oxidation of mo/al hybrid materials at high temperatures
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IOP Conference Series Materials Science and Engineering
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Oxidation of MoAl2O3 hybrid materials at hightemperaturesTo cite this article T D Nguyen et al 2011 IOP Conf Ser Mater Sci Eng 20 012015
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Oxidation of MoAl2O3 hybrid materials at high temperatures
T D Nguyen12
D Maruoka1 and M Nanko
1
1 Department of Mechanical Engineering Nagaoka University of Technology
1603-1 Kamitomioka Nagaoka 940-2188 Japan 2 Permanent Address Faculty of Materials Science and Technology
Hanoi University of Technology No1 Daicoviet Street Hanoi Vietnam
E-mail thuy-canklmailhuteduvn
Abstract In the present work the oxidation behaviour of 5 vol MoAl2O3 hybrid materials
was investigated at high temperatures in air At oxidation temperature ranging from 600 to
800oC Al2(MoO4)3 and MoO3 were developed The growth of the oxidized zone at which
metallic Mo was not observed obeyed the parabolic manner The apparent activation energy
of the parabolic rate constant was equal to 95 kJmol-1
The microstructures showed that there
were many voids in the oxidized zone Significant cracks which would be formed due to
volume expansion of Mo particles via inward diffusion of oxygen were not observed in
oxidized zone They must be the evidence of outward diffusion of cations most likely Mo6+
at
grain boundary As the temperature over 800oC the evaporation of MoO3 occurred
dramatically
1 Introduction
Alumina is a high performance structure oxide with low density high hardness high mechanical
strength high wear resistance and good thermal properties It has been widely used as the refractory
material in industry But it is also a brittle material with low fracture toughness The mechanical
properties of the Al2O3 ceramics are significantly improved by introducing some kinds of ductile metal
particles like W Ni Cu and Mo [1-4] The role of metal phases is to form plastic crack bridges across
the fracture faces which leads to a rising R-curve behaviour thus increases fracture toughness [5]
Because molybdenum has high melting point of 2623oC it would be good dispersion to improve
mechanical properties of Al2O3 at high temperatures Thus the MoAl2O3 hybrid materials are
potential candidates for applications as wear resistance components corrosion ndash barrier coating
cutting tools However the application of the MoAl2O3 would be limited by high-temperature
oxidation
In the past decade there has been many attempts to improve the mechanical properties of MoAl2O3
but there are few works on its oxidation behaviour In a concerning work Wu et al investigated the
mass loss of pure Mo and nano-MoAl2O3 materials with 10 16 and 20 vol dispersed Mo during
elevating temperatures [6] They concluded that the oxidation behaviour of the pure Mo and gt16vol
MoAl2O3 materials are similar due to the connecting oxidation product in the composites The
evaporation of MoO3 is attributed to start at 380oC proceeding together with the oxidation The mass
loss starts from 700oC for gt16 vol MoAl2O3 and from 800
oC for 10 vol MoAl2O3 respectively
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
Published under licence by IOP Publishing Ltd 1
In order to use MoAl2O3 as the structure material at high temperatures the oxidation mechanism
would be studied in detail The oxidation behaviour of the MoAl2O3 is described and the mechanism
of the growth of oxidized zone is discussed in this work
2 Experimental
The raw materials were commercial -alumina powder (Sumitomo Chemical Co AA ndash 04 average
particle size of 05 m purity 9999) and Mo powder (purity 9999 and average particle size of 07
m) Powder mixture consisting of 5 vol Mo and 95 vol Al2O3 was dry-mixed by using a
conventional ball milling process for at least 48 h in a container of 100 mm in diameter Alumina
balls of 2 mm were used to prevent contamination for a long mixing time The mass ratio of balls to
the materials was 101 After that the powder mixture was sintered at 1400oC for 5 min under 40 MPa
in vacuum by pulsed electric current sintering (PECS) The density of the sintered specimen was
measured by the Archimedes method with toluene The relative density of the sintered samples for the
tests of oxidation should be at least 99 after sintering by PECS The oxidation test was conducted at
the temperature ranging from 600 to 1200oC for 1~48 h with heating rate of 400 Kh in air The
surface of sintered specimen and the cross-section of the oxidized specimens were polished until a
mirror quality by using 05 m diamond slurry Figure 1(a) showed the microstructures of the
polished surface Mo particles were mostly distributed into the Al2O3 matrix to prepare the uniform
materials as the bright dots Some Mo agglomerates still remained as the bright area The average
Al2O3 grain size of the sintered sample was approximately 4 m which was determined from the
fracture surface shown in Figure 1(b) The analysis of phase formation was carried out by using X-ray
diffraction method The evolution of oxidation product at the different temperatures and for the
different periods of time on the surface and in the cross-section was observed and analyzed by using
scan electron microscope (SEM) The thickness of oxidized zone was also measured from the SEM
images
3 Results and Discussion
Figure 2 shows the XRD patterns of the as-sintered samples and the oxidized samples at 600 and
1200oC for 6 h The Al2O3 and Mo peaks mainly appeared After annealing at 600 700 and 800
oC
bi-oxides of Aluminum (III) and Trimolybdate (VI) with the chemical formula of Al2(MoO4)3 MoO3
Al2O2 and residual Mo peaks were identified When the temperature exceeded 1000oC there were
only Al2O3 peaks on the XRD pattern The following reactions during high-temperature oxidation of
the MoAl2O3 should occur
(a)
(b)
Figure 1 Microstructures of sintered MoAl2O3
(a) polished area (b) fracture surface in high magnification
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
2
Figure 2 X-Ray diffraction pattern of the sintered samples (a) the specimens
after oxidation at 600oC (b) and 1200
oC (c) for 6 h
2Mo + 3O2 = 2MoO3 (s) (1)
MoO3 (s) = MoO3 (g) (2)
6Mo + 9O2 + 2Al2O3 = 2Al2(MoO4)3 (s) (3)
Al2(MoO4)3 = Al2O3 + 3MoO3 (g) (4)
Here Al2(MoO4)3 is the unique bi-oxide in the Al2O3 ndash MoO3 phase diagram [7]
The microstructures of oxidized surfaces were observed by SEM as shown in Figure 3 The oxidation
product seems to be at the grain boundary of Al2O3 matrix on the sample oxidized at 700oC for 6 h as
shown in Figure 3(a) Certainly the exposed Mo particles to the air reacted with O2 to form clumps of
MoO3 on the surface as observed on low magnification images When the oxidation time was reached
to 48 h the oxidation products mostly disappeared to leave pores with bright dots (round marks)
nearby them as shown in Figure 3(b) The same morphology was also found on the surface of
specimen after oxidizing at 800oC for 6 and 48 h as shown in Figure 3(c) It means MoO3 was formed
for the short time as the main oxidation product on the surface and then it evaporated during oxidation
at 600700oC The bright dots would be Al2O3 that formed from Al2(MoO4)3 after MoO3 evaporated
by following Equation 4 The oxidation product can be seen as bright phases like a clumps as shown
in Figure 3(d) appeared again from the grain size of the Al2O3 matrix on the oxidized surface after
oxidizing at 900oC for 6 h This phenomenon can be explained as follows when the temperature was
over the eutectic temperature in the Al2O3-Al2(MO4)3 of 820oC liquid phase was generated [7] The
liquid phase in Al2O3 matrix would promote outward diffusion of Al3+
and Mo6+
cations from inside to
the surface As well Al2O3 grains would be formed as shown in Equation 4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
3
(a)
(b)
(c)
(d)
Figure 3 The surface of the specimen after oxidizing at 700oC for 12 h (a) and 48 h (b) and
at 800oC for 6 h (c) and at 900
oC for 6 h (d)
The oxidized zone was observed from the SEM images of cross-section as shown in Figure 4 Mo
particles are not observed in the oxidized zone The morphology was in agreement with the
phenomenon that already discussed from the oxidized surfaces The oxidation product grew on the
surface for the oxidized specimen at 600800oC for short oxidation time Prolonging oxidation time
most of MoO3 was evaporated from the surface The voids appeared inside the oxidized zone and
there was no crack They implied that the outward diffusion of cations Conversely if we assume that
the O2-
anion inward diffused and the reaction occurred in Al2O3 matrix the oxidation zones would be
cracked because of the large volume expansion of oxidation products to the MoAl2O3 The Pilling-
Bedworth ratios of MoO3Mo and Al2(MoO4)3 are approximately 68 and 3 respectively
Thickness of the oxidized zones measured from the SEM images of the cross-section is a function of
time as shown in Figure 5 The growth of oxidized zone at 600800oC follows a parabolic manner
tkx p2 (5)
where kp x and t are the parabolic rate constant of the growth of oxidized zone the thickness of
oxidize zone and time The kp values of 5 vol MoAl2O3 is much faster than that of 5 vol NiAl2O3
[8] and 8 vol SiCAl2O3 [9] at 12001400 oC The dependence of kp on the annealing temperature
obeyed the Arrhenius Equation
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
4
(a)
(b)
Figure 4 Cross-section of specimens oxidizing at 700oC for 48 h (a) and at 900
oC for 6 h
RT
Qkk op exp (6)
where R is the universal gas constant Q T and ko are apparent activation energy Q the absolute
temperature and a constant respectively The apparent activation energy was calculated from the slope
of this line in Figure 6 and equal to 95 kJmol-1
The value of the activation energy is much smaller
than those in NiAl2O3 and SiCAl2O3
When increasing temperature to higher than 900oC the Al2(MoO4)3 which may be located at grain
boundary of Al2O3 was decomposed to porous Al2O3 and MoO3(g) resulting in the channel for high
rate penetrating of oxygen and releasing of MoO3(g) as shown in Figure 4(b) The oxidation
behaviour of MoAl2O3 at the temperature exceeding 800oC would follow a parabolic manner
0 10 20 30 40 50 600
20
40
60
80
At 600oC
At 700oC
At 800oC
Time th
Th
ick
nes
s x
m
60 80 100 120
-15
-14
-13
-12
log
(kpm
2s-1
)
T -1
104K
-1
5 vol Ni Al2O3 [8]
8 vol SiCAl2O3 [9]
5 vol Mo Al2O3
Figure 5 Oxidized zone as a function of
time at (600800)oC
Figure 6 Dependence of parabolic rate
constant on reciprocal temperature
In case of oxidation of 5 vol NiAl2O3 at 12001350oC [8] the oxidation behaviour of NiAl2O3 is
governed by inward diffusion of O2-
along grain boundary of Al2O3 matrix As the results NiAl2O4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
5
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6
Oxidation of MoAl2O3 hybrid materials at high temperatures
T D Nguyen12
D Maruoka1 and M Nanko
1
1 Department of Mechanical Engineering Nagaoka University of Technology
1603-1 Kamitomioka Nagaoka 940-2188 Japan 2 Permanent Address Faculty of Materials Science and Technology
Hanoi University of Technology No1 Daicoviet Street Hanoi Vietnam
E-mail thuy-canklmailhuteduvn
Abstract In the present work the oxidation behaviour of 5 vol MoAl2O3 hybrid materials
was investigated at high temperatures in air At oxidation temperature ranging from 600 to
800oC Al2(MoO4)3 and MoO3 were developed The growth of the oxidized zone at which
metallic Mo was not observed obeyed the parabolic manner The apparent activation energy
of the parabolic rate constant was equal to 95 kJmol-1
The microstructures showed that there
were many voids in the oxidized zone Significant cracks which would be formed due to
volume expansion of Mo particles via inward diffusion of oxygen were not observed in
oxidized zone They must be the evidence of outward diffusion of cations most likely Mo6+
at
grain boundary As the temperature over 800oC the evaporation of MoO3 occurred
dramatically
1 Introduction
Alumina is a high performance structure oxide with low density high hardness high mechanical
strength high wear resistance and good thermal properties It has been widely used as the refractory
material in industry But it is also a brittle material with low fracture toughness The mechanical
properties of the Al2O3 ceramics are significantly improved by introducing some kinds of ductile metal
particles like W Ni Cu and Mo [1-4] The role of metal phases is to form plastic crack bridges across
the fracture faces which leads to a rising R-curve behaviour thus increases fracture toughness [5]
Because molybdenum has high melting point of 2623oC it would be good dispersion to improve
mechanical properties of Al2O3 at high temperatures Thus the MoAl2O3 hybrid materials are
potential candidates for applications as wear resistance components corrosion ndash barrier coating
cutting tools However the application of the MoAl2O3 would be limited by high-temperature
oxidation
In the past decade there has been many attempts to improve the mechanical properties of MoAl2O3
but there are few works on its oxidation behaviour In a concerning work Wu et al investigated the
mass loss of pure Mo and nano-MoAl2O3 materials with 10 16 and 20 vol dispersed Mo during
elevating temperatures [6] They concluded that the oxidation behaviour of the pure Mo and gt16vol
MoAl2O3 materials are similar due to the connecting oxidation product in the composites The
evaporation of MoO3 is attributed to start at 380oC proceeding together with the oxidation The mass
loss starts from 700oC for gt16 vol MoAl2O3 and from 800
oC for 10 vol MoAl2O3 respectively
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
Published under licence by IOP Publishing Ltd 1
In order to use MoAl2O3 as the structure material at high temperatures the oxidation mechanism
would be studied in detail The oxidation behaviour of the MoAl2O3 is described and the mechanism
of the growth of oxidized zone is discussed in this work
2 Experimental
The raw materials were commercial -alumina powder (Sumitomo Chemical Co AA ndash 04 average
particle size of 05 m purity 9999) and Mo powder (purity 9999 and average particle size of 07
m) Powder mixture consisting of 5 vol Mo and 95 vol Al2O3 was dry-mixed by using a
conventional ball milling process for at least 48 h in a container of 100 mm in diameter Alumina
balls of 2 mm were used to prevent contamination for a long mixing time The mass ratio of balls to
the materials was 101 After that the powder mixture was sintered at 1400oC for 5 min under 40 MPa
in vacuum by pulsed electric current sintering (PECS) The density of the sintered specimen was
measured by the Archimedes method with toluene The relative density of the sintered samples for the
tests of oxidation should be at least 99 after sintering by PECS The oxidation test was conducted at
the temperature ranging from 600 to 1200oC for 1~48 h with heating rate of 400 Kh in air The
surface of sintered specimen and the cross-section of the oxidized specimens were polished until a
mirror quality by using 05 m diamond slurry Figure 1(a) showed the microstructures of the
polished surface Mo particles were mostly distributed into the Al2O3 matrix to prepare the uniform
materials as the bright dots Some Mo agglomerates still remained as the bright area The average
Al2O3 grain size of the sintered sample was approximately 4 m which was determined from the
fracture surface shown in Figure 1(b) The analysis of phase formation was carried out by using X-ray
diffraction method The evolution of oxidation product at the different temperatures and for the
different periods of time on the surface and in the cross-section was observed and analyzed by using
scan electron microscope (SEM) The thickness of oxidized zone was also measured from the SEM
images
3 Results and Discussion
Figure 2 shows the XRD patterns of the as-sintered samples and the oxidized samples at 600 and
1200oC for 6 h The Al2O3 and Mo peaks mainly appeared After annealing at 600 700 and 800
oC
bi-oxides of Aluminum (III) and Trimolybdate (VI) with the chemical formula of Al2(MoO4)3 MoO3
Al2O2 and residual Mo peaks were identified When the temperature exceeded 1000oC there were
only Al2O3 peaks on the XRD pattern The following reactions during high-temperature oxidation of
the MoAl2O3 should occur
(a)
(b)
Figure 1 Microstructures of sintered MoAl2O3
(a) polished area (b) fracture surface in high magnification
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
2
Figure 2 X-Ray diffraction pattern of the sintered samples (a) the specimens
after oxidation at 600oC (b) and 1200
oC (c) for 6 h
2Mo + 3O2 = 2MoO3 (s) (1)
MoO3 (s) = MoO3 (g) (2)
6Mo + 9O2 + 2Al2O3 = 2Al2(MoO4)3 (s) (3)
Al2(MoO4)3 = Al2O3 + 3MoO3 (g) (4)
Here Al2(MoO4)3 is the unique bi-oxide in the Al2O3 ndash MoO3 phase diagram [7]
The microstructures of oxidized surfaces were observed by SEM as shown in Figure 3 The oxidation
product seems to be at the grain boundary of Al2O3 matrix on the sample oxidized at 700oC for 6 h as
shown in Figure 3(a) Certainly the exposed Mo particles to the air reacted with O2 to form clumps of
MoO3 on the surface as observed on low magnification images When the oxidation time was reached
to 48 h the oxidation products mostly disappeared to leave pores with bright dots (round marks)
nearby them as shown in Figure 3(b) The same morphology was also found on the surface of
specimen after oxidizing at 800oC for 6 and 48 h as shown in Figure 3(c) It means MoO3 was formed
for the short time as the main oxidation product on the surface and then it evaporated during oxidation
at 600700oC The bright dots would be Al2O3 that formed from Al2(MoO4)3 after MoO3 evaporated
by following Equation 4 The oxidation product can be seen as bright phases like a clumps as shown
in Figure 3(d) appeared again from the grain size of the Al2O3 matrix on the oxidized surface after
oxidizing at 900oC for 6 h This phenomenon can be explained as follows when the temperature was
over the eutectic temperature in the Al2O3-Al2(MO4)3 of 820oC liquid phase was generated [7] The
liquid phase in Al2O3 matrix would promote outward diffusion of Al3+
and Mo6+
cations from inside to
the surface As well Al2O3 grains would be formed as shown in Equation 4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
3
(a)
(b)
(c)
(d)
Figure 3 The surface of the specimen after oxidizing at 700oC for 12 h (a) and 48 h (b) and
at 800oC for 6 h (c) and at 900
oC for 6 h (d)
The oxidized zone was observed from the SEM images of cross-section as shown in Figure 4 Mo
particles are not observed in the oxidized zone The morphology was in agreement with the
phenomenon that already discussed from the oxidized surfaces The oxidation product grew on the
surface for the oxidized specimen at 600800oC for short oxidation time Prolonging oxidation time
most of MoO3 was evaporated from the surface The voids appeared inside the oxidized zone and
there was no crack They implied that the outward diffusion of cations Conversely if we assume that
the O2-
anion inward diffused and the reaction occurred in Al2O3 matrix the oxidation zones would be
cracked because of the large volume expansion of oxidation products to the MoAl2O3 The Pilling-
Bedworth ratios of MoO3Mo and Al2(MoO4)3 are approximately 68 and 3 respectively
Thickness of the oxidized zones measured from the SEM images of the cross-section is a function of
time as shown in Figure 5 The growth of oxidized zone at 600800oC follows a parabolic manner
tkx p2 (5)
where kp x and t are the parabolic rate constant of the growth of oxidized zone the thickness of
oxidize zone and time The kp values of 5 vol MoAl2O3 is much faster than that of 5 vol NiAl2O3
[8] and 8 vol SiCAl2O3 [9] at 12001400 oC The dependence of kp on the annealing temperature
obeyed the Arrhenius Equation
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
4
(a)
(b)
Figure 4 Cross-section of specimens oxidizing at 700oC for 48 h (a) and at 900
oC for 6 h
RT
Qkk op exp (6)
where R is the universal gas constant Q T and ko are apparent activation energy Q the absolute
temperature and a constant respectively The apparent activation energy was calculated from the slope
of this line in Figure 6 and equal to 95 kJmol-1
The value of the activation energy is much smaller
than those in NiAl2O3 and SiCAl2O3
When increasing temperature to higher than 900oC the Al2(MoO4)3 which may be located at grain
boundary of Al2O3 was decomposed to porous Al2O3 and MoO3(g) resulting in the channel for high
rate penetrating of oxygen and releasing of MoO3(g) as shown in Figure 4(b) The oxidation
behaviour of MoAl2O3 at the temperature exceeding 800oC would follow a parabolic manner
0 10 20 30 40 50 600
20
40
60
80
At 600oC
At 700oC
At 800oC
Time th
Th
ick
nes
s x
m
60 80 100 120
-15
-14
-13
-12
log
(kpm
2s-1
)
T -1
104K
-1
5 vol Ni Al2O3 [8]
8 vol SiCAl2O3 [9]
5 vol Mo Al2O3
Figure 5 Oxidized zone as a function of
time at (600800)oC
Figure 6 Dependence of parabolic rate
constant on reciprocal temperature
In case of oxidation of 5 vol NiAl2O3 at 12001350oC [8] the oxidation behaviour of NiAl2O3 is
governed by inward diffusion of O2-
along grain boundary of Al2O3 matrix As the results NiAl2O4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
5
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6
In order to use MoAl2O3 as the structure material at high temperatures the oxidation mechanism
would be studied in detail The oxidation behaviour of the MoAl2O3 is described and the mechanism
of the growth of oxidized zone is discussed in this work
2 Experimental
The raw materials were commercial -alumina powder (Sumitomo Chemical Co AA ndash 04 average
particle size of 05 m purity 9999) and Mo powder (purity 9999 and average particle size of 07
m) Powder mixture consisting of 5 vol Mo and 95 vol Al2O3 was dry-mixed by using a
conventional ball milling process for at least 48 h in a container of 100 mm in diameter Alumina
balls of 2 mm were used to prevent contamination for a long mixing time The mass ratio of balls to
the materials was 101 After that the powder mixture was sintered at 1400oC for 5 min under 40 MPa
in vacuum by pulsed electric current sintering (PECS) The density of the sintered specimen was
measured by the Archimedes method with toluene The relative density of the sintered samples for the
tests of oxidation should be at least 99 after sintering by PECS The oxidation test was conducted at
the temperature ranging from 600 to 1200oC for 1~48 h with heating rate of 400 Kh in air The
surface of sintered specimen and the cross-section of the oxidized specimens were polished until a
mirror quality by using 05 m diamond slurry Figure 1(a) showed the microstructures of the
polished surface Mo particles were mostly distributed into the Al2O3 matrix to prepare the uniform
materials as the bright dots Some Mo agglomerates still remained as the bright area The average
Al2O3 grain size of the sintered sample was approximately 4 m which was determined from the
fracture surface shown in Figure 1(b) The analysis of phase formation was carried out by using X-ray
diffraction method The evolution of oxidation product at the different temperatures and for the
different periods of time on the surface and in the cross-section was observed and analyzed by using
scan electron microscope (SEM) The thickness of oxidized zone was also measured from the SEM
images
3 Results and Discussion
Figure 2 shows the XRD patterns of the as-sintered samples and the oxidized samples at 600 and
1200oC for 6 h The Al2O3 and Mo peaks mainly appeared After annealing at 600 700 and 800
oC
bi-oxides of Aluminum (III) and Trimolybdate (VI) with the chemical formula of Al2(MoO4)3 MoO3
Al2O2 and residual Mo peaks were identified When the temperature exceeded 1000oC there were
only Al2O3 peaks on the XRD pattern The following reactions during high-temperature oxidation of
the MoAl2O3 should occur
(a)
(b)
Figure 1 Microstructures of sintered MoAl2O3
(a) polished area (b) fracture surface in high magnification
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
2
Figure 2 X-Ray diffraction pattern of the sintered samples (a) the specimens
after oxidation at 600oC (b) and 1200
oC (c) for 6 h
2Mo + 3O2 = 2MoO3 (s) (1)
MoO3 (s) = MoO3 (g) (2)
6Mo + 9O2 + 2Al2O3 = 2Al2(MoO4)3 (s) (3)
Al2(MoO4)3 = Al2O3 + 3MoO3 (g) (4)
Here Al2(MoO4)3 is the unique bi-oxide in the Al2O3 ndash MoO3 phase diagram [7]
The microstructures of oxidized surfaces were observed by SEM as shown in Figure 3 The oxidation
product seems to be at the grain boundary of Al2O3 matrix on the sample oxidized at 700oC for 6 h as
shown in Figure 3(a) Certainly the exposed Mo particles to the air reacted with O2 to form clumps of
MoO3 on the surface as observed on low magnification images When the oxidation time was reached
to 48 h the oxidation products mostly disappeared to leave pores with bright dots (round marks)
nearby them as shown in Figure 3(b) The same morphology was also found on the surface of
specimen after oxidizing at 800oC for 6 and 48 h as shown in Figure 3(c) It means MoO3 was formed
for the short time as the main oxidation product on the surface and then it evaporated during oxidation
at 600700oC The bright dots would be Al2O3 that formed from Al2(MoO4)3 after MoO3 evaporated
by following Equation 4 The oxidation product can be seen as bright phases like a clumps as shown
in Figure 3(d) appeared again from the grain size of the Al2O3 matrix on the oxidized surface after
oxidizing at 900oC for 6 h This phenomenon can be explained as follows when the temperature was
over the eutectic temperature in the Al2O3-Al2(MO4)3 of 820oC liquid phase was generated [7] The
liquid phase in Al2O3 matrix would promote outward diffusion of Al3+
and Mo6+
cations from inside to
the surface As well Al2O3 grains would be formed as shown in Equation 4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
3
(a)
(b)
(c)
(d)
Figure 3 The surface of the specimen after oxidizing at 700oC for 12 h (a) and 48 h (b) and
at 800oC for 6 h (c) and at 900
oC for 6 h (d)
The oxidized zone was observed from the SEM images of cross-section as shown in Figure 4 Mo
particles are not observed in the oxidized zone The morphology was in agreement with the
phenomenon that already discussed from the oxidized surfaces The oxidation product grew on the
surface for the oxidized specimen at 600800oC for short oxidation time Prolonging oxidation time
most of MoO3 was evaporated from the surface The voids appeared inside the oxidized zone and
there was no crack They implied that the outward diffusion of cations Conversely if we assume that
the O2-
anion inward diffused and the reaction occurred in Al2O3 matrix the oxidation zones would be
cracked because of the large volume expansion of oxidation products to the MoAl2O3 The Pilling-
Bedworth ratios of MoO3Mo and Al2(MoO4)3 are approximately 68 and 3 respectively
Thickness of the oxidized zones measured from the SEM images of the cross-section is a function of
time as shown in Figure 5 The growth of oxidized zone at 600800oC follows a parabolic manner
tkx p2 (5)
where kp x and t are the parabolic rate constant of the growth of oxidized zone the thickness of
oxidize zone and time The kp values of 5 vol MoAl2O3 is much faster than that of 5 vol NiAl2O3
[8] and 8 vol SiCAl2O3 [9] at 12001400 oC The dependence of kp on the annealing temperature
obeyed the Arrhenius Equation
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
4
(a)
(b)
Figure 4 Cross-section of specimens oxidizing at 700oC for 48 h (a) and at 900
oC for 6 h
RT
Qkk op exp (6)
where R is the universal gas constant Q T and ko are apparent activation energy Q the absolute
temperature and a constant respectively The apparent activation energy was calculated from the slope
of this line in Figure 6 and equal to 95 kJmol-1
The value of the activation energy is much smaller
than those in NiAl2O3 and SiCAl2O3
When increasing temperature to higher than 900oC the Al2(MoO4)3 which may be located at grain
boundary of Al2O3 was decomposed to porous Al2O3 and MoO3(g) resulting in the channel for high
rate penetrating of oxygen and releasing of MoO3(g) as shown in Figure 4(b) The oxidation
behaviour of MoAl2O3 at the temperature exceeding 800oC would follow a parabolic manner
0 10 20 30 40 50 600
20
40
60
80
At 600oC
At 700oC
At 800oC
Time th
Th
ick
nes
s x
m
60 80 100 120
-15
-14
-13
-12
log
(kpm
2s-1
)
T -1
104K
-1
5 vol Ni Al2O3 [8]
8 vol SiCAl2O3 [9]
5 vol Mo Al2O3
Figure 5 Oxidized zone as a function of
time at (600800)oC
Figure 6 Dependence of parabolic rate
constant on reciprocal temperature
In case of oxidation of 5 vol NiAl2O3 at 12001350oC [8] the oxidation behaviour of NiAl2O3 is
governed by inward diffusion of O2-
along grain boundary of Al2O3 matrix As the results NiAl2O4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
5
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6
Figure 2 X-Ray diffraction pattern of the sintered samples (a) the specimens
after oxidation at 600oC (b) and 1200
oC (c) for 6 h
2Mo + 3O2 = 2MoO3 (s) (1)
MoO3 (s) = MoO3 (g) (2)
6Mo + 9O2 + 2Al2O3 = 2Al2(MoO4)3 (s) (3)
Al2(MoO4)3 = Al2O3 + 3MoO3 (g) (4)
Here Al2(MoO4)3 is the unique bi-oxide in the Al2O3 ndash MoO3 phase diagram [7]
The microstructures of oxidized surfaces were observed by SEM as shown in Figure 3 The oxidation
product seems to be at the grain boundary of Al2O3 matrix on the sample oxidized at 700oC for 6 h as
shown in Figure 3(a) Certainly the exposed Mo particles to the air reacted with O2 to form clumps of
MoO3 on the surface as observed on low magnification images When the oxidation time was reached
to 48 h the oxidation products mostly disappeared to leave pores with bright dots (round marks)
nearby them as shown in Figure 3(b) The same morphology was also found on the surface of
specimen after oxidizing at 800oC for 6 and 48 h as shown in Figure 3(c) It means MoO3 was formed
for the short time as the main oxidation product on the surface and then it evaporated during oxidation
at 600700oC The bright dots would be Al2O3 that formed from Al2(MoO4)3 after MoO3 evaporated
by following Equation 4 The oxidation product can be seen as bright phases like a clumps as shown
in Figure 3(d) appeared again from the grain size of the Al2O3 matrix on the oxidized surface after
oxidizing at 900oC for 6 h This phenomenon can be explained as follows when the temperature was
over the eutectic temperature in the Al2O3-Al2(MO4)3 of 820oC liquid phase was generated [7] The
liquid phase in Al2O3 matrix would promote outward diffusion of Al3+
and Mo6+
cations from inside to
the surface As well Al2O3 grains would be formed as shown in Equation 4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
3
(a)
(b)
(c)
(d)
Figure 3 The surface of the specimen after oxidizing at 700oC for 12 h (a) and 48 h (b) and
at 800oC for 6 h (c) and at 900
oC for 6 h (d)
The oxidized zone was observed from the SEM images of cross-section as shown in Figure 4 Mo
particles are not observed in the oxidized zone The morphology was in agreement with the
phenomenon that already discussed from the oxidized surfaces The oxidation product grew on the
surface for the oxidized specimen at 600800oC for short oxidation time Prolonging oxidation time
most of MoO3 was evaporated from the surface The voids appeared inside the oxidized zone and
there was no crack They implied that the outward diffusion of cations Conversely if we assume that
the O2-
anion inward diffused and the reaction occurred in Al2O3 matrix the oxidation zones would be
cracked because of the large volume expansion of oxidation products to the MoAl2O3 The Pilling-
Bedworth ratios of MoO3Mo and Al2(MoO4)3 are approximately 68 and 3 respectively
Thickness of the oxidized zones measured from the SEM images of the cross-section is a function of
time as shown in Figure 5 The growth of oxidized zone at 600800oC follows a parabolic manner
tkx p2 (5)
where kp x and t are the parabolic rate constant of the growth of oxidized zone the thickness of
oxidize zone and time The kp values of 5 vol MoAl2O3 is much faster than that of 5 vol NiAl2O3
[8] and 8 vol SiCAl2O3 [9] at 12001400 oC The dependence of kp on the annealing temperature
obeyed the Arrhenius Equation
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
4
(a)
(b)
Figure 4 Cross-section of specimens oxidizing at 700oC for 48 h (a) and at 900
oC for 6 h
RT
Qkk op exp (6)
where R is the universal gas constant Q T and ko are apparent activation energy Q the absolute
temperature and a constant respectively The apparent activation energy was calculated from the slope
of this line in Figure 6 and equal to 95 kJmol-1
The value of the activation energy is much smaller
than those in NiAl2O3 and SiCAl2O3
When increasing temperature to higher than 900oC the Al2(MoO4)3 which may be located at grain
boundary of Al2O3 was decomposed to porous Al2O3 and MoO3(g) resulting in the channel for high
rate penetrating of oxygen and releasing of MoO3(g) as shown in Figure 4(b) The oxidation
behaviour of MoAl2O3 at the temperature exceeding 800oC would follow a parabolic manner
0 10 20 30 40 50 600
20
40
60
80
At 600oC
At 700oC
At 800oC
Time th
Th
ick
nes
s x
m
60 80 100 120
-15
-14
-13
-12
log
(kpm
2s-1
)
T -1
104K
-1
5 vol Ni Al2O3 [8]
8 vol SiCAl2O3 [9]
5 vol Mo Al2O3
Figure 5 Oxidized zone as a function of
time at (600800)oC
Figure 6 Dependence of parabolic rate
constant on reciprocal temperature
In case of oxidation of 5 vol NiAl2O3 at 12001350oC [8] the oxidation behaviour of NiAl2O3 is
governed by inward diffusion of O2-
along grain boundary of Al2O3 matrix As the results NiAl2O4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
5
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6
(a)
(b)
(c)
(d)
Figure 3 The surface of the specimen after oxidizing at 700oC for 12 h (a) and 48 h (b) and
at 800oC for 6 h (c) and at 900
oC for 6 h (d)
The oxidized zone was observed from the SEM images of cross-section as shown in Figure 4 Mo
particles are not observed in the oxidized zone The morphology was in agreement with the
phenomenon that already discussed from the oxidized surfaces The oxidation product grew on the
surface for the oxidized specimen at 600800oC for short oxidation time Prolonging oxidation time
most of MoO3 was evaporated from the surface The voids appeared inside the oxidized zone and
there was no crack They implied that the outward diffusion of cations Conversely if we assume that
the O2-
anion inward diffused and the reaction occurred in Al2O3 matrix the oxidation zones would be
cracked because of the large volume expansion of oxidation products to the MoAl2O3 The Pilling-
Bedworth ratios of MoO3Mo and Al2(MoO4)3 are approximately 68 and 3 respectively
Thickness of the oxidized zones measured from the SEM images of the cross-section is a function of
time as shown in Figure 5 The growth of oxidized zone at 600800oC follows a parabolic manner
tkx p2 (5)
where kp x and t are the parabolic rate constant of the growth of oxidized zone the thickness of
oxidize zone and time The kp values of 5 vol MoAl2O3 is much faster than that of 5 vol NiAl2O3
[8] and 8 vol SiCAl2O3 [9] at 12001400 oC The dependence of kp on the annealing temperature
obeyed the Arrhenius Equation
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
4
(a)
(b)
Figure 4 Cross-section of specimens oxidizing at 700oC for 48 h (a) and at 900
oC for 6 h
RT
Qkk op exp (6)
where R is the universal gas constant Q T and ko are apparent activation energy Q the absolute
temperature and a constant respectively The apparent activation energy was calculated from the slope
of this line in Figure 6 and equal to 95 kJmol-1
The value of the activation energy is much smaller
than those in NiAl2O3 and SiCAl2O3
When increasing temperature to higher than 900oC the Al2(MoO4)3 which may be located at grain
boundary of Al2O3 was decomposed to porous Al2O3 and MoO3(g) resulting in the channel for high
rate penetrating of oxygen and releasing of MoO3(g) as shown in Figure 4(b) The oxidation
behaviour of MoAl2O3 at the temperature exceeding 800oC would follow a parabolic manner
0 10 20 30 40 50 600
20
40
60
80
At 600oC
At 700oC
At 800oC
Time th
Th
ick
nes
s x
m
60 80 100 120
-15
-14
-13
-12
log
(kpm
2s-1
)
T -1
104K
-1
5 vol Ni Al2O3 [8]
8 vol SiCAl2O3 [9]
5 vol Mo Al2O3
Figure 5 Oxidized zone as a function of
time at (600800)oC
Figure 6 Dependence of parabolic rate
constant on reciprocal temperature
In case of oxidation of 5 vol NiAl2O3 at 12001350oC [8] the oxidation behaviour of NiAl2O3 is
governed by inward diffusion of O2-
along grain boundary of Al2O3 matrix As the results NiAl2O4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
5
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6
(a)
(b)
Figure 4 Cross-section of specimens oxidizing at 700oC for 48 h (a) and at 900
oC for 6 h
RT
Qkk op exp (6)
where R is the universal gas constant Q T and ko are apparent activation energy Q the absolute
temperature and a constant respectively The apparent activation energy was calculated from the slope
of this line in Figure 6 and equal to 95 kJmol-1
The value of the activation energy is much smaller
than those in NiAl2O3 and SiCAl2O3
When increasing temperature to higher than 900oC the Al2(MoO4)3 which may be located at grain
boundary of Al2O3 was decomposed to porous Al2O3 and MoO3(g) resulting in the channel for high
rate penetrating of oxygen and releasing of MoO3(g) as shown in Figure 4(b) The oxidation
behaviour of MoAl2O3 at the temperature exceeding 800oC would follow a parabolic manner
0 10 20 30 40 50 600
20
40
60
80
At 600oC
At 700oC
At 800oC
Time th
Th
ick
nes
s x
m
60 80 100 120
-15
-14
-13
-12
log
(kpm
2s-1
)
T -1
104K
-1
5 vol Ni Al2O3 [8]
8 vol SiCAl2O3 [9]
5 vol Mo Al2O3
Figure 5 Oxidized zone as a function of
time at (600800)oC
Figure 6 Dependence of parabolic rate
constant on reciprocal temperature
In case of oxidation of 5 vol NiAl2O3 at 12001350oC [8] the oxidation behaviour of NiAl2O3 is
governed by inward diffusion of O2-
along grain boundary of Al2O3 matrix As the results NiAl2O4
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
5
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6
grains are formed in Al2O3 matrix they are traced around the Ni particles as partly oxidized zone and
around the voids as completed oxidized zone But in the present work the size of voids was
approximate to the size of Mo particles and the partly oxidized zone could not be observed Then two
possibilities of outward diffusion of cations were given Firstly Al2(MoO4)3 was located at grain
boundary of Al2O3 and enhance the outward diffusion of Al3+
It is reported that Al2(MoO4)3 is an Al3+
ion conductor The other possibility is rapid diffusion of Mo6+
at grain boundary of Al2O3 matrix
4 Conclusion
In this work the mechanism of oxidation of 5 vol MoAl2O3 was discussed The oxidized products
were determined as Al2(MoO4)3 and MoO3 At oxidation temperatures of 500800oC MoO3 firstly
formed on the surface mainly due to the outward diffusion of Mo6+
and Al3+
For a long holding time
the MoO3 evaporated to leave the porous Al2O3 The growth of oxidized zone followed parabolic
manner with activation energy of 95 kJmol-1
5 References
[1] Sbaizero O Pezzotti G and Nishida T 1998 J Acta Mater 46 681
[2] Sekino T and Niihara K 1997 J Mater Sci 32 3943
[3] Tuan W H and Brook R J 1990 J Eur Ceram Soc 6 31
[4] Oh S T Sekino T and Niihara K 1998 J Eur Ceram Soc 18 31
[5] Sbaizero O and Pezzottl G 2000 Acta Mater 48 985
[6] Wu T and Wei W J 2001 Scripta Mater 44 1025
[7] Dabrowska G Tabero P and Kurzawa M 2009 J Phase Equil and Diff 30 220
[8] Nanko M Nguyen T D Matsumaru K and Ishizaki K 2002 J Ceram Proc Res 3 132
[9] Luthra K L and Park H D 1990 J Amer Ceram Soc 73 1014
[10] Adachi G Imanaka N Tamura S 2001 J Alloys Comp 323-324 534
Int Symp Multifunctional Ceramic Materials Based on Nanotechnology (ISMCN2010) IOP PublishingIOP Conf Series Materials Science and Engineering 20 (2011) 012015 doi1010881757-899X201012015
6