Overaluminizing of a CoNiCrAlY Coating by Inward and Outward Diffusion Treatments
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NTRODUCTION
THERE are two main categories of non-ceramic high temperature coatings to protect the hot section of gas turbine components: diffusion aluminide and MCrAlY (M represents Ni, Co, or the both) overlay coatings. Diffusion aluminide coatings are easy and inexpensive to apply and develop the most protective oxide—aluminum oxide—during service. However, they show insufficient resistance to such degradation mechanisms as hot corrosion by fused salts,[1] thermal stresses,[2] and interdiffusion with the substrate.[3] MCrAlY overlay coatings are resistant to hot corrosion due to their high chromium content. Their compositions do not depend on the substrate and can be tailored to withstand specific corrosive environments.[4–6] The body of the coating is not an intermetallic phase like the case in diffusion aluminide coatings; therefore, a better mechanical behavior is brought about. However, MCrAlY overlay coatings have less aluminum content which makes them less resistant to high temperature oxidation in contrast to diffusion aluminide coatings.[7] In order to achieve the advantages of both aforementioned categories, a new class of high temperature coatings has been developed via combining their application processes. The normal coating procedures for these coatings (known as overaluminized MCrAlY coatings) include thermal spraying of a MCrAlY composition followed by an aluminizing process.[7] Although these compound coatings have had their share of development and the commercial versions were presented,[8,9] some primary issues had not been sufficiently SAMIRA MOHSENI BABABDANI, M. Sc. Student, and FARHAD SHAHRIARI NOGORANI, Assistant Professor, are with the Department of Materials Science and Engineering, Shiraz University of Technology, 71557-13876 Shiraz, Iran. Contact e-mail: [email protected] Manuscript submitted August 1, 2013. Article published online December 7, 2013 2116—VOLUME 45A, APRIL 2014
clarified in the literature.[10–17] For instance, the effect of the mode of formation of the aluminide top layer (i.e., inward or outward growth[18]) on the oxidation behavior of overaluminized MCrAlY coating can be mentioned. The growth mechanism of the diffusion aluminide coatings on nickel-based alloys has been the subject of some previous studies, such as those by Goward and Boone,[18] Das et al.,[19] and Xiang et al.[20] Goward and Boone[18] showed that only two main basic types of aluminide coatings can be formed on nickel-based superalloys. The first type is formed by inward diffusion of aluminum from a powder mixture of high aluminum activity. As a result, an intermetallic Ni2Al3 phase (or aluminum-rich b-NiAl phase or a mixture of these two phases) develops on the substrate and an additional heat treatment at higher temperatures is needed to convert this brittle phase to a more ductile b phase for practical use. During the heat treatment, nickel from the substrate diffuses outwardly and builds a new intermetallic b layer beneath the Ni2Al3 layer. The required aluminum for the growth of the newly formed b layer come
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