Nobuhiro Shinohara

Microstructure of alumina compact body made by slip casting

Abstract An attempt was made to understand the structural changes and particle-packing behavior of compacted alumina (α-Al 2 O 3 ) made by slip casting. Packing structure of powder particles was examined in alumina compact made by slip casting with the crossed polarized light microscope in the transmission mode. Anisotropic structure was present in the specimen. A repeated change of color was noted with the rotation of the specimen at every 45° increments in the cross-section of compact cut parallel to the flow direction of water in the casting process. The alumina particles were aligned with the longest axis normal to the flow direction. The center of the cross-section showed a rather disordered packing structure. Isotropic optical property was noted in the specimen cut perpendicular to the flow direction. Isotropic microstructure was found in all directions of the compact made by the spontaneous drying process, in which less apt water flow occurred during drying. The development of anisotropic structure was insensitive to the dispersing state of particles in the slurry.

Processing defects and their relevance to strength in alumina ceramics made by slip casting

Abstract Al2O3 made by slip casting inherently contained the elongated and the spherical shaped defects. The pores of elongated shape were formed through the liquid flow during the casting process, since they were found in all slip cast specimens and not found in the spontaneously dried specimen where no rigorous flow of water happened. The formation of these defects was insensitive to the slurry properties. The origin of spherical pores was likely due to the entrapped air bubbles during de-airing procedure. Their removal by de-airing was easy for a dispersed slurry having a low viscosity, but difficult for a flocculated slurry of high viscosity. The Weibulls plots for the flexural strengths are essentially the same in the region of high strengths. Specimens made from the flocculated slurry contain a higher concentration of the spherical pores, and some of the resultant specimens have low strength. The lower strength of those ceramics has been ascribed to more detrimental defects, i.e. the spherical ones.

Origin of strength variation of silicon nitride ceramics with CIP condition in a powder compaction process

An origin of the strength variation of silicon nitride ceramics with cold isostatic pressure (CIP) condition was investigated for a powder granule compaction process. Direct observation methods were successfully applied to examine the microstructures of granules, green bodies and their sintered specimens. These methods revealed a significant variation in pore structure with CIP condition for both green and sintered bodies. The strength variation with CIP condition was attributed to the different sizes and concentration of large pores in sintered bodies, which were developed or persisted during liquid phase sintering. The large pores were formed from the dimple in the spray-dried granule and the boundaries of the granule traces in green compacts.

Origin of the strength change of silicon nitride ceramics with the alteration of spray drying conditions

The effect of changing the spray drying conditions on the microstructure of green bodies and on the strength variation of sintered bodies was examined for silicon nitride ceramics. Novel characterization techniques were applied to observe the internal structure of granules, green bodies and the resultant ceramics. As a result, the difference of the fracture strength of sintered bodies made from the granular compaction route could be explained quantitatively by the difference of the pore size distribution in the sintered bodies. Those potential flaws were introduced by the incomplete collapse of dimples in granules and of interstices between granules. Different characteristics of granules exerted a serious influence on the size and concentration of potential flaws in green compacts and on the resultant mechanical strength of sintered bodies after densification.

Variation of the microstructure and fracture strength of cold isostatically pressed alumina ceramics with the alteration of dewaxing procedures

Influence of the alteration of CIP and dewaxing procedures on the pore structure and the fracture strength of sintered alumina ceramics was examined. Variation of the fracture strength with the process alteration could be explained by the difference in distribution of large pore defects determined by applying the direct observation techniques. Dewaxing, before CIP, varied the strength and fracture behavior of granules and was effective in promoting the uniform powder packing during consolidation, resulting in the high strength of sintered bodies.

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.

Characterization of coarse particles in alumina powders by a wet sieving method

Abstract The wet-sieving method was applied to determine the content of coarse particles in commercial alumina powders by four independent research organizations participated in the Round-Robin Testing. The results showed that those powders clearly contain 700 to several thousands ppm of coarse particles with the size >25 μm, whereas they could not be detected with the conventional particle size analyzer because of their low concentration. SEM and polarized light microscope observations revealed that the coarse particles contained several types of aggregates which survived during the production processes of alumina by the Bayer process.

Effect of dewaxing procedures of cold isostatically pressed silicon nitride ceramics on its microstructure and fracture strength

The effect of the alteration of cold isostatic pressing (CIP) and dewaxing procedures on the pore structure and fracture strength of silicon nitride ceramics was examined. The difference of the fracture strength between two sintered bodies was explained by the pore size variation in ceramics observed in thinned specimens by the transparency technique. It was explained that CIP after dewaxing lowered the compressive strength of the granules in the green body and thus contributed to making the pore size of the sintered body small.

Characterization of micro- and macrostructure of injection-molded green body by liquid immersion method

Micro- and macrostructures of injection-molded alumina body were examined by the liquid immersion method, which is a novel powerful method for characterizing them. Thin specimens for examination were cut from various regions in the green body and made transparent by an immersion liquid. They were then examined by an optical microscope in normal and/or crossed polarized mode. The structural features found by the examination include particle orientation along the flow direction in the injection molding process and few extremely large particles. Their relevance to the forming process and the densification process is discussed.

Relevance of internal structures to the fracture strength of injection-molded and sintered silicon nitride ceramics

Abstract The densification behavior and fracture strength of silicon nitride ceramics were investigated for samplesmade through the injection-molding process in comparison with thosemade by powder granule compaction. Although no clear difference was observed in the density, phase composition and microstructure of sintered bodies between the processes, the fracture strength was clearly higher for the samples made through the injection-molding route. A relatively uniform internal macrostructure was considered responsible for the improved fracture strength of the injection-molded and sintered silicon nitride.