Community Research and Development Information Service - CORDIS

Surface diffusion ZrO2-(Y,Gd)O1.5

This result comprises a technique for the study of the effects of composition on surface diffusion kinetics in thermal barrier oxides. The technique is an adaptation of Mullins' grain boundary grooving method to study surface diffusion in polycrystalline materials, with the novel feature that the materials are dense, thick (≥200µm) textured columnar oxide coatings produced by physical vapour deposition. The conditions are similar to those used in the deposition of thermal barrier coatings, but without rotation of the substrate to avoid segmentation of the columnar grain structure.

The coatings are deposited on a substrate of similar composition to minimize thermal expansion mismatch stresses that may bias the kinetics of the process. The coatings are carefully polished parallel to the substrate plane, whereupon all boundaries are essentially normal to the surface, decreasing the uncertainty in the measurement. In principle, each boundary can be fully characterized by orientation imaging microscopy of the surrounding grains and texture analysis of the coating, enabling the correlation of orientation to the kinetics of grooving. Characterization of the grooves after heat treatment is readily accomplished by atomic force microscopy (AFM). The technique was demonstrated on the standard yttria stabilized zirconia used for thermal barrier coatings, as well as an alternate coating based on gadolinium zirconate. Tests on co-doped compositions are in progress, but preliminary results already exist for non-textured polycrystalline compacts of zirconia co-doped with both Y and Gd. The results will be published in a forthcoming publication, currently under preparation.

The primary advantage of this technique is that it enables a direct and clean comparison of the effects of composition on micro-structural changes controlled by surface diffusion. This avoids some of the uncertainty of studying morphological changes in actual thermal barrier coatings, which are porous and have differences in porosity content and distribution in the as-deposited condition. Results so far reveal that the addition of Gd to YSZ slows down the surface diffusion kinetics, and that Gd zirconate has a much slower morphological evolution than the standard YSZ, which bodes well for its potential as a microstructurally stable thermal barrier material.

Reported by

University of California, Santa Barbara
1361D Engineering II, Materials Department
93106-5050 Santa Barbara, California
United States
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