Nanostructured zirconia as a strain relief interlayer - A study of microstructure and its evolution in plasma sprayed coatings.

Abstract

Nanostructured zirconia as a strain relief interlayer - a study of microstructure and its evolution Thermal barrier coatings (TBC) for the next generation gas turbines and jet engines need to work at higher temperature gradients and better accommodate strain across their thickness. The research undertaken is an effort to understand whether the morphological characteristics of coatings prepared using nanostructured material could lead to strain accommodation within a TBC.

TBCs of nanostructured partially stabilized zirconia (n-PSZ) were prepared using the Triple Torch Plasma Reactor, a reduced pressure plasma spray deposition system. A shutter mechanism was designed to capture individual splats of the spray particles. Scanning electron microscope (SEM) images of the splats were analyzed to identify morphological characteristics such as circularity and crack density. Samples of proto-coatings, i.e. samples with two to five splats deposited one on top of the other were also prepared. Reference coatings were prepared using a commercial atmospheric pressure plasma spray deposition system. The coatings were subjected to isothermal testing at different temperatures. Cross sections of representative coatings were prepared and imaged using a SEM. Image analysis techniques were used to extract the porosity information of the coatings.

A database of morphological parameters of the splat samples as a function of the plasma spray parameters was compiled. It was confirmed that the defining characteristic of a n-PSZ TBC is the presence of semi-molten feedstock particle. Cross sectional samples of these semi-molten structures were prepared and imaged. The distribution of peak heights of these semi-molten n-PSZ structures in the splat samples were found to be strongly influenced by the plasma spray deposition conditions. The peak heights are, on an average, smaller when the percentage of hydrogen in the plasma gas is increased, and even smaller in the case of a deposition with an increased arc current, indicative of stronger melting of the agglomerated feedstock particles.

Heat treatment of the coating samples indicated that a n-PSZ under-layer within a multilayered TBC had a stabilizing influence on the porosity across the coating. Thus the morphology of a n-PSZ interlayer is conducive for strain accommodation within a multi-layered TBC in plasma sprayed coatings.