Mar 4 2016
Stainless steel contains components that protect it from rough environments encountered in the defense and aerospace industries. These components need paths that allow electricity to power and interact with them. The paths require a genuine insulation seal to prevent them from contacting the metal case, which can limit the communication and power lines.
Sandia National Laboratories researcher Steve Dai has been working on bonding glass-ceramic materials to stainless steel. Strong bonds for airtight seals are important for applications in fields ranging from aerospace to defense.(Photo by Randy Montoya)
It is extremely important to have strong bonds for hermetic or airtight, seals, and Sandia National Laboratories is continuously focusing on advancing this approach.
Generally glass-ceramic composite or glass is used to isolate electrical paths. Steve Dai, chief investigator for a project on bonding glass-ceramic to stainless steel, aims to develop the fundamental science in materials and is working towards achieving efficient performance and glass-ceramic-to-metal seals with high reliability. The scientific foundation could be used to design, develop, and manufacture advanced seals.
In November, 2015 a provisional patent application was filed by Dai’s team for interfacial bonding oxides for glass-ceramic-to-metal seals. A long-lasting seal requires a strong chemical bond between the metal and the glass-ceramic, and it also needs to moderately match the coefficient of thermal expansion (CTE) between materials. The CTE represents how changes in temperature lead the changes in the size of the object. A glass-ceramic containing crystalline phases inside the original glass increases the CTE to better match the metal housing and to decrease thermal stresses.
Bonded glass-metals must be processed at extremely high temperatures, because of this “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 is strong and capable of withstanding high pressures and temperatures also has significant industrial uses, including in fuel cells and defense or aerospace applications that function in rough environments.
In high temperatures pure glass is found to shrink less when compared to metal. The mismatch leads to crimping of the metal, followed by compression of the seal. This has its own 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.
Steve Dai, Researcher, Sandia National Laboratory
Dai’s team focused on developing a chemical bond between glass-ceramics and metal, without including any production steps, by forming an interfacial bonding layer, which serves as a bridge material fixed to both glass and steel.
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.
Steve Dai, Researcher, Sandia National Laboratory
Glass-to-metal seals are developed in an inert atmosphere without oxygen, because metal takes in oxygen from the atmosphere resulting in oxidation and rust. The procedure has an inherent contradiction: An oxide is required to bond the metal to a glass-ceramic, so that the interfacial bonding layer is actually an interfacial layer of oxide.
“That’s the fundamental challenge, how do we do that?” Dai said. In a few processes, the metal is pre-oxidized, but Sandia desired to do away with this extra step.
Approach Modified Glass-Ceramic with Oxidant
Dai’s thermodynamic approach doped or modified the glass-ceramic sealant containing an oxidant, which is used as a sacrificial metal oxide. The oxidant decays and travels at high temperatures, providing oxygen to oxidize the metal chromium present in the stainless steel. The chromium oxide bond developed in the metal interface and glass-ceramic results in the production of hermetic seals.
Dai’s team created 24 potentially enhanced glass-ceramic compositions, using a wide range of metal oxides that were easy to handle and non-toxic in nature, 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?
Steve Dai, Researcher, Sandia National Laboratory
Researchers expect the doped ceramic-glass material to release oxygen at the interface and not at the glass-ceramics’ surface.
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.
Steve Dai, Researcher, Sandia National Laboratory
The team discovered two enhanced glass-ceramic compositions with good working performance. Dai stated that the compositions are a big step forward even though they are not perfect.
Basically we see a chemical bond between the glass-ceramic and the metal, and it’s a very strong bond. If we break it, we break the glass.
Steve Dai, Researcher, Sandia National Laboratory
Sandia also came up with a method for testing whether interfacial bonding is established and, if so, whether the bonding is strong enough to make sure that the glass will not break.
Taking All Factors into Account
There are several other factors that need to be taken into account. If not carefully processed the glass fastens to other surfaces and to the metal housing. This is prevented by allowing the bonding process to use graphite for the fixtures capable of holding glass-ceramic and metal pieces during the formation of the bond. However graphite fights for oxygen just like stainless steel.
Essentially, that’s a kind of competition thermodynamically. If my metal housing gets that oxygen to form the bond oxide, that’s all I want. If the graphite grabs that oxygen, it doesn’t do any good. That delicate balance of the reaction is very challenging.
Steve Dai, Researcher, Sandia National Laboratory
For the first two years of Dai’s three-year Laboratory Directed Research and Development (LDRD) project, they concentrated on the bonding process. In the final year the project focused on exploring ways to control glass-ceramic crystallization to ensure an excellent thermal match. Despite the closure of funding received from the LDRD, the project continues with other funding due to its ability to aid production. Even though the project did not focus on an immediate application, the researchers identified a near-term opportunity to enable a weapons production team with an improved thermal match between metal and glass-ceramic.
During the sealing process, glass-ceramic is sent through a crystallization phase, which permits the development of a high-expansion crystalline phase, increasing the CTE of the glass-ceramic to match the CTE metals just like stainless steel. The sudden change in volume with regard to the crystalline phase, results in an uneven distribution of the glass-ceramic expansion over the change in temperature that takes place during processing. This results in a mismatch of the thermal strain rate between the glass-ceramic and the metal.
Managing the Crystalline Phase Difficult but Important
The team focuses on handling the crystallization process by disintegrating it, to create two or three high-expansion crystalline phases, along with sudden volume changes in individual phases occurring at temperatures several hundred degrees apart. A better understanding of what temperatures are needed to form specific crystalline phases is essential for the concept.
We try to do two or more multiple crystallizations to smooth the thermal strain of the glass-ceramic. As a result, you no longer have this nonlinear, almost step-like strain change in the glass-ceramic. It’s a more near-linear strain curve and matches a lot better to the metals.
Steve Dai, Researcher, Sandia National Laboratory
It is a challenging task to handle several crystallization phases at extremely high temperatures.
We need to learn that part of the process to make sure we have a good balance of all the phases, have them all crystallize in the right sequence and ideally in the right proportion.
Steve Dai, Researcher, Sandia National Laboratory
Dai believes that all of the hard work will result in enhancing hermetic seals in a consistent manner.
Dai’s team are creating verification methods to check if the process is suitable for production applications. Researchers will then go on to examine if the process delivers the expected results in a consistent manner.
Once we reach that point, we will make sure the right specifications are in place and that the processed parts have certain properties so that the production agency can do the process on a continual basis using their equipment.
Steve Dai, Researcher, Sandia National Laboratory