Ceramics are some of the most common materials on Earth, valued for their versatility in a wide range of applications. Advanced technical ceramics are the next level of ceramics. They are incredibly hard, lightweight, and wear-resistant. These properties make them ideal for advanced aerospace applications, where they are used in critical components such as engine parts and sensors for aviation communication systems. In the aerospace industry, many components are classified as high-stress parts that need to endure extreme pressure and temperature changes, leaving no room for failure.
So what’s the difference between a normal ceramic and a technical ceramic?
The main difference lies in the purity of the ceramics in the material blend. Technical ceramics, like high-purity alumina, are made with materials of higher purity to enhance their properties. High-purity alumina is made up of over 99.99% alumina, making it exceptionally durable and resistant to wear, high temperatures, and mechanical stress, properties that are critical to ensure reliable performance under the harsh conditions of aerospace environments.
In this article, we will explore how high-purity alumina is used as a technical ceramic, particularly for aerospace applications, and why it is the material of choice for high-performance aerospace components.
How is High Purity Alumina Used in Technical Ceramics?
Technical alumina ceramics are composed of different material blends, such as alumina and silicate in aluminosilicate or alumina and zirconia in zirconia-toughened alumina. However, the key material is high-purity alumina, also known as high-purity aluminum oxide (Al2O3). It is known for its high hardness and wear resistance, making it highly durable. In fact, alumina ceramics can only be shaped through diamond grinding owing to its very high hardness.
Alumina is a white crystalline powder that is derived from bauxite, a naturally occurring mineral rich in aluminum oxide. Standard alumina is produced through the Bayer process, which begins by crushing the bauxite ore and mixing it with caustic soda to dissolve the alumina. The resulting solution is then filtered to remove impurities, allowing the alumina to precipitate. Finally, the precipitate is washed and heated to eliminate any remaining moisture.
As discussed in our previous article, “Can High Purity Alumina Be Produced Sustainably?,” achieving ultra-high purity levels, such as 5N HPA (≥99.999% purity), requires a more advanced refining process than the Bayer method. This is crucial for high-purity alumina used in technical ceramics, such as Polar Performance Materials’ HPA-M-HT (5N HPA with a 2–6 µm particle size) and HPA-M5 (5N HPA with a 3–5 µm particle size).
Once the key component (high-purity alumina) is produced, HPA technical ceramics can be processed using advanced manufacturing techniques, such as isostatic pressing, injection molding, extrusion, and casting. These methods allow HPA to be shaped into components with precise dimensions, making it suitable for a wide range of applications.
Why HPA Technical Ceramics are Ideal for Aerospace Applications
Alumina ceramic is prized for its outstanding durability and resistance to wear, chemical attack, erosion, corrosion, and high temperatures. These qualities make it an ideal material for use in harsh environments where traditional metals or polymers would degrade. Even under extreme conditions, alumina retains its strength and stability. As discussed in High Purity Alumina vs. Traditional Alumina: Why Purity Matters in High-Performance Applications, high-purity alumina exhibits further property improvements, making it even more effective in demanding applications.
High Hardness
The high hardness of high-purity alumina makes it stand out as a material for technical ceramics. HPA is one of the hardest materials known with a hardness of 9 on the Mohs Scale. This high hardness is crucial for materials that need to withstand extreme mechanical stress. Unlike metals, which often lose their strength under high temperatures, alumina ceramics retain their integrity when cooled back to normal temperatures without suffering from the structural damage that typically affects metals.
▼Vickers hardness (HV) vs. Mohs hardness of the 10 hardest materials.
High Purity Alumina stands out as the second hardest material, following diamond, highlighting its exceptional mechanical strength.
The hardness of alumina contributes directly to its abrasion resistance, which is important for applications involving constant friction, impact, or contact with abrasive materials. For example, wear-resistant components, such as bearings, valves, and rotors, benefit from alumina’s ability to endure prolonged usage without significant wear.
The strength and hardness of alumina increase further with higher purity levels. Having said that, high-purity alumina (purity> 99.99%) exhibits significantly improved mechanical properties that make it suitable for the most demanding environments and highly valued in applications such as aerospace, electronics, and industrial machinery.
Excellent Thermal Properties
Another key property of high-purity alumina is its exceptional high-temperature resistance. It can withstand temperatures as high as 1650 °C (2900°F) in atmospheric conditions and 2000 °C (3000°F) under vacuum. As a good thermal insulator, alumina has a high thermal resistivity, meaning it resists the flow of heat and can function in environments where other materials might degrade or fail. As alumina purity increases, its thermal resistivity decreases, allowing heat to flow more easily. For example, 94% alumina has a thermal resistivity of 0.0556 K⋅m/W, while high-purity alumina (above 99.99%) has a thermal resistivity of 0.02857 K⋅m/W, making it better for applications that require effective heat dissipation.
▼Thermal resistivity (ρ) as the reciprocal of thermal conductivity (k).
The graph shows the thermal resistivity of alumina with varying purity levels, highlighting the decrease in ρ as alumina purity increases, indicating improved thermal conductivity at higher purity.
High-purity alumina has a melting point of 2050 °C (3722 °F), making it an ideal material for technical ceramics exposed to extreme temperatures. Combined with its excellent thermal stability, this high melting point makes it suitable for applications such as in turbine blades, exhausts, temperature sensors, and other components exposed to intense heat in aircraft. Lastly, the ability of alumina to resist thermal shock, or the rapid expansion or contraction of materials when subjected to sudden temperature changes, also contributes to its reliability in these applications.
High Dielectric Strength
High-purity alumina also exhibits high dielectric strength, which is the maximum voltage needed to cause a dielectric breakdown through the material (Volts per unit thickness). The high dielectric strength and electrical insulating properties of high-purity alumina are particularly useful in components that need to maintain electrical integrity in extreme conditions, such as capacitors, resistors, and insulators used in high-voltage applications. Moreover, alumina’s ability to withstand both high temperatures and electrical stress makes it particularly valuable in aerospace electronics, communication systems, and sensors.
▼Dielectric strength of alumina as a function of purity and sample thickness.
The first graph shows the increase in dielectric strength with higher alumina purity at a fixed sample thickness of 635 µm. The second graph illustrates the inverse relationship between dielectric strength and sample thickness for high purity alumina (>99.5%), where thinner samples exhibit higher dielectric strength.
Data from Talbi, F., Lalam, F., & Malec, D. (2010). Dielectric breakdown characteristics of alumina. 2010 10th IEEE International Conference on Solid Dielectrics.
Excellent Chemical Stability
HPA is chemically inert. This means HPA ceramics do not react with most acids, alkalis, or other corrosive substances, even at elevated temperatures. That chemical stability makes them resistant to corrosion and oxidation. This property is critical for components exposed to harsh environments such as outer space, atmospheric re-entry, or extreme industrial conditions. This chemical stability ensures that alumina remains unaffected by environmental stressors such as saltwater exposure, which can quickly degrade other materials.
High Purity Alumina in Technical Ceramics for Aerospace
High Purity Alumina Ceramics for Temperature and Pressure Sensors
In aircraft engines, components can experience rapid and extreme temperature and pressure changes. It is critical to monitor the temperature in real-time to ensure optimal performance and safety.
Temperature sensors, used in parts like turbine blades and combustion chambers, ensure temperatures stay within safe limits. Alumina-based temperature sensors are ideal for this application due to their accuracy, reliability, and ability to withstand high thermal gradients. The dielectric properties of alumina also help with the insulation and durability of these sensors in harsh environments, where high temperatures and electrical interference are common.
High-purity alumina ceramics are also ideal for pressure sensors in aerospace applications. These sensors must withstand extreme temperature fluctuations and chemical exposure. In critical applications like measuring cabin pressure or fuel system pressure, HPA ceramics are chosen for their superior thermal stability and ability to function reliably under high thermal gradients.
High Purity Alumina Ceramics for Structural and Turbine Components
The high hardness of high-purity alumina ceramics makes them an excellent base material for structural components in aircraft. In addition to their outstanding mechanical properties, their superior thermal characteristics enable them to resist thermal shocks from rapid temperature fluctuations. As a result, they are ideal for use in bearings, seals, and even protective plates for military aircraft.
Exhaust nozzles in aircraft and spacecraft engines direct exhaust gases at high velocities to generate thrust. Given the extreme temperatures and aggressive conditions they can endure, high-purity alumina ceramics offer significant advantages. The material’s hardness and abrasion resistance protect it from erosion caused by the high-velocity exhaust gases.
Additionally, HPA ceramics can withstand the intense heat of these gases while providing excellent thermal insulation, helping protect internal components by minimizing heat transfer. This combination of properties makes HPA ceramics an ideal choice for exhaust nozzles in demanding aerospace applications.
HPA Ceramics for Heat Shields
Heat shields protect an aircraft and its interior components from extreme heat, particularly during high-speed flight or re-entry into Earth’s atmosphere. High-purity alumina ceramics are used in these applications to safeguard the aircraft and its sensitive parts, effectively preventing excessive heat from causing damage and ensuring the integrity of the aircraft in these extreme conditions.
High Purity Alumina Grades for Advanced Technical Ceramics
High purity alumina ceramics represent a pivotal advancement in materials science, offering remarkable strength, thermal stability, and chemical resistance that make them ideal for aerospace applications. From temperature and pressure sensors to structural components and critical parts like turbine blades and exhaust nozzles, HPA ceramics are crucial for ensuring the performance, safety, and longevity of aerospace technologies. Their exceptional hardness, resistance to wear, and high thermal resistivity allow them to endure the extreme pressures, temperatures, and environmental conditions encountered in the aerospace sector.
As technology advances, Polar Performance Materials’ finely-tuned high purity alumina products, such as HPA-SDF advanced 5N HPA, with high crystallinity and controlled specific surface area, will play a vital role in emerging applications for technical ceramics like pressure sensors and heat shields.
Looking to take your technical ceramics to the next level with optimized HPA grades? CAPLINQ is here to help. Our team offers tailored solutions to enhance performance and efficiency. Get in touch today to improve your advanced ceramics!
