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Porosity is one factor that creates a propagation zone for these imbalances. It also interferes with the ability of the armor plate to dissipate some of the kinetic-to-sound energy during the initial strike. Recent manufacturing advances in reaction bonding and hot pressing processes have eliminated much of the unwanted porosity. However, defects do remain, and a fully dense ceramic tile has not yet been achieved.
Material ProblemsCurrently, up to a few hundred parts per million (ppm) of unwanted elements can be present in the crystal lattice that comprises the B4C unit cell. Tables 1-3 denote averages from five random samples of the basic feedstocks (graphite, boric acid and petroleum coke) used to manufacture B4C powder. One can quickly note from the elements in red that some wide swings exist in the amounts of impurities, as well as unwanted elements like calcium, manganese, sodium and silicon.
These impurities, or debris, act to expand the lattice structure, thereby weakening the crystal, and the remaining debris in the crystal does not add to the hardness or fracture toughness of the B4C powder. The debris interferes with crystal grain growth, causes lattice expansion of the B4C crystal, and makes the final powder susceptible to voids and porosity that can inhibit the manufacturer from producing a fully dense armor plate (see Figure 1).
Other impurities, like metals, are less of a problem as they are "forced" from the center of the melt through diffusion and become part of the shell next to the furnace wall. This shell can be discarded after cooling.
Improved ProcessingA program is currently underway, in cooperation with a major B4C powder producer, to improve the base materials in the melt. It is possible to pre-process the starting materials for armor powder fabrication to reduce the amount of unwanted impurities, thereby creating a boron carbide powder that reaches its theoretical density of 2.52 g/cc (see Figure 2).
Having a fully dense powder ready for pressing into a protective armor piece can increase the hardness of front and back plates in personnel protection. Increased hardness is needed to erode the projectile on impact, and to continue to erode the projectile even if fracture of the plate occurs. Improving the starting powder can also improve the overall ballistic performance by making the powder able to withstand the high transient stresses created at the point of impact, as long as there is a solid crystal to flex with the instantaneous load.
Composite AdvancesRecently, using high-temperature/high-pressure techniques, ceramic B4C pieces have been produced with an outer cover of diamond. This diamond outer cover is designed specifically to provide additional hardness (10.0 mohs scale) to defeat the tungsten or ceramic projectile, or to provide additional protection against multiple projectile hits or improvised explosive devices.
The purpose of the diamond outer cover, which is completely bonded to the B4C powder in a solid piece, is to completely erode the tip of all types of projectiles before the projectile has a chance to invade the B4C portion of the armor plate. Eroding the projectile increases its dwell time, thereby permitting the comminuting of the entire projectile. Because the surface energies of the two species of materials are very similar, high-temperature/high-pressure, hot pressing or reaction bonding can be achieved with a small increase in cost (mainly due to the cost of the synthetic diamond).
Using a semi-empirical model for diamond surface energies (Helmholtz energy of a plane), most of the adhesion of diamond onto the B4C powder is accomplished through dipole-to-dipole interaction and can be attributed to Van der Walls forces. It has been calculated that the diamond plane 111 contains an estimated 1.83 x 1019 carbon bonds per square meter, and the surface energy is estimated at 5659 mJ/m2 . Put simply, this means that the diamond is more than ready to react at normal processing temperatures/pressures in reaction-bonded systems with its neighbor on the periodic table to produce a possible lightweight ceramic piece with the ability to withstand current ballistic threats.
Ongoing ResearchOne current investigation involves adding diamond powders to the mix before pressing in either system of plate fabrication previously mentioned. It is thought that there can be a new orientation (hybridization) between the B4C rhombohedral crystal and synthetic cubo-octohedral crystal, thus creating a material with higher toughness and fracture resistance.
In another study, a high-quality larger-crystal diamond (200 to 600 microns) is being pressed onto the surface of the B4C piece. The thought process here is that the larger crystals provide the hardness and toughness needed against initial impact while maintaining good fracture resistance with the finer boron carbide powder (30 to 50 microns).
Ultimately, preventing loss of life is the goal. Rigorous testing by the armed forces on all new materials will need to be done before any new material is approved and distributed to military personnel.
For more information regarding improving armor performance, contact Warren/Amplex Superabrasives at 1401 E. Lackawanna St., Olyphant, PA 18447-0177; (570) 383-3261; fax (570) 383-3218; e-mail firstname.lastname@example.org ; or visit http://www.warrenamplex.com .
For Further Reading1. J.F. Smith, "Crystallogarphy and Phase Equilbria," J of Phase Equlibra & Diffusion, Vol. 26, No. 6 2004, pp. 497- 503.
2. "Applied Ceramic Technology," Ceramic Products Development, Vol. 1, 2005.
3. "Surviac Bulletin," Dept. of Defense, Vol. 27, 2006.
4. St. Gobain Research and Testing laboratory, Northborough, Mass., 2006.
5. L. Cartz, "Nondestructive Testing," ASM International, Materials Park, Ohio, 1995, pp. 135-136.
6. A.B. Tugrul, "Capillarity Effect Analysis for Alternative Liquid Penetrant Chemicals," NDT & E International, Vol. 30, No. 1, Elsevier Science Ltd., Oxford England, February 1997, pp. 19-23.