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Magnetorheological (MR) fluids, suspension of magnetic pure carbonyl iron (CI) in non magnetic carrier, were prepared with and without magnetic CI nanoparticle additive in this study. Initially, the magnetic CI nanoparticle additive was synthesized in a rather simple process of decomposition of penta carbonyl iron (Fe(CO) 5 ) using oleyl amine and kerosene. Magnetic property and morphology of the synthesized magnetic CI nanoparticles were confirmed via vibration sample magnetometer (VSM) and transmission electron microscopy (TEM), respectively. MR fluids, prepared as a mixture of pure CI and CI nanoparticle additive of different weight ratio in carrier fluid, was investigated under different external magnetic field strengths via a rotational rheometer. Sedimentation of the MR fluid was characterized by an optical analyzer, Turbiscan. Their flow behaviors at a steady shear mode were examined with and without magnetic CI nanoparticle additive under magnetic field strength. The MR fluids with magnetic CI nanoparticles added demonstrated slightly higher yield behaviors, suggesting that pure CI and CI nanoparticle additive were being oriented in the magnetic field direction under an applied magnetic field and with much strengthened structure.Index Terms-Additive, carbonyl iron, magnetorheological fluid, nanoparticle.

While magnetorheological fluids can be used for ultra-precise polishing, for example, of advanced optical components, oxidation of metallic particles in water-based magnetorheological fluids causes irregular polishing behavior. In this study, carbonyl iron microspheres were initially coated with silica to prevent oxidation and were used to polish BK7 glass. In addition, their rheological and sedimentation characterizations were investigated. Material removal and surface roughness were analyzed to investigate the surface quality and optimal experimental conditions of polishing wheel speed and magnetic field intensity. The maximum material removal was 0.95 µm at 95.52 kA/m magnetic field intensity and 1854 mm/s wheel speed. A very fine surface roughness of 0.87 nm was achieved using the silica-coated magnetorheological fluid at 47.76 kA/m magnetic field intensity and 1854 mm/s wheel speed.