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WO2013019875A1 - Procédé et système de polissage de sphères creuses solides - Google Patents

Procédé et système de polissage de sphères creuses solides Download PDF

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Publication number
WO2013019875A1
WO2013019875A1 PCT/US2012/049184 US2012049184W WO2013019875A1 WO 2013019875 A1 WO2013019875 A1 WO 2013019875A1 US 2012049184 W US2012049184 W US 2012049184W WO 2013019875 A1 WO2013019875 A1 WO 2013019875A1
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WO
WIPO (PCT)
Prior art keywords
media
polishing
container
capsule
load bearing
Prior art date
Application number
PCT/US2012/049184
Other languages
English (en)
Inventor
Tayyab I. SURATWALA
William A. Steele
Michael D. Feit
Michael Stadermann
Jim E. FAIR
Kuang Jen Wu
Kari MORENA
Kuo-Chun Chen
Abbas KIKROO
Kelly YOUNGBLOOD
Original Assignee
Lawrence Livermore National Security, Llc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lawrence Livermore National Security, Llc filed Critical Lawrence Livermore National Security, Llc
Publication of WO2013019875A1 publication Critical patent/WO2013019875A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B11/00—Machines or devices designed for grinding spherical surfaces or parts of spherical surfaces on work; Accessories therefor
    • B24B11/02—Machines or devices designed for grinding spherical surfaces or parts of spherical surfaces on work; Accessories therefor for grinding balls
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B31/00—Machines or devices designed for polishing or abrading surfaces on work by means of tumbling apparatus or other apparatus in which the work and/or the abrasive material is loose; Accessories therefor
    • B24B31/02—Machines or devices designed for polishing or abrading surfaces on work by means of tumbling apparatus or other apparatus in which the work and/or the abrasive material is loose; Accessories therefor involving rotary barrels

Definitions

  • Plastic shells are used in Inertial Confinement Fusion (ICF) laser targets.
  • ICF Inertial Confinement Fusion
  • the capsule absorbs energy and compresses/implodes, causing the hydrogen isotope fuel within the capsule to greatly increase in density.
  • the surface quality of the capsules is controlled.
  • As-made capsules have bumps and dimples (as high as several microns and tens of microns wide) as a consequence of the deposition process for making the capsules. These bumps greatly degrade the ability for the capsule to compress uniformly. Therefore, there is a need in the art for improved methods and systems for improving capsule surface quality.
  • embodiments of the present invention relate to methods and systems for polishing solid or hollow spheres, for example, hollow plastic spheres.
  • the invention has been applied to a tumble finishing process in which local planarization of surface non-uniformities on an external surface of an object is performed.
  • the methods and systems described herein are applicable to the processing and fabrication of a wide variety of optical materials suitable for use with high power laser and amplifier systems.
  • a new method for polishing and achieving local planarization on precision spherical, plastic capsules is provided.
  • Such capsules have various applications, such as ablators used in high-peak-power laser targets for fusion energy research.
  • the as-manufactured ablators contain many shallow domes (many 100's of nm high and a few l O's of ⁇ wide) on the outer surface which are undesirable due to their contribution to instabilities during implosion.
  • These capsules were polished (i.e., Tumble Finished) by rotating a cylindrical vial containing the capsule, many borosilicate glass or zirconia media, and an aqueous-based colloidal silica polishing slurry.
  • the relative media/capsule motions cause multiple, random sliding spherical-spherical Hertzian contacts, resulting in material removal, and possibly plastic deformation, on the capsule.
  • the domes were observed to locally planarize (i.e., converge to lower heights).
  • the correct kinematics i.e., the characteristics of the media/capsule motions
  • the vial rotation rate and the fill fraction of media and slurry high velocity downward circumferential media motions were avoided, preventing fracturing of the fragile capsules.
  • the resulting post-polished surface roughness on the capsule was found to scale with the initial media surface roughness.
  • pre-polishing the media greatly reduced the roughness of the media and thus the roughness of the polished capsule.
  • a material removal model is described based on the Preston model and spherical- spherical Hertzian contacts which shows reasonable agreement with measured average removal rates of 35 ⁇ 1 5 nm/day and which serves as a valuable tool to scale the polishing behavior with changes in process variables. Narrow domes were observed to planarize more rapidly than wider domes.
  • a local planarization convergence model is also described, based on the concept of workpiece-lap mismatch where the local pressure, and hence removal, varies with the gap at the interface contact. The calculated rate and shape evolution of various size isolated domes compares well with the experimental data.
  • a method of polishing an object includes placing the object in a container and placing a load bearing media different than the object in the container. The method also includes placing a polishing compound in the container and rotating the container about an axis of rotation.
  • an apparatus for polishing an object includes a generally cylindrical container characterized by an internal volume and the object disposed in the container. The apparatus also includes a plurality of load bearing media disposed in the container. The plurality load bearing media are different from the object. The apparatus further includes a polishing compound disposed in the container and a rotation device operable to rotate the container about an axis of rotation.
  • the present invention provides methods for polishing and smoothing hollow and solid plastic spheres by tumbling in a cylindrical container in the presence of media and slurry.
  • the media typically solid glass spheres
  • the aqueous slurry contains polishing particles (typically colloidal silica), which remove material from the surface of the shell.
  • polishing particles typically colloidal silica
  • this polishing method and process has been specifically tailored: 1 ) not to cause significant scratching, 2) not to modify the overall roundness of the shell/sphere, 3) not to fracture the shell/sphere, and 4) not to embed polishing particles into the sphere surface.
  • the smoothness of these plastic shells is an important parameter for plastic shells used in targets for high power fusion lasers. These plastic shells are designed to compress and implode during the laser firing. Increasing the surface smoothness can dramatically reduce the asymmetry of the capsule compression/implosion. Symmetric capsule
  • compression/implosion is a critical criterion for creating a target with a high probability of achieving ignition.
  • This polishing method can also be applied to polishing or smoothing of plastic shells or spheres used for a wide variety sphere manufacturing.
  • the present invention has applications and uses for polishing capsules for fusion laser targets and well as commercial/industrial sphere manufacturing, such as bearings or hollow spheres, especially for cases when the spheres are fragile and surface smoothness requirements are high.
  • FIG. 1 is a photo of a capsule shell (2 mm diameter) used as an ablator in high- peak-power laser targets for fusion energy research;
  • FIGS. 2A - 2C illustrate SEM micrographs of the capsule surface as-deposited, after conventional polishing, and after Tumble Finishing performed according to an embodiment of the present invention, respectively;
  • FIG. 3 is a photo of an acrylic vial cylinder (60 mm long) containing colloidal silica slurry, glass media, and a single capsule according to an embodiment of the present invention;
  • FIG. 4A is a schematic diagram of an AFM spheremapper for characterizing capsule surface and shape
  • FIG. 4B is a schematic of a PSDI scanning interferometer for characterizing isolated defects (i.e., domes);
  • FIG. 5 is a schematic diagram illustrating characteristic motions of the capsule and media during Tumble Finishing according to an embodiment of the present invention
  • FIG. 6 is a plot illustrating circumferential motion fraction (f m ) as function of vial rotation rate (R v ), media fill fraction (f,), and slurry fill fraction (f s ) according to an embodiment of the present invention
  • FIGS. 7A and 7B are SEM images of a capsule surface after Tumble Finishing without a pre-polished media/vial and with a pre-polished media/vial according to an embodiment of the present invention, respectively;
  • FIG. 8 is a plot illustrating measured areal pit density as function of media roughness according to an embodiment of the present invention.
  • FIGS. 9A and 9B are optical micrographs of the capsule surface before and after Tumble Finishing according to an embodiment of the present invention, respectively.
  • FIGS. 9C and 9D are plots of the isolated defect distribution on the same capsule as measured by PSDI before and after Tumble Finishing according to an embodiment of the present invention, respectively;
  • FIG. 9E is a plot showing comparison of capsule roughness before and after Tumble Finishing as function of mode number (i.e., Power Spectral Density) from the average of 3 full circumference AFM lines scans according to an embodiment of the present invention.
  • mode number i.e., Power Spectral Density
  • FIG. 1 OA is a PSDI image and a corresponding line scan of a capsule surface before (left) and after (right) Tumble Finishing of an isolated, narrow dome on the capsule.
  • FIG. 1 OB is a PSDI image and a corresponding line scan of a capsule surface before (left) and after (right) Tumble Finishing of an isolated, wide dome on the capsule.
  • FIGS. 1 1A and 1 IB are schematic diagrams illustrating relevant parameters and dynamics of the media and capsule for a cascading motion during approach and at contact during Tumble Finishing according to an embodiment of the present invention, respectively.
  • FIG. 12 is a plot illustrating the calculated load at contact (P), the contact zone diameter (2a), and the removal rate (dh/dt) for a range of possible relative velocities and values according to an embodiment of the present invention
  • FIGS. 13 A - 13D are x-lineouts of 3D simulations of the surface evolution of various dome and dimple defects during Tumble Finishing according to an embodiment of the present invention
  • FIG. 14 is a plot of calculated post-Tumble Finishing heights of various measured domes and dimples (using Eq. l i b and initial measured heights (h 0 ) and width (w 0 )) cross plotted with the measured final height of the same corresponding domes and dimples.
  • the dashed line corresponds to a 1 -to-l correlation between calculated and measured values; and
  • FIG. 1 5 is a simplified flowchart illustrating a method of polishing an object according to an embodiment of the present invention.
  • Typical ablators are hollow, spherical plastic capsules ( ⁇ 2 mm in diameter) used on ignition targets in high-peak-power laser systems such as the National Ignition Facility (NIF).
  • NIF National Ignition Facility
  • isolated defects typically bumps which are 100's of nm high and 10's of ⁇ in width
  • tumble finishing is performed by rotating a cylindrical vial (-25 mm in diameter) containing the capsule, glass media ( ⁇ 2 mm in diameter), and an aqueous-based colloidal silica (-50 nm) slurry.
  • PSDI phase shifting diffractive interferometry
  • AFM atomic force microscopy
  • achieving bump removal and planarization without fracturing, scratching, or causing impact damage is provided by embodiments of the present invention.
  • Parameters utilized herein include use of the appropriate media (e.g., material, size & surface roughness), kinematics, and fill fraction of media & slurry. Taller, wider bumps were found to planarize and converge more slowly than shorter, narrow bumps.
  • Preston model applied to small tool polishing and an isolated defect convergence model the rate of local planarization can be quantitatively computed and compared with the experimental data.
  • the Tumble Finishing process can be implemented as a standard production process for making ablators and contributes to improved ablator production yield by reducing Mixed Mass (a metric used to describe the degree of contribution of the isolated defects to the instability of the implosion).
  • NIF NIF Specifications for the NIF include the specification that ignition capsules are characterized by a small number of large surface defects in order to minimize the amount of ablator material mixed into the hot spot at ignition.
  • the inventors have quantified contributing isolated defects using a phase shifting diffraction interferometer (PSDI) technique and process the ignition capsules as described herein to reduce these features.
  • PSDI phase shifting diffraction interferometer
  • Capsules are tumble finished in a process developed for ablator targets (also referred to as capsules) but applicable to a wider variety of objects including a variety of optical elements.
  • capsules may have a number of defects with widths larger than thirty microns and heights greater than 600 nm.
  • capsules are polished in four day increments based on the largest twenty-five isolated defects, individual domes, and clusters of domes. These processes have improved the capsule yields.
  • Embodiments of the present invention are also useful in removing residual debris, minimizing scratches, and increasing production rates.
  • methods and systems are provided for locally planarizing spherical surfaces in which the surface roughness is reduced without substantially modifying the initial shape of the object being polished.
  • the surfaces do not need to be spherical and other surfaces including elliptical surfaces are included within the scope of the present invention, which provides for local planarization of a variety of media.
  • FIG. 1 is a photo of a capsule shell (2 mm diameter) used as an ablator in high-peak-power laser targets for fusion energy research.
  • ablators which can have dimensions on the order of millimeters (e.g., 2 mm in diameter and -190 ⁇ thick) are injected with isotopes of hydrogen that are frozen as a layer on the inner surface, which compress through laser inertial confinement.
  • the capsules are fabricated using plasma- assisted chemical vapor deposition (PA-CVD), where hydrogen and trans-2-butene are broken down to form an amorphous polymer coating on a pre-fabricated spherical poly-a- methylstyrene (PAMS) substrate (called mandrels) produced by micro-encapsulation. Later, the mandrels are removed through thermal decomposition.
  • PA-CVD plasma- assisted chemical vapor deposition
  • PAMS poly-a- methylstyrene
  • the capsules have stringent surface roughness and isolated defect requirements, since these can contribute to
  • FIGS. 2A - 2C illustrate SEM micrographs of the capsule surface as-deposited, after conventional polishing of the as-deposited capsule, and after Tumble Finishing of the as- deposited capsule performed according to an embodiment of the present invention, respectively.
  • the inventors believe, without limiting embodiments of the present invention, that these domes are caused by small particle or asperity precursors present on the mandrel which grow into a dome during PA-CVD.
  • Tumble Finishing offers several advantages over conventional polishing techniques for planarization: 1 ) since the polishing system is hermetically sealed, the capsule is less prone to scratching (as long as incoming materials are free of rogue particles and surface asperities), 2) unlike conventional polishing, capsule mounting is not needed, thereby minimizing capsule deformation and mounting-interface-induced scratching and 3) the process is relatively simple and low cost.
  • the tumble finishing process does not modify the overall shape (i.e., sphericity) of the items being polished, which can occur with some conventional polishing techniques.
  • capsules used in some applications of interest are largely spherical and thereby, only local planarization is performed.
  • Embodiments of the present invention utilize a Tumble Finishing process suitable for the polishing of hollow capsules, providing process improvements associated with the kinematics and media roughness. Utilizing the processes described herein, including optimized processes, the domes can be locally planarized with minimal degradation to the overall capsule roughness as illustrated in FIG. 2C.
  • a material removal model is described (based on the Preston material removal concept and sliding spherical-spherical Hertzian contact) along with a dome convergence model (based on workpiece-lap mismatch contribution to non-uniform pressure distribution) that compare well with experimental data.
  • ablators or capsules can be made using the micro-encapsulation method for the mandrel (used as the substrate) and PA-CVD deposition for the capsule.
  • media to be polished e.g., a capsule
  • a chamber with polishing media, a polishing compound, and a lubricant.
  • a solution of colloidal silica (50 nm; Blue colloidal silica suspension available from Allied High Tech Products, Inc., Collinso Dominguez, CA) serving as the polishing compound is diluted l Ox with de-ionized water ( 18 ⁇ ) with the addition of a lubricant (e.g., 0.25 wt% Micro-90 ® soap solution available from International Products Corp, Burlington, NJ).
  • a lubricant e.g., 0.25 wt% Micro-90 ® soap solution available from International Products Corp, Burlington, NJ.
  • the solution is prepared and prefiltered using a 0.45 ⁇ point-of-use filter.
  • the capsule, solution, and polishing media e.g., 2.4 mm diameter, Grade 48 borosilicate glass available from Winstead Precision Ball Comp.
  • zirconia (Grade 3) available from Grainger, Inc.
  • FIG. 3 is a photo of an acrylic vial cylinder (60 mm long) used as a container 310, also referred to as a chamber, to contain one or more objects (e.g., one or more capsules), load bearing media (e.g., glass polishing media), and a polishing compound (e.g., colloidal silica slurry) according to an embodiment of the present invention.
  • the acrylic vial is inserted into a cylindrical sleeve (e.g., 1 10 mm in diameter).
  • the unit is placed on a tumbler (e.g., a C&M Topline three bar tumbler) and rotated at various rotation rates for a predetermined period, for example, 96 hours.
  • a tumbler e.g., a C&M Topline three bar tumbler
  • f b 0.8, the vial contains ⁇ 1700 media.
  • the vial, media and capsule were cleaned pre- and post- polishing by soaking them in de-ionized water under ultrasonic agitation and aggressively rinsing with de-ionized water and air drying.
  • the media was pre-polished by Tumble Finishing at 100 rpm in cerium oxide slurry (Hastilite PO at Baume 9 available from Universal Photonics of
  • the surface roughness (i.e., peak to valley) of the polishing media is reduced using pre-polishing to provide polishing media having a surface roughness less than 100 nm RMS, less than 50 nm RMS, or less than 40 nm RMS.
  • the surface morphology of the capsule is characterized pre- and post- Tumble Finishing using various techniques to characterize the material removal properties.
  • the capsule mass was measured gravimetrically ( ⁇ 0.0005 gm) after equilibrating to fixed relative humidity and temperature to determine material removal rate.
  • FIG. 4A is a schematic diagram of an AFM spheremapper for characterizing capsule surface and shape.
  • FIG. 4B is a schematic of a PSDI scanning interferometer for characterizing isolated defects (i.e., domes).
  • Bright field optical microscopy using a Nikon Optiphot was performed to obtain general surface characteristics of the capsule surface.
  • a spheremapper, an Atomic Force Microscope configured for characterizing spherical surfaces was used to determine circumferential roughness lineouts and create power spectral density curves of the capsule surface.
  • Phase Shifting Diffractive Interferometry (PSDI) was performed to image the whole capsule surface and determine the isolated defect counts, locations, and size characteristics.
  • PSDI Phase Shifting Diffractive Interferometry
  • the media were characterized using white light interferometry (Veeco Wyko NT9800) to determine their average surface roughness.
  • the inventors have analyzed the media kinematics to study the motions of the media (e.g., number of collisions, velocities, media path, and the like), i.e., the kinematics, during Tumble Finishing.
  • the media kinematics may 1) influence the overall material removal rate on the capsule and 2) influence the randomness of contact with the capsule surface.
  • the capsule will float, requiring specific kinematic conditions to overcome, 2) the capsule is prone to liquid surface tension adhesion particularly with the walls of the vial, and 3) the capsule is more prone to breakage during impact.
  • the addition of the surfactant also referred to as a lubricant, reduces or minimizes the motion of the polishing media from the path 510 followed by polishing media in FIG. 5.
  • Table 1 illustrates the measured characteristics of media motions and of capsule survivability as function of the media fill fraction (3 ⁇ 4), slurry fill fraction (f s ), and vial rotation rate (R v ) during ten runs according to an embodiment of the present invention.
  • FIG. 5 is a schematic diagram illustrating characteristic motions of a single capsule being polished and the polishing media during Tumble Finishing according to an embodiment of the present invention.
  • FIG. 5 illustrates the container 310, which can also be referred to as a chamber or polishing vial.
  • the container contains one or more capsules 1 10 and a plurality of load bearing media 505, which can also be referred to as polishing media.
  • the load bearing media are solid glass spheres.
  • a plurality of metallic spheres or glass, ceramic, or metallic shells i.e., hollow spheres
  • a polishing compound 530 in solution also referred to as a slurry, such as colloidal silica in an aqueous solution is also illustrated in FIG. 5.
  • the polishing compound facilitates polishing of the object by the load bearing media.
  • a single object to be polished i.e., a single capsule
  • embodiments of the present invention are not limited to the polishing of a single object, but can be applied to multiple objects to be polished.
  • the polishing media 505 also referred to as the load bearing media, can have a range of sizes and material characteristics.
  • the polishing media are substantially the same size as the object being polished, for example, a diameter ranging from about 200 ⁇ to 4 mm, more particularly, between 200 ⁇ and 2 mm.
  • the diameter of the polishing media are within a predetermined percentage of the diameter of the object being polished, for example, within 100%.
  • the capsule is 2 mm in diameter and the polishing media are 2 mm in diameter.
  • the polishing process can be analyzed in terms of various layers of polishing media (illustrated by arrows indicating the direction and velocity of motion of the polishing media and the capsule).
  • the polishing media adjacent the wall of the chamber are moving upward with high velocity.
  • the next layer of polishing media farther from the wall is still moving upward, but with a reduced velocity.
  • the polishing media are moving downward with a high velocity.
  • the capsule is illustrated as surrounded by polishing media moving in a downward direction with an intermediate velocity. The contact between the polishing media and the capsule is a function of these directions and velocities and results in removal of material through polishing.
  • ends 320 of the vial are curved into a rounded shape, which the inventor believe, without limiting embodiments of the present invention, prevent the media and/or slurry from sticking in squared off corner regions.
  • the container can be fabricated using an acrylic material having a generally cylindrical shape, with the end portions of the container being characterized by a curvature as illustrated in FIG. 3.
  • the curvature can have elements that are spherical, hyperbolic, or the like.
  • the cascading velocities are increased (e.g., maximized) without any direct collisions due to the capsule's fragile nature. This can be contrasted with ball milling, which strives to maximize the collisions from circumferential motions and impact energy.
  • the circumferential motion occurs at a critical velocity given by:
  • the other characteristic motion, cascading motion generally occurred in layers (n ⁇ ) with characteristic angles ( ⁇ ) and velocities (v casc ) at each vial rotation rate (R v ) and fill fraction (3 ⁇ 4, f s ).
  • the velocity was lowest in the center of layers and increased outwards (see FIG. 5).
  • the maximum cascading velocity (v casc ) is noted for each of the kinematic conditions explored in Table 1 .
  • the fill fractions for the polishing media fill fraction (ft,) and the fill fraction for the slurry (f s ) are not additive, but are measured including both media and slurry.
  • FIG. 6 is a plot illustrating circumferential motion fraction (f m ) as function of vial rotation rate (R v ), polishing media fill fraction (ft), and slurry fill fraction (f s ) according to an embodiment of the present invention.
  • ft vial rotation rate
  • f s slurry fill fraction
  • the higher fill fraction is believed to increase surface tension induced pull from the nearest neighbor media, overcoming the surface tension between the media and vial wall, and thus preventing the circumferential motion of the media. Additionally, the inventors believe, without limiting the present invention, that the addition of a surfactant or lubricant contributes to the reduction of circumferential motion of the polishing media as a result of decreased surface tension between the walls of the container and the polishing media.
  • Embodiments of the present invention contrast with conventional ball milling processes in which the fill fraction of the media and slurry is small (e.g., less than 50%) to encourage circumferential motion of the media and resulting high velocity impact between the media.
  • the process regime utilized by embodiments of the present invention differs greatly from conventional polishing processes since high fill factors are utilized to reduce or prevent circumferential motion of the media and utilize cascading motion to provide a low impact polishing process.
  • An issue that is addressed by embodiments of the present invention is providing a Tumble Finishing process in which isolated surface defects (i.e., domes) are planarized without introducing undesirable surface features, such as pits and scratches, due to rogue particles or asperities from external contamination, from corrosion products of the media/vial, and/or from the roughness of the media and vial.
  • the rogue particles were minimized using stringent cleaning or filtering processes of the media, slurry, and vial, and selecting media materials that would not corrode (i.e., glass, ceramic, or the like).
  • the roughness of the media was shown to significantly affect the amount of pitting and scratching observed on the capsule surface.
  • embodiments of the present invention utilize a closed system illustrated by the sealed container 3 10 in FIG. 3.
  • the humidity inside the polishing system can be maintained at a value higher than the ambient humidity, for example, higher than 80%, higher than 85%, higher than 90%, higher than 95%, higher than 97%, higher than 98%, higher than 99% and up to 100%.
  • the humidity is provided at a high level to prevent substantial drying of the slurry in the system. The lack of drying in the environment prevents the formation of the hard agglomerates and the associated scratching. Additional discussion related to the use of sealed and/or high humidity environments to prevent damage from rogue particles and agglomerates is provided in International Patent Application No. PCT/US2012/029837, filed on March 20, 2012, the disclosure of which is hereby incorporated by reference in its entirety for all purposes.
  • FIGS. 7A and 7B are SEM images of a capsule surface after Tumble Finishing without a pre-polished media/vial and with a pre-polished media/vial according to an embodiment of the present invention, respectively.
  • FIG. 7A some examples of the observed pits and scratches are shown as measured by scanning electron microscopy. The pits were typically ⁇ 10 ⁇ wide and several hundred nm deep.
  • FIG. 8 is a plot illustrating measured areal pit density as function of media roughness according to an embodiment of the present invention.
  • the line represents a linear regression fit to the data.
  • other high modulus media characterized by suitable density and beneficial corrosion properties, including stainless steel, tungsten carbide, other oxide-based media including other glasses, zirconium oxide, zirconia, aluminum oxide, silicon nitride, or the like can be utilized according to alternative embodiments.
  • mj is the initial mass before polishing after equilibration to 45% RH
  • m f is the mass after polishing and equilibration to 45% RH
  • t is the polishing time
  • PCH is the density of the capsule material
  • Am 0 is the mass uptake of a control capsule from storage in liquid water for time t
  • r s is the radius of the capsule shell.
  • FIGS. 9A and 9B are optical micrographs of the capsule surface before and after Tumble Finishing according to an embodiment of the present invention respectively.
  • FIGS. 9C and 9D are plots of the isolated defect distribution on the same capsule as measured by PSDI before and after Tumble Finishing according to an embodiment of the present invention, respectively.
  • FIG. 9E is a plot showing comparison of capsule roughness before and after Tumble Finishing as function of mode number (i.e., Power Spectral Density) from the average of 3 full circumference AFM lines scans according to an embodiment of the present invention, respectively.
  • mode number i.e., Power Spectral Density
  • FIGS. 9A and 9B optical microscope images are shown of a portion of the capsule surface, illustrating the removal of the majority of the pre-existing domes on the capsule surface.
  • FIGS. 9C and 9D show the height and width distribution of each of the domes over the whole 4 ⁇ capsule surface pre- and post- Tumble Finishing, as analyzed by PSDI.
  • the lines in the figures are contour lines representing a fixed severity in contribution of these domes to instability of the ablator implosion (referred to a "mix mass") for fusion energy applications. In other words, taller and wider domes have a larger mix mass. It is desirable to have domes within the two (positive and negative) contour lines.
  • the capsule had over 15,000 identified domes before Tumble Finishing and only 1 1 8 after, illustrating the high degree of local planarization achieved.
  • the peak to valley roughness (RMS) was measured as 7.2 ⁇ for the data presented in FIG. 9C and was reduced to a peak to valley roughness (RMS) of only 1.5 ⁇ after polishing as shown in FIG. 9D.
  • FIG. 9E compares the power spectral density (PSD) as a function of mode number for the same capsule pre- and post- Tumble Finishing, as measured by the AFM
  • Mode number is defined as the spatial scale length across the circumference of the capsule (i.e., mode 1 is a full circumference ( ⁇ 6.3.1 mm) and mode 1000 is a scale length 1/1000 of the circumference (6.3 ⁇ )).
  • the dashed line in FIG. 9E represents a target specification for the PSD, derived based on minimizing the instability of the implosion when such a capsule is used in ICF applications. No observable change in the capsule low mode shape occurred, confirming that the Tumble Finishing process is gentle enough not to change the overall sphericity of the capsule. Also, due to the reduction of the domes, the high mode roughness is improved. Finally, the mid modes are slightly degraded after Tumble Finishing under some conditions, likely due to slight variations in the random contacts during polishing resulting in small removal variations at the spatial scale lengths greater than the contact size.
  • FIG. 1 OA is a PSDI image and a corresponding line scan of a capsule surface before (left) and after (right) Tumble Finishing of an isolated, narrow dome on the capsule.
  • FIG. 10B is a PSDI image and a corresponding line scan of a capsule surface before (left) and after (right) Tumble Finishing of an isolated, wide dome on the capsule.
  • 2D profiles and lineouts of two isolated specific domes are shown pre- and post- Tumble Finishing as measured by PSDI. Both domes decreased in height. However, the narrow dome converged much further to planarity than the wider dome. In both FIGS.
  • colloidal silica is used as a polishing compound, but other polishing compounds can be utilized, including alumina, diamond abrasives, combinations thereof, or the like.
  • the inventors have studied the chemical and/or mechanical removal mechanism by modifying the Preston model as employed in relation to material removal on glass, silicon, and other ceramic materials to extend the model to describe removal on plastic. In the general form, material removal is described as: dh
  • dh/dt is the time average removal rate at some given time t and position x,y on the workpiece
  • ⁇ is the friction coefficient which is a function of the relative velocity (v r ) at the workpiece/lap interface
  • ⁇ is the pressure distribution resulting from the applied pressure ( ⁇ 0 ) and the nature of the workpiece/lap contact
  • k p is the Preston constant, which is the value that describes the amount of material removal per unit velocity and pressure. In other words, the Preston constant describes the relative rate of removal of a given polishing particle on the workpiece and houses all the complex microscopic/molecular level interactions during polishing.
  • FIGS. 1 1 A and 1 IB are schematic diagrams illustrating relevant parameters and dynamics of the media and capsule for a cascading motion during approach and at contact during Tumble Finishing according to an embodiment of the present invention, respectively.
  • This mechanism is analogous to small tool polishing, which is often used for fabricating optical components.
  • the Preston equation for the spatial and time average removal rate can be re-written for Tumble Finishing in the following form:
  • f c is fraction of the capsule surface area in contact with media for each media/capsule contact, and f, is the fraction of time the media is in contact with the capsule. Note that the contacts are assumed to be perfectly random over the surface of the capsule, removing the spatial dependence shown in Eq. (3).
  • Hertzian sphere-on-sphere contact mechanics can be used to quantitatively evaluate material removal (described by Eq. (4)).
  • the peak load (P) at contact is due to two contributions: 1 ) the force at impact when a single media contacts and the capsule, and 2) the effective weight of the media in the layers above.
  • the former can be determined by equating the kinetic energy of media with that of elastic strain energy at maximum penetration into the capsule.
  • the latter can be estimated by the weight of the average number of media above the capsule.
  • the peak load at contact can be described as:
  • p m is the mass density of the media
  • sin 6> is the normal component of the relative velocity v r at contact
  • g is the gravitation constant (9.80 m 2 /s)
  • nL is the number of layers
  • r m is the media radius
  • r c is the composite radius of the capsule shell and media (defined below).
  • k' is material constant given by:
  • ECH is the modulus of the raw CH material of the capsule
  • t s is the shell thickness
  • r s is the shell radius
  • the contact zone radius (a) is given by:
  • FIG. 12 is a plot illustrating the calculated load at contact (P), the contact zone diameter (2a), and the removal rate (dh/dt) for a range of possible relative velocities and values according to an embodiment of the present invention. Using literature values for the
  • dh/dt is the spatial and time average thickness removal rate defined in Eq. (4)
  • hj(x,y,t) is the surface height on a given point on the surface
  • L is a characteristic length.
  • the second term in the parenthesis describes the workpiece-lap mismatch effect on the material removal in terms of the local curvature of the surface of the workpiece. In other words, a negative curvature feature on the surface (i.e., a peak) will see enhanced removal, and a positive curvature feature on the surface (i.e., a valley) will reduce removal.
  • the L term incorporates the effect of both the size of the contact zone and relative stiffness of the tool.
  • the formalism described by Eq. (11 ) has also been used to describe material removal on surface from chemical etching processes.
  • FIGS. 13 A - 13D are x-lineouts of 3D simulations of the surface evolution of various dome and dimple defects during Tumble Finishing according to an embodiment of the present invention.
  • results are shown for the numerical solution of Eq. (1 1) for various 3D isolated defects (i.e., a narrow dome, a wide dome, two intersecting domes, and a narrow dimple).
  • the narrow domes converged more quickly than the wider domes (see FIGS. 13A and 13B in comparison with FIGS. 10A and 10B).
  • the intersecting domes have a more complicated behavior; with removal, the domes merge and the resulting dome has an effective width which is larger than that of either initial dome and which slows down the convergence rate as shown in FIG. 13C. Finally, a dimple feature (i.e. a valley) will also converge by widening and reducing is relative depth as shown in FIG. 13D.
  • FIG. 13A is for a Narrow Gaussian dome (600nm high; 16.5 ⁇ wide).
  • FIG. 13B is for a Wide Gaussian dome (600 nm high; 50 ⁇ wide).
  • FIG. 13C is for Multiple domes.
  • FIG. 13D is for a Narrow Dimple (600 nm deep; 16.5 ⁇ wide).
  • Eq. (1 1 ) can also be solved analytically by approximating the shape of the domes as Gaussian with initial height h 0 and full width half maximum (FWHM) width w 0 .
  • the solution for the height relative to the average baseline surface height at time t at distance r from the dome center is given by:
  • Eq. (12b) demonstrates that the convergence rate will decrease as width of the dome increases.
  • FIG. 14 is a plot of calculated post-Tumble Finishing heights of various measured domes and dimples (using Eq. (l i b) and initial measured heights (h 0 ) and width (w 0 )) cross plotted with the measured final height of the same corresponding domes and dimples.
  • the dashed line corresponds to a 1 -to- l correlation between calculated and measured values.
  • the time to polish is determined using the above equations and measurements of the topology, including surface features, prior to polishing, computing a polishing time as a function of the widths and heights of the features.
  • the largest features are used to compute the polishing time.
  • the measured final height of the dome after polishing is cross plotted against the predicted final height of the dome after polishing.
  • the dashed line represents a 1 -to-l correspondence between measured and calculated heights.
  • the model shows a reasonably good agreement with experimental results over a wide range of isolated dome heights and even some dimples.
  • embodiments of the present invention provide a novel method, which can be referred to as Tumble Finishing, for polishing and planarizing isolated features (domes) on spherical capsules.
  • Tumble Finishing successfully removes material and locally planarizes domes on the surface.
  • a material removal model developed by the inventors (based on the Preston material removal concept and sliding spherical-spherical Hertzian contact) predicts an average thickness removal rate values similar to that measured.
  • a local planarization rate convergence model based on the concept of workpiece-lap mismatch where the local pressure, and hence removal, varies with the gap at the contact interface was developed by the inventors and is described herein. The calculated rate and shape evolution of various sized isolated domes compare well with the experimental data.
  • the Tumble Finishing process is well suited for treating capsule ablators used in Targets for High-Peak-Power laser systems for fusion energy research such as National Ignition Facility to improve local surface planarization and to help in reducing instabilities during implosion.
  • FIG. 15 is a simplified flowchart illustrating a method of polishing an object according to an embodiment of the present invention.
  • the method 1500 includes placing the object in a container ( 1510) and placing a load bearing media different than the object in the container (1512).
  • the object can be a hollow plastic media, for instance, a hollow plastic sphere.
  • the load bearing media can include a plurality of oxide-based spheres (e.g., glass spheres) having similar size to the object, i.e., the diameter of the load bearing media is substantially equal to the diameter of the object.
  • the method includes pre-polishing the load bearing media prior to placing the load bearing media in the container with the object.
  • the pre-polished load bearing media can be characterized by a surface roughness less than 100 nm RMS.
  • the load bearing media can also be pre-polished, with the term load bearing media including pre-polished load bearing media.
  • the method also includes placing a polishing compound in the container (1514) and rotating the container about an axis of rotation ( 1516).
  • the method also includes placing a surfactant in the container.
  • the container is an acrylic material having a generally cylindrical shape. The end portions of the container are characterized by a curvature.
  • the container is characterized by an internal volume and the object, the load bearing media, and the polishing compound fill greater than 50% of the internal volume, for example, greater than 55% of the internal volume, greater than 60%o of the internal volume, greater than 65% of the internal volume, greater than 70% of the internal volume, greater than 75% of the internal volume, greater than 80% of the internal volume, greater than 85% of the internal volume.
  • the method contains sealing the container.
  • FIG. 15 provides a particular method of polishing an object according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in FIG. 15 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Finish Polishing, Edge Sharpening, And Grinding By Specific Grinding Devices (AREA)

Abstract

La présente invention concerne un procédé de polissage d'un objet consistant à placer l'objet dans un contenant et à placer un support porteur différent de l'objet dans le contenant. Le procédé consiste également à placer un composé de polissage dans le contenant et à faire tourner le contenant autour d'un axe de rotation.
PCT/US2012/049184 2011-08-02 2012-08-01 Procédé et système de polissage de sphères creuses solides WO2013019875A1 (fr)

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US20160340236A1 (en) * 2015-04-27 2016-11-24 Ford Global Technologies, Llc Surface treatment of glass bubbles
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CN110142684A (zh) * 2019-07-04 2019-08-20 中国工程物理研究院激光聚变研究中心 空心微球表面抛光装置及方法
CN110142684B (zh) * 2019-07-04 2023-12-26 中国工程物理研究院激光聚变研究中心 空心微球表面抛光装置及方法
CN111702561A (zh) * 2020-06-09 2020-09-25 石家庄新世纪胶囊有限公司 一种空心胶囊抛光工艺及胶囊抛光装置

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