Segmented Structural Ceramics

The selection of hypersonic IR window materials is generally driven by the specific application, from which the necessary transmission, thermal, and structural performance requirements can be derived.  In considering the flight environment, there are three general problem areas in IR window design and these are a hot window (emission), rain and/or sleet erosion, and thermal shock failure.  Of these three, thermal shock sensitivity imposes the most critical requirement on the window material. Existing ceramic materials offer several attractive qualities for hypersonic windows (e.g., high melting temperature, thermal-chemical compatibility, and good transmission properties); however, many ceramics possess poor thermal shock resistance and consequently require innovative designs for successful implementation.  Improving their thermal shock resistance of ceramic IR window materials was the basis of the recently completed seedling effort and this work, along with parallel efforts on optical modeling, fabrication, characterization, and representative testing will continue in the proposed program. Generally speaking, there is no “one material” that exists that will provide the desirable electrical and physical properties to protect the seeker over a variety of trajectories.  However, when consideration is given to architected metallic reinforced ceramics, or the introduction of composite window designs in the form of Segmented Structural Ceramics (SSC), the possibilities of improved aperture designs become intriguing. The objective of this proposed program is to develop a complete solution for the needs of hypersonic apertures.  This will be demonstrated during the Phase II research and development work plan by completing tasks related to: simultaneously addressing thermal-structural and optical responses in the design process; fabrication efforts for both monolithic and SSC windows to produce coupons and hardware of representative geometries; material characterization of critical properties; representative hardware testing in relevant environments; integration and trade studies; post-test data correlation studies and design improvements; and fabrication maturation.  The importance of this proposed effort is the development of technology that can drastically improve the thermal shock tolerance of hypersonic windows.  The anticipated findings of this effort will be a marked improvement in the thermal shock resistance of the SSC composite aperture with minimal/manageable effects on the transmission performance.  The technical feasibility of improving the thermal shock resistance with this approach is considered to be well demonstrated and this is supported based on the favorable testing results sited in the literature and the supporting analytical studies from the seedling effort.  With respect to the transmission behavior of the SSC concept, the technical feasibility is encouraging based on the waveguide analogy work performed in the seedling.