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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

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