Excellent CMAS resistance of a newly developed equiatomic high entropy (Dy_1/4Ho_1/4Tm_1/4Yb_1/4)₂Si₂O₇ ceramic pyrosilicate

In this work, novel equiatomic high entropy (Dy_1/4Ho_1/4Tm_1/4Yb_1/4)₂Si₂O₇ or (4RE_1/4)₂Si₂O₇ ceramic pyrosilicate was fabricated through a single solid solution method to use as environmental barrier coating. The SEM analysis of high entropy powders shows the homogenous mixing and XRD proves the formation of single β-phase after milling and sintering. The coefficient of thermal expansion was reported as (2.3–4.8 × 10⁻⁶ K⁻¹) from 400 K⁻¹ to 1723 K⁻¹. The ultra-low thermal diffusivity (0.4 mm² s⁻¹) and thermal conductivity (0.8 W/m°C) were reported at 1500 °C for this novel HE ceramic disilicate. The as fabricated (4RE_1/4)₂Si₂O₇ pyrosilicate shows an excellent CMAS resistant for even up to 48 h and negligible amount of Ca is able to penetrate in the substrate. Rare earth disilicate species with intermediate radii such as Tm³⁺ helps in maintaining phase stability along with passive element Yb³⁺ of smaller radii which also protect the interface from severe CMAS attack. However, the rare earth species with larger radii such as Dy³⁺ and Ho³⁺ actively take part in apatite formation leading to reduced corrosion activity of CMAS melt by changing its composition. This result confirms the application of (4RE_1/4)₂Si₂O₇ as a potential candidate to be used as protecting coating material in harsh combustion environments.