Development and Evaluation of Transparent, Aligned Polycrystalline Alumina as an Infrared Window Candidate for Hypersonic Flight

Hypersonic flight is the key to unlocking a nation’s strategic advantage in this century’s military theater. Military powerhouses such as the United States, Russia, India, China, Australia, and the EU publicly possess hypersonic weapons capabilities. Such technology enables intercontinental travel orders of magnitude faster than conventional flights. A trip halfway across the world would take not twenty hours, but two. However, the level of thermal and chemical load the aircraft and these electronic equipment experience while at such high speeds cause them to fail. Thus, ceramic window materials are used to act as a barrier between the hypersonic flight environment and this sensitive electronic equipment. Such materials need to be both mechanically robust, but transparent within the relevant infrared ranges used for target detection. Single-crystal sapphire (alumina) is an infrared window material readily available, plentiful, and easy to microstructurally control and manufacture, but not optimal. Its transparency range is limited to the optical and near-infrared, while it exhibits poor mechanical and dielectric strength. Polycrystalline alumina (PCA) has recently been shown to possess more favorable infrared window characteristics as opposed to its single-crystal counterpart. This is achieved by processing using a platelet powder morphology in a single processing step – hot-pressing. Full densification (> 99.5%) of PCA samples was achieved, demonstrating maximum of 84% optical transparency, but accompanied by grain growth (60+µm), resulting in lower mechanical strength. This research thus works on a two-fold approach to minimizing the grain growth of PCA. Optical tests demonstrated favorable results for lowering isothermal temperatures to reduce grain growth. Weibull values of m = 28.8 and m = 9.7 from 4 point-flexure tests were obtained (ASTM 1161a). Thermal loading via ablation testing compared PCA samples to industry alternatives (single-crystal sapphire) and (equiaxed alumina). Ablation tests revealed the benefit of polycrystalline alumina over sapphire. The benefit of lower isothermal sintering temperatures for reduced grain growth resulted in higher peak load before failure, resulting in greater characteristic strength and minimal transmission lost during a minute of oxyacetylene heat flux exposure. Finally, additional work was done on nanoceramic MgO-Y₂O₃, in a ceramic-processing method like that of PCA. These findings will also be discussed.