Materials Science Forum Vols. 534-536

Abstract: Powder compaction is a well-established process for manufacturing a wide range of products, including engineering components and pharmaceutical tablets. During powder compaction, the compacts (green bodies or tablets) produced need to sustain their integrity during the process and possess certain strength. Any defects are hence not tolerable during the production. Therefore, understanding failure mechanisms during powder compaction is of practical significance. In this paper, the mechanisms for one typical failure, capping, during the compaction of pharmaceutical powders were explored. Both experimental and numerical investigations were performed. For the experimental study, an instrumented hydraulic press (a compaction simulator) with an instrumented die has been used, which enable the material properties to be extracted for real pharmaceutical powders. Close attentions have been paid to the occurrence of capping during the compaction. An X-ray Computed Microtomography system has also used to examine the internal failure patterns of the tablets produced. Finite element (FE) methods have also been used to analyse the powder compaction. The experimental and numerical studies have shown that the shear bands developed at the early stage of unloading appear to be responsible for the occurrence of capping. It has also been found that the capping patterns depend on the compact shape.

Abstract: This research is aimed at determining the thermal expansion coefficient, α(T), along three directions, X, Y, and Z, of a graphite sample pressed along the direction of weak interactions (Z), over the temperature range [25-500°C]. The experiment results showed that pressing completely alters the dilatometric behavior of the material, and the shape of the α(T) curve changes significantly from one direction to another. Comparing αX, αY, and αZ, a strong anisotropy in the thermal expansion coefficient is observed. This is in good agreement with results published by other researchers who have shown that the electrical conductivity and thermal conductibility of a similar material exhibits strong anisotropic behavior. Pressing has also led to anisotropy in the basal plane, i.e. between αX and αY which have opposite signs. [Pressing results in a significant decrease in α(T) along the Y axis, which is considerably lower than that reported in the literature. αZ is clearly higher that of non-pressed graphite. The wide divergence among the three coefficients is believed to be attributed to the intensification of the membrane effect.

Abstract: Carbon nanotubes (CNTs) have been the subject of intensive study for applications in the fields of nanotechnologies in recent years due to their superior mechanical, electric, optical and electronic properties. Because of their exceptionally small diameters (≈ several nm) as well as their high Young’s modulus (≈ 1 TPa), tensile strength (≈ 200 GPa) and high elongation (10-30%) in addition to a high chemical stability, CNTs are attractive reinforcement materials for light weight and high strength metal matrix composites. In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve full density of CNT/metal matrix composites with superior mechanical properties by improved particle bonding and least grain growth, which were considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering. ECAP (equal channel angular pressing), the most promising method in SPD, was used for the CNT/Cu powder consolidation. The powder ECAP processing with 1, 2, 4 and 8 route C passes was conducted at room temperature. It was found by mechanical testing of the consolidated CNT/Cu that high mechanical strength could be achieved effectively as a result of the Cu matrix strengthening and improved particle bonding during ECAP. The ECAP processing of powders is a viable method to achieve fully density CNT-Cu nanocomposites.

Abstract: High-Speed Centrifugal Compaction Process (HCP) is one of slip-using compacting method originally developed for processing of oxide ceramics. In this study, we are going to apply the HCP to ultra-fine (0.1 micron) WC powder. Organic liquid of heptane was chosen as dispersing media to avoid possible oxidation of WC. For slip preparation, addition of sorbitan-monostearate (SMS) dramatically improved state of dispersion. The mixing apparatus also was in consideration. The slips mixed by conventional ball mill or turbula mill were scarcely densified by the HCP. Only the slips mixed by high energy planetary ball mill were packed up to 55% by the HCP, and sintered to almost full density at 1673 K without any sintering aids. This sintered compact marked Vickers hardness of Hv 2750 at maximum.

Abstract: In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders with least grain growth, which is considered as a bottle neck of the bottom-up method that uses the conventional powder metallurgy of compaction and sintering. ECAP (Equal channel angular pressing), one of the most promising method in SPD, was used for the powder consolidation. In the ECAP process of not only solid but also powder metals, it is important to get a good understanding of the density as well as internal stress, strain and strain rate distribution. We investigated the consolidation, plastic deformation and microstructure evolution behavior of the metallic powders during ECAP using an experimental method. It was found that high mechanical strength could be achieved effectively due to the well bonded powder contact surface during ECAP process of gas atomized Al-Si powders. The experimental results show that SPD processing of powders is a viable method to achieve both fully density and nanostructured materials.

Abstract: Densification behavior of iron powder under cold stepped compaction was studied. Experimental data were also obtained for iron powder under cold stepped compaction. The elastoplastic constitutive equation based on the yield function of Shima and Oyane was implemented into a finite element program (ABAQUS) to simulate compaction responses of iron powder during cold stepped compaction. Finite element results were compared with experimental data for densification, deformed geometry and density distribution. The agreement between finite element results and experimental data was very good for iron powder. The distributions of hydrostatic pressure and the Mises stress of iron powder under cold stepped compaction were also studied.

Abstract: An apparatus measuring changes of various forces directly and continuously was developed by a way of direct touch between powders and transmitting force component, which can be used to study forces state of powders during warm compaction. Using the apparatus, warm compaction processes of iron-based powder materials containing different lubricants at different temperatures were studied. Results show that densification of the powder materials can be divided into four stages, in which powder movement changes from robustness to weakness, while its degree of plastic deformation changes from weakness to robustness. The proposed densification mechanism may provide an insight into understanding of warm compaction process.

Abstract: The high pressure compaction without internal lubricant and the high green density even with the pore free density were achieved by the newly developed die wall lubricant for warm compaction. This developed die wall lubricated warm compaction followed by high temperature sintering resulted in improved mechanical property and almost no dimensional change. In this paper, the effects of increasing the green density on the sintered density, the dimensional change and the mechanical property are mainly discussed.

Abstract: In order to develop the SUS304L housing by powder metallurgy for an optical device useful for the FTTH communication system, the optimum compacting pressure and sintering temperature were investigated using granulated powder as the material to satisfy high air-tightness and high laser-weldability. Then the laser-welding test of specimen made under the optimum condition was carried out to observe welding sputters.

Abstract: Granular feedstock for dry pressing is prepared by spray drying oxide powders with polymeric additives. The interactions between polymeric additives in suspension or during drying are usually considered negligible. However, recent studies demonstrate that interactions between polymers usually do occur and can adversely affect product performance. Flory-Huggins calculations predict phase separation of Na-PMAA and Na-PAA with PVA and homogeneous mixing of Na-silicate with PVA. These predictions were confirmed by the light scattering studies on polymer solutions and morphology studies on dried polymer solutions. This reveals that the interaction between polymers can lead to either a homogeneous or phase-separated solution, depending on the functional groups and chain length of polymers. PVA binder in spray-dried granules is stained and its location within the spray dried granule is observed verifying the effect of other polymers on binder migration and case-hardening of spray dried granules. The results indicate polymeric additives that prevent binder migration, potentially eliminating case-hardening in spray dried granules and the subsequent problems observed during compaction.