Shai N. Shafrir

Shear stress in magnetorheological finishing for glasses

We report in situ, simultaneous measurements of both drag and normal forces in magnetorheological finishing (MRF) for what is believed to be the first time, using a spot taking machine (STM) as a test bed to take MRF spots on stationary parts. The measurements are carried out over the entire area where material is being removed, i.e., the projected area of the MRF removal function/spot on the part surface, using a dual force sensor. This approach experimentally addresses the mechanisms governing material removal in MRF for optical glasses in terms of the hydrodynamic pressure and shear stress, applied by the hydrodynamic flow of magnetorheological fluid at the gap between the part surface and the STM wheel. This work demonstrates that the volumetric removal rate shows a positive linear dependence on shear stress. Shear stress exhibits a positive linear dependence on a material figure of merit that depends upon Youngs modulus, fracture toughness, and hardness. A modified Prestons equation is proposed that better estimates MRF material removal rate for optical glasses by incorporating mechanical properties, shear stress, and velocity.

Synthesis and corrosion study of zirconia-coated carbonyl iron particles

This paper describes the surface modification of micrometer-sized magnetic carbonyl iron particles (CI) with zirconia from zirconium(IV) butoxide using a sol-gel method. Zirconia shells with various thicknesses and different grain sizes and shapes are coated on the surface of CI particles by changing the reaction conditions, such as the amounts of zirconia sol, nitric acid, and CI particles. A silica adhesive layer made from 3-aminopropyl trimethoxysilane (APTMS) can be introduced first onto the surface of CI particles in order to adjust both the size and the shape of zirconia crystals, and thus the roughness of the coating. The microanalyses on these coated particles are studied by field-emission scanning electron microscopy (FE-SEM) and X-ray-diffraction (XRD). Accelerated acid corrosion and air oxidation tests indicate that the coating process dramatically improved oxidation and acid corrosion resistances, which are critical issues in various applications of CI magnetic particles.

Subsurface damage and microstructure development in precision microground hard ceramics using magnetorheological finishing spots

We demonstrate the use of spots taken with magnetorheological finishing (MRF) for estimating subsurface damage (SSD) depth from deterministic microgrinding for three hard ceramics: aluminum oxynitride (Al(23)O(27)N(5)/ALON), polycrystalline alumina (Al(2)O(3)/PCA), and chemical vapor deposited (CVD) silicon carbide (Si(4)C/SiC). Using various microscopy techniques to characterize the surfaces, we find that the evolution of surface microroughness with the amount of material removed shows two stages. In the first, the damaged layer and SSD induced by microgrinding are removed, and the surface microroughness reaches a low value. Peak-to-valley (p-v) surface microroughness induced from grinding gives a measure of the SSD depth in the first stage. With the removal of additional material, a second stage develops, wherein the interaction of MRF and the materials microstructure is revealed. We study the development of this texture for these hard ceramics with the use of power spectral density to characterize surface features.

Toward Magnetorheological Finishing of Magnetic Materials

Magnetorheological finishing (MRF) is a precision optical finishing process traditionally limited to processing only nonmagnetic materials, e.g., optical glasses, ceramics, polymers, and metals. Here we demonstrate that MRF can be used for material removal from magnetic material surfaces. Our approach is to place an MRF spot on machined surfaces of magnetic WC-Co materials. The resulting surface roughness is comparable to that produced on nonmagnetic materials. This spotting technique may be used to evaluate the depth of subsurface damage, or deformed layer, induced by earlier manufacturing steps, such as grinding and lapping.

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.

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.

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.

Optical materials micromechanical property database: fracture toughness and ductility

We describe the construction of elements of an optical materials property database. The database reports micromechanical properties (Youngs 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.

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.

Micromechanical contributions to material removal and surface finish

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-Youngs 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.