NOVEL PROCESSING OF BORON CARBIDE (B4C): PLASMA SYNTHESIZEDNANO POWDERS AND PRESSURELESS SINTERING FORMING OF COMPLEX

SHAPES

James Campbell, Melissa Klusewitz, Jerry LaSalvia, and Ernest ChinU.S. Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5069

Robert Speyer and Namtae ChoGeorgia Institute of Technology

Atlanta, GA 30332

Noel Vanier, Cheng-Hung Hung, Edward Abbott and Peter Votruba-DrzalPPG Industries

4325 Rosanna DriveAllison Park, Pennsylvania 15101

William Coblenz and Toni MarcheauxDARPA, Defense Sciences Office

3701 N. Fairfax DriveArlington, VA 22203-1714

ABSTRACT

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 of 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 domestically produced startingpowder.

I. INTRODUCTION

Boron carbide is an important armor ceramic that isprimarily used in personnel protection applications due toits very low density and its ability to defeat small annsthreats. Boron carbide has a density of 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 (SAPI) 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 of 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 ofelectricity required to manufacture the material. It isformed in an electric arc furnace as a large billet ofmaterial, 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 of sintering aids directly into the powder.

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4. TITLE AND SUBTITLE Novel Processing Of Boron Carbide (B4c): Plasma Synthesized NanoPowders And Pressureless Sintering Forming Of Complex Shapes

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Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

2. TRADITIONAL B4C POWDERS

U.S. Boron Carbide Imports Top 5 Countries

3. PPG NANOPOWDERS

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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 in 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 B20) 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:

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Year

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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.

demonstrated bere will allow for tbe domestic productionof B,C powders. It is targeted tbat powders produced bytbis process may be less expensive wben produced inlarge quantities, wben compared to $50/lb wbicb is acommon cost for armor-grade powder produced bytraditional metbods.

Initial powders produced by tbe process were veryuniform and with a very fine grain size, but they showedgreat difficulty in sintering to high density, even attemptsto hot press were unsuccessful. Therefore, sintering aidswere added directly to the plasma. These sintering aidswere added as oxides and sometimes more than one wereadded together. A large number of sintering aids anddopants were investigated and densification studies wereconducted. The studies looked at densification ratesthroughout the sintering scbedule and to determine whichcombination of furnace conditions (hold times, ramp rate,atmosphere, etc) would provide the highest densities for aparticular powder system. The goal would be to producesintered densities that would be high enough so that apost-sinter hot isostatic press (HIP) could be done to closethe remaining porosity and allow for full density to beachieved. Georgia Tech conducted a nwnber ofdilatometry studies on these powders Figure 3 shows atypical densification experiment for one of the PPGcompositions, where the temperature profile with a ramprate to 230QoC is shown as well as the dimensionalchanges that were measured by the dilatometer. In theupper temperature ramp, rapid densification is apparent,which slows at the isotbermal soaking. The nano-sizedgrains are very unstable at tbese temperatures; theirmelting temperatures are significantly lowered, resultingin near-instantaneous grain growth at the temperaturescorresponding to the momentary expansions shown in thesintering curves. The final result often is a B4C withmicron-sized grains; typical to what is observed whenconventionally produced powders are used.

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1000

PPO # 161 optimization-N 500

o SO 100 ISO 200 250 300 350

Time (min)

Figure 3. Dilatometry showing dimensional changes overtime for the shown furnace temperature profile

4. B,C HOT PRESSING

After B,C powder is produced. regardless of how it wasmanufactured. it has to densified to form a useful armorproduct. Traditionally, all B,C is manufactured throughhot pressing, in which the powder is densified at a hightemperature with the concurrent application of uniaxialpressure via graphite dies. In the US, there are two maincommercial manufacturers of boron carbide: Ceradyne,Costa Mesa, CA and BAE-Advanced Ceramics Division.Vista, CA. Verco Materials, Atlanta, GA is a start-upcompany from the same Georgia Tecb research group tbatconducted the dilatometry studies on the PPG powders.They are manufacturing complex-sbaped B,C throughpressureless sintering with a subsequent HIP step. Theyare producing armor tiles and extremity armor usingconventional powders, and supplying them to a number ofarmor system suppliers and laboratories. Verco has alsoproduced armor tiles using the PPG nanopowders, whicbwere tested at ARL.

Traditional bot pressing is a batch process, where the"green" (undensified) powder compacts are formedthrough some method witb the ceramic powder andloaded into a bot pressing die. The die and powder arebrought up to the sintering temperature and a load isapplied to the die. Due to tbe very high temperaturerequired for sintering, graphite tooling and specializedfurnaces are required. Many of these non-oxide ceramicsrequire sintering temperatures in excess of2000°C. As thesize of the ceramic part increases, the load required forthe desired pressure becomes quite large. Since bodyarmor inserts are the most common B4C partsmanufactured, the load frames in the hot pressingequipment needs to be quite large. To increasethroughput, commercial vendors typically stack a numberof plates together with machined graphite spacers andapply pressure to the entire stack. Although parts ofsimple geometry are most easily manufactured by hotpressing, the commercial vendors have introducedforming methods to allow for curved plates to bemanufactured, such as a Small Arms Protective Insert(SAPI) plate. When armor tiles are manufactured by hotpressing, billets of the material are produced and the tilesground and sectioned. B,C is a very brittle material withlow fracture toughness. Therefore, it is very difficult tomachine and prone to edge chipping, which addsadditional expensive to bot pressed B,C tiles.

Hot pressing allows for the use of coarser ceramic powderand less reliance on sintering aids to form a fully densepart. Therefore, the powders may be more expensive forsintering; however, the expendable furnace and any post­densification machining would add more expense to a hotpressed part. Dve,all, it is anticipated that it would be

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 in fracture surfaces; therefore suggestingthat these are not polishing artifact", It 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 in flexure testing.

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

5. B,C PRESSURELESS SINTERING AND HIPING

Georgia 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. A 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 allow for themanufacture of complex-shaped pieces for conformablebody armor, such as shoulder inserts and neck protectors.

Figure 5. Complex shaped part of B,C manufactured byVerco Material via pressureless sintering usingconventional powders.

Summary

Boron Carbide is an important armor ceramic that is usedprimarily in personnel protection due to its low density.Pressureless sintering techniques developed by VercoMaterials for B,C would allow for lower costs and theability to form conformable sbapes. Traditional B,Cpowders are all imported and are rather expensive. PPGis manufacturing B4C powders through a different route.These 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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