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.

The development of coating elaboration processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coatings properties. This is particularly important when considering the plasma treatment of submicron or nanosized particles in Suspension Plasma Spraying (SPS). Submicron particles closely follow plasma instabilities and have non-homogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in dc plasma torches, showing dominant fluctuation frequencies between 4 – 6 kHz, are linked to pressure oscillations in cathode cavity in the rear part of the plasma torch. These resonant oscillations are linked to plasma torch geometry. In this paper, first, we will present a method to isolate the different oscillations modes in measured arc voltage and pressure signals by the use of signal processing methods. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters.

One of the goals of this study is to better understand how suspension plasma spraying parameters, particularly plasma gas mixtures, influence layer formation. Another goal is to produce finely structured layers of Al 2 O 3 -ZrO 2 with a wide range of architectures. To that end, a simple theoretical model is used to describe the operating conditions of the plasma torch and the influence of spraying parameters is expressed in terms of the shape and size of spray beads.

Suspension plasma spraying (SPS) is a fairly recent technology that is able to process sub-micrometric-sized feedstock particles and permits the deposition of layers thinner (from 5 to 50 µm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 µm. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and evaporation. Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers. Nevertheless, the coupling between the parameters controlling the coating microstructure and properties are not yet fully identified. The aim of this study is to further understand the influence of parameters controlling the manufacturing mechanisms of SPS alumina coatings, in particular the spray patterns influence.

Intermediate temperature - solid oxy-fuel cells (IT-SOFCs) include in their design a solid electrolyte layer made of yttria-partially stabilized zirconia (Y-PSZ), an ionic conductor, through which oxygen ions diffuse. This layer needs to fulfill several characteristics among which a low leakage rate corresponding to a non-connected pore network and a low level of stacking defects such as microcracks or globular pores. Moreover, the thickness of this layer needs to be as low as possible (about 20 µm) in order to limit ohmic losses. Suspension plasma spraying (SPS) appears as a potential technological route to manufacture such layers structured at micrometric or sub-micrometric scales. In SPS, a stabilized suspension, made of a liquid, solid particles and a dispersant, is injected within the plasma flow. The liquid is very quickly fragmented and then vaporized and the individual particles, or the particle agglomerates, depending on the average size and morphology of the solid feedstock, are heated and simultaneously accelerated towards the substrate surface where they impact, spread and solidify, analogously in a first approximation to larger particles, to form a layer. The architecture of the layer is very closely related to plasma operating parameters (from which derive plasma flow stability), from the suspension characteristics, in particular the feedstock particle size distribution and from the suspension injection parameters. This work aims at presenting recent developments made to optimize some of these operating parameters to maximize the electrolyte layer characteristics.

A new thermal spray process is under development in order to produce thin coatings (thickness lower than 50 µm), with a fine microstructure (grain size smaller than 1 µm). It consists in injecting in a Direct Current (D.C.) plasma jet a suspension containing submicronic particles of the material to be deposited. To study the interaction between the plasma jet and the suspension, a system based on a pendulum allows the collection of particles at different distances from the injection point. In this paper the effect of substrate temperature upon the formation of micrometric zirconia splats was studied, glass and stainless steel were used as substrate materials.

This paper describes a new spraying process that uses a special torch to inject aluminum oxide suspensions into a dc plasma jet. Based on the results obtained, a general mechanism of action is proposed in which factors such as acceleration, impact behavior, and heat transfer phenomena play a role. Paper includes a German-language abstract.