Sharp leading edges
required for hypersonic vehicles improve the maneuverability as well as reduce
aerodynamic drag. However, due to the sharp design, increased surface
temperatures require materials that can withstand these extreme conditions.
Ultra-high temperature ceramics are a material group being considered for the
leading-edge material, specifically ZrB<sub>2</sub>/SiC (ZBS) which has a high
thermal shock resistance, melting temperature, and thermal conductivity.
Studies done by Tan et. al. has shown that adding samarium (Sm) as a dopant to
ZBS has an emittance of 0.9 at 1600<sup>o</sup>C and develop oxide scales that
have excellent ablation performance. However, it remained unknown how the Sm
doped oxide scale formed as well as how the emittance and ablation performance
are affected by the microstructure. This study investigates the oxide scale
development of 3 mol% doped Sm-ZBS billets as well as how differences in
microstructure affect the emittance and ablation performance. Samples were
prepared via chemical infiltration of samarium nitrate into spray-dried powders
of 80 vol.% ZrB<sub>2</sub>/20 vol.% SiC; powders were then pressed into billets
and pressureless sintered. Samples cut and polished from these billets were
then oxidized for 10, 60, or 300 s, respectively, using an oxyacetylene torch.
X-ray diffraction was used to determine the sequence of oxidation of Sm-ZBS,
beginning with the formation of ZrO<sub>2</sub> and Sm<sub>2</sub>O<sub>3</sub>.
The final oxide scale was determined to be c<sub>1</sub>-Sm<sub>0.2</sub>Zr<sub>0.8</sub>O<sub>1.9</sub>,
with a melting temperature exceeding 2500<sup>o</sup>C. SEM and EDS were also
used to investigate the microstructural formation that occurs from the bursting
of convection cells. Samples with different microstructures revealed similar
topographical microstructures post-ablation due to the sequence of the oxide
formation. However, samples with rougher surfaces and higher porosities had a
higher concentration of trapped glass in the cross-sectional oxide scale. It
was also found that due to differences in heating the sample during emittance
testing compared to ablation testing, the oxide developed was identical for all
the samples. It was also found that variances in microstructure had no effect
on the spectral emittance of Sm-ZBS at ultra-high temperatures. The fabrication
of c<sub>1</sub>-Sm<sub>0.2</sub>Zr<sub>0.8</sub>O<sub>1.9</sub> (SZO) as a
bulk billet was also investigated to use as a thermal barrier coating (TBC) in
replacement of Sm-ZBS.
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8029220 |
| Date | 14 May 2019 |
| Creators | Anneliese E Brenner (6623978) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/MICROSTRUCTURAL_INVESTIGATIONS_OF_SAMARIUM-DOPED_ZIRCONIUM_DIBORIDE_FOR_HYPERSONIC_APPLICATIONS/8029220 |
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