JP6209615B2 - Glass substrate for magnetic disk, method for manufacturing glass substrate for magnetic disk, and magnetic disk - Google Patents

Description

  The present invention relates to a glass substrate for a magnetic disk, a method for manufacturing a glass substrate for a magnetic disk, and a magnetic disk.

  Conventionally, a glass substrate is suitably used for a magnetic disk used as one of information recording media. Today, in response to a request for an increase in storage capacity in a hard disk drive device, the density of magnetic recording has been increased. In recent years, a recording medium for an energy-assisted magnetic recording method such as a heat-assisted magnetic recording method in which an iron-platinum magnetic layer is formed on a glass substrate has been proposed in order to increase the magnetic recording density of a magnetic disk. The glass substrate used for this energy-assisted recording medium is exposed to a high temperature of about 700 ° C. during the formation of the iron-platinum magnetic layer, so that the glass transition is maintained while maintaining the mechanical strength of the glass substrate. A high point temperature is required.

On the other hand, it is also known to form a compressive stress layer on a glass substrate by subjecting the glass substrate to a chemical strengthening treatment in order to increase mechanical strength. The compressive stress value of the compressive stress layer and the depth of the compressive stress layer are obtained by the Babinet corrector method. A method of providing a glass substrate having a compressive stress or tensile stress profile suitable for a glass substrate for a magnetic disk from the compressive stress value obtained by the Babinet corrector method and the depth of the compressive stress layer is also known. For example, in order to have high impact resistance, the outermost stress layer indentation length of the main surface of the glass substrate is 49.1 μm or less, and the stress profile by the Babinet corrector method is between the main surface and the compressive stress. A glass substrate subjected to a chemical strengthening treatment is known in which a value y determined by θ is equal to or shorter than the outermost stress layer indentation length when θ is an angle formed (Patent Document 1).
In addition, a glass substrate for a data storage medium that is excellent in impact resistance and has a stable disk shape and a manufacturing method thereof are also known. Specifically, the manufacturing method of the said glass substrate includes the chemical strengthening process process which immerses the glass for substrates in mixed molten salt, and forms a compression layer on the surface and back surface of this glass for substrates. At this time, the glass for a substrate contains lithium ions as an alkali component, the mixed molten salt contains 1 to 6% of sodium nitrate, potassium nitrate and lithium nitrate in terms of mass percentage, and the glass for a substrate is mixed into a molten salt at 325 ° C. It is immersed in a treatment time of 30 minutes or less at a treatment temperature of 475 ° C. or less and satisfies the following formula. T is a processing temperature (unit: K), and t is a processing time (unit: second). 1900 ≦ T × log (t ² ) ≦ 2900. In the glass substrate, it is also known that the mechanical strength can be increased by setting the fracture toughness value measured by an indenter press-fitting method based on JIS R1607 to 1.2 or more (Patent Document 2).

JP 2009-099251 A JP 2011-134367 A

However, in the glass material used for the magnetic disk glass substrate obtained by the above-mentioned known technique, the glass transition temperature is generally 600 ° C. or lower, so that the conventional magnetic disk glass substrate is used as the energy assist type recording medium. When used, the substrate is exposed to the high temperature, and the substrate is likely to warp or crack. For this reason, the conventional glass substrate cannot be applied to the glass substrate for the energy-assisted recording medium even if the mechanical strength at room temperature is high.
In order to increase the glass transition temperature, it is known to reduce the Li ₂ O content. However, in a glass substrate using such a glass, the Li content is small, so Na, K, etc. Even when a chemical treatment is performed using a known chemically strengthened molten salt solution containing bismuth, there is a case where a compressive stress layer having a mechanical strength increased to the extent required for a glass substrate for a magnetic disk cannot be formed. .

  Accordingly, the present invention provides a magnetic disk glass substrate having a mechanical strength suitable for a magnetic disk, a method for manufacturing the magnetic disk glass substrate, and a magnetic disk using the magnetic disk glass substrate. With the goal.

(Form 1)
One aspect of the present invention is a glass substrate for a magnetic disk having the following configuration.
The material of the glass substrate is amorphous aluminosilicate glass, and the glass substrate for magnetic disk contains 0.1 to 1% of Li ₂ O in terms of mol% as a glass composition,
Fracture toughness value K _IC [MPa · m ^1/2 ] is 1 [MPa · m ^1/2 ] or more,
A compressive stress layer by chemical strengthening is provided on the glass substrate surface, and the depth D [μm] of the compressive stress layer satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm].
The fracture toughness value K _IC [MPa · m ^1/2 ] is preferably 2.5 [MPa · m ^1/2 ] or less. The depth D is preferably 150 μm or less.

(Form 2)
The glass substrate for magnetic disk according to mode 1, wherein the glass substrate for magnetic disk has a transition temperature Tg of 650 ° C. or higher.

(Form 3)
Also,
As a glass composition, in mol% display,
The SiO _2, 55~78%,
Li ₂ O, 0.1-1%,
Na ₂ O, 2-15%,
MgO, CaO, SrO and BaO in a total of 10 to 25%,
Including
The glass substrate for a magnetic disk according to mode 1 or 2, wherein the molar ratio of the content of CaO to the total content of MgO, CaO, SrO, and BaO (CaO / (MgO + CaO + SrO + BaO)) is 0.20 or less.

(Form 4)
The glass substrate for magnetic disks according to any one of Embodiments 1 to 3, wherein the thickness of the glass substrate for magnetic disks is 0.3 to 1.5 mm.
At this time, when the glass substrate for magnetic disk has a nominal size of 2.5 inches or more, it is 0.5 mm or more.

(Form 5)
The glass substrate for magnetic disks according to any one of Embodiments 1 to 4, which is composed of aluminosilicate glass not containing Ca as a glass composition of the glass substrate for magnetic disks.

(Form 6)
A magnetic disk comprising: the glass substrate for a magnetic disk according to any one of forms 1 to 5; and a magnetic layer formed on a surface of the glass substrate.

(Form 7)
A method of manufacturing a glass substrate for a magnetic disk,
A process for producing a glass substrate, which is a disk-shaped glass substrate and made of amorphous aluminosilicate glass, containing 0.1 to 1% of Li ₂ O in terms of mol%,
When the glass substrate is chemically strengthened, the fracture toughness value K _IC [MPa · m ^1/2 ] is 1 [MPa · m ^1/2 ] or more and is provided on the glass substrate surface by the chemical strengthening. And a process of performing the chemical strengthening so that the depth D [μm] of the compressive stress layer satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. A method for producing a glass substrate for a disk.

(Form 8)
In the chemical strengthening treatment, the glass substrate is immersed in a mixed molten salt liquid containing KNO ₃ and NaNO ₃ , and the mixed molten salt contains 55 to 85 mass% of KNO _3. Of manufacturing a glass substrate for magnetic disk.

(Form 9)
The glass substrate after the chemical strengthening treatment is
As a glass composition, in mol% display,
The SiO _2, 55~78%,
Li ₂ O, 0.1 to 1%,
Na ₂ O, 2-15%,
MgO, CaO, SrO and BaO in a total of 10 to 25%,
Including
The manufacturing method of the glass substrate for magnetic disks of the form 7 or 8 whose molar ratio (CaO / (MgO + CaO + SrO + BaO)) of CaO content with respect to the total content of MgO, CaO, SrO and BaO is 0.20 or less.

(Form 10)
The plate | board thickness of the said glass substrate is a manufacturing method of the glass substrate for magnetic discs as described in any one of form 7-9 which is 0.3-1.5 mm.
In that case, the plate thickness is preferably 0.5 mm or more.

(Form 11)
The said glass substrate is a manufacturing method of the glass substrate for magnetic discs as described in any one of the forms 7-10 comprised with the aluminosilicate glass which does not contain Ca.

  According to the above-described aspect, it is possible to provide a glass substrate for a magnetic disk that surely has mechanical strength suitable for the magnetic disk.

It is a perspective view of the glass substrate for magnetic discs of this embodiment. It is a figure which shows the correspondence of the fracture toughness value before the flaw addition process of a glass substrate, and mechanical strength (bending strength). It is a figure explaining the compression stress layer and tensile stress layer of the glass substrate of this embodiment. It is a figure explaining the test method of mechanical strength (bending strength). It is a figure which shows the range of the glass substrate of this embodiment.

  Hereinafter, the glass substrate for magnetic disks of this embodiment will be described in detail.

[Magnetic disk glass substrate]
FIG. 1 is a perspective view of a magnetic disk glass substrate 10 (hereinafter referred to as a glass substrate 10) of this embodiment. As shown in FIG. 1, the glass substrate 10 has a disc shape and a ring shape in which a central portion is cut into a concentric shape. In the glass substrate 10, at least the surface of the glass substrate is chemically strengthened, and a compressive stress layer is formed on the surface of the glass plate 10.
The glass substrate 10 contains 0.1 to 1% of Li ₂ O as a glass composition in terms of mol%. As mechanical properties of the glass substrate 10, the fracture toughness value K _IC [MPa · m ^1/2 ] is 1 [MPa · m ^1/2 ] or more. A compressive stress layer by chemical strengthening is provided on the surface of the glass substrate 10, and the depth D [μm] of the compressive stress layer satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. More preferably, the fracture toughness value K _IC [MPa · m ^1/2 ] is 1.2 [MPa · m ^1/2 ] or more, and the compressive stress layer depth D [μm] is D ≧ 70 · K _IC ^{−. 1.6} and D ≧ 35 [μm] are satisfied.
It is preferable that the glass transition point temperature Tg of the glass of such a glass substrate 10 is 650 degreeC or more. In this case, the glass substrate 10 can be used for various glass substrates for magnetic disks, but can be preferably used for an energy assist type magnetic disk glass substrate.
The fracture toughness value K _IC [MPa · m ^1/2 ] is preferably 2.5 [MPa · m ^1/2 ] or less. The depth D is preferably 150 μm or less.

When a magnetic disk is manufactured using the glass substrate 10, the magnetic disk is manufactured by forming at least a magnetic layer on the main surface of the glass substrate 10.
The magnetic layer includes, for example, a magnetic material whose main component is an alloy of Fe and / or Co and Pt. The magnetic disk having this magnetic layer is suitably used for an energy assist type magnetic disk. Examples of the magnetic material mainly composed of an alloy of Fe and / or Co and Pt include an Fe—Pt magnetic material, a Co—Pt magnetic material, and an Fe—Co—Pt magnetic material.

  When the magnetic layer is formed, a magnetic material mainly composed of an alloy of Fe and / or Co and Pt is formed on the main surface of the glass substrate, and then an annealing process is performed. Here, the film formation temperature of the magnetic material is usually a high temperature exceeding 500 ° C. Furthermore, since these magnetic materials have the same crystal orientation after film formation, the annealing treatment is performed at a temperature exceeding the film formation temperature. Therefore, when a magnetic layer is formed using an Fe—Pt magnetic material, a Co—Pt magnetic material, or an Fe—Co—Pt magnetic material, the glass substrate is exposed to the high temperature. It is preferable that the glass substrate 10 of this embodiment is 650 degreeC or more as a glass transition point temperature from the point which has the outstanding heat resistance. The preferable lower limit of the glass transition temperature Tg is 660 ° C., the more preferable lower limit is 665 ° C., the still more preferable lower limit is 670 ° C., and the still more preferable lower limit is 675 ° C. However, if the glass transition temperature Tg is excessively increased, the chemical strengthening treatment temperature described later increases, and the molten salt undergoes thermal decomposition during chemical strengthening and corrodes the surface of the glass substrate. It is preferable to set it as 740 degreeC. The glass transition temperature Tg is almost constant before and after chemical strengthening. A glass substrate 10 made of glass having such a glass transition temperature Tg is formed of a Fe—Pt magnetic material, a Co—Pt magnetic material, or a magnetic layer of Fe—Co—Pt magnetic material on the glass substrate 10. Even after annealing and annealing, it has high flatness.

In order to make the glass transition temperature Tg higher than before, it is realized by adjusting the glass composition. Specifically, the glass transition temperature Tg can be increased by suppressing the content of alkali metals such as Li and Na.
In this respect, the glass used for the glass substrate 10 of the present embodiment contains 0.1 to 1% of Li ₂ O in terms of mol%. The content of Li ₂ O is, preferably 0.1 to 0.6%, more preferably from 0.1 to 0.3%.

With regard to the glass substrate having the above-described glass composition, the inventor of the present application variously studied the formation of a compressive stress layer that satisfies the mechanical strength required for a magnetic disk, paying attention to the chemical strengthening treatment used in the manufacturing method.
Generally, a known bending strength is used as the mechanical strength of the glass substrate for magnetic disks. The bending strength of the glass substrate for magnetic disks is the load at which the glass substrate breaks when a steel ball larger than the circular hole opened at the center of the glass substrate is placed on the circular hole and a load is gradually applied to the steel ball. The value is determined by the load. It can be said that this bending strength test method simulates the mechanical strength of a glass substrate for a magnetic disk fixed to a rotating spindle in a hard disk drive device. On the other hand, the fracture toughness value K _IC [MPa · m ^1/2 ] measured by an indenter press-fitting method based on JIS R 1607 has been conventionally used as an index of the mechanical strength of the glass substrate.

FIG. 2 shows a new glass substrate manufactured as a glass substrate for a magnetic disk, that is, a glass substrate in a state where the inner end surface around the circular hole of the glass substrate and the peripheral portion of the circular hole are not damaged as described later. The relationship between the fracture toughness value K _IC [MPa · m ^1/2 ] and the bending strength value was investigated. As can be seen from FIG. 2, it can be seen that the bending strength increases roughly as the fracture toughness value K _IC increases. However, the above correspondence is a result of using a glass substrate that is a new article and that has no scratches on the inner end surface around the circular hole of the glass substrate and the peripheral part of the circular hole on the main surface of the glass substrate. . The inner end surface around the circular hole provided at the center of the actual magnetic disk glass substrate and the peripheral portion of the circular hole on the main surface of the glass substrate are fixed to various members before being fixed to the rotating spindle of the hard disk drive device. May come into contact with minor scratches. For example, when the glass substrate is positioned while being in contact with the rotating spindle itself for fixing to the rotating spindle, or when the glass substrate is sampled and inspected by a contact sensor, the inner end surface and the main surface around the circular hole of the glass substrate are checked. The peripheral part of the circular hole may be damaged. In particular, the next-generation magnetic recording by the energy-assisted magnetic recording system has a very high track density, and therefore it is necessary to position the magnetic disk with respect to the spindle particularly strictly in order to suppress tracking errors. And it is easy to be damaged around the circular hole of a glass substrate at the time of this adjustment operation. As described above, the present inventor has found that the correspondence shown in FIG. 2 may not always be established due to scratches that occur before the glass substrate is actually incorporated into the hard disk drive device. In such a case, since the occurrence rate (defective rate) at which the bending strength is reduced is not within the allowable range, the yield of the magnetic disk glass substrate is reduced. Then, in particular Li 2 _O glass substrate a low content of, it found that the phenomenon is observed remarkably.

The inventor of the present application has determined that another index is required in addition to the fracture toughness value K _IC for such failure of the correspondence. Therefore, while variously changing the glass composition of the glass substrate and the conditions of chemical treatment applied to the glass substrate, the glass substrate becomes a rejected product that cannot be provided as a product because the bending strength is reduced due to the formation of scratches on the inner end face of the glass substrate. The defect rate was investigated, and an indicator that affects the defect rate was studied earnestly.
As a result, in order to obtain a glass substrate that does not warp even when the glass substrate is exposed to a high temperature of about 700 ° C., and that does not break even after being incorporated into the HDD after various adjustments, the content of Li ₂ O is 0.1. It was found that the defect rate of the glass substrate can be reduced by adjusting the chemical strengthening treatment conditions in the glass substrate of ˜1 mol%. And in the glass substrate made into the acceptance product in the screening using the acceptance rate (= 1-defective rate) of the glass substrate, the fracture toughness value K _IC is 1 [MPa · m ^1/2 ] or more, and chemical It has been found that the depth of the compressive stress layer formed on the glass surface by strengthening needs to be greater than a certain value and larger than a value determined based on the fracture toughness value K _IC .

That is, the fracture toughness value K _IC [MPa · m ^1/2 ] is 1 [MPa · m ¹ ] in the glass substrate 10 composed of extremely small glass with a Li ₂ O content of 0.1 to 1 mol%. ^{/ 2} ] or more, a compressive stress layer formed by chemical strengthening is provided on the glass substrate surface, and the depth D [μm] of the compressive stress layer is D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. Satisfied. Thus, by defining the fracture toughness value K _IC of the glass substrate and the depth D of the compressive stress layer, the glass substrate surely has the mechanical strength suitable for the magnetic disk. Further, when the fracture toughness value K _IC and the depth D of the compressive stress layer satisfy the above ranges, and the content of Li ₂ O is 0.1 to 1 mol%, the mechanical strength suitable for the magnetic disk is obtained. It was found that the glass substrate was provided with certainty. Hereinafter, the fracture toughness value K _IC and the depth D of the compressive stress layer will be described.

(Fracture toughness value K _IC )
The fracture toughness value K _IC [MPa · m ^1/2 ] is obtained by pressing a sharp diamond indenter (Vickers indenter) of a well-known Vickers hardness meter into a glass substrate with a load P [N] to form indentations and cracks on the glass substrate. It is obtained by. When the Young's modulus of the glass substrate is E [Pa], the indent diagonal length is d [m], and the half length of the surface crack is a [m], the fracture toughness value K _Ic [MPa · m ^1/2 ] is expressed by the following formula: It is represented by
K _IC = 0.026 · E ^1/2 · P ^1/2 · (d / 2) / a ^3/2
Unless otherwise specified, the fracture toughness value K _IC means a fracture toughness value measured with a load P of 9.81 N (1000 gf). The fracture toughness value changes depending on the glass composition and also on the chemical strengthening conditions. Therefore, in order to obtain a chemically strengthened glass substrate for a magnetic disk, the fracture toughness depends on the composition adjustment and the chemical strengthening treatment conditions. The value can be in the desired range.

(Compressive stress layer)
FIG. 3 is a diagram for explaining the compressive stress layer and the tensile stress layer of the glass substrate 10.
Since the glass substrate 10 of the present embodiment is chemically strengthened, the compressive stress layer 14 is sandwiched between the compressive stress layers 14 on both sides of the main surface 12 on the main surface on both sides of the glass substrate 10. A tensile stress layer 16 is formed on the surface. The compressive stress layer 14 and the tensile stress layer 16 are formed by chemically strengthening the glass substrate 10.

In general, when the glass constituting the glass substrate is immersed in a mixed molten salt of sodium salt and potassium salt and chemically strengthened, Li ions in the glass and Na ions in the molten salt undergo ion exchange, and the glass The Na ions therein and K ions in the molten salt are ion-exchanged to form a compressive stress layer near the surface and a tensile stress layer inside the glass. However, since the glass substrate 10 of the present embodiment uses a glass containing Li ₂ O of 1 mol% or less as the glass composition, the depth D of the compressive stress layer 14 is small and the maximum value of the compressive stress is extremely high. In general, in a glass substrate using glass containing more than 1 mol% of Li ₂ O, Li ions in the glass and Na ions in the molten salt are ion-exchanged, and Na ions in the glass and K ions in the molten salt are exchanged. As a result of ion exchange, a gentle stress distribution is formed. However, like the glass substrate 10 of this embodiment, the glass with a low Li ₂ O content contains little Li ions that are ion-exchanged with Na ⁺ ions in the molten salt. On the other hand, Na ions in the glass and K ions in the molten salt undergo ion exchange. Since the diffusion rate of alkali metal ions in the glass is smaller as the ion radius is larger, K ions that enter the glass by ion exchange do not diffuse into the glass and remain on the surface 12 of the glass. For this reason, the depth D of the compressive stress layer 14 of the glass substrate 10 containing only 1 mol% or less of Li ₂ O is shallow. Such a compressive stress layer can generally be measured by the Babinet method. However, in the glass substrate 10 of the present embodiment, the depth D of the compressive stress layer 14 is shallow, but the maximum value of the compressive stress becomes extremely large, and it is difficult to measure the value.
When the content of Li ₂ O is extremely low, it is difficult to obtain the effect of chemical strengthening. Therefore, the lower limit of the content of Li ₂ O is 0.1% in terms of mol%. A preferable range of the content of Li ₂ O is 0.1 to 0.6% in terms of mol%.
In the glass substrate 10 of the present embodiment, the depth D of the compressive stress layer 14 satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm].

As described above, the glass substrate 10 that defines the fracture toughness value K _IC and the depth D of the compressive stress layer 14 can be produced by a method for manufacturing a glass blank for a magnetic disk described later.
The plate thickness of the glass substrate 10 of the present embodiment is preferably 0.3 to 1.5 mm from the viewpoint of securing the strength of the glass substrate 10. The plate thickness is more preferably 0.5 mm or more.

(Test and evaluation of mechanical strength of glass substrate)
When the glass substrate 10 is incorporated in the hard disk drive device, the glass substrate 10 is fixed while being in contact with the rotary spindle, so that the inner end surface of the glass substrate 10 is easily damaged. Further, the position of the glass substrate is finely adjusted before being fixed to the rotating spindle. Further, the inner end face of the glass substrate 10 is fixed to a jig in order to record servo tracks before being incorporated into the hard disk drive device. At this time, the inner end surface of the glass substrate 10 is easily scratched. Such scratches reduce the mechanical strength (bending strength).
FIG. 4 is a diagram for explaining a test method of mechanical strength (bending strength). In the test of mechanical strength (bending strength), when a hard sphere 18 having a diameter larger than the circular hole is placed in a circular hole in the center of the glass substrate 10 and a load is applied to the hard sphere 18 below, the glass substrate 10 Find the minimum load that will break. The glass substrate is evaluated by this minimum load. Such a test of mechanical strength (bending strength) is performed by placing a glass substrate in a predetermined position on the inner end surface of the glass substrate 10 in advance through a rotating spindle and then removing it from the rotating spindle, that is, applying a scratch. Done after. By this scratching process, the glass substrate 10 reproduces the state of the glass substrate when incorporated in the hard disk drive device as described above.
As can be seen from FIG. 2 which shows the correspondence between the fracture toughness value K _IC and the mechanical strength (bending strength) of the glass substrate before the above-described scratching treatment, in the case of the glass substrate 10 having no scratches on the inner end face, The toughness value K _IC and the mechanical strength (bending strength) correspond relatively. However, in the glass substrate 10 in which the inner end face is scratched, the correspondence between the fracture toughness value K _IC and the mechanical strength (bending strength) is lowered.
For this purpose, in this embodiment, the mechanical strength of the glass substrate is evaluated after performing the above-described scratching treatment.

Evaluation of the mechanical strength (bending strength) of such a glass substrate was performed about the glass substrate 10 of several conditions produced by changing various conditions of a glass composition and a chemical strengthening process. When the mechanical strength (bending strength) is evaluated, the glass substrate has a bending strength of 95% or more, which satisfies the condition that the bending strength of the glass substrate satisfies 60 N or more. A plurality of conditions were classified as conditions that ensure that the mechanical strength suitable for the magnetic disk was reliably provided, and when the pass rate was less than 95%, the conditions were that the mechanical strength was inappropriate for the magnetic disk. FIG. 5 is a diagram showing the results of the above classification, and is a diagram showing a range of the glass substrate 10 in which the pass rate regarding the mechanical strength (bending strength) is 95% or more. In FIG. 5, among the glass substrates having a glass composition with a Li ₂ O content of 1 mol% or less, the hatched area is a range of glass substrates having a pass rate of 95% or more, that is, a machine suitable for a magnetic disk. It is the range of the glass substrate which is surely provided with a mechanical strength (bending strength). In the shaded region, the fracture toughness value K _IC is 1 [MPa · m ^1/2 ] or more, the depth D of the compressive stress layer 14 is D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. It is a satisfactory area. That is, the fracture toughness value K _IC [MPa · m ^1/2 ] of the glass substrate is 1 [MPa · m ^1/2 ] or more, and a compressive stress layer formed by chemical strengthening is provided on the glass substrate surface. The glass substrate having a depth D [μm] satisfying D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm] is a glass substrate having mechanical strength suitable for a magnetic disk. The fracture toughness value K _IC [MPa · m ^1/2 ] is 1.2 [MPa · m ^1/2 ] or more, and the depth D of the compressive stress layer is D ≧ 70 · K _IC ^−1.6 and D It is more preferable to satisfy ≧ 35 [μm].

Glass composition of such glass substrate _10, when a Li 2 O 0.1 to 1 mol%, the molten salt of the chemical strengthening treatment, the compression caused by the chemical strengthening a mixed salt containing KNO ₃ and NaNO ₃ This is preferable in that the stress layer can be appropriately formed.

(Chemical strengthening treatment)
The chemical strengthening treatment is a treatment in which a glass substrate is immersed in a high-temperature molten salt to exchange ions in the molten salt with ions in the glass. The molten salt preferably contains 55 to 85% by mass of KNO ₃ from the viewpoint of ensuring the mechanical strength. In this case, the content of NaNO ₃ is preferably 15 to 45% by mass. The content of KNO _{3 in} the molten salt is 60 to 80% by mass, and the content of NaNO ₃ is more preferably 20 to 40% by mass. By setting the content of KNO ₃ to 75% by mass or more, the fracture toughness value K _IC is increased. The content of KNO _{3 in} the molten salt is particularly preferably 75 to 85% by mass, and the content of NaNO ₃ is particularly preferably 15 to 25% by mass.

The treatment time for immersing in the molten salt in chemical strengthening needs to be adjusted appropriately depending on the temperature of the molten salt, but it is 2 hours or more in the range of 400 to 600 ° C., and 4 hours or more in order to form the deeper compressive stress layer 14. Preferably there is. However, when immersed in the molten salt for a long time, the fracture toughness value K _IC is lowered due to stress relaxation, and the processing time for chemical strengthening becomes long, so the processing time for immersing the glass substrate 10 in the molten salt is More preferably, it is 20 hours or less. From the above viewpoint, it is more preferably 4 to 16 hours. If the Li ₂ O content in the glass substrate is 0.1 to 1 mol%, it is difficult to obtain the effect of chemical strengthening, but by performing chemical strengthening treatment over a relatively high temperature and a long time, K _IC and compressive stress layer depth D satisfy the above range, and a glass substrate having high durability when scratched can be formed. When the Li ₂ O content in the glass substrate increases, the Li content in the glass substrate exchanged with K in the molten salt in the chemical strengthening treatment increases. In addition, in the present embodiment, since the chemical strengthening process is performed for a long time as described above, K ions can easily penetrate deeply into the glass substrate instead of Li ions. Therefore, although thicker the thickness of the compressive stress layer can be by chemical strengthening treatment, the stress value at the surface of the compressive stress layer tends to relax, it may not be possible to improve the K _IC. Therefore, Li 2 _O content in the glass substrate is less than 1 mol%.
Further, the processing temperature of the molten salt in chemical strengthening is preferably 400 ° C. or higher, and more preferably 450 ° C. or higher in order to obtain the K _IC and the depth D of the compressive stress layer 14 in a short time. On the other hand, if the treatment temperature is too high, the nitrate used for the chemical strengthening treatment will be decomposed, so that the temperature is preferably 570 ° C. or lower. The treatment temperature of the molten salt is, for example, 400 to 570 ° C, more preferably 450 to 550 ° C.
Note that the fracture toughness value K _IC [MPa · m ^1/2 ] is 2.5 [MPa · m ^1/2 ] or less from the viewpoint of suppressing the treatment time of the chemical strengthening treatment from being longer than necessary. It is preferable that For the same reason, the depth D is preferably 150 μm or less.
In addition, when the temperature of the glass substrate is returned to room temperature after being immersed in the molten salt for a predetermined time, the glass substrate is cooled at a slow cooling rate of 30 ° C. or less per minute until the temperature of the glass substrate reaches at least 200 ° C. It is preferable from the point which makes fracture toughness value K _IC and depth D into the said range. In addition, if the cooling rate is too high, minute chipping and cracks are likely to occur at the edge of the glass substrate, and the mechanical strength may be lowered. Therefore, the cooling rate is such that the temperature of the glass substrate is at least 200. The temperature is preferably 30 ° C. or less per minute until the temperature reaches 0 ° C.
In order for the glass substrate 10 of the present embodiment to satisfy the above-described range of the fracture toughness value K _IC and the depth D of the compressive stress layer 14 to satisfy the above range, the glass composition and chemical strengthening treatment conditions ( The composition of the molten salt, processing time, processing temperature, cooling rate) may be adjusted as appropriate.

(Glass composition)
Examples of the glass composition of the glass substrate 10 of the present embodiment include the following aluminosilicate glasses. That is, in terms of mol%, the glass substrate 10 is
The SiO _2, 55~78%,
Li ₂ O, 0.1-1%,
Na ₂ O, 2-15%,
MgO, CaO, SrO and BaO in a total of 10 to 25%,
Including.
The molar ratio CaO / (MgO + CaO + SrO + BaO) of the CaO content to the total content of MgO, CaO, SrO and BaO is 0.20 or less. By setting the molar ratio CaO / (MgO + CaO + SrO + BaO) to 0.20 or less, it is possible to suppress the decrease in the mechanical strength. By maintaining the molar ratio CaO / (MgO + CaO + SrO + BaO) preferably at 0.18 or less, more preferably at 0.16 or less, and particularly preferably at 0.15 or less, the efficiency of ion exchange is maintained (degradation of the molten salt hardly occurs). ) And mechanical strength can be maintained. In particular, the glass substrate is preferably made of aluminosilicate glass that does not contain Ca in terms of maintaining the efficiency of ion exchange (deteriorating the molten salt is difficult to occur). The glass is more preferably an amorphous aluminosilicate glass from the viewpoint of reducing surface roughness and ease of chemical strengthening.

(Glass substrate size)
The size of the glass substrate 10 is not particularly limited, but the plate thickness is, for example, 0.3 to 1.5 mm. The glass substrate 10 has a diameter used for a magnetic disk having a nominal diameter of 2.5 inches, 1 inch, 1.8 inches, 3 inches, 3.5 inches, and the like.

(Method for producing glass substrate for magnetic disk)
A method for manufacturing such a glass substrate 10 will be described below.
First, a glass blank that is a material for a plate-shaped magnetic disk glass substrate having a pair of main surfaces is molded. Next, the rough grinding process of this glass blank is performed. Thereafter, the glass blank is subjected to a shape processing treatment and an end surface polishing treatment. Then, the precise grinding process which uses a fixed abrasive for the glass substrate obtained from the glass blank is performed. Thereafter, a first polishing process, a chemical strengthening process, and a second polishing process are performed on the glass substrate. In the present embodiment, the above process is performed. However, it is not necessary to perform all the above processes, and these processes may not be performed as appropriate. Hereinafter, each process will be described.

(A) Glass blank molding process In the molding of a glass blank, for example, a press molding method can be used. A circular glass blank can be obtained by the press molding method. Furthermore, a glass blank can be manufactured using well-known manufacturing methods, such as a downdraw method, a redraw method, and a fusion method. A disk-shaped glass blank serving as a base of the magnetic disk glass substrate can be obtained by appropriately shaping the plate-shaped glass blank made by these known production methods.

(B) Coarse grinding process In the coarse grinding process, specifically, the glass blank is mainly held on both sides of the glass blank while being held in a holding hole provided in a holding member (carrier) mounted on a well-known double-side grinding apparatus. Surface grinding is performed. For example, loose abrasive grains are used as the abrasive. In the rough grinding process, the glass blank is ground so as to approximate the target plate thickness dimension and the flatness of the main surface. The rough grinding process is performed according to the dimensional accuracy or surface roughness of the molded glass blank, and may not be performed depending on the case.

(C) Shape processing processing Next, shape processing processing is performed. In the shape processing, after forming the glass blank, a circular hole is formed using a known processing method to obtain a disk-shaped glass substrate having a circular hole. Thereafter, the end surface of the glass substrate is chamfered. Thereby, a side wall surface orthogonal to the main surface and an inclined surface (intervening surface) connecting the side wall surface and the main surface are formed on the end surface of the glass substrate.

(D) End surface polishing treatment Next, an end surface polishing treatment of the glass substrate is performed. The end surface polishing process is a process for performing polishing by supplying a polishing liquid containing loose abrasive grains between the polishing brush and the end surface of the glass substrate and relatively moving the polishing brush and the end surface of the glass substrate. In the end surface polishing, the inner end surface and the outer end surface of the glass substrate are to be polished, and the inner end surface and the outer end surface are in a mirror state.

(E) Fine grinding treatment Next, fine grinding treatment is performed on the main surfaces on both sides of the glass substrate. Specifically, the main surface of the glass substrate is ground using a well-known double-side grinding apparatus. For example, a fixed abrasive is provided on a surface plate to grind the glass substrate. Specifically, the main surfaces on both sides of the glass substrate are ground while holding the glass substrate in a holding hole provided in a carrier which is a holding member of a double-side grinding apparatus.
In the grinding process of the present embodiment, the grinding surface containing the fixed abrasive and the main surface of the glass substrate are brought into contact with each other to grind the main surface of the glass substrate. However, grinding using loose abrasive grains may be performed.

(F) 1st grinding | polishing processing Next, 1st grinding | polishing processing is performed to the main surface of the both sides of a glass substrate. Specifically, the main surfaces on both sides of the glass substrate 10 are polished while holding the outer peripheral side end face of the glass substrate in a holding hole provided in the carrier of the polishing apparatus. The first polishing process uses a polishing pad attached to a surface plate using loose abrasive grains. The first polishing removes cracks and distortions remaining on the main surface when, for example, grinding with fixed abrasive grains is performed while adjusting the thickness of the glass substrate 10. In the first polishing, it is possible to reduce the surface roughness of the main surface, for example, the arithmetic average roughness Ra, while preventing the shape of the end portion of the main surface from excessively dropping or protruding.
The free abrasive grains used for the first polishing are not particularly limited. For example, cerium oxide abrasive grains or zirconia abrasive grains are used.
Although the kind in particular of a polishing pad is not restrict | limited, For example, a hard foaming urethane resin polisher is used.

(G) Chemical strengthening process Next, the glass substrate is subjected to the above-described chemical strengthening process using a molten salt. As the molten salt, for example, a molten salt obtained by heating a mixture of potassium nitrate and sodium nitrate can be used. Further, potassium nitrate or sodium nitrate can be used alone. Then, by immersing the glass substrate in the molten salt, Li ions and Na ions in the glass composition on the surface layer of the glass substrate 10 are respectively Na ions and K ions having a relatively large ion radius in the chemical strengthening solution. Are replaced with each other to form a compressive stress layer on the surface of the glass substrate 10 and strengthen the glass substrate 10.
The timing of performing the chemical strengthening treatment can be determined as appropriate. However, if the polishing treatment is performed after the chemical strengthening treatment, the foreign matter fixed to the surface of the glass substrate by the chemical strengthening treatment can be removed together with the smoothing of the surface. This is particularly preferable because it can be performed.

(H) Second polishing (mirror polishing) treatment Next, the glass substrate after the chemical strengthening treatment is subjected to second polishing. The second polishing is intended for mirror polishing of the main surface. Also in the second polishing, a well-known double-side polishing apparatus is used. By carrying out like this, the roughness of the main surface can be reduced, preventing the shape of the edge part of the main surface of the glass substrate 10 from dropping excessively or projecting. The second polishing process is different from the first polishing process in terms of free abrasive grains, and the particle size of the free abrasive grains used in the second polishing process is smaller than the particle size of the first polishing process. Further, the hardness of the resin polisher of the polishing pad used in the second polishing process is preferably lower than the hardness of the resin polisher of the polishing pad used in the first polishing process.
That is, the polishing pad 10 tends to sink due to the pressure during polishing by the second polishing.

  As the free abrasive grains used for the second polishing treatment, for example, fine particles such as colloidal silica are used. The allowance for the second polishing treatment is preferably 2 μm or less in terms of plate thickness in order not to lose the effect of the compressive stress layer formed during the chemical strengthening treatment. By cleaning the polished glass substrate, a glass substrate for a magnetic disk can be obtained. In this way, the glass substrate 10 that has been subjected to the second polishing process becomes a magnetic disk glass substrate.

[Experimental example]
In order to investigate the effect of this embodiment, a glass raw material was prepared so as to have the glass composition described above to produce a glass substrate, and the glass substrate was chemically strengthened under various conditions. The glass composition of the glass substrate is SiO ₂ 65.5 mol%, Al ₂ O ₃ 5 mol%, Li ₂ O 1 mol%, Na ₂ O 9 mol%, MgO 16 mol%, ZrO ₂ 3.5 mol%, Was the basic composition. At that time, constant content of Li ₂ O, Na ₂ O, i.e. waved three levels the content of Li ₂ O as a 10 mol%. The above-mentioned fracture toughness value K _IC was measured for the glass substrate subjected to chemical strengthening, and the depth D of the compressive stress layer was measured using the well-known Babinet corrector method. Moreover, the mechanical strength (bending strength [N]) of the glass substrate which performed the above-mentioned damage | wound addition process was investigated by the method mentioned above. In the test of mechanical strength (bending strength), 100 glass substrates treated with the same glass composition and the same chemical strengthening conditions were examined as test objects, and the glass substrate was passed when the bending strength was 60 N or more. The rate was determined. The condition that the pass rate of the glass substrate was 95% or more was set as a condition suitable for the magnetic disk.
Table 1 below shows the details of conditions in which various contents of Li ₂ O in glass, molten salt composition (content ratio of KNO ₃ and NaNO ₃ ), molten salt temperature and chemical strengthening treatment time were changed, and destruction of the glass substrate. The toughness value K _IC , the compression stress layer depth D, the bending strength (average value), and the acceptance rate are shown.

The hatched area shown in FIG. 5 is a list of conditions suitable for the magnetic disk, with the results shown in Table 1 as the conditions suitable for the magnetic disk, where the pass rate is 95% or more. In FIG. 5, a circle mark means that the pass rate is 98% or more, a triangle mark means that the pass rate is 95% or more and less than 98%, and a cross mark indicates that the pass rate is less than 95%. Means that. Therefore, the circles and the triangles indicate that they are suitable for magnetic disks. From this, in order to ensure that a glass substrate containing 0.1 to 1% of Li ₂ O in terms of glass composition has a mechanical strength (bending strength) suitable for a magnetic disk, fracture toughness is required. The value K _IC is 1 [MPa · m ^1/2 ] or more, and the depth D of the compressive stress layer 14 needs to satisfy D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. I understand that. More preferably, the fracture toughness value K _IC [MPa · m ^1/2 ] is 1.2 [MPa · m ^1/2 ] or more, and the depth D [μm] of the compressive stress layer 14 is D ≧ 70 ·. This is a range satisfying K _IC ^-1.6 and D ≧ 35 [μm].
Incidentally, Li ₂ O, with the basic composition, except for Na ₂ O, the Li ₂ O content 0 mol%, examples of the content of Na ₂ O is 10 mol%, and the content of Li ₂ O In the case of an example in which the content of 0.05 mol% and Na ₂ O was 9.95 mol%, the depth D was 5 μm or less, and the pass rates were all less than 95%. This is presumably because the content of Li ₂ O was too small and ion exchange was not performed well in the chemical strengthening treatment.
Further, when the above basic composition excluding Li ₂ O and Na ₂ O is used, when the content of Li ₂ O is 1.5 mol% and the content of Na ₂ O is 8.5 mol%, the glass transition temperature T _g is less than 650 ℃.

As mentioned above, when the glass substrate 10 of this embodiment is put together,
(1) a glass substrate using a glass containing 0.1% to 1% of Li ₂ O in mol%, the fracture toughness value _K IC [MPa ^{· m 1/2]} is 1 [MPa · ^{m 1 / 2} ] or more, and a compressive stress layer formed by chemical strengthening is provided on the glass substrate surface, and the depth D [μm] of the compressive stress layer satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. A satisfactory glass substrate has a glass transition temperature higher than that of a conventional glass substrate and reliably has mechanical strength (bending strength) suitable for a magnetic disk. In addition, in order to satisfy the above-mentioned ranges of the fracture toughness value K _IC and the compressive stress layer depth D, and to ensure the mechanical strength (bending strength), 0.1 to 1% of Li ₂ O is contained. I also found it necessary to do.
(2) By setting the glass transition temperature to 650 ° C. or higher, it can be suitably used as a glass substrate for an energy-assisted magnetic disk.
(3) In mol%, SiO ₂ is 55 to 78%, Li ₂ O is 0.1 to 1%, Na ₂ O is ₂ to 15%, MgO, CaO, SrO and BaO in total. A glass substrate using 10 to 25% glass, wherein the molar ratio of the content of CaO to the total content of MgO, CaO, SrO and BaO is CaO / (MgO + CaO + SrO + BaO) is 0.20 or less; The glass substrate using the aluminosilicate glass not containing Ca can maintain the ion exchange efficiency in the chemical strengthening in the same manner as in the prior art and can maintain the same mechanical strength as in the prior art.
(4) In chemical strengthening, when a glass substrate is immersed in a mixed molten salt solution of KNO ₃ and NaNO ₃ , the mixed molten salt contains 55 to 85 mass% of KNO ₃ , so that the depth of the compression stress layer is increased. D can be realized.

  As described above, the glass substrate for magnetic disk, the method for producing the glass substrate for magnetic disk, and the magnetic disk of the present invention have been described in detail. Of course, various improvements and changes may be made in the range.

10 Glass substrate for magnetic disk 12 Surface 14 Compressive stress layer 16 Tensile stress layer 18 Steel ball

Claims (9)

A glass substrate for a magnetic disk,
The material of the glass substrate is amorphous aluminosilicate glass, and the glass composition contains 0.1 to 1% of Li ₂ O in terms of mol%,
Fracture toughness value K _IC [MPa · m ^1/2 ] is 1 [MPa · m ^1/2 ] or more,
A compressive stress layer formed by chemical strengthening is provided on the surface of the glass substrate, and the depth D [μm] of the compressive stress layer satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. A magnetic disk glass substrate.   The glass substrate for magnetic disks according to claim 1, wherein the glass transition temperature Tg is 650 ° C or higher. As a glass composition, in mol% display,
The SiO _2, 55~78%,
Li ₂ O, 0.1-1%,
Na ₂ O, 2-15%,
MgO, CaO, SrO and BaO in a total of 10 to 25%,
Including
3. The glass substrate for a magnetic disk according to claim 1, wherein the molar ratio of the content of CaO to the total content of MgO, CaO, SrO, and BaO (CaO / (MgO + CaO + SrO + BaO)) is 0.20 or less.   The glass substrate for magnetic disks of any one of Claims 1-3 comprised with the aluminosilicate glass which does not contain Ca as a glass composition.   A magnetic disk comprising the glass substrate for a magnetic disk according to claim 1 and a magnetic layer formed on a surface of the glass substrate. A method of manufacturing a glass substrate for a magnetic disk,
A process for producing a glass substrate, which is a disk-shaped glass substrate and made of amorphous aluminosilicate glass, containing 0.1 to 1% of Li ₂ O in terms of mol%,
When the glass substrate is chemically strengthened, the fracture toughness value K _IC [MPa · m ^1/2 ] is 1 [MPa · m ^1/2 ] or more and is provided on the glass substrate surface by the chemical strengthening. And a process of performing the chemical strengthening so that the depth D [μm] of the compressive stress layer satisfies D ≧ 57 · K _IC ^−1.6 and D ≧ 20 [μm]. Method. The chemical strengthening process is a process of immersing the glass substrate in a mixed molten salt solution containing KNO ₃ and NaNO ₃ , and the mixed molten salt contains 55 to 85 mass% of KNO _3. The manufacturing method of the glass substrate for magnetic disks of description. The glass substrate after the chemical strengthening treatment is
As a glass composition, in mol% display,
The SiO _2, 55~78%,
Li ₂ O, 0.1 to 1%,
Na ₂ O, 2-15%,
MgO, CaO, SrO and BaO in a total of 10 to 25%,
Including
The method for producing a glass substrate for a magnetic disk according to claim 6 or 7, wherein the molar ratio of the content of CaO to the total content of MgO, CaO, SrO and BaO (CaO / (MgO + CaO + SrO + BaO)) is 0.20 or less. .   The said glass substrate is a manufacturing method of the glass substrate for magnetic discs of any one of Claims 6-8 comprised with the aluminosilicate glass which does not contain Ca.

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