Thin-Film Synthesis and Cyclic Oxidation Behavior of B2-RuAl
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1128-U06-10
Thin-Film Synthesis and Cyclic Oxidation Behavior of B2-RuAl Karsten Woll, Rama K.S. Chinnam and Frank Mücklich Functional Materials, Dept. for Materials Science and Engineering, Saarland University, Campus, 66123 Saarbrücken, Germany ABSTRACT B2-RuAl as a potential intermetallic for thin-film applications at high temperatures is studied with respect to thin-film synthesis and cyclic oxidation behavior. Using the multilayer approach, single phase RuAl thin films were fabricated. The phase sequence from the elements Ru and Al goes through RuAl6 to the final product RuAl. To understand the reaction mechanism calorimetric as well as kinetic experiments were performed. The cyclic oxidation behavior is characterized at 1200 °C up to 47 h. The morphology of the grown alumina shows no cracks or regions of spallation which indicates the good cyclic oxidation behavior. Compressive stresses in the oxidation scale of about 1.2 GPa at maximum were determined.
INTRODUCTION One group of materials that are used for protection purposes from extreme environments are intermetallic compounds based on single phase B2 aluminides [1]. Crucial factors for the application at high temperature, e.g. in thermal barrier systems [2] or protective coatings which avoid material interactions [3,4], are good oxidation as well as high creep resistance [5]. Recent experiments indicated that RuAl shows excellent oxidation properties due to the formation of a thin but compact protective Al2O3 scale [6]. During oxidation, only the thermodynamic stable αAl2O3 layer forms on the surface with a parabolic kinetic. Tryon et al. additionally revealed that the thermal expansion coefficient of RuAl is nearly equal to that of Al2O3, whereas the coefficient of NiAl exceeds that of alumina about one order of magnitude [7]. Thermal induced stresses at the interface between alumina and RuAl are therefore expected to be lower and crack initiation due to thermal loads should be significantly reduced. Moreover, RuAl with its extraordinary high melting point of 2060 °C appears as a candidate material that exhibits high creep resistance. Therefore, the RuAl intermetallic compound is a potential material for applications at high temperatures [8,9]. RuAl obviously finds prospective application as a thin film or coating. However, there are only few systematic studies concerning the thin-film fabrication of RuAl [10]. Therefore, the detailed study of its synthesis is the first purpose of this paper. For this, the multilayer approach was chosen where individual Ru and Al layers were staggered. The thickness of each layer is in the order of tens of nanometers. Annealing such multilayers results in the formation of intermetallic phases. The application of intermetallics at high temperatures requires a good cyclic oxidation resistance which is the second purpose of this paper. Good isothermal oxidation behavior is an essential precondition which is given for RuAl [6, 11]. Cao et al. studied the cyclic oxidation behavior of near single-phase RuAl and two-phase RuAl-
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