Metallic mesh thin film coatings have been used for many years to provide electromagnetic interference (EMI) shielding
on infrared windows and domes. The level of EMI shielding effectiveness (SE) of metallic mesh coatings when used in a
high frequency application is understood and characterized. Conversely, the level of SE of these metallic mesh coatings
when used in a low frequency application has been called into question. In a recent study, we applied an appropriately
designed metallic mesh coating to a sapphire window, mounted that window in a fixture, and tested the SE of the
window assembly over a frequency range that envelopes the various military platforms covered in MIL-STD-461 (10
kHz to 18 GHz) for a radiated emissions test. The test plan was devised in such a way as to independently assess the
individual contributions of the aperture, the mounting, and the metallic mesh coating to the total shielding. The results of
our testing will be described in this paper. Additionally, the test results will be compared to the predicted SE for both the
aperture and the metallic mesh coated window in order to validate the predictive model. Finally, an assessment of the
appropriateness of the use of metallic mesh coatings for EMI shielding in a low and/or broad range frequency
application will be made.
Advances in optical manufacturing and testing technologies for sapphire material are required to support the increasing use of large-aperture sapphire panels as windscreens for various electro-optical system applications. Single surface grinding is a crucial process step in both the figuring and finishing of optical components. Improper grinding can make subsequent polishing operations more difficult and time consuming. Poor grinding can also lead to the introduction of surface stress and sub-surface damage which can affect critical opto-mechanical performance characteristics such as strength and durability. Initial efforts have been completed at Exotic Electro-Optics under the funding of the Office of Naval Research and the Air Force Research Laboratory to investigate a number of process enhancements in the grinding of a-plane sapphire panels. The information gained from this study will ultimately provide a better understanding of the overall manufacturing process leading to optimized process time and cost. EEO has completed two sets of twelve-run Plackett-Burman designs of experiment (DOE) to study the effects of fundamental grinding parameters on sapphire panel surfaces. The relative importance of specific process parameters on window characteristics including surface roughness, stress, sub-surface damage are reported.
Advancements in optical manufacturing and testing technologies for sapphire material are required to support the increasing use of large aperture sapphire panels as windscreens for various electro-optical system applications. It is well known that the grinding and polishing operations employed to create optical surfaces leads to the introduction of surface stress and sub-surface damage which can affect critical opto-mechanical performance characteristics such as strength and durability. Traditional methods for measuring these defects are destructive and, therefore, unsuitable as in-process, high volume inspection tools. A number of non-destructive optical techniques were investigated at Exotic Electro-Optics under funding by the Office of Naval Research and the Air Force Research Laboratory including Raman spectroscopy, laser polarimetry and the Twyman effect to characterize process-induced defects in sapphire panels. Preliminary experimental results using these techniques have shown that surface stress and sub-surface damage may be non-destructively measured. Raman spectroscopy has shown promise in quantifying surface stress, laser polarimetry is of questionable utility and the Twyman effect may be used qualitatively to monitor relative stress and sub-surface damage. This information will ultimately provide a better understanding of the overall manufacturing process leading to optimized process time and cost.
Exotic Electro-Optics (EEO) has completed a study of how the edge finish of an A-plane sapphire sample affects its flexural strength when tested using the 4-point bend test method. Flexural bar samples were fabricated out of a sapphire panel that was polished to production quality using EEO's standard production methods. All samples were configured to meet the requirements for a C-size sample as defined by ASTM C-1161. The only difference between the three sample groups was the edge finish applied to the sample - conventionally ground, fine ground or a commercial polish edge finish. The edge finish on each sample was quantitatively characterized prior to strength testing. All samples were visually inspected prior to testing to identify any potential fracture initiation points. The samples were then tested using an Instron Universal tester per ASTM C-1161 in the UDRI Ceramics and Glasses Laboratory. After testing, a visual inspection was performed to identify the fracture initiation surface and location. Observations confirmed that all sample data was valid (all fractures initiated inside the two inner load dowels), no fractures were initiated on the edges, and no fractures initiated at any of the suspect sites noted in the pre-test visual inspection. The data was post processed using standard statistical and Weibull analysis methodologies. The results showed no significant difference when comparing the flexural strength of the three edge finish groups. The data suggest that the surface quality of the planar surfaces and the bevels is more critical than the finish of the full edge.
Exotic Electro-Optics (EEO) recently completed a series of MIL-STD-810F, Method 510.4 sand erosion tests at multiple commercial testing sites. During this testing process, it became apparent that no two environmental test vendors are alike, even if MIL-STD-810F is specified in all cases. Three different test laboratories performing the same Method 510.4 sand test on identically fabricated samples yielded three different results. Ultimately, it is the responsibility of the Engineer to confirm that the test vendor’s equipment, processes, and procedures produce a test environment that is applicable and a result that is accurate based upon the customer specified test requirement and the MIL-STD-810 methodology. Some critical factors that determine the utility of a test are particle concentration, air velocity, particle size and composition, and the ability to maintain these parameters over test duration of up to 90 minutes. EEO has identified a number of parametric details critical to maximizing the stability and accuracy of MIL-STD-810F, Method 510.4, Procedure II sand testing. These strategies will be presented.