Hélène Ageorges

Plasma spraying of stainless-steel particles coated with an alumina shell

Abstract The effect of an alumina coating, obtained by mechanofusion, on stainless-steel particles used in plasma spraying has been studied by examining sprayed particles in mid-flight and their resulting splats and coatings. The mean size of the injected powders is about 65 μm and the thickness of the alumina shell 4 μm. The results show that without preheating the substrate the splats of both types of powder are extensively fingered and become circular when the substrate surface is preheated over 200°C. For the case of the stainless steel/alumina composite splats, Energy dispersive spectroscopy (EDS) analysis of the distribution of the various elements shows that the alumina is either spread exactly on the stainless-steel splat or is dispersed in pieces and frozen over the surface of the stainless-steel splat. The first case corresponds to well molten particles where, after their flight in the plasma jet, all the alumina shell has flowed to the tail of the particle; the second case is related to particles which have still an alumina shell uniformly distributed around the stainless-steel core. Finally, a composite stainless steel/alumina coating sprayed on a rough ( R a ∼6.7±0.3 μm) stainless-steel substrate preheated to 400°C is compared with a pure stainless-steel coating. Both hardness and cohesion are found to improve for the alumina-coated particles.

Suspension Plasma Spraying of Alumina Coatings: Process and Coating Structure

Summary form only given. Due to the large volume fraction of the internal interfaces, coatings structured at the nanoscale should exhibit better properties than conventional coatings structured at the microscale. However, when processing by thermal plasmas such feedstock, several questions arise: (i) how feeding the plasma jet with nanosized powders? (ii) how keeping their nanostructured structures when melting them? (iii) how controlling the growth of coatings grain? Suspension plasma spraying (SPS) appears as a technology permitting to circumvent those difficulties. It consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.1 and 1 mum through an injector of diameter in the order of one hundred micrometers. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and then evaporation (lasting, in average, two orders of magnitude longer than fragmentation). Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Compared to plasma spraying of micron-sized particles, SPS exhibit several major differences: (i) a more pronounced sensitivity to arc root fluctuations requiring to adapt operating parameters in order to operate the spray gun in its take over mode unless to have inhomogeneous process of the suspension by the plasma: (ii) a shorter spray distance since small particles decelerate faster than bigger ones; (iii) a higher thermal flux transmitted from the plasma flow to the substrate, between 5 to 10 times higher than the heat flux transmitted in conventional plasma spraying. In consequence, coatings manufactured by SPS differ from conventional coatings by: (i) a more pronounced sensitivity to the particle size distribution: narrow-sized ones are absolutely required to produce dense layers; (ii) lamella characteristic dimensions smaller than the ones encountered in conventional plasma spraying (with lower flattening ratio due to lower particle Reynolds numbers upon impact) with almost no peripheral splashing (due to lower particle adjusted Sommerfeld numbers upon impact); (iii) lower residual stress levels within lamellae which limit the development of intralamellar cracks; (iv) a denser structure with almost no pore connectivity. This paper aims at presenting the SPS of alumina coatings in terms of: (i) suspension characteristics on the coating architecture; (ii) process optimization, emphasizing the predominant role of the arc root fluctuations on the coating architecture; (iii) resulting coating architectures, in particular in terms of their pore network architecture.

Composite Coatings Elaborated by Plasma Spraying of Dry Coated Particles

The influence of dry particle coating on the properties of coatings produced by d.c. arc plasma spraying is reported. A mechanofusion process is used to coat coarser metallic particles with fine ceramic particles without using either binders or solvents. The key parameters affecting the mechanofusion process and the corresponding plasma spraying method have been varied in order to increase the hardness of the resulting composite coatings. Efforts have been made to disperse homogeneously hard particles (α-Al2O3, SiC) into a metallic matrix (316L stainless steel) and check if it is possible to limit the oxidation of metallic particles during their flight in the plasma jet flowing in air. The hardness of resulting composite coatings depends on the metallic particle size even when the hard ceramic particles are homogenously dispersed into the metallic matrix. Spraying mechanofused powder composed of finer stainless steel particles (64 ,m), results in finer structured deposits that show a higher oxide content. On the contrary, a low oxidation rate of the metallic matrix is observed when coarser metallic particles (120 ,m), covered by a binary layer of α-Al2O3 and SiC are sprayed.

Estudio de la estructura y las propiedades mecánicas en un recubrimiento de circona estabilizada con 8% en mol de itria elaborado por proyección térmica por plasma a partir de suspensiones

El estudio de recubrimientos nanoestructurados fabricados mediante proyeccion termica ha generado un gran interes en las ultimas dos decadas debido a su considerable mejora en sus propiedades sobre todo en las propiedades fisicas y mecanicas. Esta mejora resulta de reducir los tamanos de grano de su materia prima a escalas de 100 a 1000 veces mas bajas comparadas con los recubrimientos convencionales microestructurados. Asi, estos recubrimientos con estructuras a nivel nano y/o submicrometrica estan empezando a ser usados en la industria gracias a sus sobresalientes propiedades y es fundamental el estudio de sus propiedades mecanicas para determinadas aplicaciones. En la actualidad, la indentacion es la principal tecnica de caracterizacion de recubrimientos y peliculas delgadas, a pesar de estar altamente influenciada por los defectos propios del recubrimiento en la vecindad de la indentacion. La nanoindentacion y la microindentacion se presentan como la mejor opcion para evaluar el comportamiento mecanico de estos recubrimientos nanoestructurados. Por esta razon, en este trabajo se analizo el efecto de la estructura de un recubrimiento de circona estabilizada con 8% en mol de itria (8YSZ) elaborado por proyeccion termica por plasma a partir de suspensiones nanometricas (SPS) y su relacion con sus propiedades mecanicas (dureza y modulo de elasticidad) medidas por las tecnicas de nanoindentacion y microindentacion. El analisis de la estructura de la seccion transversal mostro que el recubrimiento exhibe una estructura bimodal, la cual esta compuesta por una zona con particulas nanometricas semifundidas (ZS) y lamelas con mayor grado de fusion (ZF). Los ensayos de nanoindentacion mostraron una distribucion de Weibull bimodal de sus propiedades mecanicas, la cual caracteristica de este tipo de recubrimientos; mientras que la dureza (HV) y el modulo de elasticidad evaluados por microindentacion, presentaron una distribucion monomodal. Estos resultados de microindentacion estuvieron influenciados por el area de contacto en las zonas indentadas en la estructura bimodal del recubrimiento.

The Influence of Dry Particle Coating Parameters on Thermal Coatings Properties

The physical properties of coatings elaborated by plasma spraying, especially the mechanical properties are strongly influenced by some fifty operating parameters of the spraying process. Several studies have been conducted to correlate these operating parameters with the coating microstructure, via the behavior of molten particles in flight to be impacted against the surface substrate, well known as splats. Then, it is expected to build coatings with tailored properties for mechanical and even thermal applications (Fauchais & Vardelle, 2000).

Mechanofusion Processing of Metal-Oxide Composite Powders for Plasma Spraying

Composite particles destined to build plasma sprayed coatings, are prepared by the mechanofusion process (MF). These particles consist of a stainless steel core particle coated by finer particles of alumina. Changes induced by the MF process are monitored by SEM, DRX, and laser granulometry, revealing that the dry particle coating process is governed by agglomeration and rolling phenomena. Simultaneously, the MF performance is controlled by the operating parameters such as the compression gap, the mass ratio of host to guest particle, and the powder input rate. The mechanical energy input leads to a nearly rounded shape of the final composite particles; however, no formation of new phases or components decomposition is detected by XRD analysis. The resulting composite powder features optimal characteristics, concerning particle shape and phases distribution, to be plasma sprayed in air.

Present knowledge in suspension plasma spraying

Suspension plasma spraying (SPS) consists in injecting a non-Newtonian liquid in a d.c. plasma jet where it is fragmented and then vaporized. The sub-micrometric or nanometric particles contained in the suspension are then accelerated, heated, partially or totally melted before flattening onto the substrate to form the coating. Such coatings are finely structured and present better thermo-mechanical properties than conventionally sprayed ones. Indeed, coating properties are strongly linked to: the interaction between the suspension and the plasma jet that depends on the liquid jet or injected drop average diameters and velocities, the suspension surface tension and its viscosity, the momentum density of the plasma jet and its fluctuations (linked to the arc root fluctuations). These phenomena control the liquid fragmentation followed by resulting droplets vaporization (two orders of magnitude slower) and acceleration; (1) the heat and momentum transfers to the solid particles contained within the droplets (related to their size distribution and propensity to agglomerate). These transfers are relatively complex since their particle average size is below a few tenths of micrometer, value for which the Knudsen effect and thermophoresis are promoted and since their dimensionless Stokes number close or lower than unity will make them to follow the fluid flow rather than impacting onto the substrate; (2) the rather short spray distance (30-50 mm) used to account for the very low inertia of the sprayed particles resulting in heat fluxes that can reach 30 MJ.m-2 (more than one order of magnitude higher than in conventional spraying) and thus play a key role on deposited layer transient temperatures; (3) the spray pattern for which it is very important to avoid the entrapment of untreated or poorly treated particles that very significantly affects the layer structure by promoting stacking defects. This paper presents what is our present knowledge in this field together with the available tools implemented to characterize the plasma-liquid interaction and the coating formation.

Suspension plasma spraying: Process parameters and resulting coating architecture

Due to the large volume fraction of the internal interfaces, finely structured coatings (nano- or submicronsized) should exhibit better properties than the ones structured at a microscale. Suspension plasma spraying (SPS) appears as a technology permitting to manufacture such coatings and consisting in injecting within a plasma jet a liquid suspension of solid particles. Compared to plasma spraying of micron-sized particles, SPS exhibit several major differences: i) a more pronounced sensitivity to arc root fluctuations requiring to adapt operating parameters; ii) a shorter spray distance; iii) a higher thermal flux transmitted from the plasma jet to the substrate. Several operating parameters, including suspension characteristics and suspension injection parameters, play relevant roles in the suspension processing and the resulting coating architecture.

Low friction stainless steel coatings graphite doped elaborated by air plasma sprayed

A new process has been developed to incorporate graphite particles into a stainless steel coating during its formation. Four means have been tested to inject the graphite particles outside the plasma jet and its plume: graphite suspension, a graphite rod rubbed on the rotating sample, powder injection close to the substrate with an injector, or a specially designed guide. The last process has been shown to be the most versatile and the most easily controllable. It allows the incorporation of between 2 and 12 vol.% of graphite particles (2–15 µm) within the plasma sprayed stainless steel coatings. A volume fraction of 2% seems to give the best results with a slight decrease (6%) of the coating hardness. This volume fraction also gave the best results in dry friction on the pin-on-disk apparatus. Depending on the sliding velocity (0.1–0.5 m/s) and loads (3.7–28 N), the dry friction coefficient against a 100C6 pin is reduced by between 1.5 and 4 compared with that obtained with plasma sprayed stainless steel.