Stability and CMAS Resistance of Ytterbium‐Silicate/Hafnate EBCs/TBC for SiC Composites

Abstract

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 Yb 2 Si 2 O 7 and Yb 4 Hf 3 O 12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb 2 SiO 5 in contact with Yb 4 Hf 3 O 12 do not experience similar degradation. The outer Yb 4 Hf 3 O 12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine-type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.

Effects of cation substitution and temperature on the interaction between thermal barrier oxides and molten CMAS

“…Within the surface reaction layer faceted apatite and globular fluorite grains, all of order 1 m, are embedded into much larger garnet grains suggesting that the garnet grows more slowly, engulfing the finer grains. To confirm this hypothesis, previously suggested for both Yb-and Y-based coatings, 13 Fig. 5(a) and (b), are comparable to the size of the apatite and fluorite grains.…”

“…Based on comparison of the penetration depth they concluded that both YZO and YbZO are more resistant to infiltration than GdZO, suggesting that coatings using the smaller RE cations might offer better performance. 16 Results by the current authors, meanwhile, showed that YbO 1.5 -based coatings are only moderately resistant to CMAS infiltration at 1300 • C. 13 In this case it was hypothesized that the ineffective CMAS mitigation observed with YbHO is related to a high Yb concentration in the re-precipitated fluorite and the apparently sluggish crystallization of a garnet silicate in addition to apatite. Testing the same YbHO coatings at 1500 • C resulted primarily in the growth of an aluminosilicate in the cuspidine family rather than apatite.…”

“…The CMAS composition 33CaO-9MgO-13AlO 1.5 -45SiO 2 with a melting temperature of ∼1235 • C was used for consistency with earlier studies. 7,13,18 Thin disks of crystalline CMAS were prepared using the method of Grant et al 19 to achieve an average loading of 15 ± 2 mg/cm 2 over the covered area. The CMAS wafers were placed in the center of the specimen and the assemblies were annealed for 4 h at either 1300 • C or 1500 • C; the samples were heated and cooled at 10 • C/min unless otherwise noted.…”

“…Applying this analysis to the compositions studied herein, the most potent thermal barrier oxides would ideally have the lowest ratio of RE:(M + Ca) for fluorite while maximizing Ca:(RE + M) 13 YZO, 24 and GdZO 7 (all at 1300 • C), SmZO at 1250 • C, 26 and NdZO + C 24 A 17 S 59 at 1200 • C. 25 Dotted lines in (a) represent the nominal ␦/pyrochlore compositions while those in (b) indicate the he defect-free Ca 2 RE 2 (SiO 4 ) 6 O 2 apatite composition and the theoretical limits of Ca solubility based on crystal chemistry. in the apatite.…”

“…9,10 Coatings based on the related ␦-phase (RE 4 (Zr/Hf) 3 O 12 ), formed by the smaller REs, have received attention as part of multilayer, CMAS resistant thermal/environmental barrier coatings (T/EBC) for SiC-based ceramic matrix composites. [11][12][13] The latter system takes advantage of the phase stability 14 and low CTE 15 of (Lu/Yb) 2 Si 2 O 7 for the EBC application and the trend toward lower CTE for smaller RE cations and hafnate phases, summarized elsewhere, 12 to minimize the CTE mismatch with the SiC substrate.…”