Research progress on thermal protection coatings on niobium alloy surfaces

With the continuous improvement of solid rocket engine technology, the service environment faced by its hot end components, such as nozzles, combustion chambers and gas rudders, is becoming increasingly harsh. These components need to work under extreme conditions of ultra-high temperature (2000~3000℃) and supersonic oxygen-rich combustion flow (Mach number 3~5). Therefore, the requirements for hot end component materials are getting higher and higher. Traditional high-temperature structural materials, such as nickel-based high-temperature alloys, have a maximum continuous use temperature of only 1100℃, which can no longer meet the rapid development needs of modern industry, and there is an urgent need to find alternative materials.

Niobium alloys have attracted much attention due to their high melting point (2467℃), low density (8.57 g/cm³) and good machinability. Studies have found that niobium alloys can still maintain high strength in a service environment of 1400℃, which is much higher than nickel-based high-temperature alloys. Therefore, they are considered to be ideal alternative materials and have been widely used in rocket engine nozzles and other components. However, niobium alloys are prone to react with oxygen at high temperatures, resulting in a decrease in mechanical properties, so improving their high-temperature oxidation resistance becomes the key.

In order to solve the oxidation problem of niobium alloys, researchers at home and abroad have adopted two methods: alloying and surface coating technology. Alloying generates a dense oxide protective film by adding elements such as Si, Al, and Hf, but it will reduce the high-temperature mechanical properties. In contrast, surface coating technology is more efficient and economical. It isolates high-temperature combustion flow and protects the substrate material by preparing a coating on the surface of niobium alloy.

At present, the anti-oxidation and ablation coating materials on the surface of niobium alloys mainly include high-temperature alloy coatings, precious metal coatings, aluminide coatings, and silicide ceramic materials. However, these coatings have limited protective effects in ultra-high temperature environments, so researchers have tried to prepare composite thermal protection structures, such as a combination of anti-oxidation and ablation layer + thermal barrier coating.

1. Research status of niobium alloy anti-oxidation and ablation coatings

The ideal niobium alloy anti-oxidation coating should have the characteristics of high melting point, low oxygen diffusion coefficient, thermal stability, matching with the substrate thermal expansion coefficient, no solid solution formation, and self-healing ability. At present, research mainly focuses on metal-based coatings and silicide coatings.

1.1 Metal-based anti-oxidation and ablative coatings

High-temperature alloy coatings, precious metal coatings and aluminide coatings are common metal-based coatings. High-temperature alloy coatings are prone to peeling and failure at high temperatures, precious metal coatings are too expensive and have a large difference in thermal expansion coefficient with the substrate, and aluminide coatings have poor high-temperature mechanical properties.

1.2 Silicon-based anti-oxidation and ablative ceramic coatings

Silicide coatings have good high-temperature oxidation resistance and can form a viscous SiO2 layer with self-healing ability at high temperatures. Nb-Si, Ti-Cr-Si and Mo-Si systems are the most studied silicide coating systems. The oxidation resistance and thermal cycle life of these coatings at high temperatures have been significantly improved.

2. Development trend of thermal barrier coatings

The stable operating temperature of traditional YSZ thermal barrier coatings is only 1200℃, which cannot meet the thermal protection requirements of niobium alloys. Therefore, researchers began to pay attention to new thermal barrier coating materials, such as ZrO2-based ceramics, rare earth zirconate ceramics and perovskite structural ceramics.

2.1 ZrO2-based ceramic coating

YSZ is the most widely used thermal barrier coating material, but its phase stability and sintering resistance at high temperatures need to be improved. Through doping modification, the service temperature of YSZ coating can be increased.

2.2 Rare earth zirconate ceramic coating

Rare earth zirconate ceramics have high melting points and low thermal conductivity. The problem of their low thermal expansion coefficient needs to be solved, but it has become a research hotspot.

2.3 Perovskite structural ceramic coating

Perovskite structural ceramics have high melting points and low thermal conductivity, but their fracture toughness and thermal shock resistance limit their applications. Researchers have prepared perovskite structural ceramics with better performance through modification.

3. Outlook

Niobium alloys have broad application prospects in aerospace and other fields, and research on their antioxidant properties is of great significance. At present, thermal protection is achieved by compounding multiple single-function coatings, but there is a problem of weak coating interface. Future research should optimize the composite coating structure, introduce a transition layer or design a gradient structure, or design a multifunctional coating through powder modification to improve the functional integration of thermal protective coatings.