Flattening of composite powder particles during thermal spraying
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Flattening of Composite Powder Particles during Thermal Spraying V. V. Sobolev, J.M. Guilemany, and A.J. Marffn An investigation is conducted of the time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of composites (agglomerated) powder particles at smooth and rough surfaces during thermal spraying. Considerations include the splat solidification, droplet mass loss, solid volume fraction, and variations of splat-surface friction and splat solidification velocity due to the presence of the solid phase. Effective values of the droplet viscosity, impact velocity, and Reynolds number taking into account characteristic features of the flattening process are introduced, and analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are established. Results can be used to predict the splat flattening parameters.
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Keywords compositeparticle, flattening, solid phase, splat-surface friction, thermal spraying, volume fraction
1. Introduction Composite powders consisting of a solid phase (carbides, oxides, etc.) and a metallic binder (nickel, chromium, cobalt, etc.) play an important role in the development of thermally sprayed coatings with increased wear and corrosion resistance (Ref 1). The structure and properties of these coatings depend essentially on the dynamics of flattening of the melted particles (droplets) of composite powders on a substrate surface. The dynamics of the droplet flattening upon impact have been studied experimentally and theoretically in a number of papers, which are reviewed in Ref 2 and in more recent publications (Ref 3-9). The results obtained enable prediction of the splat dimensions, which significantly influence its further cooling and solidification (Ref 10-13). This article investigates the flattening dynamics of composite powder particles during thermal spraying and presents analytical formulas that permit estimation of this process during practice. These formulas consider the roughness of the substrate surface, the splat solidification, and mass loss due to splashing and crater formation in the surface where the flattening occurs.
2. General Ideas Consider an agglomerate composite particle consisting of small, high-melting-point solid components (e.g., carbides) and a binding metal. Assume that during thermal spraying the binder melts and this particle of radius Rp impinges normally with a velocity U onto the surface of a substrate or previously deposited coating layer and forms a cylindrical splat of radius R and thickness b, which vary with time (t) during flattening. Assume further that the solid components are markedly smaller than the splat thickness and that the surface roughness eo and a liquidV.V. Sobolev, J.M. Guilemany, and A.J. Martin, Metalurgia FfsicaCiencia de Materiales, Centro de Proyecci6n Trrmica, Departamento de lngenierfa Qufmica y Metalurgia, Universidad de Barcelona, Mart/ i Franqurs, 1,08028 Barcelona, Spain.
Journal of Thermal Spray Technology
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