Jessica E. DeGroote

Removal rate model for magnetorheological finishing of glass

Magnetorheological finishing (MRF) is a deterministic subaperture polishing process. The process uses a magnetorheological (MR) fluid that consists of micrometer-sized, spherical, magnetic carbonyl iron (CI) particles, nonmagnetic polishing abrasives, water, and stabilizers. Material removal occurs when the CI and nonmagnetic polishing abrasives shear material off the surface being polished. We introduce a new MRF material removal rate model for glass. This model contains terms for the near surface mechanical properties of glass, drag force, polishing abrasive size and concentration, chemical durability of the glass, MR fluid pH, and the glass composition. We introduce quantitative chemical predictors for the first time, to the best of our knowledge, into an MRF removal rate model. We validate individual terms in our model separately and then combine all of the terms to show the whole MRF material removal model compared with experimental data. All of our experimental data were obtained using nanodiamond MR fluids and a set of six optical glasses.

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.

Quantitative characterization of optical polishing pitch

Optical polishing pitch has properties that can be quantitatively examined. These properties may be used to check batches of pitch for consistency, or to evaluate products for differences. In this work we explore the hardness, softening point and viscosity of pitch. The testing methods involved require little preparation, have quick turnaround time, use less than 200 g of pitch, and produce statistically significant results. The tests include: Shore A Durometer hardness test, ASTM Mettler softening point method, and dynamic viscosity measurements via a novel falling needle viscometer. Results from all three tests are given as averages with standard deviations for a variety of wood-based and petroleum-based products.

Magnetorheological fluid template for basic studies of mechanical-chemical effects during polishing

We developed a new magnetorheological (MR) fluid for studying the relative contributions of mechanics and chemistry in polishing hard materials. The base carrier fluid is a mixture of two non-aqueous liquids. At conventional carbonyl iron (CI) magnetic particle concentrations, removal rates with this formulation were unacceptably low for the polycrystalline optical ceramic aluminum oxynitride (ALON). We overcame this problem by creating a high magnetic solids concentration suspension consisting of a blend of large and small CI particles. Our test bed for experiments was a magnetorheological finishing (MRF) spot-taking machine (STM) that can only polish spots into a non-rotating part. We demonstrated that, using this new MR fluid formulation, we could substantially increase peak removal rates on ALON with small additions of nonmagnetic, nanodiamond abrasives. Material removal with this fluid was assumed to be predominately driven by mechanics. With the addition of small amounts of DI water to the base fluid containing nanodiamonds, the peak removal rate showed an additional increase, presumably due to the altered fluid rheology and possibly chemical interactions. It is possible, however, that this result is due to increased fluid viscosity as well. Interestingly, the microtexture on the surfaces of the ALON grains (albeit-two different ALON parts) showed distinctly different features when spotted with nanodiamonds or with nanodiamonds and water, and an understanding of this phenomenon is the goal of future work. In this paper we describe the difficult fluid viscosity issues that were addressed in creating a viable, high removal rate, non-aqueous MR fluid template that could be pumped in the STM for several days of experiments.

Effects of nanodiamond abrasive friability in experimental MR fluids with phosphate laser glass LHG-8 and other optical glasses

Research is currently being conducted to better understand the role that nanodiamond abrasives play in the removal process of Magnetorheological Finishing (MRF). The following presents removal rate data for a set of six optical glasses that were spotted (not polished out) with four different MR fluids, as well as texturing/smoothing data for phosphate laser glass LHG-8. Three of the fluids contained nanodiamonds with varying friability levels and the fourth fluid was an abrasive-free fluid that was used as a baseline for comparison. The medium friability nanodiamonds were found to be the most efficient in removing material on LHG-8, and the three silicate glasses, FS, BK-7 and FD-60. The high friability nanodiamond fluid was the most efficient for removal with the titanium and fluro- phosphate glasses EFDS-1 and FCD-1. With this nanodiamond the removal rates of all six glasses followed a mechanical figure of merit. The presence of nanodiamonds in the MR fluid greatly affected the surface texture of LHG-8. The abrasive-free MR fluid caused severe pitting that was either reduced or eliminated once the nanodiamonds were added to the fluid.

The Role of Nanodiamonds in the Polishing Zone During Magnetorheological Finishing (MRF)

In this work we discuss the role that nanodiamond abrasives play in magnetorheological finishing. We hypothesize that, as the nanodiamond MR fluid is introduced to the magnetic field, the micron sized spherical carbonyl iron (CI) particles are pulled down towards the rotating wheel, leaving a thin layer of nanodiamonds at the surface of the stiffened MR fluid ribbon. Our experimental results shown here support this hypothesis. We also show that surface roughness values inside MRF spots show a strong correlation with the near surface mechanical properties of the glass substrates and with drag force.

Material removal rate model for magnetorheological finishing (MRF) of optical glasses with nanodiamond MR fluid

We present a material removal rate model for MRF of optical glasses using nanodiamond MR fluid. The new model incorporates terms for drag force, polishing particle properties, chemical durability and glass composition into an existing model that contains only terms for the glass mechanical properties. Experimental results for six optical glasses are given that support this model.

Power spectral density plots inside MRF spots made with a polishing abrasive-free MR fluid

We present power spectral density (PSD) data measured inside magnetorheological finishing (MRF) spots in orthogonal directions. MRF spots exhibit a distinct grooving pattern that varies for each fluid/material combination. This spot analysis may provide new insights on the material removal process. Issues associated with taking orthogonal PSD measurements are also discussed.

Adding Chemistry and Glass Composition Data into a Mechanical Material Removal Model for Magnetorheological Finishing (MRF)

Chemical durability and glass composition terms were developed for the MRF material removal process. Results indicate that chemistry plays a significant role in the MRF process with nanodiamond fluid.

Using Mechanics and Polishing Particle Properties to Model Material Removal for Magnetorheological Finishing (MRF) of Optical Glasses

A material removal rate model for Magnetorheological Finishing is introduced. Results show a strong linear dependence between material removal rates and drag force specific to glass type as removal rates increase with nanodiamond concentration.