<title>Material removal during magnetorheological finishing (MRF)</title>

Abstract

Magnetorheological Finishing (MRF) is a newly developed and recently commercialized method for finishing optical components. The MR fluid consists of a water based suspension of carbonyl iron (CI), nonmagnetic polishing abrasives, and small amounts of stabilizers. MRF uses both mechanics and chemistry to smooth glass surfaces to less than 10 angstrom rms. Mechanics are responsible for the microscratching of the glass surface which is initially 'softened' by hydration from water in the MR fluid. Experiments were performed to study the separate roles of mechanical abrasion and chemical softening during MRF. Chemical effects were suppressed by introducing a nonaqueous fluid in place of the water. A novel nanoindentation technique was used to determine CI particle hardness, which varied by a factor of five. The mechanics of removal were then investigated with soft and hard CI powders working against soft and hard optical glasses. Chemistry was then 'turned on' by the addition of a small amount of water to the system. Preliminary results of these experiments are presented here.