Loss of mechanical strength at high temperature limits the thermal shock resistance of sapphire IR windows and domes. The critical weakness is loss of compressive strength along the c-axis of the crystal. Compression causes twinning in the crystal. When twins intersect, a fracture forms at the intersection. The fracture initiates mechanical failure if sufficient tensile stress is present. Twinning is a property of the perfect crystal. Strategies for increasing the high- temperature compressive strength of sapphire are based on introducing crystal defects to inhibit twinning. Doping with Mg²⁺ or Ti⁴⁺ can double the compressive strength at 600 degrees C. Ion implantation increases the tensile strength by a factor of 2 at 300 degrees C. Placing a thin sheet of graphite between sapphire and the test fixture in mechanical tests apparently reduces contact compressive stresses and increases the apparent strength of sapphire in many kinds of mechanical tests at 600 degrees C.

Single crystal sapphire shows a rapid loss of c-axis compressive strength at high temperature. The strength degradation is attributed to the rhombohedral twinning which leads to crack initiation and propagation. This work takes a critical look at the rhombohedral twinning in sapphire via controlled c-axis compression tests. It is shown that surface/subsurface damage due to machining has an important role in controlling the twin initiation process. The removal of the machining damage results in a more than two-fold increase in the compressive strength of sapphire at 600 degrees C.

Sapphire's strength at elevated temperature is highly dependent on the test conditions. Tests that involve compressive forces at localized contact point can cause failure strengths due to the rhombohedral twinning. High contact stress can result at the load points due to roughness of surfaces. A thin sheet of Grafoil serves as a complaint layer between the load surface and the specimen and reduces the contact stress, and this increased the compression strength by a factor of 4 and the biaxial flexure of c oriented specimens by 2X at 600 degrees C. The strength reported by different testing facilities was comparable when Grafoil was used. The use of Grafoil has made it possible to evalute the effect of process parameters on the compressive and biaxial flexure strength at 600 degrees C.

Rhombohedral twinning is responsible for loss of c-axis compressive strength of sapphire at high temperatures. Loss of strength is shown to be due to cracking caused by intersecting twins. Single crystal sapphire was doped with both titanium and titanium ions in concentrations from 0.05 percent wt. to 0.25 percent wt. It is shown that both valence state and concentration of titanium are important is lowing down twin boundary movement and increasing the c-axis compressive strength of sapphire. Titanium doping did not cause any change in thermal conductivity and IR transmission of sapphire.

Heat treatments of finished sapphire compression and biaxial flexure specimens increased sapphire's high temperature strength. Heat treatments of sapphire specimens at 1450 degrees C for 48 hours in an air atmosphere enriched with oxygen increased the compression strength by 60 percent and biaxial flexure strength at 600 degrees C by 45 percent over untreated samples.

The 4-point flexure strength of sapphire in two different crystal orientations was measured between 227 and 600 degrees C. Two types of coupons were fabricated in a manner intended to mimic the way a hemisphere dome would be made. Bar 1 fails in tension at low temperature and from c-axis compression at elevated temperature. Bar 2 fails in tension at all temperatures. As-polished Bar 1 and Bar 2 in the present work ar only about half as strong as comparable bars from previous work. Annealing increased the strength of Bar 2 by 50 percent at all temperature and Bar 1 by 25 percent at low temperature. Annealing did not strengthen Bar 1 at 600 degrees C, at which temperature it fails in compression. A compressive coating had little effect on the strength of annealed bars.

Of current interest in the missile community is the high- temperature mechanical behavior of single-crystal sapphire. For endo-atmospheric IR transparent windows, single crystal sapphire is the material of choice. However, sapphire has been found to undergo a significant change in the mechanical properties, leading to a potential fracture of the window at the high temperatures encountered during typical flight conditions. A critical figure of merit used in considering a material's usefulness as a dome material is its thermal shock resistance.

High quality measurements of the IR refractive index of sapphire, above room temperature, have not been previously reported. Such knowledge is crucial for performance studies involving sapphire optical components, such as a window or lens. The temperature dependent refractive index of the o- ray of sapphire is measured by determining the change in free spectral range of sapphire etalons with temperature. The experimental temperatures range from room temperature to 720 C. One sample is used with the c-axis normal to the sample surface allowing measurement of the o-ray refractive index.

Sapphire is an ideal visible-MWIR window due to its excellent optical and mechanical properties and its availability in large sizes up to 340-mm diameter boules. Anticipated applications for new, high performance optical systems call for even larger, 450-750 mm diameter, windows. The present effort has focused on producing 500-mm diameter sapphire boules using the Heat Exchanger Method. Three experimental growth runs demonstrated the feasibility of producing 500-mm diameter sapphire boules. Completely crack- free boules have not been grown, but large size sapphire pieces up to 400 mm by 280 mm have yielded from these experimental runs.

Sapphire is being used more frequently as an optical window material in conjunction with optical sensors operations in the UV and in the visible to MWIR bands. This paper addresses two related development efforts. The first is associated with the development of a thick Al₂O₃ coating to be deposited over gridded sapphire windows operating in the visible to MWIR optical band. This coating serves as an environmental barrier - in particular to protect the grids against high-speed rain impact. The second is associated with the optimization of the optical transmittance properties of the environmental barrier for UV applications. This paper describes each area of development, the resulting coatings, the highly successful performance results achieved.

Sapphire is an ideal optical material and is in used for window and dome applications. The anisotropic properties of sapphire affect the production of high-quality components. Out of the three major orientations, c-axis, a-axis or m- axis, the c-axis is preferred for optical applications as it is the zero birefringence orientation. This orientation is difficult to grow with high quality. Therefore, components are fabricated by sectioning from the sides of a- or m- boules. The anisotropic properties also present problems in grinding and polishing windows for precision optical applications. The degree of difficulty varies with the orientation selected. For hemispherical domes involving polishing of several orientations, it is difficult to achieve a good figure. The choice for larger diameter windows is limited to a- or m-orientation; the m-orientation may be preferable due to the geometry of fabrication-induced stress.

There are several crystalline materials that transmit electromagnetic radiation in the visible and IR portion of the spectrum. At this time, single-crystal sapphire, aluminum oxynitride (ALON), and spinel show promise for applications, including advanced electromagnetic windows and transparent armor. These applications require materials with high strength, hardness, and the ability to withstand high temperatures. Because of lower processing temperatures and shorter processing times, it is reasonable to assume that spinel should ultimately be less costly to produce than ALON or sapphire. Despite many attempts to commercialize spinel, it is not available today as an optical materials due to difficulties in reliably obtaining the desired transparently. To help develop a commercial source for transparent spinel, the US Army Research Laboratory and Ceramic Composites Inc. of Annapolis have signed a Cooperative Research and Development Agreement on the 'Development and Dual-Use Assessment of Transparent Spinel'. The advent of commercially available, highly pure spinel powders should lead to improvements in processing spinel to transparency. This investigation compares the advantages and limitations of hot-pressing, microwave sintering, and rate- controlled sintering and compares the limited property data available from each of these fabrication techniques.

A common window material for midwave IR systems is polycrystalline magnesium fluoride. This material is now only available from a French manufacturer, Ceramiques Techniques Desmarquest. The optical constant database on this material is based on samples produced by Kodak and Bausch and Lomb. Because the optical constants are extrinsically dominated in the midwave IR, a new characterization study is needed. The IR absorption coefficient is presented as a function of temperature and frequency. Also, the scatter properties are determined by the bidirectional scatter distribution function.

In the application of laser ignition to large caliber cannons, a critical element is the window into the cannon chamber to admit the laser energy. This window must repeatedly withstand a particularly harsh environment of highly reactive high temperature combustion products from the gun propeller at pressures up to 440 MPa. Failure of the window can be caused by either thermal gradients in the window or mechanical force, or a combustion. Previous successes with single-crystal sapphire have sometimes been limited by window deterioration modes suggestive of crystalline behavior. Samples of ALON have been fabricated in the same design as the standard sapphire windows and were qualified for gun testing. This process involves a series of experiments in a closed chamber where gun propellant is burned to generate an environment similar to that inside the gun. Windows mounted in two methods have been tested. One of these windows has survived the full pre-gun test series with no visible damage.

This paper will report on the properties and applications of polished polycrystalline diamond optical components produced by Chemical Vapor Deposition and fabricated in sizes and geometries suited to several IR window and some applications. Data will be presented on optical quality CVD diamond materials that has been synthesized in a production plant and then optically finished sufficiently flat and parallel for operation as passive IR imaging windows or exit windows for high power CO_s lasers. The processing tolerances achieved on the windows as well as data on the material's transmission and IR imaging properties will be presented. The current state of the art in the manufacture of CVD diamond optical components will be presented and we report for the first time: (1) precision, planar IR imaging windows up to 120 mm in diameter, (2) 70mm diameter, fully polished hemispherical domes, and (3) shallow diamond lenses suitable for use as laser output-coupling windows. Polished CVD diamond windows and domes are very well suited to exploitation in adverse environments; in particular, for use as protective windows in passive long wave IR imaging applications. It is essential that the precision and finish of the components are suitable for such uses and this paper will describe the characterization of the geometrical tolerances that can currently be achieved with CVD diamond as well as other relevant properties.

Micron-size cracks and voids produced during grinding and polishing may be responsible for sapphire's loss of flexure strength at elevated temperatures. The ability to fill these micron-size voids and cracks with a compressive coating may remove the crack-initiating defects that lead to loss of strength. It is well known that compression of c-axis sapphire can cause twinning on rhombohedral crystal p;lanes, especially at elevated temperatures. If twins on two different planes intersect, a crack will form and the sapphire will fail in tension. In ring-on-ring biaxial flexure test, microtwins could form where the load rings contact the c-axis sapphire. A coating should mitigate the very high compressive stress produced at the surface of the sapphire by the load rings. Results presented here for ring- on-ring biaxial flexure test show that compressive coatings increase the fracture strength of c-axis sapphire by a factor of about 1.95 at 600 degrees C. Additional results show that the coating do not significantly strengthen sapphire at ambient temperature. This is not surprising since sapphire already is very strong in compression and tension at ambient temperature.

The protection of IR windows in airborne FLIR sensor systems against erosion resulting from raindrop and particle impact is accomplished by means of a front surface coating. The wavelength ranges required are 8-14 micrometers , where diamond competes with boron phosphide based coatings for the protection of multispectral zinc sulphide used as a window for multiple detectors. This paper describes progress in the development of diamond coatings for germanium windows, including selection and deposition of durable front surface antireflection layers. The diamond layers are deposited by microwave plasma CVD techniques at 500 degrees C. For the multispectral application, hard oxynitride coatings have been developed both as stand-alone coatings and as interlayers for diamond coatings. The multispectral coatings and the antireflection coatings are deposited by a sputtering process, applicable to flats and domes. In both cases, structured surfaces at appropriate scales are used to improve optical transmission and mechanical adhesion.

Boron phosphide (BP) films deposited by plasma assisted chemical vapor deposition are known to be very effective in protecting substrate materials from solid and liquid particle erosion encountered in high speed flight. This paper describes the result obtained by protecting the 3-5 micrometers band substrate material silicon and the soft 8-11.5 micrometers band substrate materials zinc selenide. In addition results are presented to show that germanium can be very effectively protected from the corrosive effects of salt water and salt fog.

Diamond is an ultra-durable material with high thermal conductivity and good transmission in the visible, near IR and far IR wavebands. Advances in the performance of synthetic diamond made by chemical vapor deposition promise an expanding range of applications for the material. An example is in advanced airborne windows and domes for high- speed flight, either as a window or as a protective coating for other IR window materials. Diamond has sufficient durability to withstand high-speed impact by solid particles and raindrops and a high level of thermal conductivity to minimize the effect of thermal shock due to aerodynamic heating. However, diamond is subject to oxidation in air at temperatures greater than 750 degrees C. After only a few seconds exposure at such temperatures the diamond surface becomes severely etched, and the optical transmission is degraded. Very high-speed flight can lead to temperatures in excess of 800 degrees C. This means that, in high temperature applications, a coating is required which can protect the diamond surface from exposure to air. In addition the coating must have excellent adhesion and mechanical durability, and itself be resistant to impact. Using sputtered coatings based on aluminium nitride, we have demonstrated complete protection for extended exposures at temperatures up to 1000 degrees C. The coatings also have excellent mechanical durability as demonstrated by particle erosion test.

The structural, optical and mechanical properties of CVD diamond grown using chemistries on the H-CO tie line have been investigated. A microwave plasma CVD system has been used with methane and ethylene containing gas chemistries to grow free standing optical quality diamond layers. When these feed stock gases are combined with carbon dioxide and hydrogen they enable the H-CO tie line to be traversed up to the central region of the Bachmann growth diagram. The structural properties were assessed using SEM, cathodoluminescence, Raman spectroscopy and x-ray diffraction techniques. The optical properties were assessed using several techniques including measurements of spectral emissivity over a range of temperatures and the role of nitrogen impurities identified. The trends in the optical characteristics will be discussed in relation to differences in mechanical properties with a view of evaluating the viability of using oxygen-based chemistries for the fabrication of diamond components.

Applications of optics have traditionally conformed to the difficulties and limitations imposed by the practical limits on designing and fabricating optical systems. The operational efficiency of devices, especially military systems, has frequently been compromised by the need to accommodate an adverse environment in a non-optical manner.

This paper discusses a novel approach to correct conformal missile dome aberrations with the addition of unique correction elements. For purposes of this study, an elliptical conformal dome with a fineness ratio of 1.0 and an index of refraction of 1.7 is used. A rotationally symmetric element, referred to as a 'fixed corrector', is capable o f some correction for roll-nod or azimuth- elevation gimbaling schemes. A non-rotationally symmetric corrector, referred to as an 'arch corrector' is capable of correcting roll-nod gimbaled missile seekers. Both methods are compared and the performance in terms of aberrations versus gimbal angle are reported.

Aerodynamic requirements on airborne optical system have brought about the need to develop lower drag conformal domes. Because these domes typically deviate greatly from spherical surface descriptions, large amounts of aberrations are induced which vary with line of sight through the dome. Several solutions to this problem have been investigated, one of which is the use of translating phase plates to dynamically dial in the appropriate amount of aberration correction. Axially translating phase plates can be described as two nominally plane parallel phase plates with matched aspheric surfaces on their inner surfaces. When placed in contact, they behave as a single plane parallel plate, but when an axial separation is introduced, the optical ray passing through the first plate intersects the second plate at a different location resulting in both a change in optical path length and a set of induced aberrations. A mathematical derivation of the aberrations generated is performed for Zernike polynomial surfaces in the presence of both converging and collimated beams. Code V is used to verify the derived expression and the theory is used to describe the results of a previous conformal optics aberration correction technique.

Conformal domes, designed with a high aspect ratio for aerodynamic purposes, present design, fabrication, and test difficulties to the optical engineer. Traditional domes have been spherical to facilitate fabrication and testing of the domes and the design of imaging optics.

Aspheric shape ZnS domes were fabricated by a scalable and cost-effective chemical vapor deposition (CVD) process to demonstrate the feasibility of producing aerodynamic domes that conform to the shape of the missile body. These domes provide enhanced performance by substantially reducing the missile drag, although they also present issues of CVD deposition, optical fabrication to the required figure and finish, particularly the inside surface, and metrology. Domes were fabricated on 'male' mandrels in a CVD chamber to produce net-shape or precision replicated inside surface and then diamond turned to produce surfaces with figure of a fraction of a wave and finish of 180 angstrom RMS. Important issues involved in near-net-shaping and precision replication of ZnS domes are discussed and data on mandrel and release coating materials, degree of replication achieved and mandrel durability are presented.

Infrared (IR) imaging systems are a critical component of military aircraft operations for navigation, surveillance, and target acquisition. A limiting feature in the readiness and life-cycle costs for such IR systems is the durability of the exposed optical window in the harsh land and sea environments associated with military operations. The Air Force Research Laboratory, Materials Directorate, has been conducting a program to develop a high durability, repairable JR window to both extend the operation life-cycle of the transparencies and to permit the reuse of the optical materials for significant cost reductions. The development effort is focused on a composite JR window composed of a highly JR transparent base material with a removable durable bonded cladding for erosion and abrasion protection. The development activities have included evaluation and selection of the optical materials, assessment of high durability coatings, refinement of the optical adhesive and bonding process, as well as laboratory and flight testing.

Optical windows consisting of layers of different materials, or layers made by chemical vapor deposition, usually exhibit substantial residual stresses. These stresses are caused by growth strains, in addition to thermal strains, and originate from the bonding of the layers, which generates internal forces and moments that must be balanced to achieve mechanical equilibrium. For elastically isotropic structures, this leads to plane contraction/expansion accompanied by spherical deformation, thus resulting in a partial relaxation of the stresses. There is a vast amount of literature relating to this topic; analytical solutions have been proposed for thin films on a thick substrate, but a closed-form solution for multiple layers of arbitrary thickness has only been available since 1987 and has not yet been fully exploited. It is the purpose of this contribution to take advantage of Townsend's model for deriving 'user- friendly' equations that properly describe the strains, the stresses, and the curvature of coated optical window blanks.

Addition of a side mounted IR seeker, to an existing missile design, introduces new issues involving the aerodynamic flow over the optical window and its near field effect on the ability of the seeker to view the target. Image aberration, distortion and boresight shift vary according to flow conditions and the thermal state of the window system. A detailed analysis of the aerodynamic flow and its aero-optic effect for a side mounted IR window was performed to quantify target image degradation, window heating and bending, and window structural failure probability due to aerothermal and aero-optical effects.

Recent improvements in engineered polymeric material compositions and advances in processing methodologies developed and patented at Raytheon Systems Company have produced long wave IR windows at exceptionally low costs. These UV stabilized, high strength windows incorporating subwavelength structured antireflection surfaces are enabling IR imaging systems to penetrate commercial markets and will reduce the cost of systems delivered to the military. The optical and mechanical properties of these windows will be discussed in detail with reference to the short and long-term impact on military IR imaging systems.

High strength edge bonds have been achieved between individual sapphire components, showing promise for fabricating window blanks up to 600 mm diameter or larger in size. Several bonding methods were investigated, with a directed-energy diffusion-bonding method yielding components with bond fracture strengths of 200 MPa. Bonded sapphire components 600 mm long and 3 mm thick with a 75 mm wide bond line have been produced. When polished, the bonded windows show no degradation in transmittance or transmitted wavefront quality. Process scale up to larger bonds lines is planned. Mechanical and optical characterizations of sub- scale edge-bonded sapphire windows are presented.

The Cavendish Laboratory has developed extensive facilities for studies of liquid and solid particle erosion. This paper describes the high-speed liquid impact erosion of thin CVD diamond discs and the variation with grain sizes of the absolute damage threshold velocity (ADTV), viz., the threshold below which the specimen shows no damage. All specimens fail by rear surface cracking and there is shown to be a shallow dependence of rear surface ADTV on grain size. Fracture propagation in CVD diamond has also been monitored using a specially-designed double-torsion apparatus and data for K_1C are presented. Tentatively, the results suggest that finer-grained CVD diamond exhibits a higher fracture toughness, although the differences are slight even over a fourfold variation in the mean grain size. No preference for intergranular fracture was observed and one may conclude from this that the grain boundaries themselves do not seriously weaken the material. The large pre-existing flaws, both within and between grains, whose size varies the grain size are believed to be the dominant source of weakness.

Arc jet testing of the Hera modified ballistic reentry vehicle - 1E (MBRV-1E) nosetip was conducted in June of 1998. The tests were conducted in the Air Force's Arnold Engineering Development Center HEAT-H1 arc plasma test facility in Tullahoma, Tennessee. The MBRV-1 vehicle is a separating short- to medium-range target. The MBRV-1E nosetip incorporates a custom designed quartz dome that is integrated into the nosetip stagnation region. The dome was bonded to the baseline nosetip material, a well characterized carbon-carbon composite material, using a silica based ceramic bond materials. The objectives of the test were to demonstrate the thermal performance and structural integrity of the nosetip design by exposing tip to arc plasma-heated flow simulating the reentry flight environment. Pre-test analysis of the Dynasil dome performed using finite element analysis predicted the dome would survive the test conditions with no failures. Post-test inspection of the dome revealed a hard, opaque coating on the outer surface of the dome. Once removed, the dome was shown to have numerous surface cracks near the stagnation region. In addition to the surface cracks, significant pitting on the surface was observed through both an optical microscope and a scanning electron microscope. Post-test analyses were performed to determine the cause of these surface cracks. It was concluded that the cracks occurred during cooldown, and were a result of significant strength degradation which was caused by the surface pitting.

Emissivity measurements of ZnS, Sapphire, ALON, MgO, and Yttria were performed in 3.9-4.0 micrometers and 4.4-4.9 micrometers bands, for temperatures between 300 degrees C and 600 degrees C. The average radiance was measured over each waveband. Emissivity was calculated as the ratio of the radiance of the sample to that of a black body source at the same temperature. The results of the emissivity measurements for the above-mentioned materials will be reported. Measurement techniques that allowed increasing the dynamic range of the measurement and significantly reducing the noise will be discussed.

Sapphire is a commonly used dome material. It is uniaxial and features different propagation properties depending on propagation direction and crystallographic orientation to the window boundary. These differences are particularly important for a hemispherical dome geometry. Prior work examined a semi-infinite medium. However, to be practical, a medium of finite thickness must be considered. The purpose of this paper is to present the results of a detailed calculation of the direcitonal emissivity of a finite sheet of uniaxial crystal located in an isotropic environment when the parallel plane surface so the crystal sample have an arbitrary orientation with respect to the crystallographic axes. Furthermore, the permittivities of the uniaxial crystal are allowed to have a complex part. Transmission measurements between 1400 cm^-1 and 2500 cm^-1 using a Fourier transform spectrometer are used in conjunction with the present theoretical result to determine the room temperature multiphonon absorption coefficient for the extraordinary-ray of sapphire. Application of the theory to a dome geometry is described.