A multi-component microstructure model is applied in ultrasmall-angle X-ray scattering (USAXS) studies of the processing microstructure property relationships for two groups of plasma-sprayed yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs); one with various controlled processing conditions (temperature, spray velocity, etc.) and the other with three different feedstock morphologies. Microstructural details (anisotropy, porosity, pore size, etc.) are related both to the processing conditions and to the properties (e.g, thermal conductivity and elastic modulus), and presented graphically in a new kind of plot known as a T-V-M diagram , designed to clarify the salient processing microstructure property relationships. It is found that the degree of microstructural anisotropy is pore-size dependent, being stronger for larger pores, and is more sensitive to the feedstock morphology than to the condition of the impacting particles. With respect to the powder feedstock, the microstructural anisotropy increases for coatings sprayed from fused-and-crushed (F&C), to agglomerated-and-sintered (A&S), to hollow-spherical-particle (HOSP) feedstock, when sprayed under nominally identical processing conditions. The USAXS microstructure analysis indicates the presence of two broad size distributions of inter-lamellar pores, which, combined, account for a major portion of the total pore volume (70 % to 80 %). The total porosity is usually found to increase with decreasing particle temperature or velocity, except for one sample. For different feedstock, the highest coating porosity is found for the HOSP coating. Across all of the coatings, a negative linear relationship between thermal conductivity and total porosity is also found for all except one specimen, and the reasons for this exception are discussed. Comparison of the new USAXS analysis is made with earlier small angle neutron scattering results for TBCs, and the implications are considered for a more general application of this metrology in TBC microstructure design.
Citation: Journal of the American Ceramic Society
Pub Type: Journals
feedstock morphology, microstructure characterization, plasma spray, ultrasmall-angle x-ray scattering, yttria-stablized zirconia