Mechanical Properties of Reactive Air Brazed (RAB) Metal/Ceramic Joints. Part 1: Visco-Plastic Deformation of Silver-Based Reactive Air Brazes

Development of planar solid oxide fuel cell systems for automotive application is mostly driven by light-weight design and durability issues. The joining technology utilized to seal ceramic and metallic stack components plays a key role. Rapid and frequent start-up capability is required for customer satisfaction. Furthermore, operational security of the system and passenger safety precondition reliable hermeticity of the seals. Metallic joints based on reactive air brazing (RAB), utilizing silver brazes, are preferred to glass ceramics because of their potentially higher failure tolerance. Mechanical design data of full-scale RAB joints, especially at high temperature, are scarce and for this reason up to now only estimated from literature data of bulk silver. Detailed deformation and damage mechanisms at high temperature are largely unknown, what further complicates target oriented braze development and evolution in stack design. A new testing technique (symmetric double shear testing) was proposed and successfully carried out for the characterization of full-scale braze joints. Mechanical testing was accompanied by scanning electron microscopy examination of microstructure. High temperature deformation and damage of the joints was uncovered to be influenced by both interfaces and matrix performance. The braze matrix governs elastic-plastic deformation, while the maximum strength is determined by the braze/joining partner interfaces. The creep properties of the joints were found to correlate to that of bulk silver