As-deposited mixed zone in thermally grown oxide beneath a thermal barriercoating

Gas turbine designers are increasingly using electron-beam physical vapor d eposited (EB-PVD) thermal barrier coatings (TBC) to meet the challenge of h igher efficiency gas turbine engine requirements. A key feature for expandi ng the use of TBCs is increased spallation life and reduced spallation life variability. Such a coating system comprises a substrate (Ni-based single crystal alloy), a bond coat (diffusion aluminide or MCrAlY), a ceramic (7 w t.% yttria stabilized zirconia), and a thin thermally grown oxide (TGO) bet ween the bond coat and the ceramic. The TGO is intended to be alpha -alumin a. but evidence reported by other researchers suggests that in some cases t he alpha -deposited TGO may not be entirely ot-alumina. The thin nature of the TGO in as-deposited TBCs (< 0.5 mum) makes analysis of the phases prese nt and morphology difficult. Advancements in transmission electron microsco py (TEM) sample preparation and photo-stimulated luminescence spectroscopy (PSLS) have allowed higher quality and easier characterization of the TGO. In this study, EB-PVD TBCs were applied to platinum-aluminide bond coats on a Ni-based superalloy. Three types of coatings were produced by changing o ne PVD process variable. The as-processed TGO layer was characterized utili zing scanning transmission electron microscopy (STEM) and PSLS for each of the three coating process variables used. Results of this work found that t he TGO comprised two sublayers; (1) a continuous layer of gamma -Al2O3 betw een the mixed oxide zone and the bond coat; and (2) a mixed oxide zone betw een the continuous gamma -Al2O3 and the TBC layer. An explanation for the c reation of the mixed oxide zone found in these TGO morphologies is presente d. (C) 2001 Elsevier Science B.V. All rights reserved.