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Non-oxide, high-performance technical ceramics that are formed simultaneously under high pressure and temperature have been shown to achieve high mass efficiencies (i.e., significant weight savings) compared to armor metals and sintered ceramics against a variety of threats due to their low density, high hardness and high elastic modulus. Hot-pressed boron carbide (typically B4C) is used for personnel protection in the form of plate inserts, for aircraft protection as panels and seats, in ships, and in armored land vehicles as seats. For protection against more lethal threats, such as heavy machine gun and medium cannon threats, hot-pressed silicon carbide (SiC) is typically the material of choice. Hot-pressed silicon carbide is also used in armored vehicles as an appliqu‚, along with hot-pressed silicon nitride (Si3N4). (Titanium diboride [TiB2], while as effective ballistically, is not as widely used for ceramic armor, primarily for the lack of alternative market applications.)
Regardless of the ceramic material used, ceramic armor is damaged on impact, and this damage propagation affects the subsequent ceramic performance. Extensive research has shown that this damage is caused by the activation of preexisting defects by the shear and tensile forces that are generated on impact.1 The resulting behavior of the ceramic is a complicated combination of the integrated responses of the damaged and undamaged regions. Since a ceramic is rarely, if ever, used as a stand-alone armor, the mechanical response of the entire system determines the degree to which the damage is generated and how well the damage is confined so that the armor continues to be effective against subsequent impacts.
Investigating ImprovementsAs mentioned previously, the current state-of-the-art armor ceramic for protection against light threats (up to 7.62 mm) is hot-pressed B4C, while SiC is the ceramic of choice for defeating larger-caliber threats. Efforts to improve the performance of both materials are being carried out at Ceradyne, Inc., particularly with regard to the hardness and fracture toughness of the ceramics. It is believed that hardness is a measure of the potential ballistic performance of the ceramic, and that fracture toughness is a measure of how much of this potential can be realized.
The reasons for the effectiveness of B4C against low-caliber threats and SiC against larger-caliber threats are not fully understood and are the subject of several ongoing studies. Extensive investigations have been carried out into possible structural changes in boron carbide at impact pressures in the 18 to 21 GPa range. It is believed that the ballistic performance of boron carbide might be improved by increasing the material's "ductility" or fracture toughness.
Since no simple relationship exists between hardness, fracture toughness and ballistic performance, measuring ballistic performance is essential to developing better armor ceramics. To that end, new techniques are also being developed to efficiently and reliably screen the ballistic performance of various silicon carbides. Mechanical properties and ballistic performance data for two materials from the SiC ballistic performance study-a B4C-SiC and a TiB2-SiC particulate composite-are shown in Table 2. A significant improvement in hardness was achieved by adding B4C particulates. However, the fracture toughness of this composite decreased compared to the baseline SiC composition. Adding the TiB2 particles resulted in a significant improvement in fracture toughness while not significantly affecting the hardness. However, the ballistic performance of both materials was poorer than the baseline hot-pressed SiC.
Other approaches to improve the ballistic performance of silicon carbide through microstructure tailoring are also being investigated in the program.
Continuing AdvancesUntil recently, the best-performing SiC materials for armor applications were hot-pressed grades. However, sintered grades are more desirable, since sintering is more cost-effective than hot pressing. Conventionally sintered silicon carbides used additives that promoted solid-state sintering. Recently, liquid phase sintering has been used to sinter silicon carbide. Through the proper control of composition and processing, a pore-free material with superior mechanical properties is now being made on a commercial scale.* The ballistic performance of this liquid-phase sintered SiC material is significantly improved compared to the conventionally sintered, pore-free SiC material and is comparable to superior hot-pressed grades of SiC (see Figure 4).
As the needs of the U.S. military continue to evolve, ceramic materials and processes will be improved to meet these needs and provide improved protection at reduced costs. c
For More InformationFor more information about hot-pressed and sintered ceramic armor materials, contact Ceradyne Inc., 3169 Redhill Ave., Costa Mesa, CA 92626; (714) 549-0421; fax (714) 549-5787; e-mail firstname.lastname@example.org; or visit www.ceradyne.com.
Some of the work reported in this article was funded by the U.S. Army Research Laboratory through contracts DAAD17-02-C-0015 and DAAD17-03-C-0025.
References 1. Progress in Ceramic Armor, G. Geiger (ed), Westerville, OH, American Ceramic Society, 2004. 2. Ceramic Armor Materials by Design, J.W. McCauley et al. (eds), Westerville, OH, American Ceramic Society, 2002.
1. Progress in Ceramic Armor, G. Geiger (ed), Westerville, OH, American Ceramic Society, 2004.
2. Ceramic Armor Materials by Design, J.W. McCauley et al. (eds), Westerville, OH, American Ceramic Society, 2002.