Characterization of Microstructure and Thermal Properties of YSZ Coatings Obtained by Axial Suspension Plasma Spraying (
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Ashish Ganvir, Nicholas Curry, Stefan Bjo¨rklund, Nicolaie Markocsan, and Per Nyle´n (Submitted February 20, 2015; in revised form May 3, 2015) The paper aims at demonstrating various microstructures which can be obtained using the suspension spraying technique and their respective significance in enhancing the thermal insulation property of a thermal barrier coating. Three different types of coating microstructures are discussed which were produced by the Axial Suspension Plasma Spraying. Detailed characterization of coatings was then performed. Optical and scanning electron microscopy were utilized for microstructure evaluations; x-ray diffraction for phase analysis; water impregnation, image analysis, and mercury intrusion porosimetry for porosity analysis, and laser flash analysis for thermal diffusivity measurements were used. The results showed that Axial Suspension Plasma Spraying can generate vertically cracked, porous, and feathery columnar-type microstructures. Pore size distribution was found in micron, submicron, and nanometer range. Higher overall porosity, the lower density of vertical cracks or inter-column spacing, and higher inter-pass porosity favored thermal insulation property of the coating. Significant increase in thermal diffusivity and conductivity was found at higher temperature, which is believed to be due to the pore rearrangement (sintering and pore coarsening). Thermal conductivity values for these coatings were also compared with electron beam physical vapor deposition (EBPVD) thermal barrier coatings from the literature and found to be much lower.
Keywords
axial injection, columnar and feathery columnar microstructure, micron, MIP, submicron, and nanometer pores, suspension plasma spraying, thermal barrier coatings, thermal diffusivity and conductivity, vertical cracks
1. Introduction Suspension spraying is a relatively new technique to produce coatings with a variety of microstructures (Ref 1, 2). Conventional way of spraying solid powder as feedstock using Atmospheric Plasma Spraying (APS) puts limitation on the powder size, where it is difficult to spray fine powders. Suspension spraying is an alternative where the suspension of a specific powder is used as feedstock, which makes it possible to spray submicron- and nanometer-sized powder particles (Ref 3). Suspension spraying can produce highly porous (Ref 4) coatings with different kinds of microstructures such as columnar type (Ref 5, 6), feathery columnar type (Ref 6), and vertically cracked type (Ref 4, 7). Sus-
Ashish Ganvir, Nicholas Curry, Stefan Bjo¨rklund, Nicolaie Markocsan, and Per Nyle´n, Department of Engineering Science, University West, 46186 Trollha¨ttan, Sweden. Contact e-mails: [email protected] and ashish. [email protected].
Journal of Thermal Spray Technology
pension sprayed coatings have also shown lower thermal conductivity than conventional APS coatings (Ref 4, 8). In a previous screening study, it was shown that feasible coating microstructures can be achieved by using Suspension High Velocity Oxy-Fuel (
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