Isao Kondoh

Centrifugal casting of Al2O3–15 wt.%ZrO2 ceramic composites

Abstract Aqueous ceramic slurries with different solid contents (60, 70 and 78 wt.%) were prepared by mixing 85 wt.% low-temperature-sinterable high purity α-alumina and 15 wt.% ZrO 2 (3 mol% Y 2 O 3 ) in distilled water and a small amount of poly-carboxylic ammonium as the dispersion agent. Consolidation was performed by using a high-speed centrifugal casting process at 4000 rpm for 1 h. The result showed that the dispersion agent had a great influence on the viscosities of the slurries. It was also observed that the green and sintered densities of the Al 2 O 3 –15 wt.%ZrO 2 composites increased with the solid contents. Phase segregation could be effectively eliminated when the solid content of the slurry was 78 wt.%.

Nitridation of silicon compact in a gas mixture of nitrogen and hydrogen

Silicon powder compacts formed by slip casting were reaction-sintered at various heating rates in a mixed nitrogen-hydrogen atmosphere containing 0 to 80vol% hydrogen. The rate of gas consumption measured in the temperature from 1050° to 1400°C gave two peaks. With increasing hydrogen content the nitriding fraction increased a maximum value at 40vol% hydrogen. The hydrogen content also influenced the bending-rupture strength of sintered bodies, and the maximum bending rupture strength was obtained at about 25vol% hydrogen. The nitridation was almost independent of the heating rate.

Effect of hot-pressing time on physical and tribological properties of silicon nitride with MgO additive

Silicon Nitride containing 5wt% MgO was hot-pressed at 1800°C and 300kg/cm2, and physical and tribological properties of sintered bodies were measured as a function of hot-pressing time. The bulk density increased with hot-pressing time up to 60min. The fracture toughness increased and the Vickers hardness decreased with increasing hot-pressing time, but these values remained essentially constant after 60min sintering. The coefficient of friction on a hot-pressed silicon carbide disk did not change, but the specific wear rate decreased rapidly with hot-pressing time up to 60min, and approached a constant value, 5×10-8mm2/kg. The bodies hot-pressed up to 30min consisted of mixed small and relatively large grains. But the grain growth occurred and fibrous microstructures were fully developed after 60min sintering, and only β-phase was detected by X-ray diffraction. As a conclusion, the physical and tribological properties of sintered silicon nitride were almost constant after 60min hot-pressing, which was consistent with the change of microstructure accompanied by α→β phase transformation.