PSI - Issue 65

V.A. Bryzgalov et al. / Procedia Structural Integrity 65 (2024) 25–31 Bryzgalov V.A., Korznikova E.A./ Structural Integrity Procedia 00 (2024) 000–000

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ductility compared to diffusion coatings, which can be attributed to the formation of a β+γ two-phase microstructure comprising intermetallic phases surrounded by a metallic nickel- or cobalt-based gamma solid solution matrix (Figure 4).

Fig. 4. Schematic of the coating/substrate interface composition (Chen and Barman, 2018).

The main advantage over diffusion coatings is the significant amount of Cr in MCrAlY which enhances the corrosion and oxidation resistance. Regarding oxidation resistance, NiCrAlY shows the best protection, whereas MCrAlY systems have higher resistance to oxidation when sulfur is present in the air. One of the most prevalent techniques for the application of overlay coatings is the thermal spray method. Thermal spraying is a process whereby a material with a high melting point is applied to a component using a plasma jet or supersonic flow of a gas stream in the high velocity oxygen fuel (HVOF) (Chen and Barman, 2018) or D-gun spraying (DGS) (Singh et al., 2012) methods. The plasma or energy-carrying medium is used to heat or melt the feedstock material, which is then injected into the plasma in the form of a powder, rod or wire. The high pressure and velocity of the heated feedstock cause particle droplets to flatten upon impact and build up subsequent layers of droplets (Fig. 5).

Fig. 5. Schematic representation of thermal spraying method (Kuroda et al., 2008).

The aforementioned method of coating is most frequently employed for coating superalloys with thermal-barrier coatings. Thermal-barrier coatings (TBC) are essential for protecting superalloys used in high-temperature environments from oxidation and thermal degradation. TBC consists of the ceramic layer, typically made from yttria-stabilized zirconia (YSZ), which provides thermal insulation, and the bond coat, which is usually MCrAlY alloy, offers oxidation and corrosion resistance and improves adhesion between the ceramic topcoat and the substrate (Fig. 7). Although less prevalent than YSZ (Banerjee et al., 2023; Pasupuleti et al., 2019; Viswanathan et al., 2015) chromium-based TBCs have gained recognition for their efficacy in temperature regions below 900°C, where they form a protective layer against hot corrosion. The most significant characteristics of Cr ₂ O ₃ are its capacity to form a robust, insoluble, and adherent layer in an oxidizing atmosphere below 1000°C, making it an ideal material for applications requiring protection against wear and corrosion (Bürgel et al. (1987); Wood et al. (1966)). Additionally, these coatings exhibit superior mechanical properties (Yang et al. (2006)).