No abstract

The calcium rare‐earth (RE) silicate oxyapatite, Ca2RE8(SiO4)6O2 (RE = Yb, Er, Y, Dy, Nd, Gd, and Sm), powders were synthesized by the solid‐state reaction method and characterized by X‐ray diffraction (XRD), Raman spectroscopy, and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high‐temperature oxide melt calorimetry in molten 2PbO–B2O3 solvent at 805°C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the REs increases, which indicates increasing energetic stability in this order.

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.