High-Temperature Oxidation of Nano-Ni Dispersed Al2O3 Composites in Air

Makoto Nanko, Masahiro Mizumo, Miku Watanabe, Koji Matsumaru and Kozo Ishizaki

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AZojomo (ISSN 1833-122X) Volume 2 May 2006

Topics Covered

Abstract

Keywords

Introduction

Experimental

Preparation of Sample

Oxidation of Sample

Results and Discussion

Conclusions

Acknowledgements

References

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Abstract

The oxidation behavior of Al₂O₃-based nano-composites with a 5 vol% Ni particle dispersoid was investigated at 1200 – 1350˚C in air.  The oxidized zone consisted of Al₂O₃ matrix and NiAl₂O₄ spinel grains.  A continuous NiAl₂O₄ layer was also observed on the surface of the composites.  The oxidized zone followed parabolic growth.  Thus the mass transport in the oxidized zone was the rate-controlling step.  The parabolic rate constant of the nano-composites was 3 times faster than Al₂O₃ composites dispersed with 10 μm Ni particles.

Keywords

High-Temperature Oxidation, Ni, Al₂O₃, Nano-Composites, Kinetics

Introduction

New high-temperature materials are required to increase energy conversion efficiency of thermal cycles such as gas turbine engines.  Functionally graded materials (referred to as FGMs) have received attention as the next generation of high-temperature materials.  Since at high temperatures oxygen can pass through an oxide matrix, dispersed metal particles are oxidized in the matrix.  The metallic dispersoid expands due to oxidation and gives rise to stress in the matrix.  Finally the composite is fractured.  To apply FGMs at high temperatures, oxidation/corrosion resistance is very important.

Nanko et al. [1] reported high-temperature oxidation of partially stabilized zirconia (PSZ) composites with Ni particles (Ni/PSZ).  Due to oxidation of Ni particles, the PSZ matrix had cracked.  The cracked region grew proportional to oxidation time.  High-temperature oxidation of Al₂O₃ matrix composites with Ni particles (Ni/Al₂O₃) was also reported [2,3].  Nanko et al. [2] reported that since Al₂O₃ has excellent mechanical properties and low diffusivities of ions, Al₂O₃ composites dispersed with metallic Ni particles (Ni/Al₂O₃) have higher oxidation resistance than the Ni/PSZ.  Oxidized zone of Ni/Al₂O₃ has a structure that shows NiAl₂O₄ grains being dispersed in an Al₂O₃ matrix with a surface NiAl₂O₄ layer.   Growth of the oxidized zone followed the parabolic law, which meant that mass transport in the oxidized zone was rate-controlling.  The oxidized zone had not cracked.  However, voids had formed in the region with NiAl₂O₄ grains.  Those voids were formed by outward diffusion of cations during oxidation of Ni.

Wang et al. [3] reported oxidation of Ni/Al₂O₃ at temperatures from 1000 to 1300ºC.  They studied the oxidation behavior of the composites with finer Ni particles (2-5 μm) than our previous work (10 μm) [2].  Oxidation behavior obeyed the parabolic law.  Oxidation rate increases with increasing Ni volume fraction.  High-temperature oxidation of MgO composites with 10 μm Ni particles was also investigated [4].

Ceramic-based nano-composites with metallic dispersoid have excellent mechanical properties such as high fracture strength and high fracture toughness [5].  In order to increase mechanical strength of FGMs, nano metal particles should be dispersed in the ceramic matrix of the composite.  However, high-temperature oxidation has not been studied on nano-composites made of ceramic matrix and metallic dispersoid.  Previously research on high-temperature oxidation was performed on only “macro-composites”, as mentioned above.  In this paper, high-temperature oxidation of nano-Ni dispersed Al₂O₃ composites are discussed and compared with Ni /Al₂O₃ macro-composites.  Ni/Al₂O₃ nano-composites are attractive materials with superior mechanical properties [5, 6].

Experimental

Preparation of Sample

For preparing the nano-composite powder, a method with Ni(NO₃)₂ solution as a source for Ni dispersoid was adopted, which was similar to that reported by Sekino et al. [6].  A slurry was prepared by mixing the commercial Al₂O₃ powder (average particle size: 0.5 μm, purity: 99.99%) and aqueous solution with 50 mass %-Ni(NO₃)₂·6H₂O as 5 vol% Ni after reduction.  Mixture of Al₂O₃ and nano-NiO powder was prepared by dropping the slurry to a glass tube heated at 300˚C.  The powder mixture was reduced at 600˚C for 12 h in a stream of Ar-1%H₂ gas mixture.  The reduced powder was consolidated in a die by a pulsed electric current pressure-sintering technique at 1400˚C under 55 MPa pressure for 5 min holding time in vacuum. The sintered samples attained a density of at least 99 % of theoretical value.  Sample surface was ground by #2000 (12 μm) SiC-abrasive papers and then polished by 4 μm-diamond slurry.  Figure 1 shows the SEM photograph of the as-sintered specimen.  There are few small pores.  Fine white particles with sub-microns in diameter are Ni particles that are dispersed homogenously.

Oxidation of Sample

The sample was put on the alumina balls (3 mm in diameter) in an alumina crucible and oxidized at temperature ranging from 1200 and 1350˚C in air.  The heating and cooling rates in the oxidation experiments were 400 K/h.  Phase identification of the samples was carried out by X-ray diffraction (XRD).  Microstructure of samples was observed by scanning electron microscopy (SEM).  Thickness of oxidized zone was determined by observing microstructure using SEM.

Results and Discussion

Figure 2 shows the SEM photograph of the sample oxidized at 1300˚C for 3 d.  In the region from surface to the depth of 200 μm, no Ni particles are observed, but grains larger than Ni particles were seen.  In this region, fine voids with sub-microns in diameter also appear.  A continuous surface layer, which has the same color of the grains dispersed in the region to the depth of 200 μm, is also observed as shown in Figure 2 (b).  Based on the XRD results, the surface layer and the grains located in this region are made of NiAl₂O₄ spinel, which is the oxidation product of Ni particles and Al₂O₃ matrix.  In this study, a region from the sample surface to the oxidation front is defined as an oxidized zone.  The oxidized zone is similar to the oxidized zone of the Ni/Al₂O₃ macro-composites as described in the previous reports [2, 3].

With increasing oxidation time, thickness of oxidized zone increases.  Figure 3 shows the time dependence of the thickness of the oxidized zone of Ni/Al₂O₃ nano-composites, x, at various oxidation temperatures.  With increasing oxidation temperature, thickness of the oxidized zone increases.

Figure 4 shows the parabolic plots on growth of the oxidized zone of Ni/ Al₂O₃ nano-composite.  Growth of the oxidized zone obeys the parabolic law, which can be expressed as follows:

X² = K_pt         (1)

where k_p is the parabolic rate constant. Because the oxidized zone is dense as shown in Figure 2, mass transport through the oxidized zone is the dominant of the growth processes of the oxidized zone.

Figure 5 shows comparison of the parabolic rate constant of high-temperature oxidation of Ni/Al₂O₃ with these of Al₂O₃ forming alloys.  The parabolic rate constant of high-temperature oxidation of Ni/Al₂O₃ nano-composite is given by:

 (2)

The oxidation rate of Ni/Al₂O₃ nano-composite is 3 times higher than the rate of Ni/Al₂O₃ macro-composites.  The apparent activation energy on growth of oxidized zone of Ni/Al₂O₃ composite is similar to one of the Ni/Al₂O₃ macro-composites [2].

In general, ceramic-based nano-composites consisting of fine metallic dispersoid have finer grain size of matrix because the fine dispersoid acts as an inhibiter of grain growth of matrix during sintering.  As described in the previous report [2], diffusion along grain boundary of oxidized zone, which is mainly Al₂O₃, are the rate-controlling process of growth of the oxidized zone.  Flux of diffusion along grain boundary is proportional to the reciprocal of grain size.  Faster oxidation rate of Ni/Al₂O₃ nano-composite is probably caused by finer grains of Al₂O₃ matrix of oxidized zone than Ni/Al₂O₃ macro-composites.

Conclusions

The oxidation behavior of Al₂O₃-based nano-composites with a 5 vol% Ni particle dispersoid was investigated at temperatures ranging from 1200 to 1350ºC in air.  The oxidized zone consisted of Al₂O₃ matrix and NiAl₂O₄ grains with surface NiAl₂O₄ layer.  Voids with sub-microns in size are observed in the oxidized zone.  The growth of the oxidized zone followed a parabolic law, which meant that mass transport in the oxidized zone was the rate-controlling process.  The value of the parabolic rate constant was 3 times faster than Al₂O₃ composites dispersed with 10 μm Ni particles.

Acknowledgements

The authors wish to express their gratitude to the Japanese government for partially supporting this work through the 21st Century Centers of Excellence (COE) Program of the Ministry of Education, Culture, Sports, Science and Technology.

References

1.       M. Nanko, M. Yoshimura and T. Maruyama, “High Temperature Oxidation of Y₂O₃ Partially-Stabilized ZrO₂ Composites Dispersed with Ni Particles”, Mater. Trans., 44 [4] (2003)736-742.

2.       M. Nanko, T. Nguyen Dang, K. Matsumaru and K. Ishizaki, “High Temperature Oxidation of Al₂O₃-Based Composites with Ni Particle Dispersion”, J. Ceram Proc. Res., 3 [3] (2002) 132-135.

3.       T.C. Wang, R.Z. Chen and W.H. Tuan, “Oxidation Resistance of Ni-Toughened Al₂O₃”, J. Euro. Ceram. Soc., 23 [6] (2003) 927-934.

4.       M. Nanko, A. Sakashita, K. Matsumaru and K. Ishizaki, “High Temperature Oxidation of A MgO Composites with Ni Particle Dispersion”, Adv. Technol. Mater. Mater. Process. J., 6 [1] (2004) 43-46.

5.       T. Sekino, S. Ethh, H. Kondo, Y-H. Choa and K. Niihara, “Transition Metal Dispersed Oxide Ceramic Nanocomposities with Multiple Functions”, Key Eng. Mater., 161-163, (1999) 489-492.

6.       T. Sekino, T. Nakajima, S. Ueda and K. Niihara, “Reduction and Sintering of a Nickel-Dispersed-Alumina Composite and its Propertoies”, J. Amer. Ceram. Soc., 80 [5] (1997) 1139-48.

7.       K. L. Luthra and H. D. Park, “Oxidation of Silicon Carbide-Reinforced Oxide-Matrix Composites at 1375 to 1575ºC”, J. Amer. Ceram. Soc., 73 [4] (1990) 1014-1023.

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