Investigation Of High Temperature Performance Of Niobium-Based And Nickel-Based Composite Protective Coatings

High temperature protective coatings play a vital role in the surface protection of aviation engines and gas turbine blades, but intensive element interdiffusion between coating and alloy substrate during the high temperature oxidation and hot corrosion environment shortens the service life of the coatings. To solve the problem, adding the diffusion barrier as interlayer between the coating and substrate is an effective way. MCrAlY overlayer coatings are the most commonly used protective coatings. However, it is rarely reported the behavior of MCrAlY coatings with a diffusion barrier in the hot corrosion conditions. Niobium alloys are considered to be the most promising as widely used high temperature structure materials, but poor oxidation resistant severely restricts their application. Protective coatings coated on the niobium alloys surface is an excellent method to improve the oxidation resistant of niobium alloys.Al/NbCrAl composite coatings and NbCrAl coatings with or without CrON interlayer were prepared by direct current reactive magnetron sputtering, and NiCrAlYSi coatings with or without a diffusion barrier were deposited by one-step arc ion plating technique. The coatings microstructure and morphology were studied, element interdiffusion between coating and substrate, high temperature and hot corrosion behaviors, and the mechanism of diffusion barrier were analyzed and discussed. Results in this paper can be drawn as follows:1. After vacuum heat treatment at 900℃ and 1000℃, NbCrAl coatings with CrON diffusion barrier maintain continuous and dense, and have the good adhesion to the coating and substrate as well. The formation of the close-packed hexagonal structure Al2O3 phases in CrON diffusion barrier after high temperature treatment effectively inhibits the element interdiffusion,2. Exposed at 1000℃ for 10h, Al/NbCrAl coatings show the lowest weight gain. Al/NbCrAl coatings surface mainly generates continuous, dense and Al2O3 scales, and the interfacial adhesion of coating systems present favorable, so they have the stronger oxidation resistant capacity and high temperature protective performance. NbCrAl coating with lower Al content mainly composes of the Cr2O3 and CrNbO4 phases, the Al element in coating is easily consumed during oxidation process; NbCrAl coating with higher Al content forms Al2O3 and more spinel phases and the coating is porous columnar structure. The oxidation resistance ability of NbCrAl coating systems can be improved with the increase of alloying Al content, but it will increase the mismatch of thermal expansion coefficient between coating and substrate, prone to appearing the cracks and spallation of the surface oxide scales when the excessive stress exists in coating interface. Therefore, NbCrAl coatings are relatively limited for the oxidation resistance and high temperature protective capacity.3. Owing to the formation of continuous and dense Al2O3 scales, NiCrAlYSi coatings with or without a diffusion barrier protect the 900℃ high temperature hot corrosion from destruction and enhance the hot corrosion resistance of DSM11 substrate.4. During the isothermal hot corrosion, NiCrAlYSi coating with a diffusion barrier significantly suppress the interdiffusion between the coating and substrate and extend the service life, which is better than the single NiCrAlYSi coating. During the cyclic hot corrosion, the barrier layer interface between NiCrAlYSi/DB composite coatings and substrate is inclined to cracking, which reduces the diffusion barrier’s efficiency, leading to the earlier failure than that NiCrAlYSi coating without a diffusion barrier.