Stability and CMAS resistance of Ytterbium-silicate/hafnate EBCs/TBC for SiC composites David L. Poerschkea*, Derek D. Hass§, Susie Eustis§, Gareth G.E. Seward♯ Jason S. Van Sluytman*, Carlos G. Levi*

*Materials Department, University of California, Santa Barbara, CA 93106-5050 § Directed Vapor Technologies International, Charlottesville, VA ♯ Department of Earth Science, University of California, Santa Barbara, CA 93106-9630

Multilayer ytterbium-hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC-based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits (CMAS). The study revealed that reactions occurring at the interface between Yb2Si2O7 and Yb4Hf3O12 layers promote coating delamination following isothermal annealing for 100h/1500ºC while coating architectures involving Yb2SiO5 in contact with Yb4Hf3O12 do not experience similar degradation. The outer Yb4Hf3O12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300ºC and 1500ºC. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300ºC, or a cuspidine-type aluminosilicate at 1500ºC, in addition to the apatite and reprecipitated fluorite phases observed in related systems. Keywords: thermal barrier coatings (TBC), environmental barrier coatings (EBC), thermodynamic equilibrium, silicates, layered ceramics

a

Author to whom correspondence should be addressed: [email protected] Accepted for publication in the Journal of the American Ceramic Society

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INTRODUCTION

The degradation of thermal (TBC) and environmental (EBC) barrier coatings by molten silicate deposits is considered a fundamental obstacle to achieving higher operating temperatures and improved efficiency in gas turbine engines1. These deposits originate from siliceous debris ingested with the intake air which, at temperatures in the engine cycle above ~1150°C2, give rise to calcium-magnesium-aluminosilicate (CMAS) melts with other minor cations. CMAS damage of TBCs is fundamentally thermomechanical in origin. It involves melt penetration into the features of the microstructure designed to provide compliance, namely the intercolumnar gaps of coatings produced by electron-beam physical vapor deposition (EB-PVD)3 or the network of pores and vertical cracks in air plasma-spray (APS) coatings4. The melt penetrates to a depth dependent on the thermal gradient across the TBC, wherein it freezes and stiffens the coating, leading to exfoliation and/or delamination upon subsequent thermal cycling. While in contact with the melt the coating dissolves and reprecipitates as crystalline phases closer to local equilibrium with the liquid phase1. CMAS degradation of EBCs, designed to prevent hydroxide-assisted volatilization of the SiO2 thermally grown oxide (TGO)5,6, is primarily thermochemical. Reactions with molten deposits convert the coating material into a combination of crystalline and glassy phases that are arguably less resistant to volatilization and more likely to flow in the high velocity combustion gases7. Rare earth silicate EBCs (RE2Si2O7 or RE2SiO5 where RE is Sc, Y, or one of the lanthanides) appear to be more resistant to attack than barium-strontium-aluminosilicate (BSAS) but the dissolution of any silicate coating contributes SiO2 to the melt thereby reducing the efficiency of silicate crystallization reactions7,8. The lack of a proven strategy to improve the intrinsic CMAS resistance of silicate EBCs motivates the development of alternate protection approaches. Proposed mitigation strategies for TBCs rely on the thermochemical interaction between the coating and melt to minimize the penetration depth by rapidly crystallizing the melt1. The

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approach is exemplified by Gd2Zr2O7 (GdZO) coatings that limit penetration to tens of microns under conditions that lead to complete infiltration of conventional yttria stabilized zirconia (YSZ) coatings9,10. This behavior is attributed to the formation of an apatite silicate, nominally Ca2Gd8(SiO4)6O2, that, combined with reprecipitated fluorite, blocks the infiltration channels10. It has been suggested that similar reactions are effective in other RE zirconates, and by extension hafnates, as long as the RE concentration is sufficient to produce a substantial apatite volume1,11. With this understanding an improved coating system for CMCs could involve applying a layer of a RE-zirconate or -hafnate to a RE silicate to mitigate the thermochemical degradation of the latter5,12. This coating architecture is shown schematically in Figure 1 and has been termed a T/EBCb. The interaction with CMAS would ideally be confined to the outermost layer without degrading the performance of the EBC or the structural component. In addition, the materials and architecture selected for the T/EBC should satisfy other design criteria. The EBC, which must be dense to serve as a vapor barrier, should exhibit low SiO2 activity, be phase stable over the operating temperature range, thermodynamically compatible with the TGO and TBC, and minimize the thermal strains generated due to CTE mismatch with SiC. The TBC layer must be phase stable and compatible with the EBC but the option for application of a segmented porous microstructure with low in-plane modulus relaxes the requirement for CTE matching. The recent study of the HfO2-SiO2-YbO1.5 system12 provides a thermodynamic foundation for the conceptual design of a higher temperature coating for SiC composites. The 1500ºC isothermal section, shown in Figure 2, suggests two stable coating architectures. In both cases ytterbium disilicate (Yb2Si2O7, YbDS, Table 1) is required adjacent the SiO2 thermally grown oxide (TGO). A narrow range of cubic YbO1.5-stabilized HfO2 solid solution (fluorite, F) compositions ranging from approximately Yb28Hf72c to Yb38Hf62 may be applied directly to the b

c

The dense layer of silicate(s) serving as a H2O/O2 barrier is referred to as the ‘EBC’ while ‘TBC’ describes the segmented zirconate/hafnate layer intended to mitigate CMAS attack and provide thermal protection. These designations are based on the historic development of the separate coating concepts; functional overlap exists in the present application. Compositions are reported in terms of mole percent of single-cation based oxide formulae: Yb28Hf72 represents a mixture of 28mol% YbO1.5 and 72mol% HfO2.

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YbDS to maintain thermochemical equilibrium at the EBC||TBC interface. Increasing the YbO1.5 concentration in the TBC for improved CMAS resistance requires an ytterbium monosilicate (Yb2SiO5, YbMS) diffusion barrier between the disilicate and either the δ-phase (Yb4Hf3O12, δYbHO) or H3 (Yb6HfO11, H3-YbHO) hafnates. Given the relatively larger, anisotropic CTE mismatch between RE monosilicates and SiC13 compared to the γ-phase RE disilicates14, adding this additional layer complicates the management of thermal stresses. The optimal YbMS interlayer thickness must balance the need for a thin layer to minimize stresses against the benefits of a thicker diffusion barrier to limit outward SiO2 transport. The deposition and performance of such ytterbium-hafnate/silicate T/EBCs has not been reported. This investigation extends the phase equilibrium study by evaluating the thermochemical interaction between coating layers while testing the concept that T/EBC systems could provide resistance to CMAS attack. The work involved two principal activities. Coatings comprised of ytterbium-silicate EBCs and -hafnate TBCs deposited on SiC substrates were annealed to assess the effect of coating architecture on interface durability. These multilayer T/EBCs were also exposed to CMAS melts under isothermal conditions to characterize the crystallization reactions and their potential to arrest the melt infiltration. 2.0

EXPERIMENTAL METHODS

The coatings were deposited on monolithic SiC substrates (Hexaloy-SA, Saint Gobain, Niagara Falls, NY) by electron beam directed vapor deposition (EB-DVD). This approach utilizes a supersonic gas jet to direct and transport a thermally evaporated vapor cloud onto the rotating substrates, as described elsewhere15. The ytterbium silicate EBC layers were deposited using YbO1.5 and SiO2 sources due to vapor pressure differences between the species. This multisource evaporation was enabled by the use of high-speed EB scanning (up to 100kHz) and a small beam spot size (