Mechanisms of rare‐earth monosilicate reaction with calcium–magnesium aluminosilicate

Abstract Reactions of calcium–magnesium aluminosilicate (CMAS) with single‐cation rare‐earth monosilicates (REMS) at 1300°C for times up to 24 h were investigated to determine reaction mechanisms. Eight single‐cation REMSs were investigated: La 2 SiO 5 , Nd 2 SiO 5 , Gd 2 SiO 5 , Dy 2 SiO 5 , Y 2 SiO 5 , Er 2 SiO 5 , Yb 2 SiO 5 , and Lu 2 SiO 5 . REMS with the smallest and largest cations, from either end of the lanthanide series, produced denser layers of apatite and underwent less recession, whereas the middle of the rare‐earths experienced much more recession and produced thick, low density apatite product layers. These reaction morphologies are attributed to competition between rates of REMS dissolution and apatite precipitation, which govern the outcome of REMS + CMAS reactions. The relative differences between these rates produce the range of REMS degradation observed when investigated as a function of rare‐earth cation. Three key reaction mechanisms are identified: dissolution‐limited, precipitation‐limited, and RE‐apatite mediated, which themselves depend on the underlying stabilities of a particular RE cation's RE 9.33 ◻ 0.67 (SiO 4 ) 6 O 2 and Ca 2 RE 8 (SiO 4 ) 6 O 2 apatite phases.