Components housed in stainless steel for protection against extreme environments seen in the aerospace and defense industries require paths for electricity to power them and communicate with them. Those paths in turn need a reliable insulation seal to prevent contact with the metal case that could short out the power and communication lines.

Strong bonds between materials for airtight, or hermetic, seals are crucial, and Sandia National Laboratories continues to advance how that’s done.

Typically, material used to isolate electrical paths is either glass or a glass-ceramic composite. Work by Steve Dai, principal investigator for a project on bonding glass-ceramic to stainless steel, aims to develop fundamental science in materials and processing for high performance and high reliability glass-ceramic-to-metal seals. That scientific foundation then could be used in designing, developing and manufacturing next-generation seals.

Dai’s team filed a provisional patent application in November for interfacial bonding oxides for glass-ceramic-to-metal seals.

A durable seal needs a strong chemical bond between the glass-ceramic and the metal and a close match of the coefficient of thermal expansion (CTE) between materials. The CTE defines how an object’s size changes as temperatures change. A glass-ceramic with crystalline phases formed inside the original glass increases the CTE to better match the metal housing and reduce thermal stresses.

Since bonded glass-metals must be processed at very high temperatures, “we need to manage the thermal mismatch very carefully to make sure during any stage in the sealing process there’s no tensile stress or tension on the glass that will cause a crack or unrecoverable separation from the metal housing,” Dai said.

Potential industrial uses seen

A seal that’s strong at high temperatures and pressures also has potential industrial uses, such as in fuel cells and aerospace or defense applications that operate in extreme environments.

Pure glass shrinks less in high temperatures than metal does. The mismatch causes metal to crimp, compressing the seal. That has both advantages and disadvantages. “The good thing is you don’t have to have very good bonding because there’s a lot of compression; the downside is that there could be too much compression, which could crack the glass over time,” Dai said.

Read more: New ways of looking at glass-to-metal seals