New military requirements have reinvigorated the need for transparent magnesium aluminate (MgAl2O4) spinel. Surmet has developed a process that yields high quality transparent spinel at production scale. Several issues related to the extreme requirements of processing ultrafine spinel powders are described. Transmission data is presented for a significant dataset of parts made by this process.
More recently, the process has been expanded to include a capability for producing domes for the Joint Common Missile program. Domes at nominal 6” and 7” diameter have been successfully fabricated. Despite early challenges related to the forming portion of the process, a repeatable capability for these domes has been demonstrated.
Several challenges remain in spinel processing in order to support additional military requirements. In particular, the strength of the material needs further improvement. Also, improvements in optical quality with regard to inclusions are needed.
Aluminum Oxynitride (ALONTM Optical Ceramic) is a transparent ceramic material which combines transparency from the UV to the MWIR with excellent mechanical properties. ALON’s optical and mechanical properties are isotropic by virtue of its cubic crystalline structure. Consequently, ALON is transparent in its polycrystalline form and can be made by conventional powder processing techniques. This combination of properties and manufacturability make ALON suitable for a range of applications from IR windows, domes and lenses to transparent armor.
The technology for producing transparent ALON was developed at Raytheon and has been transferred to Surmet Corporation where it is currently in production. Surmet is currently selling ALON into a number of military (e.g., windows and domes) and commercial (e.g., supermarket scanner windows) applications.
The capability to manufacture large ALON windows for both sensor window and armor applications is in place. ALON windows up to 20x30 inches have been fabricated. In addition, the capability to shape and polish these large and curved windows is being developed and demonstrated at Surmet. Complex shapes, both hyper-hemispherical and conformal, are also under development and will be described.
Durable coatings of silicon-carbon-oxy-nitride (a.k.a. SiCON) are being developed to protect high-speed missile windows from the environmental loads during flight. Originally developed at Rockwell Scientific Corporation (RSC) these coatings exhibited substantial promise, but were difficult to deposit. Under a DoD DARPA SBIR Phase I program, Surmet Corporation, working closely with RSC, is depositing these coatings using an innovative vacuum vapor deposition process. High rate of coating deposition and the ease of manipulating the process variables, make Surmet’s process suitable for the deposition of substantially thick films (up to 30 μm) with precisely controlled chemistry. Initial work has shown encouraging results, and the refinement of the coating and coating process is still underway. Coupons of SiN and SiCON coatings with varying thickness on a variety of substrates such as Si-wafer, ZnS and ALON were fabricated and used for the study. This paper will present and discuss the results of SiN and SiCON coatings deposition and characterization (physical, mechanical and optical properties) as a basis for evaluating their suitability for high speed missile windows application.
Commerically available yttrium oxide nanopowders were evaluated as starting materials for preparation of transparent materials. The objective is an yttria optical ceramic exhibiting approximately one micrometer grain size to provide increased strength and thermal shock resistance. Three vendors were selected to provide nanoscale powders for testing and evaluation. They were compared to a conventional (5 μm) powder previously used to prepare optical quality ceramic yttria. While all of the selected nanopowders had impurity levels that were too high to allow processing to full transparency, two of the samples were processed to full density and moderate transparency was produced in one. In preparation for processing via Hot Isostatic Press (HIP) samples were sintered to a closed pore state at temperatures as low as 1400 °C, and with soak times as short as 12 minutes at 1550 °C. The use of ultrasonic attenuation as a technique for measuring particle size distributions in slurries was explored and found to be an invaluable tool when colloidally processing nanopowders. Finally, the areas most important for continued improvements were identified.
Aluminum Oxynitride or ALON optical ceramic is transparent material, developed and patented by Raytheon, which is very similar to sapphire, being comprised mostly of Al2O3 with a small amount of additional nitrogen. This nitrogen addition has the effect of producing a cubic material whose optical and mechanical properties are isotropic. Importantly, this means that it can be produced by powder processing methods, which are scalable to larger sizes, and at lower prices than can be achieved by the single crystal growth techniques that are used to grow sapphire. Furthermore, its isotropic properties make it much easier to grind and polish than sapphire. Recently, the interest in ALON optical ceramic has grown substantially following impressive results in ballistic testing. Ballistic laminates, containing ALON layers, have demonstrated protection against armor piercing rounds, at half the areal density and thickness of conventional ballistic laminates. ALON plates as large as 14x20in are being produced, under Air Force funding, for evaluation as IR windows and transparent armor, using conventional powder processing techniques. The production processes themselves are now being scaled to produce large pieces and large quantities of ALON optical ceramic.
Thin films were deposited onto ZnS as a means of strengthening and toughening. Fracture strength increases of
up to 40% have been observed. These mechanical property enhancements were found to be related to the
compressive stress in the deposited films. Rain erosion resistance is also enhanced through the application of
compressively stressed thin film coatings.
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