Rare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for resistance against attack by molten ca
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Rare-earth pyrosilicate solid-solution environmentalbarrier coating ceramics for resistance against attack by molten calcia–magnesia–aluminosilicate (CMAS) glass Laura R. Turcer, Nitin P. Padturea) School of Engineering Brown University, Providence, Rhode Island 02912, USA a) Address all correspondence to this author. e-mail: [email protected] Received: 21 February 2020; accepted: 11 May 2020
High-temperature (1500 °C) interactions of promising environmental-barrier coating (EBC) ceramics in the rare-earth (RE) pyrosilicate system, Yb(2-x)YxSi2O7 (x = 0, 0.2, 1, or 2), with three different calcia–magnesia– aluminosiliate (CMAS) glass compositions, are explored. Only the Ca/Si ratio is varied in the CMAS: 0.76, 0.44, or 0.10. Interaction between the highest Ca/Si CMAS and the EBC ceramic with the lowest x (=0, Yb2Si2O7) promotes no reaction but the formation of “blister” cracks. In contrast, the highest x (=2, Y2Si2O7) promotes the formation of an apatite reaction product, but no “blister” cracks. Observationally, it is found that a decrease in the CMAS Ca/Si ratio (0.76–0.10) and a decrease in Y-content decreases the propensity for reaction crystallization (apatite formation) and “blister” cracks. These results are rationalized based on the relative affinities between Ca2+ in the CMAS and Y3+ or Yb3+ in the EBC ceramics, suggesting a way to tune the CMAS interactions in RE pyrosilicate solid solutions.
Introduction The hot section of gas turbine engines can operate at maximum temperatures exceeding 1500 °C as a result of the use of ceramic thermal-barrier coatings (TBCs) on Ni-based superalloy components, in conjunction with air cooling [1–4]. This has resulted in outstanding power and efficiency gains in current gas turbine engines used for aircraft propulsion and landbased power generation. TBCs are being developed with even higher temperature capabilities and lower thermal conductivities [3,4]. However, Ni-based superalloys have not kept up with TBC development, requiring more aggressive cooling of the metallic components [3,4], resulting in efficiency losses [4,5]. Therefore, hot section materials with inherently higher temperature capabilities are needed. In this context, ceramic matrix composites (CMCs), typically comprised of SiC-based matrix and fibers, are showing great promise for replacing Ni-based superalloys in the engine’s hot section [4–7]. Some Ni-based superalloy hot section components (stationary) have already been replaced by CMCs in gas turbine engines that are in-service commercially, both for aircraft propulsion and power generation. However, the oxygen and steam present in the high-velocity, high-pressure hot gas stream in the
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engine cause the SiC-based CMCs to undergo active oxidation and recession [4,8–10]. Thus, SiC-based CMCs need to be protected by ceramic environmental-barrier coatings (EBCs) [4,10–13]. Unlike TBCs, EBCs must be dense in order to prevent ingression of the hot gas stream. Consequently, EBCs must have a good coefficient of therm
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