Influence of bond coat surface roughness on the structure of axial suspension plasma spray thermal barrier coatings — Thermal and lifetime performance Nicholas Curry a, ⁎, Zhaolin Tang b , Nicolaie Markocsan a , Per Nylén a a University West, Trollhättan, Sweden b Northwest Mettech Corp., Vancouver, Canada abstract article info Available online xxxx Keywords: Thermal barrier coatings Suspension plasma spray Thermal conductivity Thermo-cyclic fatigue Thermal shock Suspension plasma spraying has become a very promising candidate for the production of strain tolerant coatings for the gas turbine industry. Under certain process conditions suspension plasma spraying (SPS) generates column-like structures in the produced coatings. While a mechanism for column formation has been suggested previously based on columns forming on surface asperities, the effect of modification of surface structures on SPS coating properties has not been investigated. In this study, the surface topography of bond coats within a TBC system were modified by the combination of polishing and surface grit blasting. Yttria stabilized zirconia coatings were deposited using an axial feed suspen- sion plasma spray gun. The surface topography of the resultant coatings was characterized using striped light projection. Samples were tested for thermo-cyclic fatigue lifetime at 1100 °C during 1 hour cycles. Thermal shock performance was evaluated using the burner rig test and thermal conductivity evaluated using the laser flash analysis. The results indicate that columnar SPS coating microstructure is strongly influenced by surface to- pography. Test results suggest that control of surface topography may be an important factor to improve the per- formance of SPS coatings. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Thermal barrier coatings (TBC's) have been in use for the protection of gas turbine hot section components for several decades [1,2]. For the last few decades the accepted industrial application methods for TBC coatings has been electron beam physical vapor deposition (EB-PVD) or plasma spraying (APS) [3]. EB-PVD has been favored for applications that receive significant thermal shock during service such as turbine blades and vanes [3]. Conventional porous APS ceramics tend to show poor lifetimes in applications of thermal shock. Applying APS coatings with segmented vertical cracks has been used industrially to generate strain tolerant plasma sprayed coatings [4,5]. However, these coatings tend to compromise thermal properties of the coating. Due to the rela- tive cost of EB-PVD coatings, there is a great interest in developing coat- ings of similar columnar morphology using lower cost techniques [5]. Recent developments of the plasma spray process have allowed the feeding of sub-micron powder in suspension or even solution precur- sors to form coatings [6]. Such coatings have the advantage of much finer scale microstructure features compared to those of conventional APS coatings. For TBC applications the feature of interest is the generation of coatings that form vertical cracks [7] or even truly colum- nar structures [8]. The formation of columnar structures during SPS deposition can be related to the atomization of suspension droplets and their interaction with the plasma jet. If process conditions and suspension parameters are controlled, atomization of the suspension can yield droplets in the range of a micron [9]. Berghaus et al. [10] proposed that if droplets gen- erated by atomization in the plasma are smaller than a few microns in diameter, then their trajectory of deposition is influenced dramatically by the plasma flow close to the sample surface. This would result in an effective shallow impact angle for the particles. VanEvery et al. [11] fur- ther proposed that for such small droplets or particles, their deposition trajectory would result in the formation of columnar structures on sur- face asperities. Furthermore, differences in structure of SPS coatings have been noted when deposited on bond coats produced by different techniques with different inherent surface roughness [12]. Assuming that the proposed mechanism for column formation holds true, then the authors propose that column size (density) will be influenced by the surface topography of the substrate or bond coat surface in the case of a TBC system. In that case there is potential to control the struc- ture of the SPS layer by manipulation of the interface on which it is to be deposited. This work presents initial spraying of SPS coatings on APS bond coats with modified surface roughness. Production of second stage coatings Surface & Coatings Technology xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +43 664 5589579. E-mail address: nicholascurry84@gmail.com (N. Curry). SCT-19701; No of Pages 9 http://dx.doi.org/10.1016/j.surfcoat.2014.08.067 0257-8972/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Please cite this article as: N. Curry, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.08.067