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The effects of hard-magnetic and soft-magnetic ferrite-based nanoparticles in magnetorheological fluids (MRFs) were observed. The hard-magnetic nanoparticles, represented by CoFe2O4, and the soft-magnetic nanoparticles, represented by MnFe2O4, were synthesized using co-precipitation methods. Before analyzing the effects of the nanoparticles in the MRFs, their characteristics were observed. The morphology of both types of nanoparticles was analyzed using FESEM, and the magnetic characteristics were tested using VSM. Three types of MRF samples were prepared by mixing carbonyl iron particles (CIP) into silicone oil: MRF without nanoparticles (MRF CIP), MRF with CoFe2O4 nanoparticles (MRF CIP+CoFe2O4), and MRF with MnFe2O4 nanoparticles (MRF CIP+MnFe2O4). The concentration of the nanoparticles was 1wt%, while the dispersed particle concentrations were maintained at 70wt%. The magnetic characterization of the MRF samples was carried out using VSM. Rheological properties were tested using a rheometer with three testing methods: continuous mode, oscillatory mode, and on-off state mode. In the continuous mode, the rotational plate on the rheometer was rotated at a shear rate from 0.001/s to 300/s. In the oscillatory mode, the rotational plate was rotated back and forth with varying rotational angles, resulting in a linear strain of MRF from 0.01% to 100%. On-off state mode was carried out similar to continuous mode, but the strain of MRF was held constant at 100%. Then, the magnetic field was suddenly applied for a few seconds and then abruptly removed. Sedimentation stability was tested by settling the MRF in a measuring tube at room temperature, and the height of sedimentation ratio was measured hourly to observe the deposition speed of the MRF. In this paper, it is obtained that the addition of both hard magnetic and soft magnetic nanoparticles reduces the magnetic saturation of CIP MRF. Although the magnetic saturation is reduced, the shear stress of the MRF increases. The addition of nanoparticles also causes the sedimentation stability to improve. However, the addition of nanoparticles has an undesirable effect on the responsiveness of the MRF.

MRF,Nanoparticles,Hard-magnetic,Soft-magnetic,Rheological,Sedimentation