Nickel–Boron Nanolayer-Coated Boron Carbide Pressureless Sintering

Kathy Lu*,w and Xiaojing Zhu

Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg,Virginia 24061

Karthik Nagarathnam

Utron Kinetics Inc., Manassas, Virginia 20109

Sintering of pure B4C and Ni2B nanolayer-coated B4C wasstudied from 13001 to 16001C, with the holding time at the peaktemperatures being 2 or 10 h. Compacts were made by uniaxialdie compaction and combustion-driven compaction. Pure B4Csample shows less sintering at all conditions. Ni2B-coated B4Csample shows more extensive densification, neck formation, andgrain shape accommodation. The combustion driven compactionprocess accelerates sintering by offering higher green density tostart with. The Ni2B species on the B4C particle surfaces meltsinto a nickel–boron-containing liquid phase during heating, re-mains as liquid during sintering, and then transforms into Ni4B3

and NiB during cooling. High-resolution composition analysisshows that there is no nickel diffusion into bulk B4C during thesintering process. However, there is boron diffusion into theNi2B coating layer. Carbon diffusion cannot be directly mea-sured but is believed to be a simultaneous process as borondiffusion. A multievent sintering process has been proposed toexplain the observations.

I. Introduction

THE B4C is the third hardest material known (Knoop hard-ness: 2800 kg/mm), ranking after diamond and cubic boron

nitride.1 In addition to the high hardness, its unique properties,such as good chemical resistance, low theoretical density (2.52 g/cm3), and neutron absorption properties make B4C an impor-tant material in different applications. Because of its excellentabrasion resistance, B4C can be used for dressing diamond tools,hot-pressed shot blast nozzles, and ceramic tooling dies.2 Be-cause of its high hardness and light weight, B4C is a preferredmaterial for armors.3 B4C is also used as an absorbent for neu-tron radiation in nuclear power plants in the form of shielding,control rod, and shut down pellets.4

In spite of these very attractive properties, it is difficult tosinter B4C compacts. This is because of the high melting point(24501C) of B4C and the low boron and carbon diffusion mo-bility resulting from the covalent bonding. To overcome theabove barriers, pressure sintering such as hot pressing (B21001C,30–40 MPa) and spark plasma sintering (14501C, 1750 A, and79 MPa) has been used.5,6 Even though pressure sintering has tosome extent achieved full densification, the process is slow, ex-pensive, and considerably limited in design flexibility.7 Pressure-

less sintering, with zero external pressure, simple equipment, andprocessing flexibility, remains to be the most desired mainstreammanufacturing process. In order to achieve high density by pres-sureless sintering, additives such as TiB2

8,9 and TiO210 were in-

corporated into B4C matrix during sintering. Pressurelesssintering of B4C with carbon doping was carried out.11–14 Thesintering kinetics of boron and B4C mixture was investigated.15

The B4C size distribution was found to determine the micro-structure of the sintered pellet and monodisperse B4C powderenabled higher densities. Graphite-coated B4C nanopowder(B50:50 volume ratio) sintering was carried out.16 The presenceof B2O3 kept B4C particles separated until its volatilization.Pressureless sintering of B4C with carbon, TiB2, and ZrO2 wasstudied in order to obtain dense pellets for use in neutron shield-ing.17 The addition of ZrO2 was found to be effective in loweringthe sintering temperature from 23751 to 22751C. TiB2 was alsofound to be beneficial for sintering densification.

If a thin, low melting temperature layer can be applied ontothe B4C particle surfaces, it should considerably facilitate thesintering of the B4C particles. When the thin layer is controlledat a very low amount and distributes homogeneously on the B4Cparticle surfaces, it should not cause significant performancedegradation, thus making B4C processing more flexible. Nickelis one of the metals that has been studied and used extensively incoatings because of its unique properties such as good ductility,lubricity, and excellent corrosion and wear resistance. If nickelcan be distributed onto B4C particle surfaces uniformly in theform of a thin layer, it presents considerable potential to improvethe sinterability of B4C. Nickel–boron (Ni–B) was depositedonto WC and VC particles by electroless coating technique.18 At4001–12001C, nickel and Ni3B phases formed. Even though theNi–B-coated powders could be precursors for liquid phase sinte-ring, the wetting behavior of the molten coating was differentbased on the characteristics of the carbide skeleton. The segre-gation of the Ni–B matrix from the VC core particles at 10351Cadversely affected densification. In the case of WC sintering,similar effect was only found when the temperature was rela-tively high (11501–12001C). Based on the similarity among WC,VC, and B4C, it is conjectured that electroless coating of Ni–Bnanolayer onto B4C particles might have the potential to facil-itate the diffusion of the boron and carbon species during sinte-ring. Under such motivation, we have carried out extensivework to understand the electroless coating process of Ni–Bnanolayer onto B4C particle surfaces and the reduction of thecorresponding Ni2O3 and B2O3 oxides in the nanolayer.19–23

In this work, sintering of pure B4C samples and Ni2B nano-layer-coated B4C samples was carried out. The samples wereobtained by uniaxial die compaction and combustion drivencompaction (CDC) processes. The effect of the Ni2B coatingon the sintering behavior and microstructures of B4C has beenexamined using scanning electron microscopy (SEM) and densitymeasurement. Phase evolution and diffusion in the Ni2B-coated

E. Olevsky—contributing editor

Presented at the International Conference Sintering 2008, November 16–20, 2008,San Diego, USA.

This work was supported by the National Science Foundation under grant no. DMI-0620621.

*Member, The Amercian Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: klu@vt.edu

Manuscript No. 25125. Received August 19, 2008; approved December 1, 2008.

Journal

J. Am. Ceram. Soc., 92 [7] 1500–1505 (2009)

DOI: 10.1111/j.1551-2916.2009.02926.x

r 2009 The American Ceramic Society

1500

B4C sintering system were studied using X-ray diffraction(XRD), energy-dispersive spectroscopy (EDS), and transmis-sion electron microscopy (TEM). A multievent sintering processhas been proposed to explain the observations.

II. Experimental Procedure

The pure B4C powder (H.C. Starck Inc., Newton, MA) used hadmonodisperse particle size distribution (Horiba, LA-950, Irvine,CA). The average particle size was about 2.27 mm.24 Method ofproducing the Ni–B nanolayer onto B4C particles by electrolesscoating was reported before.20,24 The coating layer was contin-uous and completely covered the B4C particle surfaces. Theas-coated Ni–B nanolayer on the B4C particle surfaces had amesh-like structure and was 50–70 nm thick. Ni–B nodules wererandomly embedded in the mesh structure. Ni:B elemental ratiowas 2.5:1 for the nanolayer. Before compaction, the Ni–B nano-layer-coated B4C particles were thermally treated in an Ar–H2

atmosphere to reduce the accompanying Ni2O3 and B2O3.19 At

8001C, the Ni–B nanolayer evolved into Ni2B. The Ni2B nano-layer on the B4C particle surfaces had around 50 nm thicknessbut some particle-like features were present. The cross-sectionSEM image of the Ni2B nanolayer-coated B4C particles hasbeen given elsewhere.25 The densities of pure B4C and Ni2B-coated B4C powders were measured by pycnometry (AccuPyc1330, MicroMeritics, Norcross, GA).

Both pure B4C powder and Ni2B-coated B4C powder wereused for the sintering study. Two compaction processes wereused to prepare green samples: uniaxial die compaction (TMS810 Material Test System, MTS Systems Corporation, EdenPrairie, MN) and CDC (Model 300-Ton, Utron Kinetics,Manassas, VA). The samples had disk shape with 25.4 mm di-ameter and 1.4 mm thickness. For the uniaxial die compactionprocess, the peak pressure was 180–200 MPa. For the CDCprocess, the details were reported before.26,27 In brief, the cham-ber beneath the upper ram was first filled with a mixture ofmethane and air. As the chamber was being filled, the ram

moved down and precompressed the powder. When the sealedgas mixture was ignited, chemical energy was converted to ki-netic energy and the pressure in the chamber rose toB2.02 GPawithin a few 100 ms. The ram was pushed down on the powderat an extremely high speed, realizing the compaction.26

A tube furnace (Rapid Temp Model 1730-20HT, CM Fur-naces, Bloomfield, NJ) was used to sinter the compacts. Duringthe heating cycle, the temperature was first increased to 8001C ata rate of 2001C/h. Above 8001C, the heating rate was changed to1001C/h. After the temperature reached the peak value, it washeld for 2 h. Then the furnace was cooled down with the samerates. During the entire sintering cycle, argon gas was sentthrough the furnace with a rate of 1.18 L/s.

Green and sintered densities were obtained by measuring thedimensions and weight of the samples. XRD (X’Pert PROdiffractometer, PANalytical B.V., EA Almelo, the Netherlands)was carried out to identify the crystalline phases in the sinteredsamples obtained from different conditions. The XRD experi-ment step size was 0.051 and the dwell time per step was 100 s.The scan rate was 0.0631/s with CuKa radiation (l5 1.5406 A)and a nickel filter. A field emission SEM (LEO 1550, Carl ZeissMicroImaging Inc., Thornwood, NY) was used to examine thefracture surfaces. A TEM (JEOL 2010F, 200 kV TEM, JEOLLtd., Tokyo, Japan) was used to image the Ni2B-coated B4Csample. The EDS module (Noran System 6, Thermo Nano-Trace, Waltham, MA) attached to the TEM was used to exam-ine chemical compositions in different regions of the sinteredB4C samples such as the neck region and the free surface regionof B4C grains.

III. Results and Discussion

(1) Microstructure Evolution

A comparison between pure B4C and Ni2B nanolayer-coatedB4C samples made by the uniaxial die compaction process(Figs. 1(a) and (c)) and the CDC process (Figs. 1(b) and (d))demonstrates microstructure differences after sintering. The

Fig. 1. Scanning electron microscopic images of fracture surfaces of B4C compacts after being sintered at 16001C for 10 h: (a) pure B4C made byuniaxial die compaction, (b) pure B4C made by combustion driven compaction (CDC), (c) Ni2B nanolayer-coated B4C made by uniaxial die compaction,and (d) Ni2B nanolayer-coated B4C made by CDC.

July 2009 Boron Carbide Sintering 1501

sintering condition was 16001C with a holding time of 10 h.As seen, little intergrain bonding is observed for the pure B4Ccompacts from both the uniaxial die compaction and the CDCprocesses (Figs. 1(a) and (b)). Only some grain shape accom-modation occurs. In contrast, the Ni2B-coated B4C compactsshow substantial neck formation, grain size increase, and grainshape accommodation for both compaction techniques (Figs.1(c) and (d)). There is also clear small grain disappearance, ademonstration of Ostwald ripening. This means that the Ni2Bnanolayer facilitates B4C sintering.

Sintering temperature effect on the microstructures of Ni2B-coated B4C samples can be seen in Figs. 2 and 3. Figure 2 showsthe fracture surface of the Ni2B-coated B4C powder compactsmade by the uniaxial die compaction process after being sinteredat different temperatures. Figure 3 shows the fracture surface ofthe Ni2B-coated B4C powder compacts made by the CDC pro-cess after being sintered at different temperatures. The sinteringtime is 2 h at each sintering temperature. At 13001C sinteringtemperature, the number of small surface particles decreases tovery few and the size of the surface particles increases. Also,there are liquid phase formation and grain size increase. At thesame time, B4C grain shape rounding is observed and substan-tial necking occurs (Figs. 2(b) and 3(b)). When the sinteringtemperature is 16001C, the small surface particles cannot be ob-served; substantial necking and grain growth continue to occur;and B4C grains are bonded with each other. The grain surfacelooks more uniform and the second surface phase cannot bedistinguished anymore (Figs. 2(c) and 3(c)).

The microstructure evolution processes observed in Figs. 2and 3 can be explained as follows. Ni2B has 11251C meltingtemperature.19,28 When the sintering temperature reaches themelting point of Ni2B, a Ni–B liquid solution forms and wetsB4C surfaces.29 The disappearance of the surface particles is theresult of the melting of the Ni2B species at sintering temperaturehigher than 11251C. The liquid phase enhances the diffusion ofthe boron and carbon species from B4C. With continued sinte-ring to 13001C, boron and carbon atoms diffuse into the liquidphase, results in grain neck formation, and leads to less sharp

B4C edges and corners. Because the solubility of B4C grainschanges inversely with the grain size, small grains consumethemselves and disappear, followed by reprecipitation on thelarge B4C grains. This process results in smoother B4C grainsurfaces, as seen in Figs. 2(b) and 3(b). As the sintering temper-ature continues to increase to 16001C, grain surfaces becomefaceted and the grain size is much larger with the presence of alarge portion of grain boundaries (Figs. 2(c) and 3(c)). Theabove understanding is supported by the B4C surface nanolayerevolution (Fig. 4). As sintering temperature increases to 13001C(2 h of sintering) and 16001C (10 h of sintering), the small sur-face particles disappear and the nanolayer on the B4C surfacebecomes more uniform with a thickness of roughly 100–150 nm.Also, the B4C grain edges are smoother as sintering prolongs.

The impact of the Ni2B coating and the compaction tech-niques on the microstructures of the sintered B4C samples canalso be seen from the compact density comparison (Table I).The pycnometer density of pure B4C powder is measured to be2.46 g/cm3. The pycnometer density of Ni2B-coated B4C powderis measured to be 2.83 g/cm3. These values are the average ofthree measurements for each powder. The green densities of thecompacts from the pure B4C and Ni2B-coated B4C powders areB54.5% from the uniaxial die compaction process. The greendensities of the compacts from the pure B4C and Ni2B-coatedB4C powders are 64.9% and 73.3% from the CDC process.After sintering at 16001C for 10 h, the densities are 59.1% forthe pure B4C compact and 68.4% for the Ni2B nanolayer-coatedB4C compact from the uniaxial die compaction process, 76.8%for the pure B4C compact and 83.8% for the Ni2B nanolayer-coated B4C compact from the CDC process. This demonstratesthat the CDC process substantially increases B4C sintering dens-ification, contributed by the much higher green densities fromthe technique. The sintering improvement from the Ni2B nano-layer and the CDC technique is also demonstrated by the rel-ative density increase differences: 3.8% for the pure B4Ccompact and 14.6% for the Ni2B-coated B4C compact fromthe uniaxial die compaction process, 11.9% for the pure B4Ccompact and 10.8% for the Ni2B-coated B4C compact from the

Fig. 2. Scanning electron microscopic images of fracture surface of Ni2B-coated B4C compacts made by uniaxial die compaction: (a) green state,(b) 13001C sintering for 2 h, and (c) 16001C sintering for 2 h.

1502 Journal of the American Ceramic Society—Lu et al. Vol. 92, No. 7

CDC process. The last density increase value is lower becausethe sample is at a more advanced sintering stage. Also, it shouldbe mentioned that there is still a large amount of porosity ex-isting in the B4C samples after sintering at 16001C for 10 h.Higher sintering temperature is needed for full densification ofthe B4C compacts.

(2) Phase Evolution

To examine the phase evolution of the studied B4C systems, it isimportant to separate the effects of different species. In thisstudy, the first step is to examine if there is any phase change forB4C itself. Figure 5 shows the XRD patterns of as-received pureB4C powder and pure B4C compact after being sintered at16001C for 10 h. The pure B4C compact was made by the uni-axial die compaction process. All the peaks from the two sam-ples match with each other. This means that the sinteringprocess does not change B4C structure or composition. Anyphase change to be observed should involve the Ni2B nanolayer.

Figure 5 also shows the XRD patterns of sintered, Ni2B-coated B4C samples made by the uniaxial die compaction pro-cess: 13001C for 2 h, 16001C for 2 h, and 16001C for 10 h. The

XRD patterns of the Ni2B-coated B4C sample at green state bythe uniaxial die compaction process and the Ni2B-coated B4Csample made by the CDC process after 16001C sintering for 10 hare also included for comparison. For the green state Ni2B-coated B4C sample, the XRD results show that Ni2B exists. Allthe sintered B4C samples, however, show that Ni2B has evolvedinto Ni4B3 and NiB. The B4C phase stays unchanged. Thereseems to be more NiB formation at more advanced sinteringstate, as indicated by the XRD pattern of the Ni2B-coated B4Csample from the CDC process after 16001C, 10 h of sintering.These observations mean that Ni2B is unstable at high sinteringtemperatures. During the sintering process, the Ni2B coating onB4C surfaces melts into a Ni–B liquid solution at 11251C, whichspreads on the B4C grain surfaces.29 This liquid phase acts as arapid diffusion path for B4C. With continued sintering temper-ature increase, the Ni–B liquid solution persists. During cooling,Ni4B3 and NiB solid phases form when the temperature reachesapproximately 10181C. This Ni2B phase evolution is alsoaccompanied by substantial and continuous weight loss(9–12 wt%), depending on the sintering temperatures. Theweight loss is a result of pure nickel formation and instant evap-oration, as reported before.18 This explains why at higher sinte-ring temperatures nickel evaporation is more extensive and NiB

Fig. 4. Scanning electron microscopic images of Ga1 beam cross-sectioned surfaces of Ni2B-coated B4C compacts after sintering at(a) 13001C for 2 h, and (b) 16001C for 10 h.

Fig. 3. Scanning electron microscopic images of fracture surface of Ni2B-coated B4C compacts made by combustion driven compaction: (a) green state,(b) 13001C sintering for 2 h, and (c) 16001C sintering for 2 h.

Table I. Compact Density Comparison at Green State andAfter 16001C, 10 h Sintering

Uniaxial die compaction

Combustion-driven

compaction

B4C

(%)

Ni2B-Coated B4C

(%)

B4C

(%)

Ni2B-Coated B4C

(%)

Green state 54.3 54.8 64.9 73.3Sintered state 59.1 68.4 76.8 83.8Relative densityincrease

3.8 14.6 11.9 10.8

July 2009 Boron Carbide Sintering 1503

content increases as shown in Fig. 5. Also, Fig. 5 shows thatthere is no chemical reaction between Ni2B and B4C, betweenNi4B3 and B4C, or between NiB and B4C.

(3) Diffusion During Sintering Process

While it is clear that the Ni2B-coated B4C sample undergoesliquid phase sintering, there is no knowledge about the inter-diffusion between B4C and Ni–B liquid phase at high temper-atures or simply the solubility of B4C in the Ni–B liquid phase.Because the Ni–B coating is very thin and exists on each andevery B4C grain surface, high-resolution composition analysistechnique is necessary in order to understand the interactionbetween B4C and Ni–B species. TEM and the EDS module at-tached on the TEM present to be the most promising techniqueto evaluate boron and nickel distribution in the sintered, Ni2B-coated B4C sample with nanometer resolution. Compositionalchange data on B4C grain surfaces and at B4C grain boundariescan be obtained. The Ni2B-coated B4C compact from the CDCprocess after being sintered at 16001C for 10 h is analyzed be-cause this processing condition should induce the most extensivediffusion.

Figure 6 shows the SEM images and the EDS intensity pro-files of Ni2B-coated B4C sample after being combustion drivencompacted and sintered at 16001C for 10 h. Figure 6(a) result isfrom grain interior to grain surface, which includes the Ni–Bcoating layer. Figure 6(b) result is from one grain interiorthrough the neck to the other grain interior. The two arrowsindicate the line mapping direction. By examining the EDSdata, compositional profiles for boron and nickel elements alongthe transition regions are obtained. Zero position in Fig. 6 isthe B4C grain and Ni–B nanolayer boundary, represented by thebright dots on each line. Negative coordinate corresponds to thepoints inside the B4C grain and positive coordinate correspondsto the points in the Ni–B nanolayer. In Fig. 6(b) the zero po-sition lies on the boundary that separates the right B4C grainand the Ni–B nanolayer that also extends to the left B4C grain.Only the intensities of boron and nickel are given because car-bon-containing compound was used during the EDS samplepreparation, which interferes with the analysis of the carbonin B4C.

Figure 6(a) shows that boron intensity decreases and nickelintensity increases along the direction indicated by the arrow.The nonzero intensity of boron at the end of the line is contrib-uted by the boron in the Ni–B coating layer. The boron intensitydecrease comes from boron diffusion from the grain into theNi–B coating layer. This result means that there is no nickeldiffusion from the coating layer into B4C grain under 16001C,10 h sintering condition. Figure 6(b) shows that nickel intensityincreases and then decreases while the boron intensity showsthe opposite trend. This indicates that the nickel peak pointshould be the middle of the neck region and the lower intensity

sides should fall into the two B4C grains. The neck region isthe nickel-rich region, consistent with the observation fromFig. 6(a). The thickness of the Ni–B layer indicated byFig. 6(b) is slightly over 300 nm. This thicker Ni–B layer couldjust be a result of some localized or preferential distribution ofNi–B species at the grain neck. Ni2B species melts during thetemperature increase to 16001C and forms a Ni–B liquid phasebetween B4C grains. This liquid phase enhances the diffusionof B4C.

The sintering process of Ni2B-coated B4C powder compactcan be described as follows. During the heating process, theNi2B nanolayer on B4C surfaces melts when the temperaturereaches the melting point of Ni2B (11251C) and Ni–B liquid so-lution forms. At sintering temperatures of 13001 and 16001C,especially at 16001C, 10 h sintering condition, this liquid phaseacts as a rapid diffusion path for boron and carbon atoms. Theboron and carbon species diffuse away from small B4C grains aswell as the sharp corners and edges of B4C grains and dissolvesinto the liquid Ni–B phase. As the sintering process continues,the dissolution–reprecipitation of B4C continues, resulting in theneck formation. This dissolution–diffusion–precipitation pro-cess continues until the liquid phase disappears. As a result,sintered B4C sample density increases and intergrain bondingforms. The same diffusion process happens on the free B4C sur-faces coated with the Ni2B layer but does not contribute to theB4C sample density increase. When the sintering is over, the B4Csurface phases transform to a smooth morphology, surroundingthe B4C grains, as seen in Figs. 2 and 3.

Even though the Ni2B nanolayer-coated B4C system under-goes liquid phase sintering, the liquid phase content is low andB4C grains experience very limited rearrangement. The diffusiondistance through the Ni–B liquid phase is only about 150 nmlong. The Ni–B liquid phase stays in-between the B4C grains,exerts a capillary sintering pressure, and facilitates diffusion.This leads to more extensive densification of the Ni2B-coatedB4C system. The result is consistent with nickel coated Al2O3

and SiC particle sintering.30 The Ni–B nanolayer plays criticalroles in boron and carbon diffusion, grain shape accommoda-tion, and densification.

Fig. 5. X-ray diffraction patterns of pure B4C and Ni2B-coated B4Csamples sintered at different conditions.

0

500

1000

1500

2000

2500

3000

3500

4000

0 25 50 75 100 125 150Position, nm

Inte

nsi

ty, c

ou

nts

B

Ni

0

1000

2000

3000

4000

5000

6000

7000

–200 0 200 400 600 800 1000 1200Position, nm

Inte

nsi

ty, c

ou

nts

B

Ni

B4C

(a)

(b)

Fig. 6. Transmission electron microscopic images and energy-disper-sive spectroscopic line mapping of Ni2B-coated B4C sample after16001C, 10 h sintering: (a) grain surface, (b) grain neck.

1504 Journal of the American Ceramic Society—Lu et al. Vol. 92, No. 7

IV. Conclusions

This study is focused on sintering of pure B4C samples andNi2B-coated B4C samples. The samples are obtained by uniaxialdie compaction and CDC processes. Ni2B nanolayer-coatedB4C compact undergoes liquid-phase sintering process withthe Ni–B nanolayer facilitating neck formation, densification,and grain shape accommodation in comparison with pure B4Csample. The original Ni2B phase on the B4C particle surfacestransforms into liquid phase during heating, stays as liquid dur-ing sintering, and then evolves into Ni4B3 and NiB during cool-ing but there is no phase change for the pure B4C sample duringsintering. There is no nickel diffusion into bulk B4C. However,boron and carbon diffuse into the Ni–B coating layer. A mi-crostructural evolution mechanism has been proposed to explainthe multievent sintering process.

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