Papers by Author: Yuichi Yoshizawa

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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.
Authors: Hiroyuki Miyazaki, Hideki Hyuga, Yuichi Yoshizawa, Kiyoshi Hirao, Tatsuki Ohji
Abstract: Effect of microstructure of silicon nitride on the fracture toughness, KIc evaluated by the IF method was studied with various indentation loads ranging from 49 N to 490 N, since practical assessment of fracture toughness of small Si3N4 parts is needed in the ceramic ball bearing market. The plot of KIc against the as-indented crack length revealed the rising R-curve behavior for the coarse Si3N4 and slight R-curve for the fine Si3N4. By comparing KIc estimated from the SEPB and IF methods using 4 different equations, it was revealed that the IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. These results were discussed in conjunction with their R-curve behavior and the effective crack length in the SEPB specimens.
Authors: Hiroyuki Miyazaki, Hideki Hyuga, Yuichi Yoshizawa, Kiyoshi Hirao, Tatsuki Ohji
Abstract: The influence of two measuring conditions, the elapsed time after indentation and the condition of edge of an indenter, on the indentation fracture toughness of silicon nitrides was assessed. No slow crack-growth after unloading was confirmed by optical microscopic observation of a crack tip induced by the indentation, which led to the negligible difference in fracture toughness measured at 1 and 30 min after the indentation. Measurements with relatively new and used indenters gave almost the same fracture toughness data, indicating that the crack lengths were hardly affected by the slight damage of the corner of the indenter. It was suggested that the large scattering of the indentation fracture toughness reported by the round-robin tests such as VAMAS was not originated from these factors.
Authors: Hiroyuki Miyazaki, Yuichi Yoshizawa, Kiyoshi Hirao
Abstract: A bundle of feedrod composed of ordinary arranged alumina and zirconia green rods was co-extruded through a 6:1 reduction die. The volume fraction of zirconia phase was varied from 10 to 88 vol%. After the first co-extrusion, the individual pieces were bundled and co-extruded again, reducing the lateral size of each phase and multiplying the number of continuous monofilaments. After a 3rd extrusion step and sintering at 1600oC, crack-free composites with a fiber diameter of ~50 μm were obtained for all compositions. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPam1/2 was attained in the 3rd co-extruded composite which consisted of 53 vol% alumina and 47 vol% zirconia. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.
Authors: Manabu Fukushima, You Zhou, Yuichi Yoshizawa, Hiroyuki Miyazaki, Kiyoshi Hirao
Abstract: Mesoporous silicon carbide with and without SiC filler was prepared from the pyrolysis of polycarbosilane (PCS), and the relationship between pyrolysis and properties of micro/meso pores was investigated. The surface area of pyrolyzed PCS without filler significantly decreased from 268 to 0.5m2/g with increasing pyrolysis temperature, and disappearance of the micropores was observed. In contrast, the pyrolyzed PCS with filler showed the retention of micro/mesopores, regardless of pyrolysis temperature. In addition, high surface area ranging from 120 to 180m2/g and a bimodal pore distribution were observed in pyrolyzed PCS with filler. FT-IR showed the decomposition of Si-H, Si-CH3 and Si-CH2-Si in the PCS network during pyrolysis, which led to the evolution of hydrogen and methane gas. However, the obtained pores might be larger than the size of evolved gases. Thus, though micro/meso pores and large surface area were retained by the addition of filler, coalescence between pores could occur during pyrolysis.
Authors: Mark Ian Jones, Ron Etzion, Jim Metson, You Zhou, Hideki Hyuga, Yuichi Yoshizawa, Kiyoshi Hirao
Abstract: The widely used Si3N4-SiC sidewall refractories for aluminum smelting cells, and β SiAlON-SiC composites that can be potentially used for this purpose, have been produced by reaction bonding and their corrosion performance assessed in simulated aluminum electrochemical cell conditions. The formation of the Si3N4 and SiAlON phases were studied by reaction bonding of silicon powders in a nitrogen atmosphere at low temperatures to promote the formation of silicon nitride, followed by a higher heating step to produce β SiAlON composites of different composition. The corrosion performance was studied in a laboratory scale aluminum electrolysis cell where samples were exposed to both liquid attack from molten salt bath and corrosive gas attack. The corrosion resistance of the samples was shown to be dependent on the composition but more importantly on the environment during corrosion, with samples in the gas phase showing higher corrosion.
Authors: Taketo Sakuma, Yuichi Yoshizawa
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