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 CoveredAbstractKeywordsIntroductionExperimental ProcedureResults and DiscussionConclusionAcknowledgementReferencesContact Details
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.
Ni, Al2O3, Nanocomposites, Crack-Healing, Oxidation, Hot Isostatic Pressing (HIPing)
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.
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
|O2-HIP, 1200 ºC for 1 h
|Air, 1200 ºC for 1 h
|Ar-1%H2, 1200 ºC for 6 h
Figure 3. XRD patterns of nano-Ni/Al2O3. A: alumina, N: nickel and s: NiAl2O4
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.
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.
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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 (2011) 30-33.