Characterization, Processing, and Consolidation of 

Nanoscale Tungsten Powder 

by Bradley R. Klotz, Franklyn R. Kellogg, Eric M. Klier, Robert J. Dowding, 

and Kyu C. Cho 

ARL-TR-5045 December 2009 

Approved for public release; distribution is unlimited. 
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Army Research Laboratory 

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ARL-TR-5045 December 2009 

Characterization, Processing, and Consolidation of 

Nanoscale Tungsten Powder 

Bradley R. Klotz 

Dynamic Science, Inc. 

Franklyn R. Kellogg 

Data Matrix Solutions 

Eric M. Klier, Robert J. Dowding, and Kyu C. Cho 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
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Characterization, Processing, and Consolidation of Nanoscale Tungsten Powder 

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Bradley R. Klotz,* Franklyn R. Kellogg,† Eric M. Klier, Robert J. Dowding, and 

Kyu C. Cho 

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1L162618AH80 

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Dynamic Science, Inc., Aberdeen Proving Ground, MD 21005-5069 

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Data Matrix Solutions, Aberdeen Proving Ground, MD 21005-5069 

14. ABSTRACT 

Nanoscale tungsten powder was consolidated by the Plasma Pressure Compaction (P2C) method. The starting powder was first 

characterized to determine particle size, morphology, and presence of chemical impurities. The characterization data led to 

efforts to deagglomerate the as-received powder through milling and experiments into hydrogen reduction of oxygen present in 

the powder. After consolidation, densities of the resulting samples were measured, microstructural features (including grain 

size) were examined, and chemical analysis was carried out to detect the presence of impurities. The study concluded that it 

was possible to consolidate the nanocrystalline powder and maintain nano-sized grains. However, retention of initial grain size 

came at the expense of sample density. 

15. SUBJECT TERMS

tungsten, nanomaterials, powder consolidation 

16. SECURITY CLASSIFICATION OF: 

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OF ABSTRACT 

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Bradley R. Klotz 

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iii

Contents 

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Experimental Procedure 2

3. Results and Discussion 4

4. Conclusions 24

5. References 25 

Distribution List 26 
iv

List of Figures 

Figure 1. The P2C apparatus: (a) schematic and (b) photograph of sample chamber. ...................4

Figure 2. FESEM images of as-received tungsten powder. ............................................................5

Figure 3. Particle size distributions for as-received tungsten powder as measured on the (a) 

Horiba LA-910 and (b) Horiba LB-500. ....................................................................................6

Figure 4. X-ray diffraction pattern for as-received tungsten powder. ............................................8

Figure 5. Micrographs showing the effects of milling time. Samples were milled with 50 

pellets at 200 RPM for (a) 1, (b) 2, (c) 4, and (d) 8 h. ...............................................................9

Figure 6. Micrographs showing the effects of the number of milling pellets used. All 

samples were milled for 1 h at 200 RPM with (a) 25 10-mm tungsten carbide balls, (b) 50 

10-mm tungsten carbide balls, and (c) 75 10-mm tungsten carbide balls. ..............................10

Figure 7. Micrographs showing the effects of milling RPM on powder. All samples were 

milled with 50 pellets for 1 h at (a) 100, (b) 200, and (c) 300 RPM. ......................................11

Figure 8. Particle size distribution plots for (a) unmilled powder, (b) powder milled 1 h with 

50 balls at 200 RPM, and (c) powder milled 2 h with 50 balls at 200 RPM. ..........................12

Figure 9. Plot of mean particle size vs. milling time. ...................................................................13

Figure 10. Graph of mean particle size change with amount of milling media. ...........................14

Figure 11. Graph of mean particle size change with mill RPM. ...................................................14

Figure 12. Hydrogen reduction curve for tungsten. ......................................................................17

Figure 13. FESEM micrographs of hydrogen reduced compacts (a) unheated, (b) 650, (c) 

750, and (d) 850 °C. .................................................................................................................17

Figure 14. FESEM micrographs of P2C samples consolidated at 1600 °C and 100 MPa using 

(a) milled powder reduced in a graphite die, (b) milled powder reduced in an alumina 

boat, (c) unreduced milled powder, and (d) unreduced unmilled powder. ..............................20

Figure 15. FESEM micrographs of P2C samples consolidated at (a) 1100, (b) 1200, and (c) 

1300 °C. ...................................................................................................................................21

Figure 16. Samples reduced in P2C chamber prior to consolidation. ...........................................22

Figure 17. FESEM images showing an (a) oxide free zone and (b) oxide containing region in 

P2

C consolidated samples. .......................................................................................................23
v

List of Tables 

Table 1. BET surface area values. ..................................................................................................5

Table 2. Elemental analysis of tungsten powder. ...........................................................................7

Table 3. Chemical analysis of milled tungsten powders...............................................................15

Table 4. BET surface area measurements for hydrogen reduced samples. ..................................15

Table 5. Density of tungsten measured by helium pycnometry. ...................................................18

Table 6. Densities of P2C consolidated samples with varying starting powder conditions. .........19

Table 7. Densities of P2C samples processed at lower temperatures. ...........................................21
vi

Acknowledgments 

This research was supported in part by an appointment to the research participation program at 

the U.S. Army Research Laboratory (ARL) administered by the Oak Ridge Institute for Science 

and Education through an interagency agreement between the U.S. Department of Energy and 

ARL. 
1

1. Introduction 

The high density of tungsten (W) makes it desirable for use in anti-armor applications, in 

particular as a replacement material for depleted uranium (DU) in kinetic energy (KE) 

penetrators. Unstable plastic deformation and flow softening of DU produce shear localization 

in the form of adiabatic shear bands in the KE penetrator upon impact, resulting in the formation 

of a chisel-nose on the penetrator tip and enhanced penetration performance (1). The tendency 

of conventional grain size W to exhibit plastic stability and resistance to flow localization limits 

its applicability in these areas (2). However, it has been shown that when tungsten grains are 

refined to 500 nm or less by severe plastic deformation techniques, significant flow localization 

is exhibited and strain rate sensitivity is reduced (3). Furthermore, for W with grain sizes of 

~100 nm produced by high pressure torsion, localized shearing is observed (4). Thus, it is 

possible that W with grain sizes in the nano-crystalline (<100 nm) or ultra-fine grain (<500 nm) 

regime could exhibit the failure behavior required for use in anti-armor applications. 

The consolidation of nano-sized tungsten powder is one method of producing bulk 

nanocrystalline parts. A requirement for this approach is the ability to obtain large quantities of 

good quality tungsten nanopowder that can then be consolidated by a densification technique that 

will produce a part with high density while retaining the initial grain size. Densification 

behavior of tungsten nanopowder is reliant on the properties of the initial powder, such as 

chemical purity, particle size, size distribution, morphology, and degree of agglomeration. 

Detailed characterization of starting powder is therefore important to assess its suitability for use 

in the production of bulk tungsten parts. Furthermore, efforts have already been undertaken at 

the U.S. Army Research Laboratory (ARL) to consolidate fine-grained tungsten by the Plasma 

Pressure Compaction (P2

C) method (5). The P2C process works by subjecting a powder sample 

to an electrical current with very high amperage, enabling rapid heating rates and very short high 

temperature soak times which could result in rapid consolidation with little grain growth. The 

previous studies showed that submicron tungsten powder could be consolidated by P2

C to 97% 

theoretical density. However, the final grain size of the samples was approximately ten times 

larger than the initial powder size. Perhaps the use of nanocrystalline tungsten powders could 

provide for a sample with final grain size in the nanocrystalline or ultra-fine grain regime.

A purportedly nanocrystalline tungsten powder has become commercially available. The 

following report details efforts to evaluate this tungsten powder for use in the consolidation of 

bulk nanocrystalline samples. Particle size, morphology, degree of agglomeration, and chemical 

purity of the starting powder were examined using a variety of powder characterization 

techniques. Several different methods of treating the powder prior to consolidation were also 

undertaken to improve suitability of the powder for the production of high density samples. 
2

Consolidation of the powder was carried out by the P2C method, and resulting samples were 

examined to evaluate properties such as density, grain size, and chemical purity. 

2. Experimental Procedure 

Approximately 25 kg of the nanocrystalline tungsten powder were obtained. A particle size of 

45 nm was calculated for the powder using the vendor-supplied BET surface area value of 

6.86 m2/g. The size and morphology of the as-received powder were observed on a Hitachi 

S-4700 Field Emission Scanning Electron Microscope (FESEM) with energy dispersive x-ray 

spectroscopy (EDS) capabilities. Surface area of the powder was measured on a Micromeritics 

ASAP 2000 Brunauer, Emmet, and Teller (BET) Surface Area Analyzer. Samples of the powder 

were also dispersed in a water-glycerol mixture, and the particle size distribution (PSD) was 

determined by laser scattering techniques on both a Horiba LA-910 Laser Scattering Particle 

Size Distribution Analyzer and a Horiba LB-500 Dynamic Light Scattering Particle Size 

Analyzer. Elemental chemical analysis of the powder was carried out for ARL by Luvak, Inc. of 

Boylston, MA, and the results were compared to those provided by the manufacturer, as well as 

measurement data provided by Kennametal, Inc of Latrobe, PA. In addition, an x-ray diffraction 

(XRD) pattern to detect the presence of impurity phases in the powder was generated on a 

Phillips X-Ray Diffractometer with Phillips X-Pert Analysis software. 

Both electron microscopy and PSD measurement showed that the powder consisted of very large 

agglomerates of fine powder. To break up these agglomerates, a rudimentary milling procedure 

was applied. Different milling conditions were used in an effort to determine the best milling 

parameters. For the first experiment, 100 g of tungsten powder was placed in a 250-mL tungsten 

carbide jar with 50 10-mm diameter tungsten carbide balls and milled on a Retsch Type PM 400 

Planetary Ball Mill at 200 RPM for 8 h. A small amount of powder was removed from the jar 

after 1, 2, and 4 h to observe the effect of milling time on particle size. Additional samples of 

powder were also milled with 25 and 75 balls to examine the effect of amount of milling media, 

as well as at speeds of 100 and 300 RPM to examine the effect of mill speed. Milled powder 

was characterized using the same techniques detailed for the starting powder, including chemical 

analysis after milling to determine if any impurities were introduced through the milling process. 

After chemical analysis detected the presence of a large amount of oxygen in both the starting 

and milled powders, experiments on hydrogen reduction of the powder were carried out by 

heating samples of the powder in a tube furnace under hydrogen atmosphere. Surface area of the 

reduced powder samples was measured by the BET method to determine whether sintering of the 
3

nanopowders had occurred during the reduction process. The presence of oxide phases could 

also result in the starting powder having a density lower than that of pure tungsten. Thus, a 

Micromeritics AccuPyc 1330 helium pycnometer was used to measure the density of the powder. 

The principle used in helium pycnometry is the same as Archimedes’ method for density 

determination, except water is replaced with helium gas, making it possible to accurately 

determine densities of powders. The tungsten powders were loaded into a 10 cm3 aluminum 

cylinder and placed in the pycnometer. The powder was tested in three conditions: as received 

powder, as received powder that was baked overnight at 120 °C to drive off absorbed moisture, 

and powder that was planetary milled and baked over night at 115 °C. Pycnometer testing was 

repeated until the results were reproducible (based upon the size of the standard deviations). 

The development of P2

C processing parameters for tungsten have been described previously 

(5–6). For this study, 30–50 g of W powder were packed into 1-in-diameter graphite dies and 

placed between two water cooled platens in the chamber of the P2

C apparatus (figure 1). 

Samples were loaded at a pressure of 22 MPa. Prior to consolidation, the chamber was pumped 

down to 10 millitorr or less. All consolidation runs were carried out under vacuum, and certain 

samples were also pre-treated in the P2C chamber under hydrogen. The samples were first 

subjected to a pulsed current with 10-s on/off times. Pulsed currents of 200, 400, 600, 800, and 

1000 A were applied in a step-wise fashion with 5-min intervals at each pulsing step. The 

temperature of the sample was monitored at the outside surface of the die with an optical 

pyrometer, which did not register a temperature until the die wall reached 636 °C. The powder 

compact itself inside the die has been shown to be at a much higher temperature (7). The pulsing 

parameters were run until the pyrometer started to read a temperature. For most samples, this 

occurred after being at 1000 A for 5 min. Once the pyrometer became active, the pressure was 

increased to 100 MPa, the power supply was switched to constant current mode, and the current 

was rapidly increased to 5000 A. The applied current of 5000 A was maintained until the 

sintering temperature was reached, at which point the current was rapidly lowered to zero, 

providing for a dwell time at temperature of essentially 0. The sample was then allowed to cool 

under vacuum. 

For samples which underwent hydrogen pre-treatment, the powder was placed in a graphite die 

and loaded into the P2

C. The sample was then held at 22 MPa inside the P2C apparatus at a 

chamber pressure of ~5 psi under a constant flow of hydrogen and heated by graphite resistance 

heaters in the chamber. A small hole was drilled into the graphite die and a thermocouple was 

inserted to measure the temperature of the sample. This thermocouple was then used to control 

the temperature of the resistance heaters. Samples were heated under hydrogen to the desired 

temperature and held there for a set time, in this case 380 min. After reduction, the chamber was 

backfilled with argon and then pumped down to a vacuum level of 5 millitorr before the 

consolidation was performed as detailed previously. 
4

Figure 1. The P2C apparatus: (a) schematic and (b) photograph of sample chamber. 

After consolidation, samples were ground to remove graphite or tungsten carbide on the surface, 

and density was measured by Archimedes’ method. Samples were then sectioned perpendicular 

to the pressing direction to provide specimens for scanning electron microscopy and chemical 

analysis. 

3. Results and Discussion 

FESEM micrographs of the as-received tungsten powder are shown in figure 2. The lowermagnification micrographs show very large, rectangular prism-shaped aggregates, some as large 

as 10–20 m. Higher magnification images show these large aggregates to be comprised of 

individual fine particles on the order of 100–200 nm that are joined together. These large “hard” 

agglomerates of solidly joined particles will most likely need to be broken up to enhance the 

sinterability of the starting powder. The nature of the joining between the particles means that 

some sort of mechanical milling procedure will have to be performed to break the larger 

aggregates down to the individual fine particles. 

BET surface area values for the powder measured at ARL, as well as those obtained from the 

vendor and Kennametal, Inc., are shown in table 1. The three measurements were relatively 

close to each other, with the ARL-generated value showing the highest surface area at 5.17 m2

/g. 

Back-calculating for a particle diameter using each BET surface area values resulted in a particle 

diameter range of 40–60 nm for the three measurements. All three values were lower than the 

100–200 nm value for the individual particles estimated from the FESEM photos, but the nature 

of the agglomeration in the powder makes getting accurate particle diameter values from surface 

area values difficult. 

(a) (b) 
5

Figure 2. FESEM images of as-received tungsten powder. 

Table 1. BET surface area values. 

Vendor Supplied Kennametal Measured ARL Measured 

BET surface area 6.86 m2/g 7.78 m2

/g 5.17 m2/g 

Particle diameter 45 nm 40 nm 60 nm 
6

Particle size distribution plots of the as-received powder obtained from both the Horiba LA-910 

and LB-500 instruments are shown in figure 3. The LA-910 is able to detect a large range of 

particle sizes, so it is able to distinguish the large agglomerates observed by FESEM as 

evidenced by the peak centered at ~10 m. The plot also shows that the powder contains 

particles with sizes as small as ~500 nm. The LB-500 has a much smaller range of particle size 

detection capabilities and is more suited to detecting very fine particles. The sample of ZY 

powder was shown to have fine particles with a size range centered between 400 and 500 nm, 

with some particles as small as 200 nm present. The generated distributions appeared to agree 

with the size of the particles observed by FESEM. 

Figure 3. Particle size distributions for as-received tungsten powder as measured on the (a) Horiba LA-910 and 

(b) Horiba LB-500. 

The results of chemical analysis of the tungsten powder are shown in table 2. The amount of 

sulfur and carbon were determined by combustion infrared detection, oxygen content was 

measured by inert gas fusion, and all other elements were measured by direct current plasma 

emission spectroscopy. The analysis showed a relatively high amount of oxygen in the powder. 

As a result of the high oxygen content, powder reduction experiments were undertaken.

(a)

(b)
7

Table 2. Elemental analysis of tungsten powder. 

Element Vendor Supplied 

(Weight-Percent) 

Kennametal Measured

(Weight-Percent) 

ARL Measured 

(Weight-Percent) 

Al <0.0005 — <0.0005 

As <0.0008 — <0.0020 

Bi <0.0001 — <0.0020 

C not supplied <0.01 0.029 

Ca 0.0010 — 0.0010 

Cd <0.0001 — <0.0005 

Co <0.0005 — <0.0005 

Cr 0.0013 — <0.0005 

Cu 0.0001 — <0.0005 

Fe 0.0039 <0.025 0.0026 

K 0.0013 — <0.0005 

Mg <0.0003 — <0.0005 

Mn <0.0005 — <0.0005 

Mo 0.0015 — 0.0020 

Na 0.0025 — 0.0032 

Ni 0.0007 <0.025 0.0021 

O 0.9400 0.98 1.40 

P 0.0005 — <0.0020 

Pb <0.0001 — <0.0010 

S 0.0017 — 0.0025 

Sb <0.0006 — <0.0020 

Si 0.0028 — 0.0023 

Sn 0.0004 — <0.0020 

Ti <0.0005 — <0.0005 

V <0.0005 <0.025 <0.0005 

An XRD pattern for the powder is shown in figure 4. No significant amounts of other phases 

were detected, as only peaks corresponding to tungsten were shown. The large amount of 

oxygen detected in the powder by chemical analysis could therefore be present as elemental 

oxygen on the surface of the powders. If tungsten oxide phases are present, the quantity was not 

significant enough to be detected by XRD.
8

Figure 4. X-ray diffraction pattern for as-received tungsten powder. 

Based on the results of the FESEM and particle size distribution analyses, planetary ball milling 

of the as-received powder was performed to try and obtain a starting powder with the finest 

particle size possible. After 1 h of milling, the rectangular agglomerates were seen to be mostly 

broken up, but the powder was still highly agglomerated (figure 5). Further milling up to 8 h did 

not appear to change the appearance of the powder. Milling the powder for 1 h with different 

amounts of milling media (figure 6) and at different RPMs (figure 7) also appeared to be 

effective in breaking up the initial agglomerates. However, from FESEM observations alone, it 

is not clear enough to determine which set of milling parameters did the best job of breaking 

down the powders. Particle size distribution analysis was also performed to better reveal the 

changes in the particle size of the powder achieved through planetary milling, and to determine 

which combination of milling parameters is best for achieving powder in the most desirable 

condition for consolidation. 

As with the starting powder, the particle size distribution of the milled powders was measured 

using two different laser light scattering methods. Plots of particle size distribution as 

determined by the Horiba LA-910 particle size analyzer for two samples of the milled powder, as 

well as a plot of the as-received powder for comparison, are shown in figure 8. Many of the 

large aggregates appeared to be broken up after 1 h of milling with 50 balls at 200 RPM, as 

evidenced by the formation of a peak at around 500–600 nm and a shift of the low end of the 

distribution to ~100 nm (figure 8b). The peaks at higher particle size still present for the milled 

sample could either be due to some large aggregates still remaining after milling or the formation 

of agglomerates of fine particles as observed in the FESEM images. Milling for longer than 1 h 

with the same mill speed and media did not appear to further reduce the size of the finest 

particles or completely remove the large agglomerates (figure 8c).
9

Figure 5. Micrographs showing the effects of milling time. Samples were milled with 50 pellets at 

200 RPM for (a) 1, (b) 2, (c) 4, and (d) 8 h.
10

Figure 6. Micrographs showing the effects of the number of milling pellets used. All samples were 

milled for 1 h at 200 RPM with (a) 25 10-mm tungsten carbide balls, (b) 50 10-mm tungsten 

carbide balls, and (c) 75 10-mm tungsten carbide balls. 

(a) 

(b) 

(c) 
11

Figure 7. Micrographs showing the effects of milling RPM on powder. All samples were milled with 

50 pellets for 1 h at (a) 100, (b) 200, and (c) 300 RPM. 

(a) 

(b) 

(c) 
12

Figure 8. Particle size distribution plots for (a) unmilled powder, (b) powder milled 1 h with 50 balls at 200 RPM, 

and (c) powder milled 2 h with 50 balls at 200 RPM. 

(a)

(b)

(c) 
13

Data generated by the LB-500 dynamic light scattering method, which is able to better measure 

very fine particles, showed that there was a significant reduction in mean particle size of the fine 

particles in all the milled samples compared to the as-received powder. Mean particle size was 

seen to decrease most dramatically after 1 h of milling and changed very little as milling time 

increased (figure 9). The data also showed that using 50 balls reduced the average particle size 

more than when 25 or 75 balls were used (figure 10). It is believed that using 75 pellets caused 

the particles to re-agglomerate, leading to a larger mean particle size. The RPM at which the 

mill is operated was also seen to have a large effect on particle size (figure 11). The mean 

particle size when running at 300 RPM was reduced from ~440 to ~74 nm, which was slightly 

lower than the ~88 nm size achieved when 200 RPM was used. At 100 RPM, the mean particle 

size decreased only to ~260 nm, showing that there is insufficient energy in the system to 

accomplish a significant decrease in particle size at that speed. Thus, the particle size 

distribution data collected suggests that the best milling procedure was 1 h of milling at 300 

RPM with 50 balls. However, the milling jar was found to be too hot to handle immediately 

after milling at 300 RPM, and, after cooling, the jar expelled gas laced with tungsten particles 

into the air when opened. This made it undesirable to utilize the 300 RPM mill speed. 

Figure 9. Plot of mean particle size vs. milling time.

ZY Tungsten Particle Size vs Milling Time

100.0000

150.0000

200.0000

250.0000

300.0000

350.0000

400.0000

450.0000

500.0000

550.0000

600.0000

650.0000

700.0000

750.0000

0123456789

Hours of Milling Time

Ave. Particle Size (nanometers)
14

Figure 10. Graph of mean particle size change with amount of milling media. 

Figure 11. Graph of mean particle size change with mill RPM.

Particle size after milling with different amounts of balls

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

475

500

525

550

Mean Particle Size (nanometers)

0 balls

25 balls

50 balls

75 balls

Effect of RPMs on Size with 50 pellets

0

25

50

75

100

125

150

175

200

225

250

275

Average particle size (nanometers)

100 RPMs

200RPMs

300RPMs
15

Powder samples were sent for chemical analysis to attempt to detect contamination from the 

tungsten carbide milling media and any further oxygen picked up by the powder during the 

milling process. Carbon was tested for via combustion infrared detection, cobalt content was 

measured by direct current plasma emission spectroscopy, and oxygen was detected by inert gas 

fusion. The results show that carbon and cobalt levels did not change very much during milling, 

while oxygen levels were seen to increase (table 3). The data shows that the most effective 

milling procedure (50 balls for 1 h at 300 RPM) led to an increase in oxygen content from 1.4 

weight-percent oxygen in the as-received powder to ~1.78 weight-percent in the milled 

specimens. Using 50 balls for 1 h at 200 RPM led to an increase from 1.4 to 1.74 weight-percent 

oxygen. Since the use of both 200 and 300 RPM mill speeds produced samples with a similar 

increase in oxygen content, as well as similar average particle sizes, it was chosen to mill the 

powder using 50 balls for 1 h at 200 RPM, due to the fact that handling the milling jar afterward 

was safer and easier than when running at the higher speed. 

Table 3. Chemical analysis of milled tungsten powders. 

Milling Conditions Carbon 

(Weight-Percent)

Oxygen 

(Weight-Percent) 

Cobalt 

(Weight-Percent) 

Un-milled 0.029 1.40 0.010 

25 balls, 200 RPM 0.024 1.47 0.010 

50 balls, 200 RPM 0.026 1.74 0.0070 

75 balls, 200 RPM 0.026 1.81 0.0078 

50 pellets, 100 RPM 0.025 1.49 0.0092 

50 pellets, 300 RPM 0.029 1.78 0.0083 

The presence of oxygen in the powder both before and after milling made it necessary to reduce 

the powder under a hydrogen atmosphere prior to compaction. A reduction time and temperature 

combination needed to be determined that would allow for adequate reduction of the oxygen in 

the powder without causing any additional agglomeration or sintering of the particles. Green 

bodies were made by pressing 25 g of milled tungsten powder to 4 ton in a 1-in stainless steel 

die. A control sample of as-received powder was also pressed. The green bodies would then be 

placed in a tube furnace and heated under hydrogen. In previous work with larger grained 

tungsten powder, it was determined that the best method for tungsten reduction was to place the 

powder in a flat alumina boat in a tube furnace and soak at 850 °C for 2 h under hydrogen (5). 

Thus, the first compact was heated to 850 °C and held for 2 h in a hydrogen atmosphere. The 

results of BET surface area measurement of the reduced samples are shown in table 4. The 

specimen reduced at 850 °C showed a decrease in surface area when compared with an unheated 

compact, indicating that sintering of the tungsten particles may have occurred during the 

reduction process. The decision was made to reduce the temperature to 750 °C while keeping 

the hold time the same. The first sample made at 750 °C spontaneously reacted when it was 

Table 4. BET surface area measurements for hydrogen reduced samples.
16

Reduction Temperature 

(°C) 

Reduction Time 

(min) 

BET Surface Area 

(m2

/g) 

unheated 0 5.1677 ± 0.0786 

850 120 3.5352 ± 0.0407 

750 120 4.6788 ± 0.0462 

650 380 5.3574 ± 0.0681 

pulled from the furnace. It changed color from purple to brown, then to orange and yellow, 

finally ending up as a bluish-green pile of ash. XRD confirmed that several different tungsten 

oxides were present in this ash. A second compact run under the exact same parameters survived 

the heating cycle and was removed from the furnace intact. For the sample reduced at 750 °C 

that survived the furnace treatment, the BET measurements also showed a slight decrease in 

surface area. The experiment was repeated at 650 °C for 2 h. Two samples made at this 

temperature also reacted upon removal from the furnace to became tungsten oxide. At first, it 

was assumed that this reaction was being fueled by residual water and oxygen in the sample due 

to incomplete reduction. A more likely explanation is that the samples reacted due to the fact 

that the tungsten oxide present in the powders, which acted as a passivation layer, had been 

removed, and the particles had not sintered enough to overcome the reactivity that nano-sized 

particles often have due to their high surface area. The samples treated at the higher 

temperatures did not react because either they had sintered enough to increase their surface area, 

or there was still enough tungsten oxide present to act as a passivating layer. 

In order to determine which set of time and temperature parameters would not result in 

spontaneous reactions, an empirical formula developed from industrial processes used to reduce 

tungsten oxide to tungsten powder was used to calculate the required reduction times at lower 

temperatures (8). The equation is: 

 t = (6.66*1020)ρ T-5.7(f h)1.6, (1) 

where t is the reduction time (seconds), ρ is the bulk density of oxide (grams per cubic 

centimeter), T is the reduction temperature in Kelvin, f is the fraction of oxide reduced, and h is 

the bed depth in centimeters. This formula is not an exact fit for this experiment because it 

assumes that the starting powder is tungsten oxide, whereas in this case the starting powder is 

tungsten with 1.4 weight-percent oxygen. By assuming that the 2-h reduction at 850 °C leads to 

a fully reduced sample, and by using the same alumina boat and green body size for each 

experiment, it is possible to combine the unknown variables (ρ, f, and h) and solve for them 

figure 12 shows a plot of reduction temperature vs. time required to fully reduce a tungsten 

sample. Following these results, a sample was reduced under hydrogen at 650 °C for 6 h and 

20 min. It survived the reduction process and showed a BET surface area very similar to the 

unreduced green body sample (see table 4). FESEM micrographs of the reduced tungsten 

compacts are shown in figure 13. Evidence of particle coalescence and sintering can be seen in 

the 850 °C sample, while the 650 and 750 °C samples appear similar to the unheated sample.
17

Figure 12. Hydrogen reduction curve for tungsten. 

Figure 13. FESEM micrographs of hydrogen reduced compacts (a) unheated, (b) 650, (c) 750, and 

(d) 850 °C.

Hydrogen Reduction Time and Temperature Curve for Tungsten

0

2

4

6

8

10

12

14

16

18

450 550 650 750 850 950 1050

Temp (C)

Time (Hr)

(a) (b)

(c) (d)
18

Based on these results, reduction under ultra-high purity (UHP) hydrogen with a dew point of –

71 °C at 650 °C for 6 h and 20 min was found to produce a compact with the least amount of 

sintering when compared to an unreduced green body sample. 

Results of density measurement for the tungsten powder are shown in table 5. In all of the 

conditions tested, the powder showed density values lower than that of pure tungsten (19.3 g/cm3

). 

Table 5. Density of tungsten measured by helium pycnometry. 

Condition Average Density 

(g/cm3

) 

Standard Deviation 

(g/cm3) 

As-received 16.9 0.02 

Baked 16.48 0.06 

Milled and baked 15.97 0.02 

Initially, four samples of tungsten were consolidated to compare the effect of different starting 

powder conditions on the final sample. All samples were run in the P2C apparatus under the 

same pulsing and consolidation conditions. Densities for these samples are shown in table 6. 

The powder for the first P2

C consolidation sample was milled and reduced using the ideal 

milling and reduction conditions as determined previously. However, rather than reducing the 

powder in the tube furnace as a pressed compact, the powder was loaded into a graphite die prior 

to reduction. The powder was pressed inside the die, the punches were removed, and the entire 

die was inserted into the reduction furnace. This was done to facilitate easier loading into the 

P2C. After reduction in the tube furnace, the die was placed in the P2C press and processed at 

100 MPa and a die temperature of 1600 °C. The final density for the first sample was 

16.96 g/cm3 (87.9% theoretical density) with grain sizes as observed by FESEM varying 

between 1 and 10 μm (figure 14a). The sample also had tungsten oxide identified by EDS in the 

light gray phase present between the tungsten grains. The P2

C process was then repeated, this 

time with the powder reduction done in an alumina boat to allow for the tungsten powder to be 

spread out, providing a shorter diffusion path for the hydrogen and a better escape pathway for 

the water vapor formed. The resulting P2

C sample (figure 14b) still had grain sizes between 1 

and 10 μm, as well as the presence of tungsten oxide, but it had a slight increase in density to 

17.4 g/cm3

 (90.2% theoretical). The P2C parameters were repeated using unreduced milled 

tungsten powder (figure 14c) and un-milled, unreduced powder (figure 14d). The respective 

densities were 16.58 (85.9% theoretical) and 16.37 g/cm3

 (84.8% theoretical). Both samples also 

had similar grain size, as well as tungsten oxide present at tungsten grain boundaries. The 

sample made with unmilled powder also appeared to contain more porosity and a less uniform 

microstructure, likely due to the presence of the large agglomerates which would normally be 

broken up with milling.
19

Table 6. Densities of P2C consolidated samples with varying starting powder conditions. 

Powder Conditions Reduction Conditions Density 

(g/cm3

) 

Theoretical Density 

(%) 

Oxygen 

(Weight-Percent) 

Milled Reduced in die at 

650 °C for 380 min 

16.96 87.9 NA 

Milled Reduced in boat at 

650 °C for 380 min 

17.40 90.2 0.27 

Milled Unreduced 16.58 85.9 0.42 

Unmilled Unreduced 16.37 84.8 0.24 

NA = not available. 

Three of these consolidated samples were also sent for measurement of oxygen content via inert 

gas fusion. The weight percent oxygen measured for each sample is also shown in table 6. The 

consolidated sample of milled powder reduced in a boat prior to consolidation (90.2% dense) 

was found to have 0.27 weight-percent oxygen, while the unreduced, milled sample (85.9% 

dense) contained 0.42 weight-percent oxygen. However, the unreduced, unmilled powder 

(84.8% dense) sample contained 0.24 weight-percent oxygen. Based on these results, it appeared 

that the oxygen picked up by the powder during the milling step resulted in extra oxygen present 

in the consolidated sample even after reduction and consolidation. A more sophisticated milling 

procedure which disperses the tungsten powder in a solvent such as isopropanol prior to milling 

could help to prevent the addition of oxygen. 

At this point, in an attempt to determine P2

C processing conditions that would provide for 

samples with final grain size in the ultra-fine or nano regime, a series of lower temperature P2C 

runs were undertaken to attempt to reduce grain growth during consolidation. To facilitate faster 

sample production, milling and reduction of the powder prior to consolidation was discontinued 

until the desired grain size was achieved. Unreduced, un-milled powder was P2C processed at 

final temperatures of 1100, 1200, and 1300 °C using the same pulsing parameters and pressure as 

before. Density values for these samples are shown in table 7. FESEM micrographs are shown 

in figure 15. The 1100 °C sample was seen to retain the fine grain size (<100 nm) of the starting 

powder. However, the density of this sample was only 12.31 g/cm3 (63.8% theoretical). At 

1200 °C, the density of the consolidated sample was 16.54 cm3

 (85.7% theoretical), which is 

similar to what was achieved using 1600 °C. The grains size of this sample ranged from 200 to 

500 nm. At 1300 °C, the sample density was 16.03 cm3

 (83.1% of theoretical) and the grains 

were slightly larger than the 1200 °C sample. It was decided to try reduction of the powder prior 

to P2

C consolidation at 1200 °C to increase the final density of the samples while retaining the 

fine grain size.
20

Figure 14. FESEM micrographs of P2C samples consolidated at 1600 °C and 100 MPa using (a) 

milled powder reduced in a graphite die, (b) milled powder reduced in an alumina 

boat, (c) unreduced milled powder, and (d) unreduced unmilled powder. 

(a) 

(b) 

(c) 

(d)
21

Table 7. Densities of P2C samples processed at lower temperatures. 

Powder Condition 

Consolidation 

Temperature

(°C) 

Density 

(g/cm3

) 

Theoretical 

Density 

(%) 

Unmilled, unreduced 1100 12.31 63.8 

Unmilled, unreduced 1200 16.54 85.7 

Unmilled, unreduced 1300 16.03 83.1 

Unmilled, reduced in P2C 1200 15.44 80.0 

Unmilled, reduced in P2C 1200 15.88 82.3 

Figure 15. FESEM micrographs of P2C samples consolidated at (a) 1100, (b) 1200, and (c) 1300 °C.

(a) 

(b) 

(c)
22

However, when attempts were made to reduce the un-milled powder in a tube furnace under the 

previously-developed conditions of 650 °C for 6 h and 20 min, the samples spontaneously 

reacted to form tungsten oxide upon extraction from the furnace. It was unclear why the 

reduction parameters at 650 °C did not work as before. It is possible that, due to the powder 

being in the un-milled condition, the lower oxygen content in the initial powder resulted in a 

lower final oxide content and made the powder more reactive subsequent to the reduction step. 

Due to these issues with reduction of the powder in the tube furnace, hydrogen reduction runs 

were performed in the P2

C prior to consolidation. Two samples were run under identical 

consolidation conditions, both being reduced in the P2

C chamber under hydrogen at 650 °C for 

380 min prior to consolidation. Density values for these samples are also shown in table 7, and 

FESEM images of the microstructure of each sample can be seen in figure 16. Reduction in the 

P2C did not appear to have any significant effects on grain size, but did result in slightly lower 

densities than for the previous unreduced sample consolidated at 1200 °C. It is not clear why 

these samples had lower density values than the previous samples. It is possible that some 

partial sintering occurred during the reduction process, creating large agglomerates that hindered 

sintering during the final consolidation step. 

Figure 16. Samples reduced in P2C chamber prior to consolidation.

(a) 

(b) 
23

FESEM observation revealed that the samples reduced in the P2C were found to have some areas 

with porosity present, but little or no tungsten oxide (figure 17). The oxide free areas were 

always within 500 μm of the edge of the sample. Reexamination of previous P2C consolidated 

samples that were not subjected to a hydrogen environment in the chamber prior to consolidation 

also showed similar phenomena. Prior to the use of hydrogen, the standard industrial practice to 

reduce tungsten oxide to pure tungsten metal involved the use of carbon as a reducing agent (8). 

Thus, it seems probable that the oxide free zone is not a result of hydrogen reduction, but is 

formed due to contact between the tungsten powder compact and either the graphite die itself or 

carbonaceous vapors produced by the die during the P2C process. The samples reduced in the 

P2C may have had large oxide free areas because of the extra time that the graphite die was in 

contact with the tungsten powder at elevated temperatures. 

(a) (b)

Figure 17. FESEM images showing an (a) oxide free zone and (b) oxide containing region in P2C consolidated 

samples. 
24

4. Conclusions 

The purpose of this study was to characterize a commercially available nano-tungsten powder 

and evaluate its behavior when consolidated using the Plasma Pressure Compaction (P2

C) 

method with the goal of achieving a fully dense sample with a final grain size in the 

nanocrystalline or ultra-fine grain regime. Characterization revealed a need for processing of the 

powder prior to consolidation. Mechanical milling was performed to reduce agglomeration of 

the nanoparticles, and hydrogen reduction of the powder prior to insertion into the P2C apparatus 

was introduced to reduce oxygen contamination. Milling was found to break up agglomerates 

and reduce the mean particle size of the powder, providing for a more uniform microstructure 

and an increase in the final density of the consolidated samples. The external hydrogen 

reduction process prior to P2

C consolidation was shown to be capable of increasing the density 

of the samples, but the reduction step has yet to be effectively applied in-situ during P2C 

processing. Although it was possible to produce bulk samples with ultra-fine and nano-sized 

grains using the nano-scale powder, it was not possible to maintain the nanocrystallinity of the 

consolidated samples while achieving high density. 
25

5. References 

1. Magness, L. S.; Farrand, T. G. Deformation Behavior and its Relationship to the Penetration 

Performance of High Density Kinetic Energy Penetrator Materials. Proceedings of the 17th 

Army Science Conference, 1990, pp 149–165. 

2. Cai, W. D.; Li, Y.; Dowding, R. J.; Mohamed, F. A.; Lavernia, E. J. A Review of TungstenBased Alloys as Kinetic Energy Penetrator Materials. Reviews in Particulate Materials

1995, 3, 71–132. 

3. Wei, Q.; Jiao, T.; Ramesh, K. T.; Ma, E.; Kecskes, L. J.; Magness, L.; Dowding, R.; 

Kazykhanov, V. U.; Valiev, R. Z. Mechanical Behavior and Dynamic Failure of HighStrength Ultrafine Grained Tungsten under Uniaxial Compression. Acta Materialia 2006, 54

(1), 77–87. 

4. Wei, Q.; Zhang, H. T.; Schuster, B.; Ramesh, K. T.; Ma, E.; Valiev, R. Z.; Kecskes, L. J.; 

Cho, K. C.; Dowding, R. J.; Magness, L. Microstructure and Mechanical Properties of 

Super-Strong Nanocrystalline Tungsten by High-Pressure Torsion. Acta Materialia 2006, 54

(15), 4079–4089. 

5. Cho, K.; Kellogg, F.; Klotz, B.; Dowding, R. Plasma Pressure Compaction (P2

C) of 

Submicron Size Tungsten Powder. Proceedings of the International Conference on 

Tungsten, Refractory, and Hardmetals VI, 2006. 

6. Cho, K.; Woodman, R.; Klotz B.; Dowding, R. Plasma Pressure Compaction of Tungsten 

Powders. Materials and Manufacturing Processes 2004, 19 (4), 619–630. 

7. McWilliams, B.; Zavaliangos, A.; Cho, K. C.; Dowding, R. J. The Modeling of ElectricCurrent-Assisted Sintering to Produce Bulk Nanocrystalline Tungsten. JOM: Journal of the 

Minerals, Materials, and Metals Society 2006, 58 (4), 67–71. 

8. Yih, S.; Wang, C. Tungsten: Sources, Metallurgy, Properties, and Applications; Plenum 

Press: New York, 1979; p 137. 
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