Sapphire is an ideal choice for the transparent dome element in higher speed missile systems because of its high transmission, high strength, and thermal shock resistance. Sapphire domes for various missile systems are being produced but fabrication technology development is required for cost reduction and improved performance. Major costs of dome production are in fabrication. It has been shown that grinding to near-finished size is important to reduce fabrication costs and improve quality of finished domes. With this approach, optical distortion problems related to the anisotropic properties and subsurface damage can be minimized. Subsurface damage has been shown to reduce sapphire's strength. The depth of subsurface damage has been quantified and it has been shown that room temperature strength can be increased with a post-polish heat treatment.

The strength of sapphire decreases more rapidly with increasing temperature than does the strength of polycrystalline alumina and many other ceramics. Twinning on the rhombohedral plane (1102) at elevated temperature induced by compression along the crystallographic c-axis [0001] appears to initiate failure and accounts for the decreased strength. The tensile strength of sapphire along the (alpha) - [1120] or c-axes is constant to within approximately 30% between 20 degree(s) and 800 degree(s)C. Compressive strength along the (alpha) -axis is also constant to within approximately 20%. However, compressive strength along the c-axis falls by > 95% (from 2000 MPa to less than 100 MPa) between 20 degree(s) and 800 degree(s)C.

Aluminum oxynitride (ALON) is a polycrystalline ceramic material useful for windows, domes, and other optical elements. It is transparent in the visible to mid-wavelength infrared. Data is presented on the affects of impacts with sand and water particles on polished surfaces. Velocities up to 690 m/sec were used.

The affect of sample position and processing technique on the vacuum ultra-violet transmission and biaxial bending strength of magnesium fluoride crystal specimens has been investigated. It has been established that the transmittance at Lyman alpha for samples cut from the bottom third of a crystal remain reasonably constant but thereafter transmission begins to fall, the lowest transmission being from the top of the crystal. Three sets of samples cut from the same crystal but processed in different ways demonstrated that the processing techniques do not affect the measured transmittance. The results of the strength testing showed no correlation with position in the crystal but the type of processing significantly affected the strength measurements. Average strengths varied from 123 MPa to 167 Mpa depending on the processing technique.

Transparent (beta) -SiC has been fabricated by the pyrolysis of methyltrichlorosilane in a hot wall chemical vapor deposition reactor. Characterization of material indicates that transparent SiC is a theoretically dense, highly pure (99.9996%), highly oriented (111), (beta) -phase (cubic) material possessing high optical transmission in the wavelength region 0.5 - 6 micrometers , a high value of hardness, electrical resistivity and thermal conductivity, and a low value of thermal expansion coefficient. These properties make it a good candidate material for use as domes and windows in severe environments such as high speed missiles, laser, combustion and space systems. Important properties of transparent SiC were also compared with those of the opaque SiC to determine if optical transmission can be correlated with other material properties. A good correlation was obtained between material transmission and microstructure, crystallographic structure and electrical resistivity.

A glass ceramic with high transmission values up to 4.5 micrometers and with extremely low thermal expansion is presented. In addition a large variety in shaping processes, for instance blocks of large dimensions, tubes, domes, and capillaries, are addressed. Furthermore a sintered cordierite glass ceramic is presented combining a low value in relative length change, a high temperature stability and a high transparency in the visible and median range infrared region. The properties include a high Young's modulus and a high Knoop hardness relative to normal IR-glasses. The application of the powder processing technique can be used successfully to prepare fully dense, complex-shaped articles.

Electro-optical (EO) systems, the platforms they perform on, and their missions continue to place increasing requirements on the infrared windows and domes associated with these systems. Supersonic flight, observability, EMI shielding, sensor range, multispectral sensors, environmental degradation resistance (sand and rain erosion resistance), and affordability are some of the requirements that are rendering most current IR window and dome technologies inadequate. Texas Instruments (TI), through both IR&D and DoD contract work, has been developing enabling IR materials technology to address these critical needs. Specifically, our work on CVD diamond, gallium arsenide (GaAs), gallium phosphide (GaP), and polymers is described and compared with othe IR window and dome materials. Trade studies involving thermal shock, transmission, absorption, emission, strength, durability, protective coatings, EMI shielding, transmitted wavefront distortion, and material status are presented. High-speed IR domes for use with 8- to 12 micrometers sensors on Mach 3 or greater missiles will generally require diamond; slower missiles could use ZnS, GaAs, and possibly GaP. For Mach 3 or greater missile systems with IR sensors operating in the 3- to 5-micrometers range, GaP is the most promising material, with its higher thermal shock resistance and lower absorption and, therefore, lower emission at elevated temperatures at 3 to 5 micrometers than sapphire, spinel, yttria, ALON, Si or ZnS. For multispectral use (3 to 5 and 8 to 12 micrometers ) at supersonic speeds, GaP and multispectral ZnS are the candidates; low supersonic use could include GaAs; and subsonic use could include ZnSe. For IR windows and domes where EMI shielding is required, GaAs offers the highest shielding of any window with or without metal grids. Si, Ge, and GaP offer bulk electrical conductivity like GaAs, but, because of the intrinsic behavior of the carriers, cannot offer the same level of shielding. Durability is a growing concern on all IR windows and domes. Various coatings for rain and sand erosion including diamond, BP, and TI GaP, amorphous carbon and novel Ti IR polymers are discussed.

Renewed interest in the use of gallium arsenide as an external window material for passive IR imaging systems (FLIRS) has revived efforts to solve problems preventing its use. Among the most serious problems are size, doping to provide EMI protection, and coating to protect against erosion. This paper is a second work discussing the efforts at Amorphous Materials to solve these problems.

Germanium and silicon optical transmissive components are widely used in infrared lenses, filter substrates, etc. We demonstrate that severe loss can be induced in some germanium samples by illumination in the visible or near infrared with power densities in the region of watts per square centimeter. The absorption arises from light hole to heavy hole inter valence band transitions. The strength of the absorption induced depends on a number of parameters not normally controlled in optical applications, such as minority carrier lifetime and surface recombination velocity. The effect is very much weaker in silicon.

The erosion resistance of thin film epitaxial phosphide coatings has been shown to be affected by substrate crystal orientation when deposited on Ge substrates. GaP was systematically studied using optical microscopy, FTIR spectrophotometry, image analysis and x-ray diffraction. Microstructural aspects of the surface fractures indicated a clear crystallographic preference for erosion. X-ray diffraction identified the (111) plane as having the preferred crystal orientation for erosion resistance and variations of the (100) as the erosion prone planes. Removal of the hard carbon coating was observed in a large number of samples resulting in measurable damage to the GaP layer. Crack morphology was distinctly different in (111) and {100} crystals. (111) planes displayed erosion as discrete isolated pits and {100} planes developed very intricate orthogonal crack networks with pits at the intersections. Depth of damage varied as a function of crystal orientation and non (111) coating sections suffered some substrate exposure. Subsections of (111) protected Ge had an average depth of damage (measured vertically) of 0.47 mm and non (111) subsections had an average depth of damage of 2.3 mm. In most specimens, complex crack networks developed within 10 minutes of exposure to the simulated rain environment. The resultant GaP coating loss averaged 18 percent.

Germanium witness samples were impacted with the NAWCADWAR modified Cambridge liquid jet device introducing varying levels of damage about the center of each sample. Surface damage statistics were collected, scatter measurements were made at 0.67 micrometers and the samples were failed in tension using a bi-axial flexure test setup. The level and character of the damage was correlated with the reflected scatter measurements as a function of local stress and flaw size distribution. Bi-axial flexure data was analyzed to predict fracture stress and the probability of failure of the germanium samples. The mechanical data were then correlated with the scatter data in order to correlate the BRDF with the material failure. The BRDF measurements were taken in several different orientations in order to study the differences in scatter character for the in-plane and out-of-plane conditions.

Birefringence is an important factor in determining the imaging quality of visible and infrared optical systems. This paper presents residual birefringence data obtained at 0.6328 micrometers and 10.591 micrometers from several ZnSe and multispectral grade ZnS windows. Refractive index inhomogeneity tests were also performed on the samples at 0.6328 micrometers and their results are given. Residual birefringence data at 0.6328 micrometers is compared to data at 10.591 micrometers .

Because of its excellent thermal-mechanical properties, diamond is a promising infrared window material. With the development of chemical vapor deposition (CVD) diamond technology, diamond windows and domes are becoming a practical reality. The infrared transmittance of type IIa and CVD diamond was characterized as a function of temperature, and the room-temperature ultraviolet transmittance of type IIa diamond was also measured. These experimental results were interpreted in terms of intrinsic and extrinsic lattice vibration models and the Urbach tail and weak absorption tail models. The first measurements of the temperature variation of the index of refraction in the 10-micrometers region for CVD diamond were obtained on a sample that showed strong modulation due to interference. Transmittance was investigated in most of the transparent range of diamond, although the 8- to 12-micrometers region is emphasized.

Diamond is a covalent material with no fundamental infrared active vibrational modes. Intrinsic diamond is completely transparent below the band gap except for two-, three-, and higher-order multiphonon absorption bands located in the mid-IR. Multiphonon absorption band models have been successfully developed for ionic materials; this paper reports the first application of this model to a purely covalent material. To obtain a fit to the experimental data, the phonon density-of-states function required considerable modification relative to the other ionic materials studied. The Gaussian density-of-states parameters were significantly modified relative to typical values for ionic materials. However, the two-phonon red wing is poorly modeled by this density-of-states function. Diamond absorptance near 10 micrometers is of particular interest because many infrared sensors operate at this atmospheric window. Evidence indicates that the absorption in this region is caused by two-phonon acoustic-acoustic interactions. In most materials, pure acoustic multiphonon absorption is not measurable because it is obscured by strong one-phonon optical mode absorption. Diamond has very high acoustic frequencies owing to its strong bonds, and the lack of fundamental absorption unmasks the pure acoustic contribution. This acoustic contribution is modeled by applying a Debye acoustic phonon density-of-states distribution function. Good agreement with experimental data is obtained.

Through a joint effort between Texas Instruments (TI) and Olin Aerospace Company (OAC), a supersonic hydrogen arc jet for high-growth-rate chemical vapor deposition (CVD) of thick layers of optical quality diamond has been developed for flat substrates as large as 8 inches in diameter and for hemispherical domes measuring 2.5 inches in diameter. An overview of this deposition system is presented, along with optical and structural analysis of the subsequently deposited material. Efforts to monitor the plasma chemistry, along with in-depth computer modeling of the gas phase chemistry and hydrodynamics of this supersonic jet, in order to produce a higher-growth-rate process while maintaining optical quality, are discussed. The current status of the overall performance of this deposition system and the resultant material are presented in relation to the requirements for high-speed-missile domes and windows.

Diamond IR windows synthesized by low pressure activated vapor deposition techniques have been demonstrated by a number of organizations in recent times. Most of the optical properties of such windows have been shown to be identical to those of natural diamond. However for the practical utility of such windows the economics of manufacture have to be considerably improved. In particular the growth rates for large area windows have to be increased. Among the techniques being developed for the synthesis of diamond films and bulk diamond slabs the combustion flame synthesis process has some distinct advantages in terms of achieving high growth rates and low manufacturing costs attended by high quality. Using this approach optical quality diamond windows have been fabricated at growth rates of 5 to 10 microns per hour. This process also has the flexibility to control the microstructure of diamond to suppress columnar growth of diamond crystals and thus enhance mechanical properties. The current status of this technology is discussed in this paper including a presentation of the microstructural, optical, electrical properties and rain erosion data of combustion synthesized windows and coatings.

A 2-D axisymmetric model of a microwave plasma ball reactor, as used in plasma assisted chemical vapor deposition systems, has been developed. The model can be broken down into two main parts. Firstly, a model of the microwave coupling to the discharge and secondly, a collision-radiative-diffusion model of the discharge plasma. Self-consistent solutions are obtained incorporating these two components of the model. The model is able to compute the position, size, and shape of the plasma ball in the reactor in addition to the general discharge properties such as 2-D excited state density, electron density, and electron temperature profiles. In addition to describing the key features of the model, results are presented from initial calculations performed for a hydrogen discharge plasma. These results are compared to experimental measurements to show the predictive capabilities of the model. Also discussed are the latest improvements in the model, which allow it to deal with admixtures of methane and hydrogen. Particular emphasis is put on the key species for diamond deposition and how their concentrations vary with reactor conditions. The model represents a valuable tool for the optimization of diamond deposition systems, and as such, further extensions to the model to improve its predictive capabilities are discussed.

As grown, CVD diamond windows have low infra-red absorption, but are highly scattering in transmission due to the presence of crystalline facets on the material surface. Conventionally, these facets are removed by mechanical polishing techniques which are slow, not easily adapted to complex shapes, and which can lead to mechanical damage and loss of strength. In this work, an attempt has been made to use a patented plasma etching and deposition method to polish CVD diamond window material optically flat. Low pressure radio frequency (rf) discharges of a variety of plasma etchant gases (Ar, H₂ CCl₄, SF₆, CO₂) have been used to etch the diamond surface. Etch rates of 2000 angstroms/minute can be obtained using carefully optimized etch chemistries. It has been shown that plasma etching the diamond window under conditions which give a high self-induced dc bias causes preferential sputtering of the edges of microcrystallites and hence polishes the diamond surface flat. Certain plasma chemistries, notably those involving chlorine, have also been found to flatten the surface by preferentially removing the crystalline facets. By plasma depositing silicon oxide on the window material it is possible to planarize the surface prior to a plasma etch stage and then plasma etch away the silicon oxide and diamond in a subsequent etch stage so smoothing the diamond surface. The affect of these polishing methods on a variety of CVD diamond films is discussed and the limitations of the technique addressed.

This paper describes a new method for significantly improving diamond film quality and growth rate on insulating substrates and thin films. The usual method of abrading the substrate surface with diamond particles yields good quality CVD diamond films at reasonable deposition rates on semiconducting materials like silicon. However, on insulating materials like fused silica and sapphire, the conventional method of diamond seeding and surface abrasion almost always results in slow growth rates and poor quality films. Current in-house diamond nucleation and growth studies have focused on depositing CVD diamond on substrates such as fused silica, sapphire, and glass ceramics. Diamond was grown successfully on these types of materials using a sacrificial metal layer method called metal induced nucleation of diamond (MIND). This technique offers a way to deposit diamond on glassy materials with improved adhesion and at lower deposition temperatures (less than 650 degree(s)C). In addition, the MIND technique can be used in combination with metal masking and conventional etching to deposit patterns of diamond. The MIND method was combined with another in-house developed technique called sputtered refractory interlayer nucleation technique (SPRINT). Diamond-crystallite size and orientation can be controlled with SPRINT to fabricate low-scatter diamond films. Both techniques are discussed. A reliable, efficient method for growing diamond on insulating materials significantly enhances the feasibility for practical applications of CVD diamond technology. For example, further development of the MIND technique may provide low-scatter, protective diamond films on sapphire and glass ceramics for visible-wavelength windows and missile domes. For electronic applications, reduction in the growth temperature makes CVD diamond more compatible with existing semiconductor processes. The lower growth temperature also helps to alleviate diffusion problems in metal alloys and facilitates the application of diamond coatings to cutting-tool inserts.

Clear diamond windows with thicknesses of 0.3 - 1.0 mm and diameters up to 6 cm have now been produced. In the 8 - 14 micrometers infrared region, the absorption coefficient is as low as 0.1 - 0.3 cm^-1, optical scatter is below 1%, and emissivity is below 3% at 500 degree(s)C. Microwave dielectric properties, thermal properties, and most mechanical properties of chemical-vapor-deposited diamond are equivalent to those of Type IIa natural diamond. The mechanical strength of 0.5 - 1 mm thick chemical-vapor-deposited diamond attained so far is an order of magnitude lower than that of natural diamond, and is governed by microscopic cracks and defects.

Optical, and dielectric properties of free-standing plates of polycrystalline diamond grown by chemical vapor deposition (CVD) are reported and compared with Type IIa natural single crystal diamond specimens. Ultra-violet, visible, and Fourier transform infrared spectroscopies have been used to assess the optical quality of the material. It has been found that over most of the spectral range, except at short wavelengths close to the fundamental edge, the transmission of the CVD plates is almost indistinguishable from that of Type IIa natural diamond. In the visible and ultra-violet the transmission is reduced due to a combination of scattering and true absorption. The imaging potential of CVD diamond at 10.6 micrometers wavelength has been assessed by measurements of modulation transfer function (MTF). The intrinsic optical quality of the material is adequate for imaging in the infrared region but improvements are needed to planarize the optical surfaces in order to minimize astigmatism and lensing. Measurements of dielectric constant and dielectric loss tangent were performed at 36 GHz, 72 GHz, and 144 GHz microwave frequencies using an open resonator technique. Bulk values of dielectric loss tangent as low as 73 X 10^-6 have been observed. There is evidence that these values may still be affected by surface effects and that the true value for the bulk dielectric loss tangent in high quality CVD diamond plates studied in this paper could be as low as 30 X 10^-6 or lower over a wide temperature range up to 250 degree(s)C, the lowest value of loss tangent so far reported for CVD diamond.

Raman spectroscopy measures molecular vibrations by analyzing the frequency components of scattered laser light. It will provide information about the composition, crystallinity, stress, and spatial distribution of the various types of carbon found in diamond films. The general composition of diamond films can be investigated by monitoring bands in the Raman spectrum at 1332 cm^-1 for crystalline diamond, 1580 cm^-1 for graphite, and a broad band around 1350 cm^-1 for amorphous material. The bandwidth of the 1332 cm^-1 band is indicative of the crystal quality. Stress variations in diamond result in wavenumber shifts of the 1332 cm^-1 band in the Raman spectrum. The internal stress in differently oriented diamond films has been investigated using Raman microscopy and was found to vary along the length of a crystallite. Using Raman mapping, it is possible to determine the spatial distribution of diamond and non-diamond carbon on the surface of a diamond film. The resulting gray scale images allow the regions of high diamond concentration to be identified.

Studies of the short-time oxidative degradation of diamond have been conducted to better define the limits of application at high temperature. The infrared (IR) and visible optical performance of polished chemical vapor deposition (CVD) diamond windows were degraded after heating in a furnace at 800 degree(s)C in air for 75 s, while heating at 700 degree(s)C for 75 s produced little change. The 800 degree(s)C heating resulted in increased light scatter visually, and microscopic etched features were revealed by optical microscopy, scanning electron microscopy, and Talystep surface profiles. Many of the etched features are concentrated at grain boundaries while some features appear to reflect residual damage from mechanical polishing that could not be seen before the heat treatment. The IR transmission of samples heated in air at 800 degree(s)C was reduced due to scatter. The scatter at 10.6 micrometers increased from 0.8% for unheated material to 2.8% for material heated at 800 degree(s)C for 75 s and 6.2% for material heated at 800 degree(s)C for 255 s. Single crystal (110) natural diamond was also heated in air. Heating for 4 min at 800 degree(s)C caused little change in IR transmission, while heating for 9 min at 800 degree(s)C caused a significant loss in IR transmission.

The magnitude of the residual stresses in thick samples of polycrystalline diamond were measured by the sine squared, angular resolved x-ray diffraction (XRD) technique and by detailed analysis of electron channeling patterns from individual grains in polished diamond films using a scanning electron microscope. The XRD measurements were made on samples produced by both plasma torch and microwave plasma low pressure growth techniques with a range of microstructures. Results show that residual levels of stress +/- 0.3 GPa can be generated inside the thinner films on substrates by thermal expansion mismatches, while average residual stress in free standing 0.5 to 2 mm thick diamond plates is negligible. Within the individual grains of the thicker films, localized stress variations on the order of +/- 0.5 to 0.8 GPa can be distinguished.

A three-dimensional, high fidelity, dynamic finite element computational model has been developed to describe the impact of a waterdrop on a structured target. This approach allows the interaction between the waterdrop and the target to be described for both normal and oblique impacts. The target can be geometrically complex and the individual components can have a broad range of constitutive behavior. The computer model allows parametric variations in the geometric and material properties of the target to be carried out to determine the most productive directions for improving its rain erosion resistance. The computer analyses minimize the need for fabricating and testing numerous material iterations to optimize a layered window candidate. Examples of the computational results are presented.

The materials applied to infra-red window materials in the form of thin coatings for mechanical protection or for optical reasons, can often only be produced in that form. This means that characterizing the mechanical properties of these materials requires very specialized techniques. A range of such techniques have been developed at the Cavendish Laboratory for determining the thermal expansion coefficient, internal stress, thermal shock resistance, hardness, and Young's modulus as a function of temperature of such films and results from these experiments are given.

The multiple impact jet apparatus (MIJA) has generated a large amount of data on the damage inflicted on a material by high velocity liquid impact. The results presented in this paper include data on several protective coating systems, together with results from a study on the rain erosion resistance of natural and artificial diamond. Damage mechanisms resulting from liquid impact have been investigated by both high speed photography, and a video system for monitoring the extent of multiple impact damage. The results of these investigations also are discussed.

A solid particle erosion facility has been used to study the erosion resistance of a range of infrared materials. The materials include zinc sulphide, germanium, calcium lanthanum sulphide, natural diamond, and chemical vapor deposited (CVD) diamond coatings. The paper concentrates on providing quantitative data on erosion rates and the processes of material loss.

The use of diamond to coat ZnS discs of 1' diameter has been reported previously. In order to make this process commercially useful, diameters of greater than 4' are required. The process of scaling up requires issues such as uniformity, morphology, and IR quality of the coating to be addressed. The main advantages of a diamond coated IR window compared with a bulk diamond window are that the relatively thin coating would have good transparency in the mid IR region (3 - 5 micrometers ) and that the cost may be considerably lower. The latter would certainly be the case if the coating did not require post growth polishing. It has been estimated that in the next generation 8 - 12' diameter diamond windows, the required polishing may well be the most expensive part of the production process. This paper describes work using an 8' diamond coating system, and looks at issues such as IR transparency, surface morphology (surface scatter) and thickness uniformity, and how this process can be applied to coating large area IR windows such as ZnS and Ge. Evidence is presented that coatings of 10 - 20 micrometers thickness can be applied having good IR transparency with a sufficiently smooth as-grown surface such that scattering in the IR is negligible. This is compared with free-standing disks of mm thickness where surface scatter on the as grown face makes polishing essential.

LWIR windows are exposed to harsh conditions during high speed flight. These include high speed rain drop impact, sand abrasion, corrosion, and aerothermodynamic load. With the possible exception of diamond, there are no LWIR transparent window/dome materials which can withstand these various environments. Rain erosion protective (REP) and oxide based abrasion resistant/oxidation resistant durable antireflection (DAR) coatings have been developed for LWIR applications. These coatings have demonstrated a substantial degree of raindrop impact protection (i.e., damage threshold velocities of approximately Mach 1, for 2 mm equivalent waterdrop impacts at normal incidence). The combination of REP + DAR coating have also demonstrated excellent resistance to sand abrasion in simulated flight environments. The high degree of abrasion resistance makes the DAR coatings applicable to ground based systems, using ZnS and ZnSe, windows as well. An additional advantage of the Raytheon REP + DAR combination is that they are transparent from the visible to the LWIR (8 to 12 mm), making them suitable for applications requiring broadband transparency. Furthermore, the DAR coatings have protected ZnS substrates from oxidation at temperatures up to 1000 degree(s)C. The combination of ZnS REP coating and DAR coating are ideally suited for protecting high speed LWIR missiles from rain and sand damage during captive carry, as well as protecting the dome/window from oxidation during high speed flight. Data are presented to demonstrate the rain/sand and oxidation protection provided by these coatings. The REP and DAR coatings have been scaled up to coat windows and domes for far infrared applications.

This paper describes a novel infrared (IR)-transparent polymer engineered at Texas Instruments to protect IR windows. The polymer has greater than 80% transmission in the 8- to 12-micrometer range and can be combined with antireflection coatings to yield highly transparent IR window systems. The novel molecular structure of the polymer imparts a high, in-plane elastic modulus/strength to the coating, yet produces a lower-modulus material perpendicular to the plane. This very inexpensive coating can protect any IR window from both supersonic normal-incidence waterdrop erosion and damage by 200-mph blowing sand.

Pilkington Optronics (Barr & Stroud), in conjunction with the Defence Research Agency (Malvern, UK), has an ongoing development program for ultradurable coatings. Such coatings enhance environmental durability of infrared (IR) transmissive windows and domes for severe environments, such as those encountered in airborne systems. In particular, these coatings are required to provide protection against high velocity rain and sand impact. This program has to date produced one of the most effective sand and rain protective coatings, based on phosphide materials, specifically boron and gallium phosphide. The phosphide coatings are incorporated into anti-reflective (AR) multilayers, providing high transmission over the required IR waveband. Such AR coatings have been shown to be very effective in protecting windows and dome materials from rain and sand/dust impact damage. Results of single and multiple water jet impact tests, whirling arm rain erosion and sand erosion are presented. Current performance of AR coatings incorporating BP or GaP is presented. Combining BP and GaP in a composite structure, thereby maximizing IR transmission while maintaining sand and rain erosion protection, is described.

Some applications of diamond infrared windows will require protective optical coatings which not only boost IR transmittance but also protect the diamond surface from oxidation, as diamond surfaces are observed to oxidize in air above 700 degree(s)C. Three different concepts of protective optical coatings on diamond were evaluated using a calibrated plasma heating source, to determine how well they protect diamond surfaces from oxidation at high temperatures. Two of the three coating designs protected diamond surfaces from oxidation to peak temperatures exceeding 1300 degree(s)C, and to temperatures exceeding 800 degree(s)C for periods up to 10s. Microscopic examination of the films shows that the coatings underwent some degree of morphological change, but afforded diamond excellent oxidation protection nonetheless.

Antireflection coated ZnS and Ge substrates erode under severe operational environmental conditions. High velocity water drop impact and high velocity sand particle impact are primarily military concerns that originated with the advent of faster aircraft. High speed flight through rain and sand storms seriously erodes forward facing components such as infrared transmitting windows and/or domes. This erosion of windows and/or domes causes reduction in transmission, resulting in the reduction of detection and recognition sensitivity of the electro-optical sensor. A single film of one quarterwave thick hard-carbon coating has been used on germanium to increase optical transmission (reducing Fresnel's reflection losses on Ge surface) as well as to reduce rain and sand impact damage to some extent, at a lower speed. At high speed, the damage becomes more severe, resulting in unacceptable large transmission losses. Recently, new hard carbon coatings have been developed for Ge which have substantially increased the damage threshold of the coated substrates. The rain erosion test was performed at Wright-Patterson AFB facility in Dayton, Ohio, and the sand tests were performed at PDA Engineering in Santa Ana, California. In addition, a multilayer AR coating utilizing hard carbon film as one of the low index films has also been developed at Hughes for ZnS substrates. The optical properties, rain erosion, sand erosion, and sand abrasion test result of these coatings are also presented in this paper.

Electromagnetic interference (EMI) shielding of windows and domes in electro-optical (EO) systems can be accomplished by transparent electrically conductive (EC) coatings or patterned opaque EC coatings. The shielding effectiveness (SE) of these two methods was studied both analytically and experimentally. Shielding due to both absorption and reflection as a function of frequency was evaluated. Infrared EO systems often use windows or domes made of semiconductive material. These semiconductive materials also provide some EMI shielding for the system. The SE of some typical exemplary semiconductive windows was calculated. The broadband shielding benefits of using EC coatings deposited on semiconductive window substrates were examined.

A high performance, electrically conductive indium-based film for application to visible/near IR windows has been developed. It provides a low sheet resistance of 7 ohms/square along with high optical transmission (88% average from 400 to 1200 nm with the back surface AR coating). These films have been shown to be highly durable. Their combined performance is superior to the best conductive films reported in the literature. This new film is production ready and can be applied to substrates up to 20 inches in diameter.

Electromagnetic windows generally require low dc resistance for radar performance and high optical throughput for visibility. For obtaining high optical throughput and desired EMI attenuation, the use of metal grids on windows should be considered. This paper reviews many of the properties of EMI grids that are useful to an EMI window designer.

A central depository for electromagnetic (EM) window material property data and information about survivability issues is necessary due to a vast amount of data being available which is dispersed throughout the EM community. The purpose of this paper is to discuss the development of a material property database and handbook on EM window materials. The proposed format of the handbook/database is given as are the key topics that are included.

Basic concepts of statistically distributed flaws and moisture-enhanced growth of cracks under stress are used in a nonparametric bootstrap analysis to assess the reliability of dual-pane glass aircraft windows. Statistical distributions of window strengths, evaluated by rapid loading in an inert environment, are analyzed as three-parameter Weibull distributions using a maximum likelihood procedure. Strength distributions for a variety of pane surface conditions are evaluated, for example, as-ground and polished surfaces, which are typical of a `protected' inner window pane, and outer pane surfaces with various types of simulated in-service `damage,' e.g., airborne dust impact, windblown sand, and cleaning/handling scratches. The crack growth parameters needed to assess the time-dependent crack growth behavior are determined at room temperature in a water environment via dynamic fatigue, or constant stressing rate tests on indented specimens. Predicted lifetimes at a 95% confidence level are ascertained for various window scenarios at a 99% survival probability via a Monte Carlo simulation using a nonparametric bootstrap procedure.

A failsafe design of a BK-7 glass window for commercial aircraft use is presented which meets the stringent requirements of the Federal Air Regulations. This may be the first ever commercially approved all glass design for aircraft use in the USA. An all glass design is often essential for both visible and near infrared photography in order to meet the stringent specifications and resolution demanded by high acuity optical systems. However, glass is subject to slow crack growth and static fatigue in the presence of moisture and tensile surface flaws. Exposed surfaces on aircraft are subject to scratches from handling and cleaning, as well as impact due to hail, windblown sand, and high-altitude high-velocity airborne dust. Additionally, defects are present due to the manufacturing process itself. Such flaws may grow quickly in the presence of high stress, as induced by cabin internal pressure, thermal gradients and soaks, aircraft racking loads, and aerodynamic pressure. In order to yield an acceptable design, a dual pane glass concept is presented which exhibits a safe life in excess of 10,000 hours and is also failsafe in the unlikely event of a catastrophic failure of one of the panes. Reliability of the design is based on extensive coupon testing by the National Institute of Standards and Technology, validating crack growth parameters and survival probability. These coupons were subject to induced scratches, hail, sand, and dust impact (performed by General Research Corporation). A full-scale failsafe test is also made to show the ability of the dual pane design to withstand maximum loading in the event of a single pane failure.

Analysis of 68 condemned LANTIRN navigation pod FLIR windows was undertaken to determine the nature and extent of damage to these windows. Visual and low-magnification examinations using reflected and transmitted light conditions were performed, as well as profilometry and scanning electron microscopy (SEM) examination on selected specimens. A number of primary modes were found which accounted for the majority of the failures seen in this population of windows. These modes were: high energy impacts due to large objects such as hail, birds, and runway debris; interaction of the residual stress state at the interface of the bulk ZnSe/ZnS coating with rain and bug strikes; and opacification due to sand erosion and atmospheric etching. Machining damage and misoriented window installation were also found. Windows which had seen appreciable hours of service were almost completely devoid of AR coating on the forward face. A navigation pod, which houses the window, was also obtained to determine if the window installation contributed to the causes of failure. Suggestions to improve the reliability of the present window material were listed.

The purpose of this paper is to provide a firm analytical base for assessing the performance degradation that affects infrared (IR) missile seekers as a result of aerodynamic heating of the protective window or dome. On assuming that the system under consideration operates in a pulsed mode and is limited only by random fluctuations in the arrival rate of background photons, difficulties encountered in evaluating the signal-to-noise ratio can be avoided by writing SNR equals H_eff/NEI, where H_eff represents an effective target irradiance and NEI is the system's noise-equivalent irradiance at the peak-response wavelength of the detector. The NEI characterizes the capability of the system and is best expressed in terms of a `dark-system NEI,' which refers to the noise-equivalent irradiance in a laboratory environment, and a `photon noise factor' that relates noise originating from the background scene, the infrared window, and the detector surroundings to the photon noise of a dark system. A set of equations is derived for evaluating the performance degradation resulting from the photon exitance of the `hot window,' taking into account the effect of radiation- shielding procedures. The missile window problem is then formulated in a general but compact form compatible with the state of the art of IR technology. An illustration is provided in the context of assessing the feasibility of using pyramidal domes for dual-mode air-to-air missile applications; it is shown how to evaluate the IR performance degradation induced by aerodynamic heating of such domes under conditions that are representative of anticipated thermally most severe trajectories for internal as well as external missile-carry configurations.

Protective windows and domes on air vehicles such as aircraft and missiles must be efficient aerodynamically, and also they must be acceptable from an optical standpoint. Flat windows have essentially no effect to the optical performance, however they are extremely blunt and not efficient aerodynamically. Concentric domes are reasonably efficient aerodynamically, and if the imaging sensor optics gimbals around the center of curvature of the concentric dome, then the optical aberrations can be easily and effectively corrected for all fields of regard away from the nose. For high speed applications such as with missiles, concentric domes have been a standard for many years. Unfortunately, for extremely high speed applications where aerodynamically induced drag is a problem, concentric domes are simply not adequate, and more aerodynamically efficient shapes such as tangent ogives, must be used. For many applications using today's state of the art IR focal plane arrays the optical performance must be close to diffraction limited. While correction of the residual optical aberrations of a concentric dome is quite trivial, the highly aspheric shape of a tangent ogive introduces significant asymmetrical aberrations which change dramatically with field of regard. In this paper we discuss some recent developments using binary optics for correcting these optical aberrations. With the approaches outlined herein, the heretofore impossible task of imaging through a tangent ogive pointed dome is now shown to be possible.

A Delphi technological forecast has been conducted for optical window materials in the 2 - 25 micron range for hypersonic interceptor application in the endoatmospheric regime. Materials technology results include near-term, mid-term, and far-term forecasts. Results also include cooling, manufacturing, bonding, and coating technologies. Scientific breakthroughs and technology advancements in the commercial and military communities are discussed in the context of electro-optical systems.

Faceted seeker domes have attributes which make them desirable for use as apertures for many types of missiles, and they exhibit only minor loss in electro-optical performance over counterpart hemispherical domes. Results of tests demonstrating transmission and modulation transfer function of faceted domes developed for use with midwave infrared sensors are discussed.

In the development of infrared seeker systems, window heating effects that result from missile flight aerodynamics are critical design considerations. For typical launch scenarios, window temperature rises rapidly during the missile's initial launch phase and usually reaches temperatures in excess of 300 degree(s)C by flight termination. This paper formulates an analytical approach for calculating window emittance and radiance on the seeker's focal plane as determined by window temperature profile, spectral transmission band, window physical parameters, and optical seeker design. Radiance levels on the seeker's detector array are calculated for two window materials, sapphire and gallium phosphide, that efficiently transmit in the spectral bands from 3.0 to 4.0 microns and 3.0 to 5.0 microns.

Environmental conditions may adversely affect the optical performance of windows used in electro-optic (EO) imaging systems. Recent advances in the sophistication of optical software packages, coupled with finite element analysis (FEA) techniques, enabled the construction of a detailed window model. This model was used to analyze the window parameters that are affected by travelling through the atmosphere at high velocities. This paper presents the results of an analysis performed for a single-pane window used for operation in the mid infra-red (MIR) spectral region.

The possibility of using selectively low emission to reduce thermal signature of domes is pointed out. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High density, polycrystalline beryllium oxide is identified as a material with low emittance in the primary atmospheric window 8 - 13 micrometers , owing to a strong reststrahlen band. Bulk reflectance spectra are reported for ceramic BeO of three grades and are used to calculate the average, bulk near normal emittance over various possible detector ranges as well as the radiance for a 50 degree(s)C BeO surface. The results vary: 0.3 - 0.6 and 10 - 37 W/m², sr respectively, depending upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3 - 5 and 8 - 13 micrometers ranges. The possibility to reduce the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured numerical results for a 0.8 micrometers silicon overlayer on BeO are reported.

Infrared domes on guided missiles are aerodynamically heated nonuniformly. Because the index of refraction is temperature dependent, the temperature gradient can cause image blur and bore-sight error. The temperature dependence of the infrared refractive index has not been directly measured in sapphire, a popular dome material. However, an accurate model of the temperature-dependent infrared refractive index can be obtained from visible measurements of the refractive index, far-infrared reflectance measurements, and infrared measurements of the absorption coefficient. Visible measurements determine the contribution to the refractive index from electronic transitions. Far-infrared measurements determine the contributions from fundamental lattice vibrations (phonons). Infrared absorption data are used to determine parameters in a multiphonon sum band model. By applying the Hilbert transform to this multiphonon absorption model, a model for the multiphonon refractivity is obtained. Two- and three-phonon contributions to the refractive index are important for an accurate model that includes temperature dependence. Results for the ordinary ray and extraordinary ray are obtained.

A mosaic design for a CVD diamond window is described as an alternate solution to a monolithic diamond window. Experimental results are shown for two different brazing techniques to bond four, one inch square diamond tiles to ceramic frames. The two brazing techniques used are (1) conventional brazing where the entire assembly is heated in an inert atmosphere to a temperature dictated by the braze material melting temperature and (2) high power, high frequency pulsed microwave brazing. The benefits of microwave brazing, such as selective heating of the different components, optimization of the mechanical properties, the size of the brazed joints and efficiency, are briefly discussed.

The infrared transmittance of a high-optical-quality chemical vapor deposition (CVD) diamond sample was independently measured by The Johns Hopkins University Applied Physics Laboratory, Rockwell International, and Texas Instruments, Inc., from room temperature to 780 K.At0.75 mm thick, the sample is one of the thickest high-optical-quality CVD diamond samples produced so far. Measurements were taken with both dual-beam grating spectrometers and a Fourier transform spectrometer so that experimental artifacts of a particular instrument could be identified and reduced. The data sets were in good agreement from room temperature to 780 K. This work definitively establishes the high-temperature optical properties of CVD diamond.

Measurements of the influence of sand/dust impact erosion on FLIR ZnS, coated with a range of durable coatings, have been made using spectral transmission. This activity has been carried out as part of a program to identify a sand/dust and rain erosion protective coating, for use on the Martin Marietta LANTIRN E-O system front element ZnS/ZnSe laminate window. Assessments have been carried out against MIL STD 810E sand/dust erosion specification, using the Defence Nuclear Agency sand erosion facility. A range of sand particle diameters (< 38 to 177 micrometers ), and velocities 45 to 206 m/s), has been investigated. Three anti- reflective durable coatings have been tested: thorium fluoride, diamond like carbon (DLC -- supplied from three separate vendors), and boron phosphide (BP) overcoated with DLC. BP/DLC showed no significant erosion damage or change in transmission for the range of test conditions, while DLC and ThF₄ exhibited significant erosion damage and consequent transmission reduction. A basic theory of solid particle impact is used to model transmission reduction in relation to coating hardness, velocity, and size of impacting sand particles. The concept of `erosion parameter and threshold velocity' is introduced, which provides a means of predicting transmission reduction for a range of sand erosion conditions.

Pilkington Optronics (Barr & Stroud Ltd) has an ongoing development and pre-production activity for ultra-durable coatings. Such coatings provide enhancement of environmental durability of infrared transmissive windows and domes on airborne platforms. This activity places particular emphasis on providing protection against rain and solid particle impact at airborne velocities. This program has produced a very effective rain and sand erosion protective, anti-reflective multilayer, based on boron phosphide overcoated with diamond like carbon (DLC/BP). This coating has been demonstrated on a range of infra-red (IR) materials: germanium, FLIR ZnS, TUFTRAN, silicon and gallium arsenide. This paper describes a pre- production program of work to coat the external, convex face of the F14 aircraft IR search and track dome with DLC/BP. This is a 9 inch diameter, extended hemispherical germanium dome. Results are presented of coating uniformity, optical performance, UDRI whirling arm rain erosion, single water jet impact assessment and fracture toughness enhancement.

Thick germanium carbide (GeC) and boron phosphide (BP) films are successfully grown on various zinc sulfide and germanium substrates at temperatures up to 450 degree(s)C by reactive radio-frequency sputtering (RRFS). The sputtering conditions are respectively a germanium target within a medium of methane-argon for GeC films and a high density boron target in a sputtering medium of phosphine-argon for BP films. The rain erosion resistance of GeC and BP films protected or not by diamond-like carbon (DLC) coating on top are measured for water drop diameter of 1.2 mm or 2 mm with an impact velocity ranging from 210 m/s to 265 m/s on the Saab-Scania whirling-arm rig facilities (Linkoping, Sweden). Rain erosion resistance of BP films for a wavelength band in the 8 micrometers to 10 micrometers range shows no damage for a speed up to 250 m/s with an exposure time up to 10 min, whereas the GeC rain erosion resistance shows no damage up to 235 m/s for the same exposure time. The transmission of each film is well correlated to its optical absorption at 10.6 micrometers . The GeC absorption can be reduced down to 40 cm^-1 whereas the BP absorption stays around 220 cm^-1 for sputtered films. So the compromise between the optical performance and the rain erosion resistance can be achieved by the use of GeC or BP films.

Quantitative measurements of high speed rain drop impact damage to FLIR grade ZnS are reported for oblique angles of incidence. Specimens of ZnS and PMMA calibration witness pieces were tested at normal incidence and a 10 degree(s), 20 degree(s), 30 degree(s), and 45 degree(s) angle of incidence (AOI) as measured from the normal. The number of drop impacts on the PMMA increase slightly at the small angles over the value observed at 0 degrees. The density of impact events observed on the ZnS specimens increases more dramatically with small angles of incidence before dropping markedly at 30 degree(s) and 45 degree(s). Geometric considerations suggest a cos (theta) dependence. However, differences in air flow around the specimens may be responsible for the anomalous observations. The enhanced effects observed for the damage to the ZnS are not unique and support earlier observations by Gorham and Fields (Wear 41 (1977) 213 - 222). Measurements of the fracture stress also correlate with the angle of incidence. There is a reduction in strength at 10 degree(s) over normal incidence but thereafter the failure stress increases dramatically with increasing angle, which is indicative of a reduction in the magnitude of raindrop induced flaws. Specimens were tested at two velocities, 168 m/s and 210 m/s.