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.

AZoJomo - The AZO Journal of Materials Online - SEM photograph of as-sintered Ni/Al2O3 nano-composite

Figure 1. SEM photograph of as-sintered Ni/Al2O3 nano-composite.
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].

AZoJomo - The AZO Journal of Materials Online - SEM photograph of the sample oxidized at 1300˚C for 3 d. (a) low magnification

(a)

AZoJomo - The AZO Journal of Materials Online - SEM photograph of the sample oxidized at 1300˚C for 3 d. high magnification near the surface

(b)

Figure 2. SEM photograph of the sample oxidized at 1300˚C for 3 d.
(a) low magnification and (b) high magnification near the surface.

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.

AZoJomo - The AZO Journal of Materials Online - Time dependence of the thickness of the oxidized zone of Ni/Al2O3 nano-composites, x, at various oxidation temperatures

Figure 3. Time dependence of the thickness of the oxidized zone of Ni/Al2O3 nano-composites, x, at various oxidation temperatures.

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 = Kpt (1)

where kp 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.

AZoJomo - The AZO Journal of Materials Online - Parabolic plot on growth of oxidized zone of Ni/Al2O3 nano-composites

Figure 4. Parabolic plot on growth of oxidized zone of Ni/Al2O3 nano-composites.

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].

AZoJomo - The AZO Journal of Materials Online - Temperature dependence of parabolic rate constant on growth of oxidized zone of Ni/Al2O3 nano-composites

Figure 5. Temperature dependence of parabolic rate constant on growth of oxidized zone of Ni/Al2O3 nano-composites.

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.