Microstructure of as-coated thermal barrier coatings with varying lifetimes

Abstract

The microstructures of the thermally grown oxide (TGO) in as-coated electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) were examined by scanning transmission electron microscopy (STEM) as a function of demonstrated TBC lifetime in a thermally cycled environment. As-coated TBC specimens were selected for analysis based on individual statistical significance in overall lifetime variation (i.e. early failures to long-lasting). Weibull probability distribution developed through cyclic furnace tests (CFT) served as the medium to determine statistical significance. For selected TBC specimens that represent the variation in lifetime, site-specific specimen preparation for STEM was carried out by focused ion beam (FIB) equipped with in-situ lift-out (INLO) technique. Differences in microstructure of TGO particularly in the mixed-oxide and continuous-oxide zone were examined and correlated to the lifetime characteristics of the TBCs. The TGO for durable TBC consisted of fine grained, uniformly thick, and dense mixed-oxide zone in addition to a uniform and smooth microstructural transition into the continuous oxide zone.