From the current research, the high temperature protective coating of
niobium based alloy is mainly divided into the following five systems: heat resistant alloy coating. Precious metal coating. Ceramic coating. Aluminide coating and silicide coating.
Heat resistant alloy is the first system to be developed for the coating of niobium-based alloy, mainly including iron, nickel, cobalt metals and alloys. Heat resistant alloy coating with niobium alloy bonding force is bad, the high temperature erosion easy peeling, the porous coating for oxygen provides a large number of diffusion channels, up to more than 900 ℃ the rapid oxidation, failure in matrix, and this kind of coating due to the defects of the component itself, not suitable for niobium alloy of high temperature anti-oxidation protection.
Precious metals not only have good corrosion resistance, but also have ductility to adapt to the deformation caused by matrix elastoplastic deformation or high temperature creep. However, the precious metal coating is limited in practical application because of its high cost.
There are many kinds of ceramic coatings and their uses are not the same. If a layer of SiO2 or Al2O3 film can be prepared directly on the surface of the alloy, it can isolate the gas medium and reduce the oxidation rate of the alloy, but the SiO2 or Al2O3 film must be very dense. The thermal expansion coefficient of ceramic coating is different from that of Nb alloy substrate, so it is easy to crack under thermal cycling condition.
Aluminide coatings were developed in the 1950s and are still widely used today, accounting for about 90% of the total high temperature protective coatings. The diffusion aluminizing process for preparing aluminide coatings is mature and the methods are diversified. The coatings have good oxidation resistance, but the high temperature mechanical properties are poor, and it is easy to form defects under thermal shock, and mechanical deformation will accelerate the failure of the coatings.
Silicide coating has excellent oxidation resistance and good thermal stability, and the operating temperature can reach 1600℃. Some scholars at home and abroad have prepared silicified coatings with good comprehensive properties by various methods. The United States and other dozens of research units developed dozens of niobium alloy protective coating, including silicide coating.
