The Effect of Undercooling on Solidification of YSZ Splats
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JTTEE5 17:646–654 DOI: 10.1007/s11666-008-9238-5 1059-9630/$19.00 ASM International
The Effect of Undercooling on Solidification of YSZ Splats Haibo Liu, Markus Bussmann, and Javad Mostaghimi (Submitted May 21, 2008; in revised form September 2, 2008) We consider the rapid solidification of a molten YSZ particle, by solving the so-called hyperbolic equations for heat and mass transfer. The hyperbolic model predicts the interface undercooling (due to thermal and solutal effects) and velocity as a function of time, as well as the yttria redistribution within the solid phase. Results are then compared to corresponding ones that we obtained from a parabolic model, to assess the extent to which YSZ solidification is influenced by nonequilibrium effects. Results indicate that these effects are limited to the early part of the solidification process when undercooling is most significant. At this stage, the interface velocity is unsteady, and solute redistribution is most evident. As solidification decelerates, the nonequilibrium effects wane and solidification can then be properly modeled as an equilibrium process.
Keywords
hyperbolic, interface tracking, nonequilibrium, rapid solidification, relaxation time, undercooling, YSZ
development of new products and techniques requires a high investment. The simulation of spray coating formation can help optimize a design, and so save time and money.
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1. Introduction Thermal spray coating is an enabling technology, which has wide application in many industries including the gas turbine, aerospace, automotive, and biomedical (Ref 1). Zirconia-based ceramic coatings in particular are used as effective thermal barriers for metal structures in gas turbines (Ref 2, 3). These coatings can delay thermally induced failure, thus increasing the durability of machine components. In the plasma spray process, yttria-stabilized zirconia (YSZ) powders are injected into a plasma jet, particles melt and accelerate until they collide with a substrate, spread, and solidify. The substrate material can vary, depending on use. Deposition of high quality coatings requires knowledge that involves materials and interface science, fluid mechanics, and heat transfer. This multidisciplinary character of the spray coating process implies that the
C C0 Cp b D h JC kf ke m me Q T Tm T0 VD Vi
concentration (wt.%) initial concentration (wt.%) specific heat capacity (J kg-1 K-1) splat thickness (m) mass diffusivity (m2 s-1) heat transfer coefficient (W m-2 K-1) concentration flux (wt.% m s-1) nonequilibrium partition coefficient equilibrium partition coefficient slope of nonequilibrium liquidus (K wt.%-1) slope of equilibrium liquidus (K wt.%-1) latent heat of solidification (J kg-1) temperature (K) equilibrium melting temperature (K) initial temperature (K) mass diffusion velocity (m s-1) interface velocity (m s-1) Greek Symbols
This article is an invited paper selected from presentations at the 2008 International Thermal Spray Conference and has been expanded from the original presenta
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