Components housed in stainless steel for protection against the extreme environments in aerospace, defense and some industrial 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 hermetic seals are crucial, and researchers at Sandia National Laboratories continue to advance how that is done.
Typically, the material used to isolate electrical paths is 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 glass-ceramic-to-metal seals.
A durable seal needs a strong chemical bond between the glass-ceramic and the metal as well as 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.
Because bonded glass-metals must be processed at 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. A seal that is strong at high temperatures and pressures also has potential industrial uses.
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.
His team looked at making a chemical bond between metal and glass-ceramics, without adding steps to production, by establishing an interfacial bonding layer, a bridge material that bonds to both steel and glass. “It’s very difficult because these are two very dissimilar materials, a piece of steel and a piece of glass-ceramic. They hardly share anything,” Dai said.
Glass-to-metal seals are processed in an inert atmosphere devoid of oxygen because metal grabs oxygen from the atmosphere, leading to oxidation and rust. But the process contains an inherent contradiction: A metal bond to a glass-ceramic requires an oxide, so the interfacial bonding layer is really an interfacial oxide layer.