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Polycrystalline spinel serves as an alternative to materials such as sapphire and magnesium fluoride that are currently
being used in electromagnetic window applications such as missile domes, where high strength, high hardness and high
transmittance in the visible and infrared spectra are required. The cubic crystal lattice of spinel imparts an isotropy to the
bulk optical property, which eliminates optical distortion due to birefringence that occurs in sapphire and other non-cubic
materials. The current study is to find a reliable manufacturing process to produce large magnesium aluminate spinel
domes from powder consolidation efficiently. A binder-less dry ball milling process was used to deflocculate the spinel
powder to increase its fluidity in an effort to ease the shape-forming. Dry ball milling time trials were conducted at
several intervals to determine the appropriate level of time required to break up both the hard and soft agglomerates
associated with the virgin spinel powder. The common problems encountered in dry powder shape-forming are crack
growth and delamination of the green body during cold isostatic pressing (CIPing). The cracking and the delamination
are due to the buildup of stress gradients on the green body that are created by the frictional force between the powder
and the die wall or mold wall. To understand the stresses during the CIPing process, a finite element analysis of stresses
on the green body was conducted. The simulation was used to evaluate the effect of die tooling and process
characteristics on the development of stress gradients in the green body dome. Additionally, the effect of friction
between the die wall and powder was examined by the simulation. It was found that by mitigating the frictional forces,
cracking and delamination on the green body could be eliminated. A stepped-pressure CIPing technique was developed
to reduce stress gradient build-up during CIPing. Also, oleic acid lubricant was applied to the die wall to reduce the wall
friction between the powder and the die itself. As a result of these two above-mentioned methods, it was demonstrated
that it is possible to consolidate a binder-free powder into large defect-free domes.
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