Verlag des Forschungszentrums Jülich
JUEL-4205
Siegert, Roberto
A Novel Process for the Liquid Feedstock Plasma Spray of Ceramic Coatings with Nanostructural Features
190 S., 2006
This study is focused in the process optimization and implementation of the liquid feedstock
plasma spray process conducted at atmospheric pressure, in which the feedstock is injected
into a free expanding plasma jet generated by industrial-type DC torches, in the form of a
suspension or solution-precursor. In here, a liquid carrier is used to transport feedstock
particles with sizes falling into the submicrometric and nanometric range, into the useful
region of the plasma jets. This condition brings about a series of important changes in the
microstructure of the deposited coatings, highlighted by a reduction in coating-building splats
of up to 3 orders magnitude as compare to those observed in standard plasma spray
process, and by a fine porosity with pore sizes in the nanometer range.
A universal apparatus for the precise dosage of the liquid feedstock was developed
considering the geometrical constraints of the different torches under consideration.
Moreover, various injection port designs, mainly grouped as atomizers or mechanical
injectors, were analyzed. A series of experiments, mostly based on the characterization of
the sprayed feedstock and resulting coatings, were design to isolate the most relevant
process parameters.
The results from the splats analysis were used to spatially resolve the trajectories of the
feedstock particles resulting from the interaction of the liquid carrier with the flow of the
plasma jet. SEM analysis of the splats and sprayed bulk material morphologies collected at
different standoff distances, was used to study the process of agglomeration and
consolidation processes that follow the fragmentation and evaporation of the feedstock
droplets. Image analysis tools were used to quantify and grouped into size classes the
detected splats. These results were to establish the correlations between feedstock
properties and the particle size distributions of the sprayed feedstock, which were positively
correlated to spray and process efficiencies, which lay bellow 25% and are relatively low
when comparing to the efficiency observed in conventional plasma spray. A model that
describes the fragmentation of the liquid feedstock and its evaporation was formulated. The
droplets size and time scales are proven to be in good agreement with the experimental
results.
Furthermore, the properties of the various coatings sprayed with the developed apparatus
were correlated to results from the splat analysis. The study of the cross-section and fracture
surfaces of the coatings served to establish the links between the coating microstructural
features and interaction of the liquid feedstock with the plasma jet, providing useful
information for the validation of the theoretical model..Abstract II
Macroscopic and microstructural properties of the sprayed coatings are in good agreement
with the results previously reported in literature, which are characterized by a columnar
growth between multi-lamellar structures reach in segmentation cracks and with density
values of about 4 to 4.3 g·cm-3. The resulting coatings are mainly porous, with porosity
values in excess of 30%. Moreover, various areas of implementation in which the new
microstructural features inherent to the LFPS process, have being explored. These areas
were mainly the spraying of thermal barrier coatings and the deposition of the various
functional layers of the solid-oxide fuel-cell systems, with an emphasis in the development of
gas-tight electrolyte layer based on yttrium-stabilized zirconia.
A process for the deposit of dense gas-tight layers with an average thickness of 50 μm
resulted from the parametrical study, which resulted in a patent application. This method had
the peculiarity of producing a bistratified layer system, in which the upper region of the
coating remelts due to the high thermal flux between the plasma jet and the substrate. In
here, the formation of the bistratified layer system during the uninterrupted spray process, is
inherent to the set of torch and feedstock parameters.
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