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Special high temperature protective coating

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3.1 new concept coating
This kind of coating introduces some basic theories of materials science, physical chemistry, solid diffusion, high temperature oxidation and other disciplines into the coating design, forming a unique high temperature coating system.

3.1.1 high temperature microcrystalline coating
Lou Hanyi and Wang Fuhui developed a new kind of high temperature alloy protective coating, namely, high temperature alloy microcrystalline coating. Different from the traditional high temperature protective coating, the composition of microcrystalline coating and base alloy is exactly the same, so the mechanical properties of the coating and base alloy are avoided to decline due to the mutual diffusion at high temperature. At the same time, the grain size of the coating is 20-100 nm, which can not only promote the selective oxidation of A1, but also improve the adhesion of the oxide film. The oxidation behavior of co-30cr-5al alloy and its sputtered microcrystalline coating in 1100 ℃ air shows that protective Al2O3 film is formed on the surface of the alloy in the first 25 hours of oxidation, and the weight of the alloy increases rapidly after 25 hours due to the cracking and peeling of Al2O3 film and the formation of Cr2O3 film. Compared with the former, sputtered microcrystalline coating shows excellent protection. After 100 hours of oxidation The oxide film is still a uniform and dense Al2O3 film, without separation of oxide film and matrix, cracking and peeling.

3.1.2 EQ coating
When the traditional high temperature protective coating (β - NiAl) and MCrAlY coating are used in the nickel base single crystal superalloy, because the content of refractory metals in the single crystal alloy is much higher than that of the traditional superalloy, the mutual diffusion between the coating and the substrate results in the formation of harmful SRZ zone at the interface between the coating and the substrate, which significantly reduces the creep fracture life of the nickel base single crystal alloy. Kawagishi et al. And Sato et al. Proposed to prepare EQ coating (equilibrium coating) to inhibit the formation of SRZ. Nickel base superalloy is composed of γ and γ′ phases, which are in equilibrium. Therefore, the chemical potential of the elements in the two phases is equal. If the γ′ phase in the alloy is used as EQ coating material, the chemical potential difference of the elements in the mutual diffusion between the coating and the substrate is zero, so the coating can be inhibited Mutual diffusion of layer and substrate. However, the oxidation resistance of the coating is limited. When the oxidation time is long, it is easy to degenerate into γ and γ′ phases.

3.1.3 functional gradient coating
FGM is the application of FGM in coating / substrate system. The basic idea of FGM is to prepare two or more different materials into composite materials with gradient distribution of components (or / and structures) in a certain direction, so that the materials can not achieve the function of non gradient structure. The application of FGM concept in coating / substrate system is to solve the interface problem. In the coating / substrate system, when the coating and substrate and / or the coating materials are different, the interface of different materials will produce serious mismatch near the interface due to the sudden change of the material properties (thermal expansion coefficient, elastic modulus, etc.), which will increase the driving force of the structure peeling. In order to alleviate the mismatch, a functional gradient layer can be introduced between the two materials, in which the composition (and / or structure) of the two materials continuously changes along the thickness, so as to reduce and overcome the performance mismatch of the joint and reduce the stress field. As mentioned before, the thermal barrier coating is composed of 8% Y2O3-ZrO2 ceramic top layer and MCrAlY metal bonding layer. The mismatch of ceramic and metal materials causes the ceramic layer to peel off during the thermal cycle. The gradient coating is prepared between the ceramic top layer and the metal bonding layer by plasma spraying, which makes the ceramic and metal in the layer The composition gradient along the thickness direction can alleviate the mismatch of ceramic / metal interface. Although the current results are not very satisfactory, but in this area of exploration has continued.

3.1.4 smart coating
The smart coatings proposed by Nicholls et al. Is a component gradient coating system that can make the best response to corrosion in a wide temperature range and complex corrosion environment, providing corrosion protection for high-temperature components of industrial and marine gas turbines. The coating has the function of resisting both high temperature oxidation and low temperature thermal corrosion. The coating is based on MCrAlY coating, and the outer layer is rich in aluminum. In the high temperature oxidation environment above 900 ℃ and the type I thermal corrosion condition above 800 ℃, Al2O3 film can be formed rapidly and play a protective role. The middle layer is rich in chromium, which can be used as a diffusion barrier layer to prevent the expansion of aluminum from aluminum rich layer to the substrate at high temperature When the temperature is low, Cr2O3 can be formed rapidly under the condition of 600-800 ℃ type II hot corrosion, and the corrosion rate can be reduced. At this time, the outer aluminum rich layer fails to form Al2O3 film rapidly in this environment. This kind of coating should have a good application prospect in industry and marine gas turbine.

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