Upper temperature limit of environmental barrier coatings based on mullite and BSAS

Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al(2)O(3).SiO2) or mullite + SAS ((1 -x)BaO.xSrO.Al2O3.2SiO(2), 0 less than or equal to x less than or equal to 1) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt-infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low-melting (similar to1300degreesC) glass, which can cause the spallation of the EBC. At temperatures greater than similar to1400degreesC BSAS suffers significant recession via volatilization in water-vapor-containing atmospheres. Both reactions can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300degreesC indicates the sintering of EBC.