Gadolinium oxide solubility in molten silicate: dissolution mechanism and stability of Ca2Gd8(SiO4)6O2 and Ca3Gd2(Si3O9)2 silicate phases

Improvement of the calcium-magnesium aluminosilicate (CMAS) infiltration mitigation concept in thermal barrier coatings (TBC) requires fundamental data on thermochemical reaction involving rare-earth oxide candidate as ZrO2 alloying element. This study investigates, through a model approach, Gd2O3 dissolution at 1200 °C in a synthetic model CAS melt (64.4SiO2-9.3Al2O3-26.4CaO mol. %) and the stability of the precipitated Gd-apatite Ca2Gd8(SiO4)6O2 and Ca3Gd2(Si3O9)2 cyclosilicate phases. The two Gd-rich silicates have been synthesized by solid-state reaction and then characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The interactions of Gd2O3, Gd-apatite and Gd-cyclosilicate with CAS have been observed by scanning electron microscope (SEM) after various times of contact at 1200 ̊C, giving information about dissolution/precipitation processes. Dissolution kinetics has been evaluated by electron probe microanalysis (EPMA) measurements in the CAS melt. A discussion is finally provided concerning the thermodynamic stability of all phases of the system and confronted with kinetics considerations.