A novel sapphire (99.99% Al2O single crystal) dome growth technique is described whereby a dome of a nominal radius of 38 to 40 mm is produced. A modified Edge-Defined Film-Fed Growth (EFGR ) technique is used to directly grow a dome blank with a wall thickness of 2.5 to 3 mm which requires a minimum of mechanical finishing and polishing. Total integrated scatter (TIS) results for the polished dome are reported for .6328 um and 3.39 um wavelengths. An evaluation of striae and bulk inhomogeneities is also given.

The strengths of sapphire disks of two different crystallographic orientations and bars of three different orientations were measured as a function of temperature in ring-on- ring flexure or 4-point bending. One set of disks (OO cut) had the crystallographic C -axis normal to the flat surface which contained the crystallographic a- and m -axes. The average strength of these disks dropped from 154 ksi at 20°C to 21 ksi at 800°C. In another set of disks (900 cut), the crystallographic c- and m -axes were parallel to the flat surface. The average strength of these disks dropped from 84 ksi at 20°C to 48 ksi at 800°C. The strength of sapphire bars whose tensile axis was the crystallographic m axis dropped from 103 ksi at 20°C to 86 ksi at 1400°C. The strength of sapphire bars whose tensile axis was the crystallographic a-axis dropped from 113 ksi at 20°C to 74 ksi at 1400°C. The strength of sapphire bars whose tensile axis was the crystallographic c-axis dropped from 153 ksi at 20°C to 35 ksi at 1400°C.

A method is described to make multiple measurements of the strength of a single hemispherical ceramic dome with a ring-on-ring flexure test of disklike sections removed from the dome by a core drill. Preliminary results show that dome cores exhibit lower strength than flat disks, because the domes have larger critical surface flaws than those present on disks. Careful attention to polishing of domes is required to realize maximum strength from the domes. The fracture toughness of optical quality yttria disks and hemispherical domes was found to be approximately 0.7 MPa'I m.

Experiments aimed at improving the physical properties of transparent polycrystalline lanthanastrengthened yttria (LSY) infrared windows and domes were conducted. The objective was to enhance the thermal shock resistance for aggressive aerothermal environments. The approach included improving the average equibiaxial flexure strength, Weibull modulus, and other relevant physical properties. Initial results of an extensive study on polishing and post-fabrication treatment along with improved powder processing showed an -30% strength improvement without sacrifice in optical properties, leading to an appreciable increase in the calculated survivability. LSY with a low lanthana content significantly enhanced predicted survivability.

Non-doped yttria (Y203) ceramics for 3-5Mm JR transmission have been experimentally made by a powder processing technique. The optical, mechanical, and thermal properties have been measured. Some samples showed fairly good transmittance properties. In addition, decreasing the grain size of yttria proved effective in improving the flexural strength.

Different structures of scattering defects have been identified in aluminate of magnesium and Yttrium oxide by mean of Laser Scanning Tomography with a probe beam at wavelength of 1,06 μm. These results are compared with scanning electronic microscopy micrographs obtained on the same samples. Conclusions are made about the nature of scattering defects and the usefullness of LST for characterizing the quality of ceramics.

Knowledge of the scatter characteristics of candidate infrared sensor dome materials is necessary for the evaluation of image quality and susceptibility to bright off-axis sources. For polycrystalline materials in particular, the scattering levels are high enough to warrant concern. To evaluate the effects of scatter on image quality, estimates of the window Point Spread Function (PSF), or its transform, the Optical Transfer Function (OTF) are required. Additionally, estimates of the material scatter cross-section per unit volume are essential for determining flare susceptibility. Experimental procedures and models in use at JHU/APL allow the determination of each. Measurement results are provided for samples of A1203 (ordinary ray), Y203, LaO3-doped Y203, MgAL2O4, and ALON. Applications of these results are illustrated for planar windows having arbitrary orientations with respect to the optical axis.

The temperature coefficients of refractive index for various crystalline and polycrystalline materials, Al203 (ordinary ray), Y203, LaO3-dOped Y203, ALON, and MgA12O4 were determined from measurements of optical thickness as a function of temperature using a Michelson interferometer operating at 0.633 pm. For the temperature range of 23°C to 500°C, the first order coefficients ranged from 8.28x10I0C for pure yttria to 14.6x10/°C for ALON. Measurements of NaC1 and A1203 samples using this technique are in agreement with published data.

The development of sulfide materials as infrared-transmitting optical ceramics is limited by intrinsic optical properties, thermomechanical properties, and considerations of chemical stability. Screening procedures with respect to band gap, electronic absorption, chemical stability, and refractory character reduced the set of all sulfides to about a dozen structural families. Systematic relationships were developed between crystal chemistry and phonon absorption edge, vibrational modes frequencies, and coefficient of thermal expansion which allow possible ranges of properties to be estimated. It is concluded that improved materials are possible but that radically improved new materials are unlikely.

A series of ZnS-rich wurtzite solid solutions containing 8, 12 and 16 mol% Ga2S3 has been prepared. The influences of the fabrication conditions and microstructure on the mechanical properties were examined. It has been demonstrated that ZniGa2Si +2,is tougher and harder than pure ZnS due to a solid-solution strengthening effect. The hardness of the solid solutions increases with increasing concentration of Ga2S3and decreasing grain size, the grain-size dependence being of the Hall-Petch type. Finer-grained solid solutions with moderate concentrations of Ga2S3 have the highest fracture toughness. Whereas the hardness increases monotonically with increasing mole fraction of Ga2S3, the fracture toughness is a maximum at around 12% Ga2S3.

Infrared transparent conductive diffused layers have been integrated into germanium windows using an ion- implantation/diffusion technique. These layers are nominally 25 microns thick with sheet resistivities of 5-10 ohms/square. The excellent transmission characteristics of the germanium windows are maintained, with maximum transmission degradation of only 3 and 5% in the 3-5 and 8-12 micron bandpasses, respectively.

It has previously been determined that absorption of infrared radiation in germanium due to free carriers can be controlled to some extent by doping. As a result, the use of germanium in infrared systems can be extended to thermal environments not possible before. This can be accomplished by using lower resistivity germanium at elevated temperatures instead of the standard optical grade germanium used near room temperature (25°). In this work, the absorption of 3 to 11.9 tm radiation by germanium of various resistivities was determined at several discrete temperatures from room temperature to 120°C. The germanium samples ranged in resistivity from 0.1 to 40 ohm-cm. The data from these measurements were used to formulate graphical representations of the relationships between room temperature resistivity, absorption coefficient, wavelength and temperature.

Diamond, beryllia, and cubic zirconia are potential optical window materials that feature high strength, hardness, and high melting temperatures. Diamond, in particular, is receiving considerable attention concerning the fabrication of polycrystalline films and windows. To fully appreciate the potential of these materials as infrared windows, the optical properties must be accurately known. Optical phonons, both single and multiple, determine the intrinsic absorption properties and infrared-active single-phonon transitions contribute to the index of refraction. Both beryllia and cubic zirconia are composed of ionic bonds. This means that dipole moments will exist for some of the vibrational modes and they will be infrared active. These modes can be observed in reflection spectra. The higher harmonics and combination bands of the one-phonon band form the multiphonon band absorption. The region of the three-phonon band determines the beginning of infrared transparency in these materials. Intrinsic diamond is composed of covalent bonds with a high degree of symmetry and hence has no infrared active one phonon bands. However, lattice defects produce weak one-phonon absorption bands in the region of 1000 cm. Also, multiphonon combination bands of inactive one-phonon and Raman bands produce significant absorption beyond 1800 cm. Experimental reflection spectra of beryllia and cubic zirconia from 200 to 1200 cm, and experimental transmission spectra of intrinsic type ila diamond from 500 to 5000 cm are presented. These experimental data are used to determine the necessary optical parameters for models of the complex index of refraction as a function of frequency and temperature.

Thin films were deposited onto ZnS as a means of strengthening and toughening. Fracture strength increases of up to 40% have been observed. These mechanical property enhancements were found to be related to the compressive stress in the deposited films. Rain erosion resistance is also enhanced through the application of compressively stressed thin film coatings.

Since materials with the best optical properties have low-fracture toughness, the need for protective coatings for IR windows and domes has been recognized for a long time. The emergence of technology for deposition of polycrystalline diamond (PCD) coatings by microwave-assisted chemical vapor deposition presents an opportunity to provide protective coatings for IR materials. Results of deposition of PCD films on various substrates will be presented. Diamond film morphology can be tailored from very smooth (35-Å RMS roughness) to textured with very large crystallites (20-μm diameter) with deposition rates from 0.5 to 2 μm/hr. Substrate surface preparation using a nonabrasive refractory interlayer technique will be described. Film morphology, characterization, and polishing that uses a chemical polishing technique will also be discussed.

Boron phosphide is shown to be an excellent component of coatings which protect infra-red substrate materials from harsh military environments. Substrate materials used in the 8-l2j.m band tend to be brittle, weak and susceptible to various forms of damage. Erosion due to rain impact during high speed flight is one important type of such damage. Results are presented which demonstrate that coatings containing boron phosphide can be used to protect substrate materials such as ZnS and Ge from the effects of rain impact. Anti-reflection properties of the coatings are also considered and results presented for both single and multilayer coatings. The other materials used in the multilayers are, like boron phosphide, deposited by plasma assisted chemical vapour deposition (PACVD) which results in a very durable coating system.

Infrared coatings are required on both surfaces of infrared domes to increase transmission and enhance system performance. These domes are integral parts of either a Missile System or an electro-optical sensor attached to military vehicles. The convex surface ofthe dome is exposed to the harsh environment ofthe outside world, and therefore requires a coating which not only increases transmission but is able to withstand humidity, fluids, exhaust, sand and rain environments. This paper deals with the techniques that were developed to adapt a rain erosion resistant coating for ZnS Optical Flats Windows to curved surfaces. The fabrication process testing and results are described in this paper.

The use of wire grid structures to block EMI radiation is well known. For the past several years Battelle has been developing photolithographic techniques and computer models to predict the performance of wire grid structures on IR transmitting windows. The amount of IR radiation transmitted through non-resonant grid structures is limited to the percent open area of the window. As the wires of the grid structure are placed closer together the area of the window not obscured by the grid decreases. Resonant grid (mesh) structures have the potential for superior JR transmission at specific wavelengths, while providing adequate EMI attenuation. The polarization of the IR radiation becomes important when non-normal angles of incidence are considered. A computer model has been developed to predict the performance of wire grid structures on windows for various angles of incidence and polarization. This model has been experimentally verified for a number of cases and has been shown to be useful in designing windows incorporating EMI shielding. The results of the modeling for several cases, along with the experimental verification, are presented. The limitations of such techniques are also discussed.

The growing need for computational codes to optimize windowed hypersonic vehicle designs has required a more complete understanding of aero-optical effects produced in the flight environment. Teledyne Brown Engineering (TBE) has determined the mixing/shear layer created by the cooling of a window on a hypersonic vehicle to be one of the largest contributors to aero-optical distortion. A novel experimental setup is currently being developed that effectively reproduces this mixing/shear layer and thus its significant aero-optical effects. In addition, various test techniques have been devised to investigate these phenomenon, as well as to produce simulated flight data required for validation of the aero-optic optimizing codes.

Advances made in the field of aero-optical system modeling are applied in the technology of super- or hypersonic vehicles carrying optical seekers. The fundamentals discussed form a basis for performance predictions of these airborne optical systems. The focus is the inherent image degradation due to aerodynamic mixing layers. This paper examines the aero-optic properties of supersonic mixing layers. Recent experimental results on the aerophysics of supersonic mixing layers is combined with statistical aero-optics theory to create empirical equations governing the image degradation resulting from light propagating through a classical mixing layer. This model results in simple expressions for blur circle size, Strehl loss, jitter and boresight error. Expressions for the turbulent Modulation Transfer Function (MTF) and the Point Spread Function (PSF), assuming a diffraction-limited optical system, can also be determined.

Aerodynamic flow surrounding a missile or aircraft in flight can degrade the performance of on-board optical sensor systems. The minimum resolvable spot or blur circle is a measure of optical system performance. The blur circle size may change by orders of magnitude throughout the course of the vehicle flight trajectory due to aerodynamic perturbations. This paper examines the wavelength dependence of blur circle size. It is shown that in many cases an optimum wavelength exists at which the blur circle size is minimized. Expressions are given for depicting the Point Spread Function shape and wavelength at which blur is minimized. The optimization expressions presented are suitable for use on a desk top computer or calculator.

A successful model of the aerodynamic heat transfer to nose-mounted IR domes on hypersonic missile interceptors is presented. This paper describes a transitional flow heating algorithm, and presents a model for predicting the onset of boundary layer transition on a hemispherical forebody. In addition, this paper discusses the results of temperature predictions for IR domes, including a description of the analytical models and favorable comparisons with experimental data taken at Mach 4.6.

Twenty years ago, work was initiated to consider the feasibility of using pyramidal domes for dual-mode air-to-air missile applications. The present contribution concerns the degradation in the performance of the infrared subsystem that results from the aerodynamic heating of such domes, under conditions representative of anticipated most-severe launch environments for internal and external missile-carry configurations. The problem is formulated in a general but compact form compatible with the state of the art of contemporary infrared technology.

The infrared radiant intensity and radiative contrast produced by the aerodynamic and solar heating of missile noses have been obtained by computer simulating the corresponding temperature distributions and by taking into account the effects of both atmosphere (absorption and self-emission) and target geometry (distance, aspect and elevation angles). Six target classes have been considered, including cruise missiles, air-to-air and surface-to-surface missiles, and ballistic missiles. The comparison between radiative quantities in the 3 to 5 μm and S to 12 μm atmospheric transmittance windows gives the optimum spectral band for target detection in each point of its trajectory.

Optical window systems protect sensor systems from the aerodynamic and atmospheric environments. The aerodynamic heating on an optical window located about 1 foot downstream from the spherical nose tip is analyzed. The vehicle flies at a constant Mach number and at a constant altitude. The Mach numbers range from 1.5 to 5 and the altitudes from 10 to 30 km. The mechanical responses are calculated for yttria and lanthana-doped yttria, infrared window materials currently being developed for the 3 to 5 im applications. The results show that the yttria window temperature is about 1000 K at Mach 5 and 10 km altitude for an emissivity of 0.04 at 5 μm Steady state and constant altitude flight are assumed.

Dr. Genichi Taguchi, a prominent quality consultant, reduced a branch of statistics known as "Design of Experiments" to a cookbook methodology that can be employed by any competent engineer. This technique has been extensively employed by Japanese manufacturers, and is widely credited with helping them attain their current level of success in low cost, high quality product design and fabrication. Although this technique was originally put forth as a tool to streamline the determination of improved production processes, it can also be applied to a wide range of engineering problems. As part of an internal research project, this method of experimental design has been adapted to window trade studies and materials research. Two of these analyses are presented herein, and have been chosen to illustrate the breadth of applications to which the Taguchi method can be utilized.

A review of the existing rain erosion test capabilities has shown that it is difficult to obtain multiple waterdrop impacts on materials over an extended range of supersonic velocities which are both well-documented and at moderate cost. Recognition of this deficiency motivated the development of a facility located at General Research Corporation (GRC) in Santa Barbara, CA to provide controlled multiple1 simulated waterdrop impact conditions for impact velocities from 300 to 1700 ms (up to Mach 5). A collection of particles, selected for their capacity to simulate waterdrop impact damage, are propelled at the target using a 50 mm powder gun. The development and anticipated capability of this facility is described. A discussion is then presented of the planned test program designed to formulate and validate damage models for electromagnetic (EM) window materials in the supersonic flight regime.

The advantages of firing liquid jets of water at stationary specimens to study the effects of single drop impact have been well documented. The development of a multiple impact jet apparatus (MIJA) at the Cavendish Laboratory now allows a controlled study ofthe multiple liquid impact process in a similar way. The design ofthe MIJA apparatus has been established around the existing nozzle design technology for the single impact gun. This design allows the creation of liquid jets of a high quality: spherically fronted; symmetrical; reproducible and with a stable coherent core. In this way the MLJA apparatus can simulate the impact of 2 to 10mm diameter drops over a range of velocities up to 400 m s1 and with a repetition rate of -1O min^-1. The apparatus is entirely automated with a sophisticated computer control maintaining an accurate record of experiments performed and the impact positions and velocities. The computer allows impact arrays of any type to be studied and restrains the spread of jet velocities to a 2% standard deviation around that selected. The first part of this paper describes the design and performance characteristics ofthe two pieces of jet apparatus. The final sections then look at some preliminary damage results.

Recent wind tunnel tests have been used to establish baseline characteristics of the effect of small particle impacts on two sapphire window plates. These two window plates have been exposed to wind tunnel flowfields of approximately Mach 10 at a simulated altitude of 50,000 feet. Test conditions indicate a free stream velocity, of approximately 4800 ft/sec. Within the wind tunnel flowfield, small particles were carried along and have appeared as impact sites on the exposed surface of each window plate. Examination of the impacted sites indicate that the particles were composed of Deirin (a hard acetal homo-polymer) or stainless steel, both of which resulted in small but measurable damage to each window plate. Characterization of the plate surface appearance and impact damage was conducted at two locations: 1) the University of Alabama in Huntsville, (UAH), Optical Measurements Laboratory (OML) and, 2) Oak Ridge National Laboratories (ORNL), Optical Component Characterization Laboratory (OCCL).

In flight, infrared domes must withstand heating and mechanical siresses. Therefore, the rain erosion resistance to be considered in predicting the lifetime of the dome must be different from the one usually measured in ground test facilities without these environmental conditions. The purpose of the present study was to measure the effect temperature and stress have on the erosion resistance of materials. A ground test facility was used that was modified to create these environmental conditions. Special supports and electronics were designed and made to heat and bend the samples. Significant variations occur in material resistances as a result of temperature, while the effect of stresses seems to be catastrophic.

This paper summarizes a test program to verify the Balageas erosion model for Slip Cast Fused Silica in a flight-like erosive environment. The test program is summarized with particular attention paid to documenting the erosive environment. The Balageas model was found to over predict the erosion for these tests and a revised model which gives reasonable agreement with the data is proposed.

For a given image brightness, IRST range is limited by background noise. Mid-IA radiation from the sun contributes to background noise, even when the sun is well out of the field of view, because the illuminated IRST window scatters unwanted background radiation onto the detector. The level of scatter depends strongly on window degradation due to contamination and actual window damage incurred in flight, as well as inherent flaws in the window material. This paper presents scatter data taken at .67, 3.39 and 1 0.6 microns in a laboratory setting from several damaged germanium window samples. The data is compared to scatter levels from undamaged windows of other materials. Estimates made of the corresponding reduction in lAST range indicate that this can be a serious effect. Instrumentation has been developed that allows measurement of reflective scatter from aircraft sensor windows on the flight line. These measurements are compared to those obtained in the laboratory.