Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

High temperature resistant joints between ceramics and metals by brazing

Research was carried out on the production of ceramic-metal joints. The common brazing alloys were unsuitable for joining silicon nitride to the nickel-based alloys. A method which showed significant promise involved eutectic brazing, in which a transient liquid phase, the nickel silicide eutectic, is formed. For the joining of silicon carbide both to itself and to metals, a family of BraSiC brazing alloys has been developed and optimized brazing parameters were identified. With respect to strain relieving interlayers, a number of novel geometry interlayers with a range of stiffness/deformability was developed for the silicon nitride - metal joints. It was not possible to use the same types of interlayer for the silicon carbide joints. A dual component, tungsten-nickel solid interlayer was selected for these joints. Testing showed that the mechanical strength of all of the joints produced tended to be relatively low (maximum 30 MPa). An inverse relationship existed between interlayer stiffness and joint strength. The silicon carbide material had excellent resistance to hot corrosive attack, but the tungsten in the interlayer failed due to oxidation. High frequency ultrasonics and computer-aided microfocus x-ray tomography were complementary methods for the non-destructive examination of joints. Finite element analysis provided a useful design aid for both interlayer structures and ceramic-metal joints. Good qualitative correlations were found between the predicted stress levels and those measured by the x-ray diffraction. The benefits of using strain-relieving interlayers, in particular the novel geometries, were confirmed by the residual stress measurements. The development of the joining technique for the gas turbine application did not progress sufficiently far to permit the production of any prototypes. However, mock-up/prototypes of the internal structure of the ceramic heat exchanger were produced using the BraSiC alloys.

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