Porosity investigation of yttria-stabilized zirconia topcoats using NMR cryoporometry

Axial suspension plasma spraying (ASPS) is a new, innovative plasma spray technique using a feedstock consisting of fine powder particles suspended in a liquid. With ASPS, thermal barrier coating (TBC) topcoats with columnar structures have been produced that are built up by fine powder particles. The microstructure consists of micro-, meso-, and macro-pores. Due to the wide pore size range including nano-porosity it is challenging to measure porosity and pore size distribution in TBC topcoats. However, it is important to characterize the porous structure as it affects the thermal conductivity. Nuclear magnetic resonance (NMR) cryoporometry is a promising method for performing such measurements because of its capability of measuring pores down to nanometer size and providing information about the pore geometry. The aim of this paper is to introduce NMR cryoporometry as a new characterization technique for determining porosity, pore size distribution and pore geometry of TBC topcoats produced by ASPS. The study includes the comparison of two different yttria-stabilized zirconia topcoats and NMR cryoporometry is complemented by microstructural characterization using scanning electron microscopy.