Henry J. Romanofsky

Polishing PMMA and other optical polymers with magnetorheological finishing

Magnetorheological finishing (MRF) was used to polish as-molded or diamond turned surfaces of several optical polymers. Materials included polymethylmethacrylate (PMMA), cyclic olefin polymer (COP), polycarbonate (PC), and polystyrene (PS). Parts were nominally plano circular discs of various diameters (~40 mm to 75 mm) and thicknesses (2.5 mm to 25 mm). Polishing trials were conducted with standard CeO2-based and nanodiamond-based MR fluids, or with MR fluids containing SnO2, ZrO2, Al2O3, TiO2, or SiO2. Excellent results were obtained for PMMA using a ZrO2-based MR fluid. The diamond turned plano surface of a 38-mm diameter by 8-mm thick puck was improved from an initial p-v wave front error of 4.5 µm to 0.35 µm with two figure correction runs. The average rms surface roughness was reduced from 3.8 nm to 0.47 nm, and the diamond turning marks were eliminated. Mounting and thermalization of polymer parts for in-process and final metrology was found to be a challenge.

Minimizing artifact formation in magnetorheological finishing of chemical vapor deposition ZnS flats

The polishing performance of magnetorheological (MR) fluids prepared with a variety of magnetic and nonmagnetic ingredients was studied on four types of initial surface for chemical vapor deposition (CVD) ZnS flats from domestic and foreign sources. The results showed that it was possible to greatly improve smoothing performance of magnetorheological finishing (MRF) by altering the fluid composition, with the best results obtained for nanoalumina abrasive used with soft carbonyl iron and altered MR fluid chemistry. Surface roughness did not exceed 20 nm peak to valley and 2 nm rms after removal of 2 microm of material. The formation of orange peel and the exposure of a pebblelike structure inherent in ZnS from the CVD process were suppressed.

Exploring anisotropy in removal rate for single crystal sapphire using MRF

Magnetorheological (MR) fluids with two types of abrasives (diamond and alumina/spinel) were used to study anisotropy in removal rate for C-cut single crystal sapphire. Interferometrically flat, basal plates (0001) employed in the experiments were characterized by different degree of C- axis (small) tilt from normal. The removal rate anisotropy depends on the type of abrasive, with anisotropy being more pronounced for the alumina/spinel abrasive. The anisotropy exhibited 2-fold symmetry, with the C-axis lying in the plane of symmetry. Roughness was found to depend on the basal plate orientation and the type of abrasive used. Diamonds improved the initial surface roughness of a polished plate regardless of orientation, while alumina/spinel abrasives increased the roughness, especially in the down-the-steps direction of fluid flow with respect to basal plane inclination. The results of this polishing experiment are in agreement with earlier studies of anisotropy observed in wear experiments on spherical surfaces of single crystal sapphire along different crystallographic orientations.

Effects of Changes in Fluid Composition on Magnetorheological Finishing (MRF) of Glasses and Crystals

Magnetorheological (MR) fluids for fine finishing of optical components consist of magnetic carbonyl iron (CI) particles and nonmagnetic polishing abrasives in a carrier liquid. With a solids loading of over 40 volume percent, these fluids incorporate surfactants and other additives to help stabilize them against sedimentation and corrosion during commercial finishing operations. Several experiments have been conducted with variations in the three primary ingredients that demonstrate the effectiveness of magnetorheological finishing (MRF) for a variety of glasses and crystals.

Advanced zirconia-coated carbonyl-iron particles for acidic magnetorheological finishing of chemical-vapor-deposited ZnS and other IR materials

We present a modified version of zirconia-coated carbonyl-iron (CI) particles that were invented at the University of Rochester in 2008. The amount of zirconia on the coating is increased to further protect the iron particles from corrosion when introduced to an acidic environment. Five low-pH, magnetorheological (MR) fluids were made with five acids: acetic, hydrochloric, nitric, phosphoric, and hydrofluoric. All fluids were based on the modified zirconia-coated CI particles. Off-line viscosity and pH stability were measured for all acidic MR fluids to determine the ideal fluid composition for acidic MR finishing of chemical-vapor–deposited (CVD) zinc sulfide (ZnS) and other infrared (IR) optical materials, such as hot-isostatic-pressed (HIP) ZnS, CVD zinc selenide (ZnSe), and magnesium fluoride (MgF2). Results show significant reduction in surface artifacts (millimeter-size, pebble-like structures on the finished surface) for several standard-grade CVD ZnS substrates and good surface roughness for the non-CVD MgF2 substrate when MR finished with our advanced acidic MR fluid.

Magnetorheological finishing of chemical-vapor deposited zinc sulfide via chemically and mechanically modified fluids.

We describe the anisotropy in the material removal rate (MRR) of the polycrystalline, chemical-vapor deposited zinc sulfide (ZnS). We define the polycrystalline anisotropy via microhardness and chemical erosion tests for four crystallographic orientations of ZnS: (100), (110), (111), and (311). Anisotropy in the MRR was studied under magnetorheological finishing (MRF) conditions. Three chemically and mechanically modified magnetorheological (MR) fluids at pH values of 4, 5, and 6 were used to test the MRR variations among the four single-crystal planes. When polishing the single-crystal planes and the polycrystalline with pH 5 and pH 6 MR fluids, variations were found in the MRR among the four single-crystal planes and surface artifacts were observed on the polycrystalline material. When polishing the single-crystal planes and the polycrystalline with the modified MR fluid at pH 4, however, minimal variation was observed in the MRR among the four orientations and a reduction in surface artifacts was achieved on the polycrystalline material.

Contributions of nanodiamond abrasives and deionized water in magnetorheological finishing of aluminum oxynitriden

Magnetorheological finishing (MRF) is a sub-aperture deterministic process for fabricating high-precision optics by removing material and smoothing the surface. The goal of this work is to study the relative contribution of nanodiamonds and water in material removal for MRF of aluminum oxynitride ceramic (ALON) based upon a nonaqueous magnetorheological (MR) fluid. Removal was enhanced by a high carbonyl iron concentration and the addition of nanodiamond abrasives. Small amounts of deionized (DI) water were introduced into the nonaqueous MR fluid to further influence the material removal process. Material removal data were collected with a spot-taking machine. Drag force (Fd) and normal force (Fn) before and after adding nanodiamonds or DI water were measured with a dual load cell. Both drag force and normal force were insensitive to the addition of nanodiamonds but increased with DI water content in the nonaqueous MR fluid. Shear stress (i.e., drag force divided by spot area) was calculated, and examined as a function of nanodiamond concentration and DI water concentration. Volumetric removal rate increased with increasing shear stress, which was shown to be a result of increasing viscosity after adding nanodiamonds and DI water. This work demonstrates that removal rate for a hard ceramic with MRF can be enhanced by adding DI water into a nonaqueous MR fluid.

Polishing of pre-polished CVD ZnS flats with altered magnetorheological (MR) fluids

Magnetorheological (MR) fluid composition and chemistry were altered to study the finishing of pre-polished CVD ZnS flats. Surface roughness did not exceed 2 nm rms after removing as much as 2 ?m of material.

In situ Drag Force Measurements in MRF of Optical Glasses

A spotting technique using the magnetorheological finishing (MRF) process is applied to measurements of drag force for optical glasses. In situ measurement results are reported as a function of substrate surface roughness.

Corrosion Resistant Zirconia Coated Carbonyl Iron Particle-Based Magnetorheological Fluid

Zirconia coated carbonyl iron particle-based magnetorheological fluid was developed for magnetorheological finishing. Particles were coated via sol-gel synthesis. Spot polishing tests were performed over 3 weeks with no signs of fluid degradation or corrosion.