Infrared imaging and precision guidance systems are distinguished by their high imaging accuracy, robust concealment, and resilience to interference. These characteristics make them a pivotal area of contemporary and future warfare. The infrared window is an integral part of the infrared guidance system, being responsible for transmitting target signals, preserving aerodynamic configurations, and protecting internal precision optoelectronic components. Consequently, the window material must possess a high optical transmittance across the operational band and exhibit the superior strength and hardness.

Hypersonic vehicles have challenges to infrared window materials, including extreme thermal shock, aerodynamic overload, degradation of optical and mechanical performance at high temperatures, and self-heat radiation that can interfere with target signals. At present, no material fully satisfies the demands of cutting-edge systems, making it imperative to develop a window material that offers a low radiation, a high strength, and a broadband transmittance at elevated temperatures.

Commonly utilized infrared window materials encompass sapphire, spinel, AlON, magnesium fluoride, yttrium oxide, and Y₂O₃-MgO nanocomposite ceramic materials. Sapphire also as α-alumina monocrystal is characterized by its high strength and broad high transmittance properties. It boasts a well-established industrial chain and is extensively employed in various aircraft models. While polycrystalline alumina transparent ceramics offer lower production costs, compared to sapphire, and they have yet to reach the same level of application. Spinel and AlON both possess cubic structures and demonstrate great transmission rates from near-ultraviolet to mid-infrared spectrums. AlON exhibits superior mechanical properties akin to those of sapphire, making them popular choices for aircraft infrared windows. However, these three materials share a significant drawback, i.e., an excessively high heat emissivity at high temperatures. This can lead to thermal barrier complications due to spontaneous radiation interference with target signals under intense aerodynamic heating. In addition, their shorter infrared absorption cut-off edges also result in pronounced transmission rate reductions at 5 micrometers, following a high-temperature blue shift. Note that sapphire and spinel both present challenges related to their inadequate mechanical performance at high temperatures. Researches on zirconia ceramics as infrared windows are limited, but their high bending strength and decent transmission performance are noteworthy. Y₂O₃ and MgF₂ with the cubic structure exhibit superior transmission in the 3-5 micrometer band due to their low phonon energy and minimal spontaneous radiation coefficient. Their further infrared absorption cutoff edge mitigates an impact of high temperature on the mid-infrared wave transmission. However, their mechanical properties do not withstand the intense heat shock encountered at a hypersonic speed. Y₂O₃-MgO nanocomposite ceramics, characterized by extremely small grain size from the pinning effect, combine a high transmission in mid-infrared, low radiation and high strength at high temperatures. This makes them an ideal material for infrared windows in hypersonic aircraft.