|
|||||
|
|
Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials |
|
| Authors: | Jamesa L Stokes Bryan J Harder Valerie L Wiesner Douglas E Wolfe |
| Affiliation: | 1. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA;2. NASA Glenn Research Center, Cleveland, OH, USA;3. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA
The Applied Research Laboratory, The Pennsylvania State University, University Park, PA, USA Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA, USA |
| 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+3x/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. |
| Keywords: | CMAS environmental barrier coatings (EBCs) glass rare earth disilicate |