Electrophoretically deposited alumina as protective overlay for thermal barrier coatings against CMAS degradation

TBCs are increasingly susceptible to degradation by airborne CMAS deposits. In order to mitigate the CMAS attack, we fabricated a dense, crack-free alumina overlay for TBCs by electrophoretic deposition (EPD) technique. Overlay coatings of controlled thickness were successfully fabricated for YSZ TBCs, by EPD followed by controlled sintering at 1200 degrees C. YSZ with alumina overlay coatings were tested for CMAS attack at 1300 degrees C. Detailed examination of microstructural changes and phase evolution in CMAS tested specimens was performed by X-ray diffraction and electron microscopy. Dense alumina overlay produced by EPD was found to physically suppress the infiltration of CMAS. Furthermore, CMAS was found to crystallize into anorthite (CaAl(2)Si(2)O(8)) and MgAl(2)O(4) spinel by chemically interacting with EPID alpha-Al(2)O(3). A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite platelets (CaAl(2)Si(2)O(8)) and localized enrichment of Mg promoted the formation of MgAl(2)O(4) spinel. EPD alumina overlay on commercial-production TBCs retained its adhesion and structural integrity after 20cycles of 1-hour furnace thermal cycle test at 1100 degrees C. (C) 2009 Elsevier B.V. All rights reserved.