Enhanced Durability, High-Transmission, ZnS Windows Incorporating AR Microstructures

Many military systems utilize the material Zinc Sulfide (ZnS) for windows and domes because of its high transmission and low absorption of infrared light, combined with its relatively low cost. Multi-spectral, or clear ZnS (ClearTran) has a wide spectral transmission range that is particularly useful for multimode missile seekers such as the Joint Common Missile. One long-standing issue with ClearTran is its low environmental durability that necessitates the use of some form of hardening to protect the window or dome from damage due to rain and sand impacts. Typical hardening methods involve the application of thin-film material coatings that must also serve as an anti-reflection (AR) treatment. Such coatings however limit the transmission bandwidth forcing a tradeoff between the level of transmission required and the expected lifetime. As mission profiles become more demanding the further utility of ZnS will depend upon the continued evolution and development of hardening strategies to improve dome durability under increasingly adverse environmental conditions. In a recent experimental study, an innovative AR treatment based on surface relief microstructures was shown to have great potential for increasing the survivability of ZnS windows operating in abrasive environments. This Phase I project proposes to further investigate the durability of AR microstructures built in ClearTran windows that meet the wide bandwidth, high transmission performance requirements of multimode seekers. Multiple design variants that combine AR microstructure technology with hard oxide coatings will be fabricated in ZnS coupons and subjected to a set of rain and sand exposure conditions set by the Government. Six coupons of the most promising designs will be delivered to the Government for further erosion testing. In a Phase I Option program, a statistically meaningful number of coupons will be fabricated with the most durable AR microstructures found during the Phase I work, and further rain and sand erosion testing will be conducted. Also in the Phase I Option program, AR microstructures will be fabricated in a small-scale ClearTran lens surface to demonstrate the ability to apply AR microstructures to dome surfaces during a Phase II program. A close collaboration with the multimode seeker vendor Raytheon Missile Systems, will be maintained throughout the multi-phase program to ensure the rapid transition of the technology into Army systems.