss the differences in terms of the enthalpy gains and losses. It also explains why an unphysical simulation artifact of electron temperature lower than the heavy species temperature can occur in simulated plasma flow. The solution, then, consists in associating the ionization part of enthalpy to electrons and selecting the appropriate source of the data of radiation heat loss. However, negligible thermal non-equilibrium persists even in the hot core of electric arc, which ensures that the heavy species are heated up by collisions with electrons. The flexibility of the open-source software allows all the necessary modifications and adjustments to achieve satisfactory simulation results. Thus, the paper could be considered as a manual for development of a plasma spray torch model.

Suspension plasma spraying has aroused much research efforts over the two last decades to describe and understand the in-flight mechanisms of suspension plasma processing by developing advanced diagnostics and models. This research field has highlighted the intricate and prevailing dependence of the coating properties, and especially the rich variety of attainable microstructures, on the thermal and dynamic histories of the submicron particles in the plasma jet. This paper gives an overview of the major key points to control from the plasma torch operation up to the coating build-up. The main diagnostics methods are examined and the most advanced models as well. The perspectives focus on the key issues to improve the basic understanding of the in-flight phenomena. They also insist on a necessary multiscale approach to combine the diagnostics and the models in order to form relevant data flows for machine learning algorithms and better process prediction and reliability.

In DC plasma spray torches, anode erosion is a common concern. It mainly depends on the heat flux brought by the arc and on the dimensions and residence time of the arc attachment to a given location on the anode wall. The latter depend, to a great extent, on the attachment mode of the arc on the anode wall. This paper compares the anode arc attachment modes predicted by an LTE (Local Thermodynamic Equilibrium) and 2-T (two-temperature) arc models that include the electrodes in the computational domain. It deals with a commercial cascaded-anode plasma torch operated at high current (500 A) and low gas flow rate (60 NLPM of argon). It shows that the LTE model predicted a constricted anode arc attachment that moves on the anode ring, while the 2-T model predicted a diffuse and steady arc attachment. The comparison between the predicted and measured arc voltage showed that the 2-T prediction is closer to the actual voltage. Also, the post-mortem observation of a new anode ring of the actual plasma torch operated under the same conditions for a short time confirmed a diffuse arc attachment on a new anode.

The cascaded-anode plasma torch makes it possible to get a longer and more stable plasma jet with higher specific enthalpy than conventional plasma torches. It is now used widely, but there are still few models of the cascaded-anode plasma torch. This study developed a 3-D time-dependent model that couples the gas phase and electrodes by encompassing the electromagnetic and heat equations both in the electrodes and gas phase. The model was applied to a commercial plasma spray gun equipped with a single cathode, single cylindrical anode and an inter-electrode insert to fix the average arc length. This paper examines the effect of the boundary conditions for the magnetic vector potential and electric current density on the electromagnetic, velocity and temperature fields of the plasma jet. The model predictions showed that, for such plasma torches where the arc is close to walls, the Biot and Savart formalism is required at the domain boundaries for the magnetic vector potential. They also showed that similar plasma fields could be obtained by imposing an electric current density profile at the cathode tip or by including the electrodes in the computational domain. However, this profile has to be chosen according to the specific design of the cathode, which is not obvious when the cathode has a design different from that of conventional plasma torches with sharp conical tip or rounded tip.

Two common concerns in DC plasma torches are stability of plasma jet and anode erosion. The challenge is how to get a stable plasma jet with minimal anode erosion. This study tackles this question by using either a swirling gas injection or an external axial magnetic field applied to the Oerlikon SinplexPro™ plasma torch. A 3-D, time-dependent MHD model of the plasma torch operation was used to predict the value of the external magnetic field and its effect on the heat flux to the anode and plasma jet stability. The special feature of the model is to couple the gas phase and electrodes that makes it possible to follow the anode temperature evolution. For specific operation conditions (anode of Ø9 mm, 500 A, Ar 60 NLPM), the model predicted that the maximal value of the azimuthal self-magnetic field inducted by the arc current was 0.055 T; it also showed that an external magnetic field of 0.05 to 0.1 T could make it possible to limit the anode erosion without noticeably disturbing the plasma jet issuing from the plasma torch. We expect this approach to help to better understand the arc behavior in commercial plasma torches and control anode erosion.

An original low-temperature atmospheric pressure plasma-enhanced chemical vapor deposition process was used to deposit titanium dioxide thin films The parametric study in dynamic mode deposition aimed at growing an ideal columnar film composed of aligned anatase monocrystals as solar cell photoanode, previously obtained on silicon wafers in static mode deposition. A process parameters optimization was necessary to deposit onto thermally sensitive glass/FTO substrates. In this paper, the morphology, crystallinity and optical transmission of the coatings have been studied. The coatings display a columnar cauliflower-like structure, composed of TiO2 amorphous particles assembly. After deposition, the light transmission properties of the substrate were reduced. As a solution, an ultrasound bath cleaning was set up to enhance the transmitted light through the photoanode.

An atmospheric pressure plasma-enhanced chemical vapor deposition process using a microwave plasma torch has been used for titania thin film synthesis. A dynamic deposition mode was set up to cover a square centimeter surface with a nanostructured TiO2 film. The process parameters were studied and optimized to control the coating crystallinity and morphology and limit the formation of powder in the plasma phase. Contrary to static deposition, the substrate movement promotes a film growth by particles agglomeration in the reference conditions, leading to a cauliflower-like morphology. Then, the precursor proportion in the plasma appears to be determinant in the TiO2 film microstructure. For a precursor flow rate beyond 0.2 slpm, the titania nanoparticles formation in the gas phase is promoted and the thin film is growing by particles agglomeration, leading to a columnar cauliflower-like morphology. At a flow rate of 0.2 slpm, the growth by surface reaction is promoted and the TiO2 film is columnar, where each column is an anatase crystal. After the optimization of the substrate holder movement, it was possible to deposit this last microstructure homogeneously on a square centimeter surface.

The Journal of Thermal Spray Technology could not exist without the ongoing contributions of guest editors and reviewers, who ensure the highest quality of our journal. Manuscripts are reviewed for technical excellence, quality, and relevance by competent reviewers. The editors identify and select these reviewers based on their demonstrable and recognized research expertise, professional experience, critical judgment and foresight, and forward thinking in their field of expertise.