*Originally published at uwspacerace.com/blog.
With extraordinary strength, thermal resistance, and chemical properties, ceramics impact our lives on a daily basis — particularly in the space-launch world.
One of the most widely-utilized ceramics is Silicon Nitride (Si3N4).
From the space shuttle to medical implants, engineers deploy this advanced ceramic in the most extreme applications.
This brief article focuses on how aerospace engineers use Si3N4 to aid mankind’s mission to explore the solar system.
Specifically, in this article I’ll discuss:
- Properties and Structure of Si3N4
- How Si3N4 is Made
- The Space Shuttle as a Case Study
- Other Applications for Si3N4
Let’s dive in.
Properties and Structure
With nearly 100 years of scientific investigation of Si3N4 the basic science (structure and properties) and processing technologies of the ceramic are remarkably well understood and described in literature.
Some authors argue that we know more about Si3N4 than any other structural ceramic.
(Disclaimer: technical information to follow, you can skip ahead to the case study section of this post for the practical applications and a fun story about Si3N4.)
There are three known phases of Si3N4:
- trigonal α-Si3N4
- hexagonal β-Si3N4
- cubic γ-Si3N4.
Figure 1 shows the unit cells for each of these three phases.
The α-Si3N4 takes on a ABCDABCD stacking sequence (along the c-axis of the unit cell), resulting in a unit cell that is two times larger than the ABAB stacked β-Si3N4.
The longer stacking sequence of α-Si3N4 induces more strain in the crystal structure while the β-Si3N4 is nearly strain free.
Figure 1 clearly shows distorted bond angles in the α-Si3N4 structure that cause high strain relative to the β- phase. This strain causes the α- phase to be less stable than the β- phase.
Significantly less stable than both α and β phases, Cubic γ-Si3N4 requires high pressures and temperatures to synthesize.
Cubic γ-Si3N4 demonstrates a spinel-derivative crystal structure with two silicon atoms octahedrally coordinated and one silicon atom tetrahedrally coordinated by four nitrogen.
The rest of this report focuses on the commonly used β- phase of Si3N4, which boasts high thermal stability, good shock resistance compared to other ceramics, good insulating properties, and is highly chemically inert (only etchable by HF and H2SO4).
How It’s Made – Synthesis:
Given its extensive application set, there are many ways to produce Si3N4.
For use as an integrated circuit insulator or dielectric, Si3N4 can be grown on a substrate using Chemical Vapor Deposition to give a crystalline or amorphous structure.
When considering space applications, Si3N4 powder can be turned into fully-dense β-Si3N4 via hot isostatic pressing. This synthesis method gives rise to superior mechanical properties, particularly hardness and strength.
Additionally, a similarly mechanical robustness can be achieved by forming β-Si3N4 via sintering.
Case Study: The Space Shuttle
The United States Space Launch System (AKA: the space shuttle) is arguably the most advanced, complex, breakthrough work of technology every conceived and built
Weighing over 2,000 tons and capable of delivering 35,000 pounds to the International Space Station, these behemoth machines required highly capable and robust engines.
To accomplish this task, the space shuttle was outfitted with three reusable Aerojet-Rocketdyne RS-25 engines, as well as a series of external boosters.
We’ll focus on the critical role that Silicon Nitride played in each of the RS25 engines.
During liftoff, each of the RS-25 engines would burn for eight-and-a-half minutes, combusting over 500,000 gallons of cryogenic liquid hydrogen and liquid oxygen, accelerating the space shuttle from rest to over 17,000 miles per hour.
The space shuttle — and particularly its engines — experienced immense loads and temperature extremes (from the cold extreme of -200oC to the extreme 3,300oC heat of the shuttle’s combustion chamber).
Due to these extreme loads, NASA engineers constantly upgraded the shuttle subsystems with cutting edge materials.
That’s where the aerospace-wonder-material Silicon Nitride fits into the picture.
In July 1995, NASA first redesigned and flew the shuttle’s turbo pump, the critical equipment for moving the supercooled propellants from their individual tanks to the combustion chamber on the shuttle.
Among the upgrades to the turbo pump were revamped ball bearings, now made of silicon nitride rather than steel (depicted in Figure 2).
Impressively, the Si3N4 ball bearings were 30% harder and 40% lighter than steel, critical improvements since increased reusability and payload efficiency dramatically reduce cost.
Figure 3, taken from a NASA analysis report, shows a forty percent gain in runtime for these new Si3N4 bearings, compared to their predecessor steel bearings.
Other Applications
Silicon Nitride has been deployed as the main material in several other aerospace applications. Notably, NASA and other space agencies have developed thrusters and engines made entirely of Si3N4.
Other Si3N4-based technologies with second-order implications on the aerospace industry include: high-precision cutting tools for machining other advanced materials, Atomic Force Microscope tips for materials analysis (see figure 4), and integrated circuit insulators and dielectrics for both development and flight hardware.
But applications for Si3N4 bearings extend beyond the extreme space shuttle operating environment to the everyday. You can find them in skateboard and roller blade bearings, wind turbines, prosthetic hips, and many industrial applications.
Conclusion
Silicon Nitride is practically a wonder material that has transformed nearly every aspect of day-to-day life from communications satellites in orbit, thanks to Si3N4 space-launch hardware, to the computer chip in your smartphone, and even to a loved ones prosthetic hip.
While not discussed explicitly in this short report, the cost of Si3N4 has dramatically fallen over the past several decades, resulting in a significant uptick in consumer market adoption of the materials
For example, it’s estimated that automakers roll out over 300,000 sintered silicon nitride turbochargers every year in their consumer-facing vehicles.
It’s clear that Si3N4 has made major contributions to society, but there’s still an abundance of new, untapped opportunities for engineers and designers ready to leverage the powerful properties of Silicon Nitride.
Through our work at UW Space Race, we are training the next generation of aerospace engineers, entrepreneurs and leaders to tap into these unprecedented opportunities, advancing materials science, aeronautics, and impacting the future of technology.
How will you support this BOLD initiative as we go where no students have gone before (to the Karman line!)? Let us know in the comments.
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