Aluminum oxynitride (or AlON) is a ceramic composed of aluminum, oxygen and nitrogen. The material is optically transparent (> 80%) in the ultraviolet, visible and half-wave ranges of the electromagnetic spectrum. It is manufactured abroad by Surmet Corporation under the ALON brand. Recently, Russian scientists have developed a technology for producing transparent aluminum, which is somewhat different from imported analogues.
Description
The development of a unique alloy has opened up new prospects in the defense industry, science and construction. According to official figures, ALON:
- 4 times stronger than tempered quartz glass ;
- 85% harder than sapphire;
- almost 15% more reliable than spinel made from magnesium aluminate.
By the way, the spinel mineral is a direct competitor to transparent aluminum and is inferior to oxynitride in a number of ways.
ALON is the hardest commercial representative of polycrystalline transparent ceramics. The combination of optical and mechanical properties makes this material a leading candidate for lightweight high-performance armored products, such as bulletproof and explosion-proof glass, elements for infrared optical systems. Also, aluminum shock-resistant transparent windows, portholes, plates, domes, rods, tubes and other products are obtained from aluminum oxynitride using traditional technologies for processing ceramic powders.
Mechanical properties
Aluminum oxynitride has outstanding characteristics:
- Elastic modulus: 334 GPa.
- Shear modulus: 135 GPa.
- Poisson's ratio: 0.24.
- Knoop hardness: 1800 kg / mm 2 at a load of 0.2 kg.
- Fracture Resistance: 2 MPa · m 1/2 .
- Bending Strength: 0.38-0.7 GPa.
- Compressive Strength: 2.68 GPa.
Optical and thermal properties
When testing transparent aluminum, the following indicators were obtained:
- Heat capacity: 0.781 J / K.
- Thermal conductivity: 12.3 W / (m · K).
- The coefficient of thermal expansion: 4.7 × 10 -6 / ° C.
- Range of transparency: 200-5000 nm.
ALON is also resistant to radiation and damage by various acids, alkalis and water.
Getting
Transparent aluminum oxynitride is made by powder sintering, like other ceramic materials. While the US Navy is busy developing a new bulletproof material called artificial spinel, Surmet Corporation is already releasing its own version of bulletproof glass called ALON.
The special powder developed in the Raytheon laboratories is laid in molds and kept at very high temperatures. The composition of the mixture may vary slightly: the aluminum content is from about 30% to 36%, which slightly affects the characteristics (the discrepancy is only 1-2%).
The heating process causes rapid thinning and cooling of the powder, leaving the molecules freely arranged, as if they were still in liquid form. It is this crystalline structure that provides transparent aluminum with a level of strength and scratch resistance comparable to sapphire.
Manufactured products are subjected to heat treatment (compaction) at elevated temperatures, followed by grinding and polishing until transparency is achieved. The material can withstand temperatures up to 2100 ° C in inert gases. Grinding and polishing significantly improve impact resistance and other mechanical properties.
Domestic analogue
Russian scientists created transparent aluminum in 2017. According to experts from NRNU MEPhI, the production technology has been significantly improved. In the manufacture of compacts, the spark-plasma sintering technique is used.
Unlike foreign colleagues, domestic developers do not pass the electric discharge through an external heating element, but directly through the mold. Scientists say that domestic transparent armor in terms of strength characteristics is comparable to cubic zirconia, but at the same time it has high impact strength.
Comparison of aluminum armor with bulletproof glass
Traditional bulletproof glass consists of several layers of polycarbonate sandwiched between two layers of glass. Similarly, transparent aluminum armor consists of three layers:
- the outer layer is aluminum oxynitride;
- middle layer - glass;
- the back layer is a polymer substrate.
However, the similarity ends there. Aluminum armor can stop the same bullets from a small-caliber weapon as traditional bulletproof glass, but it will still be transparent even after a shot without characteristic cracks. In addition, the strength of ALON is much higher.
Aluminum oxynitride armor can be made in almost any shape. She is not afraid of sand, gravel or dust. Resistance to abrasive materials is very high. Despite the excellent properties of transparent aluminum, this material is not widely used. The biggest deterrent is cost (3-5 times more expensive than traditional bulletproof glass). ALON is currently used primarily for lenses for surveillance devices and missile sensors.