JP5015184B2 - Information recording medium glass blank, information recording medium substrate manufacturing method, and information recording medium manufacturing method - Google Patents

Description

The present invention relates to an information recording medium glass blank, information recording method of manufacturing a medium substrate, and to a method of manufacturing an information recording medium.

  As a typical method for producing a substrate for an information recording medium such as a magnetic disk, a glass semi-product (so-called glass blank) produced by pressing molten glass with a mold is produced (direct press method), and this glass blank. A method of performing post-processing such as grinding / polishing is known.

  However, a glass blank used for an information recording medium substrate such as a magnetic disk is required to be thinner in order to reduce a grinding / polishing allowance in a subsequent process. Furthermore, flatness, smoothness, and parallelism are also important. In order to satisfy these requirements, a glass blank having a thick portion at the center has been proposed (see Patent Document 1). In this technique, a part of the glass gob that has been cooled to a temperature range in which viscous flow is difficult due to contact with the lower mold surface during the time required to form the total amount of glass gob is mainly used when processing a glass blank into a substrate. Since it is not necessary to stretch to the thickness required as a product at the time of pressing after being removed by drilling, it is not enough to stretch the glass lump of the planar size required as a product by making the center part thick. Can be suppressed. For this reason, the thickness can be further reduced.

  In addition, it has been proposed that a glass blank having a thick portion at the center is used also for the purpose of improving productivity and reducing the cost of center hole processing (see Patent Document 2). In this technique, a glass blank heated to a high temperature is rapidly cooled to form a center hole by generating a crack along a stepped portion between a thick portion and a thin portion.

Japanese Patent No. 4133309 (Claim 1, paragraph numbers 0009, 0014, 0015, etc.)

Japanese Patent No. 4133091 (Claim 1, paragraph numbers 0003, 0004, etc.)

  On the other hand, as a glass blank having a thick portion at the center as described above, a glass blank having a thinner thickness than the outer diameter, that is, when the outer diameter is R and the thickness is T, R When a glass blank with a large / T is produced, cracks are generated in the part where the wall thickness near the center of the glass blank changes immediately after pressing without intentional quenching for the purpose of center hole processing. May end up.

The present invention has been made in view of the above circumstances, and even when a thick portion is provided at the center, it is easy to reduce the thickness of portions other than the thick portion, and cracks are generated after pressing. glass blank for an information recording medium capable of suppressing provided (hereinafter, sometimes simply referred to as glass blank), in parallel beauty, a manufacturing method of the production method and the information recording medium of the information recording medium substrate using the glass blank The task is to do.

The above-mentioned subject is achieved by the following present invention. That is,
The glass blank of the present invention has a disk-like thick part, and a ring-like part provided so as to surround the thick part, and a thin part having a smaller thickness than the thick part. meat portion, Rutotomoni provided so as to form a convex only for hand side of the thin portion, the glass blank forming the other side are all flush, so that a thin portion and a thick portion forming a step The inclined surface that continues from the flat surface portion of the thin-walled portion to the topmost portion of the thick-walled portion has a curved surface that forms a surface continuous with the flat surface portion.

  In one embodiment of the glass blank of the present invention, the curved surface is a part of any one of the curves whose cross-sectional outline is selected from a perfect circular curve, an elliptical curve, a parabola and a hyperbola, It is preferable that the plane portion is a tangent line of the curve, and the boundary between the plane portion and the curved surface is a contact point of the tangent line.

In one embodiment of the glass blank of the present invention, it is preferable that the curved surface has a cross-sectional outline part of a perfect circular curve, and the curvature radius of the curved surface satisfies the following formula (1).
Formula (1) 0.5 × Rb ≦ R ≦ Rb
[In Formula (1), R represents the curvature radius of a curved surface. Also, Rb is the outermost circumference of the thick portion on the surface that coincides with the first point on the outermost circumference of the thickest portion and the flat portion of the thin portion in the cross section of the glass blank cut in the diameter direction. From the intersection of the first straight line orthogonal to bisect the straight line connecting the second point on the side and the second straight line passing through the second point and orthogonal to the plane portion, It means the shortest distance to the second point. ]

  In one embodiment of the glass blank of the present invention, it is preferable that the thick portion is provided at the central portion of the thin portion so as to be convex only with respect to one surface of the thin portion.

Method for producing a glass blank of the present invention, as one of the types of the upper and lower molds in contact with the glass softened at the time of press forming, comprising a pressing surface having a substantially circular recess in the central portion, at least After using a mold having a curved surface in which the inclined surface extending from the flat portion outside the concave portion to the bottom of the concave portion forms a surface continuous with the flat portion, and supplying softened glass onto the lower die, the upper die and the lower die A glass blank is produced through at least a pressing step of press-molding a softened glass between the two.

The method of manufacturing a substrate for an information recording medium of the present invention to include a thick portion provided at the center portion of the glass blank of the present invention, the center hole to form a center hole by cutting the central portion of the glass blank A substrate for an information recording medium is manufactured through at least a forming step.

The manufacturing method of the information recording medium of the present invention includes a center hole forming step of forming a central hole by cutting the central part of the glass blank so as to include a thick part provided at the central part of the glass blank of the present invention. An information recording medium is manufactured through at least an information recording layer forming step of forming an information recording layer on at least one side of the information recording medium substrate after the information recording medium substrate is manufactured at least.

According to the present invention, even when a thick part having a large thickness is provided at the center, it is easy to reduce the thickness of the part other than the thick part, and it is possible to suppress the occurrence of cracks after pressing. glass blank, in parallel beauty, it is possible to provide a manufacturing method of the production method and the information recording medium of the substrate for an information recording medium using the glass blank.

It is explanatory drawing for demonstrating parameter Rb used in order to define the numerical range of the curvature radius R of the curved surface formed in the glass blank which concerns on embodiment of this invention. It is a schematic cross section which shows an example of the glass blank of this embodiment. It is a schematic cross section which shows the other example of the glass blank of this embodiment, FIG. 3 (A) is a figure which shows the embodiment in which the whole inclined surface consists only of a curved surface, FIG.3 (B) It is a figure which shows the embodiment in which an inclined surface is comprised from the combination of a curved surface and a plane. It is a schematic cross section which shows the other example of the glass blank of this embodiment.

(Glass blank and its manufacturing method)
-Glass blank-
The glass blank of the present embodiment has a disk-shaped thick part, and a ring-shaped part provided so as to surround the periphery of the thick part, and a thin part that is thinner than the thick part, In the glass blank provided so that the thick wall portion is convex with respect to at least one surface of the thin wall portion, the thin wall portion is formed on the side surface formed so that the thin wall portion and the thick wall portion form a step. The inclined surface that continues from the flat portion to the top of the thick portion has a curved surface that forms a surface that is continuous with the flat portion. Therefore, even if the glass blank of this embodiment has a thick part with a thick wall at the center, it is easy to reduce the thickness of the part other than the thick part, and cracks are generated after pressing. Can be suppressed.

In addition, in the glass blank of this embodiment, the reason that the effect mentioned above is acquired is as follows. First, when producing a glass blank, it is inevitable that the amount of heat shrinkage of the thick portion is larger than the amount of heat shrinkage of the thin portion in the process of cooling the glass blank in a high temperature state immediately after pressing. The tensile stress generated inside the glass blank due to such imbalance of heat shrinkage is the surface where the thin and thick parts have a step, and the thickness changes from the thin part to the thick part. region near to concentrate on (and innermost periphery of the planar portion of the thin portion, the boundary vicinity of the outermost periphery of the thick portion) easily. Such stress concentration is considered to cause cracking of the glass blank after pressing.

  Considering such a mechanism, the fundamental solution for cracking is to eliminate the wall thickness change itself. That is, it is not providing the thick part which becomes thick in a glass blank. However, if the glass blank is not provided with a thick wall portion, a portion of the glass gob where heat is lost due to contact with the surface of the lower mold before pressing and the viscosity flow is significantly reduced significantly hinders thinning. As a result, a thin glass blank cannot be obtained.

  Considering these points, (1) the stress concentration near the region where the thickness changes from the thin portion to the thick portion can be relaxed, and (2) the area and thickness of the thick portion can be sufficiently secured. It is considered necessary. Here, (1) As a method of relieving stress concentration, there is a method of further reducing the inclination angle of the inclined surface that continues from the flat portion of the thin portion to the topmost portion of the thick portion. However, in this method, the degree of relaxation of stress concentration depends on the inclination angle. That is, if the inclination angle is made smaller in order to suppress the occurrence of cracks, the diameter of the top of the thick-walled portion becomes smaller in proportion to the thickness if sufficient thickness of the thick-walled portion that can be secured within the center drilling range is taken. It will be. Therefore, the influence of the contact surface that has been in contact with the lower mold surface for a long time before pressing and is cooled cannot be excluded, and it becomes difficult to obtain a thin glass blank.

  However, in the glass blank of this embodiment, the inclined surface that continues from the flat portion of the thin portion to the top of the thick portion has a curved surface that forms a surface that is continuous with the flat portion. Here, “the inclined surface forms a surface that is continuous with the flat surface portion” more precisely means that the glass includes a central axis of a substantially disk-shaped glass blank having a thick portion at the central portion in the diameter direction. In the cut surface which cut | disconnected the blank, it means that the inclination of the outline continuing from the plane part of a thin part to an inclined surface changes continuously from the flat part of a thin part to an inclined surface. For this reason, the angle formed by the intersecting portions of the plane portions and the inclined surfaces that extend from each other may be considered to be substantially 0 °, and an inflection point obtained by simply tilting the inclined surface with respect to the plane portion. The effect is greatly different from the case of having a small inclination angle, and the glass blank of the present embodiment can further disperse and relax the local stress concentration. For this reason, the crack suppression effect is dramatically improved. Moreover, in the glass blank of this embodiment, since the boundary part of a plane part and an inclined surface continues so that a curved surface may comprise, the angle of the inclined surface (except the area | region which becomes the curved surface by the side of a plane part) with respect to a plane part Can be arbitrarily set in a range of 45 ° or less, and more preferably in a range of 20 ° or less. For this reason, the area of the thick portion within the limited center drilling range for crack suppression can be ensured to the maximum, and as a result, thinning is easy. Thus, in the glass blank of this embodiment, the improvement effect of the crack generation after a press is very large, and also in addition to this, thickness reduction of parts other than a thick part can also be achieved.

  The curved surface provided so as to continue from the flat surface portion to the inclined surface has a cross-sectional contour line selected from known curves such as a perfect circular curve, an elliptical curve, a parabola, and a hyperbola. Preferably, the plane portion is a tangent to the curve, and the boundary between the plane portion and the curved surface is a contact point of the tangent. Among these, it is preferable that the contour line of the curved cross section is a part of a parabola or a hyperbola from the viewpoint that the curvature change is more gradual and the crack suppressing effect can be further enhanced. In the present specification, “a cross section of a curved surface or an inclined surface partially including a curved surface” means a cut surface cut to include the central axis of a substantially disk-shaped glass blank.

In addition, when the outline of the cross section of a curved surface is a part of a perfect circular curve, it is preferable that the curvature radius of the curved surface satisfies the following formula (1).
Formula (1) 0.5 × Rb ≦ R ≦ Rb
Here, in Formula (1), R represents the curvature radius of a curved surface. Also, Rb is the outermost circumference of the thick portion on the surface that coincides with the first point on the outermost circumference of the thickest portion and the flat portion of the thin portion in the cross section of the glass blank cut in the diameter direction. From the intersection of the first straight line orthogonal to bisect the straight line connecting the second point on the side and the second straight line passing through the second point and orthogonal to the plane portion, It means the shortest distance to the second point.

  Here, the parameter Rb used to define the numerical range of the curvature radius R of the curved surface will be described with reference to the drawings. FIG. 1 is an explanatory diagram for explaining a parameter Rb used to define a numerical range of a curvature radius R of a curved surface. Specifically, in the vicinity of a thick portion when a glass blank is cut in a diameter direction. It shows about sectional drawing. Here, in FIG. 1, the first point P <b> 1 is a point on the outermost peripheral side of the topmost part 12 of the thick part 10, and the second point P <b> 2 is on a plane coinciding with the planar part 22 of the thin part 20. It is a point on the outermost peripheral side of the thick portion 10, and the first straight line L1 is a straight line orthogonal to the straight line L3 connecting the first point P1 and the second point P2 so as to bisect, The second straight line L2 passes through the second point P2 and is a straight line orthogonal to the plane portion 22, and the intersection C is a point where the first straight line L1 and the second straight line L2 intersect. . Rb means the shortest distance from the intersection C to the second point P2. Here, when it is assumed that the thick portion 10 is provided in an area inside the straight line L3, the thick portion 10 within the radius Rb is removed from the intersection C as a starting point. A curved surface 14 continuous with the portion 22 is formed.

  In addition, when the curvature radius R of a curved surface satisfy | fills Formula (1), it is easy to make a crack suppression effect and thinning balance compatible. The radius of curvature R is more preferably 0.7 × Rb or more, and the radius of curvature R = Rb is most preferable. In any case, the center of the perfect circle for forming the curved surface 14 is located on the second straight line L2. Further, the straight line L3 connecting the first point P1 and the second point P2 preferably intersects the flat surface portion 22 of the thin portion 20 so as to form an angle of 45 ° or less, and the angle of 20 ° or less. It is more preferable to intersect so as to form. Thereby, the crack suppression effect can be acquired reliably. In the case of R = Rb, the inclined surface of the thick wall portion 12 is composed only of the curved surface 14, and the contour line of the cross section of the curved surface 14 forms a part of a perfect circular curve (that is, an arc having a radius of curvature R). ). In the case of R <Rb, the inclined surface of the thick portion 12 includes a curved surface 14 in which the outline of the cross section includes the second point P2, and a plane in which the outline of the cross section includes the first point P1 ( (Not shown in FIG. 1). Here, the inclination of the outline (straight line) of the cross section of the plane is a tangent at the point where the curved surface 14 and the plane intersect (that is, the other end of the arc having the radius of curvature Rb having the second point P2 at one end). It matches the slope of.

In addition, the thick part may be provided at the central part of the thin part so as to be convex with respect to both surfaces of the thin part, but only with respect to one surface of the thin part. and et provided in the center portion of the thin portion. In Rukoto provided only on one side of the glass blank as the thick portion forms a convex, it is possible to more reliably suppress the occurrence of cracks due to the crack.

  The dimension of each part of the glass blank of this embodiment is not specifically limited, It can select suitably according to the dimension of each part of the board | substrate for information recording media produced by processing a glass blank. However, the thickness of the thin portion is preferably 0.9 mm or less, and 0.8 mm or less in order to further reduce the polishing allowance when producing the substrate by post-processing such as polishing the glass blank. More preferably. Since the glass blank of this embodiment has a thick part in the center part, it is easy to make the thickness of a thin part into the said range.

  In consideration of the diameter of the substrate finally produced using the glass blank, the glass blank for a so-called 2.5 inch substrate (diameter = 65 mm) preferably has a thickness of 0.9 mm or less, and 0 .8 mm or less is more preferable. In a glass blank for a 1.8 inch substrate (diameter = 48 mm), the thickness of the thin portion is preferably 0.77 mm or less, and more preferably 0.67 mm or less. In a glass blank for a 1.0 inch substrate (diameter = 27.4 mm), the thickness of the thin portion is preferably 0.65 mm or less, and more preferably 0.55 mm or less. In a glass blank for a 0.85-inch substrate (diameter = 21.6 mm), the thickness of the thin portion is preferably 0.65 mm or less, and more preferably 0.55 mm or less. The lower limit of the thickness of the thin portion is not particularly limited regardless of the size of the target substrate, but is practically 0.55 mm or more.

  Further, in the thick part, the longer the time from when the molten glass gob contacts the lower mold surface to when the pressing is performed, the thicker the glass layer that is brought into contact with the lower mold at a lower temperature to increase the viscosity. For this reason, it is necessary to ensure the thickness of a thick part to some extent. However, when the thickness of the thick portion increases, the cooling of the thick portion is delayed as compared with the thin portion. For this reason, when the thick portion is excessively thick, there is a possibility that an increase in internal stress during cooling is caused. Therefore, the thickness of the thick portion is appropriately adjusted in consideration of these points.

  The diameter in the cross section assuming that the thick part was cut along the same plane as the flat part of the thin part (hereinafter, may be abbreviated as “the diameter of the base part of the thick part”) is finally determined using a glass blank. There is no particular limitation as long as it is the same as or smaller than the diameter of the center hole provided in the substrate to be manufactured. However, when the center hole is formed in the center portion of the glass blank, it is necessary to consider the thickness of the cutter in consideration of cutting by scribing. In this case, the diameter of the base portion of the thick portion is preferably 2 mm or less in the diameter of the center hole provided in the substrate, and more preferably 3 mm or less in the diameter of the center hole provided in the substrate. For example, in the case of a glass blank for a 2.5-inch substrate (center hole diameter = 20.0 mm), it can be said that the diameter of the base portion of the thick portion is preferably 18 mm or less. In addition, the lower limit value of the diameter of the base portion of the thick-walled portion is not particularly limited, but has a diameter that can secure an area in a range where the molten glass gob is in a temperature range where the viscous flow is difficult due to contact with the lower mold surface. There is a need.

-Specific examples of glass blanks-
Next, specific examples of the glass blank of this embodiment will be described with reference to the drawings. FIG. 2 is a schematic cross-sectional view showing an example of the glass blank of the present embodiment, specifically, a cross-sectional view when the glass blank is cut in the diameter direction. Note that the cross-sectional view shown in FIG. The glass blank 1 shown in FIG. 2 has a disk-shaped thick part 10 and a ring-shaped (annular) thin part 20 surrounding the thick part 10. The thick portion 10 is thicker than the thin portion 20, and is provided at the center of the thin portion 20 so as to protrude from one surface of the thin portion 20. The other surface is all flush. The inclined surface that continues from the flat portion 22 of the thin portion 20 to the topmost portion 12 of the thick portion 10 has a curved surface 14 that forms a surface that is continuous with the flat portion 22. The inclined surface that continues from the flat portion 22 to the topmost portion 12 of the thick portion 10 may be entirely composed of the curved surface 14 as shown in FIG. As shown, only the flat surface portion 22 side may be configured from the curved surface 16A, and the topmost portion 12 side of the thick portion 10 may be configured from the flat surface 16B.

  In addition, although the thick part 10 is provided in the center part of the glass blank 1, the glass blank of this embodiment may provide a 2nd thick part along a peripheral part. Thus, when a glass blank is produced by pressing, the heat capacity of the stretched tip of the softened glass lump stretched by pressing can be sufficiently secured, so that it becomes easier to press the softened glass lump thinner. . In this case, as shown in FIG. 4, the inclined surface that continues from the flat portion 22 of the thin portion 20 to the topmost portion 32 of the second thick portion 30 also has a curved surface 34 that forms a surface continuous with the flat portion 22. It is preferable that Thereby, even if it provides a 2nd thick part along the peripheral part of a glass blank, generation | occurrence | production of a crack can be suppressed.

-Manufacturing method of glass blank-
Glass blank of the present embodiment described above, as one type of upper and lower molds in contact with the glass softened at the time of press forming, comprising a pressing surface having a substantially circular recess in the central portion, at least After using a mold having a curved surface in which the inclined surface extending from the flat portion outside the concave portion to the bottom of the concave portion forms a surface continuous with the flat portion, and supplying softened glass onto the lower die, the upper die and the lower die The glass can be produced through at least a pressing step of press-molding a softened glass.

  In the direct press method, as the mold used in the pressing process, in addition to the upper mold and the lower mold used to form both sides of the glass blank, the glass lump in the softened state at the time of pressing is stretched in the press surface direction. In some cases, a body mold is used in combination for regulation. Also in the production of the glass blank of the present embodiment, a press method (non-side free press method) using a restricting member such as a barrel shape that restricts free stretching of the softened glass lump in the press surface direction is adopted. However, it is more preferable to employ a press method (side free press method) that does not use a regulating member such as a body mold. The reason for this is that in the side free press method, free stretching of the softened glass lump in the press surface direction during pressing is not regulated. Therefore, even if the volume of the softened glass lump supplied onto the lower mold press surface slightly varies, the volume of the glass lump is maintained while keeping the distance between the upper and lower press surfaces constant during pressing. Variations can be absorbed as variations in the outer diameter of the glass blank. Therefore, when the side free press method is adopted, the thickness variation between the glass blanks can be made smaller than when the non-side free press method is adopted, and as a result, the load of the polishing process, which is a subsequent process, is further reduced. be able to.

  Next, the typical example of the manufacturing method of the glass blank of this embodiment including a press process is demonstrated below. First, molten glass made of these glass materials that has been melted, clarified, and stirred and homogenized is continuously discharged from the outflow nozzle at a constant outflow speed, and this molten glass flow is always kept at a constant mass by a cutting machine called shear. Periodically cut to obtain a softened glass lump. The softened glass lump that has been cut is supplied (cast) onto the press surface of the lower mold waiting just under the outflow nozzle. The molten glass discharged from the outflow nozzle is in a softened state and has a viscosity of about 0.3 to 100 Pa · s. Although the temperature of the lower mold is lower than the temperature of the glass lump, the temperature of the lower mold is adjusted to a temperature at which the temperature of the glass lump suddenly drops and is not pressable. In addition, in order to improve the lubricity with respect to the press surface of the molten glass to be cast, a solid lubricant such as boron nitride (BN) powder may be attached to the lower mold press surface in advance. Good.

  The lower mold in which the cast is finished and the softened glass is placed on the press surface is transferred to a press position where the upper mold is waiting, and is pressed by the upper mold and the lower mold. At this time, the temperature of the upper and lower molds, the pressing pressure, and the pressing time take into consideration the thermal properties of the glass such as the glass transition temperature, the diameter and thickness of the glass blank to be produced, and whether or not the side free press method is used. Set as appropriate. For example, the upper mold temperature can be set to 250 to 400 ° C., and the lower mold temperature can be set to 420 to 470 ° C. The thrust during pressing can be about 3 to 7 tons, but is not particularly limited to this range, and can be adjusted as appropriate.

  When press molding is finished, the upper surface of the molded product is released from the upper mold, and the lower mold on which the molded product is placed is transferred to a take-out position. In addition, the lower mold is stopped between the press position and the take-out position, the upper surface of the molded product on the lower mold is pressed with the pressing mold, the warpage of the molded product is corrected, and each lower mold is transferred to the take-out position. May be. The molded product is cooled to near the glass transition temperature or lower than the glass transition temperature before being transferred to the take-out position. This is to prevent the molded product from being deformed by the force applied during take-out. Take-out is performed by adsorbing and holding the upper surface of the molded product with an adsorption means. The take-out molded product is rapidly cooled in the atmosphere, and then placed in an annealing furnace and annealed for strain removal. And the glass blank of this embodiment can be obtained through such a series of processes.

-Glass composition-
The glass composition of the glass blank of the present embodiment can be appropriately selected according to the substrate and information recording medium produced using the glass blank. For example, aluminosilicate glass, soda lime glass, soda aluminosilicate glass, aluminoborosilicate Glass, borosilicate glass, quartz glass, chain silicate glass, and the like can be given. These glasses may be crystallized glass that crystallizes by heat treatment.

As the aluminosilicate glass, SiO ₂ is 58% by mass to 75% by mass, Al ₂ O ₃ is 5% by mass to 23% by mass, Li ₂ O is 3% by mass to 10% by mass, Na ₂ O. May be an aluminosilicate glass containing 4 mass% or more and 13 mass% or less as a main component (however, an aluminosilicate glass not containing a phosphorus oxide). For example, SiO ₂ is 62% by mass to 75% by mass, Al ₂ O ₃ is 5% by mass to 15% by mass, Li ₂ O is 4% by mass to 10% by mass, and Na ₂ O is 4% by mass to 12% by mass. Mass% or less, ZrO ₂ contains 5.5 mass% or more and 15 mass% or less as a main component, and the mass ratio of Na ₂ O / ZrO ₂ is 0.5 or more and 2.0 or less, Al ₂ O ₃ / ZrO. It is good also as an amorphous aluminosilicate glass which does not contain the phosphorus oxide whose mass ratio of ₂ is 0.4-2.5. In addition, it is desirable that the glass does not contain an alkaline earth metal oxide such as CaO or MgO. Examples of such glass include N5 glass (trade name) manufactured by HOYA Corporation.

(Method for manufacturing substrate for information recording medium)
A center hole forming step (hereinafter referred to as “core”) in which the glass blank obtained through the above-described steps is formed by cutting the central portion of the glass blank so as to include a thick portion provided in the central portion. An information recording medium substrate can be manufactured through at least a “ring”. The thick part can be cut using a diamond cutter. Usually, in addition to the center hole forming step, a substrate for an information recording medium can be obtained by performing post-processing such as shape processing such as end face processing and double-side polishing processing. Specifically, the information recording medium substrate includes, for example, (1) a first lapping step, (2) a cutting step (coring, forming), (3) an end surface grinding step, (4) a second lapping step, ( 5) End face polishing step, (6) Main surface polishing step, (7) Chemical strengthening step and cooling step, (8) Precision cleaning step can be performed in this order. Hereinafter, these eight steps will be described more specifically. The disk-shaped glass having a diameter sufficiently larger than the diameter of the substrate is press-molded, the concentric disk-shaped glass is taken out from the disk-shaped glass by scribing, and the taken-out disk-shaped glass is removed from the above (1) to (8) The substrate for the information recording medium can also be manufactured by the processes leading to.

(1) 1st lapping process In a 1st lapping process, a disk-shaped glass base plate is obtained by lapping the both main surfaces of the glass blank after cut | disconnecting a thick part. This lapping process can be performed using alumina free abrasive grains with a double-sided lapping apparatus using a planetary gear mechanism. Specifically, the lapping platen is pressed on both sides of the glass base plate from above and below, and a grinding liquid containing free abrasive grains is supplied onto the main surface of the glass base plate, and these are moved relatively to perform lapping processing. It can be carried out. By this lapping process, a glass base plate having a flat main surface is obtained.

(2) Cutting process (coring, forming)
Next, the glass base plate is cut using a diamond cutter, and a disk-shaped glass substrate is cut out from the glass base plate. Next, using a cylindrical diamond drill, a circular hole is formed in the center of the glass substrate to obtain a donut-shaped glass substrate (coring).

(3) End surface grinding process And an inner peripheral end surface and an outer peripheral end surface are ground with a diamond grindstone, and a predetermined chamfering process is performed (forming).

(4) Second Lapping Step Next, a second lapping process is performed on both main surfaces of the obtained glass substrate in the same manner as in the first lapping step. By performing this second lapping step, it is possible to remove in advance the fine unevenness formed on the main surface in the cutting step and end surface polishing step, which are the previous steps, and shorten the subsequent polishing step on the main surface. Can be completed in time.

(5) End surface polishing step Next, the end surface of the glass substrate is mirror-polished by a brush polishing method. At this time, as the abrasive grains, a slurry (free abrasive grains) containing cerium oxide abrasive grains can be used. By this end surface polishing step, generation of particles and the like from the end surface of the glass substrate can be prevented.

(6) Main surface polishing step As the main surface polishing step, first, a first polishing step is performed. This first polishing process is mainly intended to remove scratches and distortions remaining on the main surface in the lapping process described above. In the first polishing step, the main surface is polished using a hard resin polisher by a double-side polishing apparatus having a planetary gear mechanism. For example, cerium oxide abrasive grains can be used as the polishing liquid. And the glass substrate which finished this 1st grinding | polishing process is immersed in each washing tank of a neutral detergent, a pure water, and IPA (isopropyl alcohol) sequentially, and is wash | cleaned.

  Next, a second polishing step is performed as part of the main surface polishing step. The purpose of this second polishing step is to finish the main surface into a mirror surface. In the second polishing step, mirror polishing of the main surface is performed using a soft foamed resin polisher by a double-side polishing apparatus having a planetary gear mechanism. As the polishing liquid, cerium oxide abrasive grains finer than the cerium oxide abrasive grains used in the first polishing step can be used. The glass substrate that has finished the second polishing step is sequentially immersed in each cleaning bath of neutral detergent, pure water, and IPA (isopropyl alcohol) to be cleaned. An ultrasonic wave is applied to each cleaning tank.

(7) Chemical strengthening step and cooling step When the glass blank used for the production of the information recording medium substrate is made of glass containing an alkali metal such as lithium or sodium, the glass substrate that has been subjected to the lapping step and polishing step described above is used. It is preferable to apply chemical strengthening. By performing the chemical strengthening step, a high compressive stress can be generated in the surface layer portion of the information recording medium substrate. For this reason, the impact resistance of the surface of the information recording medium substrate can be improved. Such chemical strengthening treatment is very suitable for producing a magnetic recording medium in which a head for recording / reproducing information may mechanically come into contact with the surface of the information recording medium.

  The chemical strengthening is performed by preparing a chemical strengthening solution in which potassium nitrate and sodium nitrate are mixed, heating the chemical strengthening solution, preheating the cleaned glass substrate, and immersing it in the chemical strengthening solution. Thus, by immersing in a chemical strengthening solution, the lithium ion and sodium ion of the surface layer of a glass substrate are each substituted by the sodium ion and potassium ion in a chemical strengthening solution, and a glass substrate is strengthened.

  Then, the glass substrate which finished the chemical strengthening process is immersed in a water bath, cooled, and maintained for a while. Then, the cooled glass substrate is cleaned by immersing it in heated concentrated sulfuric acid. Further, the glass substrate after the sulfuric acid cleaning is cleaned by immersing in a cleaning bath of pure water and IPA (isopropyl alcohol) sequentially. In addition, an ultrasonic wave is applied to each washing tank.

(8) Precision cleaning step Next, it is preferable to carry out a precision cleaning step in order to remove abrasive residues and foreign iron-based contaminants, and to make the surface of the glass substrate smoother and cleaner. Such a precision cleaning process is very suitable for producing a magnetic recording medium in which a head for recording / reproducing information may mechanically contact the surface of the information recording medium. This is because the occurrence of head crashes and thermal asperities can be suppressed by carrying out precision cleaning. In addition, as a precision washing | cleaning process, you may make it perform a water rinse washing | cleaning and an IPA washing | cleaning process after washing | cleaning by alkaline aqueous solution.

  The surface roughness of the information recording medium produced through these series of steps can be on the order of sub-nanometers with Ra. The surface roughness can be appropriately adjusted by selecting main surface polishing conditions and cleaning conditions. The information recording medium substrate obtained through the above-described eight steps can be used for production of an information recording medium employing various known recording methods such as known magnetic recording, optical recording, and magneto-optical recording. However, it is particularly suitable for use in producing a magnetic recording medium. Further, in the case of an information recording medium substrate that is not required to have cleanness, smoothness, and impact resistance on the surface of the information recording medium substrate as much as the magnetic recording medium substrate, one of the above eight steps is performed as necessary. The steps may not be performed, and each process may be simplified or performed under rougher conditions.

(Method of manufacturing information recording medium)
The information recording medium can be manufactured by performing at least an information recording layer forming step of forming an information recording layer on at least one surface of the information recording medium substrate thus obtained. When a magnetic recording medium is manufactured, a magnetic recording layer is provided as an information recording layer. The magnetic recording medium may be either a horizontal magnetic recording system or a perpendicular magnetic recording system, but is preferably a perpendicular magnetic recording system. When a perpendicular magnetic recording type magnetic recording medium is manufactured, for example, an adhesion layer made of Cr alloy, a soft magnetic layer made of FeCoCrB alloy, an underlayer made of Ru, and CoCrPt—TiO _{2 on} both surfaces of an information recording medium substrate. A perpendicular magnetic recording layer made of an alloy, a protective layer made of hydrogenated carbon, and a lubricating layer made of perfluoropolyether can be sequentially formed in this order. The adhesion layer, the soft magnetic layer, the underlayer, and the perpendicular magnetic recording layer can be formed by a sputtering method, and the protective layer can be formed by a sputtering method or a CVD method (Chemical Vapor Deposition method). The lubricating layer can be formed by a dip coating method. Further, in-line type or single-wafer type sputtering apparatus capable of continuous film formation of each layer can be used for film formation from the adhesion layer to the protective layer, and immersion coating apparatus can be used for film formation of the lubricating layer. .

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

Example 1
After supplying the molten glass obtained by melting the aluminosilicate glass onto the press surface of the lower mold, the glass blank having the shape shown in FIG. 2 is obtained by pressing the upper mold and the lower mold in a side-free manner. A sheet was produced. The dimensions of each part of the obtained glass blank are shown in Table 1. In the production of this glass blank, a lower mold having a concave portion having a shape corresponding to the dimensional shape of the thick portion of the glass blank shown in Table 1 at the center of the press surface was used as the lower mold. The main production conditions for producing a glass blank are as follows.
Glass transition temperature Tg: 485 ° C
-Average linear expansion coefficient of glass: 95 × 10 ⁻⁷ / K (100 to 300 ° C.), 98 × 10 ⁻⁷ / K (300 to Tg ° C.), 37 × 10 ⁻⁶ / K (Tg to 530 ° C.),
・ Pressing surface temperature when supplying molten glass onto the pressing surface of the lower mold: 500 ° C.
・ Temperature of upper die press surface during pressing: 450 ℃
・ Viscosity of molten glass put on the lower mold: 40 Pa · s
・ Pressing time (time for applying pressure to glass): 1 second or less ・ Material constituting upper and lower pressing surfaces: cast iron ・ Glass blank temperature when taking out glass blank from lower die: about 520 ℃
・ Glass blank leaving environment after take-out: Put into a slow cooling furnace held around the strain point temperature, hold the temperature for a predetermined time and then slowly cool to room temperature

(Comparative Example 1)
A glass blank having the same dimensional shape as that of FIG. 2 was prepared except that the inclined surface of the step portion between the thick portion and the thin portion was not a curved surface but a substantially flat surface. Specifically, using a lower mold having a recess in the center of the press surface so that a glass blank having the dimensions shown in Table 1 is obtained, the pressure during pressing is the same thickness as the thin portion of the glass blank of Example 1. 200 glass blanks were produced in the same manner as in Example 1 except that the glass blank was adjusted so as to be obtained. In the present specification, “the inclined surface is substantially flat” means that the radius of curvature R is ∞ .

(Comparative Example 2)
As shown in Table 1, 200 glass blanks were produced in the same manner as in Comparative Example 1 except that the inclination angle θ was changed.

<Evaluation>
In each Example and Comparative Example, Table 1 shows the results of evaluating the number of cracks generated (including those in which cracking occurred due to cracks) when 200 glass blanks were produced. In addition to these results, the relative pressing thrust in Comparative Example 1 and Comparative Example 2 when the pressing thrust during pressing in Example 1 is set to 100 is also shown.

DESCRIPTION OF SYMBOLS 1 Glass blank 10 Thick part 12 Top part 14 Curved surface 16A Curved surface 16B Plane 20 Thin part 22 Plane part 30 Second thick part 32 Top part 34 Curved surface

Claims (6)

A disk-shaped thick part,
A ring-like shape provided so as to surround the thick-walled portion, and a thin-walled portion having a smaller thickness than the thick-walled portion,
In the glass blank for information recording medium in which the thick part is provided so as to be convex only with respect to one surface of the thin part, and the other surface is all flush.
In the surface on which the thin portion and the thick portion are formed to form a step, an inclined surface that continues from the flat portion of the thin portion to the topmost portion of the thick portion is continuous with the flat portion. A glass blank for an information recording medium, characterized by having a curved surface forming a surface. The curved surface is a part of any one of the curved lines whose cross-sectional contour line is selected from a perfect circular curve, an elliptical curve, a parabola and a hyperbola,
The planar portion is tangent to the curve, and
The glass blank for an information recording medium according to claim 1, wherein a boundary between the flat portion and the curved surface is a contact point of the tangent line. In the curved surface, the outline of the cross section is a part of a perfect circular curve,
The glass blank for an information recording medium according to claim 1 or 2, wherein a radius of curvature of the curved surface satisfies the following formula (1).
Formula (1) 0.5 × Rb ≦ R ≦ Rb
[In Formula (1), R represents the curvature radius of the said curved surface. In addition, Rb is the cross section obtained by cutting the glass blank in the diameter direction, the first point on the outermost peripheral side of the topmost part of the thick part, and the thick part on the surface that coincides with the flat part of the thin part. A first straight line orthogonal to bisect a straight line connecting the second point on the outermost peripheral side of the second straight line, and a second straight line passing through the second point and orthogonal to the plane portion Means the shortest distance from the intersection point to the second point. ]   The thick part is provided in a central part of the thin part so as to be convex only with respect to one surface of the thin part. The glass blank for information recording media of description.   The center part of the said glass blank is cut | disconnected so that the thick part provided in the center part of the glass blank selected from the glass blank for information recording media as described in any one of Claims 1-4 may be included. A method for producing an information recording medium substrate, comprising producing an information recording medium substrate through at least a center hole forming step of forming a center hole. The center part of the said glass blank is cut | disconnected so that the thick part provided in the center part of the glass blank selected from the glass blank for information recording media as described in any one of Claims 1-4 may be included. After producing the substrate for information recording medium through at least the center hole forming step of forming the center hole,
An information recording medium manufacturing method comprising manufacturing an information recording medium through at least an information recording layer forming step of forming an information recording layer on at least one side of the information recording medium substrate.

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