JP4556962B2 - Method for reducing residual stress of blank material, method for manufacturing blank material and glass substrate for information recording medium - Google Patents

Description

The present invention relates to a method for reducing a residual stress of a blank material , a method for manufacturing a blank material, and a glass substrate for an information recording medium .

  Among information recording media having a recording layer utilizing properties such as magnetism, light, and magnetomagnetism, a typical example is a magnetic disk. Conventionally, aluminum substrates have been widely used as magnetic disk substrates. However, in recent years, with the demand for reducing the flying height of the magnetic head for improving the recording density, the proportion of using a glass substrate as a magnetic disk substrate has increased. The reason for the increased use of glass substrates is that the glass substrate has better surface smoothness than the aluminum substrate and has fewer surface defects, so that the flying height of the magnetic head can be reduced.

  Such a glass substrate for a magnetic disk or the like is manufactured by subjecting a blank material, which is a glass substrate precursor, to polishing. As a glass plate used as a blank material, a method of manufacturing by press molding, a method of manufacturing by cutting a glass plate manufactured by a float method or the like is known. Among these methods, a method of producing a glass plate by directly press-molding molten glass is attracting attention because it can be expected to have particularly high productivity.

  For example, when manufacturing a glass substrate for a 2.5-inch magnetic disk, a disk-shaped glass plate having a thickness of about 1.2 mm to 1.5 mm is manufactured by press molding, and then the glass plate is heat treated. And the residual stress which a glass plate has is eased, and it is set as the blank material. In general, when a glass product is manufactured, heat treatment is performed in order to remove residual stress in which tensile stress or compressive stress exists in the glass. If the residual stress remains inside the glass, it will be deformed during polishing, or the fracture strength will be significantly reduced, and scratches on the surface governing the glass strength may grow spontaneously, leading to the destruction. The mechanical reliability of the product is significantly impaired (see Non-Patent Document 1).

  A hole is made in the center of the blank material in which the residual stress is relieved, or both surfaces are ground and polished to finish the blank material into a glass substrate for information recording media having a thickness of 0.635 mm, for example.

  In order to finish from a blank material to a glass substrate for an information recording medium, about half of the thickness is removed by grinding or polishing as described above. For this reason, even if there is a state in which the blank material is warped by the heat treatment, a desired flatness can be obtained because a processing allowance by grinding or polishing sufficient to remove the warp can be taken.

  On the other hand, since a great amount of time is spent on these grinding and polishing processes, it is conceivable to produce a thin blank material in order to perform such processes efficiently. However, the thin blank material has a large warp compared to the blank material of the conventional thickness, and the thickness of the processing allowance for correcting the warp cannot be sufficiently secured, and the glass for the information recording medium having a good flatness. There was a problem that the substrate could not be obtained.

In order to solve the above problem, there is a method of obtaining a blank material by correcting the warpage of the glass plate by sandwiching and heating a glass plate as a blank material between two flat plates. (For example, see Patent Document 1 and Patent Document 2)
"Glass Optics Handbook" First Edition, Asakura Shoten, issued July 5, 1999, page 379 JP-A-9-102125 JP 2002-87835 A

When manufacturing the glass substrate for information recording media in Patent Documents 1 and 2, the blank material provided for grinding or polishing is crystallized glass. In other words, the amorphous glass plate obtained by press molding or the like is sandwiched between the plates and heat-treated to correct the warp, and the amorphous glass is made into crystallized glass. A glass substrate for an information recording medium is obtained by subjecting the crystallized glass to grinding or polishing.

  On the other hand, the glass substrate for information recording media to be manufactured by the present inventors is not a crystallized glass but a glass substrate obtained by chemically strengthening amorphous glass.

  Taking a glass plate with a thickness of 1 mm obtained by the inventors as an example, in a blank material obtained by heat treatment sandwiched between plates to correct the warp, the warp is also corrected. Regardless, there were cases where the residual stress was not sufficiently reduced. When a glass substrate for an information recording medium is manufactured using a blank material in which the residual stress is not reduced, the information recording medium glass substrate cannot be efficiently manufactured due to new deformation or destruction such as warpage. There was a problem that a case would occur. The inventors speculate that the cause of this problem is due to the difference between the blank material being crystallized glass or amorphous glass.

  The present invention has been made in view of the above problems, and the object of the present invention is to provide a glass substrate for an information recording medium that is not deformed or broken by residual stress in the manufacturing process of the glass substrate for an information recording medium. It is to provide a method and a blank material for reducing the residual stress of a blank material provided for manufacturing.

  Said subject is solved by the following structures.

1. The molded article of amorphous glass is press-molded, viewed one by one clamping between the plates of the state of being opposed to the flat surface on the main surface the whole area of the molded product, crystallization or soften the laminate stacked without bonding In the method of reducing the residual stress of the blank material to be a glass substrate for information recording medium having a heat treatment step of correcting the warp of the glass substrate by heat treatment in a temperature range that does not,
The method for reducing residual stress of a blank material, wherein the thermal expansion coefficient As of the material forming the plate and the thermal expansion coefficient Ab of the material forming the molded body satisfy the following conditional expression:
0.80 ≦ As / Ab ≦ 1.20
2. A blank material in which residual stress is reduced by the method for reducing residual stress of the blank material according to 1 above , wherein a retardation in the thickness direction of the blank material is 50 nm / 10 mm or less. Wood.
3. 2. The method for reducing residual stress of a blank material according to 1 above, wherein, in the heat treatment step, the molded body is heated to a temperature between a glass transition point and a yield point of the glass material forming the molded body. .
4). Press molded amorphous glass compacts are sandwiched one by one between plates with a flat surface facing the entire main surface of the compact, and the stacked stack without bonding is not crystallized or softened. Having a heat treatment step of correcting warpage of the glass substrate by heat treatment in a temperature range;
In the manufacturing method of manufacturing the glass substrate for information recording medium by processing the blank material obtained in the heat treatment step,
The method for producing a glass substrate for an information recording medium, wherein the thermal expansion coefficient As of the material forming the plate and the thermal expansion coefficient Ab of the material forming the molded body satisfy the following conditional expression.
0.80 ≦ As / Ab ≦ 1.20
5. 5. The information recording medium according to 4, wherein the molded body and the plate are both disk-shaped, and the diameter of the plate is 1.01 to 1.2 times the diameter of the molded body. Method for manufacturing glass substrate.
6). 6. The method for producing a glass substrate for an information recording medium according to 4 or 5, wherein the thickness of the plate is 0.5 to 3.5 times the thickness of the molded body.
7). 7. The information recording according to any one of 4 to 6, wherein, in the heat treatment step, the molded body is heated to a temperature between a glass transition point and a yield point of the glass material forming the molded body. A method for producing a glass substrate for a medium.

  According to the present invention, in the step of heat treatment in a temperature range in which a molded body of amorphous glass is sandwiched between flat plates and not crystallized or softened, the range of thermal expansion coefficient of the plates sandwiching the molded body is molded. It is set to 0.8 to 1.2 times the thermal expansion coefficient of the body. Therefore, the stress which arises in the molded object by the difference of the thermal expansion coefficient between the molded object and board which are heat-processed is suppressed, and the blank material which could reduce the residual stress after heat processing can be obtained.

Accordingly, a method for reducing residual stress of a blank material provided for manufacturing a glass substrate for information recording medium that does not cause deformation or breakage due to residual stress in the manufacturing process of the glass substrate for information recording medium , the blank material, and the information recording medium A method for producing a glass substrate can be provided.

  Although the present invention will be described based on the illustrated embodiment, the present invention is not limited to the embodiment.

  The present invention relates to a method for reducing the residual stress of a blank material that is subjected to grinding or polishing and becomes a glass substrate for an information recording medium, which is a final product.

  A flow for manufacturing the blank is shown in FIG. First, glass in a molten state as a base material for a blank material is prepared (glass melting step), and is formed into a disk shape close to a target shape by a press molding method (press molding step). Hereinafter, the amorphous glass plate press-molded into a disk shape is referred to as a molded body. The blank is obtained by heat-treating this molded body in a temperature range that does not crystallize or soften (heat treatment step).

  More specifically, as follows. A material (glass material) necessary for constituting glass is mixed, heated to a high temperature of about 1500 ° C. and melted to obtain a molten glass (glass melting step). Next, a lower mold and an upper mold having a flat molding surface are prepared. After the lower mold and the upper mold are heated to a predetermined temperature, an appropriate amount of molten glass for supplying a blank material is supplied to the lower mold. To do. Thereafter, the molten glass supplied to the lower mold is pressed with the upper mold so as to be sandwiched between the lower mold and the molten glass is press-molded to obtain a molded body (press molding process).

  The thickness of the molded body is, for example, a 2.5 inch diameter magnetic disk glass substrate with a thickness of 0.635 mm in order to reduce the labor of grinding and polishing to obtain a glass substrate for information recording media. It is preferable that the thickness is about 0.7 mm to 1 mm. The molded body has a residual stress and may be warped. The appearance of the molded body is shown in FIG. In FIG. 1, 1 is a molded body and 3 is a main surface. The main surface 3 is front and back, but need not be distinguished.

Next, the molded body is heat treated to reduce the residual stress of the molded body and correct the warp (heat treatment step). Thus, a favorable blank material can be obtained for the production of a glass substrate for an information recording medium. The blank material obtained by subjecting the molded body 1 to heat treatment is then provided with a hole in the center, and the main surface 3 is ground and polished so that the main surface is flattened and smoothed. It can be set as the glass substrate for media. The state of the glass substrate for information recording media is shown in FIG. Reference numeral 24 denotes a hole, 26 denotes an inner peripheral end face, 28 denotes an outer peripheral end face, and 22 denotes a main surface in a mirror state after being polished. Further, as shown in FIG. 3, by providing a recording layer 30 directly or indirectly on one main surface 22 of the information recording medium glass substrate 20, information provided with the information recording medium glass substrate 20 and the recording layer 30 is provided. The recording medium D can be obtained. The recording layer 30 may be provided on both surfaces of the information recording medium glass substrate 20.

In the blank material according to the present invention, a molded body by press molding having a shape approximate to the final product is prepared in advance, and the molded body is heat-treated to reduce the residual stress along with the correction of the warp. Since the present invention is for efficiently producing a blank material provided for the production of a glass substrate for an information recording medium, heat treatment for correcting warpage and reducing residual stress is performed on a plurality of molded bodies at once. Preferably it is done.

  The compacts are sandwiched one by one between spacers that are flat plates to form a laminate in which the compacts and the spacers are alternately stacked, and this laminate is heat treated. FIG. 5A shows the state of this laminated body, and FIG. 5B shows a state in which the molded body and the spacer are separated in order to explain the structure of the laminated body. Reference numeral 1 denotes a molded body (a blank material after heat treatment), and S denotes a spacer. The material of the spacer S is preferably a material that has sufficient heat resistance at the temperature exposed by heat treatment, does not deform, and does not fuse with the molded body 1.

As shown in FIG. 1, the molded body 1 is substantially disk-shaped by press molding. The spacer S shown in FIG. 5 is preferably disk-shaped in accordance with the shape of the molded body 1, and the diameter of the spacer S is in the range of 1.01 to 1.2 times the diameter of the molded body 1. preferable. By setting the diameter of the spacer S in the above range, the flat surface S1 of the spacer S can be opposed to the entire main surface 3 of the molded body 1, so that warpage occurs over the entire main surface 3 of the molded body 1. Can be corrected. Further, since the spacer S is not larger than necessary with respect to the molded body 1, it is possible to improve the thermal efficiency during heating and cooling, and further when the laminated body B is placed in an electric furnace for heat treatment, for example. This is preferable because the necessary space can be reduced.

As for the spacers S, n + 1 are prepared for the number n (n is an integer of 1 or more) of the molded bodies 1 to be heat-treated. As a result, the spacer S can be placed on the bottom, and then the molded body 1 and then the spacer S can be alternately stacked one after another to form a stacked body B with the top as the spacer S. Accordingly, since the flat surfaces S1 of the spacers S can be opposed to both the main surfaces 3 of all the molded bodies 1 constituting the laminate B, the warpage of the main surface 3 of the molded body 1 can be corrected. .

  When the molded body 1 is heat-treated, it is preferable to make the temperature distribution in the space where the laminate B is heated uniform. In order to place the laminated body B composed of as many spacers S and molded bodies 1 as possible in a space having a uniform temperature distribution, it is preferable that the spacers S have a small thickness. However, if the spacer S is too thin, deformation such as warpage of the molded body 1 cannot be sufficiently corrected. On the other hand, if it is too thick, depending on the arrangement of the laminate B and the heat source for heating, for example, if the heat source is located above and below the laminate, the heat does not sufficiently conduct to the central portion of the laminate B and the upper and lower portions. As a result, a temperature gradient occurs in the central portion, and sufficient heat treatment cannot be performed in the central portion.

  Therefore, the thickness of the spacer S is preferably in the range of 0.5 to 3.5 times the thickness of the molded body 1. By setting the thickness of the spacer S in the above range, the residual stress can be reduced and the deformation can be corrected sufficiently by sufficiently heat-treating the molded body 1 constituting the laminated body B. It is preferable that the material forming the spacer S has a high thermal conductivity so that heat can be sufficiently conducted to the central portion of the stacked body B. This will be described later.

The material forming the spacer S satisfies the following conditional expression (1), where Ab is the thermal expansion coefficient As of the glass material forming the molded body 1.
0.80 ≦ As / Ab ≦ 1.20 (1)
To correct the warpage in the molded body 1 and a spacer S constituting the laminate B, and applying a load in the range from 0.005 g / mm ² of 0.5 g / mm ^2. If it is less than 0.005 g / mm ² , the warp cannot be sufficiently corrected, and if it exceeds 0.5 g / mm ² , there is a concern that the warp may be excessive and the spacer or the molded body may be cracked. Is done. This load, the contact surface between the spacer S formed body 1 caused friction, the spacer S and the molded body 1 is not easily move relative to a direction parallel to the contact surface. As the heat treatment proceeds, the warpage is corrected, so that the contact area between the spacer S and the molded body 1 becomes larger and the friction becomes larger.

When heat treatment is performed, a temperature difference occurs between the outer peripheral portion and the central portion of the molded body 1 during the heating / cooling process. In particular, in the cooling process, when the difference in the coefficient of thermal expansion between the spacer S and the molded body 1 is large, a large stress is generated in the molded body 1 due to the difference in shrinkage between the two, and this stress remains after the heat treatment. End up. Therefore, the spacer S made of a material having a thermal expansion coefficient that satisfies the conditional expression (1) is selected, and the difference in the thermal expansion coefficient between the molded body 1 and the spacer S is limited, thereby existing in the molded body 1 after the heat treatment. Residual stress can be suppressed.

When the glass material forming the formed body 1 is aluminosilicate glass, the thermal expansion coefficient is 3.4 × 10 ⁻⁶ / K. For the aluminosilicate glass, examples of the spacer material satisfying the conditional expression (1) include silicon nitride (3.4 × 10 ⁻⁶ / K) and silicon carbide (4.0 × 10 ⁻⁶ / K). Can be mentioned. When the glass material is MEL3 (manufactured by Konica Minolta Opto Co., Ltd.), the coefficient of thermal expansion is 7.5 × 10 ⁻⁶ / K, and the spacer material satisfying the conditional expression (1) is alumina. (Thermal expansion coefficient: 6 to 7 × 10 ⁻⁶ / K) and steatite (thermal expansion coefficient: 7.4 × 10 ⁻⁶ / K). In addition, the value of the thermal expansion coefficient mentioned above and shown below is a value at normal temperature to 400 ° C. described in a document presented by a manufacturing company or the like.

  When a blank material whose residual stress is reduced by heat-treating the molded body 1 in this way is supplied to the manufacturing process of the glass substrate for information recording media, a product that does not deform or break in the manufacturing process, or does not have a decrease in breaking strength, That is, the reliability of the glass substrate for information recording media and the magnetic recording media using this glass substrate are not impaired. The residual stress in such a preferable blank material can be evaluated by measuring a retardation, which is an optical path difference between polarized components orthogonal to each other due to birefringence, using a polarizing microscope. This residual stress preferably has a retardation of 50 nm / 10 mm or less when the thickness of the blank material as a sample is converted to 10 mm. By setting the retardation to 50 nm / 10 mm or less, a blank material with sufficiently reduced residual stress can be supplied to the step of manufacturing the glass substrate for information recording medium, and the glass substrate for information recording medium is efficiently manufactured. be able to.

Furthermore, the spacer S preferably has a thermal conductivity larger than that of the molded body 1 so that heat can be sufficiently transferred to the molded body 1 constituting the laminated body B. The thermal conductivity at room temperature of the glass forming the molded body 1 is, for example, 1.2 W / (m · K) for aluminosilicate glass and 1.11 W / (m · K) for MEL3. The spacer materials mentioned above have a thermal expansion coefficient of silicon nitride (90 W / (m · K)), silicon carbide (100 to 300 W / (m · K)), alumina (Al ₂ O ₃ : 15 to 40 W /). (M · K)) and steatite (MgO · SiO ₂ : 2 W / (m · K)). Note that the thermal conductivity listed above is a representative value.

  The flat surfaces S1 of the spacers S are preferably parallel to each other, the flatness of the flat surface S1 is 10 μm or less, and the surface roughness Rmax (maximum height) is preferably in the range of 0.01 μm to 5 μm. Since the flat surfaces S1 of the spacers are parallel to each other, the stacked body B formed by stacking with the molded body 1 can be stabilized. Further, by setting the flatness to 10 μm or less, it is possible to keep the warp of the blank material 1 within a range that can be sufficiently removed in the manufacturing process of the glass substrate for information recording medium. Furthermore, by setting the surface roughness Rmax in the range of 0.01 μm to 5 μm, the heat-treated molded body 1 can be easily taken out without being fixed to the spacer S, and grinding processing in the subsequent grinding process can be performed. It can be done efficiently.

  The height of the laminated body B composed of the spacer S and the molded body 1 is such that the load on the molded body 1 at the lower stage of the laminated body B is not excessive, and stability can be maintained without the laminated body B collapsing. What is necessary is just to determine suitably by the height of heating spaces, such as a heating furnace. Although it depends on the size of the heating furnace, the number of stacking stages is 10 to 30 (one spacer and one molded body) from the viewpoint of production efficiency, stability of the stacked state of the stacked body B, and heat uniformity in the stacked body B. The range of 1) is preferred.

  In the manufacturing method of a blank material, the laminated body B which consists of the spacer S and the molded object 1 is heat-processed as 1 set. It is preferable from the viewpoint of manufacturing efficiency that the plurality of laminated bodies B are sequentially heat-treated by moving them in a heating furnace set at a predetermined temperature with a belt or a roller. Moreover, when moving a laminated body with a belt, a roller, etc. in order to heat-process continuously, you may use the jig | tool for improving the stability of the laminated body B (especially collapse by the vibration etc. during a transfer). However, if an excessive jig having a large amount of heat is used, heat is taken away by the jig, and the original heat treatment cannot be sufficiently performed on the molded body 1.

  The temperature range in which the laminated body B is heated in the heat treatment step is set to a temperature range in which crystallization or softening does not occur. Furthermore, it is preferably between the glass transition point Tg and the yield point At of the glass material forming the molded body 1 constituting the laminate B, and more preferably glass transition point Tg + 10 ° C. + 30 ° C. By maintaining the temperature within this temperature range for a certain period of time, the warping of the molded body 1 can be corrected in a moderately soft state and within a range where the shape of the molded body does not break down, and the residual stress can be efficiently reduced. can do. The temperature holding state may be constant within the previous temperature range, or may have a slope that rises or falls over time. Further, the time for maintaining the above temperature range during the heat treatment may be appropriately determined depending on the state of residual stress reduction or the state of correction of the warp of the molded body 1, but may be about 2 to 3 hours. The temperature gradient during heating / cooling is preferably in the range of 1 ° C./min to 20 ° C./min. By setting it within this range, the molded body 1 and the spacer S due to thermal shock are not damaged, heating and cooling can be performed efficiently, and the residual stress of the blank can be efficiently reduced.

(Manufacturing process of glass substrate for information recording medium)
The manufacturing method of the glass substrate for information recording media is demonstrated using the flowchart of FIG. In the blank material that has undergone the heat treatment described above, if necessary, a hole is made in the center with a core drill or the like (coring step). In the coring step, it is preferable from the viewpoint of manufacturing efficiency that the blank material is laminated. Since the warpage of the blank material is corrected by the above heat treatment, the blank material can be stacked and fixed in a stable state, and the drilling process by the core drill can be performed efficiently. Henceforth, the blank material which a process advances is called a glass substrate.

  Next, both surfaces of the glass substrate are polished to preliminarily adjust the overall shape of the glass substrate, that is, the parallelism, flatness, and thickness of the glass substrate (first lapping step). Next, after grinding and chamfering the outer peripheral end surface and inner peripheral end surface of the glass substrate, and finely adjusting the outer diameter and roundness of the glass substrate, the inner diameter of the hole, and the concentricity between the glass substrate and the hole ( Inner / outer diameter processing step), and polishing the inner peripheral end surface of the glass substrate to remove fine scratches (inner peripheral end surface processing step).

  Next, both surfaces of the glass substrate are polished again to finely adjust the parallelism, flatness and thickness of the glass substrate (second lapping step). And the outer periphery end surface of a glass substrate is grind | polished and a fine crack etc. are removed (outer periphery end surface processing process).

  Next, after washing the glass substrate, the glass substrate is immersed in a chemical strengthening solution to form a chemically strengthened layer on the glass substrate (chemical strengthening step). The chemical strengthening method in the chemical strengthening step is not particularly limited as long as it is a conventionally known chemical strengthening method. For example, low-temperature type chemistry in which ion exchange is performed in a temperature range not exceeding the glass transition point Tg from the viewpoint of the glass transition point Tg. Strengthening is preferred. Examples of the alkali molten salt used for chemical strengthening include potassium nitrate, sodium nitrate, and nitrates obtained by mixing them. Thereafter, polishing is performed to precisely finish the surface of the glass substrate (polishing process). And the glass substrate 20 for information recording media shown in FIG. 2 as a product is obtained through a washing | cleaning process and an inspection process.

  In the manufacturing process of the information recording medium glass substrate, in order to obtain a 2.5 inch diameter information recording medium glass substrate, assuming that the thickness of the blank is 1 mm, about 0.7 mm at the end of the second lapping process. It becomes a predetermined 0.635 mm at the end of the polishing process. By introducing a blank material that has undergone a heat treatment to reduce residual stress into this manufacturing process, the glass substrate will not be warped or deformed in the lapping process or polishing process, and will also be damaged during heating in the chemical strengthening process. There is nothing to do. Therefore, a good glass substrate for information recording media can be produced efficiently.

Example 1
A circular shaped body was prepared by press molding the melted MEL3. The molded body had a diameter of about 67 mm and a thickness of about 0.9 mm.

The thermal expansion coefficient of MEL3 was 7.5 × 10 ⁻⁶ / K, and the thermal conductivity was 1.1 W / (m · K).

Spacers were prepared for sandwiching the molded body to heat-treat. As the spacer material, alumina AP960 (thermal expansion coefficient: 7.3 × 10 ⁻⁶ / K, thermal conductivity coefficient: 25 W / (m · K)) was used. The size of the spacer was 70 mm in diameter and 1 mm in thickness. The flatness of the spacer was 10 μm or less, and the surface roughness Rmax (maximum height) was in the range of 0.5 μm to 1.2 μm. In addition, the surf coder SEF3500 (made by Kosaka Laboratory) was used for the measurement of flatness and surface roughness.

As shown in FIG. 5, the molded body and the spacer prepared above are the spacer S at the bottom, and then the molded body 1, and this is repeated 20 times in order to make the combination of one spacer and one molded body one stage. Six sets of 20 layers of laminates B were prepared. A spacer S was placed on the top. Further, the load applied to the molded body 1 was adjusted to the range of 0.005 g / mm ² of 0.5 g / mm ^2.

  Then, it heated up to 500 degreeC which is the glass transition point Tg of MEL3 at a temperature increase rate of 5 degree-C / min, and then hold | maintained 500 degreeC for 3 hours, Then, it cooled at 5 degree-C / min.

  After cooling to room temperature, the blank material 1 was taken out from the laminated body B. At this time, the spacer S and the blank material 1 were not fixed, and the blank material 1 could be easily taken out.

  As a result of measuring the retardation of the blank using a polarizing microscope, all the blanks were in the range of 23 nm / 10 mm to 32 nm / 10 mm. Moreover, although the flatness (warpage) of the molded body exceeded 20 μm, it was 10 μm or less in the blank material after the heat treatment. Therefore, it was confirmed that a blank material in which the residual stress was sufficiently reduced and the warpage was corrected could be obtained.

(Example 2)
A molded body made of MEL3 as in Example 1 except that alumina AP90T (thermal expansion coefficient: 6.2 × 10 ⁻⁶ / K, thermal conductivity: 15 W / (m · K)) was used as the spacer material. Was heat treated.

  As a result of measuring the retardation of the blank using a polarizing microscope, all the blanks were in the range of 31 nm / 10 mm to 44 nm / 10 mm. Moreover, although the flatness (warpage) of the molded body exceeded 20 μm, it was 10 μm or less in the blank material after the heat treatment. Therefore, it was confirmed that a blank material in which the residual stress was sufficiently reduced and the warpage was corrected could be obtained.

(Comparative Example 1)
A molded body made of MEL3 as in Example 1 except that aluminum nitride (thermal expansion coefficient: 4.5 × 10 ⁻⁶ / K, thermal conductivity: 170 W / (m · K)) is used as the spacer material. Was heat treated.

  After cooling to room temperature, the retardation was measured using a polarizing microscope. As a result, all blank materials were in the range of 59 nm / 10 mm to 72 nm / 10 mm, and did not become 50 nm / 10 mm or less. The flatness (warpage) before heat treatment exceeded 20 μm, but after heat treatment was 10 μm or less. Therefore, although the warp has been corrected, the residual stress could not be reduced sufficiently.

(Comparative Example 2)
A molded body made of MEL3 as in Example 1 except that forsterite (thermal expansion coefficient: 9.5 × 10 ⁻⁶ / K, thermal conductivity: 3 W / (m · K)) is used as the spacer material. Was heat treated.

  After cooling to room temperature, the retardation was measured using a polarizing microscope. As a result, all blank materials were in the range of 56 nm / 10 mm to 67 nm / 10 mm, and did not become 50 nm / 10 mm or less. The flatness (warpage) before heat treatment exceeded 20 μm, but after heat treatment was 10 μm or less. Accordingly, although the warpage is corrected, the residual stress cannot be reduced.

(Example 3)
ME-X02 (manufactured by Konica Minolta Opto Co., Ltd., thermal expansion coefficient: 6.1 × 10 ⁻⁶ / K, thermal conductivity: 1.14 W / (m · K)) as a glass material, and alumina AP960 as a spacer material Except for using (thermal expansion coefficient: 7.3 × 10 ⁻⁶ / K, thermal conductivity coefficient: 25 W / (m · K)), the molded body made of ME-X02 was heat-treated in the same manner as in Example 1.

  After cooling to room temperature, the retardation was measured using a polarizing microscope. As a result, all blank materials were in the range of 37 nm / 10 mm to 43 nm / 10 mm, and became 50 nm / 10 mm or less. The flatness (warpage) before heat treatment exceeded 20 μm, but after heat treatment was 10 μm or less. Therefore, it was confirmed that a blank material in which the residual stress was sufficiently reduced and the warpage was corrected could be obtained.

(Comparative Example 3)
ME-X02 (manufactured by Konica Minolta Opto Co., Ltd., thermal expansion coefficient: 6.1 × 10 ⁻⁶ / K, thermal conductivity: 1.14 W / (m · K)) for glass material, aluminum nitride for spacer material A molded body made of ME-X02 was heat-treated in the same manner as in Example 1 except that (thermal expansion coefficient: 4.5 × 10 ⁻⁶ / K, thermal conductivity: 170 W / (m · K)) was used.

  After cooling to room temperature, the retardation was measured using a polarizing microscope. As a result, all blank materials were in the range of 55 nm / 10 mm to 59 nm / 10 mm, and did not become 50 nm / 10 mm or less. The flatness (warpage) before heat treatment exceeded 20 μm, but after heat treatment was 10 μm or less. Accordingly, although the warpage is corrected, the residual stress cannot be reduced.

  Table 1 shows the molded body materials, spacer materials, values of conditional expression (1), and retardations of Examples 1 to 3 and Comparative Examples 1 to 3.

It is a figure which shows a molded object. It is a figure which shows the glass for information recording media. It is a figure which shows an example of the magnetic recording medium provided with the magnetic layer on the main surface of the glass substrate for information recording media. It is a figure which shows the example of the manufacturing process of a blank material with a flowchart. It is a figure explaining a laminated body. It is a figure which shows the example of the manufacturing process of the glass substrate for recording media using a blank material with a flowchart.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Molded body 3, 22 Main surface 20 Glass substrate for information recording medium 30 Recording layer B Laminated body D Magnetic recording medium S Spacer S1 Flat surface

Claims (7)

The molded article of amorphous glass is press-molded, viewed one by one clamping between the plates of the state of being opposed to the flat surface on the main surface the whole area of the molded product, crystallization or soften the laminate stacked without bonding In the method of reducing the residual stress of the blank material to be a glass substrate for information recording medium having a heat treatment step of correcting the warp of the glass substrate by heat treatment in a temperature range that does not,
The method for reducing residual stress of a blank material, wherein the thermal expansion coefficient As of the material forming the plate and the thermal expansion coefficient Ab of the material forming the molded body satisfy the following conditional expression:
0.80 ≦ As / Ab ≦ 1.20   It is the blank material by which the residual stress was reduced by the method of reducing the residual stress of the blank material of Claim 1, Comprising: The retardation of the thickness direction in the said blank material is 50 nm / 10mm or less, It is characterized by the above-mentioned. Blank material. 2. The residual stress of the blank material according to claim 1, wherein in the heat treatment step, the formed body is heated to a temperature between a glass transition point and a yield point of the glass material forming the formed body. Method. Press molded amorphous glass compacts are sandwiched one by one between plates with a flat surface facing the entire main surface of the compact, and the stacked stack without bonding is not crystallized or softened. Having a heat treatment step of correcting warpage of the glass substrate by heat treatment in a temperature range;
  In the manufacturing method of manufacturing the glass substrate for information recording medium by processing the blank material obtained in the heat treatment step,
  The method for producing a glass substrate for an information recording medium, wherein the thermal expansion coefficient As of the material forming the plate and the thermal expansion coefficient Ab of the material forming the molded body satisfy the following conditional expression.
    0.80 ≦ As / Ab ≦ 1.20 5. The information recording according to claim 4, wherein the molded body and the plate are both disk-shaped, and the diameter of the plate is 1.01 to 1.2 times the diameter of the molded body. A method for producing a glass substrate for a medium. 6. The method for producing a glass substrate for an information recording medium according to claim 4, wherein the thickness of the plate is 0.5 to 3.5 times the thickness of the molded body. The information according to any one of claims 4 to 6, wherein in the heat treatment step, the formed body is heated to a temperature between a glass transition point and a yield point of the glass material forming the formed body. A method for producing a glass substrate for a recording medium.

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