Reactions of molten silicate deposits with yttrium monosilicate

Abstract

The article addresses effects of silicate deposit composition on reactions with yttrium monosilicate (YMS), a candidate environmental barrier coating for aero-engine components. Computed phase equilibria are used to predict the nature and relative proportions of reaction products and the extent of YMS consumption upon reaction with twelve deposits of varying composition at 1300°C. These predictions are compared with results of a corresponding experimental study on three exemplary deposits.Although the nature and sequence of reaction products formed (typically apatite and yttrium disilicate) depend on the Ca:Si ratio of the deposit, the degree of consumption of YMS at equilibrium is relatively insensitive to deposit composition and is predicted to proceed to a greater extent than that in yttrium disilicate. However, sluggish reaction kinetics associated with the formation of a thin apatite layer above the YMS prevents reactions from reaching their terminal equilibrium states within the experimental times investigated (250 hours). For deposit loadings of 18 mg/cm 2 (corresponding to a thickness of about 100 µm), the degree of consumption following 250 hours exposures is only about 10%-40% of the predicted terminal values, depending on deposit composition. K E Y W O R D Schemical reactions, CMAS, Environmental barrier coating, yttrium silicate 2920 | SUMMERS Et al.

Resistance of pure and mixed rare earth silicates against calcium‐magnesium‐aluminosilicate (CMAS): A comparative study

“…30 With respect to recession, it can be assumed that the dense fringe acts as a protective layer against CMAS infiltration or Ca 2+ diffusion. This assumption is supported by the results of Summers et al 37 They observed a dense fringe at the interface of the reaction zone and remaining YbMS as well as reduced recession of the MS, which was only 50% of the amount they predicted.…”

“…Both reported that with increasing Ca:Si ratio, the formation of apatite Ca 2 RE 8 (SiO 4 ) 6 O 2 phases as the primary reaction product is promoted. In addition, the recession became more pronounced as more RE silicate material was necessary to stop the CMAS attack 35,37 . Furthermore, Stokes reported that at high Ca:Si ratios above 0.72 only apatite is formed and that for lower ratios the silicates of smaller RE cations, such as Yb, have a tendency to form additional metastable reaction products 35 …”

“…In recent studies, Stokes et al 35 and Summers et al 36,37 noted the high impact of the Ca:Si ratio in the CMAS compositions on the resulting reaction product. Both reported that with increasing Ca:Si ratio, the formation of apatite Ca 2 RE 8 (SiO 4 ) 6 O 2 phases as the primary reaction product is promoted.…”

“…In addition, the recession became more pronounced as more RE silicate material was necessary to stop the CMAS attack. 35,37 Furthermore, Stokes reported that at high Ca:Si ratios above 0.72 only apatite is formed and that for lower ratios the silicates of smaller RE cations, such as Yb, have a tendency to form additional metastable reaction products. 35 Comparing these studies, the factors dominating the reaction rates remain unclear and a ranking with respect to stability concerning CMAS-induced recession cannot be established.…”

Corrosion behavior of calcium–magnesium–aluminosilicate (CMAS) on sintered Gd2SiO5 for environmental barrier coatings

“…Therefore, rare earth (RE)-doped silicates, which have very good oxidation resistance and can withstand high temperatures, are attracting attention for this kind of application. [22][23][24][25][26]…”