A New Alloy for Satellite Skeletons
Materials scientist Timothy Stephenson has created a new material that combines the strength of ceramics with the thermal-dimensional stability of a 100-year-old metal alloy.
Funded by Goddard’s Internal Research and Development (IRAD) program, Stephenson’s material is ideal for building super-stable, lightweight “skeletons” that support satellite mirrors and other instruments. While developing the new alloy, Stephenson also developed a recipe for creating metal alloys with specific thermal expansion properties that could be used for satellite brackets and base plates. The components would offer the same bend and stretch as the instruments they support.
Mixing Ceramic with Invar
Along with aluminum, stainless steel, and other commonly known metals, engineers often build satellite parts from an alloy called Invar. Invented more than 100 years ago by Swiss Nobel Prize winner Charles Édouard Guillaume, Invar is short for invariable because, unlike most other metals, this nickel-iron alloy expands very little when it is heated. Likewise, it also contracts very little when it’s cool.
Consequently, Invar is used in everything from satellites to toasters and is great for any application where the instrument needs to keep its shape during extreme temperature changes. But Invar is heavy, and it could be stronger and stiffer. “Every pound you save, you can save on everything else,” Stephenson says.
With his IRAD funding, Stephenson began experimenting with ways to create an Invar-like material rigid enough to be shaped into a lightweight structure but still capable of enduring the temperature extremes found in space. He thought ceramic might be an ingredient worth investigating. “So the question became, how much ceramic could you put in it and still have it maintain its properties,” Stephenson says.
A Dirty Process – But a Proof of Concept
Stephenson’s first experiment, carried out at the Michigan Technological University, mixed different combinations of nickel-iron Invar with a silicon-nitride ceramic. His colleagues placed the powdered ingredients into a container about the size of a coffee can and then mechanically mixed them with marble-sized metal balls and a metal stirrer. When the powders were uniform, technicians sucked out the air inside the metal can and then pressurized it in a process called hot isostatic pressing or HIP, for short. “That means it’s compressed from all directions at a high temperature,” Stephenson says. “That fuses it all together.”
The result was a ceramic-Invar alloy — a proof of concept that these materials could be combined and offer the same coefficient of thermal expansion or CTE as Invar. “It was a finger-sized sample made of iron powder, mixed in proportion with the nickel powder and a bit of ceramic. We actually got a higher specific stiffness alloy with the desired CTE,” Stephenson says.
However, Stephenson’s sample was “dirty,” he says. Metal from either the balls, the stirrer, or the can contaminated the material.
So, Stephenson tried a non-mechanical alloying process.
Hot Off the (Isostatic) Presses
Working under a second IRAD, Stephenson acquired fine powders of Invar and silicon nitride, which Bodycote in Andover, Mass., stirred together like a cake mix and then pressurized.
The finished product was about the size of a large loaf of bread, but Stephenson believes the process could yield any shape an engineer would need. “It’s possible that the satellite components wouldn’t have to be machined into shape, but instead created whole using net-shape HIP. We could make a mold of whatever the engineers want, like a ring or something with a curve,” he says. “We could then put the powder in, create the alloy, and then pop out the finished piece.”
Now that he has created the new alloy, Stephenson has begun securing support for a contractor to open up the canisters and machine out samples of the new material. Those samples will be put to the test to ensure their composition and characteristics.
But they’ll also undergo a second, more informal test. Stephenson wants to give the engineers who design, construct, and build satellites something they can hold in their hands. “These projects are science-driven, they’re mission-driven,” Stephenson says. “So, if you want to get something new accepted and used, you need to get the best tools and materials in the right hands.”
The Office of the Chief Technologist is involved in a variety of projects, missions, and technologies.