NOVEL PROCESSING OF BORON CARBIDE (B4C): PLASMA SYNTHESIZED NANO POWDERS AND PRESSURELESS SINTERING...

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NOVEL PROCESSING OF BORON CARBIDE B4C PLASMA SYNTHESIZEDNANO POWDERS AND PRESSURELESS SINTERING FORMING OF COMPLEXSHAPES

James Campbell, Melissa Klusewitz, Jerry LaSalvia, and Ernest ChinU.S. Army Research LaboratoryAberdeen Proving Ground, MD 21005-5069Robert Speyer and Namtae Choeorgia Institute TechnologyAtlanta, GA 30332

Noel Vanier, Cheng-Hung Hung, Edward Abbott and Peter Votruba-DrzalPPG Industries4325 Rosanna DriveAllison Park, Pennsylvania 15101William Coblenz and Toni MarcheauxDARPA, Defense Sciences Office3701 N Fairfax DriveArlington, VA 22203-1714

STR CT

Boron carbide, most often used in personnel armorsystems, is rather difficult to form and is almost alwaysdensified from ceramic powders under heat and pressure.Recent developments by PPG and Georgia Tech areallowing for the full densification nano-sized powdersthrough a pressureless sintering route. PPG ismanufacturing these powders in a plasma processingtechnique, and they are less expensive than traditionallyproduced powders. The powders are fonned directly inthe plasma, and they can be doped with densification aids.Pressureless sintering teclmiques developed by GeorgiaTech offers a low-cost processing route for B4C that waspreviously unavailable. Together both efforts will lead toboron carbide with shapes and sizes unavailable today andusing a fine-grained domest ically produced startingpowder.

INTRODUCTION

oron carbide is an important armor ceramic that isprimarily used personnel protection applications due toits very low density and its ability to defeat small a nn sthreats. Boron carbide has a density 2.5 glcm ,compared to 3.2 glcm for silicon carbide SiC) and 3.9 gIem for aluminum oxide AI,O,).

Traditionally, boron carbide B,C) annor is manufacturedthrough the hot pressing of ceramic powders, whichrequires the application of uniaxial pressure during thesintering densification process. This processing routeallows for only very simple shapes to be fonned, such asflat plates or those with some curvature, such as a SmallArms Protective Insert SAP I plate. Until recently, it hasbeen very difficult to densify this ceramic to full densityvia a pressureless sintering route. Sintering would givethe ability to make components with much morecomplicated geometries, such as conformable body armorand helmet liners. Georgia Institute Technology GlT)has developed a sintering approach with a follow-on hotisostatic gas) pressing HIP) to produce pressurelesssintered, additive-free, B,C components at full density.Currently, almost all B,C powders used in armorapplications are imported due the high amounts electricity required to manufacture the material. isformed in an electric arc furnace as a large billet material, which is then ground down into the powdersused to make ceramic bodies. PPG offers an alternateapproach to making the powders. The starting materialsare reacted in a plasma and powders are formed directly.These powders are nano-sized and the process allows forthe addition sintering aids directly into the powder.


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Novel Processing Of Boron Carbide (B4c): Plasma Synthesized Nano

Powders And Pressureless Sintering Forming Of Complex Shapes

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U.S. Army Research Laboratory Aberdeen Proving Ground, MD

21005-5069

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2. TRADITIONAL B4C POWDERSU.S. Boron Carbide Imports Top 5 Countries

3. PPG NANOPOWDERS

Germ YDel lOlJkrajne. c . -

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For this process, the powder is formed directly from thestarting materials, and the final powder should have amore consistent chemistry and less contamination, sincethere are no grinding operations. Currently, the powderswere manufactured in a laboratory scale plasma systemand larger systems are plarmed. Cost estimates show thatpowders produced through this method would beconsiderably less expensive than B,C powdersmanufactured through traditional methods. Sincepowders are manufactured directly, the costs associatedwith grinding disappear. It also eliminates contaminationof the powder with the materials in grinding medium andgrinding vessel. The high electric costs and pollutionassociated of the traditional electric arc furnace havelimited the amount of domestic production of B4Cpowders. A process such as the gas plasma system

The B4C powders are then collected. Depending on theresidence time n the reactor, powders of nano-scaleparticle sizes are formed. Reactor parameters must bechosen such that the reaction has time to progress tocompletion but the time/temperature/pressure profile mustminimize reversal of the reaction. The powders arecollected in a filter system and the powder is then washedto remove any residual B2 that remains.

2B,O,+ 7C B,C+ 6CO

PPG is producing their B4C powders through a verydifferent method, in which the ceramic powder is formedby directly by reacting the precursors in a gas. The rawmaterials are vaporized and atomized in the thermalplasma, along with any sintering aids. The basic reactionwould be a boron precursor, such as boron oxide B,O,),and a carbon source carbon or a hydrocarbon) arevaporized and atomized in a thermal plasma. As theplasma cools as it moves through the reactor, B4C powderis formed through the following reaction:

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007Year

Figure2. B,C powder imports as a function of countryand year

Figure 1. A large B4C ingot is manufactured in anAcheson furnace, and is subsequently ground into powderof various grades.

Like SiC, B,C is a man-made material; the ceramicpowder is typically produced through the carbothermalreduction of boron oxide in an electric arc Acheson)furnace, Figure 1. The end product is a large ingot of B4Cthat must be pulverized and ground down into the desiredgrit and powder sizes. Both the high temperature furnaceoperation and the subsequent grinding operations make ita very energy-intensive approach to making B4C powder;and this accounts for the high cost of the powder and thelimited powder production capacity in the United States.After the B,C powder is ground and sized, only a portionof the powder is of armor grade quality, since there isvariability of the material in the ingot. The remaindermust be used for other applications such as the abrasivesindustry. Nearly all B,C powder is imported, with Chinabecoming a major player in the B4C powder business.Figure 2 shows the two main suppliers of boron carbide tothe United States are China and Germany. It should benoted that these numbers are for all grades of powder, notjust armor-grade material. But it is interesting to note thatmajor military procurement programs for body aITIlOrwere going on during this period of rapid increases inpowder imports. Typically, armor-grade B4C powder hasa cost of approximately 50/lb, while a sintered aluminumoxide might be 5/lb for the [mished tile.


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considerably less expensive to manufacture parts, such asa SAPI plate, using a pressureless sintered technique.

Figure 4. B4C plates formed via hot pressing with acurved shape.Commercial B4C shows a large number of lenticulargraphitic inclusions Figure 5 Inclusions of this type arealso observed fracture surfaces; therefore suggestingthat these are not polishing artifact , is difficult toascertain how these inclusions are introduced into thematerial - in the starting powders during green fonning orduring densification. Interestingly the ballistic propertiesdon t seem to be particularly sensitive to these inclusions.However these inclusions do affect the mechanicalproperties of the materials and they act as crack initiationpoints flexure testing

Figure 5 SEM micrograph showing large lenticulargraphitic inclusions in commercial hot pressed B,C

5. CPRESSURELESS SINTERING N HIPINGGeorgia Tech and Verco Materials have developedmethods to manufacture B,C without the aid of uniaxial

pressure during densification. They have been able todevelop sintering profiles that allow for high densities tobe achieved. By understanding when chemical reactionsoccur, when compounds breakdown and how materialsout-gas a sintering schedule can be developed thatminimizes time at temperature ranges where unfavorabletransformations occur. differential di1atometer wasused to develop these sintering schedules and understandwhen these reactions occur similar to the data shown inFigure 2. They have attained closed-porositymicrostructures which can then be successfully post-hotisostatically pressed to a zero-porosity state. HIPinginvolves heating the part in the presence of high-pressureinert gas which applies a hydrostatic pressure on the partwhich is shape preserving. HIPing closes the remainingporosity and theoretical density can be achieved. HIPingis done commercially in very large vessels and largenumber of parts can be densified at one time. Therefore,this step adds marginal cost to the overall process.A major opportunity for pressureless sintering is parts ofcomplex geometry, which cannot be produced by hotpressing. The undensified green bodies for these partsare made by established ceramic processing methods,such as extrusion injection molding or slip casting.Figure 6 shows a throat and upward blast protection B,Cinsert that Verco Materials produced via pressurelesssintering. This processing route would al low for themanufacture of complex-shaped pieces for conformablebody armor, such as shoulder inserts and neck protectors.

Figure Complex shaped part of B,C manufactured byVerco Material via pressureless sintering usingconventional powders.


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ummary oronCarbide is an important armor ceramic that is usedprimarily in personnel protection due to its low density.Pressureless sintering techniques developed y VercoMaterials for B C would allow for lower costs nd theability to form conformable sbapes. Traditional B Cpowders re all imported and are rather expensive. PPGis manufacturing powders through a different routeThese powders are nano sized and offer the opportunity toproduce price competive powders domestically. ThePPG powders have been densified to full density byVerca Materials


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09/2212008 NOVEL PROCESSING OF BORONCARBIDE (B4C): PLASMA SYNTIfESIZ3. Author(s) 4. Office Symbol(s) 5. Telephone Nr(s)James Campbell, Melissa Klusewitz et a1 AMSRD-ARL-WM-MD 4 0896

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NOVEL PROCESSING OF BORON CARBIDE (B4C): PLASMA SYNTHESIZED NANO POWDERS AND PRESSURELESS SINTERING...

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