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Silicate layer is key to low temperature bonding of silicon carbide

A method for bonding silicon carbide has been developed at the University of Glasgow. This image shows a join between silicon ingots that was made in a similar way. Credit: Christian Killow; Univ. Glasgow.

An online story in The Engineer last week reiterated for me the practical benefits of basic science research.

Researchers at University of Glasgow, Scotland, have been working on building equipment and instruments for studying gravity, both on the ground and in space. The team is led by UG professor in the School of Physics and Astronomy, Sheila Rowan. According to the Institute for Gravitational Research website, the group’s work “is targeted at the development of detectors and signal analysis methods to search for gravitational waves from astrophysical sources. Gravitational waves — waves in the curvature of space-time — are a prediction of General Relativity.” The website goes on to describe work the group is doing on detection techniques based on kilometer-scale laser interferometry and other highly sensitive instruments. The instruments require components that are made by precision manufacturing of highly stable materials. Silicon carbide, for example.

Silicon carbide is an attractive material for space and other applications that require strong, lightweight structures. A researcher on the team, Christian Killow, says in the story, “Silicon carbide is very hard and very tough. It’s quite brittle but it’s very good at absorbing impact.”

But, it is not easy to build with. Killow continues, “It just kind of sits there and does nothing, so when you want to stick something to it, it’s not very easy to do.”

The Glasgow work builds on hydroxide catalysis work first pioneered and patented by Jason Gwo while he was working at at Lawrence Livermore National Lab when he was looking for a way to join very flat fused silica pieces for a telescope assembly. According to the researchers, Gwo realized that “bonding may occur between flat surfaces of a number of materials if a silicate-like network can be created between the surfaces.”

That is, the molecular structure of the surfaces can be altered in such a way as to encourage a chemical bond between them.

The resulting bonding interfaces are very thin, less than 100 nm. According to Killow, “There are a lot of things that can go wrong, but when the bonds go well they very nearly inherit the bulk strength of the materials that they’re bonding.”

The chemical bonding mechanism of hydroxide bonding is a three-step process: hydration and etching, polymerization and dehydration. Gwo used alkaline bonding solution, such as sodium or potassium hydroxide, or sodium silicate. The solution etches into the silica and causes polymerization of the surface, causing chains of molecules to form between the joining surfaces.

Chains of hydroxide molecules form naturally on the surfaces of many materials, but not on SiC. The Glasgow group formed a silica layer on the SiC surface, which provided a surface for attaching hydroxide ions by applying a hydroxide-containing bonding solution. The interaction between the hydroxide ions on the bonding surfaces creates the bond in the same way that Gwo joined fused silica pieces.

The process can be used to join SiC to itself or any number of dissimilar materials, such as sapphire, aluminum, silicon and zinc. The thinness and stability of the joint make it especially well suited for manufacturing high-precision equipment and instruments.

According to the story, the bonding process can be tailored to specific applications. But, Killow offers the caveat, “The logistics of bonding things is a strange process. It will either fail spectacularly or really work quite well. … We’re still developing the process and refining it.”

In an unusual move, the researchers have made the technology available without charge through the Easy Access IP program at UG.