The Impact of a Partially Molten YSZ Particle
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ommy Che-Ming Wu, Markus Bussmann, and Javad Mostaghimi (Submitted February 13, 2009; in revised form June 18, 2009) In plasma and high velocity oxy-fuel spraying, a certain portion of the injected powder is often not fully molten upon impact onto a substrate. We present numerical results of the impact of a partially molten yttria-stabilized zirconia particle, a material widely used as a thermal barrier. We consider an idealized scenario: the axisymmetric impact of a particle with a solid core, examining only the flow of the molten material around the core. The numerical method is based on the immersed boundary method for treating the interaction of solids and fluid within a computational domain. We present the results of a set of simulations, varying the particle and solid core diameters, and the impact velocity. When the core diameter is small relative to the particle, the impact behavior is similar to the impact of a fully molten droplet. When the core is larger, it promotes considerable splashing, which is undesirable.
Keywords
properties of coatings, splats cooling, spray depositions
1. Introduction Thermal-sprayed coatings are widely used for diverse applications including, for example, to protect substrates from corrosion, wear, and high temperature; to enhance the biocompatability of implants; and to fabricate solid oxide fuel cells (Ref 1). In general, material in powder form is injected into a heat source (e.g., a plasma or combustion jet); the particles are entrained by the jet, accelerate and melt, and upon impact onto a substrate, spread and solidify into lamellar splats. A coating is the agglomeration of many such splats, piled one on top of another. A common characteristic of thermal-sprayed coatings is the presence of a small fraction of particles that were not entirely molten upon impact. Figure 1 is a micrograph of a cross section of a yttria-stabilized zirconia (YSZ) coating produced by atmospheric plasma spraying (SG-100 torch, Praxair). In the middle of the figure are several particles that impacted as solid, evidenced by the fact that close inspection reveals that the particle microstructure is equiaxed, which is characteristic of YSZ powder, in contrast with the columnar microstructure visible elsewhere in the cross section, that forms during the rapid solidification of molten YSZ; this also suggests that the particles are not simply fragments of a molten particle that splashed on impact. Also, note that the particles are not spherical,
Tommy Che-Ming Wu, Markus Bussmann, and Javad Mostaghimi, Centre for Advanced Coatings Technology, University of Toronto, Toronto, ON, Canada; and Department of Mechanical and Industrial Engineering, University of Toronto, 5 KingÕs College Road, Toronto, ON M5S 3G8, Canada. Contact e-mail: mostag@mie. utoronto.ca.
Journal of Thermal Spray Technology
perhaps because they deformed on impact and/or because YSZ is not a pure material. There are several reasons why a small fraction of particles will not completely melt (Ref 2, 3): some particles are simply too
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