<p dir="ltr">During atmospheric re-entry, the hypersonic leading edges can experience enormous heat fluxes, with surface temperatures greater than 1600℃ expected. While carbon/carbon (C/C) is a candidate material for leading edge structures, it is prone to oxidation and ablation damage above 500℃. Ablation-resistant coatings can protect the C/C, while emissivity can be engineered to lower the leading-edge surface temperature via radiative cooling. In this dissertation, a novel bilayer coating system and a multilayer coating system based on individual layers consisting of ultra-high temperature ceramics (borides, carbides), refractory oxides (zirconia), and rare-earth oxide as emissivity modifiers were applied to a C/C surface via pack cementation and plasma spray. Ablation tests were performed to evaluate the efficacy of the multilayer coatings in simulated high heat flux environments. <a href="" target="_blank">The spectral emittance of the rare-earth modified topcoat ZrO<sub>2</sub> was measured at high temperatures up to 1200</a>℃ using a benchtop emissometer. ZrO<sub>2</sub> stabilized with 6 mol% Sm<sub>2</sub>O<sub>3</sub> demonstrated a maximum spectral emissivity of 0.99 at λ = 12.5 µm proving its effectiveness in cooling the leading edge surface through enhanced thermal radiation.</p><p dir="ltr"><a href="" target="_blank">The bilayer coating system comprised of Sm<sub>2</sub>O<sub>3</sub>-stabilized ZrO<sub>2</sub> topcoat layer and SiC intermediate sublayer on C/C. </a><a href="" target="_blank">This coating significantly improved the ablation resistance of C/C by reducing the mass ablation rate by ~71%. Despite a significant thermal expansion coefficient mismatch between the substrate and the coating, a well-defined mechanical adhesion characterized by the anchors was observed in pre- and post-ablated coating microstructures, indicating their influence on improving ablation resistance.</a></p><p dir="ltr"><a href="" target="_blank">The multilayer coating architecture consisted of SiC, ZrB<sub>2</sub>-SiC, ZrC-ZrO<sub>2</sub> sublayers and a Sm<sub>2</sub>O<sub>3</sub>-ZrO<sub>2</sub> topcoat. The as-sprayed coating microstructure demonstrated well-defined adhesion between the layers and the substrate without forming major voids or cracks. The multilayer coating with optimized</a> sublayer thickness demonstrated excellent ablation and mass erosion resistance as they reduced the mass ablation rate of C/C by ~90% after being subjected to an aggressive oxyacetylene torch heating for 60 s. During testing, the Sm<sub>2</sub>O<sub>3</sub>-stabilized ZrO<sub>2</sub> topcoat acted as oxygen and thermal barrier, protecting the underlying sublayers from oxidation-induced damage while maintaining a constant surface temperature of ~2100 ℃. Additionally, the high spectral emittance of topcoat material contributed to efficient outward heat transfer via thermal radiation from the external surface while maintaining a constant temperature.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/24353518 |
| Date | 18 October 2023 |
| Creators | Abdullah Al Saad (17201221) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/_b_Design_and_Evaluation_of_High_Emissivity_Coatings_for_Carbon_Carbon_Composites_b_/24353518 |
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