Aluminum oxynitride (AlON) transparent ceramic has several advantages such as high strength, hardness, corrosion resistance, and good thermal shock resistance. It also has a wide transmission range and high linear transmittance, which makes it highly promising for both defense and civilian applications. It is considered by the U.S. military as "one of the most important defense materials of the 21st century."

Unlike glass, the primary raw materials for AlON are not silica or polymers, but aluminum, oxygen, and nitrogen. There are various methods to produce AlON, involving sintering aluminum oxynitride powder at high temperatures, followed by polishing to achieve transparency. Its hardness is four times that of quartz glass, and it can withstand temperatures up to 2150°C, making it stronger than bulletproof glass. In the movie "Jurassic World," the spherical tour vehicles are made from AlON.

Why is AlON ceramic transparent?

AlON, as a novel material, is composed of a unidirectional structure, and its crystal structure is "isotropic" in a cubic crystal system. This can be understood as the grains within the material being neatly aligned, coupled with the surface being mechanically polished, naturally preventing any scattering of light.

What is the hardness of AlON ceramic?

AlON ceramic (aluminum oxynitride) has a very high hardness, typically around 8.0 on the Mohs scale. This makes it harder than most metals and regular glass, making it very suitable for high wear resistance and protective applications.

AlON ceramic exhibits outstanding comprehensive properties that are unmatched by many other ceramic materials. It possesses excellent linear transmittance across ultraviolet, visible, and near-infrared wavelengths, making it one of the most widely used materials for infrared transparency and a preferred choice for infrared windows.

Additionally, one of the greatest potential applications of AlON ceramic is in transparent armor. It can be used as bulletproof windows for armored vehicles and helicopters. Due to its wide bandgap, low photon energy, and high-temperature stability, AlON ceramic is suitable as a substrate for luminescent materials, applicable in areas such as laser devices, fiber optic communications, and optical data storage. Beyond military applications, its high hardness, durability, and exceptional chemical resistance make it ideal for manufacturing POS system windows, as well as casings for precision instruments, high-end watches, prisms, various protective eyepieces, observation windows in high-temperature and high-pressure equipment, and windshields.

The preparation methods of AlON ceramics (aluminum oxynitride) are mainly divided into two categories: reaction sintering and two-step preparation methods.

Reaction Sintering Method

The one-step reaction sintering method involves using commercially available high-purity Al2O3 and AlN as raw materials. These are mixed in a certain ratio, with an appropriate amount of sintering additives added. The mixture is ball milled, dried, and then directly used for solid-phase sintering to form AlON ceramics. This one-step preparation method has a simple process flow, avoids the complexity of raw material powder preparation, and has a relatively low cost.

Two-Step Sintering Method

The key to the two-step sintering preparation method is the synthesis of high-purity γ-Al2O3 powder. The mainstream methods for powder synthesis include:

1. Carbothermal reduction nitridation method: This method involves mixing alumina powder with a reducing agent carbon powder in a certain ratio, and heating it to a specific temperature under a flowing N2 atmosphere to prepare the desired γ-AlON product. This method is cost-effective, produces highly active powder, and is suitable for large-scale production of AlON ceramics. However, the process is complex, the synthesized powder may vary, and it is not suitable for making large-sized AlON ceramics.

2. High-temperature solid-state reaction method: This method involves a chemical reaction of mixed Al2O3 and AlN powders at temperatures above 1650°C to produce AlON powder, continuously introducing nitrogen during high-temperature sintering. The high-temperature solid-state reaction method has a simple process flow and can avoid problems such as particle agglomeration due to prolonged sintering. However, it requires the use of high-purity, ultra-fine AlN raw material powders to ensure high reactivity, resulting in higher production costs.