Microstructure control of multi-layered EBC prepared by dual electron beam PVD
November 5-9, 2017
Environmental barrier coatings (EBCs) can play important roles in enabling SiC fiber reinforced SiC matrix composites (SiC/SiC) for application to advanced hot-section components in airplane engines. EBCs must exhibit superior environmental shielding durability and excellent volatilization resistance in the combustion environment containing water vapor. Therefore, a multilayered structure is applied when designing EBCs. Yb silicates are promising substances for EBC use. Volatilization resistance of Yb2SiO5 is superior to Yb2Si2O7. Thermal expansion coefficient of Yb2Si2O7 is closer to SiC/SiC composites than Yb2SiO5 and it exhibits a single phase up to about 1873 K. Mullite has higher oxygen shielding performance than Yb2Si2O7. Thus, we design EBC which is composed of a bond layer, dense mullite oxygen shielding layer, compositional-gradient dense Yb silicate layer showing water vapor shielding function, and Yb2SiO5 layer with segmented structure. The function of the Yb2SiO5 layer is reduction of thermal stresses during thermal cycling. Such an environmental shielding capability depends greatly on both the compositions and microstructures of the layers. Therefore, the coating processes used to make the EBCs should allow precise control of these factors. While it is difficult to strictly control the compositions of such complex oxides using conventional plasma spray deposition techniques due to incongruent evaporation of raw powders, dual electron beam physical vapor deposition (EB-PVD) is a potential process for constructing the complex oxides layer with controlled compositions as well as microstructures. We recently reported that formation of Yb2Si2O7 layer by dual EB-PVD . In the present study, we investigated the in-situ formation of a dense mullite layer, compositional gradient layer from Yb2Si2O7 to Yb2SiO5 with dense structure, and Yb2SiO5 layer with segmented structure via dual EB-PVD, simultaneously heating the substrate.
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Taishi Yokoi, Norio Yamaguchi, Satoshi Kitaoka, and Masasuke Takata, "Microstructure control of multi-layered EBC prepared by dual electron beam PVD" in "Advanced Ceramic Matrix Composites: Science and Technology of Materials, Design, Applications, Performance and Integration", Yutaka Kagawa, Tokyo University of Technology, Japan Dongming Zhu, NASA Glenn Research Center, USA Ram Darolia, GE Aviation (retired), USA Rishi Raj, University of Colorado, Boulder, USA Eds, ECI Symposium Series, (2017). https://dc.engconfintl.org/acmc/10