Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

Jamesa L. Stokes; Bryan J. Harder; Valerie L. Wiesner; Douglas E. Wolfe

doi:10.1111/jace.16694

Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

Jamesa L. Stokes, Bryan J. Harder, Valerie L. Wiesner, Douglas E. Wolfe

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62 Scopus citations

Abstract

Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si-based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE-disilicates (RE₂Si₂O₇, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium-magnesium-aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE₂Si₂O₇, SiO₂, and a Ca₂₊ _yRE₈₊ _x(SiO₄)₆O₂₊₃ _x _/2+ _y apatite-type silicate. RE₂Si₂O₇ formation was favored in interactions with CMAS having low CaO:SiO₂ ratios. Increased reactivity was observed for higher CaO:SiO₂ ratios in CMAS combined with larger RE³⁺ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO₂ was dependent on both thermodynamic equilibrium at low CaO:SiO₂ ratios and sequestration of silicate modifiers at higher CaO:SiO₂ ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.

Original language	English (US)
Pages (from-to)	622-634
Number of pages	13
Journal	Journal of the American Ceramic Society
Volume	103
Issue number	1
DOIs	https://doi.org/10.1111/jace.16694
State	Published - Jan 1 2020

All Science Journal Classification (ASJC) codes

Ceramics and Composites
Materials Chemistry

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title = "Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials",

abstract = "Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si-based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE-disilicates (RE2Si2O7, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium-magnesium-aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE2Si2O7, SiO2, and a Ca2+ yRE8+ x(SiO4)6O2+3 x /2+ y apatite-type silicate. RE2Si2O7 formation was favored in interactions with CMAS having low CaO:SiO2 ratios. Increased reactivity was observed for higher CaO:SiO2 ratios in CMAS combined with larger RE3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO2 was dependent on both thermodynamic equilibrium at low CaO:SiO2 ratios and sequestration of silicate modifiers at higher CaO:SiO2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.",

author = "Stokes, {Jamesa L.} and Harder, {Bryan J.} and Wiesner, {Valerie L.} and Wolfe, {Douglas E.}",

note = "Publisher Copyright: Published 2019. This article is a U.S. Government work and is in the public domain in the USA.",

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AU - Stokes, Jamesa L.

AU - Harder, Bryan J.

AU - Wiesner, Valerie L.

AU - Wolfe, Douglas E.

N1 - Publisher Copyright: Published 2019. This article is a U.S. Government work and is in the public domain in the USA.

PY - 2020/1/1

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N2 - Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si-based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE-disilicates (RE2Si2O7, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium-magnesium-aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE2Si2O7, SiO2, and a Ca2+ yRE8+ x(SiO4)6O2+3 x /2+ y apatite-type silicate. RE2Si2O7 formation was favored in interactions with CMAS having low CaO:SiO2 ratios. Increased reactivity was observed for higher CaO:SiO2 ratios in CMAS combined with larger RE3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO2 was dependent on both thermodynamic equilibrium at low CaO:SiO2 ratios and sequestration of silicate modifiers at higher CaO:SiO2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.

AB - Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si-based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE-disilicates (RE2Si2O7, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium-magnesium-aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE2Si2O7, SiO2, and a Ca2+ yRE8+ x(SiO4)6O2+3 x /2+ y apatite-type silicate. RE2Si2O7 formation was favored in interactions with CMAS having low CaO:SiO2 ratios. Increased reactivity was observed for higher CaO:SiO2 ratios in CMAS combined with larger RE3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO2 was dependent on both thermodynamic equilibrium at low CaO:SiO2 ratios and sequestration of silicate modifiers at higher CaO:SiO2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.

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Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

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