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 3-D time-dependent and two-temperature model of electric arc combined with a cathode sheath model is applied to the commercial cascaded-anode plasma torch SinplexPro. 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 spacecharge 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 of 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 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 cathode sheath model results in a higher constriction of the cathode arc attachment, more plausible cathode surface temperature distribution, more reliable prediction of the torch voltage, cooling loss, and more consistent thermal balance in the torch.

Most models used to simulate plasma torch operation consider only the gas domain with the electrodes included as boundary conditions, imposing a current density profile and temperature at the cathode tip. In order to achieve more realistic simulations, it is necessary to include the electrodes and arc column in the calculation domain. This paper discusses some of the issues, factors, and considerations involved in modeling arc-cathode and arc-anode interactions and their effect on the plasma jet. Descriptions of the arc column and plasma jet are based on the coupling of electromagnetic and fluid equations and require thermodynamic, transport, and radiative property values of the gas mixture. In order to capture the stochastic behavior of the arc, it is assumed that the model is 3D and time-dependent.

In this study, a hybrid plasma spraying process is used to produce particle-reinforced metal-matrix composite coatings on 316 stainless steel. Two injectors are mounted at the output of the plasma gun, one feeding a nickel-base alloy powder, the other feeding a suspension of alumina nanoparticles. Different feed rates, suspension compositions, and alumina particle contents are used and their effects on microstructure, microhardness, porosity, adhesion, and wear behavior are assessed.

Coupling of the electromagnetic and heat transfer phenomena in a non-transferred arc plasma torch is generally based on a current density profile and a temperature imposed on the cathode surface. However, it is not possible to observe the current density profile experimentally. To eliminate this boundary condition and be able to predict the arc dynamics in the plasma torch, the electrodes were included in the computational domain, the arc current was imposed on the rear surface of the cathode, and the electromagnetism and energy conservation equations for the fluid and the electrodes were coupled. The solution of this system of equations was implemented in a CFD computer code to model various plasma torch operating conditions. The model predictions for various arc currents were consistent and indicated that such a model could be applied with confidence to plasma torches of different geometries, such as cascaded-anode plasma torches.

In nuclear plants, the replacement of hardfacing Stellite, a cobalt-base alloy, on parts of the piping system in connection with the reactor has been investigated since the late 60’s. Various Fe-base or Ni-base alloys, Co-free or with a low content of Co, have been developed but their mechanical properties are generally lower than that of Stellites. The 4th generation nuclear plants impose additional or more stringent requirements for hardfacing materials. Plasma transferred arc (PTA) coatings of cobalt-free nickel-base alloys with the addition of sub-micrometric or micrometric alumina particles are thought to be a potential solution for tribological applications in the primary system of sodium-cooled fast reactors. In this study, PTA coatings of nickel-base alloys reinforced with alumina particles were deposited on 316L stainless steel substrates. The examination of coatings revealed a refinement of the microstructure. Under the conditions of the study, the addition of alumina particles did not improve the micro-hardness of coatings but improve their resistance to abrasive wear.

This paper describes the development of a numerical model and explains how it is used to investigate arc-cathode interactions in a plasma arc torch. The model is based on magnetohydrodynamic (MHD) theory and couples Navier-Stokes equations for a nonisothermal fluid with Maxwell’s equations for electromagnetic fields. The equations account for the internal geometry of the torch as well as arc current and gas type and flow rate. They are solved using CFD code and relevant boundary conditions and are shown to provide insight on arc dynamics and the effect of cathode shape on arc behavior.

This paper presents a new approach to overcome arc plasma instabilities in suspension plasma spraying. The method employs a dc plasma torch with a large cathode cavity. This design modification reduces the Helmholtz frequency of the torch, resulting in a new oscillation mode with a very regular voltage signal. In the experiments, droplets of a TiO 2 suspension are injected into the pulsed laminar arc jet in synch with the Helmholtz resonant frequency. Interactions between the plasma and droplets are observed by time-resolved imagine techniques and are discussed.

This study examines the fundamental reactions that, in the solution plasma spraying process, lead to the conversion of the precursor salts to solid material that is deposited onto the substrate. The study specifically focused on the phenomena occurring in-flight and the effect of plasma jet treatment on the mechanical and thermal treatment of the solution injected in the form of a liquid jet. The evolution of precursor droplets in the plasma flow was investigated “in situ” using a shadowgraphy technique. The morphology and structure of material deposited onto smooth stainless steel substrates during single scan experiments were characterized by SEM, GI-XRD and micro-Raman spectroscopy and were correlated to the in-flight observations, in order to evaluate the effect of the plasma-forming gas and solution solvent.

The use of liquid precursors in plasma spraying makes it possible to produce coatings with more refined microstructures than in conventional plasma spraying. Depending on the injection device, the liquid feedstock is injected into the plasma jet in the form of liquid jet or droplets. The instabilities on the liquid-gas interface cause the mechanical break-up of liquids into drops that are subjected to further break-up until the droplets reach a stable state or evaporate. The process break-up may strongly influence the size, trajectories and, therefore, treatment of the droplets in the plasma medium. This study deals with the experimental observation of liquid break-up under plasma spray conditions when using a conventional DC plasma torch with radial injection by means of a pneumatic injection system that can deliver either liquid stream or blobs.

Plasma spraying using liquid feedstock makes it possible to produce thin coatings (< 100 µm) with more refined microstructures than in conventional plasma spraying. However, the low density of the feedstock droplets makes them very sensitive to the instantaneous characteristics of the fluctuating plasma jet at the location where they are injected. In this study the interactions between the fluctuating plasma jet and droplets are explored by using numerical simulations. The computations are based on a three-dimensional and time-dependent model of the plasma jet that couples the dynamic behavior of the arc inside the torch and the plasma jet issuing from the plasma torch. The turbulence that develops in the jet flow issuing in air is modeled by a Large Eddy Simulation model that computes the largest structures of the flow which carry most of the energy and momentum.

A key aspect of the operation of conventional non-transferred Direct Current (DC) plasma torches is the random motion of the arc inside the nozzle. Various plasma gun designs have been developed to limit the arc fluctuations without increasing the heat load to the anode wall that results in surface erosion and anode wear. However, construction of these plasma torches is highly complex while the conventional DC plasma torch consists of small number of elements and is simple to manufacture and maintain. A better understanding of the behavior of the arc-anode attachment and the way it depends on the operating conditions may help to design and operate conventional plasma torches so that the fluctuation of the time-voltage and therefore the time-enthalpy variation, is as low as possible with a fluctuation frequency adapted to the time characteristic of the powder particles in the plasma jet. This study deals with a three-dimensional (3-D) time-dependent modeling of the arc and plasma generation in such a torch operating under the so-called “restrike” mode. The latter is characterized by rather large voltage fluctuations, corresponding to a broad range of conditions used in the manufacturing of plasma coatings. The mathematical model is based on the simultaneous solution of the conservation equations of mass, momentum, energy, electric current and electromagnetism equations. It makes it possible to predict the effect of the operating parameters of the plasma torch on the motion of the anode root attachment over the anode surface, and the time-evolution of arc voltage and flow fields in the nozzle.