| Abstrakt: |
Small amounts of polymers (ca. 1 wt.‐%) are often used in the processing of ceramics to bond the particles. The purpose is to enhance mechanical properties, notably the strength, of intermediate or green products obtained after consolidation of the powder. In this paper, the influence of polymers on mechanical properties of green (Mn, Zn)‐ferrite products is investigated. Frequently used polymers are methyl methacrylate ‐ ethyl acrylate copolymer and poly(vinyl alcohol) (PVA). Because PVA generally produces low‐strength compacts and is sensitive to moisture, a blend of the copolymer and PVA was used. The glass transition temperatures (Tg) of the polymers and green products were determined. The good correspondence between the Tg's indicates that the polymers dominate the mechanical behaviour when tensile stresses are applied. In practice, compacts usually have an axisymmetric shape and are subject to diametral compression during handling. Therefore, ring‐shaped compacts were crushed at different velocities. The crushing velocity has a strong influence on the compact strength when the copolymer is used and only a small effect when the compact contains the PVA blend. This indicates that the copolymer can absorb a rapid buildup of stress through plastic deformation. By contrast, the rate of plastic deformation in the PVA blend is relatively slow. Further insight is obtained by subjecting rectangular compacts to three‐point bending. By interpreting the experimental data with a mechanical model, the deformation of compacts containing acrylic is found to be elasto‐plastic. With compacts containing the PVA blend, a yield point is observed. This indicates that elasto‐plastic deformation is preceded by elastic deformation. |
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