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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8 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 = "Funding Information: The authors thank Dr. Richard Rogers for XRD assistance and Valerie Muldoon for sample preparation and fabrication. This work was supported under the NASA Aeronautics Scholarship and Advanced STEM Training and Research (AS&ASTAR) Graduate Fellowship and the NASA Pathways Intern Employment Program (IEP). 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 - Funding Information: The authors thank Dr. Richard Rogers for XRD assistance and Valerie Muldoon for sample preparation and fabrication. This work was supported under the NASA Aeronautics Scholarship and Advanced STEM Training and Research (AS&ASTAR) Graduate Fellowship and the NASA Pathways Intern Employment Program (IEP). 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

Y1 - 2020/1/1

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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