Magnetorheological elastomers (MRE) are increasing in popularity in many applications because of their ability to change stiffness by applying a magnetic field. Instead of liquid-based 1 K and 2 K silicone, thermoplastic elastomers (TPE), based on styrene-butadiene-styrene block copolymers, have been investigated as matrix material. Three different carbonyl iron particles (CIPs) with different surface treatments were used as magneto active filler material. For the sample fabrication, the thermoplastic pressing method was used, and the MR effect under static and dynamic load was investigated. We show that for filler contents above 40 vol.-%, the linear relationship between powder content and the magnetorheological effect is no longer valid. We showed how the SiO2 and phosphate coating of the CIPs affects the saturation magnetization and the shear modulus of MRE composites. A combined silica phosphate coating resulted in a higher shear modulus, and therefore, the MR effect decreased, while coating with SiO2 only improved the MR effect. The highest performance was achieved at low deformations; a static MR effect of 73% and a dynamic MR effect of 126% were recorded. It was also shown that a lower melting viscosity of the TPE matrix helps to increase the static MR effect of anisotropic MREs, while low shear modulus is crucial for achieving high dynamic MR. The knowledge from TPE-based magnetic composites will open up new opportunities for processing such as injection molding, extrusion, and fused deposition modeling (FDM).
Industrial wood cutting is a highly demanding application for cutting tool materials as a wedge angle of ~50°, a tip radius in the 1 m range, and high wear, temperature and corrosion resistance are needed. For this application Si3N4 based ceramic matrix composites (CMC) were developed. Cutting tests showed that reinforcing Si3N4 with 30 wt.% SiC gives a good balance between fracture toughness and wear resistance. The use of an yttria / lanthana sintering aid system resulted in a fine-grained microstructure without degrading the fracture toughness. Post heat treatment was essential for the integrity of the cutting edge. Finally, cutting tests proved that the CMC cutting tips had a 3-fold lifetime compared to tungsten carbide.
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