Calcium-magnesium-alumina-silicate (CMAS) resistance characteristics of LnPO4 (Ln = Nd, Sm, Gd) thermal barrier oxides

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO4 (Ln = Nd, Sm, Gd) compounds to CMAS attack at 1250 °C. Due to the chemical reaction between molten CMAS and LnPO4, a dense, crack-free reaction layer, mainly composed of Ca3Ln7(PO4)(SiO4)5O2 apatite, CaAl2Si2O8 and MgAl2O4, was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO4 (Ln = Nd, Sm, Gd) decreased in the sequence of NdPO4, SmPO4 and GdPO4. GdPO4 had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO4.