The online version of ACerS’ International Journal of Applied Ceramic Technology has a new story that reveals many of the problems scientists and engineers face when designing the tips of missiles – called domes – used by primarily by the military, and the results of some interesting research on a new dome material. The gist of the paper in ACT is that a group of Saint-Gobain researchers have found that ultra pure α-alumina powder may provide a superior material for making these domes.
Consider, however, the problems that must be addressed in coming up with a dome material. Besides providing an aerodynamic leading surface for air-to-air and air-to-ground missiles, a missile dome shields an array of sensors that control various systems within the missile. Among other things, some of these sensors are used to detect a variety of electromagnetic radiation (e.g., in both visible and infrared ranges).
The best systems must be able to differentiate, for example, between the signature of the exhaust of a jet engine and the signature of decoy flares that a targeted airplane might jettison. Thus the domes have to be functionally transparent in a fairly wide range of the spectrum. Likewise, the shape and thickness of the domes must be such that they don’t distort any of the incoming electromagnetic radiation.
To complicate things even further, the dome must be able to withstand enormous mechanical stresses and thermal shocks. At the start of a missile launch, friction causes the outside leading surface of the front most portion of the dome to heat more rapidly than the rest. This hot front surface expands more than the cooler internal portions of the doom, and soon there is significant stress between the expanded and the unexpanded material. If the stress exceeds the mechanical strength of the material, the dome shatters.
Some materials work well at lower speeds, but new missiles will soon be rocketing at +Mach 4 levels.
Cost and ability to manufacture/machine are factors, too.
A commonly used material for domes is monocrystalline alumina (i.e., sapphire) and polycrystalline magnesium fluoride. Sapphire is costly, difficult to shape and prone to chipping. Large boules of sapphire must be drawn and then machined, which is an art in itself. A sapphire dome that has all of the required properties is typically 4-5 mm thick. The benefit of sapphire is that is a relative transparent material in the 0.25–5 μm range. However, transparency declines significantly at wavelengths higher than 5 μm.
The transparency of MgF2 is more limited than sapphire, but does very well in the 2–5 μm range, It is much less expensive than sapphire. Unfortunately, MgF2 isn’t a particularly rugged material.
So, the question is this: Is there some alternative material with good mechanical/thermo-mechanical strength and transparent in the key IR and visible ranges that is easy to work with and cheaper than sapphire, and a thermal shock resistance better than magnesium fluoride will be of use for this kind of application?
Guillaume Bernard-Granger, Christian Guizard and Nathalie Monchalin, from Saint-Gobain’s Laboratoire de Synthèse et Fonctionnalisation des Céramiques, say domes made of a dense and submicronic form α-alumina powder may be a good and relatively inexpensive alternative.
In brief, they were able to document that their alumina material has good transparency in the visible and mid-infrared ranges and can be formed using mold-and-sintering processes rather than complex and delicate machining. Just as importantly, a dome can be made with a thickness as low as 1 mm and still survive Mach 4 speeds.
Overall, this is just a great example of how advanced ceramics, using ultrapure feedstock refined at the submicron and nano levels is providing new, customizable solutions to engineering problems. Read the paper for the details.