Int. J. Appl. Ceram. Technol., 1 [3] 203-04 (2004)

Ceramic Product Development and Commercialization

In the early 1960s, the Department of AdvancedResearch Project Agency (ARPA) funded a project at theLawrence Radiation Laboratory (LRL) to provide a fun-damental understanding of lightweight armor. The ap-plication of materials to lightweight armor is unusualbecause materials are used in regions of material failure,that is, if the armor does not fail for a given ballistic threat,it could be made lighter. Penetration is the process ofmoving material aside. The material properties, apart fromdensity, responsible for resisting a change in shape froman applied load, are the elastic bulk and shear moduliiwhich determine the material hardness. The parameter Yfrom the Von Mise model of material failure is used tomeasure the compressive stress where the material hard-ness changes value. Thus an armor material must possesshigh elastic modulii and a high value of Y compared tothe ballistic threat materials. The elastic modulii can bedetermined from sound speed experiments and thestrength parameter Y from Hugoniot elastic limit experi-ments. Low-density ceramics are the materials of choicesince ceramics possess elastic shear modulii two to threetimes greater than steel and compressive yield strengthsup to ten times greater.

The LRL project used experiments with flash radi-ography and calculations with a computer simulation pro-gram1 to follow step by step the penetration of a steelprojectile into ceramic armor. The composite armor con-sisted of a ceramic faceplate backed by a plate of alumi-num alloy or fiberglass. The calculations showed that thecatastrophic breakup of the ceramic first occurs on theside opposite the impact from a tensile failure. The cal-

culations also established that a slight increase in ductil-ity in a ceramic had an important effect on the time offailure. Over 20 different ceramics were analyzed for com-pressive failure strength, elastic modulii, and ballistic lim-its. Ballistic limit was defined as the maximum velocitythat could be defeated for a 30-caliber steel projectile. Itwas noted that some ceramics performed better than ex-pected for their material hardness and strength param-eters. Separate measurements determined that these ce-ramics had superior strain-to-failure properties under thepressure of impact. The possibility that high compres-sive strength could be traded for ductility led to researchon cermets, a combination of ceramic and metal powdersconsolidated by techniques like “hot pressing.” Materi-als that are very hard in compression are weak in tension.With cermet technology, it is possible to vary the metalcontent in a ceramic gradually to achieve the desired duc-tility at the surface opposite impact. We refer to thesematerials as graded cermets.2,3 Introducing ductilitythrough the thickness of a ceramic material can also beachieved by other methods.

The selection of lightweight materials was always ofprimary importance in armor systems due to several factors:

1) The lower the weight, the thicker the ceramicarmor.

2) The thicker the ceramic armor, the larger thearea the load was spread over the backup plate.

3) The thicker the armor, the more erosion of theprojectile during penetration.

4) The more the erosion, the less mass and kineticenergy of the projectile.

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Lawrence Livermore National Laboratory, Livermore, CA

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204 Vol. 1, No. 3, 2004International Journal of Applied Ceramic TechnologyLandingham and Wilkins

5) The less the projectile mass and kinetic energy,the easier to stop with the backup plate.

The hardness of the front plate needed to be higherthan the hardness of the projectile point to break anderode the point of the projectile. This erosion of the pro-jectile will continue even after the ceramic plate is frac-tured so long as the ceramic pieces are held in place bythe backup plate. Improved ceramic performance is ob-tained by improved backplate support and lateral con-finement in addition to improved fracture toughness.

The above improvements to the ceramic front plateare difficult to achieve in a lightweight armor system dueto the constraints on space and weight. While a few se-lect systems have show these improvements, it is difficultto incorporate all improvements into one armor systemand still be cost-effective and outperform monolithic ce-ramics like hot-pressed B4C and SiC front plates on fibercomposite backup plates. The Be4B-Be system was use-ful in demonstrating the principles of the above penetra-tion mechanics but would be too expensive and has tox-icity risks.

LRL did identify one armor system of sufficient po-tential to warrant further development. This system,boron carbide-aluminum cermet (B4C-Al), has the de-sired properties described above. B4C-Al cermet armor

uses less B4C powder (approximately 40% less) than hot-pressed B4C. This cermet can be fabricated at relativelylow temperatures (1100°C) with no applied pressure incomparison to hot-pressed B4C or SiC at greater than1900°C and 2-4,000 psi applied pressure. The fabrica-tion process, molten metal infiltration of aluminum al-loys into B4C powder compacts, is very fast (minutes)even for thick tiles (8-in. thick). Methods have been de-veloped to form graded tiles with high B4C loading atthe impact face and high aluminum contents at the backside. Graded armor formed by increasing the grain size ofthe B4C powder resulted in lower ballistic performances.Current efforts are under consideration to revive this tech-nology as a partial substitute for hot-pressed B4C andSiC armor since the process favors mass production. Whilehot-pressed B4C has shown a sensitivity to high-velocityimpacts, the cermet has not. Other systems need to beidentified and investigated as substitute materials that canbe mass-produced at low temperatures using inexpensivematerials (SiC-Si alloys, SiC-Al alloys, etc.).

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1. M. L. Wilkins, Computer Simulation of Dynamic Phenomena, Springer, 1968.2. M. L. Wilkins, R.L. Landingham, C.A. Honodel, Fifth Progress Report of

Light Armor Program, Lawrence Radiation Laboratory, UCRL-50980, 1971.3. R.L. Landingham and A.W. Casey, Final Report of the Light Armor Materials

Program, UCRL-57269, September 15, 1972.

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