First-principles calculations and thermal-mechanical experimental studies on middle-entropy rare-earth disilicates

Dense environmental barrier coatings (EBCs) are urgently needed to protect SiC-based ceramic matrix composites (SiC CMCs) in future gas turbine engines, and multi-component rare-earth (Re) disilicates are attracting attention due to their excellent thermal-mechanical properties and chemical compatibility. In this paper, first-principles calculations were performed on ten different crystal structures and elastic constants of (Re0.5Yb0.5)2Si2O7 (ReSc, Y, Ho, Er, Tm, Yb, Lu) and (ScxYb1-x)2Si2O7 (x = 0, 0.25, 0.5, 0.75, 1.00) disilicate materials. Meanwhile, five bulk disilicate ceramics of Yb2Si2O7, (Sc0.5Yb0.5)2Si2O7, (Er0.5Yb0.5)2Si2O7, (Sc0.33Er0.33Yb0.34)2Si2O7, and (Sc0.25Ho0.25Er0.25Yb0.25)2Si2O7 were prepared and tested for thermal-mechanical properties. The prepared bulk ceramics exhibit superior thermal stability at 1823 K, with no phase transformation or grain coarsening occurring. Compared to Yb2Si2O7, (Sc0.5Yb0.5)2Si2O7 exhibits a smaller unit cell volume, average Re–O bond length, grain size, and thermal conductivity. It also shows increased ion mass disorder, lattice distortion, ion radius disorder, coefficient of thermal expansion (CTE), microhardness, and Young's modulus. However, (Er0.5Yb0.5)2Si2O7 mainly exhibits opposite behaviors. Most of the thermal-mechanical properties of (Sc0.33Er0.33Yb0.34)2Si2O7 and (Sc0.25Ho0.25Er0.25Yb0.25)2Si2O7 are intermediate between those of (Sc0.5Yb0.5)2Si2O7 and (Er0.5Yb0.5)2Si2O7. These findings of this study may accelerate the development of medium-entropy or high-entropy disilicates with controlled polymorphic phases and customized thermal-mechanical properties.