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.
The material removal in magnetorheological finishing (MRF) is known to be controlled by shear stress, λ, which equals
drag force, Fd, divided by spot area, As. However, it is unclear how the normal force, Fn, affects the material removal in
MRF and how the measured ratio of drag force to normal force Fd/Fn, equivalent to coefficient of friction, is related to
material removal. This work studies, for the first time for MRF, the normal force and the measured ratio Fd/Fn as a function
of material mechanical properties. Experimental data were obtained by taking spots on a variety of materials including
optical glasses and hard ceramics with a spot-taking machine (STM). Drag force and normal force were measured with a
dual load cell. Drag force decreases linearly with increasing material hardness. In contrast, normal force increases with
hardness for glasses, saturating at high hardness values for ceramics. Volumetric removal rate decreases with normal force
across all materials. The measured ratio Fd/Fn shows a strong negative linear correlation with material hardness. Hard
materials exhibit a low "coefficient of friction". The volumetric removal rate increases with the measured ratio Fd/Fn which
is also correlated with shear stress, indicating that the measured ratio Fd/Fn is a useful measure of material removal in MRF.
Aqueous magnetorheological (MR) polishing fluids used in magnetorheological finishing (MRF) have a high solids
concentration consisting of magnetic carbonyl iron particles and nonmagnetic polishing abrasives. The properties of MR
polishing fluids are affected over time by corrosion of CI particles. Here we report on MRF spotting experiments
performed on optical glasses using a zirconia coated carbonyl iron (CI) particle-based MR fluid. The zirconia coated
magnetic CI particles were prepared via sol-gel synthesis in kg quantities. The coating layer was ~50-100 nm thick,
faceted in surface structure, and well adhered. Coated particles showed long term stability against aqueous corrosion.
"Free" nano-crystalline zirconia polishing abrasives were co-generated in the coating process, resulting in an abrasivecharged
powder for MRF. A viable MR fluid was prepared simply by adding water. Spot polishing tests were performed
on a variety of optical glasses over a period of 3 weeks with no signs of MR fluid degradation or corrosion. Stable
material removal rates and smooth surfaces inside spots were obtained.
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.
We report on use of the magnetorheological finishing (MRF) spotting technique to estimate subsurface damage (SSD)
depth resulting from deterministic microgrinding for polycrystalline alumina (PCA). With various microscopy
techniques, we show how surface roughness evolves with the amount of material removed by an MRF spot. Two stages
are identified. In the first stage the induced damaged layer and associated SSD from microgrinding are removed,
reaching an optimal value of surface roughness. Here, the initial peak-to-valley (p-v) surface roughness from grinding
gives a measure of the SSD depth found by spotting. In the second stage, where more material is removed from the nonrotating
surface, the resulting surface roughness begins to show the interaction between MRF abrasive particles and the
material's microstructure (crystal grains), i.e., the "MRF signature" for a specific material. We can examine the "MRF
signature" across grains using power spectral density and characterize surface features that contribute to surface
We study material removal mechanisms of commercially available hard optical materials, with respect to
their micromechanical properties, as well as their response to different manufacturing techniques. The
materials of interest are heterogeneous materials such as Ni-based (nonmagnetic), Co-based (magnetic),
and binderless tungsten carbides, in addition to other hard optical ceramics such as ALON, polycrystalline
alumina (PCA), and silicon carbide (SiC). Our experimental work is performed in three stages,
emphasizing the contributions of each material’s microstructure to its mechanical response. In the first
stage, we identify and characterize material physical properties, such as E-Young's modulus, Hv-Vickers
hardness, and KIc- fracture toughness (either by microindentation techniques, previously published models,
or vendors’ data base). In the second stage, we examine the ability of these materials to be deterministically
microground and spotted with magnetorheological finishing (MRF). The evolution of the resulting surface
topography is studied using a contact profilometer, white light interferometry, scanning electron
microscopy, and atomic force microscopy. In the third stage, we demonstrate that subsurface damage
(SSD) depth can be estimated by correlating surface microroughness measurements, specifically, the peakto-
valley (p-v) microroughness, to the amount of material removed by an MRF spot.
We describe the construction of elements of an optical materials property database. The database reports micromechanical properties (Young's modulus E, hardness H, fracture toughness Kc) for many optical glasses and crystals. Glass manufacturers included are Corning, Hoya, Schott, and Ohara. The materials included are many types of optical glasses and some optical crystals and polycrystals.
Our goal is to better understand the effects of glass mechanical properties, such as hardness, Young's modulus and fracture toughness, on the material removal rate during polishing. We have focused on eleven types of commercial Ohara optical glasses that cover the entire spectrum of the glass table. Conventional polishing experiments were performed to study the correlation between the process parameters (i.e., polishing pad, nominal load and slurry particle size) to the material removal rate and surface roughness. The results demonstrate how a material figure of merit affects the glass material removal rate. We review past results acquired at the Center for Optics Manufacturing as well as the work of others. The data presented demonstrate the controllability of the polishing process based on the known mechanical properties of any glass material.