Materials Transactions, Vol. 53, No. 10 (2012) pp. 1816 to 1821
© 2012 The Japan Institute of Metals
Recovery of Mechanical Property on Nano-Co Particles Dispersed Al2O3
via High-Temperature Oxidation
Daisuke Maruoka1,+1, Tsuyoshi Itaya2,+2, Tai Misaki3,+3 and Makoto Nanko4
1
Graduate School of Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan
Department of Mechanical Engineering, Oyama National College of Technology, Oyama 323-0806, Japan
3
Department of Eco-materials Engineering, Toyama National College of Technology, Toyama 939-8630, Japan
4
Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka 940-2188, Japan
2
Recovery of mechanical strength in ¡-Al2O3-based hybrid materials with 5 vol% dispersed nano-Co particles was carried out via a thermal
oxidation process. Three Vickers indentations were conducted at 49 N for 10 s in air to introduce pre-cracks on the sample surface. The bending
strengths of as-sintered and as-cracked samples were approximately 710 and 159 MPa, respectively. Bending strength value increased to
636 MPa when as-cracked specimens with introduced surface cracks were heat-treated at 1200°C for 6 h in air. Surface cracks disappeared
completely through formation of the oxidation product, CoAl2O4. Specimens heat-treated at 1200°C for 6 h in air did not fracture through the
surface cracks introduced by the Vickers indentation during three-point bending test. Specimens heat-treated at 1200°C for 6 h in an Ar­10%H2
gas flow, in which metallic Co is stable, were broken along the surface crack introduced by the Vickers indentation. The recovery of mechanical
strength was obtained by surface cracks filling with CoAl2O4. [doi:10.2320/matertrans.MAW201202]
(Received April 24, 2012; Accepted July 27, 2012; Published September 25, 2012)
Keywords: self-healing, nanocomposite, cobalt, mechanical property, high-temperature oxidation
1.
Introduction
While ceramic materials have excellent properties such
as wear resistance, chemical stability and high-temperature
strength, their ductility and fracture toughness are insufficient
owing to a high susceptibility to surface cracking.1)
Nanocomposite materials are known as reinforced ceramic
materials with improved mechanical strength and fracture
toughness.2­19) Nanocomposite ceramics can be roughly
divided into two types with respect to microstructures: (1)
nano-particle reinforced ceramics; and (2) ceramics composed of different kinds of nano-sized particles. In the former,
metallic or non-metallic particles are intra or inter-granularly
dispersed in the ceramic matrix.2) Nano-Ni,3­8) nano-W,9,10)
nano-Mo11,12) and nano-Co particles13­17) have been reported
as metallic particle dispersoids for nanocomposites. Likewise,
nano-TiC18) and nano-SiC19) particles have been studied as
non-metallic dispersoids for nanocomposites. Hybrid materials comprising nano-Ni particles dispersed in Al2O3 (Ni/
Al2O3) have been well-investigated with regards to mechanical properties such as bending strength, hardness and fracture
toughness3­8) in nanocomposite systems containing metallic
dispersoids. Since Ni/Al2O3 contains metallic Ni particles,
it is expected to have useful properties such as magnetism.4)
Our group has previously reported20,21) that bending
strength in Ni/Al2O3 was decreased by Vickers indentation,
and that the strength was recovered by heat treatment, for
example, at 1200°C for 6 h in air. Surface cracks filled with
oxidation product, NiAl2O4. Similar phenomena have been
well-investigated22­32) for other ceramic composites containing SiC particles and whiskers, and dependence of recovery
of mechanical strength on heat-treatment time, temperature
and atmosphere has been reported.
+1
Graduate Student, Nagaoka University of Technology
Undergraduate Student, Oyama National College of Technology
+3
Undergraduate Student, Toyama National College of Technology
+2
Because recovery of mechanical strength in Ni/Al2O3 is
obtained through formation of NiAl2O4, similar recovery of
mechanical strength may also be possible in other metallic
dispersoids through formation of their oxidation products.
For example, nano-Co particles can be used instead of
Ni nanoparticles as dispersoids in Al2O3 matrix hybrid
materials.13­17) A similar oxidation compound, CoAl2O4, is
expected to form in Co/Al2O3 ceramic composite during
heat-treatment, that is, recovery of mechanical strength is
also expected to occur. Co not only has similar properties
to Ni, including melting point, density and coefficient of
thermal expansion, but also higher magnetic susceptibility
than Ni. Co/Al2O3 with high magnetism will be fixable in
magnetic vises for machining processes, as is possible with
WC-Co cemented carbides.
In the present study, recovery of mechanical strength via
thermal oxidation was investigated for nano-Co particle
dispersion-containing Al2O3 hybrid materials.
2.
Experimental Procedures
Al2O3 powder with 99.99% purity and 0.44 µm average
particle size and Co(NO3)2·6H2O were used as starting
materials, from which a 5 vol% of Co aqueous slurry was
prepared. After drying at 400°C and milling for 10 min, the
starting powder mixture was reduced at 600°C for 12 h and
then 800°C for an additional 12 h in an Ar­10%H2 gas flow.
The resulting Co/Al2O3 powder was consolidated by pulsed
electric current sintering at a die temperature of 1400°C
under 40 MPa for 5 min in vacuum. Fracture toughness was
evaluated using an indentation fracture method based on
Niihara’s equation.33) Scanning electron microscopy (SEM)
was used for microstructure observations.
Figure 1(a) illustrates the shape and sizes of the fabricated
specimens and Vickers indentation positions. Specimen
surfaces were mechanically polished using 2-µm diamond
slurry. Three Vickers indentations were produced on each
Recovery of Mechanical Property on Nano-Co Particles Dispersed Al2O3 via High-Temperature Oxidation
1817
(a)
26
3
2
1
4
13
45°
0.1
Vickers indentation
φ4
16
Unit: mm
Fig. 1 Schematic diagrams of specimen and Vickers indentations (a) and
location of three-point bending test (b).
specimen at a loading for 49 N for 10 s in air to introduce precracks on the center of their tension surface. Three-point
bending tests (16 mm span length) were carried out with a
cross-head speed of 0.5 mm/min at room temperature as
shown in Fig. 1(b). Bending strength values were averaged
for five test pieces.
Specimens without heat-treatment after Vickers indentation
are hereafter called “as-cracked specimens” because surface
cracks were formed by the Vickers indentation. The ascracked specimens were heat-treated at 1100 or 1200°C for
6 h in air, and also in an Ar­10%H2 gas flow for comparison.
Heat-treatment temperature was measured with a thermocouple located near the specimen. X-ray diffraction (XRD)
measurements were carried out for phase identification.
3.
Experimental Results
XRD analysis showed that the reduced powder mixture
was composed of ¡-Co and ¡-Al2O3 phases. Relative density
of the consolidated specimen attained over 99% of the
theoretical density calculated by the Archimedean method.
Vickers hardness and fracture toughness of the Co/Al2O3
specimen were 16.1 GPa and 5.81 MPa m0.5, respectively.
Figure 2 shows the fractured surface of the consolidated
Co/Al2O3 specimen, in which dispersed Co particles are
observed with bright contrast. Average grain sizes of the
Al2O3 matrix and Co particles were approximately 1.1 and
0.2 µm, respectively.
Figure 3 shows XRD patterns of the consolidated Co/
Al2O3 specimens with and without heat-treatment. Peaks of
assigned to ¡-Co and ¡-Al2O3 phases were detected in all
specimens (Figs. 3(a), 3(c) and 3(d)) except that heat-treated
at 1200°C for 6 h in air, as shown in Fig. 3(b). Peaks of
CoAl2O4 phase were detected in specimens heat-treated in air
(Figs. 3(b) and 3(c)), but did not appear in the specimen heattreated in an Ar­10%H2 gas flow, as shown in Fig. 3(a).
CoAl2O4 peak intensities for the specimen heat-treated at
1200°C were higher compared with those of the specimen
heat-treated at 1100°C.
Figure 4 shows the fractured surface of a specimen without
heat-treatment after the bending test. The white arrow
1 μm
Fig. 2
Fractured surface of the as-sintered specimen.
(a) 1200°C, 6 h
Ar-10%H 2
Intensity, I/ a.u.
(b)
α-Co, α-Al2O3
CoAl2O4
(b) 1200°C, 6 h, Air
(c) 1100°C, 6 h, Air
(d) As-sintered
40
50
Diffraction angle, 2θ / deg.
60
Fig. 3 XRD patterns of the consolidated Co/Al2O3 specimens with and
without heat-treatments.
Vickers indentation
50 μm
Fig. 4 SEM image of crack induced by the Vickers indentation.
indicates a Vickers indentation and the white broken line
indicates the front of the semi-elliptical crack induced by
the Vickers indentation. The diameter and depth of this
1818
D. Maruoka, T. Itaya, T. Misaki and M. Nanko
(a)
(b)
20 μm
2 μm
(d)
(c)
20 μm
2 μm
(f)
(e)
20 μm
2 μm
(h)
(g)
20 μm
2 μm
Fig. 5 SEM photographs of surfaces of as-cracked specimen (a) and (b) and specimens heat-treated at 1100°C for 6 h in air (c) and (d),
1200°C for 6 h in air (e) and (f ) and 1200°C for 6 h in an Ar­10%H2 gas flow (g) and (h).
semi-elliptical crack were approximately 200 and 100 µm,
respectively.
Figure 5 shows SEM images of the Vickers indentations
on specimens without heat-treatment (a) and (b), heat-treated
at 1100°C (c) and (d), heat-treated at 1200°C (e) and (f ) and
heat-treated in an Ar­10%H2 gas flow (g) and (h). Broken
white lines indicate the shape of the Vickers indentations,
while white arrows indicate the cracks induced by the Vickers
indentations. Figure 5(b) shows high magnification of the
area indicated with the black rectangle in Fig. 5(a). The
length and width of the surface crack formed on the specimen
without heat-treatment (Figs. 5(a) and 5(b)) were approx-
imately 60 and 0.5 µm, respectively. Figures 5(c) and 5(d)
show the indentation and surface cracks on the specimen
heat-treated at 1100°C for 6 h in air. Although surface
cracks were still observed near the corners of the Vickers
indentations (Fig. 5(c)), their widths were found to be
decreased (Fig. 5(d)). On the specimen heat-treated at
1200°C for 6 h in air (Figs. 5(e) and 5(f )), most of the
surface crack areas had closed. In contrast, surface cracks
were clearly observed on the specimen heat-treated in an Ar­
10%H2 gas flow as shown in Figs. 5(g) and 5(h).
Figure 6 shows bending strength of the as-sintered, ascracked and heat-treated specimens. The bending strength
Recovery of Mechanical Property on Nano-Co Particles Dispersed Al2O3 via High-Temperature Oxidation
Bending strength, σ / MPa
of the as-sintered specimens was approximately 710 MPa,
which was decreased to 159 MPa by the Vickers indentation.
Specimens heat-treated at 1200°C for 6 h in air exhibited a
bending strength of 636 MPa, comparable with those of as-
800
As-sintered
1200°C, 6 h
Air
600
400
1200°C, 6 h
Ar-10% H2
As-cracked
200
sintered specimens. When the specimen was instead heattreated in an Ar­H2 gas flow, its bending strength was
approximately 241 MPa.
Figure 7 shows specimen surfaces after bending tests, in
which the Vickers indentations are indicated by broken lines.
Figure 7(a) shows the as-sintered specimen, while Fig. 7(b)
shows high magnification of the area indicated with the black
rectangle in Fig. 7(a). Fracture of this specimen by the
bending test occurred across the Vickers indentation. While
in the specimen heat-treated at 1200°C for 6 h in air, fracture
occurred separate from the Vickers indentation as shown in
Figs. 7(c) and 7(d). In the case of the specimen heat-treated
in an Ar­10%H2 gas flow, fracture of the specimen was done
across the Vickers indentation as shown in Figs. 7(e) and
7(f ).
4.
0
Sample condition
Fig. 6 Bending strength of as-sintered, as-cracked and heat-treated
specimens.
Discussion
As shown in Fig. 3, no CoO or Co3O4, which are known
to be the common oxides of Co, were apparent in the
specimens. According to pseudo-phase diagram of CoO­
Al2O3 reported by Mori,34) CoAl2O4 and Al2O3 can coexist at
(b)
(a)
20 μm
100 μm
(d)
(c)
100 μm
20 μm
(f)
(e)
100 μm
1819
20 μm
Fig. 7 SEM images of bending tested specimen surfaces; as-cracked (a) and (b), 1200°C for 6 h in air (c) and (d) and 1200°C for 6 h in
an Ar­10%H2 gas flow (e) and (f ).
1820
D. Maruoka, T. Itaya, T. Misaki and M. Nanko
1200°C in 5 vol% Co/Al2O3. Because Co is oxidized to CoO
in air, only CoAl2O4 is obtained as the oxidation product after
heat-treatment in air. Specimens heat-treated at 1200°C for
6 h in an Ar­10%H2 atmosphere contained peaks of ¡-Co
and ¡-Al2O3 but no CoAl2O4 because oxygen partial pressure
was lower than that required for oxidation of Co. According
to Fig. 5, bending strength increased when CoAl2O4 was
formed on the specimen surface. Thus, recovery of
mechanical strength in Co/Al2O3 was obtained by the
formation of CoAl2O4.
Bending strength of specimens heat-treated at 1200°C for
6 h in air was recovered as shown in Fig. 6. Brittle materials
such as ceramics fracture at the largest defect causing the
largest stress concentration. Prepared specimens as-cracked
and heat-treated at 1200°C for 6 h in an Ar­10%H2 gas
flow broke along surface cracks introduced by the Vickers
indentations as shown in Figs. 7(b) and 7(f ). Specimens with
surface cracks that had disappeared completely fractured at
different positions from the indentation as shown in Fig. 7(d),
indicating that the mechanical strength of the cracked region
increased to the same level as that of the region without
cracks. Yao et al.35) have revealed the crack-healing behavior
of SiC/Si3N4 after 1 h heat-treatment in air at 1200­1400°C.
In this case, bending strength of SiC/Si3N4 was recovered
through the formation of SiO2 on the specimen surface.
The specimens were broken at different positions from the
cracks introduced by Vickers indentations during three-points
bending tests.
Specimens heat-treated at 1200°C for 6 h in an Ar­10%H2
atmosphere were recovered to a mechanical strength
approximately 50 MPa higher than that of as-cracked ones.
Thompson et al.36) reported the disappearance of surface
cracks in SiC-dispersed Al2O3 nanocomposite, in which
surface cracks introduced by the Vickers indentation were
closed in the vicinity of the crack tips after heat-treatment at
1300°C for 2 h in Ar. Because largest defect size affects the
mechanical strength of a material, decreasing of defect size
increases material mechanical strength. On the other hands,
Matsuo et al.37) reported that recovery of mechanical
strength of Al2O3 with surface cracks in diamond grinding
process. Feed rate and depth of cut were 8­10 and 8 µm,
respectively. Although bending strength value of ground
Al2O3 was 235 MPa, this value was increased to 381 MPa by
annealing at 1230°C for 1 h in air. Recovery of bending
strength occurred by relaxation of compressive residual
stress. In the present study, partial recovery of mechanical
strength in Co/Al2O3 was expected to be caused by closure
of surface cracks and/or relaxation of residual stress
introduced by the Vickers indentation. However, because
only approximately 50 MPa of bending strength, compared
with as-cracked specimens, was recovered by heat-treatment
at 1200°C for 6 h in an Ar­10%H2 atmosphere, the recovery
of mechanical strength was mainly obtained by CoAl2O4
filling of the cracks.
5.
Conclusions
Recovery of mechanical strength in ¡-Al2O3-based hybrid
materials with 5 vol% dispersed nano-Co particles (Co/
Al2O3) was investigated via thermal oxidation process. The
bending strengths of as-sintered and as-cracked specimens
were approximately 710 and 159 MPa, respectively. Bending
strength of as-cracked specimens recovered to 636 MPa
after heat-treatment at 1200°C for 6 h in air, which was
comparable in value to that of as-sintered specimens. Surface
cracks disappeared completely through formation of the
oxidation product, CoAl2O4. A bending strength value of
241 MPa was obtained for specimens heat-treated at 1200°C
for 6 h in an Ar­10%H2 gas flow, slightly higher than that of
as-cracked specimens. This partial recovery of mechanical
strength was likely caused by closure of surface cracks and/
or relaxation of residual stress introduced by the Vickers
indentation.
Specimens heat-treated at 1200°C for 6 h in air fractured
during three-point bending test at different positions from the
surface cracks introduced by the Vickers indentation. Thus,
the introduced cracks had disappeared and were no longer the
largest defect. The observed recovery of mechanical strength
was mainly obtained by filling of the surface cracks by
formation of CoAl2O4.
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