Recent works have been devoted to achieve dense and thin (<15 µm) zirconia coatings using a relatively new process, Suspension Plasma Spraying (SPS). Nevertheless, the parameters controlling the microstructure of the deposit are not yet clearly identified, particularly for the injection of suspension. Hence, the liquid penetration into the plasma has been observed with a fast shutter (10 -5 s) camera coupled with a laser flash and triggered by a defined instantaneous voltage level of the plasma torch. This paper is focused on the treatment of the suspension jet or drops according to the suspension properties (with the viscosity, particles load, injection velocity…) and depending on the different spray parameters such as the plasma forming gas mixture composition and the plasma torch design (either PTF4 or home made torch). These works have permitted the obtention of zirconia coatings with low thicknesses (~10 µm) and dense structure (~4% of porosity).

Suspension Plasma Spraying (SPS) allows depositing finely structured coatings. This paper presents an analysis of the influence of plasma instabilities which control the interaction plasma jet-zirconia suspension. A particular attention is paid to the treatment of suspension jet or drops according to the importance of voltage fluctuations (linked to those of arc root) and depending on the different spray parameters such as the plasma forming gas mixture and the suspension momentum. By observing the suspension drops injection with a fast shutter camera and a laser flash triggered by a defined transient voltage level of the plasma torch, the influence of plasma fluctuation on drops fragmentation is studied through the deviation and dispersion trajectories of droplets within the plasma jet.

This paper is devoted to the study of coatings elaborated by a process permitting Air Plasma Spraying (A.P.S.) of finely structured ceramic coatings. This process mainly consists in injecting in a d.c. plasma jet, a ceramic suspension containing sub-micron ceramic particles. Coating characteristics are close to those observed in P.E.C.V.D., but with a faster elaboration rate (. 15 µm/m2.h) and, the possibility to produce a wide range of thicknesses (1 < e < 100 µm). This paper is aimed at producing the Yttria Stabilized Zirconia electrolyte (Y.S.Z.) of Solid Oxide Fuel Cells with a thickness of 5 to 30 µm. Previous studies of this process have been devoted to the influence of the spray parameters on the structure of Y.S.Z. splats collected and have allowed determining some well adapted working conditions. The new stage described in this paper, is related to the study of the growth of Yttria Stabilized Zirconia coatings and the influence of different parameters such as the ceramic particle size distributions contained in the suspension and the heat flux imposed to the surface of the substrate and successive passes during the coating elaboration.

This paper describes a study of a plasma spray process to deposit ceramic coatings with finer structures than those that can be produced by conventional DC plasma spraying. The structures are similar to those produced by plasma-enhanced chemical vapor deposition (PECVD), but with a very fast coating rate and the ability to produce a wide range of thicknesses (1 < e < 100 µm). The process involves injecting a ceramic suspension containing submicronic ceramic particles into a DC plasma jet. The goal is to develop the process so that it can be used for the production of such coatings as yttria stabilized zirconia (YSZ), Ni-YSZ, or perovskite (for solid oxide fuel cells) or YSZ, Al 2 O 3 -YSZ, or Pyrochlore (for thermal barrier applications). The research stage described in this paper is related to the study of the growth of YSZ coatings, to calculations of the dynamic and thermal plasma-particle transfers, linked to the effect on particle flattening of the various spray parameters (arc current intensity, particle collection distance, substrate temperature, plasma gas composition, and so on). It also describes the achievement of a multi layered coating of YSZ and alumina.

The purpose of this work is to optimise the chemical composition of perovskite coatings prepared by injecting a suspension of submicrometric LaMnO 3 perovskite particles (d 50 =~ 1 µm) in a direct current (d.c.) plasma jet. The perovskite powder composition, the particle size and the plasma parameters were modified in order to diminish the manganese evaporation. The process consists in mechanically injecting a well dispersed stable suspension of submicrometric perovskite particles in a dc plasma jet. In the process, large suspension droplets (~300 µm) are sheared into tiny ones (a few µm) by the plasma jet flow. Then the solvent is evaporated and the particles melt resulting in perovskite droplets of about 1 µm impacting on the substrate, the coating resulting from their layering. Such coatings are to be used as cathodes for the SOFCs (Solid Oxide Fuel Cells). Best results were obtained by injecting a stable suspension containing a 10 mol% MnO 2 doped perovskite powder with 3 µm particle size in an Ar plasma forming gas and 300 A of current intensity.

A perovskite suspension was air plasma sprayed (APS) with a direct current (dc) plasma jet. The process conditions including the standoff distance, plasma forming gases and their ratio influence on the perovskite deposits and splats characteristics were investigated. Particularly the molten state of particles, phases obtained, morphology and composition of splats were studied. The process consists in injecting a well dispersed stable suspension of submicrometric particles. The suspension was produced by attrition milling LaMnO 3 perovskite powder (mean particle size of 0.8 µm). The plasma forming gases influences drastically the obtained phases in deposits. When the plasma gas mixture contains hydrogen the heat transfer is too high and the perovskite phase is decomposed but when the plasma forming gases is Ar/He or only Ar, just a very little quantity of perovskite is found to be decomposed in the plasma jet. With Ar/He, perovskite splats are well melted, but with argon there is a high percentage of semi-melted and non-melted particles.

For many years, a new interest in nanomaterials, with grain sizes smaller than 100nm, has emerged. This is due to the enhanced properties of the resulting sintered materials or coatings compared to those with coarser-grained materials. This paper is devoted to the feasibility to produce nanomaterial coatings by a dc plasma spray process. Until now, only thick coatings (> 100µm) have been elaborated using this technique, by injecting, with a carrier gas, micrometric particles in the plasma flow. But, it is not possible to inject too small particles (<5µm) without perturbing drastically the plasma jet by the high carrier gas flow rate necessary to give them a high enough momentum. This work presents a new dc plasma spray process, designed to elaborate alumina nanocoatings. The most important step of the process is the control of the ceramic nanometric particle penetration in the plasma. Because of their small size, a liquid, which density made the momentum transfer more efficient, replaced the carrier gas with an injector creating calibrated droplets with controlled velocity and flow rate. To study the liquid-plasma interaction, the penetration of pure water in an Ar/H 2 plasma jet was investigated by means of emission spectroscopy. The modification of temperature field together with oxygen concentration was determined quantitatively. Emission spectra were treated with a new localization method, avoiding the use of Abel's inversion implying a cylindrical symmetry, destroyed by the liquid injection. Such measurements allowed optimizing the liquid penetration in the plasma jet. Alumina nanopowders were dispersed in a liquid to form a stable suspension, which was injected in the plasma. The layered particle morphology, collected on glass substrates at different distances downstream of the injection point, was then studied.

Air entrainment in the first 30 mm of a dc Ar/ H 2 plasma jet has been studied by emission spectroscopy. The tests were conducted using 6, 7 and 10-mm diameter nozzles and plasma arc currents of 400 and 600 A. Oxygen, nitrogen, and argon spectral lines were recorded 20 and 30 mm downstream of the nozzle exit during spraying, and the corresponding atom density ratios were estimated based on plasma population temperature and volumetric emission coefficients. The results indicate that at 20 mm air entrainment is mainly due to piston flow for the 10-mm nozzle and both piston flow and engulfment for the 7-mm nozzle. At 30 mm, the engulfment process is found to have 4 to 6 times the impact that it does at 20 mm and is directly linked to the jet velocity. At both locations, the atom density ratios differ from that observed in air due to the time required to dissociate N 2 .

This paper is devoted to the investigation of the dynamics of the plasma jet, which is produced by a subsonic DC spray torch operated with an argon-helium mixture as the plasma gas. It focuses on studying the effect of some parameters which influence the arc attachment inside the anode nozzle. For this purpose, the paper uses different means of gas injection, that is straight flow injection and vortex flow injection, and anodes which have experienced different degree of wear. A heavily eroded anode is characterized by large voltage fluctuations at relatively low frequencies, while straight gas injection at high current levels led to a low average voltage with small fluctuations and to low burner performance. The results are interpreted by assuming changes in the thickness of the cold interface between the arc and the anode, and conclusions are drawn as to the voltage characteristics indicative of good torch operation. Paper includes a German-language abstract.

PTA (Plasma Transferred Arc) reclamation of aluminum alloys by hard materials with a much higher melting temperature is very difficult. This is due to the high thermal diffusivity of these al1oys. Below a critical heat flux φc nothing happens and over φc the substrate melts very rapidly contrarily to what is observed with steel substrates. That explains probably why PTA is mainly used for steel reclamation. Thus the knowledge of heat flux transferred to the anode is a critical point to develop PTA reclamation on aluminum alloys and this is the aim of this paper. An experimental set-up was built to study the heat transferred to three substrates made of different materials : cast iron for reference, aluminum alloy and copper for its high thermal conductivity. The plasma torch was a Castolin Eutectic gun and allowed to inject a sheath gas around the plasma column. The copper, aluminum alloy and cast iron substrates, easily interchangeable, were the top of a water-cooled calorimeter allowing to determine the variation of the received heat flux with the working parameters : arc current, stand off distance, plasma forming gas momentum, sheath gas composition and momentum. The determination of the arc electric field allowed to calculate the arc diameter which was compared first with pictures taken with a video camera and second, with wear traces left on the anode material. Several correlations have been established to characterize the arc voltage and the anode heat flux.

In dc plasma spray guns, the properties of the plasma forming gas largely control the characteristics of the plasma jet and the momentum and heat of the particles injected into the flow. This paper examines the effect of Ar-He-H2 mixtures on the dynamic and static behavior of plasma jets expressed in terms of arc voltage and gas velocity. Correlations between these parameters and operating variables (arc current, gas flow rate, volume composition) were established from a dimensional analysis and supported by the calculation of the thermodynamic and transport properties of the gas mixtures used in the study.