Formation of vertical cracks in solution-precursor plasma-sprayed thermal barrier coatings Liangde Xie a, ⁎ ,1 , Dianying Chen a , Eric H. Jordan b , Alper Ozturk b , Fang Wu a , Xinqing Ma c , Baki M. Cetegen b , Maurice Gell a a Department of Materials Science and Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, United States b Department of Mechanical Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, United States c Inframat Corporation, Farmington, CT 06032, United States Received 18 June 2005; accepted in revised form 12 January 2006 Available online 23 February 2006 Abstract When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spray (SPPS) process produces a microstructure containing uniformly vertical cracks. These cracks provide a high degree of strain tolerance to the ceramic top coat. In order to understand the formation of vertical crack in SPPS process, coatings of various thicknesses were deposited on a variety of substrates with vastly different thermal properties. These coatings were characterized in the as-sprayed state and after heat treatment. It has been determined that the tensile stress derived from the pyrolysis of precursor occurring during coating deposition or post heat-treatment is the major driving force for the formation of vertical cracks in SPPS TBCs. © 2006 Elsevier B.V. All rights reserved. Keywords: Thermal barrier coatings; Plasma spray; Solution precursor plasma spray; Vertical cracks 1. Introduction Ceramic thermal barrier coatings (TBCs) of 6 ∼ g wt.% Y 2 O 3 - ZrO 2 (7YSZ) are widely used to protect and to insulate metallic components in gas-turbine engines from high tem- peratures, which can prolong the lifetime of the component and/ or increase the allowable operating temperatures (see, e.g., overview articles by Miller [1,2], Jones [3], Evans et al. [4], and Padture et al. [5] and references therein). TBCs are subjected to cyclic thermal environment in service. As a result, residual stresses are generated in the coating due to the differences in the coefficient of thermal expansion (CTE) between the ceramic coating and the metallic substrate. In order to achieve high durability in TBCs, the ceramic top-coat is required to have some capability to tolerate the resulting thermal strain or to be “strain tolerant”. The two commercial processes used for the deposition of TBCs are air plasma spray (APS) and electron-beam physical- vapor deposition (EB-PVD). APS TBCs are relatively low-cost, have lower thermal conductivities relative to EB-PVD TBCs, but are generally less durable [1–5]. In order to increase the durability of APS TBCs by improving the strain tolerance of the coating, Taylor [6] developed dense vertically cracked (DVC) APS TBCs. The DVC TBCs have a density greater than 88% of the theoretical density. A majority of vertical cracks runs normal to the ceramic/metal interface and has a length greater than half of the coating thickness. Improved thermal cyclic durability has been reported for the DVC TBCs compared to the normal APS TBCs [6]. In the APS process, the coating is formed by the accumulation of splat layers deposited each time the plasma torch passes over the substrate. The vertical cracks in DVC TBCs are generated through the alignment of micro vertical cracks produced in each splat layer due to shrinkage of the deposited splats [6]. It was found that if the coating density is less than about 88% of the theoretical density, the shrinkage strain can be absorbed or compensated by the porosity, which prevents the formation of the vertical cracks [6]. The improved stain tolerance of DVC TBCs is partially offset by the higher thermal conductivity associated with the higher coating density. Surface & Coatings Technology 201 (2006) 1058 – 1064 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +1 516 338 2516; fax: +1 860 338 2488. E-mail address: liangde.xie@sulzer.com (L. Xie). 1 Currently with Sulzer Metco (US) Inc., Westbury, NY 11590, United States. 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.01.020