Optimization of High‐Temperature Environmental Barrier Coatings for 2700°F SiC/SiC Ceramic Matrix Composites

ABSTRACT The upper use temperature of state‐of‐the‐art environmental barrier coatings (EBCs) is limited by the low melting point of the silicon bond coat (BC) (1414°C). The success of next‐generation ceramic matrix composites (CMCs), targeting a 1482°C use temperature, is contingent upon the development of a new generation of EBCs with a higher temperature BC. NASA recently developed a mullite‐based BC via a slurry process to replace the silicon BC. The NASA high temperature EBC (mullite/hafnon/Yb 2 Si 2 O 7 ) exhibited about 500 h of life at 1427°C in furnace steam cycling (FSC), which is below the 1000 h life goal at 1482°C. A study was undertaken to optimize the high temperature EBC to improve its life in FSC and evaluate its performance in rigs that simulate the gas turbine environment. Two layer mullite/hafnon and the topcoat (TC) were first optimized separately, before being combined as one EBC. The main variable for the mullite/hafnon optimization was the amounts of sintering aids. Sc 2 Si 2 O 7 and HfO 2 were the two TC candidates considered to replace the Yb 2 Si 2 O 7 TC. Key factors that influenced the TC performance were CTE match and chemical compatibility with the underlying layers. This article discusses the oxidation kinetics and phase/microstructural evolution of the optimized EBC in FSC, a natural gas burner rig (NGBR), and a combustion rig (CR).