Improved IR Windows for Severe Aerothermal Environments

Current IR window materials, while offering good optical properties are often structurally inadequate in their ability to perform in ever more increasingly hostile environments. Optically transparent, hard ceramic materials such as spinel, ALON and sapphire while expensive to fabricate and finish are considered viable candidates against scratch and rain erosion and severe thermal environments. Pressureless sintering of spinel using little or no additives is being proposed as a low cost approach to producing IR windows and domes with exceptional strength and optical properties. Spinel transmits further into the MWIR than either ALON or sapphire and due to the significantly lower fabrication temperature and processing complexity, can be made at considerably less cost. Careful control of the pressureless sintering schedule allows grain refinement, with improved strength and thermal shock resistance. This research effort will establish a baseline processing protocol for net shape spinel domes and will establish cost estimates for future scale up. It is expected that the proposed pressureless sintering approach for spinel will lead to a significant cost reduction in the fabrication of future IR windows and domes. Conventional ceramic processing technologies such as slip casting may be employed enabling the low cost fabrication of ogives and other complex shapes difficult to fabricate by other low cost processing approaches. Additionally, the very near net shape capability will result in significantly reduced finishing costs over conventional grinding and polishing approaches. Transparent spinel products could become mass produced for such optical applications as fuses, small sensor windows, wear resistant transparent surfaces and thermal inspection IR lens and windows .