Excellent CMAS resistance of a newly developed equiatomic high entropy (Dy1/4Ho1/4Tm1/4Yb1/4)2Si2O7 ceramic pyrosilicate

In this work, novel equiatomic high entropy (Dy 1/4 Ho 1/4 Tm 1/4 Yb 1/4 ) 2 Si 2 O 7 or (4RE 1/4 ) 2 Si 2 O 7 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 mm 2  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 ) 2 Si 2 O 7 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 Tm 3+ helps in maintaining phase stability along with passive element Yb 3+ of smaller radii which also protect the interface from severe CMAS attack. However, the rare earth species with larger radii such as Dy 3+ and Ho 3+ 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 ) 2 Si 2 O 7 as a potential candidate to be used as protecting coating material in harsh combustion environments.