Signifigant enhancements of the flexural strength of a- and c- plane sapphire by means of "super polishing" was first reported be McHargue and Snyder [Proc. SPIE 2013, 135 (1993)]. The improvement was attributed to the removal of residual mechanical polishing damage. More recently, a comprehensive series of eperiements was carried out by Crystal Systems for the specific purpose of assessing the effects of various polishing procedures on the high-temperature strength of c-plane sapphire. Subsequent testing at room temperature confirmed that chemomechanical polishing improves both the effective strength and the strength distriution. In this contribution we take advantage of the methodology previously used by Klein et al. [Opt. Eng. 41, 3151 (2002)] to perform a correct Weibull statistical analysis of biaxial flexure-strength data genenrated in the course of Crystal Systems' investigations. We demonstrate that chemomechanical polishing procedures can improve the high-temperature characteristic strength of c-plane sapphire by 150% and the room-temperature Weibull modulus by 100%.
The world's largest sapphire boules up to 340-mm diameter are produced by the Heat Exchanger Method (HEM). In order to meet all applications, the highest purity crackle is used so the product has impurity levels very near the detectability limit of Glow Discharge Mass Spectroscopy (GDMS). The charge size of production 340-mm diameter sapphire boules was increased from 55-kg to 70-kg, and larger 380-mm diameter, 84-kg boules were produced. These boules were used to produce 315-mm diameter, 132-mm high sapphire cylinders to meet customer requirements. Efforts have been taken to produce a nearly flat top surface of HEM-grown boules with minimal undulations along the sidewalls to allow fabrication of larger sapphire pieces for production boules.
A new multiwire Fixed Abrasive Slicing Technology (FAST) was developed that combines the low kerf loss and high material utilization of multiwire slicing (MWS), low consumable costs of internal diameter (ID) slicing and low cost of multiblade slicing (MBS) technologies. Recent improvements in FAST allow it to be utilized for effective slicing of hard materials at low cost. For over two years, 2-inch diameter sapphire has been sliced in prototype mode for supplying wafers to the industry.
Various ways to increase the 600C strength of sapphire were explored, including heat treatments, doping and improvement of fabrication techniques. Different grinding and polishing procedures were performed on sapphire disks and compression specimens. Measured strength showed correlation with fabrication procedures, with less aggressive, multi-step processes resulting in the highest strength. Simple heat treatments in oxidizing atmosphere significantly improve both compressive and biaxial flexure strength of sapphire. MgO doping was found to be very effective in increasing the compressive strength of sapphire when combined with the heat treatment.
Flexure strength testing of single crystal sapphire was conducted in the 500 - 600 degree(s)C temperature range using thin sheets of Grafoil<SUP>TM</SUP> to reduce failures due to the contact stresses at the loading points. Load-point failures occur often in high temperature testing of sapphire when twinning mechanisms activate above 500 degree(s)C. Bend bars of three different orientations were tested, and the flexure strength was found to be strongly dependent on the orientation type both in Grafoil and non-Grafoil tests. Use of Grafoil increases measured flexure strength up to the factor of three due to the reduction of contact stress. The effect of using Grafoil in mechanical testing of sapphire, including flexure strength, compressive strength and biaxial flexure strength testing is discussed.
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.
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'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.
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.
Recent interest in monitoring systems requires very large optical windows that are transmitting over a wide spectral range. Some of the other requirements involve durability, high strength and robustness to withstand severe environments. Therefore, sapphire has been required for these applications. The Heat Exchanger Method (HEM)<SUP>TM</SUP> has been used to produce very large sapphire crystals primarily for optical applications. Crystals of 20 cm and 25 cm diameter have been produced in production for over 20 years. Presently, 34 cm diameter boules have been adopted in production, and 50 cm diameter sapphire growth is currently in development. Results of progress and characterization data of the boules will be presented.
Precision optical fabrication is often influenced by surface stress introduced during processing. Various steps, such as lapping, grinding, polishing and coating, can influence optical figure and transmitted wave font in sapphire optics. The Twyman effect was used as a tool to measure the variation in stress form different processes and to investigate annealing treatments. Compressive stresses were generated by all fabrication techniques; however, the magnitude of stress varied considerably. The highest stress was generated during the transition from the brittle to ductile mode of removal; the lowest stress was observed during polishing with colloidal silica. Heat treatments were successful in removing machining stress from the parts. After heat treatment at 1450 degrees C, the remaining grinding-induced stress levels were too small to measure accurately.
Sapphire has been used for many optical applications. However, smaller sizes have been used, even though the Heat Exchanger Method (HEM) has produced 20 cm diameter crystals. New generation systems require outstanding optical properties, high strength, and abrasion and thermal shock resistance. Therefore, the choice is limited to sapphire. Crystals up to 34 cm diameter, 65 kg have been grown by HEM, and it is planned to scale up the size to 50 cm diameter. In addition to larger size, the optical quality has been improved to cover the vacuum ultraviolet (VUV) and the near infrared wavelengths. Fabrication technology was advanced to fabricate larger size, higher precision optics cost effectively. Improved transmitted wavefronts and higher quality surfaces have been produced to address current applications.
Large sapphire boules, up to 34 cm diameter, 65 kg, are being grown by the heat exchanger method (HEM) and even larger sizes are sought to meet future requirements of advanced optical systems. These boules, especially in large sizes, exhibit lattice distortion and light scatter in a very narrow range. A qualitative grading system has been developed to characterize sapphire. Windows of five grades and different orientations were prepared and measured for refractive index homogeneity to evaluate transmitted wavefront distortion. The data showed that the refractive index homogeneity for all samples was in the 10<SUP>-7</SUP> (0.1 ppm) range. The fact that lattice distortion does not affect the transmitted wavefront allows fabrication of large sapphire windows in production mode at low cost.
Sapphire's loss of strength between 20 degrees and 1000 degrees Celsius depends on orientation and state of stress. The critical weakness of sapphire occurs in compression along the c-axis of the crystal. In flexure tests of sapphire that is not subject to c-axis compression, the strength actually increases between 20 degrees and 1000 degrees Celsius. Compression on the c-axis causes twinning on rhombohedral crystal planes. When twins on different planes intersect, a crack forms and the specimen is then subject to tensile failure. Doping with Mg<SUP>2+</SUP>, Ti<SUP>4+</SUP>, or introduction of a TiO<SUB>2</SUB> second phase each doubled the c-axis compressive strength of sapphire at 600 degrees Celsius, probably by inhibiting twin propagation. X-ray topography was employed to investigate the relationship between surface and bulk defects and mechanical strength in sapphire. Low angle grain boundaries were not associated with mechanical weakness. Wide, transverse scratches that are evident to x-rays, but not obvious in optical microscopy, can weaken sapphire. Topography demonstrated that annealing reduces long range strain in polished sapphire.
Large sapphire components are required to meet the challenging needs of commercial and military optical applications. However, the desirable properties of sapphire also make it difficult to grind and polish. Fabrication costs can represent 50 percent of the price of large sapphire components. Cutting and grinding studies on sapphire were carried out with four types of diamond tools to correlate tool characteristics and process parameters with the grinding mechanism. The Twyman effect was also investigated to relate to crystallographic properties of sapphire with fabrication concerns. If grinding and microgrinding techniques can be optimized, costs associated with fabrication of sapphire and other optical materials will ultimately be reduced.
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  appears to initiate failure and accounts for the decreased strength. The tensile strength of sapphire along the (alpha) -  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.
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.
Transparent dome elements for future higher speed missile systems place stringent requirements on the mechanical properties of the dome material. Currently no material meets all the requirements. Among the five candidate oxide materials, sapphire appears to be the best choice based upon material properties, thermal shock resistance and status of material production. A disadvantage of sapphire is that it must be produced in single crystal form because of its anisotropic structure. A new approach to produce near net-shaped sapphire domes from the melt is discussed. Multiple near-net-shaped sapphire domes of various sizes and curvature have been produced.