Performance of thermally sprayed Si/mullite/BSAS environmental barrier coatings exposed to thermal cycling in water vapor environment

Abstract

The ongoing development of environmental barrier coatings (EBCs) offers the prospect to implement the full potential of silicon-based ceramic matrix composites (CMCs) for high temperature structural applications, more specifically the hot zones of gas turbine engines. The current state-of-the-art EBC system comprises a Si bond coat, a mullite (Al6Si2O13) interlayer and a barium–strontium aluminosilicate (BSAS) (Ba1−xSrxAl2Si2O8; 0<x<1) crack-resistant and water vapor attack resistant top coat. In this study, fully crystalline air plasma sprayed Si/mullite/BSAS-celsian EBCs were engineered under controlled conditions on SiC substrates. The influence of water vapor corrosion on the structural and mechanical properties of a Si/Mullite/BSAS EBC architecture was assessed by furnace thermal cycle testing (i.e.; 50 and 100cycles, 2h/cycles at 1300°C in water vapor atmosphere). The elastic modulus values of the as-sprayed BSAS top coat (~75±6GPa as determined via indentation) did not exhibit major changes after thermal exposure (~78±8GPa). In addition, the BSAS layer exhibited crack healing at high temperatures, the density of cracks decreasing from 15cracks/cm in the as-sprayed state to 2cracks/cm after thermal cycling. These characteristics of the BSAS top coat were related to its glass-ceramic nature, the phase/chemical stabilities of the BSAS-celsian at high temperatures and the engineered deposition conditions at which it was deposited. The overall performance at high-temperature of this functionally graded EBC architecture is discussed and correlated to its microstructural characteristics.