Direct current plasma spraying of mechanofused alumina-steel particles

Stainless steel particles (60 $\mu$m in mean diameter) cladded with an alumina shell (2 $\mu$m thick and manufactured by mechanofusion) were sprayed with an Ar-H2 (53-7 slm) d.c. plasma jet (I = 500 A, P = 28 kW, \rho_th = 56 %). Two main types of particles were collected in flight, as close as 50 mm downstream of the nozzle exit: particles with a steel core with pieces of alumina unevenly distributed at their surface and those consisting of a spherical stainless steel particle with an alumina cap. The plasma flow was modeled by a 2D steady parabolic model and a single particle trajectory by using the 3D Boussinesq-Oseen-Basset equation. The heat transfer, within the two-layer, stainless steel cladded by alumina, particle, considered the heat propagation phenomena including phase changes. The models allowed determining the positions, along the particle trajectory, where the convective movement could occur as well as the entrainment of the liquid oxide to the leading edge of the in-flight particles. The heat transfer calculations showed the importance of the thermal contact resistance TCR between alumina and steel

Thermal Sprayed Coatings Used Against Corrosion and Corrosive Wear

International audienceCoatings have historically been developed to provide protection against corrosion and erosion that is to protect the material from chemical and physical interaction with its environment. Corrosion and wear problems are still of great relevance in a wide range of industrial applications and products as they result in the degradation and eventual failure of components and systems both in the processing and manufacturing industries and in the service life of many components. Various technologies can be used to deposit the appropriate surface protection that can resist under specific conditions. They are usually distinguished by coating thickness: deposition of thin films (below 10 to 20 μm according to authors) and deposition of thick films. The latter, mostly produced at atmospheric pressure have a thickness over 30 μm, up to several millimeters and are used when the functional performance and life of component depend on the protective layer thickness. Both coating technology can also be divided into two distinct categories: "wet" and " dry " coating methods, the crucial difference being the medium in which the deposited material is processed. The former group mainly involves electroplating, electroless plating and hot-dip galvanizing while the second includes, among others methods, vapor deposition, thermal spray techniques, brazing, or weld overlays. This chapter deals with coatings deposited by thermal spraying. It is defined by Hermanek (2001) as follows , "Thermal spraying comprises a group of coating processes in which finely divided metallic or non-metallic materials are deposited in a molten or semi-molten condition to form a coating". The processes comprise: direct current (d.c.) arcs or radio frequency (r.f.) discharges-generated plasmas, plasma transferred arcs (PTA), wire arcs, flames, high velocity oxy-fuel flames (HVOF), high velocity air-fuel flames (HVAF), detonation guns (D-gun). Another spray technology has emerged recently ; it is called cold gas-dynamic spray technology, or Cold Spray (CS). It is not really a thermal spray technology as the high energy gas flow is produced by a compressed relatively cold gas (T < 800°C) expanding in a nozzle and will not be included in this presentation

Solution and Suspension Plasma Spraying of Nanostructure Coatings

International audienceThe main motivation for coating industrial parts with a different material lies on the following needs: (1) to improve functional performance, (2) to improve the component life by reducing wear due to abrasion, erosion and/or corrosion, (3) to extend the component life by rebuilding the worn part to its original dimensions, and (4) to improve the functionality of a low-cost material by coating it with a high performance but more expensive coating. Coating technologies can be roughly divided into thin- and thick- film technologies. Thin films, with thickness of less than 20 μm can be produced by dry coating processes like Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD); they offer excellent enhancement of surface properties and are for example used in optical and electronic device and cutting tools, Davis J.R. (2004). However, most of these thin-film technologies require a reduced pressure environment and, therefore, are more expensive with a limit on the size and shape of the substrate. Thick films have a thickness over 20 μm and can be several millimeters thick. They are required when the functional performance depends on the layer thickness, e.g. in thermal barrier coatings, when high erosion and corrosion conditions result in wear and the component life depends on the layer thickness, or when the original dimensions of worn parts have to be restored. Thick film deposition methods include chemical/electro-chemical plating, brazing, weld overlays, and thermal spray. Thermal spray processes, Davis J.R. (2004), are well-established surface treatments aiming at forming a coating by stacking of lamellae resulting from impact, flattening and solidification of impinging molten particles. "Thermal spraying comprises a group of coating processes in which finely divided metallic or non-metallic materials are deposited in a molten or semi-molten condition to form a coating. The coating material may be in the form of powder, ceramic rod, wire or molten materials, Hermanek, F.J. (2001).

Influence des paramètres d'injection de poudres sur le traitement thermique des particules

International audienceEn projection plasma, la dispersion, en taille et en vitesse des particules à la sortie de l'injecteur, détermine les trajectoires des particules dans le jet plasma, et donne leurs histoires dynamiques et thermiques (vitesses, état de fusion, taux de vaporisation ...) qui sont des paramètres déterminants de la qualité de l'impact et du dépôt. Dans ce qui suit on décrit un modèle qui caractérise cette dispersion et qui tient compte de la distribution granulométrique de la poudre, des collisions inter-particulaires et des collisions avec les parois, il permet d'obtenir la distribution en vitesse et en taille des particules en sortie de l'injecteur

Etude de la solidification rapide d'une lamelle métallique en contact imparfait avec un substrat céramique

International audienceCe travail s'intéresse au problème de solidification de matériaux bicouche en régime transitoire, dont les propriétés thermophysiques dépendent de la température. Le modèle numérique utilisé est basé sur une approche enthalpique pour résoudre le problème de changement de phase dans chacun des matériaux en présence. Les résultats sont présentés pour plusieurs paramètres tel que l'épaisseur de la lamelle, la résistance thermique de contact, la nature des matériaux et leurs températures à l'impact

On generalization of electric field strength in longitudinally blown arcs

Generalization of av. elec. field strength for different discharge conditions in longitudinally blown arcs is considered. Exptl. data for distinctive devices and different gases were used for phys. modeling. Anal. showed that heat transfer processes are responsible for I-E characteristic formation. Turbulent heat transfer is the most effective for atm. pressure discharges while convection plays the main role in vacuum arcs. A generalized I-E characteristic was obtained. [on SciFinder (R)

Flow modelling and gas heating inside a profiling channel of an electric arc plasma torch

This paper presents a theor. model for the numerical anal. of the acceleration and heating of a gas by an elec. arc in a profiling channel with a diffuser anode. The thermal parameters and transport coeffs. of helium were detd. and the approximative expression for their calcn. in a broad range of temp. (0,3-100) kK and pressures are proposed. The calcn. and study of the effect of the thermal nonequil. plasma on the characteristics of the cylindrical arc were carried out. The results of the calcns. were compared with exptl. data and calcns. obtained from an equil. plasma model. [on SciFinder (R)