Microstructure and phase stability of EB-PVD alumina and alumina/zirconia for thermal barrier coating applications

This paper describes recent progress on research aimed at improving the performance of EB-PVD thermal barrier coatings (TBCs) for gas turbine applications by incorporating alumina as an oxygen diffusion barrier between the bond coat and zirconia. Two approaches are being investigated, either a single, discrete alumina layer, or a graded alumina/zirconia layer. This paper reports on preliminary experiments in which alumina was evaporated under different conditions, and the phase content and morphology of the coatings were characterized. Deposition rate and chamber pressure had a significant effect on the microstructure of the coatings; however, phase formation was essentially unaffected. In agreement with previously published results, α-Al2O3 was produced either by in situ deposition on substrates heated to temperatures near 1000 C, or by heat treatment of metastable as-deposited alumina coatings at temperatures above 1100 C. By continuously changing the vaporization ratio of alumina and zirconia as a function of deposition time, TBCs exhibiting composition gradients through the film thickness were produced. In related work, suitable source materials for alumina deposition were produced using a new sinterless powder processing route involving bimodal powder mixtures. (orig.)