Papers by Author: Toshiyuki Nishimura

Abstract: A commercial silicon nitride powder with sintering additives was ground by high-energy milling to reduce particle size. Nanometer sized powder was obtained. The powder was densified for short time by spark plasma sintering to prevent grain growth. Nanometer-grained Si3N4 ceramics were obtained. Plastic deformation of the Si3N4 nano-ceramics has been studied in compression over a wide range of strain rates and temperatures. The experimental results revealed that a transition in stress exponent, n, at each temperature. The n value decreased from ~ 2 to ~ 1 with increasing applied stress. Activation energy was also different for the two regions, decreasing from 858.2 kJ/mol in the n ~ 2 region to 571.8 kJ/mol in the n ~ 1 region. Effect of sintering additives on plastic deformation was also discussed.

Abstract: We report sintering additive systems to decrease the densification temperature of the corrosion resistant AlN-SiC-TiB2 system. Since oxide additives degrade the high temperature properties of the system, and other kinds of metallic additives may affect the formation of protective mullite during oxidation, only the constituent elements were applied as additives. Dense samples ( > 98 % relative density) could be fabricated at 1850 oC and 1900 oC by spark plasma sintering (SPS) and hot pressing method, respectively.

Abstract: There has been a great progress in the development of heat-resistant silicon carbide ceramics, owing to the better understanding of composition-microstructure-properties relations. Based on the progress, it has been possible to fabricate heat-resistant SiC ceramics with improved fracture toughness. In this paper, three rare-earth oxides (Re2O3, Re=Er, Lu, and Sc) in combination with AlN were used as sintering additives for a β-SiC containing 1 vol% α-SiC seeds. The effect of intergranular phase, using Re2O3 and AlN as sintering additives, on the microstructure and mechanical properties of liquid-phasesintered, and subsequently annealed SiC ceramics were investigated. The microstructure and mechanical properties were strongly influenced by the sintering additive composition, which determines the chemistry and structure of IGP. The strength and fracture toughness of the Lu2O3-doped SiC were ∼700 MPa at 1400oC and ∼6 MPa.m1/2 at room temperature, respectively. The beneficial effect of the new additive compositions on high-temperature strength was attributed to the crystallization of the intergranular phase.

Abstract: In previous report, we succeeded in preparing dense nano-sized ceramics with the composition of Y- α-sialon (m=1.35, n=0.675) by high-energy mechanical milling followed by spark plasma sintering. The superplastic deformation of the obtained nano-ceramics was studied in this report. A good ductility of the nano-sized ceramics has been confirmed, which arised from the nano-sized grains and large amount of transient liquid phase. The effects of deformation on the phase and microstructural evolution were also studied. The nano-sized grains promoted the formation of elongated α-sialon grains during post-annealing at 1850oC for 3 h, which would strengthen and toughen the deformed ceramics.

Abstract: Y- α-sialon (m=1.35, n=0.675) ceramics were prepared by high-energy mechanical milling followed by spark plasma sintering. The milling promoted not only liquid-phase sintering, but also phase transformation from β-Si3N4 to α-sialon. Under the same holding time of 5 min, milled powder could be completely densified at 1500oC, which is about 250oC lower than that required for as-received powder. The temperature where the phase transformation finished was 1600oC and 1750oC for milled and as-received powder, respectively. The grain size of obtained dense ceramics from milled powder was significantly decreased. Nano-sized dense ceramics have been obtained by sintering the milled powder at 1500oC for 5 min. Although 100 % α-sialon has not been achieved, the nano-sized ceramics can be used for superplastic deformation, taking advantage of small grain size and large amount of transient liquid phase.