Improved oxidation resistance of thermal barrier coatings

In order to improve the engine output and the efficiency of gas turbines, optimized thermal barrier coatings (TBCs) are required to protect the metallic components at high temperatures. In common TBC-systems, consisting of a Ni-base alloy substrate/MCrAlY-bond coat/ZrO(2)7 wt.% Y2O3 top coat, an oxide layer grows at the interface bond coat/ceramic under high temperature service, which limits the life of these coatings. In this paper the oxidation resistance of a new tripler TBC-system, consisting of a CoNiCrAlY-bond coat/Pt-modified aluminide coating/ZrO(2)7 wt.% Y(2)O(3)top coat is compared with that of a common TBC-system. The as-coated Pt-aluminide coating consists of an outer region of PtAl2 + (CoNiPt)Al followed by a single phase layer of (CoNiPt)Al. The results of the oxidation tests at 1000, 1050 and 1100 degrees C in air show excellent oxidation resistance of the tripler TBC-system with the thickest investigated Pt-aluminide coating. In particular, a 28 mu m thick Pt-aluminide coating allows the thickness of the oxide layers to be reduced up to 70% compared to the common TBC after 500 h at all examined temperatures. After heat treatment the coating systems were investigated by SEM, EDX and X-ray analysis. Annealing tests with Al2O3 powder indicate which mechanism is probably responsible for the improved oxidation resistance of platinum additions. Platinum is evidently capable of decomposing aluminum oxide at temperatures above 900 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.