Papers by Author: Takashi Akatsu

Abstract: In this paper, the internal friction behaviors of unidirectionally solidified Al2O3/YAG eutectic that are examined by the authors in passed several years are reviewed. The internal friction of this eutectic increased with increasing temperatures above 1200oC. The apparent activation energy of the eutectic sample in the range from 1100 to 1400oC was same to that of Al2O3 single crystal. Above 1200oC, the internal friction of this material drastically increased with increasing temperatures. The magnitude of the internal friction strongly depended on the elevated temperatures and the cycle numbers of the torsional loading. For the 1400oC test, the internal friction gradually increased with the loading cycles and then saturated by 103 times of the loading cycles.

Abstract: The sintering stress is related to the thermal stability of porous structure. The sintering stress for a given porous structure in equilibrium can be calculated by three methods theoretically; the energy difference method, the curvature method, and the force balance method. The sintering stresses by three different methods were exactly the same for the idealized porous materials in equilibrium, in which the pore surface had a constant curvature at any point. The porous material does not spontaneously shrink when the sintering stress becomes zero or negative. The sintering stress will be used to design optimal porous structures with improved thermal stability.

Abstract: The deformation behavior of SiO2 doped nanocrystalline monoclinic zirconia (MZP) was studied at 1323-1223 K in compression tests. The strain rate of SiO2 doped nanocrystalline MZP was slower than that of high-purity MZP by one order of magnitude. SiO2 doped nanocrystalline MZP exhibited a stress exponent n ≈ 2. The apparent activation energy for the deformation of SiO2 doped nanocrystalline MZP was characterized by a higher value than that observed for high-purity MZP. 1wt% SiO2 doped nanocrystalline MZP was deformed at constant flow stress, while the flow stress of high-purity MZP increased significantly with the strain (strain hardening). While no grain growth was observed after the compressive deformation of 1wt % SiO2 doped nanocrystalline MZP, remarkable grain growth was observed after the deformation of high-purity MZP. The addition of SiO2 into nanocrystalline MZP is effective in limiting grain growth at low temperatures

Abstract: High-strain-rate superplasticity and low-temperature superplasticity are favorable for making the use of superplastic forming for engineering ceramics even more wide spread. In this study, a silicon nitride based nanocomposite was developed for the purpose of improving the superplasticity. An amorphous powder was prepared by mechanical alloying of silicon nitride and metal titanium. A Si3N4-Si2N2O-TiN nanocomposite was fabricated by hot isostatically pressing the amorphous powder compact. A compression test was performed in the temperature range of 1573 K to 1873 K. The nanocomposite could be deformed at a strain rate of 10-2s-1, which was more than 100 times faster than that available for conventional superplastic Si3N4 at 1873 K. Furthermore, the nanocomposite was superplastically deformed in compression at low temperatures from 1573 K to 1673 K. The stress exponent and the activation energy of the nanocomposite were close to those of submicron-silicon nitride.

Abstract: In this study, the effect of composition of intergranular glass on superplastic compressive deformation of
-Si3N4 has been studied by compression tests. Oxide additives were used to form Y2O3-Al2O3-SiO2 melt and, with increasing temperatures, an oxynitride melt (Y-Al-Si-O-N) by dissolving Si3N4. The relation between flow stress and glass composition qualitatively corresponded to the effect of chemical composition on viscosity of Y2O3-Al2O3-SiO2 glass. However, the rate of increase of the flow stress was not proportional to the viscosity of Y2O3-Al2O3-SiO2 glass, probably because the composition of intergranular glass phase had changed by dissolving Si3N4 and by crystallization of Si2N2O.