Through the comprehensive understanding of the RE EBCs devel-opment, their composition has experienced a change from single to multiple layers. The trilayer coatings usually have a Si bond layer,a mullite intermediate layer, and a RE silicate top layer.
The Si-bond layer usually acts as an oxygen absorbent to prevent oxidation of silicon based non-oxide substrate. The mulite intermedi-ate layer, serves not only as a diffusion barrier against liquids and high temperature gases, but also a preventor for the reaction between thermally grown silica and top layer. The RE silicate top layer is characteristic with high melting point, low steam volatility and lowthermal conductivity, it plays an important role as thermal and water vapor insulator.
As expected, Si/mullite/rare earth silicate multilayer system can significantly improve the service life of the substrate under the aero-engine working environment. However, after long-time service at combustion conditions, the three-layer EBCs have showed a series of defects, such as the cracks generated from surface corrosion by water vapor and molten salt, interface reaction and even spallation from the substrate.Detailed studies on the high temperature water vapor corrosion of the RE silicates as well as the surface degradation behavior of those selected ceramic materials in hot gas environment have been reported. It is found that the water vapor corrosion of REdisilicate also results
in the formation of volatile silicon hydroxide as:
RE2Si2O7,+2H2O(g)→RE2SiO5+Si(OH)4(g),(RE=Y, Yb, and Lu), simi-lar to silica. In case of ScoSipO, however, the decomposition produces the pure oxides. While the high temperature oxidation of erbium silicate have been studied, there have been no reports focusing on the water vapor corrosion behavior about EroSip0, at elevated temperature.
The top layer CTE of the RE monosilicate, especially RE2SiO5,(RE=Yb, Lu, and Gd), EBCs was significantly higher than that of SiC substrate at test conditions, which resulted in the cracking between RE monosilicate and mullite(which also have a relative higher CTE than that of SiC). These cracks, however, provide a channel for oxidizing species and water vapor to the Si bond layer, rendering the coating ineffective.
In summary, the degradation of RE2SiO5 and RE2Si2O7, coatings is ascribed to CTE mismatch and high volatility.
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