Glass

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June 1, 2007
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Sapphire transparent armor systems can decrease the area density and thickness of traditional glass laminated systems without compromising ballistic performance.



Sapphire (single crystal aluminum oxide, Al2O3) is more a part of our everyday life than most people realize. From the transparent point-of-sale (POS) scanner windows at the supermarket checkout to translucent orthodontic braces, scratch-resistant watch faces and epitaxial substrates that produce light emitting diodes (LEDs), sapphire's use often goes unnoticed.

Sapphire for industrial applications is available in many sizes and shapes, including tubes, rods and sheets. Most recently, very large sapphire sheets have been developed for use as optical windows when strength, hardness (resistance to erosion), chemical inertness and high optical transmission are a necessity. Those properties are also very desirable for transparent armor, making sapphire a viable solution for lightweight, high-performance transparent armor.

Traditional transparent armor systems use laminated glass layers with a polymer backing to stop ballistic projectiles, which often consist of armor-piercing (AP) threats with a soft outer jacket and a hard armor-piercing core. Traditional glass-only systems can be quite thick and heavy due to the amount of glass needed to stop high-powered projectiles. In glass systems, where the thickness of individual glass layers ranges from 4-12 mm, the glass needs to absorb the energy from the projectile and slow down the armor-piercing core enough for subsequent glass or backing layers to catch the core. Further, when used in harsh environments (including sand and chemicals), traditional designs tend to exhibit a loss in clarity.

As threats become tougher to defeat, ceramic composites are a leading solution to reduce thickness and weight while improving protection and transparency. A recent co-development program evaluated the effectiveness of using sapphire sheets as a component of a transparent armor laminated system. The outcome of this program was a complete laminated system, fully qualified with extensive ballistic testing on standard single- and multi-shot parameters. The first commercially available transparent ceramic armor system was released in 2006.*

*Transparent armor products that incorporate sapphire components have been co-developed by Saint-Gobain Crystals and Saint-Gobain Sully. Saint-Gobain Crystals manufactures these products in the U.S., and Saint-Gobain Sully manufactures them in Europe.

Improved Performance

In comparison to traditional glass-based laminated transparent armor systems, the new sapphire transparent armor systems offer a thickness and weight savings of more than 50%. For certain systems, the weight savings exceed 60% (see Table 1).

Another key aspect of the sapphire transparent armor systems is that the sapphire actually breaks the armor-piercing core apart, resulting in smaller, lower-energy projectiles for the subsequent glass, inter-layers and backing material to defeat (see Figure 1). This is attributed to sapphire's superior Knoop hardness and fracture toughness when compared to glass and other transparent ceramic materials. The sapphire transparent armor systems were extensively tested, and partial results are provided in Table 2.

Figure 1. A 7.62 x 54R B32 API hard projectile core before (left) and after impact (right) on a sapphire strike face. Note the total destruction of the hard projectile core after contact with the sapphire transparent armor system.

In harsh operating environments, such as aircraft rotor wash or wind-blown sand, sapphire is extremely resistant to erosion. It is also unaffected by exposure to chemicals and solvents, where traditional glass-based systems would degrade and lose visible transmission. With a hardness of 9 on the Mohs scale (materials like diamond, silicon carbide and boron carbide, to name a few, are harder), sapphire is capable of withstanding harsh environments without transmission losses.

The armor system also incorporates novel interlayer material with industry-proven weathering durability under a variety of environmental conditions, along with backing material that offers excellent scratch resistance, chemical resistance and durability. The combination of an extremely durable sapphire strike face, non-yellowing interlayers, and durable backing layers significantly reduces the need for maintenance/replacement and allows for consistent high performance during use.

Superior Optical Transmission

The sapphire transparent armor systems provide a luminous transmission that surpasses traditional glass-only systems, and excellent compatibility with night vision goggles. A sapphire transparent armor system with a 29.4 mm thickness that can defeat the 7.62 x 54R B32 round has a transmission greater than 85%, with haze levels around 1% (see Table 3).

For comparison, a typical glass window that could defeat the same round would be 55 mm thick with a luminous transmission of 73% and haze around 0.6%, showing the vast improvement in transmission by using a sapphire transparent armor system. Increasing the thickness of the system by adding additional glass layers to 41.1 mm thickness only slightly reduces the transmission to around 84%, with no change in haze.

A 305 x 305 mm sapphire transparent armor system defeats a multi-hit 7.62 x 54R B32 API. The ballistic strike face is on the top, while the back side is on the bottom.

Additional Benefits

A development program is ongoing, and further designs are currently being tested against additional threat levels and environmental conditions. Through the use of current technology to leverage the excellent thermal conductivity of sapphire, a defogging capability can be added to the sapphire transparent armor systems.

In addition, custom dielectric coatings can be integrated in the systems to block specific wavelengths (i.e., laser threats) that have the potential to blind individuals on the friendly side of the windows.

For additional information regarding the sapphire transparent armor systems, e-mail jeffrey.b.rioux@saint-gobain.com or visit www.photonic.saint-gobain.com.

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