Mechanical behavior of reactive air brazed (RAB) Crofer 22 APU-Al 2 O 3 joints at ambient temperature
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Mechanical behavior of reactive air brazed (RAB) Crofer 22 APU‑Al2O3 joints at ambient temperature Wolfgang Tillmann1 · Nadeem Babar Anar2 · Lukas Wojarski1 Received: 12 February 2020 / Accepted: 26 March 2020 © Springer Nature Switzerland AG 2020
Abstract The evolution of innovative high-temperature electrochemical devices, such as high temperature solid oxide fuel cells (SOFCs), gas separators and gas reformers, consisting of metal–ceramic-joints is challenging. The seals have to be stable and gastight in isothermal high-temperature as well as in thermo-cyclic operation. Here, the reduction of porosity is the primary aim, to obtain air brazed joints with a long lifetime. In the last years, reactive air brazing (RAB) has gained rising interest for the joining of ceramic–ceramic and ceramic–metal compounds. In this paper an alternative brazing filler metal manufacturing process employing (physical vapor deposition (PVD)) is applied and its feasibility for the production of metal–ceramic composites has been investigated for Ag–4 wt%CuO. For RAB aluminum oxide with ferritic high chromium steel Crofer22APU have been joined. The pore formation in subordination of the braze and base materials can be monitored after brazing. By modifying the brazing process, the pore formation in the joints can be avoided. The microstructure of brazed joints with the developed braze foils is studied. Discussion of the results focuses on the influence of microstructural evolution on mechanical properties, the pore formation in the brazing seam and failure behavior of the brazed joints. A correlation between the process parameters brazing temperature and holding time and the achieved compound properties could be derived. Further, excellent wetting of the ceramic was obtained. The highest shear strength with 123 MPa was measured for a temperature of 1000 °C and 5 min, using the Ag4CuO alloy. Keywords Reactive air brazing (RAB) · Physical vapor deposition (PVD) · Metal–ceramic composites · Microstructure · Shear strength · Fracture
1 Introduction Ceramic–metal compounds are becoming increasingly important for a large number of technical applications [1]. High-temperature heat exchangers, gas turbines and combustion engines are examples of such advanced power generation devices. There is a growing demand to generate materials appropriate for the industrial use, especially for the operation under oxidizing conditions [2, 3]. Ceramics offer totally new capabilities due to their exceptional high-temperature material properties. These enables their
usage for many high-temperature technology [4, 5]. The fabrication of ceramic–metal compounds allows to combine the mechanical properties of these material classes. However, the joining of ceramics to themselves or to a metallic counterpart is a challenging task by reason of the insufficient wetting behavior of metallic filler brazes on ceramics [6]. The brazing process for these applications induces high internal thermal stresses during cooling due to large differences in the coefficients of the t
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