The U.S. Department of Defense (DoD) needs materials for armor windows that provide essential protection for both personnel and equipment while maintaining a high degree of transparency. To meet that need, scientists at the U.S. Naval Research Laboratory (NRL) have developed a method to fabricate 

Spinel windows can have applications as electro-optical/infrared deckhouse windows in the new class of U.S. Navy destroyers, like the USS Elmo Zumwalt pictured here, that feature a low radar signature compared with current vessels. (U.S. Navy photo courtesy of General Dynamics.)

nanocrystalline spinel that is 50% harder than the current spinel armor materials used in military vehicles. With the highest reported hardness for spinel, NRL’s nanocrystalline spinel demonstrates that the hardness of transparent ceramics can be increased simply by reducing the grain size to 28 nm. This harder spinel offers the potential for better armor windows in military vehicles, which would give improved protection and other benefits to both personnel and equipment, such as sensors.

A New Approach

To create the harder spinel, the NRL research team sintered commercial nanopowders into fully dense nanocrystalline materials. Through its development of the enhanced high-pressure sintering (EHPS) approach, the NRL team is reportedly the first to succeed in making this harder spinel, explains James Wollmershauser, Ph.D., NRL researcher and lead author in the research paper published in the January 30, 2014, issue of the journal Acta Materialia. The EHPS approach, developed largely as a result of scientist Boris Feigelson, Ph.D.’s ideas and experience in high-pressure research, uses high pressures to simultaneously retard bulk diffusion rates, break powder agglomerates, and reposition nanoparticles very close to each other to help eliminate porosity in the sintered ceramic. Researchers can then exploit the increased surface potential of nanoparticles for surface-energy-driven densification without coarsening.

Using this EHPS approach to create the nanocrystalline spinel, the NRL research team did not observe any decline in density or fracture resistance due to residual porosity. Other researchers have tried to make nanocrystalline spinel, but they have all had problems with the final product, such as a reduced density, reduced fracture resistance, or reduced transparency. Reduced density is caused by voids that cannot be removed during processing, which can reduce hardness, fracture resistance and transparency. Wollmershauser notes that some theories suggest fracture resistance should decrease when you make a ceramic material nanocrystalline. However, in their work, the NRL researchers have shown that the fracture resistance does not change, suggesting that nanocrystalline ceramics can have an equivalent toughness to microcrystalline ceramics, which is important for high window lifetimes.

Material Benefits

The Hall-Petch relationship has been used to describe the phenomenon where a material’s strength and hardness can be increased by decreasing the average crystallite grain size. However, prior experimental work has shown a breakdown in this relationship (where hardness starts reducing with decreasing grain size) for certain ceramics at ~130 nm. The NRL researchers have disproved that a breakdown in the Hall-Perch effect exists at these nanoscale grain sizes by measuring an increasing hardness down to at least a 28 nm crystallite grain size. The new high-hardness values were measured on samples with these extremely small average grain sizes.

In current applications, spinel and sapphire (which is also very hard) are used to create materials for military armor windows. A drawback with sapphire is that it is expensive to make into windows. By increasing the hardness of spinel even further, NRL researchers can make a material harder than sapphire and possibly replace sapphire windows with those made out of nanocrystalline spinel. In addition, harder nanocrystalline spinel windows can be made thinner and still meet current military specifications. This thinness translates to weight savings on the vehicle. Thus, the nanocrystalline spinel brings improvements in hardness, window thickness and weight, and cost.

A final benefit is that the NRL-developed nanocrystalline spinel is highly transparent, making it useful in ultraviolet (UV), visible and infrared optics. The armor material used by the military must be transparent so that both equipment and personnel can see. Different sensors “see” varying wavelengths of light: infrared is important for heat-seeking capabilities, while UV imaging can be used to detect threats not seen in the visible spectrum. UV detectors also have applications in space-borne astronomy missions. A single window produced using the NRL-developed nanocrystalline spinel would be transparent across many technologically important wavelengths, easing design and weight requirements.

 Beyond the use for a harder spinel in armor windows, there could be other potential DoD and civilian applications in better/stronger office windows, smartphones and tablets screens, military/civilian vehicles, space vehicles, and even extraterrestrial rovers.

For more information, visit www.nrl.navy.mil.