A comprehensive search was made to identify ceramic materials which could be employed as IR sensor windows or IRdomes in environments conducive to severe mechanical stresses. This program, started in late 1977 under the joint sponsorship of the Defense Advanced Research Projects Agency (DARPA) and the office of Naval Research (ONR), has been reported in detail in Ref. 1 - 4 through the period ending in mid 1980. The purpose of this paper is to summarize the current status of the processing technologies being developed to realize the potentials of certain advanced materials selected for development.
The optical properties in the UV, visible and IR spectral regions are reported for polycrystalline Mg A1204 fabricated by hot-pressing high purity starting powders. Room temperature mechanical properties and the strength at elevated temperatures will be reported. Data will be presented for flat discs and hemispherical domes.
A new ceramic, nitrogen-stabilized aluminum oxide spinel (ALON), has been developed at AMMRC which may have distinct advantages over sintered polycrystalline a-A1203. This material is isotropic and has been fabricated into dense, transparent bodies with the following properties: Knoop (100) hardness of 1800, elastic modulus of 46x106 psi, a room temperature fracture strength of 45x103 psi, a dielectric constant and loss tan%ent, respectively, at 10 MHz of 8.56 and 0.0004, trivial oxidation in air up to 1200 C and IR cutoff at 5.2 μm, and an average thermal expansion coefficient (a) of 7.00x10-6C -1 (25Ã‚Â°C-1000Ã‚Â°C).
Several Sources of CaLa2S4 have been evaluated as potential starting powder for fabrication of windows and domes to be used in the 8-14μ m region. Optical transmission and X-ray diffraction data will be presented along with Hot-press parameters as evidence of the difficulty being experienced in procuring starting powders which can be fabricated to dense compacts that manifest useful optical quality. Recent results suggest that the starting material problems may be resolved with continued concentrated effort.
Zinc sulphide is currently being developed as an optical window material for the visible and infrared bands. While this material is adequate for some airborne applications it is evident that a new window material with improved thermal and mechanical properties is required if 8 - 12 pm thermal systems are to be deployed without constraint in all envisaged environmental conditions and scenarios. A little known family of refractory thorium phosphide type cubic and spinel type cubic crystalline rare earth ternary sulphide compounds exists which offers potential for exhibiting better thermal and mechanical properties than ZnS. The crystal chemistry, energy gaps and in some cases melting points of these materials are known but bulk single crystal or polycrystalline samples have not been made to enable their physical properties to be assessed for airborne window applications. Particular compounds are identified for further investigation.
CaLa2S4 is a member of the family of ternary sulfides. It has the cubic Th3P4 structure, space group I-43d, with both cations on equivalent 8-coordinated sites. Powders prepared by firing oxides and carbonates in flowing H2S can be processed by a combination of hot-pressing and hot-isostatic pressing into sulfide ceramics with good infrared transmission. The vibrational absorption edge is at 17 μm. The materials are resistant to attack by water. The microhardness of the nearly 100% dense ceramics is 600 kg/mm2. Optical transmission of 55% (including reflection losses) at 14 μm has been achieved, but optical quality is extremely sensitive to processing conditions. Absorption and scattering are produced by residual pores, by second phase impurities on grain boundaries, by oxidation products such as sulfate and thiosulfate, and electronic defects arising from non-stoichiometry.
Large (7- by 16-inch) plates of semi-insulating gallium arsenide suitable for use as external infrared transmitting windows were grown using a unique compounding process. The plates were grown in an open tube system, the gallium arsenide being solidified from a stoichiometric melt. Detailed growth conditions are discussed. The optical and physical properties of this material are compared with other candidate infrared (IR) optical materials. In addition, some results of captive flight tests comparing gallium arsenide with germanium are given.
Polycarbonate has emerged in recent years as the optimum choice of material for fabrication of advanced aircraft windshields and canopies that reauire maximum performance against bird strikes. This choice is based on a combination of properties of which the most outstanding are impact strength, high temperature resistance, light weight and ease of thermoforming to complex shapes. Polycarbonate, however, requires surface protection from abrasion, ultraviolet irradiation and common chemical solvents. The most cost-effective method of polycarbonate protection and one which adds negligible weight, is by the use of thin transparent polymeric coatings. This paper describes a coating which has been developed for aircraft applications and in fact meets the stringent requirements of the General Dynamics Lightweight F-16 specification. In order to withstand high-speed rain erosion and at the same time retain the mechanical properties of polycarbonate, it has been determined that inherent flexibility of the coating together with high adhesion to polycarbonate are essential requirements for long-term durability in service environments. The conceot of windshield surface protection, screening tests and shortcomings of other coatings will be reviewed emphasizing the major differences in performance between flexible and brittle formulations.
Silicon nitride, prepared by the chemical vapor deposition (CVD) method, transmits electromagnetic energy in the visible, infrared and radar frequency ranges. In this paper, the visible and infrared transmittance properties of polished flat plate and hemispherical shell geometries are described. Integrating sphere hemispherical and in-line specular transmittance measurements indicate near intrinsic levels are being achieved in the infrared wavelength region, however, a diffuse scattering component is limiting imaging capability. Current processing research is aimed at identifying deposition conditions which will eliminate major scattering sources such as the microcrack network found in all deposits made to date. Achievement of this research goal will broaden the optical applications of this unique refractory ceramic.
A number of new improvements in the infrared optical materials, CVD zinc sulfide (ZrS) and zinc selenide (ZnSe), are discussed. These developments include the manufacture of a new water-clear ZnS with improved visible and infrared transmission, the production of a sandwich material which combines the hardness/toughness of ZnS with the infrared optical properties of ZnSe, the development of a manufacturing technique to bond wire grids to ZnS and ZnSe surfaces to provide de-icer/defogger and EMI protection, and a technique to produce small optical element arrays directly by the CVD process. Each of these new developments will be discussed and illustrated, technical issues evaluated, and current status of the technology summarized.
The pioneering work of Charles W. Frederick and George W. Morey on the design by Frederick of an "ideal photographic lens" using hypothetical glasses, and the subsequent discovery and development of rare-element borate glasses by Morey, has been resumed at Eastman Kodak. New ultra-high index, low dispersion crown glasses and companion flint glasses have been developed, based on the needs dictated by lens design studies for novel fast cine' and still camera lenses. These new glasses reduce the number of elements required in a lens while maintaining or improving lens performance. Composition studies leading to these new glasses will be discussed.
The infrared cut-off of glasses is primarily determined by the frequency of vibration of the cation-anion bonds. In order to extend infrared transmittance to longer wavelengths cations and anions of larger sizes and lower field strengths should be used; however, physical and chemical properties become poorer. For silicate glasses, this cut-off is about 5 pm. If germanium is used as the glass network former instead of silicon, the cut-oft moves out to about 6 μm. Of all types of glasses, the germanates provide the optimum combination of transmission, physical, and chemical properties. While the size of glass form-ing areas for germanates is smaller than that for silicates, properties can be varied some-what to fit a particular application. Lxpansion coefficients (25°-300°C) can vary from about 50 x 10-11°C to over 100 x 1011°C with mechanical hardness, Young's modulus, and chemical durability generally decreasing with increasing expansion. While the cut-off is due to the germanium-oxygen bond vibration, the shape of the transmission curve approaching zero transmission from about 4.5 pm to 6 pm can be significantly affected by the quantity and type and modifying oxides. An optimum glass with respect to infrared transmission, low thermal expansion, meltability, formability, and cost was selected and designated Code 9/54. Originally, this glass was melted in crucibles and pressed into domes of rather poor quality. However, more zecently a process has been devised to melt Code 9754 glass in an optical tank and then press into various shapes with standard first grade optical quality (i.e., glass contains no visible striae), a total bubble cross section of 5-0.10 mm2 per 100 cm3, and no cracks or checks. The glass has a thermal expansion (25°C-300°C) of 63.6 x 10-7PC, a Young's modulus of 8.58 x 103 kg/mm2, and a refractive index (ND) of 1.660. The minimum uncoated transmission of a 1.35 mm thick sample is 73% at 5 μm and 35% from 4.2 μm down through the visible.
Water, an ubiquitous impurity in all phases of materials processing, degrades the near-infrared transmission of metal halides and oxides. With the halides, one deals with both the oxygen and hydrogen impurities derived from H2O, while with the oxides, only the H-impurity. The electrolytic and thermal dissociation modes of H2O become significant at the much higher process temperature of the refractory metal oxides. Because of the high strength of the oxide ion as a Lewis base, halogen atoms are unable to abstract the proton of the H-impurity. However, they can displace by electron transfer the H-impurity as the unit OH, a pseudohalide. The limitation and application of the use of halogens to clean up H-impurities in metal oxides are demonstrated with the use of basic and acidic metal oxides.
Internal stresses will arise in an amorphous plastic body if it is cooled rapidly down through the glass-transition zone. This phenomenon is similar to the thermoelastic effect exhibited by conventional, i.e. inorganic, glasses and is fairly commom during the processing of plastic articles. The phenomenology of the thermoelastic effect in amorphous plastics is examined with special regard to the optical properties of the solidified material. It is found that thermal stresses may induce distinct birefringence patterns in the solid plastic body which depend on the imposed cooling rate and the rheo-optical characteristics of the material. The interpretation of these patterns should be made with caution, however, because of the questionable validity of the stress-optical law in the vicinity of the glass-transition zone. It is also found that the thermoelastic effect may give rise to density and refractive index gradients across the solid plastic body which are likely to influence its optical performance.
The mechanisms of impact damage and erosion that occur in infra-red materials subject to impact by solid particles and water drops are discussed. The elucidation of the damage mechanisms allows identification of the material properties that impart the maximum damage resistance and hence, permit the retention of good optical quality in erosive environments. The principal material properties are the fracture toughness, hardness and elastic wave velocity. Microstructural effects on these properties are briefly discussed.
Continued increases in the speed of tactical systems have forced current ceramic radome materials to perform near their operational limits for thermally induced stresses. In addition, the all-weather requirements for emerging systems and the potential for erosion and fracture from particle impacts have necessitated the development of improved radome materials for these environments. Among the concepts being developed for these applications is a class of reinforced ablative materials which consist of polytetrafluoroethylene (PTFE) filled with a borosilicate particulate or chopped glass microfiber. RT/duroid is a material of this class and has attractive thermal and electrical properties. However, an accurate definition of the ablation-erosion and thermal performance of materials is required because transmission characteristics are sensitive to radome thickness and temperature. This paper reports the results of a combined experimental-analytical program that was conducted to define the thermal-ablation and erosion performance of RT/duroid 5870M, a candidate ablative radome material. The resultant thermalablation model is demonstrated to provide excellent predictions of thermochemical ablation and in-depth thermal response. The shape change of RT/duroid 5870M models in the clear air and rain environments of Holloman Mach 5 tests is also well predicted by a computer code that uses the ablation model and an erosion model based on work by Schmitt.
A summary is presented of the various techniques for measuring the resistance to failure from thermal stresses of optical ceramics - glasses, polycrystalline ceramics and composites. This includes lengthy discussion of the more popular and more useful test techniques and recommendations for use of these tests and other techniques for treating problems with thermally induced stresses in optical materials and components.
Aerothermal testing of two full scale magnesium aluminate spinel IR domes in a hot gas facility has been conducted at the Naval Weapons Center (NWC). The objective of the testing was to provide information on the stress levels at which the spinel domes would fracture. Initial tests showed the spinel domes would survive heating conditions which had caused thermal stress fracture in thirty previously tested magnesium fluoride domes. Test conditions were typical of the aeroheating experienced following a supersonic missile launch. In subsequent testing, spinel dome fracture was accomplished by increasing the severity of the heating conditions. Through the use of computer modeling, temperature and stress levels in the domes were calculated for both heating conditions. Predicted fracture stress for the dome was found to be very sensitive to the property data which was used in the computer models.
The effective optical absorption coefficient obtained by the slope method of laser calorimetry is presented for a number of materials at a variety of laser wavelengths. The materials studied include MgF2, NaC1, KC1, LiYF4, CaF2, LiF, YAG, A1203 (sapphire), Si02, BaF2, CORTRAN 9753 and 9754 glass, ZBT glass, GGG, Mg0•A1203 (spinel), ZnSe, MgO, Si, As2S3, ZnS, and calcium aluminate glasses. The laser wavelengths studied were 1.319, 2.7, 3.8, 5.3, 9.27 and 10.6 μm.
An outline is presented demonstrating how reasonable estimates of the rain environment, the desired operational capabilities of the aircraft or missile, and laboratory erosion data can be utilized with a rain erosion damage model to provide a fairly reliable estimate of environmental degradation of infrared-transmitting windows and radomes throughout a flight trajectory. If water drop interactions with the flow field around the vehicle are ignored, the suggested approach will provide an upper bound on the extent of the water drop impact damage associated with a particular mission. The design philosophy would then be that if this level of damage can be tolerated the actual impact conditions will not be detrimental. A wide range of flight scenarios can be examined on this basis. If the upper bound computation indicates that a level of damage will result which is well beyond the limit which can be tolerated, then aerodynamic analyses of water drop interactions with the flow field around the vehicle and a more complete statistical analysis of the impact damage will have to be undertaken.
This study represents the first attempts at a systematic study of light scattering in polycrystalline materials both in the visible and infra-red region where all parameters responsible for scattering are independently varied. Light is scattered by voids and second phase particles. Light is also reflected and refracted by grain interfaces of random crystal orientations. With sufficient optical density of the sample, multiple scattering from voids, grain boundaries or second phase particles becomes the significant scattering mechanism in polycrystalline material. The diffuse scattering envelope width is measured and various scattering mechanisms are identified with respect to the way they influence this scattering width. Existing theories are reviewed and it is shown how these theories can qualitatively account for the observed behavior.
Extremely pure glasses of poly(methyl methacrylate), PΜΜA, and polystyrene, PS, have been prepared for examination of bulk scattering and absorption properties. PS, which is atactic, forms glasses which show no excess light scattering whatever; these glasses resemble liquids. PMMA, on the other hand, is predominantly syndiotactic. Glassy PΜΜA has intense forward scattering from intrinsic heterogeneities characterized by a dimension of ~70 nm. The concentration and size of these structures are controlled by tacticity and molecular weight of the macromolecules and by the temperature history of the glass. These scattering studies have been complemented by laser calorimetry to measure optical absorption over the range of wavelengths λ = 490-680 nm. Here the differences in absorption are attributed to the chemical compositions of the two glasses. From the combined effects of scattering and absorption, total losses of 100-200 dB/km are anticipated from the bulk properties of polymer glasses.
Several methods exist for significantly' improving the strength and fracture toughness of ceramics. Analysis of these techniques indicates several should be applicable for optical window materials provided the morphologies and the physical and optical properties of the microstructural constituents are properly selected. Recent experimental results support the view that one of the most promising of these involves toughening with dispersions of (Zr,Hf)02, particularly when the dispersion size and refractive index match with the matrix can be suitably controlled.
The paper presents the results of a characterization study of selected materials for possible use as laser windows. The preliminary materials selection was based on known or calculated optical properties at the wavelengths of interest and the availability of the materials in sufficient quantity and size. The characterization includes optical, mechanical, and thermal property determinations necessary for the systems designer to make proper selection of a window material based on system requirements. The materials presently under consideration include A1203 (sapphire), Si02 (fused silica), Y3A15012 (YAG), MgO, ZnS, ZnSe, CaF2, BaF2, SrF2, LiYF4 (YLF) and various fluoride glass compositions. The optical properties for the wavelengths of interest will be presented and the problems encountered when designing with brittle materials will be discussed.
The performance advantages of using silicon for actively cooled mirrors are developed through a comparison with molybdenum mirrors, the type used most extensively for high-energy CW laser applications'' In particular, maximum temperatures and distortions are related to a power loading parameter. Factors which are likely to limit the operating capabilities of both types of mirrors are considered to assess their impact on attainable performance, as compared to potential performance. Because silicOn has not been used extensively for demanding thermal structural applications, mechanical property data has been quite limited. This paper summarizes recently generated property data for single crystal. Particular emphasis was placed on short-time strength because of the paucity of such data in the literature. The test specimens used were processed in the manner expected to be employed for actual mirror components. In this way, the strength test results should represent the behavior of the brittle silicon material in its as-used form. By considering the performance potential of silicon mirrors and the properties of this material, it is possible to outline the potential advantages of such optical elements and to identify the challenges which must be met if actively cooled silicon mirrors are to become a reality.
Optical figured surfaces were formed directly by hot pressing a fluoride glass in a closed die of tungsten carbide. Microduplication of the surface finish was also obtained. A glass composed of the fluorides of barium, thorium and zirconium or hafnium, and transmitting into the infrared out to 8 microns was press formed into an optical flat above its softening temperature and below its crystallization temperature. Also, small vitreous pieces were consolidated into larger pieces under moderate heat and pressure, thus avoiding the crystallization which occurs in large batches cooled from the melt.
This paper relates a technique for increasing the optical strength of single-crystal NaCl. The 1.06-μm pulse laser-induced damage threshold was increased by a factor of 4.6 by a temperature annealing process. The starting material for this work was single-crystal "laser grade" NaCl acquired from the Harshaw Chemical Company. The bulk laser-induced damage threshold of the crystal was measured prior to and after the heat treatment using a Nd:YAG laser operated at 1.06 pm in the TEM00 spatial mode with a pulse width of 9 nsec, full width at half maximum. After the laser-induced damage threshold of the untreated crystal was meas-ured, it was mounted in a quartz tube and placed in an oven for the heat treatment. The quartz tube was continuously flushed with dry nitrogen throughout the heat treatment. The NaC1 sample was slowly heated to 794°C (approximately 7°C below its melting temperature). The sample was maintained at this temperature for a short time and was then removed from the oven to allow rapid cooling. The specimen required repolishing after the heat treatment because of surface sublimation which occurred at temperatures near melting. After repolishing, the bulk laser-induced breakdown intensity was remeasured and found to be 4.6 times greater than the value measured for the untreated crystal.
We report the results of laser-induced damage and nonlinear absorption measured for CdTe and other selected II-VI materials. These studies were conducted using pulsed 1.06 pm radiation from a Nd:YAG laser. The laser pulsewidth was varied from approximately 40 psec to 9,000 psec (9 nsec). The lasernduced breakdown irradiance measured for CdTe over this pulsewidth range scaled as tp -1/2 [tp is the laser pulsewidth (FWHM)]. Nonlinear phtoacoustic spectroscopy was used to monitorPthe nonlinear and linear absorption in the samples in which laser-induced damage thresholds were measured.