2.2 coating

Coating refers to the coating formed by direct deposition of coating materials on the alloy surface by physical or chemical means. The obvious difference between the coating and the diffusion coating is that the coating only interacts with the substrate during deposition, which can improve the adhesion of the coating, and the substrate does not participate in the formation of the coating, so the selection of the coating composition is more diverse. The coating can be metal coating and ceramic coating, and the most typical coating is MCrAlY coating.

MCrAlY coating developed in the 1970s, and now it has developed into a series of coating systems, among which m is Fe, Co, Ni or their combination, Al is used to form protective Al2O3 film, Cr is used to promote the formation of oxide film and improve the thermal corrosion resistance, y is used to improve the adhesion of oxide film, and HF, Si, Ta, re, Zr, Nb and other elements can also be added to the coating Or more to meet some specific application needs. This kind of coating is mainly composed of β phase (NiAl or coal) and γ solid solution of Ni or Co. Nickel based Ni-Cr-Al-Y coating has excellent oxidation resistance, cobalt based co-cr-al-y coating is more resistant to thermal corrosion, both of which are considered. The oxidation resistance and thermal corrosion resistance of aluminized coating, modified aluminide coating and MCrAlY coating were compared. By adjusting the composition of MCrAlY coating, oxidation resistant coating and thermal corrosion resistant coating can be prepared to meet the needs of different working environment and different base alloys. The common preparation methods of MCrAlY coating are physical vapor deposition, including electron beam physical vapor deposition (EB-PVD), sputtering, ion plating, and spraying technology, including low-pressure plasma spraying, argon plasma spraying, supersonic flame spraying, etc.

2.3 thermal barrier coating
The main function of TBCs is heat insulation, which is composed of ceramic surface with low thermal conductivity and metal bonding layer. The early thermal barrier coating is Al2O3 and ZrO2 (MgO or Cao stable) ceramic thermal insulation layer directly sprayed on the alloy surface. In the mid-1970s, the preparation of Y2O3 stable ZrO2 surface layer using NiCrAlY as the bonding layer and plasma spraying technology, and the deposition of ceramic surface layer using EB-PVD technology developed in the early 1980s are important progress in the development history of thermal barrier coating. At present, most of the bonding layers of thermal barrier coatings are MCrAlY and Pt modified aluminide coatings. The main function of metal bonding layer is to increase the bonding force between ceramic coating and matrix, improve the mismatch of thermal expansion coefficient between them, and improve the oxidation resistance of matrix. The partially stable ZrO2 (y-psz) with 8% Y2O3 has high melting point, high temperature stability, low thermal conductivity and the most close thermal expansion rate to the base material, which makes it the first choice for ceramic insulation layer. At high temperature, Al in the bonding layer reacts with oxygen diffused from the ceramic layer, forming a thermally grown oxide (TGO) between the bonding layer and the ceramic layer interface, the main component of which is α - Al2O3, effectively preventing the oxidation of the substrate.

There are many preparation methods of ceramic insulation layer, including thermal spraying, physical vapor deposition, chemical vapor deposition, sol-gel method, etc. the commonly used technologies are plasma spraying and EB-PVD. The y-psz prepared by plasma spraying is a lamellar structure, which usually contains 15% ~ 25% porosity. Therefore, it has low thermal conductivity and certain strain tolerance. It is usually applied to the parts with low requirements in aeroengine, such as combustion chamber, combustion evaporator, stator blade, etc. The y-psz layer prepared by EB-PVD is columnar crystal structure, which has high strain tolerance in the process of temperature change, so it has longer life than plasma spraying coating, but the equipment is expensive and the cost is high, so it is used in the parts with more stringent requirements in the engine, such as aviation gas turbine blades. The thermal insulation effect of TBCs can reach 175 ℃.

At present, the main challenge of TBCs application is the durability of the coating, especially the anti peeling ability of the coating, which is affected by many factors, such as the stress state in ZrO2 layer, the microstructure of the bonding layer, the thickness and stress state of TGO layer, and the fracture resistance of various interfaces between the bonding layer and TGO. At present, it is recognized that the oxidation of bonding layer is the key factor to determine the service life of EB-PVD TBCs.