Simon Goutier

Suspension and solution plasma spraying

Suspension and solution plasma spraying makes it possible to achieve coatings with fine microstructural features and is becoming a common route in laboratories to elaborate coatings a few tenths to a few hundreds of micrometres thick. This paper presents the recent developments in direct current plasma spraying of suspensions or solutions. It begins with a short description of the main plasma torches used for liquid feedstock spraying as well as the techniques used to experimentally observe droplets and particles in the plasma jet and characterize the void network of nanostructured plasma-sprayed coatings. The paper then turns to the momentum and heat transfers between fine particles and the plasma jet and the interactions between the plasma jet and a liquid in the form of a jet or drops. It concludes by linking some characteristic features of coating microstructures with the liquid processing in the plasma jet.

What Do We Know, What are the Current Limitations of Suspension Plasma Spraying?

Nano-structured coatings should exhibit better properties than micro-structured coatings because of a high volume fraction of internal interfaces. Since the mid-nineties a large body of works have been devoted to suspension and solution plasma spraying for the deposition of finely and even nanometer-structured coatings. The aim of this paper is to take stock of our present knowledge in the field of suspension plasma spraying that is, at the moment, essentially used for oxide ceramic coatings. It will first tackle the injection of the suspension in the plasma jet and the behavior of nano or sub-micro-meter particles processed in the plasma jet core involving the liquid breakup and vaporization that releases the solid particles from the solvent droplets. It will, then, deal with the plasma torches and liquid feeding systems available to suspension spraying. It will finally discuss the key characteristics of suspensions (solvent, dispersant, and particle morphologies), designing of coating microstructure, and potential industrial applications, with the developments requested to cope with these applications.

Latest Researches Advances of Plasma Spraying: From Splat to Coating Formation

The plasma spray process with solid feedstock, mainly ceramics powders, studied since the sixties is now a mature technology. The plasma jet and particle in-flight characterizations are now well established. The use of computer-aided robot trajectory allows spraying on industrial parts with complex geometries. Works about splat formation have shown the importance of: the substrate preheating over the transition temperature to get rid of adsorbates and condensates, substrate chemistry, crystal structure and substrate temperature during the whole coating process. These studies showed that coating properties strongly depend on the splat formation and layering. The first part of this work deals with a summary of conventional plasma spraying key points. The second part presents the current knowledge in plasma spraying with liquid feedstock, technology developed for about two decades with suspensions of particles below micrometers or solutions of precursors that form particles a few micrometers sized through precipitation. Coatings are finely structured and even nanostructured with properties arousing the interest of researchers. However, the technology is by far more complex than the conventional ones. The main conclusions are that models should be developed further, plasma torches and injection setups adapted, and new measuring techniques to reliably characterize these small particles must be designed.

Specific Measurements of In-Flight Droplet and Particle Behavior and Coating Microstructure in Suspension and Solution Plasma Spraying

The plasma spraying of suspensions of sub-micro- or nano-sized particles and of solutions of chemicals precursors produces finely structured coatings that have generally enhanced properties compared to conventional plasma-sprayed coatings. However, most techniques used in conventional plasma spraying are no more adapted to experimentally observe the behavior of the liquid feedstock in the plasma jet and investigate the effect of the operating conditions on liquid fragmentation in droplets, solid particles released by solvent evaporation or formed from the chemical precursors. Also, specific techniques have to be used to study the coating formation and characterize its microstructure. This paper aims to present the main techniques developed or adapted, up to now, to study the plasma-liquid feedstock interactions and characterize the coatings achieved by suspension and solution plasma spraying.

Synchronization of Suspension Plasma Spray Injection with the Arc Fluctuations

Poorly controlled heat and momentum transfers between plasma and material, plasma instabilities are some of the difficulties encountered in suspension plasma spraying. The improvement of this method is usually attempted by means of the reduction of arc fluctuations. This paper presents a new approach to the injection of reactive material in an arc jet. The principle is to produce a pulsed laminar plasma jet combined with phased injection of liquid droplets. This is achieved by the particular design of the plasma torch that works at moderate power and following a resonant mode. The droplets are injected using a piezoelectric device, based on drop-on-demand method, triggered by the voltage signal sampled at the torch connections. The results are evaluated by time-resolved imaging technique that shows how the trajectories are influenced by the moment at which the droplets penetrate the plasma jet.

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.

Sprays Used for Thermal Barrier Coatings

“Nano-sized” or “nano-structured” materials correspond to particle diameters or an internal structure with at least one dimension smaller than 100 nm. The thermal spray community has been actively involved in this area for more than 30 years. Due to the large volume fraction of the internal interfaces, nano-structured coatings exhibit better properties, especially toughness, thanks to crack arresting effect. However, nano-sized particles can be injected in plasma jets only with a liquid carrier as either suspensions (Suspension Plasma Spray, SPS) of nanometer-sized particles or solutions (Solution Plasma Spray Process, SPPS) of reacting elements forming particles. However, a few problems must be solved: How droplets and sub-micrometer particles behave in spray jets and what are the measuring tools available? Is it possible to follow the formation of splats with particles below a few micrometers? How optimizing the liquid injection and its matching with the spray torch? What is the interest to spray coatings with mixtures of micrometer and nanometer-sized particles? Among the different coatings studied many works have been related to thermal barrier coatings (TBCs) to compete, at lower price, with Electron Beam-Physical Vapor Deposition (EB-PVD) or Plasma Spray-Physical Vapor Deposition (PS-PVD) and also to achieve TBCs with a better resistance to Ca50Mg10Al10Si30 (CMAS).