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. Nations that possess such technology would not only experience a massive growth in the commercial sector, but also pose a precise and lethal military threat.

 

These aircraft travel at speeds greater than Mach V. Guiding them towards a target involves using passive and active remote sensing optoelectronics, operating in infrared. 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, and also transparent within the relevant infrared ranges used for guidance.

 

Single-crystal sapphire (alumina) is a infrared window material that has long been researched and used since the 1960s for hypersonic window application. Not however, because it is optimal, but because sapphire is readily available, plentiful, and easy to microstructurally control and manufacture. 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 optical transparency comparable to that of single-crystal sapphire. These results were accompanied by exaggerated grain growth, resulting in lower mechanical strength.

 

This research works on a two-fold approach to minimizing the grain growth of PCA, in efforts to maintain favorable optical characteristics. The first is tailoring sintering conditions, while the second is purifying powder morphology. Optical tests demonstrated favorable results, comparable to control group transparency, while mechanical tests (ASTM 1161a) calculated a high Weibull modulus for PCA. Thermal loading via ablation testing compared PCA samples to industry alternatives (single-crystal sapphire) and (equiaxed alumina).

 

Finally, additional work was done on nanoceramic MgO:Y2O3, in a ceramic-processing method like that of PCA. These findings will also be discussed.