Materials Science and Engineering A 384 (2004) 331–351 Modeling of plasma assisted formation of precipitates in zirconium containing liquid precursor droplets Alper Ozturk, Baki M. Cetegen ∗ Mechanical Engineering Department, University of Connecticut, 191 Auditorium Rd. U-3139, Storrs, CT 06269-3139, USA Received 9 January 2004; received in revised form 7 June 2004 Abstract This paper focuses on the modeling of heat and mass transfer in precursor containing droplets injected into a plasma jet and the estimation of precipitate formation in these droplets from the solute. A hybrid model is employed where the plasma temperature and velocity fields are obtained from previous experimental results and the heat and mass transfer around droplets are modeled. The precipitate formation zones from the zirconium acetate solution in these droplets are estimated based on the solute concentration field within the droplet. A simple homogeneous nucleation hypothesis is employed in predicting the regions of droplets where zirconia might precipitate. The effects of droplet size, injection velocity and angle, plasma conditions as well as the solute mass diffusivity are considered. Micrographs from single pass coating experiments give credible evidence of the presence of similar types of particle morphologies in agreement with this modeling study. © 2004 Elsevier B.V. All rights reserved. Keywords: Droplet vaporization; Precursor solution; Precursor precipitation; Plasma processing; Ceramic coatings; Thermal spray 1. Introduction Metallic and ceramic coatings are applied on various engi- neering hardware components to provide protection against wear, corrosion and high heat fluxes. Metal hardware operat- ing in harsh environments could fail prematurely due to wear, corrosion and high temperature exposure. A protective layer of coating is typically applied to provide a barrier which pro- longs component durability and the desired properties of the hardware components. Among the different coating systems, the thermal bar- rier coatings (TBCs) are commonly used to protect hardware operating in high temperature environments, such as com- bustor liners and gas turbine blades, from excessively high heat fluxes and temperatures. Among the different ceramic coating systems, yttria stabilized zirconia is widely used as thermal barrier coatings. They are conventionally applied by introducing a powder of the coating material into a plasma jet in which powder particles are melted and accelerated to- ∗ Corresponding author. Tel.: +1 860 486 2966; fax: +1 860 486 5088. E-mail address: cetegen@engr.uconn.edu (B.M. Cetegen). wards the surface to be coated. While this technology has matured over the past several decades, the recent develop- ments have focused on attaining nanometer size features in the coating microstructure for superior coating properties in terms of better service performance and spallation resistance [1]. The processing conditions of powders for this type of coating have been recently studied in the laboratory and a pilot scale production facility [2]. Processing conditions to obtain nano-structured coatings require process optimization since the nanometer size features of the original powders need to be retained in the coating which would otherwise be lost upon melting. While the recent studies have yielded considerable success in attaining nano-structured coatings using powders, a new method of producing nano-structured coatings was recently discovered and it involves production of nano-structured yttria stabilized zirconia coatings from liquid precursors in- jected into a plasma jet [3]. In this new process, schematically shown in Fig. 1, an aqueous solution of precursors (zirconium acetate, yttrium nitrate and some additives) is injected into the plasma jet in the form a spray instead of ceramic powders. Rapid heat up and evaporation of the solution droplets in 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.06.042