Silicon Nitride – An Advanced Aerospace Material

Silicon nitride’s properties make it an advanced aerospace material for extreme environments. See how silicon nitride is used in the aerospace industry.

What is Silicon Nitride?

Silicon nitride (Si3N4) is made by mixing highly refined raw powders that are formed into desired shapes. The final product is finished in precision kilns or furnaces. This is similar to the making of pottery; the word “ceramic” derives from its Greek root “kéramos,” which refers to pottery.

Dense silicon nitride is a very hard, abrasion-resistant, and corrosion-resistant solid. Unlike familiar ceramics such as porcelain or glass, silicon nitride has very high strength, with the highest fracture resistance of any advanced ceramic. This makes it a fantastic material for aerospace, as well as a component in electronics, bearings, and turbomachinery.

Properties of Silicon Nitride 

Silicon nitride material properties include high strength, high hardness, and thermal resistance. It is incredibly light for the strength it provides, making it ideal for aerospace and other applications where both weight and strength are critically important.

In addition, the thermal properties of silicon nitride make it an excellent material for aerospace applications. It has a low thermal conductivity and high melting point which allows it to resist the force of extreme temperatures. It also offers a low thermal expansion coefficient, leading to fewer dimensional changes compared to metal components.

What Makes Silicon Nitride an Advanced Material for Aerospace?

Ceramics find use in aerospace because they are lighter than metals, enabling faster speeds, reduced fuel consumption, larger payloads, and longer times in space for exploration vehicles.

Silicon nitride has many properties that make it desirable as an aerospace material:

  • Mechanical properties

Silicon nitride offers high mechanical strength (at a wide range of temperatures),  high fracture toughness, and particle impact resistance.

Even at a high temperature, silicon nitride has a high bending strength, extreme fracture toughness, and solid particle erosion resistance. This makes this material extremely durable.

  • Thermal properties

Silicon nitride is an ideal material for high-temperature applications. It is one of the few monolithic ceramic materials capable of surviving the severe thermal shock and thermal gradients generated in hydrogen/oxygen rocket engines.

Silicon nitride also exhibits unusually low thermal conductivity, which is a useful property for designers working with high temperature applications, such as missile thrusters. It also offers a high melting point and the ability to maintain performance at high temperatures. 

  • Dielectric properties

The low dielectric losses of silicon nitride make it a promising aerospace material, namely for antenna windows and high speed missile radomes, nose cones, radio windows, and more.

Because metals reflect radio waves, plastics or ceramics are used where transmission is needed. The lower the dielectric constant, the better the transmission. While plastics work great at room temperature, they do not stand up to high temperature use. This leaves ceramics. While there are ceramics with lower dielectric constants than silicon nitride, these other ceramics are not able to sustain high temperature performance.

Aerospace Applications of Silicon Nitride

Due to its highly resilient material properties, silicon nitride is used in the manufacture of high temperature components in a variety of industries. As an aerospace material, silicon nitride is ideal for traditional aircraft components such as bearings.

The dielectric properties of silicon nitride also make it a desirable material for use in the manufacture of radomes (protective coverings that shelter the antenna assemblies of a radar set), especially for aircraft. It’s critical that radomes are made of materials that minimally attenuate the signal transmitted or received by the antenna, allowing for unobstructed communication.

Depending on the size of the communication system that needs protecting, a radome can be composed partially or entirely of silicon nitride. For example, a radome can be fabricated with a built-in silicon nitride window, also known as an RF window, that would allow the transmission of radiofrequency signals to permeate the material while still providing a degree of protection that can withstand hypersonic environments, such as those experienced by a hypersonic missile. (To learn more about silicon nitride within hypersonic technology, read our post, “Silicon Nitride – A High Temperature Material for Hypersonic Flight”).

Silicon Nitride in Space Technology

Aside from use in missile radomes, silicon nitride is used in the manufacture of missile thrusters as well as spacecraft thrusters. In fact, silicon nitride has been used in space technology before: NASA and other space agencies have developed thrusters and engines made entirely of Si3N4.*

Silicon nitride was also used for the “microshutters” developed for the Near Infrared Spectrograph aboard the James Webb Space Telescope. According to NASA: The “operating temperature is cryogenic so the device has to be able to operate at extremely cold temperatures. Another challenge was developing shutters that would be able to open and close repeatedly without fatigue…Silicon nitride was chosen for use in the microshutters, because of its high strength and resistance to fatigue.”**

Other Applications  

  • Bearings 

Other applications of silicon nitride as a material for aerospace include brakes, bearings, and other wear-resistant  components.

Because of its strength, wear resistance, and shock resistance compared to other ceramics, silicon nitride is used for performance ball bearings in aviation and aerospace. (Silicon nitride bearings were used in the main engine of NASA’s Space Shuttle.)***

Since silicon nitride ball bearings are harder than metal, contact with the bearing track is reduced, resulting in less friction, higher possible speeds, and a longer lifetime overall. Ceramic ball bearings also offer higher corrosion resistance and higher operating temperature, as compared to traditional metal bearings

Silicon Nitride: Uniquely Advanced

Silicon nitride is truly a uniquely advanced material for aerospace, offering many advantages of strength, lightness, and thermal resistance. To learn more about this material and its uses in aerospace and other industries, visit our Applications page.

 

References:

*https://wisc.space/blog/silicon-nitride-and-the-space-shuttle/

**https://jwst.nasa.gov/content/about/innovations/microshutters.html

***https://en.wikipedia.org/wiki/Silicon_nitride#High-temperature_material

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