Silicon Nitride: A High Temperature Material for Hypersonic Flight

Since 1961, when Russian Major Yuri Gagarin became the first human to travel at hypersonic speed, engineers have been working to make hypersonic flight a reality. See how silicon nitride is uniquely poised to solve the technical hurdles of hypersonic conditions.

Hypersonic Meaning – What’s Hypersonic Speed? 

A fairly standard definition of hypersonic speed is one that exceeds 5 times the speed of sound, often stated as starting at speeds of Mach 5 and above.*

However, hypersonic meaning can change, because hypersonic speeds are achieved when the air molecules around the vehicle break apart or pick up an electrical charge. Because the variables surrounding that change with the aircraft and with weather conditions, there is no one set speed that means ”hypersonic” for all flights. Instead, the term refers to the point that the air molecules’ changes meaningfully affect the mechanics of flight. This is generally accepted to be Mach 5.

What is the Difference Between Supersonic and Hypersonic?

The difference between supersonic and hypersonic is as follows:   

  • Supersonic speed

Supersonic speed is a rate of travel that exceeds the speed of sound. It is also referred to as Mach 1 speed, after Moravian physicist and philosopher Ernst Mach.** Since the speed of sound depends on temperature and composition of air, supersonic speed also varies with changing altitudes and weather conditions.

A sonic boom is a sound associated with the shock waves created when an object travels through the air at supersonic speeds. Sonic booms generate enormous amounts of sound energy, sounding similar to an explosion or a thunderclap to the human ear.

  • Hypersonic speed

As we’ve seen, there is no set number associated with hypersonic meaning. But in general, hypersonic speeds are very high supersonic speeds. They are generally accepted to be Mach 5 speeds, or five times the speed of sound.

 NASA defines “high” hypersonic as any Mach number from 10 to 25, and re-entry speeds as anything greater than Mach 25 (near 17,500 mph)***.

Hypersonic Material Requirements

According to NASA’s Glenn Research center, “The chief characteristic of hypersonic aerodynamics is that the temperature of the flow around the aircraft is so great that the chemistry of the gas must be considered. At low hypersonic speeds, the molecular bonds vibrate, which changes the magnitude of the forces generated by the air on the aircraft. At higher hypersonic speeds, the molecules break apart producing an electrically charged plasma around the aircraft.” ****

Thus, hypersonic materials must be resilient to the extreme environments that these structures are subject to. Namely, speeds exceeding Mach 5 will experience extreme temperature fluctuations–often enough to melt or weaken metals. The material used in hypersonic vehicles must be able to maintain their mechanical properties within a wide range of temperatures.

Hypersonic flight vehicles and weapons used in aerospace and defense must also be able to withstand ballistic impact.

Silicon Nitride – A Game Changer for Hypersonic Technology 

Silicon nitride has a unique portfolio of material properties that make it an ideal fit for hypersonic structures, from flight vehicles to weapons. Silicon nitride is lightweight and strong, making it an ideal aerospace material.

Hypersonic flight dictates the need for new materials possessing extremely exceptional material properties. The mechanical robustness of silicon nitride along with its ability to withstand and perform at high temperatures experienced during high-velocity flights have made it the material of choice for various aerospace applications. 

  •  It maintains strength at a wide range of temperatures.

At high temperature, silicon nitride shows excellent mechanical properties including low density, high bending strength, and high abrasion wear.. In other words, this material is extremely strong and extremely tough. Silicon nitride also shows excellent thermal properties, with minimal expansion and contraction due to temperature and the ability to withstand thermal shock (fast, significant changes in temperature).

 

  • It’s resistant against particle impact and even ballistic impact. 

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 enables it to withstand severe operating conditions that may cause other ceramic materials to deform or fracture.

  • It has a low thermal conductivity.

Thermal conductivity is the intrinsic ability of a material to transfer or conduct heat. The heat transfer coefficient is a critical factor that characterizes the applicability of engineered materials to be used in any industrial application under extreme temperature requirements. Because of its unique chemical composition and microstructure, silicon nitride offers similarly low thermal conductivity as compared to metals.

 

See ‘Silicon Nitride – An Advanced Aerospace Material’ to gain a deeper understanding of the characteristics  that make silicon nitride the ideal material choice for hypersonic technology.

 Silicon Nitride for Hypersonic Technology  

Hypersonic technology covers everything from the way an aircraft is designed to the materials used. As a high temperature material for hypersonic flight, silicon nitride offers an ideal combination of strength and thermal resistance—necessary for the demands of hypersonic flight.

*https://en.wikipedia.org/wiki/Hypersonic_speed

**https://en.wikipedia.org/wiki/Mach_number

***https://www.grc.nasa.gov/www/k-12/BGP/hihyper.html

****https://www.grc.nasa.gov/www/BGH/index.html

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