A team of experts at the US Naval Research Laboratory (NRL) has developed a new way of fabricating spinel, an armor material used in the windows of military vehicles, demonstrating that the strength of transparent ceramics could be dramatically improved. The nanocrystalline spinel is 50 percent harder than the spinel currently used in armored vehicles and could result in enhanced protection for personnel.

Reducing the average size of the grains (crystallites) in a material has long been thought to improve its strength. This is known as the Hall-Petch relationship, though some previous experiments have demonstrated a breakdown in this theory when working with grains sized 130 nanometers.

Using a technique they call Enhanced High Pressure Sintering (EHPS), the researchers were able to reduce the average grain size to 28 nanometers, observing no decline in density or fracture resistance. Using this EHPS method to fabricate the spinel involves the application of high amounts of pressure (up to 6 GPa). This works to break up gatherings of nanoparticles and reposition them close to one another to minimize the formation of minute holes in the final product.

Previous efforts to manufacture nanocrystalline spinel have resulted in reduced density, reduced fracture resistance or reduced transparency. However, the work carried out by the NRL researchers indicates that nanocrystalline ceramics can be of equal strength to microcystalline ceramics.

In addition to offering greater protection for military personnel, nanocrystalline spinel could make the production of armored windows more cost effective. At present, sapphire is used in some of these applications with one downside being the associated cost. If the researchers can improve the hardness enough, they say they could replace sapphire windows with ones made from nanocrystalline spinel.

Further to military applications, the harder spinel could prove useful in making stronger windows for civilian offices and vehicles, along with more durable screens for tablets and phones.

The team's research was published in the journal Acta Materialla.

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