Stability and CMAS Resistance of Ytterbium‐Silicate/Hafnate EBCs/TBC for SiC Composites

Abstract

Multilayer ytterbium‐hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC ‐based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits ( CMAS ). The study revealed that reactions occurring at the interface between Yb 2 Si 2 O 7 and Yb 4 Hf 3 O 12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb 2 SiO 5 in contact with Yb 4 Hf 3 O 12 do not experience similar degradation. The outer Yb 4 Hf 3 O 12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine‐type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.