Papers by Author: Kiyoshi Hirao

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Authors: Yu Ping Zeng, Naoki Kondo, Kiyoshi Hirao, Hideki Kita, Tatsuki Ohji, Shuzo Kanzaki
Abstract: Silicon nitride ceramics was prepared by tape casting nonaqueous ceramic slurries, laminating the green ceramic tapes, and gas pressure sintering in nitrogen atmosphere. Lu2O3 and SiO2 were used as the sintering additives, and 3 wt.% β-Si3N4 seed was added to enhance β-Si3N4 grain growth unidirectionally. The seeded and tape-cast Si3N4 showed very good high temperature bending strength at 1500oC, when the stress applied along with the grain alignment direction. This was attributable to the formation of a high melting point grain boundary phase and the fibrous Si3N4 grains alignment. After exposure in air at 1500oC for up to 100 h, the oxidation products formed on the Si3N4 surface consist of Lu2Si2O7 and SiO2. The bending strength of the oxidized and tape-cast Si3N4 was degradation, the strength decrease was associated with the formation of new defects on the surface and the interface between the oxide layer and the Si3N4 bulk.
Authors: Zoltán Lenčéš, Pavol Šajgalík, E. Roncari, Kiyoshi Hirao
Authors: Seung Boo Jung, Yoritoshi Minamino, Hideki Araki, Toshimi Yamane, Kiyoshi Hirao, S. Saji
Authors: Yoritoshi Minamino, Hidehiro Yoshida, Seung Boo Jung, Kiyoshi Hirao, Toshimi Yamane
Authors: Mark I. Jones, Kiyoshi Hirao, Hideki Hyuga, Yukihiko Yamauchi
Abstract: The effects of microstructure and composition on the wear properties of Lu sialon ceramics have been studied under dry sliding conditions through block-on-ring wear tests. Microstructural and compositional effects on wear behaviour were studied by producing both equiaxed and elongated α sialons through the incorporation of additional oxides to promote extended liquid formation and grain growth, and by producing α / β composite materials with elongated β grains. The wear response of the materials is discussed in terms of the dominant wear mechanism under different experimental conditions. Under higher loads, where fracture dominates, materials with improved mechanical properties show better wear resistance and both the composite materials and the elongated α sialons showed lower wear rates than the equiaxed materials due to the elongated grain microstructures. Under low normal loads, fracture does not occur and the dominant wear mechanism is thought to be tribochemically assisted wear. Under these conditions, the equiaxed materials had better wear resistance than the composites, and the Lu-α sialon showed an order of magnitude lower wear rate than an equivalent Y-α sialon, thought to be due to better oxidation resistance and improved refractory nature afforded through the use of the smaller radius cation. The elongated Lu-α sialons under these low load conditions showed wear resistance that was to some extent dependent on the composition of the additional liquid phase, with high SiO2 contents leading to higher wear rates.
Authors: Toru Wakihara, Masahiro Yabuki, Junichi Tatami, Katsutoshi Komeya, Takeshi Meguro, Hideki Kita, Naoki Kondo, Kiyoshi Hirao
Abstract: Post-reaction sintering as a technique for the fabrication of Si3N4 ceramics has received much attention as a cost-effective process due to the use of cheap Si powder as a raw material. In this method, the rapid exothermic nitridation of Si results in local melting of Si to cause its agglomeration, which is expected to be a flaw after densification. Therefore, control of the exothermic reaction is needed to improve the reliability of post-reaction sintered Si3N4 ceramics. In this study, Si3N4 ceramics were fabricated by post-reaction sintering with Si3N4 or SiO2 powders in order to control the exothermic reaction. As a result, the microstructure and bending strength of Si3N4 ceramics was changed by adding these additives. In particular, the addition of SiO2 resulted in the high strength of Si3N4 ceramics. Consequently, it was found that Si3N4 and SiO2 particles played the role of diluents, and SiO2 was effective in post-reaction sintering as an oxygen donor.
Authors: Kiyoshi Hirao, Hiroyuki Hayashi, Kiyoshi Itatani, Yukihiko Yamauchi
Authors: Yu Ping Zeng, Naoki Kondo, Kiyoshi Hirao, Tatsuki Ohji, Shuzo Kanzaki
Abstract: Silicon nitride ceramics with and without β-Si3N4 seed addition were prepared via tape casting nonaqueous ceramic slurries, laminating the green ceramic tapes, and gas pressure sintering the green bodies in nitrogen atmosphere. Lu2O3 and SiO2 were used as sintering additives. The results showed that the rod-like β-Si3N4 seed was helpful to enhance β-Si3N4 grain growth unidirectionally. The elongated grains grown from seeds were preferentially oriented parallel to the casting direction, resulting in anisotropic microstructure. When a stress was applied with along the grain alignment, the bending strength of the tape-cast Si3N4 with 3 wt% β-Si3N4 seed addition measured at 1500oC was 738 MPa, which was almost the same as the room temperature bending strength 739 MPa, and the fracture energy can be improved from 301 J/m2 at room temperature to 781 J/m2 at 1500oC. The large fracture energy and bending strength at 1500oC were attributable primarily to the unidirectional alignment fibrous grains and a high melting point grain boundary phase.
Authors: Zoltán Lenčéš, Kiyoshi Hirao, Michael J. Hoffmann, Shuzo Kanzaki
Authors: Hiroyuki Miyazaki, Hideki Hyuga, Yuichi Yoshizawa, Kiyoshi Hirao, Tatsuki Ohji
Abstract: Six kinds of silicon nitrides with different microstructures were fabricated by changing the amount of sintering additives, Al2O3 and Y2O3. The hardness decreased with an increase in the amount of additives, whereas the indentation fracture resistance increased in the samples with large amount of additives due to the formation of coarse and elongated grains. The wear property of various Si3N4 ceramics was investigated in the sliding contact test without lubricant and was compared with the mechanical properties. The specific wear rate varied notably from 4x10-4 to 6x10-6 mm-3N-1m-1 depending on the compositions, which was difficult to explain directly from the hardness and fracture resistance. An indentation fracture model for material removal could correlate the wear properties with a function of hardness and fracture resistance of the materials, suggesting that the indentation model was likely to be valid for analyzing the wear behavior in this study.
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