The refractory metals used in engine thrust chambers mainly include tungsten alloys, molybdenum alloys, tantalum alloys, and niobium alloys. Their advantages and disadvantages are shown in the table below. Among them, niobium alloys have advantages such as lower density than other refractory metals, excellent high-temperature strength, good plasticity, and weldability, making them one of the most promising refractory metal materials. However, niobium alloys have poor high-temperature oxidation resistance, rapidly undergoing "pest" oxidation at around 600℃. Therefore, an anti-oxidation coating must be applied to their surface to meet the requirements of engine thrust chambers. Tungsten alloys have excellent high-temperature strength, a low coefficient of thermal expansion, high density (19.3 g/cm³), a high ductile-brittle transition temperature (above room temperature), are difficult to machine, and have poor high-temperature oxidation resistance. Molybdenum alloys have a lower density (10.2 g/cm³), excellent high-temperature creep performance, a low coefficient of thermal expansion, a high ductile-brittle transition temperature (above room temperature), are difficult to machine, have poor weldability, and oxidize above 700℃. Tantalum alloys possess excellent high-temperature strength, good thermal shock resistance, high creep strength, low coefficient of thermal expansion, good ductility and toughness, and high density (16.68 g/cm³). They undergo "pest" oxidation above 500℃. Niobium alloys have low density (8.57 g/cm³), excellent high-temperature strength, good ductility, and good weldability. They also oxidize above 600℃.
Compared to other types of high-temperature alloys, high-temperature niobium alloys have advantages such as low density, high specific strength at high temperatures (600~1600 ℃), excellent cold and hot formability, and good weldability. They can be processed into thin-walled and complex-shaped parts and are used to manufacture components such as thrust chamber extensions for attitude/orbit control engines in rocket engines, satellites, spacecraft, and missiles. They are one of the important candidate materials for aerospace structural components. In the aerospace field, niobium-hafnium and niobium-tungsten alloys are most commonly used. Currently, refractory metal thrust chambers for attitude/orbit control engines have formed two generations of products. The "first generation" refers to the niobium-hafnium alloy most commonly used in the United States (C-103), while the "second generation" refers to the niobium-tungsten alloy most commonly used in Russia and China (Nb521). Comparing the physical properties and high-temperature tensile properties of the two niobium alloys, Nb521 exhibits significantly higher high-temperature mechanical properties than C-103 alloy. At 1600 °C, its strength is 3-4 times that of C-103 alloy, and it has been successfully applied to various orbital/attitude control engine models.
Aluminide and silicide coatings are the main research systems for high-temperature protective coatings of niobium alloys. Aluminide coatings are easy to prepare, but have poor high-temperature mechanical properties, are prone to cracking and even peeling under thermal shock, and have low service temperatures (generally below 1200 °C), resulting in a short service life. They are suitable for static isothermal oxidation environments. Silicate coatings possess good thermal stability and self-healing properties, and can be used at temperatures above 1300 °C, making them the most commonly used niobium alloy coating material currently. The most commonly used niobium alloy grade in the United States is C103 (Nb-10Hf-1Ti). Other common grades include SCb291 (Nb-10W-10Ta) and FS-85 (Nb-11W-27.5Ta). Commonly used silicide coating grades include R512A (Si-20Cr-5Ti) and R512E (Si-20Cr-20Fe). Russia commonly uses Nb521 (Nb-5W-2Mo-1Zr) as its niobium alloy, and its common coating is molybdenum silicide (MoSi2).
The preparation techniques and applications of several coatings are shown in the table below. The R512A and R512E coatings developed by the United States use a slurry sintering method, which has a simple preparation process and a short production cycle, but the service temperature is generally below 1400℃. They are currently widely used in spacecraft, space shuttles, and other aircraft. The MoSi2 coating developed by Russia uses a two-step method of vacuum arc deposition and infiltration reaction. The preparation process is relatively complex and the production cycle is long, but it has better high-temperature oxidation resistance and can be used at temperatures above 1500℃. Various specifications of engine thrust chambers prepared with this coating have been widely used in satellites, spacecraft and other aircraft.
