J. F. Coudert

Suspension and solution plasma spraying of finely structured layers: potential application to SOFCs

Suspension direct current plasma spraying allows achieving finely structured coatings whose thickness is between few tens and few hundreds of micrometres. Drops (200?300??m in diameter) or liquid jets are mechanically injected in the plasma jet. With radial injection they are rapidly (a few ?s) fragmented into droplets (a few ?m in diameter). The latter are vaporized (in a few ?s) and the solid particles contained in suspension droplets are accelerated and melted by the plasma jet. As in conventional plasma spraying (CPS), much smaller splats (with diameters between 0.2 and 3??m and thicknesses between 30 and 200?nm) are arranged in layers up to form the coating. The low inertia of particles requires spray distances between 40 and 60?mm which induces plasma heat fluxes up to 22?MW?m?2 participating in coating densification. Even more than in CPS, the plasma jet fluctuations, particularly for plasmas containing di-atomic gases, perturb drops penetration and fragmentation. It has been chosen to illustrate difficulties and possibilities of this new method, through the spraying of the three layers of an element of solid oxide fuel cells. Indeed, it requires a dense stabilized zirconia electrolyte, if possible thin (15?20??m) with two porous electrodes: cathode made of perovskite prone to decomposing upon spraying and anode made of two materials (nickel and zirconia) with very different melting points. These components were obtained by spraying ethanol suspensions, with, first, LaMnO3 perovskite particles doped with 10?mol% of MnO2 and 3??m in mean diameter sprayed with pure argon to limit their decomposition and achieve porous coatings, second, Yttria (13?wt%) stabilized zirconia (YSZ) with two different particle size distributions and morphologies for which plasma compositions were adapted, producing in both cases 15??m thick and fully dense coatings, third, porous Raneigh nickel by co-spraying the YSZ suspension and solution of nickel nitrate.

Velocity Measurements for Arc Jets Produced by a DC Plasma Spray Torch

An optical method was used to determine the axial velocity of plasma jets produced by a DC plasma spray torch. Different experimental conditions were tested in order to systematically study the influence of the working parameters on the plasma velocity. In this way, the arc current ranged between 200 and 600 A, the gas flow rate between 30 and 80 slm, and the internal nozzle diameter between 6 and 10 mm; the plasma gases were either an Ar–H2mixture or N2. Rather well defined tendencies were observed and at the same time it appeared that the arc stability greatly influenced the fluctuations of the velocity.

CHARACTERIZATION OF D.C. PLASMA TORCH VOLTAGE FLUCTUATIONS

The arc root fluctuations at the anode-nozzle of a d.c. plasma spray torch with a special configuration of the electrodes allowing to work with the same gas flowrate with nozzle diameters ranging from 6 to 10 mm were systematically studied. The plasma gas was Ar/H2 (25 vol % H2), the current was varied between 200 and 600 A and the plasma gas flowrate between 24 and 80 slm. After 30–60 mn working the nozzle wall started to be sufficiently eroded to have a stagnant arc spot which lived until arcing created another one. It was shown that the life time of the upstream arc spots were 30–40 % longer than the downstream ones which could play an important role in the electrode erosion. Dimensional analysis allowed to find a relationship between the nozzle diameter D, the arc current I and gas flow rate G and the mean spot lifetime which is closely connected with the difference between D and the electrical diameter of the arc column. The comparison of voltage signal and light emission at a point of the plasma jet close to the nozzle exit on its axis allowed to determine the mean electrical field within the plasma column and the mean position of the arc root. The comparison with the electrode erosion area for well defined conditions showed a good correlation with the calculated arc root position.

Diagnostics of thermal spraying plasma jets

Direct current thermal plasma jets are strongly affected on the one hand by the arc root fluctuations at the anode, resulting in a type of pulsed flow and enhanced turbulence, and on the other hand by the entrainment of surrounding cold gas in the plasma jet. These phenomena and the resulting temperature distributions have been studied using a wide range of diagnostic techniques, including fast cameras, laser doppler anemometry (LDA), coherent anti-Stokes Raman spectroscopy (CARS), Rayleigh scattering, emission spectroscopy, Schlieren photography, enthalpy probes, and sampling probes. The information obtained by these techniques is evaluated and compared. The effect of the arc fluctuations on the spectroscopic measurements is emphasized, and the possibility of using these fluctuations to determine information on the arc behavior and the axial velocity of the jet is presented. Optimization of plasma processing of solid particles requires information about their size and surface temperature, as well as number flux, and velocity distributions at various locations in the flow field. The different statistical techniques of inflight measurements are discussed together with their limitations. A method to determine the temperature and species density of the vapor cloud or comet traveling with each particle in flight is then presented. However, such statistical measurements present ambiguities in their interpretation, which can be addressed only by additional measurements to determine the velocity, diameter, and surface temperature of a single particle in flight. Moreover, information on single particles is required to understand the coating properties, which depend strongly on the way the particles flatten and solidify upon impact. A method to obtain data related to a single particle in flight and to follow the temperature evolution of the corresponding splat upon cooling is presented. The article concludes with the description of the experimental techniques to follow the temperature evolution of the successive layers and passes. This is important because temperature distribution within the coating and substrate controls the adhesion and cohesion of coatings as well as their residual stress.

Influence of configuration and operating conditions on the electric arc instabilities of a plasma spray torch: role of acoustic resonance

A theoretical approach is proposed to explain the particularities of the power spectrum of a plasma spray torch voltage. This is founded on an acoustic resonance of the rear part of the plasma torch which is occupied by the cold gas before it reaches the arc region. The spectrum is characterized by the presence of a sharp peak in the range 3–8 kHz, with an amplitude that represents up to ±30% of the voltage mean value. The peak frequency presents an evolution that is governed by the torch operating conditions, the thermal properties of the plasma forming gas and the geometrical configuration of the electrode assembly. The theory is compared with experimental values of the peak frequency recorded for different operating conditions and torch configurations. The injection ring through which the gas is injected also plays an important role. The torch behaviour is carefully characterized in order to collect all the experimental data required for the analysis on the involved phenomena.

Plasma spraying using Ar-He-H2 gas mixtures

In plasma spraying, the properties of the plasma-forming gas largely control the characteristics of the plasma jet and the momentum, heat, and mass transfers to the particles injected in the flow. The objective of this work was to investigate the effect of gas composition on the static and dynamic behaviors of the plasma jet. The latter behaviors were investigated from measurements of arc voltage and plasma jet velocity. Ternary gas mixtures of argon, helium, and hydrogen were used. The results were expressed as correlations between arc voltage and flow velocity, and the operating parameters of the gun for a specific nozzle diameter.

Influence of Helmholtz oscillations on arc voltage fluctuations in a dc plasma spraying torch

Arc voltage fluctuations are studied for two different plasma torches. One of them is home-made, the other is a Sultzer Metco plasma torch. Both work under similar operating conditions, but they show very different voltage waveforms and spectra, depending on their electrode configurations. The first torch shows a rather intermittent behaviour and works under the restrike mode with a rather low fluctuation amplitude and with non-reproducible spectral components. The second torch also shows characteristic features related to the restrike mode, but they are superimposed on more regular oscillations which are due to pressure variations in the cold gas chamber, located upstream the arc region. This part of the torch together with the nozzle channel appears to be a Helmholtz resonator, whose resonance frequency is theoretically evaluated as a function of the torch geometry and of the operating conditions. The theoretical predictions are in very good agreement with the measured frequency of the main peak in the voltage spectrum of the second torch. A discussion about the coupling between the pressure and the voltage is proposed to explain how the torch design could influence the Helmholtz resonance.

Study of the dynamic and static behavior of de vortex plasma torches: Part II: Well-tye cathode

This work was devoted to the study of the dynamic and static behavior of de vortex plasma torch with a well-type cathode (power level below 100 kW). The dynamic behavior of the torch was characterized by the fulctuations of arc voltage and current, plasma jet radiation, and acoustic pressure. Characteristic frequencies of the arc root movement inside the torch were observed. By numerical simulation (with the numerical codeMelodie, it was shown that the position of the erosion diameter) of the axial velocity along the cathode channel near the wall. The static behavior of the torch was inverstigated for different cathode designs. The variations of voltage U with arc current I, gas flow rate G nature of the gas and cathode design were represented by semiempirical relationships established between dimensionless numbers. By dimensional analysis, the behavior of this torch was compared with that of two powerful torches: the Aerospatiale and the Plasma Energy Corporation torches.

Plasma spraying of ceramic particles in argon-hydrogen D.C. Plasma jets: Modeling and measurements of particles in flight correlation with thermophysical properties of sprayed layers

The behavior of alumina or stabilized zirconia particles in flight in Ar−H2 d. c. plasma jets (up to 40 kW) has been studied. Measurement of the temperature distributions in the plasma jets (by emission spectroscopy) has shown the importance of the electrodes and are chamber designs on the length and diameter of the jets. The important cooling effect of the surrounding air has also been shown, and the parameters controlling it have been studied. Modeling of the momentum, mass, and heat transfers between plasma and particles as well as measurements of the trajectory, velocity, surface temperature distributions, and particle evaporation have enabled us to determine the influence of the different paramters, such as size and injection velocity distributions, particle morphology,etc., on the particle molten state upon impact. These calculations and measurements on the particles in flight have been correlated to some physical properties of deposits.

Correlation of the physical properties of sprayed ceramic coatings to the temperature and velocity of the particles travelling in atmospheric plasma jets: Measurements, modelling and comparison

Abstract The aim in this paper is to present an overview for the plasma spraying of ceramics at atmospheric pressure of our knowledge of the properties and possible measurements of the plasma jets, of the possible measurements of the trajectories, velocities and surface temperatures of the particles, of the models developed and of the correlations between the measured and predicted physical and mechanical properties of the deposits. Examples of measurements are presented to illustrate the problems analysed.