The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma-sprayed 7 wt% Y2O3-ZrO2 thermal barrier coatings with annealing from 800 degrees to 1400 degrees C. Mullins's thermal grooving theories have been applied to plasma-sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain-boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000 degrees C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100 degrees C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200 degrees C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat-treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.