Study on High Temperature Oxidation Behavior of Single Crystal Superalloy Chromium Modified Aluminide Coating

A chromium modified aluminide (Cr-Al) coating was prepared on a single crystal superalloy substrate by chemical vapor deposition (CVD) method, and the high temperature oxidation behavior of the coating in air at 1050 ℃ was studied. X-ray diffraction (XRD), scanning electron microscope (SEM), energy spectrometer (EDS) and other methods were used to analyze the evolution of phase structure, microstructure and composition of Cr-Al coating during high temperature oxidation. The results show that : After oxidation at 1050 ℃ for 300 h, the oxidation kinetic curve of the coating conforms to the parabolic evolution law. The coating of Cr-Al coating improves the high temperature oxidation resistance of the base alloy; the high temperature oxidation for 200 h maintains good internal coating Surface morphology, and after 300 hours of oxidation, the coating has severe oxide film peeling; it reveals the phase transition process of Cr-Al coating degradation under high temperature oxidation conditions, and the phase structure transition process of the coating area is β- NiAl phase →β-NiAl phase(main)+γ′-Ni3Al phase(secondary); the phase structure transformation process of the diffusion zone is β-NiAl phase+precipitated phase→β-NiAl phase(main)+ γ′-Ni3Al phase (secondary) + precipitated phase →γ′-Ni3Al phase (primary) +β-NiAl phase (secondary) + precipitated phase →γ′-Ni3Al phase + precipitated phase.

With the development of high thrust-to-weight ratio aeroengines, the use environment of its turbine blades is more demanding, which puts forward higher requirements on the performance of blade materials. Materials need to take into account both mechanical properties at high temperatures and high temperature oxidation and thermal corrosion resistance. . After decades of development, turbine blades have gradually developed from polycrystalline alloys to directional casting and single crystal alloys. Among them, the single crystal superalloy eliminates the weakening factor of grain boundaries at high temperatures, making its mechanical properties at high temperatures greatly improved. Improvement. High temperature oxidation and hot corrosion are the main failure modes in the application of single crystal turbine blades. Once oxidation and corrosion occur, the matrix material will be directly damaged and the mechanical properties will drop sharply. In response to this phenomenon, the Vacuum Technology Network (http://www.chvacuum.com/) believes that a high-temperature protective coating must be applied to extend the service life of the base alloy material.

At present, aluminide coating has been widely used as a protective coating for the outer surface and inner cavity of nickel-based single crystal superalloy blades. The aluminum coating has a good bonding ability with the base alloy, and can form an aluminum oxide film with a certain self-healing ability in a high temperature oxidation environment, which can effectively alleviate the further high temperature oxidation of the coating. However, a single aluminide coating has shortcomings such as easy degradation and resistance to heat corrosion. Using a modified aluminide coating to partially replace the single aluminide coating is an effective way to overcome its shortcomings. The Cr-Al coating prepared by adding chromium to the aluminide coating can effectively reduce the degradation rate of the coating, slow down the martensite transformation process in the nickel-rich β-NiAl phase, and improve the phase structure of the coating Stability and resistance to thermal corrosion. In addition, the presence of Cr can also promote the selective oxidation of aluminum on the coating surface, thereby reducing the critical aluminum content required to form a protective aluminum oxide film.

At present, the preparation process of Cr-Al coating basically focuses on embedded co-infiltration, physical vapor deposition and electrodeposition. However, there are still few research reports on the preparation of Cr-Al coating by chemical vapor deposition (CVD). CVD technology With its own unique advantages (high product purity, good winding properties, etc.), it should be more widely used in coating preparation processes.

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In this paper, a chromium-modified aluminide coating was prepared on the nickel-based single crystal superalloy substrate by chemical vapor deposition technology. Under the high temperature oxidation condition of 1050 ℃, the oxidation kinetics, phase structure and microstructure of the Cr-Al coating A systematic study of the evolution law of composition and composition, in-depth discussion of the high-temperature oxidation behavior of Cr-Al coatings, and reveals the possible phase transition mechanism of the coatings in high-temperature oxidation environments.

A nickel-based single crystal superalloy (15 mm× 10 mm× 1.5 mm) is used as the matrix material, and its nominal composition is shown in Table 1. After the alloy substrate is polished with No. 600 metallographic sandpaper and wet sandblasted, it is ultrasonically cleaned and dried with acetone.

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Table 1 Nominal composition of nickel-based single crystal superalloy matrix (mass fraction %)

(ALUVAP CVA 190 BL L-Single) type chemical vapor deposition equipment was used to prepare Cr-Al coating. The preparation method of the Cr-Al coating is as follows: firstly deposit the Cr layer, the deposition process parameter is 1000 ℃ for 1 h, the vacuum chamber pressure is 0.005~0.02 MPa; secondly, the Al layer is directly prepared on the surface of the Cr layer, and the corresponding deposition process is React at 1050 ℃ for 3 h, and the pressure of the vacuum chamber is 0.01~0.03 MPa.

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The static oxidation test is carried out in a high-temperature muffle furnace with reference to HB 5258—2000 "Test Method for Oxidation Resistance of Steel and Superalloys", the test temperature is 1050 ℃, and the test time is 100 h, 200 h and 300 h respectively. X-ray diffractometer (XRD, Bruker D8 Advance) was used to analyze the phase structure of the coating, scanning electron microscope (SEM, FEI-Quanta 600) was used to analyze the surface and cross-sectional micro morphology of the coating, and the energy spectrometer (EDS) , Oxford INCAx-sight 6427) detects the coating composition of the relevant area.

(1) Under the same test conditions, the coating of Cr-Al coating increased the oxidation resistance of the single crystal alloy by more than 2 times;

(2) With the extension of the oxidation time, three new phases of α-Al2O3, γ′-Ni3Al and NiO appeared in the coating;

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(3) After being oxidized at 1050 ℃ for 300 h, severe scale peeling appeared on the surface of the coating and showed three different organizational morphologies;

(4) The decrease of Al content leads to the massive formation of Ni3Al phase and the formation of oxygen internal diffusion channels caused by abnormal oxidation of Ni3Al phase are one of the main reasons for the high temperature degradation of Cr-Al coatings.

Founded in 2015,Zunhua Baorui Titanium Equipment Co.,Ltd. is a manufacturer specializing in pvd vacuum ion coating equipment. The company’s products mainly include large plate coating machine, large tube collating machine, tool coating machine and LOW-E glass production line. Mr.Wang baijiang ,general manager of the company ,has been engaged in vacuum coating industry for more than 30 years. He continuously improve production technology, improve product performance and devote himself to provide customers with better product experience and higher production efficiency.