The concept of “transparent aluminum” originated in science fiction, notably Star Trek. However, this fictional idea has a real, non-metallic counterpart: Aluminum Oxynitride (AlON). This advanced ceramic material offers a unique combination of optical clarity and extreme physical strength. The true strength of this material is redefining what is possible for high-performance windows and protective enclosures.
The Material Behind the Name
Commercially known as AlON, this material is not a metal but a polycrystalline ceramic composed of aluminum, oxygen, and nitrogen. Its chemical formula is approximately Al23O27N5, making it a nitrogen-substituted form of aluminum oxide (sapphire). It is synthesized using powder-based processes involving extreme heat and pressure to form a dense, solid piece.
AlON is transparent, unlike most ceramics, due to its internal structure. It forms a cubic spinel crystal structure, meaning its optical properties are the same in all directions (isotropy). This uniformity prevents the light scattering and double refraction (birefringence) that typically make polycrystalline materials opaque. The material is clear across a wide spectrum, from the near-ultraviolet through the visible and into the mid-infrared regions.
Mechanical Properties and Fracture Resistance
AlON’s durability is defined by specific mechanical properties that distinguish it from traditional glass. Its resistance to surface scratching is quantified by its Vickers hardness (1524 to 1615 HV). This extreme hardness is coupled with a high compressive strength, measured at approximately 2.68 GPa, indicating its ability to withstand crushing forces.
AlON’s flexural strength, also known as the modulus of rupture, ranges between 0.38 and 0.7 GPa, showing its ability to resist bending before breaking. Crucial for protective applications is its fracture toughness, which measures resistance to crack propagation. AlON exhibits a fracture toughness of around 2.0 MPa·m^(1/2), significantly higher than standard glass. This toughness allows the material to resist the spread of cracks more effectively after impact than conventional transparent materials.
Strength Compared to Standard Materials
AlON is considered a high-performance material for demanding environments. It is four times harder than fused silica glass. Its elastic modulus, which indicates stiffness and resistance to deformation, is roughly five times greater than common window glass.
Compared to traditional metals, AlON is approximately three times harder than steel of the same thickness. This superior hardness and stiffness are achieved while maintaining a low density, resulting in a highly efficient strength-to-weight ratio. AlON offers performance nearly equivalent to single-crystal sapphire but can be manufactured in larger, more complex shapes at a lower cost.
In ballistic applications, the performance difference is significant. A 1.6-inch (41 mm) thick panel of AlON can stop a .50 BMG armor-piercing projectile. Achieving this protection requires a laminated glass system nearly twice as thick, specifically 3.7 inches (94 mm). This reduction in thickness and weight translates directly into armor systems that are up to 50% lighter than those using glass laminates.
Current and Emerging Uses
The unique combination of transparency, hardness, and durability makes AlON preferred for specialized defense and aerospace applications. It is widely used in transparent armor systems for military vehicles and aircraft, offering advantages over traditional bulletproof glass due to its thinness and light weight. AlON is also utilized for missile domes and sensor windows on advanced aircraft because of its clarity across infrared wavelengths and resistance to harsh environmental conditions.
In the commercial sector, AlON’s scratch resistance is leveraged in high-traffic environments. Supermarket and retail point-of-sale scanner windows frequently use AlON to withstand constant abrasion. Researchers are also exploring its potential for next-generation screen covers and protective windows in high-end consumer electronics.