High-Temperature Oxidation Treatment of Nano-Ni Composites by O2-HIPing

Makoto Nanko and Daisuke Maruoka

Presented at the 2011 International Conference on Hot Isostatic Pressing Kobe, Japan, 12-14 April 2011
Submitted: 12 April 2011, Accepted: 24 May 2011

Topics Covered

Abstract

Advanced nano-Ni/Al₂O₃ composites have great performance such as excellent mechanical strength and show crack-healing function by high-temperature oxidation treatment. Cracks are filled up by oxidation product, NiAl₂O₄, during high-temperature oxidation. As a result, mechanical strength of the composites can be recovered until the level without any cracks. In this report, high-temperature oxidation treatment of nano- Ni/Al₂O₃ composites by O₂-HIP is discussed to enhance the crack-healing. O₂-HIP treatment is expected to enhance outward diffusion of Ni to develop NiAl₂O₄.

Keywords

Ni, Al₂O₃, Nanocomposites, Crack-Healing, Oxidation, Hot Isostatic Pressing (HIPing)

Introduction

Ni/Al₂O₃ nanocomposites, i.e., Al₂O₃ composites dispersed with nano-Ni particles (referred as to nano-Ni/Al₂O₃) are one of the well-studied nanocomposites, and have excellent mechanical properties such as high bending strength and high fracture toughness [1-3]. Sekino et al. [1-3] reported that nano-Ni/Al₂O₃ has the in-situ stress-monitoring function by magnetic property change.

As a new function of nano-Ni/Al₂O₃, self crack-healing via thermal oxidation was discovered [4, 5]. In the crack-healing, cracks was filled up with the oxidation product, NiAl₂O₄, resulting in recovery of the bending strength which is decreased by the surface cracks. In cases of Al₂O₃ matrix, outward diffusion of cations though the grain boundaries of Al₂O₃ takes place during thermal oxidation process. A typical oxidation conditions for crack-healing of nano-Ni/Al₂O₃ was 1200 ºC for 6 h in air. Taking account of the speculative mechanism of the crack-healing, thermal oxidation under high oxidation pressure may decrease oxidation temperatures and oxidation time.

Hot isostatic pressing under oxygen containing gas mixture (O₂-HIP) can realize highly oxidizing atmosphere with high oxygen partial pressure. O₂-HIPing has been used for heat treatment under high oxygen partial pressure [6-8]. High oxygen potential of the atmosphere may accelerate outward diffusion of cations during oxidation of Ni/Al₂O₃. O₂-HIPing is expected to realize fast crack-disappearance of nano- Ni/Al₂O₃. In this paper, crack-disappearance of nano-Ni/Al₂O₃ via O₂ -HIPing was reported.

Experimental Procedure

Aqueous slurry with a commercial α-Al₂O₃ powder (average particle size: 0.4 μm) and Ni(NO₃)2•6H₂O as 5 vol% Ni in nano-Ni/Al₂O₃ was prepared by using an ultrasonic vibration for 10 min. Powder mixture was produced by dropping the slurry to a glass container 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 obtained powder mixture was consolidated in a graphite die by a pulsed electric current pressure-sintering technique at 1400 ˚C under 40 MPa for 5 min in a vacuum. The sintered specimens attained a density of at least 99 % of the theoretical value. Sample surface was ground by #2000 diamond abrasive papers and then polished by 2 μm-diamond slurry.

The cracks were introduced into the sintered bodies by using the Vickers hardness tester with 49 N for 10 s. The samples with cracks were annealed by O₂-HIPing (Kobelco Co. Ltd, Dr. O2 HIP) at 1200 ˚C for 1 h under 100 MPa of the Ar-20 %O₂ gas mixture. Oxygen partial pressure in the O₂-HIPing was 20 MPa. Sample temperature during O₂-HIPing was monitored using additional type K thermocouple set near the sample. The temperature difference was lower than ±2 ºC and temperature fluctuation was less than ±5 ºC. As comparison, thermal oxidation treatments in air and in a stream of Ar-1 %H₂ gas mixture were also conducted to as-cracked samples.

Sample surface was observed by SEM after the thermal oxidation treatment. Length of the un-closed cracks was measured using the scanning electron microscopy (SEM) images of the surface after heat treatments. Phase-identification for oxidized samples was conducted by X-ray diffraction (XRD).

Results and Discussion

Fig. 1 shows the fractured surface images of the as-sintered nano-Ni/Al₂O₃ by using SEM. As shown in Fig. 1 (a), there are no significant voids in the sintered sample. In high magnification view shown in Fig. 1 (b), fine white particles with sub-microns in diameter are Ni particles that are dispersed at the Al₂O₃ grain boundaries homogenously. The average Ni particle size is approximately 300 nm. Grain size of Al₂O₃ matrix was approximately 1 µm.

Fig. 2 shows the surface SEM images of (a) as-cracked nano-Ni/Al₂O₃, (b) O₂-HIPed one, (c) air-annealed one and (d) vacuum-annealed one. Thermally oxidized samples have fewer than as cracked one. No crack- disappearance was observed on the vacuum-annealed sample. O₂-HIPed nano-Ni/Al₂O₃ has higher cracks-disappearance than the air-annealed one, which is comparable with as-cracked one. These results mean that thermal oxidation process causes crack-disappearance. Table 1 shows a ratio of crack-disappearance of nano- Ni/Al₂O₃ by thermal oxidation. The O₂-HIP treatment has the highest crack-disappearance ratio of nano-Ni/Al₂O₃. The effect of O₂-HIP for crack-disappearance is caused by enhancement of outward diffusion of cations such as Ni and Al under high oxygen partial pressure.

Fig. 3 shows XRD patterns on the surface of nano-Ni/Al₂O₃. Thermally oxidized samples, that is, O₂-HIPed one and air-annealed one, contain NiAl₂O₄, which is the oxide product of nano- Ni/Al₂O₃ [4, 5 and 9]. The surface of oxidized nano-Ni/Al₂O₃ would be covered with NiAl₂O₄. Crack-disappearance of nano-Ni/Al₂O₃ via O₂-HIPing was also conducted by the formation of NiAl₂O₄.

Figure 1. Fractured surface of as-sintered nano-Ni/Al₂O₃: (a) low magnification and (b) high magnification.

Figure 2. Surface SEM images of (a) as-cracked nano-Ni/Al₂O₃, (b) O₂ -HIPed one at 1200 °C for 1 h under 100 MPa in total pressure and 20 MPa in oxygen partial pressure, (c) air-annealed one at 1200 °C for 1 h in air and (d) H₂-annealed one at 1200 °C for 6 h.

Table 1. Crack-disappearance ratio of nano-Ni/Al₂O₃ by various thermal oxidation

Figure 3. XRD patterns of nano-Ni/Al₂O₃. A: alumina, N: nickel and s: NiAl₂O₄

Conclusion

Faster crack-disappearance of Ni/Al₂O₃ nanocomposites via O₂-HIPing was observed at 1200 ºC under 100 MPa in total pressure and 20 MPa in oxygen partial pressure. The O₂-HIPing effect is most likely due to high oxygen partial pressure. O₂-HIPing is expected for faster crack-healing treatment for Ni/Al₂O₃ nanocomposites.

Acknowledgement

This study has been supported partially by Grant-In-Aid for Scientific Researches (C18560691) and (B22360284) from Japan Society for the Promotion of Science and by the Union Tools Foundation.

References

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Contact Details

Makoto Nanko and Daisuke Maruoka
Nagaoka University of Technology1603-1, Kamitomioka, Nagaoka, Niigata, 940-2188, Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13[1] (2011) 30-33.