Characterization of Interdiffusion and Resulting Abnormal Crystal Growth and Cracking During Glass-Ceramic-to-Metal Joining

Glass-ceramic (GC) -to-metal seals are often used to join electrical contact pins and header shells for high-reliability electrical feedthroughs. In this application, lithium silicate based GC is sealed to an outer stainless steel shell and a central precious metal alloy pin. This material set is selected to create a favorable coefficient of thermal expansion (CTE) mismatch (i.e. CTE_shell>CTE_GC>CTE_pin) such that a hermetic, compression seal is established after joining. Recently, abnormal large crystal growth (identified as lithium disilicate (Li₂Si₂O₅)) was observed radiating from the pin/GC interface, through the GC, towards the shell. The large crystals have a lower/incorrect CTE compared to the remainder of the GC (closer to the shell). This incorrect CTE results in the development of high hoop stress at the interface between the large crystal zone and the normal bulk GC and, if the stresses are sufficiently high, can lead to cracking starting at the crystal tips and progressing into the bulk GC. This presentation will cover characterization of the pin/GC interdiffusion that develops during sealing and resulting microstructure in both the electrical contact pin and GC, as well as implications on stress evolution in the seal (from finite element analysis (FEA)). Methods to mitigate against abnormal crystal growth and cracking will also be discussed, including recent efforts to characterize an alternative electrical pin alloy.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.