Transparent Ceramics for Armor And EM Window Applications

Recently, the U.S. Army Research Laboratory (ARL) has focused increased attention on the development of transparent armor material systems for a variety of applications. Future combat and non-combat environments will require lightweight, threat adjustable, multifunctional, and affordable armor. Current glass/polycarbonate technologies are not expected to meet the increased requirements. Results over the past few years indicate that the use of transparent crystalline ceramics greatly improve the performance of a system. These results coupled with recent processing and manufacturing advances have revitalized the interest in using transparent ceramics for armor systems. The materials currently under investigation at ARL are magnesium aluminate spinel (MgAl2O4), aluminum oxynitride spinel (AlON), single crystal sapphire (Al2O3), glasses, and glass-ceramics. The polymers under investigation are polycarbonate (PC) and polyurethane (PU). An overview of current ARL efforts in these areas, including the motivation for using transparent ceramics, the requirements, the potential applications, and the ongoing processing research will be reviewed.

Temperature and stress dependence of rhombohedral twinning behavior of sapphire single crystals was investigated. Specimens were deformed in compression along the c-axis with an Instron universal testing machine below 1100 °C in order to avoid basal twinning and dislocation slip. Rhombohedral twinning formation can be explained by nucleation of a double kink in the twinning dislocation, while twin growth can be treated as a twinning dislocation movement which is driven by atomic diffusion at elevated temperatures and by mechanical slip at relatively low temperatures. The activation enthalpy and activation volume of rhombohedral twinning dislocation and also the activation energy of twinning dislocation movement were measured using Smidt's empirical equation and Arrhenius' equation, respectively.


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