Future advanced communications systems will utilize lasers which operate at 1.55μm
backed up by an RF links. To the extent that such systems utilize a common aperture,
dual IR/RF windows and domes will be required. The durability of such windows, with
respect to rain and sand erosion damage, is an important consideration as damaged
surfaces will lead to significant optical degradation for operation at 1.55μm. This
requirement drives the materials choices toward more durable materials such as ALON®
optical ceramic, spinel and sapphire. Single layer windows, with appropriately selected
thicknesses of these materials can be used for narrow RF wavebands, but are not
adequate for applications requiring broadband RF transparency. To this end, multilayer
windows, with durable outer layers of ALON have been developed, and built. Recent
results will be presented.
The optical performance of windows and domes are subject to degradation from rain and
sand erosion damage in harsh flight environments. While durable window and dome
materials, such as ALON®, spinel and sapphire are more or less impervious to rain and
sand erosion damage in the captive carry environments, the coatings use to provide antireflection
(AR) function are not. Rain and/or sand erosion damage of the outer AR
coating leads to degradation of the windows optical performance, even when the
underlying window itself is not damaged.
Surmet has been working on design and development of physical gradient index (Moth-
Eye) structures based AR surfaces etched directly into the surface of the ALON substrate.
By eliminating the need for less durable coating materials, these structures offer high
optical performance without compromising durability. The difficulty of this approach is
that the same durability that makes ALON impervious to erosion damage makes it very
difficult to etch. Processes have been developed at Surmet which facilitate the etching of
fine deep features into ALON surfaces required for broadband AR function. Recent
results will be presented.
Future advanced communications systems will utilize lasers which operate at 1.55 μm
backed up by an RF links. To the extent that such systems utilize a common aperture,
dual IR/RF windows and domes will be required. The durability of such windows, with
respect to rain and sand erosion damage, is an important consideration as damaged
surfaces will lead to significant optical degradation for operation at 1.55 μm. This
requirement drives the materials choices toward more durable materials such as ALON® Optical ceramic, spinel and sapphire. Single layer windows, with appropriately selected
thicknesses of these materials can be used for narrow RF wavebands, but are not
adequate for applications requiring broadband RF transparency. To this end, multilayer
windows, with durable outer layers of ALON have been developed, and built. Recent
results will be presented.
Space based HgCdTe imaging devices, built on CdZnTe substrates, require radiation hardened anti-reflection (AR) treatments in order to withstand the rigors of the space environment. Conventional anti-reflection (AR) coatings provide adequate optical performance but are prone to delamination and degradation due to extreme temperature cycling and irradiation in space. Consequently, there is an intense need for improved AR technology that combines high optical performance with improved durability. Etching physical gradient or motheye structures directly into the CdZnTe eliminates the need to deposit additional layers of different materials onto the substrate, avoiding the possibility of delamination and cross contamination. Motheye AR surfaces, under development at Surmet Corporation, have demonstrated excellent broadband optical performance in the LWIR (7 to14 micron) waveband. Surmet's motheye technology involves direct etching of a regular pattern of fine features into the CdZnTe substrate, using standard lithography and dry etching techniques. The results from this ongoing research and development effort are discussed.
Thin films of selected amorphous and nano-crystalline compositions from the ternary phase diagram made up of carbon, silicon, and nitrogen, with a significant oxygen impurity, have been grown by chemical plasma reactions, ion beam deposition and plasma CVD. Characterization has focused on optical and mechanical properties to determine the compositions best suited for erosion resistant infrared (IR) optical coatings. The most desirable film compositions for the goal application are located in a compositional region around the C/Si atom ratio = 0.2 and C/N = 0.3. The most durable films have no apparent midwave infrared (IR) absorptions, an optical index of refraction ranging from 2 to 2.1, and indent hardness near 40 GPa. Thin film compositions nearer to carbon nitride show significant midwave absorption bands and lower hardness. Ambient thermal oxidation resistance increases with film nitrogen content, with the most durable films being stable at 700 degree(s)C in air.
Aluminum Oxynitride or ALON optical ceramic is transparent material, developed and patented by Raytheon, which is very similar to sapphire, being comprised mostly of Al2O3 with a small amount of additional nitrogen. This nitrogen addition has the effect of producing a cubic material whose optical and mechanical properties are isotropic. Importantly, this means that it can be produced by powder processing methods, which are scalable to larger sizes, and at lower prices than can be achieved by the single crystal growth techniques that are used to grow sapphire. Furthermore, its isotropic properties make it much easier to grind and polish than sapphire. Recently, the interest in ALON optical ceramic has grown substantially following impressive results in ballistic testing. Ballistic laminates, containing ALON layers, have demonstrated protection against armor piercing rounds, at half the areal density and thickness of conventional ballistic laminates. ALON plates as large as 14x20in are being produced, under Air Force funding, for evaluation as IR windows and transparent armor, using conventional powder processing techniques. The production processes themselves are now being scaled to produce large pieces and large quantities of ALON optical ceramic.
Infrared (IR) imaging systems are a critical component of military aircraft operations for navigation, surveillance, and target acquisition. A limiting feature in the readiness and life-cycle costs for such IR systems is the durability of the exposed optical window in the harsh land and sea environments associated with military operations. The Air Force Research Laboratory, Materials Directorate, has been conducting a program to develop a high durability, repairable JR window to both extend the operation life-cycle of the transparencies and to permit the reuse of the optical materials for significant cost reductions. The development effort is focused on a composite JR window composed of a highly JR transparent base material with a removable durable bonded cladding for erosion and abrasion protection. The development activities have included evaluation and selection of the optical materials, assessment of high durability coatings, refinement of the optical adhesive and bonding process, as well as laboratory and flight testing.
Accurate and reliable testing is paramount to the development of LWIR window materials. Without appropriate characterization and testing, improvements to existing technologies are impossible to document with certainty. Reliable and repeatable testing provides the data needed to measure advancements and identify improvements in any technology. No single test can be completely definitive, and the continuous evaluation of emerging technologies using different test methods under varying conditions is critical when evaluating a new materials' capability. The environmental testing of infrared (IR) window materials has traditionally consisted of rain erosion testing, single impact water jet testing, and sand erosion testing. While these three tests provide the materials engineer with significant insight into the durability of a window material, these tests have generally ignored the combined effect of rain and sand. This paper looks at the combined effect of rain and sand erosion on a standard LWIR window material, zinc sulfide (ZnS).
Durable coatings are used to improve the erosion resistance of high performance optical materials such ZnS. Diamond is the hardest and stiffest of all LWIR transparent materials and would make an excellent protective coating for ZnS. Direct deposition of diamond on ZnS by microwave plasma CVD has proved to be very difficult. Atomic hydrogen used in the diamond deposition process attacks and destroys ZnS very rapidly. In order to protect ZnS during the diamond deposition process protective IR transparent interlayers were developed. These layers encapsulate the ZnS and provide a nucleating surface for diamond deposition. Two different methods of nucleating diamond on these interlayers were developed to produce fully dense diamond films several microns thick. The sand erosion resistance of diamond coated ZnS was found to improve when the diamond was deposited on patterned ZnS substrates.
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