Influence of partial charge on the material removal rate during chemical polishing

Abstract

A partial charge‐based chemical polishing model has been developed, which can serve as metric for describing the relative polishing material removal rate for different combinations of slurries and workpieces. A series of controlled polishing experiments utilizing a variety of colloidal polishing slurries (SiO2, CeO2, ZrO2, MgO, Sb2O5) and optical materials [single crystals of Al2O3 (sapphire), SiC, Y3Al5O12 (YAG), CaF2, and LiB3O5 (LBO); a SiO2‐Al2O3‐P2O5‐Li2O glass ceramic (Zerodur); and glasses of SiO2:TiO2 (ULE), SiO2 (fused silica), and P2O5‐Al2O3‐K2O‐BaO (Phosphate)] was performed and its material removal rate was measured. As previously proposed by Cook (J Non‐Cryst Solids. 1990;120:152), for many polishing systems, the removal rate is governed by a series of chemical reactions which include the formation of a surface hydroxide, followed by condensation of that hydroxyl moiety with the polishing particle, and a subsequent hydrolysis reaction. The rate of condensation can often be the rate limiting step, thus it can determine the polishing material removal rate. By largely keeping the numerous other factors that influence material removal rate fixed (such as due to particle size distributions, interface interactions, pad topography, kinematics, and applied pressure), the material removal rate is shown to scale exponentially with the partial charge difference (δwp‐s) between the workpiece and polishing slurry particle for many of the slurry‐workpiece combinations indicating that condensation rate is the rate limiting step. The partial charge (δ) describes the equilibrium distribution of electron density between chemically bonded atoms and is related to the electronegativity of the atoms chemically bonded to one another. This partial charge model also explains the age‐old experimental finding of why cerium oxide is the most effective polishing slurry for chemical removal of many workpieces. Some of the slurry‐workpiece combinations that did not follow the partial charge dependence offer insight to other removal mechanisms or rate limiting reaction pathways.

Nanoplastic removal function and the mechanical nature of colloidal silica slurry polishing

“…This correlation also suggests that the difference in mechanical properties between diamond and silica have little effect on the amount of material removal. The mechanical nature of colloidal silica slurry removal mechanism is also additionally supported by the fact that it can successfully polish all of these workpiece materials, while polishing based on chemical removal performed using cerium oxide slurry resulted in certain workpiece materials with no material removal such as with SiC and sapphire …”

“…Figure shows the correlation between determined effective removal depth ( d p ) from nanoscratching in water for each of the optical materials with the corresponding measured polishing material removal rate using colloidal silica slurry under identical polishing conditions (data from Ref. ). Despite the very different measurements (nanoscratching versus polishing rate) over a wide range of materials (glasses, single crystals, and glass ceramics), a linear relationship between the two was observed given by: …”

“…Correlation between polishing rate and grinding rate for various optical materials. (data from Ref , T. Suratwala, in preparation) [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Nine different optical workpiece materials were evaluated: single crystal Al 2 O 3 (Sapphire) (a‐plane; Coastline Optics, Camarillo, CA), single crystal SiC (silicon carbide) (SiC‐6H 0001; MTI Corporation, Richmond, CA), single crystal Y 3 Al 5 O 12 (YAG) (Northrop Grumman/Synoptics, Charlotte, NC), single crystal CaF 2 (111 orientation; ISP Optics, Irvington, NY), single crystal LiB 3 O 5 (LBO) (2ω doubler cut; Coherent Crystal, New Jersey), SiO 2 ‐Al 2 O 3 ‐P 2 O 5 ‐Li 2 O glass‐ceramics (Zerodur) (Schott; Duryea, PA), SiO 2 glass (Fused Silica) (Corning 7980; Corning Inc., Corning, NY), SiO 2 :TiO 2 glass (ULE) (Corning Inc, Corning, NY), and P 2 O 5 ‐Al 2 O 3 ‐K 2 O‐BaO glass (Phosphate glass) (LHG‐8; Hoya Corporation, Milpitas, CA). These materials were polished following the Convergent Polishing method using a colloidal silica polishing slurry (Ultra‐Sol ® S27; Eminess Technologies, Scotsdale, AZ) at a 0.6 mL/min slurry feed rate (single pass) for 8 hours with 1.1 psi applied pressure, and matched 20 rpm optic and lap rotation rates . The removal rate was determined gravimetrically.…”

“…To understand, and therefore predict, the material removal rate during polishing for various materials using Equation , one needs to determine both the chemical ( d m ) and nanoplastic mechanical ( d p ) removal due to individual particles under the relevant load conditions. In a companion study, the mechanism of chemical removal and quantitative scaling trends using the partial charge difference between the workpiece and slurry particle are described . In this study, we focus on measuring and determining the nanoplastic material removal amount ( d p ) for various optical materials and correlating it both to their polishing behavior and mechanical properties.…”