Mechanical properties and fracture behavior of interfacial alumina scales on plasma-sprayed thermal barrier coatings

The mechanical properties and fracture behavior of Y-doped Al2O3 scales wer e investigated by furnace thermal cycling (to 1,150 degrees C) of plasma-sp rayed thermal barrier coatings (TBCs) with vacuum plasma-sprayed (VPS) or a ir plasma-sprayed (APS) Ni-22Cr-10Al-1Y bond coatings. No significant alter ations in Al2O3 hardness or Young's modulus (as measured by mechanical prop erties microprobe) were detected as a function of bond coat type, exposure time, or number of thermal cycles. The interfacial Al2O3 scales on VPS NiCr AlY exhibited progressive increases in localized fracture, buckling, and de lamination during thermal cycling. The concentration of arrayed lenticular voids in the columnar Al2O3 grain boundaries significantly increased during cyclic oxidation (as compared to isothermal oxidation), but only in scales which formed on convex surfaces, suggesting internal void growth was stres s-related. The amount and frequency of scare damage was higher on convex su rfaces with a relatively lame radius of curvature as compared to convex sur faces with a very small radius of curvature. Although the thermo-mechanical fracture resistance of Al2O3 scales on APS NiCrAlY was superior to scales on VPS NiCrAlY, TBC lifetimes on VPS NiCrAlY were greater by a factor of 2. Apparently, severe interfacial scale damage did not rapidly degrade the ad herence of the ceramic top coatings.