Fraunhofer-Center HTL develops soldering and joining processes for high-temperature applications. The joints include substance-to-substance, positive-fitting and/or friction-locked joints. They can be customer-specifically developed at the HTL for joint compoents made of monolithic ceramics, fiber-reinforced ceramics (CMC) or metals. The HTL has the entire process chain for the manufacture of geometrically complex ceramic components and metal-ceramic composites at its disposal (see Component design).
Construction and Design of Material Composites
For successful joining, a suitable design for joining components and connection points is vital. This is undertaken at the Fraunhofer-Center HTL employing finite element (FE) methods. By simulation of the application behavior of the respective components, the loads in the joint area are determined. The geometry of the connection is optimized, so that mechanical and thermal stresses are minimized. The reversible combination made of positive-fitting and friction-locked joints is implemented at the Fraunhofer-Center HTL using special ceramic screws (see Stepanyan, M.; Seifert, G.; Weiß, T.: Why not Using Ceramics).
Development of Solders
At the Fraunhofer-Center HTL, special crystallizing glass solders are used for the joining process. These glasses provide good wetting and low viscosity and can therefore fill the joint gap accurately. For non-oxide ceramics and metals, solders on a silicon or metallic basis are also used. The development of solders is supported by the ThermoOptical measuring devices available at the Fraunhofer-Center HTL. With the different solders, both joinings between similar materials as well as joinings between ceramic and CMC, ceramic and metals or CMC and metals can be implemented (see Gadelmeier, C.; Schmidt, J.; Göthe, M.; Jovanovic, D.: Characterization of Furnace sintered mullite and oxide ceramic matrix composites (O-CMC) by using glass solders).
High-temperature Joining Processes
Joining processes are implemented at the Fraunhofer-Center HTL using different equipment: If the focus is on low-stress heat treatment or a controlled atmosphere, joining processes are performed in the conventional way by means of furnace heating. For this purpose, furnaces with different atmospheres and usable volumes are available at the Fraunhofer-Center HTL. The process times range from several hours up to one day. Alternatively, joining can be performed using laser heating within minute cycles (see Gadelmeier, C.; Schmidt, J.; Jovanovic, D.; Zietkowski, M.; Eckardt, C.: Adhesive bonding of ceramics for complex ceramic parts in high temperatures furnaces).
Characterization of Joint Connections
The quality of the produced joined connections and material composites is evaluated at the Fraunhofer-Center HTL using different testing methods. Standard-conforming mechanical test procedures are employed for shear strength and flexural strength at room temperature and high-temperature up to 1500 °C. For evaluation of the composite components or large surface joinings, the non-destructive test procedures available at the Fraunhofer-Center HTL are ideal. For determination of the seal tightness of joined connections, at HTL the gas tightness is tested compliant to the DIN EN 1779 standard at temperatures up to 1000 °C.