Single surface annealing of glass disks

A method for annealing a glass disk is disclosed. The glass disk is placed on a base, whereby the bottom surface of the glass disk makes a contact with the base, and the top surface of the glass disk is exposed to air. The glass disk is heated with thermal energy supplied to the glass disk, the thermal energy comprising first thermal energy supplied from the air through the top surface and second thermal energy supplied from the base through the bottom surface.

FIELD OF THE INVENTION

The present invention generally relates to glass annealing and, in particular, relates to single surface annealing of glass disks.

BACKGROUND

Glass annealing is a process of slowly heating up and slowly cooling glass to relieve internal stresses after the glass is formed. Annealing of glass through slow cooling provides a homogeneous structure by reducing internal stresses to give isotropic properties. Glass which has not been annealed is prone to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by grinding, polishing and will possess a low overall strength.

During a glass annealing process, the glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at which the glass is still too hard to deform, but is soft enough for the internal stresses to relax. The glass is then allowed to heat-soak until its temperature is even throughout.

In the context of magnetic recording media manufacturing, a glass substrate in the form of a disk needs to be annealed before depositing various magnetic layers thereon. A traditional approach to performing such a glass annealing process on glass disks is by a “disk stacking annealing” process in which a stack120of glass disks122,124,126,128are provided on a thick glass base110as depicted inFIG. 1before heat is applied to the stack. In this arrangement, both top and bottom surfaces of the glass disks122,124,126,128are in contact with and receive heat from other glass disks in the stack120during the annealing process.

It has been found that such disk stacking annealing process results in defects such as diffusion, curvy scratches, ID size variations and low disk strength.

BRIEF SUMMARY OF THE INVENTION

The present disclosure addresses this and other problems by providing various systems and methods for single surface annealing of glass disks.

In certain aspects, a method for annealing a glass disk is provided. The method can comprise placing the glass disk on a base, whereby the bottom surface of the glass disk makes a contact with the base, and the top surface of the glass disk is exposed to air. The method can further comprise heating the glass disk with thermal energy supplied to the glass disk, the thermal energy comprising first thermal energy supplied from the base through the bottom surface, and second thermal energy supplied from the air through the top surface.

In certain aspects, an apparatus for annealing glass disks is provided. The apparatus can comprise a base configured to receive a glass disk thereon, whereby the bottom surface of the glass disk makes a contact with the base, and the top surface of the glass disk is exposed to air. The apparatus can further comprise a heat source configured to heat the air and the base, thereby supplying thermal energy to the glass disk.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth to provide a full understanding of the present invention. It will be apparent, however, to one ordinarily skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid unnecessarily obscuring the present invention.

FIG. 2is a diagram depicting an interior view of an oven chamber200with an arrangement designed for an exemplary single surface annealing process according to certain aspects of the subject disclosure. In the illustrated example ofFIG. 2, a base220is provided on a metal platform210. The metal platform210includes a plurality of openings205to facilitate air circulation inside the oven chamber200. In certain embodiments, the base220is a sheet of glass comprising e.g., soda lime. In some embodiments, the sheet of glass220has a thickness between about 10 and 20 mm. In some embodiments, the oven chamber200includes a computer-controlled heat source (e.g., a heating element) that allows for a precise control of temperature inside the oven chamber200. The oven chamber200can also include a fan for circulating hot air inside the chamber.

Also shown inFIG. 2are glass disks232,234,234that are placed on the glass base220. In the illustrated arrangement, the bottom surface of the glass disk (e.g.,232) is in contact with the top surface of the glass base220, while the top surface of the glass disk is exposed to air inside the oven chamber200. In the illustrated embodiment, the glass disk (e.g.,232) is an annular ring having an ID and an OD. In certain embodiments, the glass disk comprises lithium aluminum silica. In some embodiments, the glass disk has a thickness between about 0.80 and 0.82 mm.

FIG. 3is a diagram depicting another interior view of an oven chamber300with an arrangement designed for an exemplary single surface annealing process according to certain aspects of the subject disclosure. In the illustrated example ofFIG. 3, a plurality of metal platforms312,314and respective glass bases322,324placed thereon are provided inside the oven chamber300. One or more glass disks to be annealed (not shown) can be placed on each of the glass bases322,324.

FIG. 4is a diagram illustrating heat conduction and heat convection mechanisms for transferring thermal energy to a glass disk430placed on a glass base420during a single surface annealing process according to certain aspects of the subject disclosure. In the heat conduction mechanism, illustrated by the arrows401, thermal energy flows from the glass base420to the glass disk430via the physical contact made between the top surface424of the glass base420and the bottom surface432of the glass disk430. In the heat convection mechanism, illustrated by the arrows405, thermal energy flows from the hot air inside the oven chamber to the glass disk430via the top surface434of the glass disk430.

FIG. 5is a flowchart illustrating an exemplary single surface annealing process500according to certain aspects of the subject disclosure. For the sake of clarity only without the intention to limit the scope of the subject disclosure in any way, the annealing process500will be described below with references toFIGS. 2 and 4described above. The annealing process500begins at start state501and proceeds to operation510in which the glass base220is provided inside the oven chamber200. In the illustrated embodiment ofFIG. 2, the glass base220is placed on top of the metal platform210having the air openings205. The glass base220can comprise soda lime and can have a thickness between about 10 and 20 mm. In some alternative embodiments, a metal base can be used instead of the glass base210.

The annealing process500proceeds to operation520in which the glass disks232,234,236to be annealed are place on the glass base220. With reference toFIG. 4, the bottom surface432of the glass disk430makes a physical contact with the top surface424of the glass base420; and the top surface434of the glass disk430is exposed to air inside the oven chamber.

The annealing process500proceeds to “ramp-up” operation530in which the temperature inside the oven chamber is raised from a room temperature to an annealing temperature for the glass material comprising the glass disk. The annealing temperature for a glass material corresponds to a temperature at which the glass material is still too hard to deform, but is soft enough for the internal stresses to relax. In case of a lithium aluminum silica (LAS) glass disk, the annealing temperature is at or below 580 degrees Celsius. With reference toFIG. 4, during the ramp-up operation530, the glass disk430receives first thermal energy (indicated by the arrows405) supplied from the hot air through the top surface434, and second thermal energy (indicated by the arrows401) supplied from the glass base420through the bottom surface432. The ramp-up operation530continues until the temperature inside the oven chamber reaches the annealing temperature. In certain embodiments, the temperature is raised from the room temperature to the annealing temperature at a rate between about 4 and 5 degrees Celsius per minute in a duration between about 120 to 135 minutes.

The annealing process500proceeds to “heat-soaking” operation540in which the temperature inside the oven chamber is maintained at the annealing temperature for a preset amount of “soaking” time. During the soaking time, the temperature inside the glass disk430becomes uniform throughout the glass material and internal stresses in the glass material are relaxed. In certain embodiments, the soaking time lasts between about 30 and 40 minutes.

The annealing process500proceeds to “cooling” operation550in which the temperature inside the oven chamber is lowered from the annealing temperature to the room temperature. In certain embodiments, the temperature is lowered at a rate between about 120 and 165 minutes. The annealing process500ends at state509.

FIG. 6AandFIG. 6Bare diagrams illustrating respective controlled temperature profiles associated with the prior art disk stacking annealing process (labeled as “Stacking Annealing”) and the single surface annealing process (labeled as “Singular Annealing”) of the subject disclosure. As can be seen fromFIG. 6A, in the prior art disk stacking annealing process, the temperature is raised to 570 degrees Celsius, which is below the annealing temperature for the LAS glass disk, and it takes a relatively long time of about 270 minutes or above to cool the temperature of the stacked glass disks down to the room temperature. As can be seen fromFIG. 6B, in the single surface annealing process, the temperature is raised to the annealing temperature of 580 degrees Celsius for the LAS glass disk and the cooling time is substantially reduced to about 160 minutes or less.

The single surface annealing process of the subject disclosure gives the glass disk a higher bending resistance and higher overall strength. It also creates a compressive stress on the disk surface. While going through strength test (tensile stress), compressive stress counterbalances any additional tensile stress incurred.

The single surface annealing process is found to substantially reduce defects such as diffusion, curvy scratches, ID size variations and low disk strength associated with the prior art disk stacking annealing process discussed above. For example, with less disk-to-disk direct surface contact, there is no scratch that forms on the glass disk surface. By placing the glass disk individually on a thick glass base (e.g., 12 mm-thick), the heat is able to transfer from the glass base to the glass disk's bottom surface (heat conduction) while leaving the top surface exposed to the hot air in the oven chamber, thereby allowing for heat convection.

Glass disks manufactured using the single surface annealing process of the present disclosure are found to have higher bending resistance than glass disks manufactured using the prior art stacking annealing process or chemical strengthening without heat treatment. Test data shows that the single surface annealing process increases the disk strength from average of 20 kg to average 50 kg with a maximum at 80 kg.FIG. 7is a diagram that provides a comparison of the strength of a glass disk annealed using the prior art disk stacking annealing process and the strength of a glass disk annealed using the single surface annealing process of the subject disclosure. The comparison indicates a significant improvement in the disk strength resulting from the use of the single surface annealing process.

Furthermore, ID size variations are reduced from about 6.5 microns to about 4 microns. No diffusion and curvy scratches were observed in the disk surfaces. In addition, the single surface annealing process of the subject disclosure eliminates a coating process. For example, while the prior art stacking annealing process required coating with Ludox and Alumina, the single surface annealing process does not require the Alumina coating.

The description of the invention is provided to enable any person skilled in the art to practice the various embodiments described herein. While the present invention has been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention.

There may be many other ways to implement the invention. Various functions and elements described herein may be partitioned differently from those shown without departing from the spirit and scope of the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other embodiments. Thus, many changes and modifications may be made to the invention, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.

A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” The term “some” refers to one or more. Underlined and/or italicized headings and subheadings are used for convenience only, do not limit the invention, and are not referred to in connection with the interpretation of the description of the invention. All structural and functional equivalents to the elements of the various embodiments of the invention described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and intended to be encompassed by the invention. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description.