Keizo Uematsu

Sintering deformation caused by particle orientation in uniaxially and isostatically pressed alumina compacts

Effect of particle orientation on deformation during sintering is reported for model systems; one made with industrial grade low soda alumina, which has an elongated particle shape, and the other a special alumina with a spherical particle shape. To ensure the homogeneous packing density of particles, compacts were made by uniaxial pressing followed by cold isostatic pressing. The particle orientation was examined with a polarized light microscope and was found to be an important cause of sintering deformation. In a green body, for elongated shape of particles, the particle orientation occurred during uniaxial pressing, causing the anisotropic sintering shrinkage during sintering and thus the sintering deformation. No particle orientation nor shrinkage anisotropy was noted in the system made with the powder of spherical particle shape.

CHARACTERIZATION OF PROCESSING PORES AND THEIR RELEVANCE TO THE STRENGTH IN ALUMINA CERAMICS

Characterization of bulk defects was successfully accomplished in alumina with a transmission optical microscope. The characterization technique used is based on the fact that many ceramics are essentially transparent. Most defects in this particular ceramic were found to be pore. Their size distribution was found to follow a simple power function. With these characteristics of defects, the strength distribution of the ceramics was calculated with Barattas model and compared to the measured strength of the ceramics. A good agreement was found between them when the pore was assumed to be accompanied with cracks 4 times the length of the grain size.

Development of anisotropic microstructure in uniaxially pressed alumina compacts

Abstract Alumina compacts are prepared by uniaxial pressing followed by cold isostatic pressing (CIP). Anisotropic microstructures of the compacts and their sintered bodies are reported. Anisotropic grain growth with sintering time was systematically examined through direct measurement of grain dimensions on SEM micrographs in various conditions. Experimental results indicated that average grain size was always larger in the plane parallel to the direction of uniaxial pressing, than in its perpendicular plane, for all sintering conditions. The difference in the average grain size increased in these planes with sintering time. The grain growth followed the cubic law and the rate constants were 4.8×10−21 and 1.9×10−21(m3/s), for the former and the latter, respectively, in specimen uniaxial pressed at 100 MPa and CIPed at 100 MPa. Many intra-granule fractures were observed in the compact in the plane perpendicular the pressing direction comparing with its parallel plane.

Effect of heat treatment of alumina granules on the compaction behavior and properties of green and sintered bodies

Abstract The effects of the heat treatment of Al2O3 granules on the fracture behavior and compressibility of the granules, as well as on the properties of the green and sintered bodies, were examined. Heat treatment contributed to increasing the strength of granules, resulting in poor deformability. However, the hard and brittle characteristics of the heat-treated granules did not hinder the promotion of uniform powder packing and the formation of a nearly cohesive compact under high compaction pressure. Although as-spray-dried granules were more deformable during compaction, they left clear interfaces between granules in the green bodies, resulting in the preservation of large pores in the samples after sintering. The high density and small pore size in green compacts formed with heat-treated granules contributed to reducing the pore-defect size in the sintered bodies, resulting in high fracture strength.