1) The study examined the microstructure of plasma sprayed yttria coatings and found large resolidified spheres with porous centers that increased the rate of acid etching. However, these defects did not correlate to variations in dielectric breakdown voltage. 2) X-ray tomography scans correlated with SEM analysis showed the large porous defects were not uniformly deposited across the substrate. 3) Sieving out large particles from the yttria powder reduced their size in the coating but did not remove large hollow particles. Further study of coatings with sieved powder is needed to determine correlations between porosity and electrical properties.

1. Study of Porosity Structure in Plasma Sprayed Yttria Coatings
Adam McCormick, Kari McLaughlin, Ryan Nelson, Tom Statkus
Department of Materials Science & Engineering
Boise State University
[1] Simunovic, K. & Sketch, Thermal Spraying. Welding Engineering and Technology. EOLSS.
[2] Saeed Coatings, http://saeedcoatings.com/?page_id=24
[3] Jiang, X., Matejicek, J. & Sampath, S. Substrate temperature effects on the splat formation,
microstructure development and properties of plasma sprayed coatings. Mater. Sci. Eng. A 272,
189–198 (1999).
Microstructure of Coating
[5]
● Large resolidified spheres with porous centers increased the rate
of acid etching within the sample; however, no correlations could
be made between these defects and dielectric breakdown voltage
of the coating.
● Correlation of SEM analysis with x-ray tomography scans showed
large porous defects were not uniformly deposited across the
substrate due to the configuration of tooling used to manufacture
the coating.
● Large particles were sieved out of the yttria powder prior to
coating. While this did not remove the large hollow particles from
the coating, it reduced their size to 10% of the coating thickness.
● NxEdge Inc. manufactures semiconductor processing equipment
for many large semiconductor companies throughout North
America.
● Plasma sprayed ceramic coatings are applied to dry etch
equipment to protect against the harsh environment encountered
during the etching process.
● Variability has been found in the dielectric breakdown voltage and
acid corrosion quality testing of the plasma sprayed yttria coating.
● To determine root cause of these variabilities, the project studied
how the microstructure of the coating correlated with the variations
in quality testing.
● The microstructure of the formed splats are dependent on the
particles’ velocity in the plume, particle and substrate temperature,
and the chemical state (oxidation) of the particles.
● Acid etch quality testing will be performed on the samples sprayed
with sieved powder to analyze the corrosion mechanisms of the
denser coating.
● Further study of the samples sprayed with sieved powder is
needed to determine if any significant correlations exist between
the porosity and electrical properties of the coatings.
SEM cross section of yttria coating sprayed onto an aluminum substrate before acid etch (left) and after (right).
● The ceramic coating exhibits a laminar structure due to the layering of individual splats during thermal spraying.
This grain structure is exaggerated during an acid etch to show the coating’s microstructure.
● SEM imaging shows multiple large pores which have been introduced during the deposition of the coating.
Jesse Armagost from NxEdge Inc. supported and guided this project as our sponsor.
Research support from Dr. Harold Ackler and Dr. Darryl Butt as part of the Materials Science and
Engineering Department at Boise State University.
Research support was obtained from the Materials Science Undergraduate Laboratory.
Josh Eixenberger from the Biomolecular Research Center at Boise State University supported this
work through the operation of the x-ray tomography scan and image reconstruction.
Dr. Patrick Price from the Materials Science and Engineering Department at Boise State University
supported this work through x-ray tomography scan data analysis and compilation.
The project described was supported by Institutional Development Awards (IDeA) from the National
Institute of General Medical Sciences of the National Institutes of Health under Grants
#P20GM103408 and P20GM109095. We also acknowledge support from The Biomolecular
Research Center at Boise State with funding from the National Science Foundation, Grants #
0619793 and #0923535; the MJ Murdock Charitable Trust; and the Idaho State Board of Education.
● Gas is accelerated through an electrode to produce a plasma.
Ceramic powder is injected into the plasma plume and the molten
particles flatten when they impact the substrate surface.
SEM cross sections of yttria coating after etching the surface with hydrochloric acid for 60 minutes.
● The acid etching showed large particles were solidifying before impacting the substrate. This results in dense
spheres with porous centers embedded in the coating. Some large particles were measured to be as large as 40%
of the coating thickness.
● Small pores were introduced around the large particles as they did not flatten on the surface before solidifying.
The additional porosity in and around these particles increased the rate of acid etching within the coating.
X-ray tomography imaging (left) provided a three dimensional model
of the coating’s density. Low density areas (surface and pores) are
seen in red with increasing density in green, and the highest density
in blue.
● A large sphere with a porous center can be seen on the
surface of the sample.
● The model allowed for analysis of specific points and
showed the porosity structure of the yttria coating.
● Large pores, as seen in the SEM cross sections, are
shown to be consistent throughout the bulk of the
sample.
SEM cross section of yttria coating before (left) and after (right) larger particles
were sieved out of the powder. These samples show significantly reduced
porosity throughout the bulk of the coatings.
Acid Corrosion Analysis
Background & Motivation Results
The morphology of a ceramic splat varies greatly when molten particles impact
on hot (left) and cold (right) substrates.[3]
Thermal energy is used to deposit surfacing materials with a plasma spray gun
(above).[1,2]
X-Ray Tomography Analysis of Porosity
Future Work
Acknowledgements
References