Tatsuo Noma

Role of H2O on the degradation process of Y-TZP

Yttrium-doped tetragonal zirconia (Y-TZP) is expected to be a good structural ceramic material because of its high strength, high toughness and wear resistance. However, the instability (degradation) of Y-TZP around 150 to 400° C in air and above 100° C in water during ageing is a severe problem which must be solved before the direct application of this material.

Formation of diffusionlessly transformed tetragonal phases in rapid quenching of ZrO2-Y2O3 melts

ZrO2 polycrystals, partially stabilized by 2 to 7 mol% Y2O3, were arc-melted and rapidly quenched using an arc-imaging furnace with a hammer-anvil unit. Some of the specimens were further annealed at 1700° C for 3 h in air. The phases and the microstructures of these ZrO2-Y2O3 polycrystals were examined through X-ray diffraction and transmission electron microscopy. Special emphasis was placed upon the examination of the microstructure of the metastable tetragonal phase (t′ phase) which was formed by a diffusionless transformation of the high-temperature cubic phase. It was found that the t′ phase exhibits a twinned and mosaic structure made of alternating layers of twin-related variants. A comparison of the present experimental results with other related works has also been made.

Toughening in very high pressure sintered diamond-alumina composite

Alumina ceramics and dispersed diamond were sintered at a temperature and pressure where the diamond phase is stable. The fracture toughness of the composite is remarkably increased after heat-treatments in vacuum. The transformation from diamond into graphite accompanied this phenomenon. The effects of the fraction of diamond and the heat-treatment temperature are investigated.

The effects of external stress on the cubic-tetragonal transformation in rapidly quenched ZrO2-Y2O3

The effects of the external stress on the displacive cubic (c) to tetragonal (t′) transformation, taking place during rapid quenching of ZrO2-Y2O3 alloys by a hammer-anvil unit, were examined through the statistical analysis of the crystallography of the product phases. The initially formed c-phase had no strong texture with respect to the stress direction. However, specific t′ variants among crystallographically equivalent ones were formed preferentially. From these observations, it is concluded that the applied stress plays an important role on the deformation to change the lattice during the c → t′ transformation.

Formation and Phase Stability of Tetragonal Phase in Rapidly Quenched ZrO2-ErO1.5

In the system ZrO2-ErO1.5, inherent tetragonal phases of their contents of 4–12 mol.% ErO1.5 were obtained using a rapid quenching technique of their melts. The tetragonality (axial ratio c/a) decreased with ErO1.5 contents to the unity of the cubic phase with >14 mol.% ErO1.5. The transformability of these tetragonal phases into monoclinic phases correlates to the tetragonality.

Diamond-Glass Composite Processing in a Microgravity Condition

Diamond-glass composite was fabricated under microgravity in the Challenger, a space shuttle orbiter. Fracture toughness and hardness were compared with the composites fabricated on earth. Under microgravity, uniform distributions in toughness and hardness were obtained. On earth, the values decreased with the distance from the bottom of the crucible.

Fabrication of Diamond-Glass Composite under Micro-Gravity

Glasses containing a dispersion of diamond-particles were fabricated under micro-gravity and 1 G. Under micro-gravity, as the wettability between the sample and specimen capsule was significant, part of the fused specimen leaked out of the capsule. However, the microstructures and mechanical properties of the specimens fabricated in space were more isotropic than those of the samples fabricated under 1 G, and the particles were dispersed more homogeneously.