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

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<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 100 cycles, 2 h/cycles at 1300 degrees C in water vapor atmosphere). The elastic modulus values of the as-sprayed BSAS top coat (similar to 75 +/- 6 GPa as determined via indentation) did not exhibit major changes after thermal exposure (similar to 78 +/- 8 GPa). In addition, the BSAS layer exhibited crack healing at high temperatures, the density of cracks decreasing from 15 cracks/cm in the as-sprayed state to 2 cracks/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. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.