Densification behavior of Mo nanopowders prepared by mechanochemical processing

Article abstract of DOI:10.1016/j.jallcom.2008.01.149

The sintering behavior of molybdenum (Mo) nanopowder was investigated in comparison to that of commercial Mo powder. At a sintering temperature of 1400 °C, the relative density of Mo nanopowder (94%) was found to be higher than that of commercial powder (

Full text of DOI:10.1016/j.jallcom.2008.01.149

Journal of Alloys and Compounds 469 (2009) 401–405
Densification behavior of Mo nanopowders prepared
by mechanochemical processing
Gil-Su Kima, Hai Gon Kima, Dae-Gun Kimb, Sung-Tag Ohc,
Myung-Jin Sukd, Young Do Kima,∗
a Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea
b Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba 305-8565, Japan
c Department of Materials Science and Engineering, Seoul National University of Technology,
Seoul 139-743, Republic of Korea
d Department of Materials and Metallurgical Engineering, Kangwon National University, 1, Joongang-ro, Samcheok-si,
Gangwon 245-711, Republic of Korea
Received 23 March 2007; accepted 27 January 2008
Available online 19 March 2008
Abstract
The sintering behavior of molybdenum (Mo) nanopowder was investigated in comparison to that of commercial Mo powder. At a sintering
temperature of 1400 ◦C, the relative density of Mo nanopowder (94%) was found to be higher than that of commercial powder (74%). In samples
of the same relative density, the measured hardness values were 2.34 GPa for nanopowder and 1.87 GPa for commercial powder. The sintered Mo
nanopowder exhibited greater hardness compared with the commercial powders. The sintering mechanism and hardness are discussed based on
the linear shrinkage behavior and observed microstructural characteristics.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Refractory metals; High-energy ball milling; Nanopowder; Sintering; Hardness
1. Introduction
density. Because of the difficulty in achieving full-densification
of Mo, much research has been focused on the enhancement of
the sinterability. In this regard, one of the suggested processes
for enhancing sinterability is the activated sintering process in
which a metal such as Ni, Pt, Pd or Co is added to Mo [6–12].
However, these elements can cause deterioration of the electrical
and thermal properties of the Mo.
Recently, densification using ultrafine Mo powder (particle
size of several micrometers) and high compaction pressure has
been proposed to enhance the sinterability [5]. The particle size
effect on sintering is known as Herring’s Scaling Law [13] and
by this law it is expected that a nanopowder will exhibit a reduc-
tion in sintering temperature compared with a powder having
particles of conventional size. Because of the enhanced sinter-
ing behavior of fine particles, we employed Mo nanopowder
in the fabrication of dense bulk material. The characteristics
of solid-state sintering of Mo nanopowder were evaluated by
dilatometric analysis under both non-isothermal and isothermal
conditions. Furthermore, microstructural characteristics and the
microhardness of sintered Mo nanopowders were investigated.
Molybdenum (Mo) has a body-centered cubic crystal struc-
ture with a melting point of 2610 ◦C and a density of
10.22 g/cm3. This material has been used for high temperature
applications in a variety of industries. In addition, the relatively
low thermal neutron cross-section of Mo makes it suitable for
nuclearapplications. Theuniquecombinationofphysical, chem-
ical and mechanical properties of Mo make it an ideal material
for a host of engineering applications where high temperature
resistance, weight considerations and ductility are key issues
[1–5].
Generally, powder metallurgy (P/M) has been used for the
production of bulk Mo components [3–5]. High sintering tem-
peratures in the range of 1800–2000 ◦C, with long sintering
times, are required for densification above 90% of theoretical
∗
Corresponding author. Tel.: +82 2 2220 0408; fax: +82 2 2282 1976.
E-mail address: ydkim1@hanyang.ac.kr (Y.D. Kim).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.01.149

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G.-S. Kim et al. / Journal of Alloys and Compounds 469 (2009) 401–405
Fig. 1. Micrographs of (a) Mo nanopowder and (b) commercial Mo powder.
2. Experimental procedure
In addition, the nanopowder exhibited a high shrinkage rate
at low temperature as shown in Fig. 2(b), wherein the shrinkage
rate was calculated by the first derivative (dY/dT) with respect
to temperature (T). In the case of the Mo nanopowder, shrink-
age started at a lower temperature of about 600 ◦C, while the
starting temperature of shrinkage for commercial powder was
1000 ◦C. The peak temperatures for shrinkage of nanopowder
and commercial powder were measured to be 953 and 1240 ◦C,
respectively. Final relative densities of the sintered samples were
94% for the nanopowder and 74% for the commercial pow-
der. These results indicate that particle size strongly influences
the sintering behavior, and thus, enhanced sinterability can be
realized by using nanopowder.
Fig. 3 shows the relative density change of Mo nanopow-
der during (a) non-isothermal and (b) isothermal sintering. In
Fig. 3(a), density shows a tendency to rise with sintering tem-
perature. At a temperature of 1400 ◦C, a relative density of about
94% was reached. In the case of isothermal sintering at 1200 ◦C
(Fig. 3(b)), the relative density increased with increasing sinter-
ing time to 1 h and then remained the same.
Commercial MoO3 (1–10 ␮m, 99.9%) and Mo (1–2 ␮m, 99.9%) powders
were used as raw materials. MoO3 powder was high-energy ball-milled at a
milling speed of 400 rpm in an Ar atmosphere for 20 h with a Simoloyer, which
is a horizontal-type attrition mill. The ball-milled MoO3 powder was reduced
under non-isothermal conditions up to a temperature of 800 ◦C with a heating
rate of 10 ◦C/min in an H2 atmosphere with a dew point of −76 ◦C. As previously
reported, the particle size of Mo powder synthesized by this process was about
100 nm [14].
The prepared Mo nanopowder and commercial Mo powder were compacted
under a pressure of 250 MPa. Green densities were about 40 and 60% of the
theoretical density, respectively. Green compacts were sintered at a heating rate
of 10 ◦C/min up to 1400 ◦C in an H2 atmosphere using a dilatometer (TDA-H-
AP6) to measure the linear shrinkage. To observe the microstructural evolution
of sintered compacts, specimens were quenched at various temperatures during
isothermal and non-isothermal heating. Densities of the sintered Mo samples
were calculated by Archimedes’ principle. The fractograph of sintered samples
was observed by field emission scanning electron microscopy (FE-SEM).
In order to analyze the effect of the initial powder size on the microstruc-
ture and hardness of the sintered compacts with the same relative density, the
nanopowder and commercial powder were sintered under a heating rate of
10 ◦C/min at 1200 ◦C for 1 h and 1500 ◦C for 3 h until a relative density of
95% was achieved for both. The grain size of sintered samples was measured
using an Image Analyzer (UTHSCSA Image Tool). Vickers hardness tests were
performed by a microhardness tester using a load of 0.1 kg. Each hardness value
was calculated from an average of 20 indentations.
3. Results and discussion
Fig. 1 shows the morphologies of the reduced Mo powder
with a particle size of about 100 nm and commercial Mo pow-
der with relatively coarse particles of 1–2 ␮m. In a previous
experiment [14], Mo nanopowder (Fig. 1(a)) was successfully
fabricated by ball-milling and subsequent hydrogen-reduction
of MoO3 powder. The XRD analysis of the reduced Mo powder
revealed that the oxide peaks completely disappeared.
Linear shrinkage curves for nanopowder and commercial
powder are shown in Fig. 2. As clearly shown in Fig. 2(a), the
shrinkage behavior of the Mo nanopowder was quite different
from that of the commercial Mo powder. The linear shrinkage
(Y = dL/L0 × 100) of the compacts showed largely different val-
ues: 22% for the Mo nanopowder and 4% for the commercial
powder at a sintering temperature of 1400 ◦C.
Fig. 2. Linear shrinkage (a) and shrinkage rate (b) of Mo nanopowder and
commercial Mo powder with a heating rate of 10 ◦C/min.

G.-S. Kim et al. / Journal of Alloys and Compounds 469 (2009) 401–405
403
Fig. 3. Relative density of Mo nanopowder under (a) non-isothermal and (b) isothermal sintering conditions.
Fig. 4 shows the fractographs of quenched specimens at sin-
tering temperatures of 1000, 1200 and 1400 ◦C. At a sintering
temperature of 1000 ◦C (Fig. 4(a)), the sintered density was
about 53%. This microstructure corresponds to the initial-stage
sintering. The intermediate stage is characterized by simulta-
neous densification and grain growth [15]. The intermediate
stage microstructure is clearly observed in Fig. 4(b) in which
specimens have a relative density of about 85%. Final-stage
sintering is a slow process compared with the initial and interme-
diate stages. The open pores are closed and grain growth occurs
rapidly. At a sintering temperature of 1400 ◦C (Fig. 4(c)), the rel-
ative density was about 94%. The final micrograph in Fig. 4(c)
shows fracture along the grain boundaries of a compact sintered
to the final stage. Thus, it is concluded that the microstructural
evolution of nanopowder during sintering occurred in a manner
similar to that of the micro-sized powder.
Mo nanopowder and commercial Mo powder were sintered at
1200 ◦C for 1 h and 1500 ◦C for 3 h, respectively, at a heating rate
of 10 ◦C/min. In both specimens the measured relative density
was about 95%. Relative densities of commercial Mo powders
Fig. 4. Micrographs of fractured surface sintered at (a) 1000 ◦C, (b) 1200 ◦C and (c) 1400 ◦C without holding time.

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G.-S. Kim et al. / Journal of Alloys and Compounds 469 (2009) 401–405
Fig. 5. Micrographs of cross-sectioned surface of Mo nanopowder sintered at 1200 ◦C for 1 h (a) and of commercial Mo powder sintered at 1500 ◦C for 3 h. Grain
size distributions are shown in (c) and (d).
have been reported previously in the literature. Tuominen [17]
reported a relative density value of about 96% at a sintering
temperature of 1790 ◦C with a holding time of 16 h. Savin [18]
reported relative densities of about 78 and 82% at sintering tem-
peratures of 1500 and 1700 ◦C, respectively, and Sukhozhak et
al. [19] reported a relative density value about 94% at a sintering
temperature of 1400 ◦C with a holding time of 1 h. Considering
the reported sintering densities of Mo powder, it is clear that the
nanopowder requires less sintering time or lower temperature
for densification to occur to the same extent as with commercial
powder.
Fig. 5(a) and (b) shows the polished surface of the compact
for each sample. The grain size of the sintered body prepared
from Mo nanopowder was finer than that prepared from com-
mercial Mo powder. These mean grain sizes were measured to
be about 1.4 and 6.2 ␮m, respectively. Fig. 5(c) and (d) shows
the grain size distribution. In the sintered body from commercial
Mo powder, the size distribution range was wider than that of
Mo nanopowder.
was estimated. The measured hardness values were 2.34 GPa
for nanopowder and 1.87 GPa for commercial powder. Tuomi-
nen and Dahl [20] reported microhardness values of about
2.64 GPa for unsintered Mo powder extrusions, and 2.45 GPa
for conventionally sintered Mo bars. Srivatsan et al. [4] reported
hardness values of about 2.32 GPa by plasma pressure com-
paction with consolidation at a temperature of 1650 ◦C for Mo
powder (47 ␮m) at a relative density of about 98%. Consider-
ing the relatively low density of 95% in this work, it is clear
that the fine-grained sample (Fig. 5(a)) has enhanced mechani-
cal properties compared to the relatively coarse-grained sample
(Fig. 5(b)).
4. Conclusion
The densification behavior of Mo nanopowders and their
properties were evaluated by the analysis of linear shrinkage,
microstructure and hardness. At a sintering condition of 1200 ◦C
for 1 h, the relative density and hardness of the nanopowder were
measured to be 95% and 2.34 GPa, respectively. At the same
density, the grain size of sintered compacts was registered at
about 1.4 ␮m for nanopowder and 6.2 ␮m for commercial pow-
der. These results suggest that the Mo nanopowder, which was
synthesized by the ball-milling and hydrogen-reduction process
The dependence of strength or hardness, Hv, of con-
ventional polycrystalline materials on grain size is usually
described by the well-known Hall-Petch relationship as fol-
lows: Hv = Hvo + kd−1/2, where Hvo and k are constant [15,16].
Using this relationship, the Vickers hardness of sintered samples

G.-S. Kim et al. / Journal of Alloys and Compounds 469 (2009) 401–405
405
using MoO3 powders, has excellent sinterability, fine grain size
and a relatively high hardness value compared with commercial
Mo powder.
[6] R.M. German, C.A. Labombard, Int. J. Powder Metall. Powder Technol.
18 (1982) 147.
[7] P.E. Zovas, R.M. German, Metall. Trans. A 15A (1983) 1103.
[8] H. Hofmann, M. Grosskopf, M. Hofmann-Amtenbrink, G. Petzow, Powder
Metall. 29 (1986) 201.
[9] Y. Hiraoka, T. Ogusu, N. Yoshizawa, J. Alloys Compd. 381 (2004) 192.
[10] K.S. Hwang, H.S. Huang, Acta Mater. 51 (2003) 3915.
[11] K.S. Hwang, H.S. Huang, Int. J. Refract. Hard Mater. 22 (2004) 185.
[12] R.M. German, Z.A. Munir, J. Less-Common Met. 58 (1978) 61.
[13] C. Herring, J. Appl. Phys. 21 (1950) 301.
Acknowledgement
This work was supported by a Korea Research Foundation
Grant funded by the Korean Government (MOEHRD) (KRF-
2005-042-D00380).
[14] G.S. Kim, Y.J. Lee, D.G. Kim, Y.D. Kim, J. Alloys Compd. 454 (2008)
327–330.
References
[15] R.M. Greman, Sintering Theory and Practice, John Wiley & Sons,
1996.
[16] E.O. Hall, Proc. Phys. Sot. Lond. B 64 (1951) 747.
[17] S.M. Tuominen, Powder Technol. 30 (1981) 73.
[18] A.V. Savin, Powder Metall. Met. Ceram. 7 (1968) 695.
[19] V.V. Sukhozhak, V.V. Panichkina, V.V. Skorokhod, Powder Metall. Met.
Ceram. 12 (1973) 619.
[1] E.R. Braithwaite, J. Haber, Molybdenum, An Outline of its Chemistry and
Uses, Elsevier, NY, 1994.
[2] J.A. Shields Jr., E.L. Baker, Adv. Mater. Process 1 (1999) 61.
[3] E.F. Baroch, M. Ostermann, G. Patrick, Adv. Powder Metall. 5 (1991) 321.
[4] T.S. Srivatsan, B.G. Ravi, M. Petraroli, T.S. Sudarshan, Int. J. Refract. Met.
Hard Mater. 20 (2002) 181.
[20] S.M. Tuominen, J.M. Dahl, 108th AIME Annual Meeting, New Orleans,
LA, February 1979.
[5] P. Garg, S.J. Park, R.M. German, Int. J. Refract. Met. Hard Mater. 25 (2007)
16.