Ternary Ru/Al-based reactive multilayers

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The intermetallic B2 compound RuAl combines good high-temperature properties with high room-temperature ductility. The high negative heat of formation of -62 kJ/mol(atoms) enables sub-µm composites of Ru and Al to self-propagating exothermic reactions forming the B2 compound RuAl. This makes the system Ru/Al similar to other intermetallic reactive systems like Ni/Al, Pt/Al, Pd/Al and Co/Al. However, the system Ru/Al is of particular interest due to the combination of a ductile reaction product, a compositional range of existence of 4.1 % at. (at room temperature) and a good energy density (comparable to that of commercially available Ni/Al foils). This makes Ru/Al an interesting system in the field of reactive bonding with local heating limited to the region of joining. The characteristic parameter of binary reactive multilayer systems is the bilayer thickness by which the basic properties of the system (speed of reaction, ignition temperature, microstructure and morphology of the reaction product) are determined and cannot be varied independently. Aim of this project is to develop a new way to manipulate the properties of Ru/Al-based reactive multilayers by moving from binary to ternary Ru/Al/X layers (X=Ni, Pt, Hf, Ti). The focus is on lowering the relatively high ignition temperature of Ru/Al multilayers (400-575°C) to values of approximately 300°C, comparable to that of Ni/Al multilayers, thus improving the reaction in the presence of a heat sink (e.g. inside a joint gap). Because there are, to the best knowledge of the applicants, no systematic studies on ternary intermetallic reactive systems, an essential part of this project is to investigate the influence of chemical composition and stacking sequence on the basic properties of a reactive multilayer (speed of reaction, morphology and microstructure). Thereby, a better understanding of the underlying mechanisms shall be gained. To enable the investigation of large ranges of composition, combinatorial sputtering is employed. This technique allows deposition of materials libraries on a single substrate by sequential sputtering of layers with thickness gradients. The compositional range of existence of the RuAl-based B2 structure is investigated by X-ray diffraction and high-throughput electrical resistivity measurements after a thermal treatment of the ternary multilayers. Within these compositional ranges, intermetallic phase formation as a function of temperature is investigated by in-situ four point measurements of the electrical resistivity to draw conclusions on the ignition temperature. Based on the insights into the influence of composition and stacking sequence, ternary Ru/Al based reactive multilayers are developed combining the benefits of Ru/Al with a reduced ignition temperature.