"Fabrication and Performance of Silicon Nitride for Radar Frequency Windows" 

Matthew Thompson, MSE PhD Candidate 

Advisor: Professor Rodney Trice

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ABSTRACT

Silicon nitride (Si3N4) is a promising candidate for ceramic radar frequency (RF) windows with high strength, fracture toughness, and thermal shock resistance caused by the interlocked, self-reinforcing microstructure of the high temperature polymorph, β-Si3N4. Introducing porosity during slip casting has been demonstrated as one strategy to lower the dielectric constant and improve the transmission of RF signals through a material. However, there is limited data available on the performance of Si3N4 components under the high temperature, high heat flux conditions relevant to the harsh aerothermal operating environment of hypersonic flight.

In the first study, the porosity of β-Si3N4 was tailored by varying sintering aid composition during processing. β-Si3N4 samples were subjected to oxyacetylene torch testing to evaluate changes in surface temperature, emissivity, surface roughness, phase composition, and microstructural evolution of the heat-affected zone.

The low temperature polymorph, α-Si3N4, has a lower dielectric constant than β-Si3N4. Thus, a second study developed a processing map to control the phase composition of α-Si3N4 and β-Si3N4 to maintain high flexural strength and elastic modulus while improving dielectric properties. Relationships between phase composition, microstructure, and high temperature responses were identified. A third study applied microstructural and processing strategies to further investigate high temperature performance.

Overall, this dissertation addresses important gaps in understanding to manufacture ceramic components capable of meeting the demand for high-performance RF window structures at high temperature.