Thermochemical interactions between Ca2Sm8(SiO4)6O2 apatite and molten calcium–magnesium-aluminosilicate (CMAS)

Abstract

Thermochemical interactions between Ca2Sm8(SiO4)6O2 (CSmS) apatite and molten calcium-magnesium-aluminosilicate (CMAS) glass having CaO/SiO2 mole ratio 0.37 have been evaluated. CSmS apatite-CMAS diffusion couples were annealed at 1200, 1300, and 1400 °C for 1–50 h. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive x-ray spectroscopy were used to characterize the phases formed during the reaction. Formation of a distinct cyclosilicate [Ca3Sm2(Si3O9)2] layer was observed at apatite-CMAS interface in couples heat treated at 1200 °C but not at higher temperatures. The cyclosilicate layer nucleated at the apatite-CMAS reaction front, continued to grow into the residual melt and eventually detached from the apatite substrate. Residual CMAS melt became Ca-lean when cyclosilicate was present. Formation of MgCaSi2O6 diopside and dendrites of CaSiO3 wollastonite was also observed within the residual CMAS at 1200 °C. At 1300 and 1400 °C molten CMAS, due to its much lower viscosity, infiltrated the apatite substrate through open pores and along the grain boundaries. Ca2Sm8(SiO4)6O2 apatite thermal/environmental barrier coating has the potential to provide protection against CMAS attack up to about 1200 °C but not at higher temperatures.