Gilles Mariaux

Modeling the restrike mode operation of a DC plasma spray torch

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 a small number of elements and is simple to manufacture and maintain. A better understanding of the behavior of the arc-anode attachment and how it depends on operating conditions may help in the design and operation of 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 (3D) 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 electromagnetic equations. These make it possible to predict the effect of 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.

Finite Element Modeling of the Different Failure Mechanisms of a Plasma Sprayed Thermal Barrier Coatings System

A new finite element model is used to investigate catastrophic failures of a thermal barrier coatings system due to crack propagation along the interfaces between the ceramic top-coat, thermally grown oxide, and bond-coat layers, as well as between the lamellas structure of the ceramic layer. The thermo-mechanical model is designed to take into account a non-homogenous temperature distribution and the effects of the residual stresses generated during the coating process. Crack propagation is simulated using the contact tool “Debond” present in the ABAQUS finite element code. Simulations are performed with a geometry corresponding to similar or dissimilar amplitudes of asperity, and for different thicknesses of the oxide layer. The numerical results have shown that crack evolution depends crucially on the ratio of the loading rate caused by growth and swelling of the oxide layer and also on the interface roughness obtained during the spraying of coatings.

Modeling of plasma spraying of two powders

The behavior of metal and ceramic powders co-sprayed through a plasma jet was simulated using a commercial fluid dynamics model in which the particles are considered as discrete Langrangian entities. Computations were carried out for the plasma jet and the injected particles using (a) a steady-state three-dimensional (3-D) jet and (b) a simplified two-dimensional (2-D) model. An analytical method was used to estimate the appropriate injection velocities for the metal and ceramic particles, injected through opposing nozzles perpendicular to the plasma flow, so that their “mean” trajectories would impinge on the same area on the target surface. Comparison of the model projections with experimental measurements showed that this method of computation can be used to predict and control the behavior of particles of widely different properties.

Arc-Cathode Coupling in the Modeling of a Conventional DC Plasma Spray Torch

The plasma torch is the basis of the plasma spray process and understanding of the electric arc dynamics within the plasma torch is necessary for better control of torch and process instabilities. Numerical simulation is a useful tool for investigating the effect of the torch geometry and operating parameters on the electric arc characteristics provided that the model of arc dynamics is reliable and the boundary conditions of the computational domain are well founded. However, such a model should also address the intricate transient and 3D interactions between the electrically conducting fluid and electromagnetic, thermal, and acoustics phenomena. Especially, the description of the electrode regions where the electric arc connects with solid material is an important part of a realistic model of the plasma torch operation as the properties of electric arcs at atmospheric pressure depend not only on the arc plasma medium, but also on the electrodes. This paper describes the 3D and time-dependent numerical simulation of a plasma arc and is focused on the cathode boundary conditions. This model was used to investigate the differences in arc characteristics when the cathode is included into the numerical domain and coupled with the arc. The magnetic and thermal coupling between the cathode and arc made it possible to get rid of the current density boundary condition at the cathode tip that is delicate to predetermine. It also allowed a better prediction of the cathode flow jet generated by the pumping action induced by the interaction of the self-magnetic field with the electric current and so it allowed a better description of the dynamics of arc. It should be a necessary step in the development of a fully predictive model of DC plasma torch operation.

Modeling time-dependent phenomena in plasma spraying of liquid precursors*

The recently developed plasma spray processes using liquid precursors make it possible to produce finely structured coatings with a broad range of microstructures and, thus, properties. However, coating reproducibility and control of the deposition efficiency are critical to industrial acceptance of these processes. Both depend on time-dependent interactions between the plasma jet and liquid material. Transient and realistic modeling of the liquid spray process may help to increase the understanding of the process. A comprehensive model should involve the formation of the plasma jet inside the torch and the transient specific treatment (break-up, droplet collision, coalescence, evaporation, chemistry) of the liquid material in the plasma jet. If much progress has been recently made on the modeling of the interaction of arc and transverse flow in the plasma torch, further theoretical and experimental research is needed, especially in respect of liquid injection and fragmentation under plasma spray conditions.

Droplet size prediction in ultrasonic nebulization for non-oxide ceramic powder synthesis

HighlightsAerosol formation by ultrasonic nebulization is characterized for spray pyrolysis process.Experimental determination of droplet mean diameter was done for liquids with different properties.Expressions in the literature where tested and taken as a base to propose a new one adapted to the system in study.Good agreement was found between the proposed correlation and experimental results. ABSTRACT Spray pyrolysis process has been used for the synthesis of non‐oxide ceramic powders from liquid precursors in the Si/C/N system. Particles with a high thermal stability and with variable composition and size distribution have been obtained. In this process, the mechanisms involved in precursor decomposition and gas phase recombination of species are still unknown. The final aim of this work consists in improving the whole process comprehension by an experimental/modelling approach that helps to connect the synthesized particles characteristics to the precursor properties and process operating parameters. It includes the following steps: aerosol formation by a piezoelectric nebulizer, its transport and the chemical‐physical phenomena involved in the reaction processes. This paper focuses on the aerosol characterization to understand the relationship between the liquid precursor properties and the liquid droplet diameter distribution. Liquids with properties close to the precursor of interest (hexamethyldisilazane) have been used. Experiments have been performed using a shadowgraphy technique to determine the drop size distribution of the aerosol. For all operating parameters of the nebulizer device and liquids used, bimodal droplet size distributions have been obtained. Correlations proposed in the literature for the droplet size prediction by ultrasonic nebulization were used and adapted to the specific nebulizer device used in this study, showing rather good agreement with experimental values.