Influences of Al₂O₃ grain size on high-temperature oxidation of nano-Ni/Al₂O₃ composites

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

Abstract

Two 5 vol% Ni/Al₂O₃ composites with the difference in Al₂O₃ grain size were fabricated by pulsed electric current sintering technique to investigate the influence of Al₂O₃ grain size on oxidation behavior of the composites. Average Al₂O₃ grain sizes of two fabricated composites were 1.1 μm and 0.5 μm after sintering. Oxidation tests were conducted at temperatures ranging from 1100 to 1350 °C for 1–48 h in air. A thin NiAl₂O₄ layer was observed in exposed surface of samples after oxidation. An oxidized zone that consisted of Al₂O₃ matrix and NiAl₂O₄ grains was defined. Growth of the oxidized zone obeyed the parabolic law. Influences of Al₂O₃ grain size on high-temperature oxidation of the composites were discussed.

Keywords:

• Nanocomposite
• High-temperature oxidation
• Grain size
• Al₂O₃

1 Introduction

Alumina (Al₂O₃) has been well-known as a high-performance structural material and widely applied in industries because of its advantages such as excellent heat resistance, high mechanical strength and wear resistance. However, similar to other ceramics, Al₂O₃ materials have inherently strong covalent and ionic bonds that prohibit substantial dislocation motion or plastic deformation. They fail to alleviate stress concentrations that occur in front of crack tips. Hence, Al₂O₃ materials are easily fractured as a consequence of crack propagation resulting from a slight surface flaw or internal flaw. Al₂O₃ materials thus exhibit poor toughness that restricts their application. In order to increase their toughness, dispersion of non-oxide phases such as Ni, NiAl, Co and SiC in Al₂O₃ matrix were proposed [1–4]. On the other hand, the presence of the non-oxide phases gives the materials the self-healing function that occurs at high temperatures in the air [2,5–7].

Ando and his co-workers reported self-healing function that can be achieved on ceramic-based composites such as SiC/mullite, SiC/Al₂O₃ [7–11]. When non-oxide dispersoids in the matrix were oxidized at high temperatures, the oxidation product filled up the cracks and involved the reduction of stress concentrations at crack tips. As a result, the mechanical strength was recovered up to the level of as-polished samples. Furthermore, this mechanism improves their performance with longer lifetime and more reliability at high temperatures.

Similarly, nano-Ni dispersed Al₂O₃ composites also have the self-healing function at high temperatures in the air [12–14]. Maruoka et al. reported the recovery of mechanical strength for nano-Ni/Al₂O₃ that was able to achieve when the fraction of surface crack disappearance is over 50% [14]. The study indicated that the crack disappearance effectiveness depended on the formation of NiAl₂O₄-oxidation product that mostly referred to the diffusion of Ni²⁺ along grain boundaries of the Al₂O₃ matrix. The conclusion implies that finer grain size of the matrix could give a positive effect on the crack-healing function.

While self-healing effectiveness mostly depends on the diffusion of Ni²⁺ and Al³⁺ cations, the oxidation resistance of materials depends on diffusion of O²⁻ ions along grain boundaries. At high temperatures, oxygen can penetrate the oxide matrix that causes the oxidation of dispersed Ni particles within the matrix. The region consisting of oxidized nickel particles develops as increasing oxidation temperature and duration. Nanko et al. reported the oxidation behavior of Ni/Al₂O₃ composites at temperatures ranging from 1200 to 1300 °C [15], in which the growth of oxidized region was attributed to obedience of the parabolic law. It implied that the mass transport in the oxidized region was the rate-controlling process, which was the diffusions of cations and oxide ions along grain boundaries of the Al₂O₃ matrix. Maruoka et al. reported the influence of Si-doping of nano-Ni dispersed Al₂O₃ composites on high-temperature oxidation [16]. The study concluded that the growth rate of the oxidized zone was decreased effectively at 1200 °C by doping 0.1 mol% Si in Ni/Al₂O₃. Doping of Si was attributed to the change to grain boundary diffusion in polycrystalline Al₂O₃. The grain boundary diffusion in polycrystalline Al₂O₃ could be also decreased by either dopant of Y, SrO, LuO_1.5 or ZrO₂ [17,18]. Since the grain boundary diffusion is an important factor responsible for the oxidation resistance of the composite, influences of Al₂O₃ grain size on oxidation resistance of Ni/Al₂O₃ are in need of investigation for high-temperature applications.

In this study, the influences of the different structure of Al₂O₃ matrix on oxidation resistance of 5 vol% Ni/Al₂O₃ composites are discussed. The oxidation resistance of the composites was evaluated through the growth rate of oxidized zone after heat treatment in air at temperatures ranging from 1100 to 1350 °C for 1–48 h.

2 Experimental procedures

In order to fabricate two types of 5 vol% Ni dispersed Al₂O₃ composites with the difference in Al₂O₃ grain structure, two different types of Al₂O₃ powder (Sumitomo Chemical Co. Ltd, AA-04, d = 0.44 μm and Taimei Chemicals Co. Ltd, TM-DAR, d = 0.14 μm) were used as starting materials. Aqueous slurries containing Ni(NO₃)₂·6H₂O (Kojundo Chemical Laboratory Co. Ltd, purity 99.9%) and the Al₂O₃ powders were prepared by milling in a plastic bottle with Al₂O₃ balls for 24 h. The composite that was prepared from Al₂O₃ with average particle size of 0.44 μm is referred to as Ni/Al₂O₃-044, hereafter. Similarly, Ni/Al₂O₃-014 is referred to as the composite that was prepared from the Al₂O₃ with average particle size of 0.14 μm. After drying at 400 °C in a boiling flask, the powder mixtures were milled by using an alumina mortar for 15 min. These powder mixtures were heat-treated at 600 °C for 12 h in a stream of the Ar–1% H₂ gas mixture to reduce NiO to Ni metallic phase. The powder mixtures were consolidated in a graphite die by pulsed electric current sintering (PECS) at 1400 °C for Ni/Al₂O₃-044 and 1200 °C for Ni/Al₂O₃-014 under 50 MPa uniaxial pressure in a vacuum for 5 min holding time. The relative density of all the consolidated samples used in this study attained at least 99% of the theoretical density. Fig. 1 shows the fractured surfaces of the as-sintered samples. Ni particles, that could be visible as the bright contrast dots, were homogeneously dispersed in the Al₂O₃ matrix. The average particle size of these Ni particles was approximately 300 nm while the average Al₂O₃ grain size of Ni/Al₂O₃-044 was 1.1 μm, and that of Ni/Al₂O₃-014 was 0.5 μm.

Fig. 1 SEM images of fractured surface of as-sintered samples: (a) Ni/Al₂O₃-044 and (b) Ni/Al₂O₃-014.

High-temperature oxidation tests were conducted at temperatures ranging from 1100 to 1350 °C for 1 h up to 48 h in the air with a heating rate of 400 °C/h. The tested samples were put on alumina balls (3 mm in diameter) in an alumina crucible and exposed in the air at the investigated conditions. Phase identification of the samples was carried out by X-ray diffraction (XRD). Oxidation evolution of heat-treated samples was evaluated via the growth rate of oxidized zone observed on cross-sectioned surface by scanning electron microscope (SEM).

3 Results

Fig. 2 shows XRD patterns obtained from sample surface before and after oxidation tests. Before oxidation, only two dominant phases of Ni and Al₂O₃ are detected on the sample surface (Fig. 2a). After oxidation at 1200 °C for 1 h, NiAl₂O₄ phase was identified as shown in Fig. 2b. On the other hand, the intensity of signals from Ni phase was decreased. The peak intensity of NiAl₂O₄ phase which was formed after oxidation at 1200 °C for 24 h was significantly increased and replaced for the existence of Ni-metallic phase. In addition, nickel oxide phase could not be identified on oxidized samples. This phenomenon has been reported by Trumble et al. [19] that NiO did not coexist with Al₂O₃. It implies that Ni particles reacted with Al₂O₃ matrix as the following equilibrium:(1) $2\text{Ni}+2{\text{Al}}_2{\text{O}}_3+{\text{O}}_2=2{\text{NiAl}}_2{\text{O}}_4$(1)

Fig. 2 XRD patterns of nano-Ni/Al₂O₃: (a) before oxidation, (b) oxidized at 1200 °C for 1 h and (c) oxidized at 1200 °C for 24 h in air.

Fig. 3 shows the SEM images of cross-section surface of Ni/Al₂O₃-014 and Ni/Al₂O₃-044 after oxidation at 1200 °C for 24 h. In the region from the surface to the depth of 35 μm, the grains that are larger than Ni particles were oxidation product-NiAl₂O₄ formed by oxidation test (Fig. 3a). Nano-Ni particles could be visible as bright dots in the region deeper 35 μm from the surface. For the reasons above, the region composed of the NiAl₂O₄ grains and Al₂O₃ was defined as the oxidized zone. Fig. 3b shows the thickness of oxidized zone in Ni/Al₂O₃-044 which is smaller than that of Ni/Al₂O₃-014. A thin surface layer that has the same color of NiAl₂O₄ was observed on both samples as shown in Fig. 3. From XRD results, the thin surface layer formed on the sample surface was determined as the NiAl₂O₄ layer. The NiAl₂O₄ layer formed on Ni/Al₂O₃-014 after oxidation at this condition was approximately 1.5 μm which was similar to that of Ni/Al₂O₃-044.

Fig. 3 SEM images of cross-section surface of (a) Ni/Al₂O₃-014 and (b) Ni/Al₂O₃-044 sample after oxidation at 1200 °C for 24 h in air. A thin surface layer composed of NiAl₂O₄ was observed on both samples.

Fig. 4 shows the thickness of oxidized zone in Ni/Al₂O₃-014 and Ni/Al₂O₃-044 as a function of oxidation time at various temperatures. With increasing oxidation temperature, the thickness of the oxidized zone increased. The growth of oxidized zone obeyed the parabolic law:(2) $x^2=k_{\text{p}}t\text{,}$(2) where x is the thickness of oxidized zone, k_p the parabolic rate constant, and t is oxidation time. The growth rate of oxidized zone in Ni/Al₂O₃-014 was slightly higher than that of Ni/Al₂O₃-044 after oxidation at 1200 °C. Nevertheless, this manner was reversed when the samples were heat-treated at 1300 °C. The growth rate of oxidized zone in Ni/Al₂O₃-014 was slightly lower than that of Ni/Al₂O₃-004 at higher temperatures.

Fig. 4 Thickness of oxidized zone as a function of oxidation time in air at various temperatures.

Fig. 5 shows the parabolic rate constant, k_p, in Ni/Al₂O₃-014 and Ni/Al₂O₃-044 as a function of reciprocal oxidation temperature. The slope of the straight line representative for Ni/Al₂O₃-014 was similar to that of Ni/Al₂O₃-044. The values of apparent activation energy for the growth rate of oxidized zone in Ni/Al₂O₃-014 and Ni/Al₂O₃-044 were 400 kJ mol⁻¹ and 414 kJ mol⁻¹, respectively.

Fig. 5 Temperature dependence of parabolic rate constant on oxidation of Ni/Al₂O₃-014 and Ni/Al₂O₃-044.

4 Discussion

The thickness of NiAl₂O₄ layer, which was formed on the sample surface after oxidation by outward diffusion of cations is much thinner than oxidized zone as mentioned in Fig. 3. For example, the thickness of oxidized zone formed by oxidation was 30–35 μm, while the thickness of NiAl₂O₄ surface layer was approximately 1.5 μm. From the kinetic point of view, diffusion of ions and cations in the oxidized zone play the major roles in oxidation behavior of the composites. The growth of NiAl₂O₄ layer in Ni/Al₂O₃-014 and Ni/Al₂O₃-044 was similar to each other under the investigated conditions. This fact indicates that grain size of Al₂O₃ does not influence significantly on the outward diffusion of cations under the investigated conditions.

Fig. 4 shows that the growth rate of oxidized zone in Ni/Al₂O₃-014 was higher than that of Ni/Al₂O₃-044 after oxidation at 1200 °C. This fact matches with the assumption that finer Al₂O₃ grain size accelerates the inward diffusion of oxygen ions. However, the thickness of oxidized zone in Ni/Al₂O₃-014 was lower than that of Ni/Al₂O₃-044 after oxidation at 1300 °C as mentioned in Fig. 4. Fig. 6 shows SEM images observed on the fractured surface of Ni/Al₂O₃-044 sample after oxidation at 1300 °C for 12 h. The average Al₂O₃ grain size in the oxidized zone was 1.5 μm (Fig. 6b) while it was 1.3 μm in the non-oxidized zone (Fig. 6c). The existence of pores in the oxidized zone was attributed to outward diffusion of cations. Similarly, Fig. 7 shows the differences in the microstructure of Ni/Al₂O₃-014 after oxidation at 1300 °C for 12 h. The average Al₂O₃ grain size in the oxidized zone was 1.3 μm (Fig. 7b) while it was 0.75 μm in the non-oxidized zone (Fig. 7c). Taking account of Al₂O₃ microstructure after sintering, the average Al₂O₃ grain sizes in Ni/Al₂O₃-044 and Ni/Al₂O₃-014 were 1.1 μm and 0.5 μm, respectively. Ni/Al₂O₃-014 was sintered at 1200 °C which was lower than that of the investigated conditions. Oxidation at temperatures higher than sintering temperature caused grain growth of the composites. On the other hand, Ni particles dispersed in Al₂O₃ matrix act as grain growth inhibitors. Outward diffusion of cations in oxidized zone during oxidation induced the reduction of Ni concentration. Consequently, grain growth occurred during oxidation tests especially in the oxidized zone. The similarities in microstructures of Al₂O₃ matrix in oxidized zone causes the similarities of oxidation behavior. In summary, nanostructure on Ni/Al₂O₃ is effective for mechanical behavior. However, there was no significant influence in high-temperature oxidation and its related phenomena because of grain growth in their oxidation zone.

Fig. 6 SEM images of fractured surface of Ni/Al₂O₃-044 after oxidation at 1300 °C for 12 h.

Fig. 7 SEM images of fractured surface of Ni/Al₂O₃-014 after oxidation at 1300 °C for 12 h.

5 Conclusions

Influences of Al₂O₃ grain size on oxidation behavior at temperatures ranging from 1100 to 1350 °C for 1–48 h were investigated in this study. A thin NiAl₂O₄ layer was observed in exposed surface of samples after oxidation. An oxidized zone that consisted of Al₂O₃ matrix and NiAl₂O₄-oxidation product was defined. The growth of oxidized zone obeyed the parabolic law. Finer Al₂O₃ grain size caused higher growth rate of oxidized zone at heat-treated temperatures lower than 1200 °C. At higher heat-treated temperatures such as 1300 °C, Al₂O₃ grain size has less influence on oxidation behavior of the composites due to the grain growth at oxidized zone.

Acknowledgements

The authors wish to express their gratitude to the Japan Science and Technology Agency (H26-354-15) for supporting this study through the Advanced Low Carbon Technology Research and Development Program. The authors also wish to acknowledge the discussion and advice of Assoc. Prof. Shigenari Hayashi, Tokyo Institute of Technology.

Notes

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.