Recent advances in powder processing and manufacturing techniques are improving the performance of hot-pressed ceramic armor while also reducing costs.
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.
While obvious system design improvements have been achieved over the last decade, extensive work on improving the properties of the ceramic materials continues. One of the most desired variables to eliminate is porosity, so that full theoretical densities are achieved. Hot pressing creates fully dense ceramics, so the focus on ceramics processed using this technique is on the other property improvements. The best way to increase ceramic efficiencies is to extend the time that the ceramic works on a bullet to cause shattering; increase the dwell duration or dwell-penetration transition velocity (the velocity above which penetration occurs, and below which the bullet is defeated on the ceramic surface); and/or achieve a greater pressure on the penetrator during penetration to increase the erosion efficiency of the highly damaged, comminuted ceramic. Additionally, and of particular interest in personnel armor protection, is a requirement to predictably defeat multiple impacts within a single plate or array. Finally, cost, manufacturability and integration with the backing materials of the armor system must also be taken into consideration.
Figure 1. Commercially feasible hot-pressing techniques have been developed to make large B-rich boron carbide tiles with B:C ratios >7.
As 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.
(Click image for larger view) Figure 2. A significant increase in fracture toughness was observed for boron carbide with B:C ratios >7. The size of the boron carbide lattice rose with the increase in the B:C ratio of the boron carbide phase. Fracture toughness is plotted against the lattice parameter, because estimating the B:C ratio in the boron carbide phase measuring the lattice parameter is simpler and more instructive than conducting multiple complex chemical analyses.
One approach being investigated through a Small Business Innovative Research (SBIR) program funded by the Army Research Laboratory is the synthesis and processing of boron carbides with B:C ratios ranging from four to eight. The rapid carbothermal reduction (RCR) process, developed by researchers at the University of Colorado-Boulder, is being investigated to achieve this synthesis. The process requires a small amount of time (typically a matter of seconds) to heat and maintain the reactants at temperatures, which enables a particle size and composition control not allowed by conventional carbothermal reduction processes. The team has been able to successfully enrich boron carbide powders with B:C ratios of around four to greater than seven using the RCR process.
Table 1. (Click image for larger view)
The processing and characterization of B-rich boron carbide powder has also been extensively studied. Commercially feasible hot-pressing techniques have been developed to make large tiles (see Figure 1) that enable the characterization of mechanical and ballistic performance, and processing techniques to manufacture complex shapes such as those used in body armor systems are also being developed. The most important observation from the characterization of the mechanical behavior has been a significant improvement in the fracture toughness of the boron carbide for B:C ratios greater than seven (see Figure 2). The cause for this increase is under investigation. Other properties, such as the modulus of rupture and hardness, improved or remained similar to those of B4C (see Table 1). While an evaluation of the ballistic performance of B-rich boron carbides is still in progress, it is hoped that the superior mechanical properties of these materials will result in an improvement in ballistic performance over that of B4C, especially for medium to heavy threats.
(Click image for larger view) Figure 3. Several approaches were attempted to achieve a simultaneous improvement in hardness and fracture toughness in hot-pressed SiC but were unsuccessful. Any improvements in hardness were generally accompanied by a decrease in fracture toughness. However, fracture toughness could be tailored over a large range (~ 4 to 6 MPa-m1/2) without significantly affecting hardness. Evaluation of the ballistic performance of these materials is in progress.
Another SBIR study funded by the Army Research Laboratory is investigating approaches to improve the ballistic performance of hot-pressed silicon carbide, also with regard to hardness and fracture toughness. Hardness control has been achieved by adding particulates, while fracture toughness control has been achieved both by adding particulates and tailoring the material's microstructure. Increases in hardness have been generally accompanied by a decrease in fracture toughness; however, significant improvements in fracture toughness have been achieved without a significant change in hardness (see Figure 3).
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.
(Click image for larger view) Figure 4. A comparison of ballistic performance against a medium threat for several sintered SiC grades and a hot-pressed SiC grade. Superior ballistic performance has been achieved for sintered SiC, comparable to hot-pressed SiC by newly developed liquid-phase sintered SiC compositions such as EKasic T.
Until 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 Information
For 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.
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.