JP5330307B2 - Manufacturing method of glass blank, manufacturing method of magnetic recording medium substrate, and manufacturing method of magnetic recording medium - Google Patents

Description

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

  Conventionally, to manufacture a magnetic recording medium substrate (magnetic disk substrate) through a press molding process, as described in Patent Document 1, a main surface of a disk-shaped glass blank obtained by press molding is a lapping process. In this method, the flatness is increased to the level required for the substrate, the thickness deviation is reduced, and the main surface is finished by a polishing process (conventional method 1). The polishing process is a process for finishing the main surface to a smooth and defect-free surface, and cannot improve the flatness of the disk main surface and the uniformity of the plate thickness.

  As described in Patent Document 2, the magnetic disk substrate is manufactured by forming a sheet glass by a float method, a downdraw method, or the like, and hollowing out a disk-shaped glass from the sheet glass, drilling a center, and inner and outer circumferences. There is a method (conventional method 2) for performing processing and polishing of the main surface. In this method, a sheet glass having a constant plate thickness and a high flatness can be obtained, so that a wrapping step for improving the flatness and the uniformity of the plate thickness is unnecessary.

  Although the conventional method 1 is superior to the conventional method 2 in terms of the glass utilization rate, a lapping process is indispensable. Therefore, there are problems that a wrapping device is necessary, man-hours are increased, processing time is increased, and processing costs are increased.

JP 2003-54965 A JP 2003-36528 A

  In order to obtain a glass blank capable of omitting the lapping process by the conventional method 1, it is necessary to reduce the plate thickness deviation of the glass blank and increase the flatness. In the conventional method 1, the molten glass flow is flowed out from the glass outlet to the lower mold press molding surface, and the molten glass flow is cut in a state where the lower end is received by the press molding surface. The molten glass lump separated from the molten glass stream is pressed by a lower mold and an upper mold facing the mold, and formed into a thin plate shape.

  The temperature of the lower mold is maintained sufficiently lower than the outflow temperature of the molten glass so that the high-temperature glass is not fused. Therefore, from the time when the molten glass flows out onto the lower mold until the press molding starts, the glass block loses heat from the surface in contact with the lower mold, and the viscosity of the lower surface of the glass block increases locally. . As a result, a glass lump having a large viscosity distribution is press-molded, and a portion that is difficult to stretch is generated by pressing. As a result, the plate thickness deviation of the glass blank increases or the flatness decreases.

  In order to avoid such a situation, the viscosity distribution in the molten glass block just before the start of pressing may be made uniform. For that purpose, the molten glass flow flowing out from the glass outlet is cut without being held by a member having a lower temperature than the glass such as the lower mold, and the molten glass lump is separated from the molten glass flow, dropped, and the molten glass lump being dropped May be press-molded.

  However, when mass-producing glass blanks from molten glass, a glass blank with a small plate thickness deviation and excellent flatness can be obtained at the start of mass production, but the plate thickness deviation increases over time and the flatness also decreases. There was a problem.

  The present invention has been made to solve the above-described problems, and is capable of stably mass-producing a glass blank for a magnetic recording medium substrate having a small thickness deviation and high flatness by press molding molten glass. A method of manufacturing a glass blank that can be processed, and a method of manufacturing a magnetic recording medium substrate that processes a glass blank produced by the above method into a magnetic recording medium substrate without performing a lapping step. It is an object of the present invention to provide a method for producing a magnetic recording medium using the produced substrate.

  When mass producing glass blanks from molten glass, the inventor can obtain a glass blank having a small thickness deviation and excellent flatness at the start of mass production. As a result of considering the cause of the decrease, the following knowledge was obtained.

(1) The main surface of the glass blank with increased thickness deviation and reduced flatness has a central portion that is recessed from the peripheral portion on both sides, and the central portion is slightly thinner than the peripheral portion. .
(2) The degree of increase in sheet thickness deviation decreases with time and shows a tendency to converge to a constant value.
(3) The plate thickness of the glass blank is determined by the distance between the press forming surfaces facing each other at the time of press forming, that is, the surfaces on which the main surface of the thin plate glass blank is transferred, so that the press forming was initially flat. The center of the surface rises with time, the shape is transferred to the main surface of the glass blank, and the thickness deviation increases. The increase in the bulge at the center of the press molding surface decreases with time and becomes constant. This state is called a steady state.

(4) The temperature of the press mold is set to a temperature that is sufficiently lower than the temperature of the molten glass and does not hinder the elongation of the glass during pressing in order to prevent fusion of the molten glass. In press molding, since the center of the press molding surface first touches the molten glass block, the temperature distribution in the press mold becomes higher as it approaches the center of the press molding surface. The closer it is, the higher the temperature.
(5) As a result, the amount of expansion of the shallow portion of the center portion of the press mold surface of the press mold increases locally, and the center portion of the press mold surface rises or warps.
(6) The amount of heat flowing into the press mold and the amount of heat flowing out are balanced with the passage of time, and the amount of deformation of the press molding surface becomes constant, so that the thickness variation of the glass blank is stabilized in a large state. .

(7) The structure of the press mold is devised to increase the rigidity so as not to be deformed by the press pressure. For example, the thickness in the direction perpendicular to the press molding surface is increased to increase the rigidity. However, as the thickness is increased, the amount of thermal expansion of the press-molded surface increases, the amount of bulge at the center of the press-molded surface increases, and the thickness deviation of the glass blank increases.
(8) When the dimension in the direction perpendicular to the press molding surface of the press mold is reduced, that is, when the press mold is thinned, the high temperature side (press molding surface side) and the low temperature side (opposite side of the press molding surface (rear surface) )) Warp in the press mold due to the difference in thermal expansion. In the case of a thin plate-shaped press mold, the warpage is dominant in the deformation of the press molding surface.

(9) In order to flatten the shape of the press molding surface during pressing, the back surface of the press mold opposite to the press molding surface is pressed against the rigid body using the pressure during pressing, and the press mold is The press molding surface may be flattened by elastic deformation. However, since pressure is applied to the press mold through glass, if the pressure required for flattening the press molding surface is too high, it becomes difficult to flatten the press molding surface. Therefore, it is desired that the elasticity of the press mold is set so that the press molding surface is flattened by the initial pressure of the press. In order to impart such elasticity to the press mold, it is advantageous to make the press mold thin.
(10) Furthermore, in order to prevent heat from flowing from the high-temperature glass into the rigid body through the press mold and preventing the rigid body from being deformed by thermal expansion, the press mold is released when the pressure during pressing is released. And the rigid body should be separated from each other and insulated from each other.

The present invention has been made on the basis of the above-described findings in order to solve the above problems. That is,
The method for producing a glass blank of the present invention comprises separating a certain amount of molten glass from a molten glass stream flowing out from a glass outlet, press-molding a thin glass using a press mold, and removing the thin glass from the press mold. In the glass blank manufacturing method for producing a glass blank to be processed into a magnetic recording medium substrate by repeating the taking-out process, a pair of thin plate-shaped press molds are prepared, and the molten glass lump separated from the molten glass stream is dropped. Pressing the molten glass lump that is falling to produce a thin sheet glass, and pressing the surface opposite to the press molding surface of the press mold against the rigid body by the pressure when pressing the molten glass lump The press mold is elastically deformed to flatten the press mold surface and the press mold surfaces of the press mold are parallel to each other. Molding the main surface of the thin glass by transferring the shape of the surface to the molten glass gob, and wherein.

  In one embodiment of the method for producing a glass blank of the present invention, the press mold is formed by processing a surface opposite to the press molding surface flat and parallel to the press molding surface. It is preferable to press the surface opposite to the surface against the flat surface of the rigid body to flatten the press molding surface.

  It is preferable that other embodiment of the manufacturing method of the glass blank of this invention cut | disconnects the molten glass flow drooping at a glass outflow port, isolate | separates and drops a molten glass lump.

  The method for producing a magnetic recording medium substrate of the present invention is characterized by producing a magnetic recording medium substrate through at least a step of polishing the glass blank produced by the method for producing a glass blank of the present invention.

  The method for producing a magnetic recording medium of the present invention comprises at least a magnetic recording layer forming step for forming a magnetic recording layer on the magnetic recording medium produced by the method for producing a magnetic recording medium substrate of the present invention, and the magnetic recording medium A medium is produced.

  According to the present invention, it is possible to stably mass-produce a glass blank for a magnetic recording medium substrate having a small thickness deviation and high flatness by press molding molten glass. Moreover, the glass blank produced by the above method can be processed into a magnetic recording medium substrate without going through a lapping process. Furthermore, a magnetic recording medium can be manufactured using the substrate manufactured by the above method.

In an Example, it is the schematic which showed a mode that the molten glass lump was press-molded and the shape | molded thin plate glass was taken out.

[Glass blank manufacturing method]
The manufacturing method of the glass blank of this embodiment separates a certain amount of molten glass lump from the molten glass flow flowing out from the glass outlet, press-molds the thin glass using a press mold, and presses the thin glass into the press mold. In a glass blank manufacturing method for producing a glass blank for processing into a magnetic recording medium substrate, a pair of thin plate-shaped press molds are prepared and the molten glass lump separated from the molten glass stream is dropped. Making a glass sheet by press molding the falling molten glass lump,
And the surface opposite to the press molding surface of the press mold is pressed against the rigid body by the pressure when pressing the molten glass lump, and the press mold surface is flattened by elastically deforming the press mold. The main surface of the thin glass sheet is formed by transferring the shape of the press molding surface to a molten glass lump (glass) in a state where the press molding surfaces of the molding die are parallel to each other.

  The molten glass lump separated from the molten glass stream is dropped, and the molten glass lump that is falling is press-molded to produce a thin glass, thereby making the viscosity distribution of the molten glass lump just before the start of pressing uniform, Can be stretched uniformly and thinly. As described above, even if a press mold with a flat press molding surface is used, repeated pressing of high-temperature glass causes a large temperature distribution in the press mold and the center of the press molding surface rises. To occur.

  In order to prevent deformation of the press mold during pressing, the conventional idea of increasing the rigidity of the press mold is changed by 180 °, and the press mold that expands and deforms by repeatedly pressing high-temperature glass is thinned, Pressing the surface opposite the press molding surface against the rigid body by the pressure during pressing, elastically deforming the press mold to flatten the press molding surface, and transferring the flattened press molding surface shape to glass I made it. At this time, the opposing press molding surfaces are made parallel to each other. By taking such a method, a glass blank having a small plate thickness deviation and a high flatness can be formed.

  When the pressure during press molding is released, the close contact between the press mold and the rigid body is released, creating a gap between the press mold and the rigid body, reducing the amount of heat flowing from the press mold to the rigid body. The rise in the temperature of the rigid body can be suppressed. In order to suppress the temperature rise of the rigid body, the surface of the rigid body that is in close contact with the press mold may be made of a material having low thermal conductivity. Alternatively, the pressurization may be released and the press mold and the rigid body may be separated and insulated. In order to prevent the temperature of the rigid body from rising, the rigid body may be cooled.

  The press mold used in the method for producing a glass blank of the present embodiment is preferably one in which the surface opposite to the press molding surface is processed flat and parallel to the press molding surface. Hereinafter, the press molding surface side of the press mold is referred to as a front surface, and the side opposite to the press molding surface is referred to as a back surface. It is preferable that a surface (hereinafter referred to as a flattening reference surface) against which the back surface of the press body of the rigid body is pressed is processed in advance. Then, the entire area of the back surface of the press mold is pressed and brought into close contact with the flattening reference surface during pressing, and the press molding surface is flattened, that is, the flattening reference surface is used as a flatness reference surface. Is pressed against the flattening reference surface to correct the shape of the press mold so that the press molding surface becomes flat.

  The thickness of the reduced press mold, that is, the dimension in the direction perpendicular to the press molding surface, is the same as that for the production of glass blanks one after another. The thickness deviation and flatness may be measured and determined so that the thickness deviation and flatness are within the required ranges.

  The material constituting the press mold is preferably a metal or an alloy in consideration of heat resistance, workability, and durability. Among them, a metal or alloy having a heat resistant temperature of 1000 ° C. or higher, preferably 1100 ° C. or higher, when used as a press mold is more preferable. Specifically, spheroidal graphite cast iron (FCD), alloy tool steel (SKD61 etc.), high speed steel (SKH), cemented carbide, colmonoy, stellite, etc. are preferable. Such a mold material is flattened by the pressure at the time of pressing the glass by thinning. For example, when molding a glass blank having a diameter of 75 mm, a pressure of about 440 KPa may be applied to flatten a press mold having a thickness of 5 mm, and about 110 KPa to flatten a press mold having a thickness of 3 mm. Apply pressure. Thus, the thickness of the press mold may be adjusted so that the press molding surface becomes flat at a pressure suitable for glass press molding.

  However, if the thickness of the press mold is made too thin, the strength of the press mold decreases and the temperature of the rigid body tends to rise as well as the temperature of the press mold, and the flattening reference surface is deformed by thermal expansion. As a result, there is a risk of hindering flattening of the press molding surface. As described above, if the thickness deviation and flatness of the prototype are measured, and the thickness of the press mold is determined so that the thickness deviation and flatness are within the required range, the support surface of the rigid body is deformed. Can also be avoided. In addition, when using the said mold material, it is preferable that the thickness of a press-molding die shall be 1 mm or more, it is more preferable to set it as 2 mm or more, and it is further more preferable to set it as 2.5 mm or more.

  Since the press molding surface is transferred to glass and the main surface of the glass blank is formed, the surface roughness of the press molding surface and the surface roughness of the glass blank main surface are substantially equal. Since the surface roughness of the glass blank main surface is preferably in the range of 0.01 to 10 [mu] m when performing scribe processing and grinding using a diamond sheet, which will be described later, the surface roughness of the press-molded surface is also 0.1. A range of 01 to 10 μm is preferred.

  Since the falling speed of the molten glass lump just before pressing reaches several meters per second, the position of the molten glass lump at the start of pressing may vary every press forming. Therefore, by making the size of the press molding surface larger than the main surface of the glass blank to be molded, it is possible to stably produce a glass blank of the required shape even if the position of the molten glass lump at the start of pressing is shifted. It is desirable to make it. For these reasons, it is desirable that the press molding surface be flat over the entire surface and no protrusions or dents be provided. For the same reason, it is not preferable to provide a press mold with a surface for transfer molding of the outer peripheral surface of the glass blank.

  The material constituting the rigid body is preferably a metal or an alloy in consideration of heat resistance, workability and durability. A material suitable as a press mold material such as alloy tool steel (SKD61 or the like) can be used for the rigid body. Since the flattening reference surface of the rigid body is transferred to the main surface of the glass blank through a press mold, it is desirable to process it flatly, and the flatness is preferably 1 μm or less.

  In order to form the molten glass lump into a thin plate shape, it is desirable that the time from the start of pressing until the glass is pressed and stretched into a thin plate shape is within 0.1 seconds. The pressure at the time of pressing is not less than the pressure necessary for flattening the press molding surface, and may be adjusted so as to obtain a thin glass sheet having a required thickness.

  After the molten glass lump is formed into a thin plate shape, the press pressure is reduced while maintaining the pressure required for flattening the press molding surface, and the press molding surface and the glass are kept in close contact for about several seconds, and then the press is performed. Press the pressure to zero and release the pressed state against the rigid body of the press mold. Furthermore, apply pressure from the outer peripheral direction to the glass blank that is in close contact with the press molding surface to release the adhesive state, and take out the glass blank. . The glass blank is preferably taken out after being cooled to a temperature at which the glass is not deformed by an external force.

  In order to further improve the thickness deviation and flatness of the glass blank, it is desirable to further reduce the viscosity distribution of the molten glass lump at the start of the press so that the glass stretches more uniformly and thinly by the press. It is preferable to cut the molten glass flow hanging from the glass outlet and to separate and drop the molten glass lump. By doing in this way, since a molten glass is surrounded by gas with high heat insulation, for example, air | atmosphere until press start, it can prevent that one part temperature falls locally and viscosity distribution increases.

  In addition, in the manufacturing method of the glass blank of this embodiment, although it is preferable to make the viscosity of the molten glass which flows out into the range of 50dPa * s-1050dPa * s, a molten glass flow is drooped to a glass outlet as mentioned above. When the molten glass lump is separated, the viscosity at the time of outflow of the molten glass is preferably in the range of 500 dPa · s to 1050 dPa · s. When the viscosity is less than 500 dPa · s, it becomes difficult to separate a necessary amount of molten glass lump in a state where the molten glass flow is suspended in the air. For molten glass with a viscosity at the outflow of less than 500 dPa · s, the lower end of the molten glass flow is supported below the glass outlet, and the molten glass lump is accumulated after the amount of molten glass necessary to obtain the molten glass lump is accumulated. May be separated, dropped and press-molded.

  The molten glass flow may be cut by using a method of shearing the glass by abutting the tip of the opposing shear blade, or a method of shearing the glass by intersecting the opposing shear blades.

  The difference in height between the position where the molten glass lump is separated and the position where it is pressed, i.e., if the falling distance of the molten glass lump is long, the viscosity of the glass increases, and there is a risk of deviating from the viscosity range suitable for pressing. The falling speed of the lump increases and it becomes difficult to press the molten glass lump at a position suitable for pressing. Therefore, it is preferable that the falling distance of the molten glass lump is within 1000 mm. The preferable range of the drop distance is 500 mm or less, the more preferable range is 300 mm or less, and the more preferable range is 200 mm or less. The lower limit of the falling distance is that the molten glass lump in the air (in a state separated from the molten glass stream) can be pressed, and that the cutting operation and the press molding operation of the molten glass stream do not interfere with each other. It is the lower limit.

  The glass blank taken out from the press mold is subjected to annealing to reduce and remove distortion, and then sent to the subsequent process.

  According to the above method, a glass blank having a thickness deviation of 10 μm or less and a flatness of 10 μm or less can be stably produced. A preferable range of the flatness of the glass blank is 8 μm or less, a more preferable range is 6 μm or less, and a further preferable range is 4 μm or less.

  The manufacturing method of the glass blank of this embodiment is suitable for manufacturing a glass blank having a ratio of diameter to plate thickness (diameter / plate thickness) of 50 to 150. Here, the diameter is an arithmetic average of the major axis and the minor axis of the glass blank.

  As described above, since the outer peripheral surface of the glass blank is not restricted by the press mold, the outer peripheral surface is a free surface, but the roundness of the glass blank to be formed is within ± 0.5 mm.

  There is no particular limitation on the diameter of the glass blank, but the setting of the diameter is performed by adding the removal amount at the time of scribe processing and outer periphery processing when processing the magnetic recording medium substrate from the glass blank to the substrate diameter, as will be described later. It is preferable to carry out the above values.

  The glass blank has a thickness of 0.75 to 1.1 mm, preferably 0.75 to 1.0 mm, and more preferably 0.90 to 0.92 mm.

  The thickness, thickness deviation, flatness, diameter, and roundness of the glass blank may be measured using a three-dimensional measuring instrument and a micrometer.

  The composition of the glass used may be appropriately selected according to the properties required for the magnetic recording medium substrate.For example, aluminosilicate glass, soda lime glass, soda aluminosilicate glass, aluminoborosilicate glass, borosilicate glass, etc. Can be mentioned. These glasses may be crystallized glass that is crystallized by heat treatment, and may be processed into a substrate after being crystallized by heat treatment.

  Glass used for a substrate of a magnetic recording medium such as a magnetic disk is desired to have chemical durability, high rigidity, and a high thermal expansion coefficient. Furthermore, when emphasizing increasing the bending strength, it is required to have a composition that can be chemically strengthened, and when high-temperature heat treatment is performed in the manufacturing process of a magnetic recording medium, the composition must have high heat resistance. Is desired.

As glass with chemical durability, high rigidity, and high thermal expansion coefficient,
Converted to oxide standards and expressed in mol%,
The SiO ₂ 50~75%,
Al ₂ O ₃ 0 to 15%
Li ₂ O, Na ₂ O and K ₂ O in total 5 to 35%,
MgO, CaO, SrO, 0 to 35% BaO and ZnO in a total, and _{ZrO 2, TiO 2, La 2} O 3, Y 2 O 3, Ta 2 O 5, Nb 2 O 5 and HfO ₂ in total 0 A glass containing -15% can be exemplified.

  In addition, in order to improve the foaming at the time of clarification, it is desirable to add 0.1 to 3.5 mass% of Sn oxide and Ce oxide by the external total content. In this case, the mass ratio of the Sn oxide content to the total content of Sn oxide and Ce oxide (Sn oxide mass / (Sn oxide mass + Ce oxide mass)) is 0.01 to 0. .99. Hereinafter, unless otherwise specified, the content of glass components and the total content are expressed in mol%, but the contents of Sn oxide and Ce oxide are expressed in mass%.

SiO ₂ is a glass network-forming component and an essential component that functions to improve glass stability, chemical durability, and particularly acid resistance. If the content of SiO ₂ is less than 50%, the above function cannot be obtained sufficiently, and if it exceeds 75%, undissolved matter is generated in the glass, or the viscosity of the glass at the time of clarification becomes too high, and the bubbles are blown out. It becomes insufficient. Therefore, the content of SiO ₂ is preferably 50 to 75%.

Al ₂ O ₃ also contributes to the formation of a glass network, functions to improve glass stability and chemical durability, and also functions to increase the ion exchange rate during chemical strengthening. When the content of Al ₂ O ₃ exceeds 15%, the meltability of the glass is lowered and undissolved substances are likely to be generated. In addition, the thermal expansion coefficient decreases and the Young's modulus also decreases. Therefore, the content of Al ₂ O ₃ is preferably 0 to 15%.

Li ₂ O, Na ₂ O and K ₂ O serve to improve the meltability and moldability of the glass. It also serves to increase the coefficient of thermal expansion. If the content of Li ₂ O, Na ₂ O and K ₂ O is less than 5%, the above function cannot be obtained sufficiently, and if it exceeds 35%, chemical durability, particularly acid resistance is lowered, or glass The thermal stability of is reduced. Further, the glass transition temperature is lowered and the heat resistance is also lowered. Therefore, the content of Li ₂ O, Na ₂ O and K ₂ O is preferably 5 to 35%. Of Li ₂ O, Na ₂ O, and K ₂ O, Li ₂ O has the greatest effect of lowering the glass transition temperature.

  MgO, CaO, SrO, BaO, and ZnO function to improve the meltability, moldability, and Young's modulus of glass. It also functions to increase the thermal expansion coefficient and Young's modulus. However, when the total content of MgO, CaO, SrO, BaO and ZnO exceeds 35%, chemical durability and thermal stability of the glass are lowered. Therefore, the total content of MgO, CaO, SrO, BaO and ZnO is preferably 0 to 35%.

ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O _5, and HfO ₂ improve chemical durability, especially alkali resistance, increase glass transition temperature, and heat resistance Improves the Young's modulus and fracture toughness. However, if the total content of ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ exceeds 15%, the melting property of the glass is lowered, This leaves undissolved glass raw materials. Therefore, the total content of ZrO ₂ , TiO ₂ , La ₂ O ₃ , Y ₂ O ₃ , Ta ₂ O ₅ , Nb ₂ O ₅ and HfO ₂ is preferably 0 to 15%.

  The composition range included in the composition range is exemplified below. The content of glass components and the total content are expressed in mol% unless otherwise specified.

The first glass emphasizes the efficiency of chemical strengthening, and its composition range is
SiO ₂ content: 60-75%,
Al ₂ O ₃ content: 3-12%,
The total content of Li ₂ O, Na ₂ O and _{K 2} O: _23~35%,
Total content of MgO, CaO, SrO, BaO and ZnO: 0 to 5%,
_{ZrO 2, TiO 2, La 2} O 3, Y 2 O 3, Yb 2 O 3, Ta 2 O 5, Nb 2 O 5 and the total content of HfO _2: 0~7%,
It is.

The second glass emphasizes chemical durability, and its composition range is
SiO ₂ content: 60-75%,
Al ₂ O ₃ content: 1 to 15%,
Total content of Li ₂ O, Na ₂ O and K ₂ O: 15-25%,
Total content of MgO, CaO, SrO, BaO and ZnO: 1-6%,
_{ZrO 2, TiO 2, La 2} O 3, Y 2 O 3, Yb 2 O 3, Ta 2 O 5, Nb 2 O 5 and the total content of HfO _2: 1~9%,
It is.

The third glass emphasizes high rigidity, and its composition range is
SiO ₂ content: 50-70%,
Al ₂ O ₃ content: 1-8%,
Total content of Li ₂ O, Na ₂ O and K ₂ O: 12-22%
Total content of MgO, CaO, SrO, BaO and ZnO: 10-20%,
_{ZrO 2, TiO 2, La 2} O 3, Y 2 O 3, Yb 2 O 3, Ta 2 O 5, Nb 2 O 5 and the total content of HfO _2: 3~10%,
It is.

The fourth glass emphasizes high heat resistance, and its composition range is
SiO ₂ content: 50-70%,
Al ₂ O ₃ content: 1-10%,
Total content of Li ₂ O, Na ₂ O and K ₂ O: 5 to 17%
(Of which Li ₂ O content: 0 to 1%),
Total content of MgO, CaO, SrO, BaO and ZnO: 10-25%,
_{ZrO 2, TiO 2, La 2} O 3, Y 2 O 3, Yb 2 O 3, Ta 2 O 5, Nb 2 O 5 and the total content of HfO _2: 1~12%,
It is.

The fifth glass emphasizes high heat resistance, high rigidity, and high thermal expansion, and its composition range is
SiO ₂ content: 50-75%,
Al ₂ O ₃ content: 0 to 5%,
Total content of Li ₂ O, Na ₂ O and K ₂ O: 3 to 15%
(Of which Li ₂ O content: 0 to 1%),
Total content of MgO, CaO, SrO, BaO and ZnO: 14 to 35%,
_{ZrO 2, TiO 2, La 2} O 3, Y 2 O 3, Yb 2 O 3, Ta 2 O 5, Nb 2 O 5 and the total content of HfO _2: 2~9%,
It is.

[Method of manufacturing magnetic recording medium substrate]
The method of manufacturing a magnetic recording medium substrate according to the present embodiment is characterized in that a magnetic recording medium substrate is manufactured through at least a step of polishing the glass blank manufactured by the method of manufacturing a glass blank of the present embodiment. .

  First, scribing is performed on a glass blank obtained by press molding. Scribe is a glass blank that is cut into two concentric circles (inner concentric circle and outer concentric circle) with a scriber made of super steel alloy or diamond particles on the surface of the glass blank in order to make the molded glass blank into a ring shape of a predetermined size. This refers to providing a line (linear scratch). The glass blank scribed in two concentric shapes is partially heated and the outer portion of the outer concentric circle is removed due to the difference in thermal expansion of the glass. As a result, a perfect circular disk-shaped glass is obtained.

  Since the roughness of the main surface of a glass blank is 1 micrometer or less, a cutting line can be suitably provided using a scriber. In addition, when the roughness of the main surface of a glass blank exceeds 1 micrometer, since a scriber does not follow surface unevenness and a cutting line cannot be provided uniformly, scribing is performed after the main surface is smoothed.

  Next, shape processing of the scribed glass is performed. Shape processing includes chamfering (chamfering of the outer peripheral end and the inner peripheral end). In chamfering, chamfering is performed on the outer peripheral end and inner peripheral end of the ring-shaped glass with a diamond grindstone.

  Next, the end surface of the disk-shaped glass is polished. In the end surface polishing, the inner peripheral side end surface and the outer peripheral side end surface of the glass are mirror-finished by brush polishing. At this time, a slurry containing fine particles such as cerium oxide as free abrasive grains is used. Preventing the occurrence of ion precipitation that causes corrosion such as sodium and potassium by removing contamination such as dirt, damage or scratches attached to the end surface of the glass by end face polishing Can do.

  Next, 1st grinding | polishing is given to the main surface of disk-shaped glass. The first polishing is intended to remove scratches and distortions remaining on the main surface.

  The machining allowance by the first polishing is, for example, about several μm to 10 μm. Since it is not necessary to perform a grinding process with a large machining allowance, the glass is not scratched or distorted due to the grinding process. Therefore, the machining allowance in the first polishing process is small.

  In the first polishing step and the second polishing step described later, a double-side polishing apparatus is used. The double-side polishing apparatus is an apparatus that performs polishing by using a polishing pad and relatively moving a disk-shaped glass and a polishing pad.

  The double-side polishing apparatus includes a polishing carrier mounting portion having an internal gear and a sun gear that are driven to rotate at a predetermined rotation ratio, and an upper surface plate and a lower surface plate that are driven to rotate reversely with respect to the polishing carrier mounting portion. Have A polishing pad, which will be described later, is attached to the surfaces of the upper and lower surface plates facing the disk-shaped glass. The polishing carrier mounted so as to mesh with the internal gear and the sun gear revolves around the sun gear while rotating around the sun gear.

  Each of the polishing carriers holds a plurality of disc-shaped glasses. The upper surface plate is movable in the vertical direction, and presses the polishing pad against the main surfaces of the front and back surfaces of the disk-shaped glass. Then, while supplying a slurry (polishing liquid) containing abrasive grains (polishing material), the planetary gear motion of the polishing carrier and the upper surface plate and the lower surface plate rotate reversely to each other, so that the disk-shaped glass and the polishing pad The main surfaces of the front and back surfaces of the disk-shaped glass are polished.

  In the first polishing step, for example, a hard resin polisher is used as the polishing pad, and for example, cerium oxide abrasive is used as the polishing material.

  Next, the disk-shaped glass after the first polishing is chemically strengthened. As the chemical strengthening liquid, for example, a mixed liquid of potassium nitrate (60%) and sodium nitrate (40%) can be used. In chemical strengthening, the chemical strengthening liquid is heated to, for example, 300 ° C. to 400 ° C., and the cleaned glass is preheated to, for example, 200 ° C. to 300 ° C., and then the glass is placed in the chemical strengthening liquid, for example, 3 hours to 4 hours. Soaked. The immersion is preferably performed in a state of being housed in a holder so that a plurality of glasses are held at the end faces so that both main surfaces of the glass are chemically strengthened.

  Thus, by immersing the glass in the chemical strengthening solution, lithium ions and sodium ions on the surface layer of the glass are replaced with sodium ions and potassium ions having relatively large ionic radii in the chemical strengthening solution, respectively. A compressive stress layer with a thickness of ˜200 μm is formed. As a result, the glass is strengthened and has good impact resistance. Note that the chemically strengthened glass is washed. For example, after washing with sulfuric acid, washing with pure water, IPA (isopropyl alcohol), or the like.

  Next, second polishing is performed on the chemically strengthened and sufficiently cleaned glass. The machining allowance by the second polishing is, for example, about 1 μm.

  The second polishing is intended to finish the main surface into a mirror surface. In the second polishing step, as with the first polishing step, the disc-shaped glass is polished using a double-side polishing apparatus. The polishing abrasive grains contained in the polishing liquid (slurry) to be used and the composition of the polishing pad Is different. In the second polishing step, the grain size of the abrasive grains to be used is made smaller than in the first polishing step, and the hardness of the polishing pad is made softer. For example, in the second polishing process, for example, a soft foamed resin polisher is used as the polishing pad, and as the abrasive, for example, cerium oxide abrasive grains finer than the cerium oxide abrasive grains used in the first polishing process are used.

  The disk-shaped glass polished in the second polishing process is washed again. In cleaning, a neutral detergent, pure water, and IPA are used.

  By the second polishing, a magnetic disk glass substrate having a main surface flatness of 4 μm or less and a main surface roughness of 0.2 nm or less is obtained.

  In the process of producing a glass substrate for a magnetic disk from a glass blank, the lapping process for reducing the plate thickness deviation and increasing the flatness is omitted. Nevertheless, the reason why the flatness of the main surface of the substrate is excellent at 4 μm or less is that the thickness deviation of the glass blank is small and the flatness is excellent.

  Thereafter, each layer such as a magnetic layer is formed on the glass substrate for magnetic disk to produce a magnetic disk.

  In addition, although a chemical strengthening process is performed between a 1st grinding | polishing process and a 2nd grinding | polishing process, it is not limited to this order. As long as a 2nd grinding | polishing process is performed after a 1st grinding | polishing process, a chemical strengthening process can be arrange | positioned suitably. For example, the order of the first polishing process → the second polishing process → the chemical strengthening process (hereinafter, process order 1) may be used. However, in the process order 1, since the surface unevenness that may be generated by the chemical strengthening process is not removed, the process order of the first polishing process → the chemical strengthening process → the second polishing process is more preferable.

[Method of manufacturing magnetic recording medium]
The method for manufacturing a magnetic recording medium according to this embodiment includes at least a magnetic recording layer forming step for forming a magnetic recording layer on the magnetic recording medium substrate manufactured by the method for manufacturing a magnetic recording medium substrate according to this embodiment. A recording medium is produced.

  A magnetic recording medium substrate (magnetic disk) is produced by depositing a layer such as a magnetic layer on the main surface of the magnetic recording medium substrate (magnetic disk glass substrate) produced by the method described above. For example, an adhesion layer, a soft magnetic layer, a nonmagnetic underlayer, a perpendicular magnetic recording layer, a protective layer, and a lubricating layer are sequentially laminated from the main surface side of the substrate. For example, a Cr alloy is used for the adhesion layer, and functions as an adhesion layer with the glass substrate. For example, a CoTaZr alloy or the like is used for the soft magnetic layer, a granular nonmagnetic layer or the like is used for the nonmagnetic underlayer, and a granular magnetic layer or the like is used for the perpendicular magnetic recording layer. In addition, a material made of hydrogen carbon is used for the protective layer, and a fluorine resin or the like is used for the lubricating layer, for example.

More specifically, an in-line sputtering apparatus is used on the glass substrate, and CrTi adhesion layer, CoTaZr / Ru / CoTaZr soft magnetic layer, and CoCrSiO ₂ nonmagnetic granular layer are formed on both main surfaces of the glass substrate. A base layer, a CoCrPt—SiO ₂ · TiO ₂ granular magnetic layer, and a hydrogenated carbon protective film are sequentially formed. Further, a perfluoropolyether lubricating layer is formed on the formed uppermost layer by a dip method to obtain a magnetic recording medium (magnetic disk).

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to a following example.

Example 1
Raw materials such as oxides, carbonates, nitrates, and hydroxides were weighed so as to obtain a glass having the composition shown in Table 1, and mixed thoroughly to obtain a blended raw material. This raw material is put into a melting tank in a glass melting furnace, heated and melted, the obtained molten glass is flowed from the melting tank to the clarification tank, defoamed in the clarification tank, and further poured into the work tank. The mixture was stirred and homogenized in the work tank, and flowed out of the glass outflow pipe attached to the bottom of the work tank. The temperature of the melting tank, clarification tank, work tank, and glass outflow pipe is controlled, and the temperature and viscosity of the glass are maintained in the optimum state in each step.

  The molten glass flowing out from the glass outflow pipe was cast into a mold. The glass transition temperature and the liquidus temperature were measured using the obtained glass as a sample. The measuring method of glass transition temperature and liquidus temperature is shown below.

(1) Glass transition temperature Tg
The glass transition temperature Tg of each glass was measured using a thermomechanical analyzer (TMA).
(2) Liquid phase temperature Put a glass sample in a platinum crucible, hold it at a predetermined temperature for 2 hours, take it out of the furnace, cool it, observe the presence or absence of crystal precipitation with a microscope, and set the minimum temperature at which no crystal is observed to the liquid phase temperature. (LT).
Table 1 shows the glass transition temperature and liquidus temperature of each glass.

  Glass blanks were sequentially produced using these glasses. First, a pair of shear blades for cutting a molten glass flow that flows out below the outlet of the glass outflow pipe, and a press molding apparatus that press-molds a molten glass lump that is further cut and separated by the shear blade below and falls. Place.

  FIG. 1 is a schematic view showing a state in which a glass blank 4-A is produced from a molten glass 2 flowing out from an outflow pipe 1 by press molding from the horizontal direction. The molten glass stream 2 flowing down the outflow pipe 1 and flowing out from the glass outlet is cut by a pair of opposed shear blades 3, and a molten glass lump 4 corresponding to one glass blank is separated from the molten glass stream 2. And fall freely vertically downward. In addition, the outflow viscosity of the molten glass was adjusted so as to be constant in the range of 500 to 1050 dPa · s.

  The position of the press mold is adjusted so that the falling path of the molten glass lump 4 passes between the pair of press molds 5-1 and 5-2 facing each other.

  Further, the height of the press mold was adjusted so that the falling distance from when the molten glass lump was separated until it was pressed was within 200 mm.

  The press mold 5-1 and the press mold 5-2 were made of cast iron (FCD), and both the flatness of the press molding surface and the back surface was 1 μm or less, and the thickness was 5 mm.

  A rigid body 6-1 and a rigid body 6-2 made of alloy tool steel (SKD61) are disposed on the back surfaces of the press mold 5-1 and the press mold 5-2, respectively. Both the rigid body 6-1 and the rigid body 6-2 are sufficiently thicker than the press mold and have high rigidity.

  The press mold 5-1 and the rigid body 6-1, and the press mold 5-2 and the rigid body 6-2 are held by members (not shown), respectively, and the contact state is released when no press pressure is applied. When pressure is applied, the back surface of the press mold and the flattening reference surface of the rigid body are in close contact with each other.

  FIG. 1 (a) schematically shows a state in which a molten glass lump 4 enters between press molds by press molding after a glass blank has been molded several times or more to reach a steady state. It is.

  In order to press the molten glass lump 4 that is falling, two opposing press molds are driven by a driving device (not shown). In this state, since no pressure is applied to the two press molds, the two press molds slightly warp due to thermal expansion, and the initially flat press mold surface is a convex surface. There is a gap between the center of the back surface of the press mold and the flattening reference surface of the rigid body.

  FIG. 1B shows a process in which pressing is started and the molten glass lump is spread by the press forming surface of the press mold. In this state, a pressure of about 6.7 MPa is applied to the glass, the press mold 5-1 and the press mold 5-2. Due to this pressure, the press mold 5-1 is pressed against the flat planarization reference surface of the rigid body 6-1 and the press mold 5-2 is pressed against the flat planarization reference surface of the rigid body 6-2 to warp. Is corrected, and the press molding surface is flattened.

FIG. 1 (c) shows a state in which the glass has been pressed and spread to a required plate thickness as compared with FIG. 1 (b). The two press forming surfaces are brought close to each other in a parallel state up to the target plate thickness, and are stopped by a press forming surface interval regulating member (not shown). Even in this state, pressure is applied to the press mold, so that the flatness is maintained on the two press molding surfaces, and these shapes are transferred to the glass to form the main surface of the glass blank that is flat and parallel to each other. The time from the start of pressing to molding the glass into the shape of a glass blank is set to within 0.1 seconds, then the pressure is lowered to about 440 KPa for about several seconds, keeping both press-formed surfaces in close contact with the glass, and cooling the glass Let During this time, the entire back surface of the press mold is maintained in contact with the flattening reference surface, and the press mold surface is flat.
Next, the press pressure is released, the press mold and the rigid body are moved backward, the glass blank is released, and the glass blank is removed.

  In the series of steps described above, the rigid body may be cooled using a cooling medium to suppress the temperature rise.

When the diameter, roundness, plate thickness, plate thickness deviation, and flatness of the obtained glass blank were measured using a three-dimensional measuring instrument and a micrometer, the diameter was 75 mm, the roundness was within ± 0.5 mm, The plate thickness was 0.90 mm, the plate thickness deviation was 10 μm or less, and the flatness was 4 μm or less. From the above measurement result, the diameter / plate thickness ratio is 83.3.
Since the shape and dimensions of the prototype thus obtained were within the required range, the above process was repeated to mass-produce disc-shaped glass blanks made of various types of glass and sample several mass-produced products. As a result of measuring the diameter, roundness, plate thickness, plate thickness deviation, and flatness, the same measurement results were obtained for the prototype.

  Next, the thickness of the press mold was reduced from 5 mm to 3 mm, and a prototype was produced in the same manner. By reducing the thickness of the press mold by 2 mm, the pressure for maintaining the flatness of the press mold surface can be reduced to about 110 KPa. When the glass is cooled while being sandwiched between the press-molded surfaces, the glass may be damaged if a high pressure is applied excessively. By reducing the pressure to maintain the flatness of the press-molded surface, the risk of glass breakage during such a cooling process can be reduced. The glass blank is annealed to reduce and remove strain.

(Example 2)
Using the glass blank produced in Example 1, a scribing process was performed on the part that becomes the outer periphery of the magnetic disk substrate and the part that becomes the central hole. By such processing, two concentric grooves are formed on the outer side and the outer side. Next, the scribed portion is partially heated to generate a crack along the scribed groove due to the difference in thermal expansion of the glass, and the outer and inner portions of the outer concentric circle are removed. As a result, a perfect circular disk-shaped glass is obtained.

  Next, the shape of the disk-shaped glass was processed by chamfering or the like, and end face polishing was further performed.

  Next, after subjecting the main surface of the disk-shaped glass to the first polishing, the glass is immersed in a chemical strengthening solution and chemically strengthened.

  After chemical strengthening, the glass that was sufficiently washed was subjected to the second polishing. After the second polishing step, the disk-shaped glass was washed again to produce a magnetic disk glass substrate. The substrate has an outer diameter of 65 mm, a center hole diameter of 20 mm, a thickness of 0.8 mm, a main surface flatness of 4 μm or less, and a main surface roughness of 0.2 nm or less. A recording medium substrate could be obtained.

(Example 3)
On both main surfaces of the magnetic recording medium substrate (magnetic disk glass substrate) produced in Example 2, an in-line sputtering apparatus was used to sequentially deposit a CrTi adhesion layer, a CoTaZr / Ru / CoTaZr soft magnetic layer, and CoCrSiO. ₂ non-magnetic granular underlayer, CoCrPt-SiO ₂ · TiO ₂ granular magnetic layer, hydrogenated carbon protective film, and perfluoropolyether lubricating layer as the top layer by dipping method to form a magnetic recording medium (Magnetic disk) was obtained.

  When the magnetic disk thus obtained was incorporated into a hard disk drive and checked for operation, the expected performance was obtained.

DESCRIPTION OF SYMBOLS 1 Glass outlet 2 Molten glass flow 3 Shear blade 4 Molten glass lump 4-A Glass blank 5-1, 5-2 Press mold 6-1, 6-2 Rigid body

Claims (5)

A step of separating a certain amount of molten glass from the molten glass flow flowing out from the glass outlet, press-molding the thin glass using a press mold, and repeating the steps of taking out the thin glass from the press mold, and the magnetic recording medium In the manufacturing method of the glass blank which produces the glass blank for processing into a substrate,
Preparing a pair of thin plate-shaped press molds, dropping the molten glass lump separated from the molten glass stream, and press-molding the molten glass lump falling to produce the thin glass;
And, the surface opposite to the press molding surface of the press mold is pressed against the rigid body by the pressure when pressing the molten glass lump, and the press mold surface is elastically deformed to flatten the press molding surface. And, in a state where the press molding surfaces of the press mold are parallel to each other, transferring the shape of the press molding surface to the molten glass lump to mold the main surface of the thin glass plate,
A method for producing a glass blank. The press mold has a surface opposite to the press molding surface, processed flat and parallel to the press molding surface,
The method for producing a glass blank according to claim 1, wherein the surface opposite to the press-molding surface is pressed against a flat surface of the rigid body to flatten the press-molded surface.   The method for producing a glass blank according to claim 1, wherein the molten glass flow hanging from the glass outlet is cut to separate and drop the molten glass lump.   A magnetic recording medium substrate comprising a glass blank produced by the method for producing a glass blank according to any one of claims 1 to 3 at least through a step of polishing the glass blank. Manufacturing method.   A magnetic recording medium is produced through at least a magnetic recording layer forming step of forming a magnetic recording layer on the magnetic recording medium produced by the method for producing a magnetic recording medium substrate according to claim 4. A method for manufacturing a magnetic recording medium.

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