Hypersonic vehicles generally refer to aerospace vehicles with a flight speed of more than 5 times the speed of sound (Mach number greater than 5), such as hypersonic cruise missiles, hypersonic reconnaissance aircraft, and so on. In other words, it can fly at least 6120 kilometers per hour. This means that it can reach any point on the earth in a short time, and then quickly strike various military targets thousands of kilometers or even tens of thousands of kilometers away.
It is no wonder that all major military powers in the world have invested heavily in the research and development of hypersonic vehicles. Moreover, countries are stepping up the development of hypersonic vehicles with a flying speed of more than 10 Mach. For example, the US X-43A hypersonic vehicle has already reached an astonishing speed of Mach 9.8 in the 2004 flight test.
However, in the process of developing hypersonic vehicles, major powers will inevitably encounter a world problem-thermal protection. Because to achieve such high-speed flight, it is necessary to ensure that the key structural components of the aircraft such as the nose cone, wing leading edge, windward surface, leeward surface, engine nozzle, etc. can withstand severe air friction and up to 2000-3000 ℃ The hot air flow is impacted without being destroyed, that is, these parts must have excellent properties such as high temperature resistance, high strength, compression resistance, and impact resistance.
Although refractory metals such as tungsten, molybdenum, and rhenium can withstand high temperatures of more than 2,000 to 3,000 degrees, and under this temperature condition, they can maintain high strength and compressive and impact resistance. However, these high-temperature metals are Under high temperature, the fate of oxidation cannot escape. Therefore, these parts are mostly made of special materials such as ceramics, high-temperature alloys and high-temperature polymers.
Currently, most hypersonic vehicles in the United States use tungsten-based materials for their heads. For example, X-43A's very narrow and thin nose cone is made of 362.88kg high-density tungsten alloy plate, its purpose is to move the center of gravity of the aircraft forward. However, it alone accounts for 29% of the total weight of the aircraft. Moreover, tungsten, as a metal material with higher value, will bring a problem-the cost remains high. Weight and cost, these two problems will be no small obstacles for future weaponization and large-scale equipment.
In addition, in addition to using special materials to manufacture these key components, in order to improve the penetration and survivability of the hypersonic aircraft, it is necessary to put a mysterious coat on the end of the aircraft-a high temperature and oxidation resistant coating, which has also been described as "Iron shirt". In fact, high-temperature and anti-oxidation coatings are developed by researchers to improve the performance of refractory metals tungsten, molybdenum, titanium, tantalum, niobium, rhenium and their alloys under high temperature and oxygen conditions.
Through sintering, foaming, coating, electroplating, laser cladding, plasma spraying, multi-arc ion plating, chemical vapor deposition (CVD), physical vapor deposition (PVD), organic chemical vapor deposition (MOCVD), high-temperature high-speed flame spraying, Diffusion welding, double glow plasma forming and other methods are used to perform high-temperature anti-oxidation treatment on the surface of the refractory metal and its alloy to obtain a high-temperature anti-oxidation coating.
Of course, in addition to studying coating materials, many military powers are also studying non-coated ceramic metal-based high-temperature oxidation-resistant materials, the more advanced are micro-nano light ceramic metal composite technology and carbon-carbon-light ceramic composite technology.
After 15 years of R&D, the research team of China ZN University has successfully developed a new type of ceramic coating and its composite material-the new Zr- that can pave the way for the development of hypersonic vehicles. Carbon/carbon composite material modified by Ti-CB ceramic coating.
As the flight speed of hypersonic vehicles continues to increase, high-temperature resistant and anti-oxidation coatings will inevitably become the commanding heights of science and technology competed by various military powers, and also the key and difficult technical points for countries to continue to tackle.