Mechanisms of material removal in magnetorheological finishing (MRF) of glass

Magnetorheological finishing (MRF) is a relatively new technology used in precision polishing. It utilizes micron-sized magnetic carbonyl iron (CI), nonmagnetic polishing abrasives, aqueous or nonaqueous carrier fluids and hydrodynamic flow in a magnetic field to remove material from a part surface. The versatility of the MR fluid has been exploited in order to study the mechanisms of material removal in the MRF of glass.; Macroscopic aspects of MRF are studied. The design and operation of a magnetorheometer is described. This new instrument is shown to be able to accurately measure the dynamic yield stress of an MR fluid in the same field orientation used in polishing. This field orientation is shown to be an important consideration in the determination of the magnitude of the yield stress. This result is used to model the bulk flow of the MR fluid underneath a glass part during polishing, allowing for the calculation of the shear stress at the surface of the glass part using experimentally determined pressure distributions. These shear stress distributions are shown to correlate with removal profiles.; Mechanisms on the microscopic scale are also investigated. The nanohardness of the individual particles are characterized with novel nanoindentation techniques. Removal experiments show that the nanohardness of the CI is important in nonaqueous MR fluids with no abrasives, but is relatively unimportant in aqueous MR fluids and/or when nonmagnetic abrasives are present. The hydrated layer created by the chemical effects of water is shown to change the way material is removed by hard CI as the MR fluid transitions from a nonaqueous MR fluid to an aqueous MR fluid. Drag force measurements and atomic force microscope (AFM) scans demonstrate that, when added to an MR fluid, nonmagnetic abrasives move to the region between the CI and part surface and become responsible for material removal. Removal rates increase with the addition of these polishing abrasives (cerium oxide, aluminum oxide and diamond). The relative increase depends on the amount and type of abrasive used because of differences in how each interacts with the glass surface.