Surface geometry and strain energy effects in the failure of a (Ni, Pt)Al/EB-PVD thermal barrier coating Krishnakumar Vaidyanathan a,1 , Eric H. Jordan b, * , Maurice Gell a a Department of Metallurgy and Materials Engineering, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA b Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 062693139, USA Received 23 May 2003; received in revised form 24 October 2003; accepted 29 October 2003 Abstract Thermal cycling tests were conducted on a commercial yttria-stabilized zirconia electron beam-physical vapor deposited thermal barrier coating (TBC) on a platinum aluminide (b-(Ni, Pt)Al) bond coat. Surprisingly, the longest life sample lasted 10 times longer than the shortest life sample. Two distinct mechanisms have been found responsible for the observed damage initiation and pro- gression at the thermally grown oxide (TGO)/bond coat interface. The first mechanism leads to localized debonding at the TGO/ bond coat interface due to increasing out-of-plane tensile stresses at ridges that form along bond coat grain boundaries. The second mechanism is driven by cyclic plasticity of the bond coat that leads to cavity formation at the TGO/bond coat interface. The primary finding of this work is that the first mechanism, involving tensile stress at ridge tops, is life limiting. Based on this mechanism, it is demonstrated that the variation in bond coat ridge aspect ratio can explain the unusual 10 variation in observed sample life. It is proposed that ridge top spallation leads to debonds of sufficient size to result in unstable fracture driven by the strain energy stored in the TGO. The criticality of the flaw created by local debonding is supported by experimental determination of the strain energy available in the TGO through measurement of TGO stress and thickness combined with published fracture mechanics solutions of the relevant flaw geometry. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal barrier coatings; Platinum aluminide; Physical vapor deposition; Failure mechanisms 1. Introduction Thermal barrier coatings (TBCs) are ceramic coatings applied to the external surface of cooled gas turbine metal components to reduce the metal temperatures by up to 150 °C. This results in important improvements in both component life and system performance. Such coatings are typically made of yttria-stabilized zirconia (YSZ), because YSZ has a number of favorable physical and mechanical properties including a relatively low thermal conductivity, high coefficient of thermal ex- pansion, and adequate erosion and environmental re- sistance. Such coatings are used on nearly all current generation aircraft and industrial gas turbines. TBCs eventually fail by spallation of the YSZ ceramic layer. A large number of failure mechanisms have been identified that are dependent on the particular TBC system and actual strain, strain rate and temper- ature history experienced by the coated component [1– 6]. In the present paper, we present the failure mode of a specific TBC, produced by a Pt-modified nickel alumi- nide bond coat/electron beam-physical vapor deposited (EB-PVD) YSZ process [7,8]. In this particular TBC system, the failure life for nominally identical samples ranges from 190 to 1917 cycles. It will be shown that the large variation in cyclic life is related to a large variation in surface roughness, which resulted from uncontrolled factors during processing. The proposed failure scenario has two steps, initial debonding at asperities driven by localized tensile stresses, followed by total separation of the TGO driven by the strain energy stored in the TGO. Quantitative estimates of the tensile stress and strain * Corresponding author. Tel.: +1-8604862371; fax: +1-8604865088. E-mail address: jordan@engr.uconn.edu (E.H. Jordan). 1 Present address: Corning Incorporated, Corning, NY 14831, USA. Acta Materialia 52 (2004) 1107–1115 www.actamat-journals.com 1359-6454/$30.00 Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.10.043