Application-Optimized Soldering of Ceramics

The integration of ceramic components often requires joining. Brazing offers advantages such as high joint strength, thermal stability and vacuum-tight joints. By means of active filler metal brazing ceramic-ceramic and ceramic-metal joints can be produced in high quality in a single-step process. Diverse practical requirements apply to ceramic joints. If the ceramic component is to be stable at high temperatures, high thermal stability of the joint is essential. In such cases, active brazes offering high temperature resistance are required. If high mechanical strength at moderate temperatures is the primary concern, it is more cost-effective to carry out the brazing process at the lowest possible temperatures. The lower the brazing temperature, the lower the thermomechanical stresses in the joint, resulting in increased joint strength. In recent years, Fraunhofer IKTS has developed active brazes for a wide range of high-temperature applications as well as for temperature ranges below 500 °C.

High-temperature brazes

Conventional active filler metal brazing joints for ceramic materials based on Ag-Cu can be used at temperatures of up to 600–800 °C and require brazing temperatures of 900–1000 °C. At Fraunhofer IKTS, low-cost commercial raw materials from the steel industry were processed into Fe-Si-Ti brazing pastes in quantities ranging from 100 g to kilogram scale. These require brazing temperatures of 1300–1400 °C. The thermal stability of SiC-SiC joints produced using the developed Fe-Si-Ti brazes is excellent, as demonstrated by creep tests shown in Fig. 1 and by the high-temperature strength values listed in Table 1. Additional investigations at temperatures of up to 1100 °C are currently ongoing with positive results. Since they are oxidation-resistant in air, Fe-Si-Ti alloys are an alternative to precious-metal-based brazing alloys, enabling critical raw materials such as platinum, gold and silver to be substituted.

Low-temperature solders

The Al-Cu-Si alloy system is currently under development for low-temperature brazing of ceramic materials. In particular, aluminum and aluminum oxide have been successfully joined, producing initial ceramic-ceramic and ceramic-metal joints. The resulting joint properties are currently being evaluated, including strength, tightness and chemical stability. Initial results indicate that this brazing alloy could enable aluminum alloys to be joined to ceramics with significantly higher strength than is achievable using solders.

Services offered
 

• Modification of soldering alloys for specific application conditions
• Production of ceramic joints
• Concept development for composite structures, joining technologies, and integration concepts for ceramic components