New Ways of Glass-to-Metal Seals

Glass to Metal Seals

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.

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

Hengli Glass to Metal Seal Furnace

A Hengli Glass to Metal Seal Furnace

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.

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.

Approach modified glass-ceramic with oxidant

Dai's thermodynamic approach modified, or doped, the glass-ceramic sealant with an oxidant. That oxidant, serving as a sacrificial metal oxide, decomposes and migrates at high temperatures, providing oxygen to oxidize the metal chromium in the stainless steel. The chromium oxide bond formed at the glass-ceramic and metal interface results in hermetic seals.

The team made 24 potential modified glass-ceramic compositions using a variety of metal oxides that were non-toxic and reasonably easy to handle, such as cobalt oxide. "Most of the work is really saying, 'OK, how many metals from the periodic table can we use and when we dope our glass with these sacrificial metal oxides, what quantity do we need to dope it?'" Dai said.

Researchers want the doped ceramic-glass material to give up oxygen at the interface, not at the surface of glass-ceramics. "The idea is giving up oxygen in the right place. That's kind of a fine line that has to do with the properties of the materials and the way you process them," Dai said.

The team identified two modified glass-ceramic compositions that worked best. Dai said they're not perfect, but they're a big step forward. "Basically we see a chemical bond between the glass-ceramic and the metal, and it's a very strong bond," he said. "If we break it, we break the glass."

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Holmes, S. (2016, March 2). New ways of looking at glass-to-metal seals. Retrieved January 10, 2023, from