Monitoring the Phase Evolution of Yttria Stabilized Zirconia in Thermal Barrier Coatings Using the Rietveld Method

Plasma sprayed thermal barrier coating composed of tetragonal Yttria Stabilized Zirconia (YSZ) have a limited lifetime and after a certain operating time they fail, usually by spallation. One of the proposed failure mechanisms is the transformation of metastable tetragonal YSZ phase into its monoclinic polymorph, which can lead to destabilization of coating structure and increase of thermal conductivity. Study of the monoclinic content in YSZ is usually performed by X-Ray diffraction using evaluation of the intensities of a few diffraction peaks for each YSZ phase. But this method is missing some important information that can be further extracted from the X-Ray diffraction pattern using Rietveld method. Using the Rietveld method, one can estimate phase content more accurately, take into account some phases present at level even below 1%, gain information on the grain size and strains. By applying Rietveld method, we can observe that during the first stage of the coating ageing process (a) small grains of cubic YSZ are crystallizing and (b) yttria content within the tetragonal phase is lowering. The tetragonal phase can be described as a mixture of two tetragonal phases; one (t') with typical for YSZ c/a ratio and unchanged yttria content and second one (t) with an increased c/a ratio, corresponding to a lower yttria content. These observations are used for modeling the decomposition of the tetragonal YSZ, considering the segregation of yttria at YSZ phase boundaries, leading to the formation of YSZ domains with the cubic structure.