Formation of discontinuous Al 2 O 3 layers during high-temperature oxidation of RuAl alloys
- PDF / 509,131 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 77 Downloads / 234 Views
Bond coats play a crucial role in the performance of thermal barrier coating systems. Ru alloys have been identified as promising candidates; therefore, systematic studies were performed on the oxidation behavior of bulk RuAl (50–50 at.%). Isothermal oxidation and thermogravimetric analyses were performed at 1100 °C for different times ranging from 0.1 h to 500 h. Microstructural characterization was performed by scanning and transmission electron microscopy. The results showed the formation of an ␣–Al2O3 layer on top of a ␦–Ru layer. Interface instability between these layers and evaporation of gaseous Ru-oxides lead to the formation of large elongated cavities and alternating ␣–Al2O3/␦–Ru layers.
I. INTRODUCTION
The primary function of thermal barrier coatings (TBCs) is to provide a low thermal conductivity to protect the surface of the turbine airfoil’s superalloy from the hot gas in the engine.1 TBCs, which are used on nickel-based superalloy components, generally consist of an yttria-stabilized zirconia (YSZ) coating deposited onto an oxidation-resistant bond-coat (BC) alloy, which develops a thermally grown oxide (TGO).2 The BC has to fulfill two main functions: (i) protect the superalloy from oxidation since the YSZ is permeable to oxygen, and (ii) provide a bond between the deposited TBC and the underlying base alloy. Platinum-modified nickel aluminide (Ni,Pt)Al3–5 and MCrAlY (M, metal)6,7 alloys are the two major classes of BC alloys that have evolved over the years; these alloys were developed to form ␣–Al2O3,8 which is compatible with YSZ when exposed to air at high temperatures.9–11 RuAl (Fig. 1)12 is an intermetallic B2 compound with high room temperature (RT) hardness and toughness13; it has a high melting point, a coefficient of thermal expansion close to that of ␣–Al2O3,14 and a good creep resistance15 (Table I). All these characteristics render this alloy a potential candidate for high-temperature applications. To investigate the possibility of using more complex Ru-containing alloys [i.e., (Ru,Ni)Al] as an alternative to (Ni,Pt)Al BCs, fundamental studies on the
a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Zentrum für Mikro- und Nanotechnologien, 98684 Ilmenau, Germany. DOI: 10.1557/JMR.2006.0028 276
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 1, Jan 2006 Downloaded: 03 Apr 2015
oxidation behavior of binary RuAl bulk material are needed. In the present investigation, oxidation experiments have been conducted at 1100 °C, which represents the desired working temperature for BCs used in TBC systems. Even if sintered single-phase RuAl displays “good” oxidation properties up to 1000 °C, where a dense and continuous ␣–Al2O3 protective layer grows on an aluminum-depleted layer (Fig. 1) identified as ␦–Ru,16 it is suspected that the oxidation at higher temperatures may behave differently. In fact, IrAl alloys, which have structure and phase diagram17 similar to RuAl, form a discontinuous Al2O3 layer when oxidized at 1300 °C.18 Furthermore, at
