Development Of New Magnetorheological Fluids For Polishing Caf2 And Kdp

DEVELOPMENT OF NEW MAGNETORHEOLOGICAL FLUIDS FOR POLISHING CAF2 AND KDP Development of New Magnetorheological Fluids for Polishing CaF2 and KDP Magnetorheological finishing (MRF) is a novel and recently commercialized1 process for figuring and polishing plano, convex, and concave optics—both spherical and aspherical— from a wide variety of optical materials. A recently written review article provides an overview of the history, theory, and implementation of this technology.2 The utility and productivity of MRF have been proven for a wide spectrum of optical glasses and demonstrated for a variety of non-glass optical materials.3,4 A 1.0-nm smoothness with removal rates of 1 to 10 µm/min is routinely achieved. Seven years of research and development culminated in 1998 with QED Technologies’ introduction of a commercial MRF machine, designated the Q22. A focus of continuing research is the development of MR fluid compositions and operating parameters to finish optical materials with an ever-widening range of physical properties. Efforts are simultaneously made to extend our understanding of the fundamental mechanisms of material removal in the MRF process. Extremely hard, extremely soft, single-crystal, polycrystalline, or water-soluble optical materials—each presents unique challenges to the MRF process. A magnetorheological (MR) fluid is a suspension of magnetically soft ferromagnetic particles in a carrier liquid. Typically, the particles are of the order of a few microns in diameter, and their volume concentration is 30% to 40%. When exposed to a magnetic field, the viscosity and yield stress of the suspension increase several orders of magnitude. The transition is rapid and reversible. The magnetically soft media used to manufacture MR fluids, which are subsequently used in MRF, are carbonyl iron (CI) powders. They are prepared by decomposing iron pentacarbonyl,5 resulting in spherical particles of almost pure iron, typically 2 to 6 µm in diameter. Incorporating nonmagnetic polishing abrasives results in an MR polishing fluid that can be manipulated to form a renewable and compliant sub-aperture lap for optical finishing. MRF Research Platforms and Polishing Spots The Center for Optics Manufacturing (COM) has two research platforms to facilitate the continuing research of MRF: The first, commonly known as the horizontal trough LLE Review, Volume 80 machine, was the basis of the first prototype MRF machine (described and shown in Fig. 1 of Ref. 2), which is still routinely used but without the fluid circulation system. The MR fluid resides in a rotating horizontal trough. The test optic must be spherical convex. While technically overshadowed by newer machines, it continues to be very productive. Experiments can be conducted with only about 100 ml of MR fluid. In addition, the machine can be quickly cleaned to prepare for another experiment. This is particularly useful for screening experiments of new nonaqueous compositions. A new research platform, designated the spot-taking machine (STM), was designed and constructed by QED T