Patent Grant
8974268
1. Field
Embodiments relate generally to a method for finishing and strengthening edges of articles made of brittle materials.
2. Technical Background
Mechanical separation is an example of a method for cutting a glass sheet. Mechanical separation typically involves mechanically scoring a glass sheet to form a score line in the glass sheet and subsequently breaking the glass sheet along the score line. The mechanical scoring and breaking result in a glass sheet with a rough/sharp edge, which are are undesirable and makes the glass sheet vulnerable to cracking. Material can be removed from the rough/sharp edge in order to smoothen/dull the edge and reduce the glass sheet's vulnerability to cracking. Abrasive grinding can be used to mechanically remove material from the rough/sharp edge of the glass sheet. Abrasive grinding involves use of a metal grinding tool with micron-sized abrasive particles which may or may not be fixed on the tool to remove material. The mechanism of material removal using abrasive grinding is considered to involve fracturing. As a result, fracture sites can appear on the edge after grinding. The larger the abrasive particles used in the grinding, the larger the fracture sites that can appear on the edge after grinding. These fracture sites effectively become stress concentrations and fracture initiation sites, which result in a finished glass sheet having a lower edge strength than the initial glass sheet. Grinding tools with smaller abrasive particles and/or mechanical polishing tools can be used to reduce the size of the fracture sites. Mechanical polishing tools can be metal or polymer wheels. Mechanical polishing also involves use of abrasive particles, but the abrasive particles are not fixed on the polishing tool. A rough edge may be avoided by cutting the glass sheet by laser separation. However, a glass sheet that is cut by laser separation is typically not exempt from a sharp edge. Laser scoring produces sharp edges and corners that are highly susceptible to impact damage, therefore it is desirable to further shape finish laser scored edges. Typically, a polishing wheel made of a series of hard bound abrasives and/or a lap with loose slurry may be used to remove the sharp laser scored edge, e.g., by beveling or rounding the edge. Several polishing steps are typically needed to remove the sharp edge, which can significantly increase the cost of the finished glass sheet.
One embodiment is a method of preparing an edge-strengthened article comprising polishing an edge of an article having a first edge strength using magnetorheological finishing, wherein after the polishing the article has a second edge strength and the second edge strength is greater than the first edge strength.
Another embodiment is a magnetorheological polishing fluid comprising a liquid vehicle comprising an etching agent having a pH≦5, magnetizable particles suspended in the liquid vehicle, and abrasive particles suspended in the liquid vehicle.
Another embodiment is a magnetorheological polishing fluid comprising a liquid vehicle comprising an etching agent having a pH≧10, magnetizable particles suspended in the liquid vehicle, and abrasive particles suspended in the liquid vehicle.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention and together with the description serve to explain the principles and operation of the invention.
The invention can be understood from the following detailed description either alone or together with the accompanying drawing figures.
The following is a description of the figures in the accompanying drawings. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.
In the following detailed description, numerous specific details may be set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be clear to one skilled in the art when embodiments of the invention may be practiced without some or all of these specific details. In other instances, well-known features or processes may not be described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals may be used to identify common or similar elements.
During the polishing process 5, MRF removes damage from the surface being polished without imparting new damage to the surface—this is in contrast to mechanical processes that involve use of mechanical tools such as pads, wheels, and belts to apply abrasives to a surface for the purpose of removing material from the surface. MRF uses a fluid-based conformable tool, called a magnetorheological polishing fluid (MPF), for polishing. MPF can include micron-sized magnetizable particles and micron-sized to nano-sized abrasive particles suspended in a liquid vehicle. For example, the sizes of the magnetizable particles may be in a range from 1 μm to 100 μm or greater, for example, 1 μm to 150 μm, for example, 5 μm to 150 μm, for example, 5 μm to 100 μm, for example, 5 μm to 50 μm, for example, 5 μm to 25 μm, for example, 10 μm to 25 μm and the sizes of the abrasive particles may be in a range from 15 nm to 10 μm. The magnetizable particles may have a uniform or a non-uniform particle size distribution, the same or different shapes, and regular or irregular shapes. Also, the magnetizable particles may be made of a single magnetizable material or a combination of different magnetizable materials. Examples of magnetizable materials include iron, iron oxide, iron nitride, iron carbide, carbonyl iron, chromium dioxide, low-carbon steel, silicon steel, nickel, cobalt, and a combination of the preceding materials. The magnetizable particles may also be coated or encapsulated, for example, with or in a protective material. In one embodiment, the protective material is a material that is chemically and physically stable in the liquid vehicle and that does not react chemically with the magnetizable material. Examples of suitable protective materials include zirconia, alumina, and silica. Similarly, the abrasive particles may have a uniform or a non-uniform particle size distribution, the same or different shapes, and regular or irregular shapes. Also, the abrasive particles may be made of a single non-magnetizable material or a combination of different non-magnetizable materials. Examples of abrasive materials include cerium oxide, diamond, silicon carbide, alumina, zirconia, and a combination of the preceding materials. Other abrasive materials not specifically included in this list and known to be useful in polishing a surface may also be used. The liquid vehicle included in a MPF may be aqueous or non-aqueous. Examples of vehicles include mineral oil, synthetic oil, water, and ethylene glycol. The vehicles may further include stabilizers, e.g., stabilizers to inhibit corrosion of the magnetizable particles, and surfactants.
In another embodiment, a MPF that can etch while polishing is provided. The etching MPF includes magnetizable particles and abrasive particles suspended in a liquid vehicle including an etching agent. The etching agent is one that is capable of etching the material of the article and would be selected based on the material of the article. The liquid vehicle may further include a solvent for the etching agent. The liquid vehicle may further include stabilizers and surfactants. The liquid vehicle may be aqueous or non-aqueous, as described above. The magnetizable particles and abrasive particles are as described above for the non-etching MPF. The magnetizable particles may be coated or encapsulated, for example, with or in a protective material, as described above. The protective material, when used, is a material that is chemically and physically stable in the presence of the etching agent and other materials in the liquid vehicle. The protective material is also a material that does not react with the magnetizable particles. Suitable examples of protective materials are zirconia and silica.
In one embodiment, the etching agent included in the etching MPF has a pH less than or equal to 5. In one embodiment, the etching agent that has a pH less than or equal to 5 comprises an acid. In one embodiment, the etching agent is an acid. The acid may exist in liquid form or may be dissolved in a suitable solvent. Examples of suitable acids include, but are not limited to, hydrofluoric acid and sulfuric acid. The liquid vehicle may further include one or more stabilizers, e.g., a stabilizer to inhibit corrosion of the magnetizable particles. Stabilizers used in the liquid vehicle should be stable in the presence of the acid or, more generally, in the presence of the etching agent.
In another embodiment, the etching agent included in the etching MPF has a pH greater than or equal to 10. In one embodiment, the etching agent that has a pH greater than or equal to 10 comprised an alkali salt. In one embodiment, the etching agent is an alkali salt. Examples of such alkali salts include, but are not limited to, alkali hydroxides, e.g., potassium hydroxide, sodium hydroxide, and compounds containing alkali hydroxides. A detergent containing an alkali hydroxide may be used as the alkali salt in the liquid vehicle, for example. The liquid vehicle may include other materials besides alkali salts, such as surfactants and other materials that may be found in detergents.
MPF is deposited on a support surface in the form of a ribbon. Typically, the support surface is a moving surface, but the support surface may also be a fixed surface. The support surface may have a variety of shapes, e.g., spherical, cylindrical, or flat. For illustration purposes,
MRF removes material from the surface being polished by shearing. This is in contrast to the fracturing mechanism associated with mechanical processes such as mechanical grinding. With this mechanism, MRF has an opportunity to remove material from the edge without inducing new fracture sites in the edge that could lower the strength of the edge. Simultaneously, MRF removes defects from the edge that results in an increase in the strength of the edge, i.e., from the first edge strength to the second edge strength. Moreover, the MPF ribbon 8, which is fluid-based, has the ability to conform to the shape of the edge, no matter the complexity, e.g., in terms of curvature or profile, of the edge, which leads to complete, high-quality polishing of the edge. MRF is governed by several parameters, e.g., the viscosity of the MPF, the rate at which the MPF is delivered to the moving surface, the speed of the moving surface, the intensity of the magnetic field, the height of the MPF ribbon, the depth to which the edge is immersed into the MPF ribbon, and the rate at which material is removed from the edge.
Returning to
Abrasive machining may involve one or more and any combination of mechanical grinding, lapping, and polishing. These processes are mechanical in the sense that they involve contact between a solid tool and the surface being processed. Each of the grinding, lapping, and polishing may be accomplished in one or more steps. Grinding is a fixed-abrasive process, while lapping and polishing are loose-abrasive processes. Grinding may be accomplished using abrasive particles embedded in a metal or polymer bonded to a metal wheel. Alternatively, grinding may be accomplished using an expendable wheel made of an abrasive compound. In lapping, abrasive particles, typically suspended in a liquid medium, are disposed between a lap and an edge of an article. Relative motion between the lap and the edge of the article abrades material from the edge. In polishing, abrasive particles, typically suspended in a liquid medium, are applied to an edge of an article using a conformable soft pad or wheel. The conformable soft pad or wheel may be made of a polymeric material, e.g., butyl rubber, silicone, polyurethane, and natural rubber. Abrasives used in abrasive machining may be selected from, for example, alumina, silicon carbide, diamond, cubic boron nitride, and pumice.
An ion-exchange process is described in, for example, U.S. Pat. No. 5,674,790 (Araujo, Roger J.). The process typically occurs at an elevated temperature range that does not exceed the transition temperature of the glass. The process is carried out by immersing the glass in a molten bath comprising an alkali salt (typically a nitrate) with ions that are larger than that of the host alkali ions in the glass. The host alkali ions are exchanged for the larger alkali ions. For example, a glass containing Na+ may be immersed in a bath of molten potassium nitrate (KNO3). The larger K+ present in the molten bath will replace the smaller Na+ in the glass. The presence of the larger alkali ions at sites formerly occupied by small alkali ions creates a compressive stress at or near the surface of the glass and tension in the interior of the glass. The glass is removed from the molten bath and cooled down after the ion-exchange process. The ion-exchange depth, i.e., the penetration depth of the invading larger alkali ions into the glass, is typically on the order of 20 μm to 300 μm, for example, 40 μm to 300 μm and is controlled by the glass composition and immersion time.
The following examples are presented for illustration purposes only and are not intended to be construed as limiting the invention as otherwise described above.
A two-step edging process comprised mechanical lapping by hand, followed by mechanical polishing with 10-μm alumina particles for a total of 1 minute.
A two-step edging process comprised mechanical grinding with 800 grit diamond particles, followed by mechanical grinding with 3000 grit diamond particles.
A three-step edging process comprised mechanical grinding with 800 grit diamond particles, followed mechanical grinding with 3000 grit diamond particles, followed by mechanical polishing with 10-μm alumina particles.
A four-step edging process comprised mechanical grinding with 400 grit diamond particles, followed by mechanical grinding with 800 grit diamond particles, followed by mechanical grinding with 1500 grit diamond particles, followed by 3000 grit mechanical grinding for a total of 17 minutes.
A five-step edging process comprised mechanical grinding with 400 grit diamond particles, followed by mechanical grinding with 800 grit diamond particles, followed by mechanical grinding with 1500 grit diamond particles, followed by 3000 grit mechanical grinding, followed by mechanical polishing with 10-μm alumina particles.
A polishing process comprised a MRF process using a MPF having a viscosity of 44-45 centipoise and containing carbonyl iron particles and cerium oxide particles suspended in a liquid medium. Other process parameters included: MRF wheel speed at 259 rpm, electromagnet current setting at 18 amperes, ribbon height of 1.5 mm, and edge immersion depth of 0.5 mm to 0.75 mm. Material removal using the MRF was approximately 0.5 μm/side material removal.
A polishing process comprised a MRF process using MPF having a viscosity of 44-45 centipoise and containing carbonyl iron particles and diamond particles suspended in a liquid medium. Other process parameters include: MRF wheel speed at 259 rpm, electromagnet current setting at 18 amperes, ribbon height of 1.5 mm, and edge immersion depth of 0.5 mm to 0.75 mm. Material removal using the MRF was approximately 0.5 μm/side material removal.
A commercially-available ion-exchanged glass sheet was cut by laser separation. Each as cut glass sheet had a size of 60.75 mm×44.75. Each resulting glass sheet after mechanical grinding and prior to MRF had a size of 60 mm×44 mm. The edge strength of each glass sheet after cutting by laser separation was on average in a range from 600 MPa to 900 MPa. The glass sheets were subjected to an edging process according to Example 5. The edge strength of each glass sheet after edging (i.e., first edge strength) was on average in a range from 242 MPa to 299 MPa. After edging, the glass sheets were polished using MRF according to Example 6 for 1, 5, or 15 minutes. The edge strengths of the glass sheets after MRF (i.e., second edge strengths) are reported in Table 1 below. Edge strengths were measured by a horizontal 4-point bend. The results show that MRF improves the edge strengths of the glass sheets.
A commercially-available ion-exchanged glass sheet was cut to glass sheets by laser cutting. Each as cut glass sheet had a size of 60.75 mm×44.75. Each resulting glass sheet after mechanical grinding and prior to MRF had a size of 60 mm×44 mm. The edge strength of each glass sheet after laser cutting was on average in a range from 600 MPa to 900 MPa. The glass sheets were subjected to an edge process according to Example 4. After edging, the small glass sheets were polished using MRF according to Example 7. The edge strengths of the glass sheets after abrasive machining and after MRF are reported in Table 2 below. Edge strengths were measured by a horizontal 4-point bend. Again, the edge strengths improved after MRF for the glass sheets.
A commercially-available ion-exchanged glass sheet was cut by mechanical separation. The resulting glass sheets were subjected to an edging process according to Example 4. After edging, the glass sheets were polished using MRF according to Example 7. The edge strengths of the glass sheets after edging and after MRF are reported in Table 3 below. Edge strengths were measured by a horizontal 4-point bend. As in the previous examples, the edges strengths were improved after MRF.
A commercially-available ion-exchanged glass sheet was cut by laser separation. The resulting glass sheets were subjected to an edging process according to Example 1. After the edging process, the glass sheets were polished using MRF according to Example 7. The edge strengths of the glass sheets after edging and after MRF are reported separately in Table 4 below. Edge strengths were measured by a horizontal 4-point bend.
A commercially-available ion-exchanged glass sheet was cut by laser separation. The resulting glass sheets were subjected to an edging process according to Example 3. After edging, the glass sheets were polished using MRF according to Example 7. The edge strengths of the glass sheets after edging and after MRF are reported separately in Table 5 below. Edge strengths were measured by a horizontal 4-point bend.
A commercially-available ion-exchanged glass sheet was cut by laser separation. The resulting glass sheets were subjected to an edging process according to Example 2. After edging process, the glass sheets were polished using MRF according to Example 7. The edge strengths of the glass sheets after edging and after MRF are reported separately in Table 6 below. Edge strengths were measured by a horizontal 4-point bend.
A commercially-available ion-exchanged glass sheet was cut by laser separation. After laser separation, the cut glass sheets were polished using MRF according to Example 7. The edge strengths of the glass sheets after laser separation and after MRF are reported separately in Table 7 below. Edge strengths were measured by horizontal 4-point bend.
When a negative effect after MRF is observed, the likely explanation is as follows: MRF is very likely providing a positive effect or no effect after any prior mechanical edge process. The samples used to determine strength before MRF processing were destructively analyzed using 4-point bend. Those samples then represent the strength of subsequent samples before being processed with the MRF. It is very possible that strength variation before the MRF step within the same lot of samples, could result in a lower unmeasured strength before MRF, subsequently a lower strength after the MRF step.
MRF edges were produced as shown by data 22 in
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/358,611 filed on Jun. 25, 2010 the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3843472 | Toussaint et al. | Oct 1974 | A |
| 4911743 | Bagby | Mar 1990 | A |
| 5128281 | Dyer et al. | Jul 1992 | A |
| 5578238 | Weiss et al. | Nov 1996 | A |
| 5616066 | Jacobs et al. | Apr 1997 | A |
| 5674790 | Araujo | Oct 1997 | A |
| 5795212 | Jacobs et al. | Aug 1998 | A |
| 6106380 | Jacobs et al. | Aug 2000 | A |
| 6325704 | Brown et al. | Dec 2001 | B1 |
| 6402978 | Levin | Jun 2002 | B1 |
| 6761747 | Gottfried et al. | Jul 2004 | B2 |
| 6921455 | Nakano et al. | Jul 2005 | B1 |
| 6991521 | Hagan et al. | Jan 2006 | B2 |
| 7217662 | Toyota et al. | May 2007 | B2 |
| 20020042244 | Trommer | Apr 2002 | A1 |
| 20020081943 | Hendron et al. | Jun 2002 | A1 |
| 20060216552 | Ikenishi et al. | Sep 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| WO 9404313 | Mar 1994 | WO |
| Entry |
|---|
| J. DeGroote et al., “Removal rate model for magnetorheological finishing of glass”, Applied Optics, 46(32): 7927-7941. |
| M. Fox et al., “Magnetic abrasive finishing of rollers”, Ann. CIRP 43 (1994). |
| Genc et al., “Rheological Properties of Magnetorheological Fluids”, Smart Materials and Structures, 11:140-146 (2002). |
| Jacobs et al., “Magnetorheogical Finishing of IR Materials”, Proceedings SPIE 3334:258-269 (1997). |
| Jang et al., “Role of organic coating on carbonyl iron suspended particles in magnetorheological fluids”, Jnl of Appled Physics: 97(10Q912) (2005). |
| Khairy, Ahmed B.1, “Aspects of surface and edge finish by magnetoabrasive particles”, Journal of Materials Processing Technology, v 116, n 1, p. 77-83, Oct. 3, 2001. |
| Kittipoomwong et al., “Dynamic yield stress enhancement in bidisperse magnetorheological fluids” J. Rheology, 49(6) (2005). |
| Kobayashi et al. “Precision Treatment of Silicon Wafer Edge Utilizing Ultrasonically Assisted Polishing Technique”, Journal of Materials Processing Technology, 201:531-535 (2008). |
| Kozhinova et al., Minimizing artifact formation in magnetorheological finishing of chemical vapor deposition ZnS flats, Applied Optics, 44(22): 4671-4677. |
| Lambropoulos et al., “Manufacturing-induced residual stresses in optical glasses and crystals: Example of residual stress relief by magnetorheological finishing (MRF) in commercial silicon wafers”, Proceedings SPIE, 4451: 181-190 (2001). |
| Pu et al., “Studies on preparation and chemical stability of reduced iron particles encapsulated with polysiloxane nano-films”, Materials Letters 60:94-97 (2006). |
| Randi et al., “Subsurface damage in some single crystalline optical materials”, Applied Optics, 44(12):2241-2249 (2005). |
| Shafrir et al., “Zirconia-coated carbonyl-iron-particle-based magnetorheological fluid for polishing optical glasses and ceramics”, Applied Optics, 48(35):6797-6810. |
| Shen et al., “Synthesis and corrosion study of zirconia-coated coarbonyl iron particles”, Jnl of Colloid and Interface Science, 342:49-56 (2010). |
| Shorey et al., “Magnetorheological finishing of large and lightweight optics”, Proceedings SPIE, 5533: 99-107 (2004). |
| A.B. Shorey, “Mechanisms of Material Removal in Magnetorheological Finishing (MRF) of Glass”, PhD Dissertation, University of Rochester, May 2000. |
| Cheng, Hai Bin et al., Preparation of composite magnetic particles and aqueous magnetorheological fluids, Smart Materials and Structures, IOP Publishing, Ltd., Bristol, GB, vol. 18, No. 8, Aug. 1, 2009, pp. 1-6. |
| Kordonski, W.I., et al., Fundamentals of Magnetorhelogical Fluid Utilization in High Precision Finishing, Journal of intelligent Material Systems and Structures, Technomic Publishing, Lancaster, PA US., vol. 10, No. 9, Sep. 1, 1999, pp. 683-689. |
| International Search Report and Written Opinion dated Sep. 29, 2011 pertaining to International Application PCT/US2001/041586, International Filing Date Jun. 23, 2011. |
| Number | Date | Country | |
|---|---|---|---|
| 20110318994 A1 | Dec 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61358611 | Jun 2010 | US |
