Mechanism of spallation in platinum aluminide/electron beam physical vapor-deposited thermal barrier coatings
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INTRODUCTION
THERMAL barrier coatings (TBCs) have been used extensively in aircraft gas turbine engines for more than 20 years to insulate combustors and turbine vanes from the hot gas stream. The TBCs are applied by air plasma spray or by an electron beam physical vapor deposition (EBPVD) process. The EBPVD TBCs, developed and applied more recently to turbine blades, provide greater durability in aircraft gas turbine applications. The ceramic layer of choice is 7 wt pct yttria-stabilized zirconia (7YSZ) deposited on a metallic bond coat. The present article deals exclusively with an EBPVD ceramic on a platinum aluminide (Pt-Al) bond coat. Schematic drawings of the sample geometry studied and the coating cross section are shown in Figure 1. A critical role is played by a layer of alpha alumina (aAl2O3) formed between the ceramic and the bond coat. The alumina layer acts as the source of bonding between the metal and ceramic. By greatly retarding the oxygen transport rate, the alumina layer also reduces the rate of bondcoat oxidation. It is generally observed that the failure of the TBC is by spallation of the ceramic layer, typically at the bond coat to Al2O3 interface in EBPVD coatings[1] (Figure 1). This article presents a detailed description of localized spallation at grain boundaries in Pt-Al/EBPVD TBCs.
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EXPERIMENTAL PROCEDURE
The Pt-Al/EBPVD coatings were supplied for this study by a commercial turbine airfoil coating manufacturer. All samples were 25.4-mm-diameter by 3.18-mm-thick disks, as shown in Figure 1. A two-step process on the ^100& MAURICE GELL, Professor-in-Residence, and KRISHNAKUMAR VAIDYANATHAN, Graduate Assistant, Department of Metallurgy and Materials Engineering, BRENT BARBER, Graduate Assistant, and ERIC JORDAN, Professor, Department of Mechanical Engineering, and JIANGTIAN CHENG, Postdoctoral Fellow, Institute of Materials Science, are with the University of Connecticut, Storrs, CT 06269. Manuscript submitted December 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
oriented single-crystal superalloy RENE´ N5* produces the *RENE´ N5 is a trademark of General Electric Company, Fairfield, CT.
bond coat. The specimens are electroplated with 10 to 20 mg/cm2 platinum, after which they are aluminized by a chemical vapor deposition process. These coatings are then heat treated at 1079 7C, in vacuum, to form a homogeneous NiPtAl bond coat[2] and a thin adherent alumina layer, also called a thermally grown oxide (TGO). The bond-coat thickness is 30 5 3 mm, including the interdiffusion zone. The 7YSZ layer is deposited by EBPVD and has a thickness of 125 5 10 mm. The typical compositions of the coating and the substrate alloy used in this study are shown in Table I. For metallographic observations, the coated specimens are mounted in a stycast epoxy blend and vacuum impregnated. The sample is then ground, polished using a 1 mm diamond, and then final polished with 0.05 mm alumina to reveal grain boundaries. Normaski interference microscopy is used to bring out contrast in the bond-coat g
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