In the past few decades, EBCs have been used as a protective layer for Si-based non-oxide ceramics substrate that inhibits the corrosion in the presence of water vapor and molten salt. Fig.1 shows the change in the temperature capability of ceramic matrix composites with EBCs.
Fig.1.Change in the temperature capability of ceramic matrix composites with EBCs.
The earlier EBCs consist of a mullite bond layer and an yttrium-stabilized zirconia (YSZ) top coating. Mullite has low density, low thermal conductivity, high oxidation resistance and chemical compatibility, as well as similar thermal expansion coefficient to Si-based non-oxide ceramics, suitable for EBCs. However, mullite degrades seriously in combustion environment due to the selective corrosion of silica by water vapor. Different approaches have been attempted to overcome these problems. It has been found that yttrium-stabilized zirconia (YSZ) shows a better stability in water vapor. Therefore,a ZrO2-8 wt% Y203 layer was coated over mullite by plasma spray. Unfortunately, thermal stress was generated in this EBCsystem due to mismatch on the coefficient of thermal expansion(CTE), leading to delamination of the system eventually.
The second generation of EBCs is a system of Si/mullite or mullite+BSAS/BSAS(1-xBaO-xSrO-Al2O3-2SiO2,0≤X≤1). BSAS is used to replace YSZ as top coating material owing to its excellent CTE match with SiC and relatively low activity of silica when compared with mullite. Meanwhile, BSAS also shows excellent corrosion resistance, creep resistance and high strength at high temperature.However, there are some intrinsic defects in the BSAS-based EBCs. BSAS suffers significant degradation at temperatures higher than1400℃. Fig.2 shows the cross section Si/mullite/BSAS EBCs on MI CMC after 1000 h in 90% H2O-balance O2at 1300 ℃ with 1 h cycles.The crack and pores are anticipated in Fig.3(b), owing to the reaction of BSAS with SiO2on the surface of the Si layer, which forms a low melting point glass-state substances. The results demonstrate that theservice temperature is limited to be lower than 1300 ℃ in Si/mullite/BSAS EBCs.
Fig.2. Cross section of Si/mullite/BSAS on MI CMC after 1000 h in 90% H2O-balance O2 at 1300 C with 1 h cycles;(a) and (b) are from two different batches of coatings.
In order to overcome the limitations of the above two generations of EBCs and to increase the performances of Si-based non-oxide ceramics at high temperature, some advanced coating systems have been explored. RE silicates are considered as promising alternatives for the previous EBC due to their low thermal expansion coefficient andthe ability to endure temperatures higher than 1482℃for thousands of hours. Based on the longer lifetime and superior comprehensiveperformance in aero-engine working environments of RE silicates, NASA developed a third generation of EBCs system, which consists of aSi bond coat, mullite intermediate layer, and rare earth silicates top layer, as shown in Fig.3.
Fig.3.Sketch of SiC/CMC based multilayer structure of RE silicate EBCs.
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