Failure modes in plasma-sprayed thermal barrier coatings K.W. Schlichting a,1 , N.P. Padture a, *, E.H. Jordan b , M. Gell a a Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA b Department of Mechanical Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA Received 19 February 2002; received in revised form 18 April 2002 Abstract Commercial plasma-sprayed thermal barrier coatings (TBCs) were investigated in an effort to elucidate the failure modes during thermal-cycling. Residual stresses in the thermally grown oxide (TGO) was measured using the Cr 3 photoluminescence piezo- spectroscopy (PLPS) method and the microstructures of the TBCs were characterized as a function of thermal cycles. The average residual stress in the TGO was found to be of the order of 1 GPa. The average thermal-cyclic life of the TBCs was found to be /350 cycles. Microstructural observations revealed that as the TGO thickened, cracking occurred at the bond-coat/TGO interface, and in some instances cracking also occurred at the TGO/top-coat interface, but primarily at crests of bond-coat undulations. The bond- coat-TGO separation resulted in ‘layering’ of the TGO at crests due to enhanced TGO thickening in those regions. In the troughs of bond-coat undulations, cracking occurred within the top-coat when the TGO was thick. Thus, the primary failure modes in these TBCs were: (i) cracking of the bond-coat/TGO interface; (ii) cracking within the top-coat; and (iii) linking of these microcracks by fracture of the TGO. A semi-quantitative failure model has been used to rationalize some of the observed cracking modes. Based on this analysis some suggestions are made for improving TBC durability. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal barrier coatings; Plasma spray; Thermo-mechanical fatigue; Ceramics 1. Introduction Thermal barrier coatings (TBCs) are used to protect and insulate hot-section metal components in advanced gas-turbine (aircraft and power generation) and diesel engines (see e.g. overview articles by Jones [1],Evans et al. [2] and Padture et al. [3] and references therein). The use of TBCs can result in a temperature reduction of 100  /300 8C at the metal surface, thereby improving the durability of the metal component and enhancing engine performance. Current plasma-sprayed TBC systems consist of the following four-layers: (i) internally-cooled Ni-based superalloy (substrate); (ii) oxidation-resistant MCrAlY bond-coat ( /125 mm thick), where M denotes Ni and/or Co; (iii) thin thermally grown oxide (TGO) scale, primarily a-Al 2 O 3 (1  /10 mm), which forms as a result of heat-treatment and oxidation of the bond-coat; and (iv) plasma-sprayed top-coat of ZrO 2 containing 6 / 8 wt.% Y 2 O 3 (YSZ). The top-coat, which is ‘strain- tolerant’ due to the presence of microstructural defects (pores, cracks, splat-boundaries), ranges in thickness from 200 to 500 mm for gas-turbine engines and up to 2 mm for diesel engines [4]. The electron-beam physical vapor deposition (EB-PVD) process is also used to make YSZ top-coats, but for relatively thinner (125  /200 mm), high-performance TBCs (not discussed here). In some instances, TBCs fail prematurely during service by spallation, exposing the bare metal to dangerously hot gases. This issue of TBC failure has become very important in the context of future gas- turbine engines designed for improved efficiency (higher operating temperatures), durability, and reliability. Thus, the understanding of failure mechanisms is a key element in the development of both better life- prediction models and future prime-reliant TBCs. The failure of plasma-sprayed TBC under thermal cycling is a highly complex process involving an inter- play between several general phenomena listed below (see [5 /10]): (i) thermal-expansion mismatch stress; (ii) * Corresponding author. Fax:  /1-860-486-4745 E-mail address: nitin.padture@uconn.edu (N.P. Padture). 1 Present address: Pratt and Whitney, East Hartford, CT 06108, USA. Materials Science and Engineering A342 (2003) 120  /130 www.elsevier.com/locate/msea 0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5093(02)00251-4