JP6299472B2 - Alkali-free glass for magnetic recording media and glass substrate for magnetic recording media using the same - Google Patents

Description

The present invention relates to a non-alkali glass which is suitable as a substrate glass for a magnetic recording medium and substantially does not contain an alkali metal oxide and can be float-molded.
The present invention also relates to a glass substrate for a magnetic recording medium using the above alkali-free glass.

Conventionally, substrate glass for magnetic recording media, particularly those in which a metal or oxide thin film is formed on the surface, have been required to have the following characteristics as shown in Patent Document 1, for example.
(1) If an alkali metal oxide is contained, it reacts with water or carbon dioxide in the air to produce a reaction product on the surface of the substrate, so-called weather resistance is lowered, and the magnetic recording layer is deteriorated. It is preferable that there are few metal ions.
(2) In the manufacturing process of the glass substrate for magnetic recording media, polishing using a slurry containing cerium oxide abrasive grains is often performed. In addition, a strongly acidic cleaning solution having a pH of 2 or less or a strongly alkaline cleaning solution having a pH of 12 or more is used for cleaning removal of the slurry after polishing, and has sufficient chemical durability against these chemicals.
(3) There are no defects (bubbles, striae, inclusions, pits, scratches, etc.) inside and on the surface.
(4) The substrate surface after polishing or cleaning is sufficiently smooth.
(5) The specific elastic modulus is high because no warping or deflection occurs during rotation of the hard disk drive.
(6) High strength so that cracks do not occur.

In addition to the above requirements, in recent years, there are the following situations.
(7) In order to reduce the motor load and power consumption during rotation of the hard disk drive, it is required to reduce the weight of the magnetic disk, and the glass itself is desired to have a low density glass.
(8) In order to reduce the weight of the magnetic recording medium, it is desired to reduce the thickness of the substrate glass.

(9) In order to increase the temperature raising / lowering speed during film formation of the magnetic recording layer to increase the productivity and the thermal shock resistance, a glass having a small linear expansion coefficient is required.

  On the other hand, when a magnetic recording medium substrate is manufactured, the main surface is mirror-polished in order to finish the main surface of the glass substrate smoothly (see Patent Document 2). In the above mirror polishing, a polishing pad is brought into contact with the main surface of the glass substrate, and an acidic (pH 1 to 3) polishing liquid containing polishing abrasive grains is supplied to the main surface of the glass substrate. The main surface of the glass substrate is polished by relatively moving the pad.

JP 2012-106908 A JP 2007-257810 A

As the magnetic recording medium is made thinner and the recording density is increased, further reduction of warpage and deflection during rotation of the hard disk drive is required. Further, the demand for practical strength of magnetic recording media is increasing, and in order to meet this demand, it is useful to improve the fracture toughness of the substrate glass.
For this reason, the substrate glass for magnetic recording media is required to have a high specific elastic modulus in order to reduce warpage and deflection during rotation of the hard disk drive, and to have a high Young's modulus in order to improve fracture toughness. Is required.

Further, when exposed to a high temperature in the thin film forming process, a high glass transition point is required so that the deformation (thermal shrinkage) associated with glass deformation and glass structural stabilization can be minimized.
In recent years, with the increase in recording capacity of hard disk drives, higher recording density is progressing at a high pace. However, with increasing recording density, miniaturization of magnetic particles impairs thermal stability, and crosstalk and a decrease in the S / N ratio of a reproduction signal have become problems. Therefore, heat-assisted magnetic recording technology has attracted attention as a fusion technology of light and magnetism. This is a technique in which a magnetic recording layer is irradiated with a laser beam or near-field light and recorded by applying an external magnetic field in a state where the coercive force is lowered in a locally heated portion, and the recorded magnetization is read by a GMR element or the like. In addition, since recording can be performed on a medium having a high holding force, the magnetic particles can be miniaturized while maintaining thermal stability. However, in order to form a high coercive force medium as a multilayer film, it is necessary to sufficiently heat the substrate, and heat resistance is required. In the perpendicular magnetic recording system, a magnetic recording layer different from the conventional one has been proposed in order to meet the demand for higher recording density, but it is necessary to form such a magnetic recording layer at a high temperature of the substrate. There are many cases. For these reasons, a high glass transition point is required.

The object of the present invention is a high specific modulus and high Young's modulus, a high glass transition point, a high chemical durability, a low density, a high strength, and a magnetic recording medium that is easy to float. An object of the present invention is to provide an alkali glass and a glass substrate for a magnetic recording medium using the alkali glass.
Another object of the present invention is to provide a method for producing a glass substrate for a magnetic recording medium, in which deterioration of the surface roughness of the main surface of the glass substrate during mirror polishing is suppressed.

The present invention has a Young's modulus of 90 GPa or more, expressed in terms of mass% based on oxide,
SiO ₂ 52 ~65,
Al ₂ O _{3 over} 23.5-30
MgO 2.5-20,
CaO 2-30
Containing
Provided is an alkali-free glass for magnetic recording media, wherein SiO ₂ + Al ₂ O ₃ is 77 or more and 90 or less.

  Moreover, this invention provides the glass substrate for magnetic recording media using the alkali free glass for magnetic recording media of this invention.

Moreover, this invention uses the alkali free glass for magnetic recording media of this invention which has the process of carrying out the mirror polishing of the main surface of a glass substrate using the polishing pad and polishing liquid less than pH7 containing an abrasive grain. A method of manufacturing a glass substrate for a magnetic disk,
In the mirror polishing step, mirror polishing is performed under the condition that the elution rate f (μg · cm ⁻² · min ⁻¹ ) of the glass component derived from the following formula (1) satisfies the following formula (2). The manufacturing method of the glass substrate for magnetic discs characterized by these.
(1)
(In the formula (1), f ₀ = 7.93 × 10 ⁶ , and α and β are α = 0.0121 × w−1 when the SiO ₂ content w of the alkali-free glass is 60% by mass or more. .46, β = −0.0868 × w + 4.38, and when the SiO ₂ content w of the alkali-free glass is less than 60% by mass, α = 0.0562 × w−4.07, β = −0. (381 × w + 21.8, pH is the pH value of the polishing liquid, and γ = 5.81 × 10 ³ )
1 × 10 ⁻³ ≦ f ≦ 1 (2)

  The alkali-free glass of the present invention is suitable as a substrate glass for a magnetic recording medium.

FIG. 1 shows the relationship between the SiO ₂ content (w) of the alkali-free glass and the elution rate f of the glass component for the alkali-free glass mainly composed of SiO ₂ , Al ₂ O ₃ , MgO, and CaO. The (simulated) polishing liquid is implemented in three ways: 50 ° C., 70 ° C., and 90 ° C. FIG. 2 shows the relationship between the SiO ₂ content (w) of the alkali-free glass and the elution rate f of the glass component for the alkali-free glass mainly composed of SiO ₂ , Al ₂ O ₃ , MgO, and CaO. The (simulated) polishing liquid was carried out with three pH values of 1, 2, and 3. (Simulation) The pH value of the polishing liquid is one of 1, 2, and 3. FIG. 3 is a partial cross-sectional perspective view showing an outline of a double-side polishing apparatus used for mirror polishing of the main surface of the glass substrate.

Next, the composition range of each component will be described. SiO ₂ increases the Young's modulus, lowers the devitrification temperature T _L , does not increase the viscosity and does not increase the dissolution temperature, and does not escape bubbles during clarification, and there is no possibility of bubbles being mixed in. The same shall apply hereinafter unless otherwise specified). Further, in order to reduce the average coefficient of thermal expansion and lower the devitrification temperature _TL , the content is made 40% or more. Preferably it is 42 to 63%, more preferably 44 to 61%.

Al ₂ O ₃ suppresses the phase separation property of the glass, lowers the average thermal expansion coefficient, raises the glass transition point Tg, increases the Young's modulus, and lowers the compaction described later, so it exceeds 23.5%. Further, the devitrification temperature T _L does not increase, the viscosity does not increase and the melting temperature does not increase, and bubbles do not escape during clarification, and there is no possibility of bubbles being mixed in, so the content is made 30% or less. Preferably it is 24-29%, More preferably, it is 24.5-28%.

MgO needs to be contained at 2.5% or more in order to improve solubility and improve Young's modulus. However, in order to lower the devitrification temperature _TL, to reduce the compaction described later, the content is made 20% or less. Preferably it is 3 to 19%, more preferably 3.5 to 18%.

  Since CaO improves solubility and contains together with MgO, the occurrence of devitrification can be suppressed, so it is necessary to contain 2% or more. However, in order to reduce the average thermal expansion coefficient, to reduce the compaction described later, the content is made 30% or less. Preferably it is 3-29%, More preferably, it is 4-28%.

If SiO ₂ + Al ₂ O ₃ is 90% or less, the Young's modulus increases, the devitrification temperature T _L decreases, the viscosity does not increase and the melting temperature does not increase, and bubbles may not be removed during clarification and bubbles may be mixed in There is no. Moreover, if it is 70% or more, an average thermal expansion coefficient will become low and the compaction mentioned later can be made low. Preferably they are 72%-88%, More preferably, they are 74%-86%.

Other components such as the following components may be contained as long as the effects of the present invention are not hindered. The other components in this case are preferably less than 5%, more preferably less than 3%, even more preferably less than 1%, and even more preferably less than 0.5% in order to suppress a decrease in Young's modulus, etc. Particularly preferably, it is preferable not to contain substantially except unavoidable impurities. Accordingly, in the present invention, the total content of SiO ₂ , Al ₂ O ₃ , CaO, and MgO is preferably 95% or more, more preferably 96% or more, and 97% or more. More preferably, it is 99% or more, more preferably 99.5% or more. It is particularly preferred that it consists essentially of SiO ₂ , Al ₂ O ₃ , CaO and MgO, excluding unavoidable impurities.

B ₂ O ₃ can be contained in an amount of less than 5% in order to improve the melting reactivity of the glass and to lower the devitrification temperature _TL . However, if it is too much, the Young's modulus decreases. Accordingly, it is preferably less than 3%, more preferably less than 1%, even more preferably less than 0.5%, and particularly preferably not substantially contained.

SrO can be contained in an amount of less than 5% in order to improve the solubility without increasing the devitrification temperature _TL of the glass. However, if the amount is too large, the average coefficient of thermal expansion increases. Accordingly, it is preferably less than 3%, more preferably less than 1%, even more preferably less than 0.5%, and particularly preferably not substantially contained.

  BaO can be contained in an amount of less than 5% in order to improve the solubility of glass. However, if the amount is too large, the average coefficient of thermal expansion increases. Accordingly, it is preferably less than 3%, more preferably less than 1%, even more preferably less than 0.5%, and particularly preferably not substantially contained.

ZrO ₂ can be contained in an amount of less than 3% in order to improve the Young's modulus of the glass. However, if the amount is too large, the devitrification temperature T _L increases. Accordingly, it is preferably less than 2%, more preferably less than 1%, even more preferably less than 0.5%, and particularly preferably not substantially contained.

The glass of the present invention does not contain an alkali metal oxide exceeding the impurity level (ie substantially) so as not to cause deterioration of the properties of the metal or oxide thin film provided on the glass surface during the production of the magnetic recording medium. Is preferred. In order to facilitate recycling of the glass, it is preferable that PbO, As ₂ O ₃ and Sb ₂ O ₃ are not substantially contained.
In the present invention, in order to improve the solubility, clarity and formability of the glass, the total amount of ZnO, SO ₃ , Fe ₂ O ₃ , F, Cl and SnO ₂ is less than 1%, preferably 0.8%. It can be contained less than 5%, more preferably less than 0.3%, and even more preferably less than 0.1%.

  Since the alkali-free glass of the present invention has a Young's modulus of 90 GPa or more, the fracture toughness is improved, and it is suitable for a substrate glass for a magnetic recording medium in which a thin glass plate is required. 92 GPa or more is more preferable, and 94 GPa or more is more preferable.

In addition, since the alkali-free glass of the present invention reduces warping and deflection during rotation of the hard disk drive and can cope with higher density of the magnetic recording medium, the specific modulus (Young's modulus / density) is 35 GPa · cm ³ / It is preferable that it is more than g. 36 GPa · cm ³ / g or more is more preferable, and 37 GPa · cm ³ / g or more is more preferable.

The alkali-free glass of the present invention preferably has a glass transition point Tg of 740 ° C. or higher because thermal deformation during production of the magnetic recording medium can be suppressed.
When the glass transition point Tg is 740 ° C. or higher, the alkali-free glass of the present invention has an application in which the fictive temperature of the glass tends to increase in the production process (for example, a plate thickness of 0.7 mm or less, preferably 0.5 mm or less, more The glass substrate is preferably 0.3 mm or less, more preferably 0.1 mm or less.
When forming a sheet glass having a plate thickness of 0.7 mm or less, further 0.5 mm or less, further 0.3 mm or less, and further 0.1 mm or less, the drawing speed at the time of forming tends to increase. Increases and thermal deformation of the glass tends to increase. In this case, thermal deformation can be suppressed when the glass has a high glass transition point Tg.

In the alkali-free glass of the present invention, the temperature T _{2 at} which the viscosity η becomes 10 ² poise (dPa · s) is preferably 1730 ° C. or less, more preferably 1710 ° C. or less, and further preferably 1690 ° C. or less. Therefore, dissolution is relatively easy.

In the alkali-free glass of the present invention, the temperature T _{4 at} which the viscosity η becomes 10 ⁴ poise (dPa · s) is preferably 1370 ° C. or less, more preferably 1350 ° C. or less, and further preferably 1330 ° C. or less. Therefore, it can be molded by the float process.

In the present invention, glass compaction can be used as an indicator of thermal deformation.
Compaction is the glass heat shrinkage generated by relaxation of the glass structure during the heat treatment. The alkali-free glass of the present invention has a very low compaction.
In the present invention, compaction means a value measured by the method described below.
First, the target glass is melted at 1550 ° C. to 1650 ° C., and then the molten glass is poured out and cooled after being formed into a plate shape. The obtained plate glass is polished to obtain a glass plate of 100 mm × 20 mm × 1 mm.
Next, the obtained glass plate is heated to the glass transition point Tg + 70 ° C., held at this temperature for 1 minute, and then cooled to room temperature at a temperature drop rate of 40 ° C./min. Thereafter, two indentations are made on the surface of the glass plate in the long side direction at intervals A (A = 90 mm) to obtain a sample before processing.
Next, the pre-treatment sample is heated to 450 ° C. at a temperature increase rate of 100 ° C./hour, held at 450 ° C. for 2 hours, and then cooled to room temperature at a temperature decrease rate of 100 ° C./hour to obtain a sample 1 after treatment.
And the distance B1 between impressions of the sample 1 after a process is measured.
The compaction C1 is calculated from A and B1 thus obtained using the following formula.
C1 [ppm] = (A−B1) / A × 10 ⁶
The sample before treatment is heated to 600 ° C. at a temperature rising rate of 100 ° C./hour, held at 600 ° C. for 1 hour, and then cooled to room temperature at a temperature lowering rate of 100 ° C./hour to obtain sample 2 after treatment.
And the distance B2 between impressions of the sample 2 after a process is measured.
The compaction C2 is calculated from A and B2 thus obtained using the following formula.
C2 [ppm] = (A−B2) / A × 10 ⁶

  The alkali-free glass of the present invention preferably has a compaction C1 of 5 ppm or less. On the other hand, the compaction C2 is preferably 50 ppm or less. Preferably it is 47 ppm or less, More preferably, it is 44 ppm or less.

  The alkali-free glass of the present invention can be produced, for example, by the following method. The raw materials of each component that are usually used are blended so as to become target components, which are continuously charged into a melting furnace and heated to 1550 to 1650 ° C. to melt. The molten glass is formed into a predetermined plate thickness by the float method, and then the glass plate can be obtained by slow cooling and cutting.

When producing a glass substrate for a magnetic disk using the alkali-free glass of the present invention, after processing the plate glass obtained by the above procedure into a glass substrate having a predetermined shape, the main surface of the glass substrate is used as a polishing pad. Then, mirror polishing is performed using a polishing liquid containing polishing abrasive grains having a pH of less than 7.
In the present invention, mirror polishing is performed under the condition that the elution rate f (μg · cm ⁻² · min ⁻¹ ) of the glass component derived by the following formula (1) satisfies the following formula (2).
(1)
(In the formula (1), f ₀ = 7.93 × 10 ⁶ , and α and β are α = 0.0121 × w−1 when the SiO ₂ content w of the alkali-free glass is 60% by mass or more. .46, β = −0.0868 × w + 4.38, and when the SiO ₂ content w of the alkali-free glass is less than 60% by mass, α = 0.0562 × w−4.07, β = −0. (381 × w + 21.8, pH is the pH value of the polishing liquid, and γ = 5.81 × 10 ³ )
1 × 10 ⁻³ ≦ f ≦ 1 (2)

The above formula (1) represents the elution rate of the glass component with respect to the polishing liquid having a pH of less than 7 for the alkali-free glass of the present invention, that is, the alkali-free glass mainly composed of SiO ₂ , Al ₂ O ₃ , MgO, and CaO. Factors affecting f are specified as three factors: SiO ₂ content (w) of alkali-free glass, polishing liquid temperature (T), and polishing liquid pH value (pH). The effect of the component on the elution rate f is quantified and formulated.
The glass surface elution rate f derived from the above formula (1) performs mirror polishing under the condition satisfying the above formula (2), thereby suppressing the deterioration of the surface roughness of the main surface of the glass substrate. Moreover, the fluctuation | variation to the pH value of polishing liquid by elution of a glass component is suppressed, and the stable grinding | polishing processing speed is ensured.
When f is larger than 1 μg · cm ⁻² · min ⁻¹ , the surface roughness of the main surface of the glass substrate is deteriorated. Further, the elution of the glass component causes a change in the pH value of the polishing liquid, making it impossible to secure a stable polishing processing speed.
On the other hand, when f is less than 1 × 10 ⁻³ μg · cm ⁻² · min ⁻¹ , the polishing speed is remarkably reduced, so that productivity is lowered.
In the present invention, the glass component elution rate f (μg · cm ⁻² · min ⁻¹ ) derived by the following formula (1) is mirror-polished under the condition satisfying the following formula (2): Using a non-alkali glass having any composition belonging to the present invention, a glass substrate for a magnetic disk having extremely high flatness can be produced.

In order to carry out mirror polishing under the condition satisfying the above formula (2), the temperature (T) of the polishing liquid, depending on the SiO ₂ content (w) of the alkali-free glass used for the production of the glass substrate for magnetic disk, And / or the pH value (pH) of the polishing liquid may be appropriately adjusted.
FIG. 1 shows the relationship between the SiO ₂ content (w) of the alkali-free glass and the elution rate f of the glass component for the alkali-free glass mainly composed of SiO ₂ , Al ₂ O ₃ , MgO, and CaO. It is the graph which showed. The plot (point) in FIG. 1 is an actual measurement value of the elution rate obtained by the following procedure.
The elution rate f of the glass component is obtained by preparing a glass substrate having both the length and width of 40 mm and thickness of 1 mm mirror-polished and immersing this in hydrochloric acid for 5 to 45 hours, and measuring the mass difference before and after the immersion. Calculated.
As the (simulated) polishing liquid, hydrochloric acid having a pH value of 1 was used. (Simulation) The temperature (T) of the polishing liquid was 50 ° C, 70 ° C, and 90 ° C.
FIG. 2 is a graph showing the relationship between the SiO ₂ content (w) of the alkali-free glass and the elution rate f of the glass component, as in FIG. However, the temperature (T) of the (simulated) polishing liquid was 90 ° C., and the pH value of the (simulated) polishing liquid was 1, 2, and 3 types. Plots (points) in FIG. 2 are measured values of the elution rate determined by the above procedure.

The predetermined shape of the glass substrate described above is not particularly limited, but as an example, it is a disk shape having a circular hole in the center.
After processing into a glass substrate of a predetermined shape, before mirror polishing the main surface of the glass substrate, it is usually ground (lapping) on both the upper and lower main surfaces of the glass substrate using loose abrasives or fixed abrasive tools Process. Moreover, when the shape of a glass substrate is a disk shape which has a circular hole in the center part, the inner peripheral end surface and outer peripheral end surface of a glass substrate are grind | polished.

  For the mirror polishing of the main surface of the glass substrate, for example, a double-side polishing apparatus shown in FIG. 3 can be used. The double-side polishing apparatus 20 includes an upper surface plate 201 and a lower surface plate 202 disposed so as to be opposed to each other in the vertical direction, and a carrier 30 disposed therebetween. The carrier 30 holds a plurality of glass substrates 10 in its holding part. Polishing pads 40 and 50 made of resin or the like are mounted on the surfaces of the upper surface plate 201 and the lower surface plate 202 facing the glass substrate 10, respectively.

Mirror polishing of the main surface of the glass substrate using the double-side polishing apparatus 20 shown in FIG. 3 is performed according to the following procedure.
The glass substrate 10 is held between the polishing surface of the upper polishing pad 40 and the polishing surface of the lower polishing pad 50 in a state where the glass substrate 10 is held by the holding portion of the carrier 30. The polishing surfaces of the polishing pads 40 and 50 are surfaces that are in contact with the glass substrate 10 that is an object to be polished.
In a state where the polishing surfaces of the upper and lower polishing pads 40 and 50 are pressed against both main surfaces of the glass substrate 10, respectively, a polishing liquid having a pH of less than 7 containing abrasive grains is supplied to both main surfaces of the glass substrate 10. In addition, both the main surfaces of the glass substrate 10 are mirror-polished at the same time by rotating around the sun gear 203 while rotating the carrier 30 and rotating the upper surface plate 201 and the lower surface plate 202 at a predetermined rotational speed. The

  As a polishing pad, what consists of a soft or hard foamed resin is preferable, and especially the polishing pad which consists of a soft foaming urethane resin is preferable.

  The polishing liquid preferably contains silica particles having an average primary particle diameter of 1 to 80 nm as abrasive grains. The average primary particle diameter of the silica particles is preferably 1 nm or more in order to maintain the polishing rate. The average primary particle diameter of the silica particles is preferably 80 nm or less in order to reduce the surface roughness of the main plane obtained by polishing to an appropriate value. The average primary particle diameter of the silica particles is more preferably in the range of 1 to 60 nm, further preferably in the range of 1 to 50 nm, and particularly preferably in the range of 1 to 40 nm. The average primary particle size can be measured using a laser diffraction / scattering particle size distribution meter, a dynamic light scattering particle size distribution measuring device, or an electron microscope.

Part of the silica particles contained in the polishing liquid may be present as aggregated particles (secondary or tertiary particles). The average particle size of the silica particles in the polishing liquid can be measured using a dynamic light scattering type particle size distribution analyzer (for example, product name: UPA-EX150 manufactured by Nikkiso Co., Ltd.). The average particle diameter (D ₅₀ ) of the silica particles is obtained by measuring the primary particle diameter and the secondary or larger particle diameter. The average particle diameter (D ₅₀ ) of the silica particles in the polishing liquid thus measured is preferably in the range of 10 to 40 nm. D ₅₀ is a volume-based cumulative 50% particle size. That is, it is the particle size at which the cumulative value becomes 50% in the cumulative curve in which the particle size distribution is obtained on a volume basis and the total volume is 100%.

  The polishing liquid contains water as a dispersion medium for the abrasive grains. Although there is no particular limitation on water, it is preferable to use pure water, ultrapure water, ion-exchanged water, etc. from the viewpoint of influence on other components described later, contamination of impurities, and less influence on pH and the like. . And when an abrasive grain is a silica particle, it is preferable that the content rate (concentration) of the silica particle in polishing liquid shall be 3-30 mass%. When the content ratio of the silica particles is less than 3% by mass, it is difficult to obtain a sufficient polishing rate. Moreover, when a content rate exceeds 30 mass%, when adjusting the pH value of polishing liquid to less than 7 in the procedure mentioned later, it will become easy to aggregate a silica particle. As for the content rate of a silica particle, 5-25 mass% is more preferable, 7-20 mass% is further more preferable, and 10-18 mass% is especially preferable.

  In the present invention, the reason why a polishing liquid having a pH value of less than 7 is used is that when a polishing liquid having a pH value of 7 or more is used, the polishing rate becomes low and sufficient productivity cannot be increased. is there. As described above, the pH value of the polishing liquid is appropriately adjusted in order to perform mirror polishing under the condition satisfying the above formula (2), but the pH value is preferably in the range of 0.5 to 6, and the pH value is A range of 0.5 to 5 is more preferable, and a pH value of 1 to 4.5 is particularly preferable.

In order to make the pH value of the polishing liquid less than 7, water as a dispersion medium for the polishing abrasive grains contains an inorganic acid or an organic acid.
Examples of inorganic acids include hydrochloric acid, nitric acid, phosphoric acid, and hydrofluoric acid. Of these, sulfuric acid or hydrochloric acid is preferable because it is easily available and has little influence on the user, the environment, and the like.
Examples of the organic acid include ascorbic acid, citric acid, succinic acid, malic acid, tartaric acid, fumaric acid, maleic acid and phthalic acid. As the organic acid, a carboxylic acid having a carboxylic acid group can be preferably used. A divalent or higher polyvalent carboxylic acid having two or more carboxylic acid groups is more preferable. The polyvalent carboxylic acid having a valence of 2 or more functions to improve the polishing rate by a complex forming action and to suppress the generation of polishing scratches by suppressing the aggregation of abrasive grains. That is, the polyvalent carboxylic acid having a valence of 2 or more contributes to an increase in the polishing rate by capturing metal ions generated during mirror polishing of the glass substrate to form a complex (chelate), and the silica particles. It works to suppress aggregation.
Specific examples of the divalent or higher polyvalent carboxylic acid include citric acid, succinic acid, malic acid, tartaric acid, fumaric acid, maleic acid and phthalic acid. In particular, citric acid is preferred.

When the surface roughness of the main surface of the glass substrate reaches a predetermined value, the mirror polishing is finished.
The target surface roughness has, for example, an arithmetic average roughness (Ra) of 0.4 nm or less and a maximum peak height (Rp) of 2 nm or less. The arithmetic average roughness (Ra) is preferably less than 0.2 nm. The maximum peak height (Rp) is preferably 1.5 nm or less.

  The glass substrate after mirror polishing is cleaned (for example, precision cleaning) to obtain a magnetic disk glass substrate. In the cleaning of the glass substrate after mirror polishing, for example, after scrub cleaning using a detergent, ultrasonic cleaning in a state immersed in a detergent solution and ultrasonic cleaning in a state immersed in pure water are sequentially performed. Drying after washing is performed, for example, by vapor drying with isopropyl alcohol vapor. A magnetic disk is manufactured by forming a thin film such as a magnetic layer on the main surface of the glass substrate for magnetic disk thus obtained.

Hereinafter, Examples 2, 3, 5 to 10 , and 13 to 16 are Examples, Examples 1 , 4 , 11, and 12 are Reference Examples, and Examples 17 to 19 are Comparative Examples. The raw material of each component was prepared so that it might become a target composition, and it melt | dissolved at the temperature of 1550-1650 degreeC using the platinum crucible. In melting, the mixture was stirred using a platinum stirrer to homogenize the glass. Next, the molten glass was poured out, formed into a plate shape, and then slowly cooled.

In Tables 1 to 3, the glass composition (unit: mass%), density ρ (g / cm ³ ), Young's modulus E (GPa) (measured by the ultrasonic method), specific modulus E / ρ (GPa · cm ³ / g), glass transition point Tg (unit: ° C.), the glass viscosity η is 10 ² poise temperature T ₂ (unit: ° C.), the temperature T ₄ of the glass viscosity η is 10 ⁴ poise (unit: ° C.) , And compaction C1 and C2 (measured by the method described above, unit: ppm).
In Tables 1 to 3, the values shown in parentheses are calculated values.

As is clear from the table, all the glasses of the examples have a high Young's modulus of 90 GPa or higher, a specific modulus of 35 GPa · cm ³ / g or higher, and a glass transition point Tg of 740 ° C. or higher. Further, T ₂ is 1730 ° C. or lower and T ₄ is 1370 ° C. or lower. Moreover, the compaction C1 is 5 ppm or less, and the compaction C2 is 50 ppm or less.

For Examples 4 and 12, the elution rate of the glass component was measured by the procedure described in paragraph [0035]. The pH values of the (simulated) polishing liquid, the temperature (T) of the (simulated) polishing liquid, and the measured values of the elution rate of the glass component are as follows. In addition, the elution of the glass component derived by applying the SiO ₂ content (w) of the alkali-free glass in this case, the temperature (T) of the polishing liquid, and the pH value (pH) of the polishing liquid to Equation (1) The rate f is also shown.
Example 4-1
(Simulation) pH value of polishing liquid: 1
Polishing liquid temperature: 50 ° C
Elution rate of glass component: 6.6 μg · cm ⁻² · min ⁻¹
f: 5.7 μg · cm ⁻² · min ⁻¹
Example 4-2
(Simulation) pH value of polishing liquid: 1
Polishing liquid temperature: 70 ° C
Elution rate of glass component: 21.9 μg · cm ⁻² · min ⁻¹
f: 15.9 μg · cm ⁻² · min ⁻¹
Example 4-3
(Simulation) pH value of polishing liquid: 1
Polishing liquid temperature: 90 ° C
Elution rate of glass component: 61.7 μg · cm ⁻² · min ⁻¹
f: 40 μg · cm ⁻² · min ⁻¹
Example 4-4
(Simulation) pH value of polishing liquid: 2
Polishing liquid temperature: 90 ° C
Elution rate of glass component: 2.6 μg · cm ⁻² · min ⁻¹
f: 2.1 μg · cm ⁻² · min ⁻¹
Example 4-5
(Simulation) pH value of polishing liquid: 3
Polishing liquid temperature: 90 ° C
Elution rate of glass component: 4.9 × 10 ⁻² μg · cm ⁻² · min ⁻¹
f: 0.1 μg · cm ⁻² · min ⁻¹
Example 12-1
(Simulation) pH value of polishing liquid: 1
Polishing liquid temperature: 50 ° C
Elution rate of glass component: 4.6 μg · cm ⁻² · min ⁻¹
f: 3.8 μg · cm ⁻² · min ⁻¹
Example 12-2
(Simulation) pH value of polishing liquid: 1
Polishing liquid temperature: 70 ° C
Elution rate of glass component: 15.4 μg · cm ⁻² · min ⁻¹
f: 10.9 μg · cm ⁻² · min ⁻¹
Example 12-3
(Simulation) pH value of polishing liquid: 1
Polishing liquid temperature: 90 ° C
Elution rate of glass component: 45.8 μg · cm ⁻² · min ⁻¹
f: 27.8 μg · cm ⁻² · min ⁻¹
Example 12-4
(Simulation) pH value of polishing liquid: 2
Polishing liquid temperature: 90 ° C
Elution rate of glass component: 1.6 μg · cm ⁻² · min ⁻¹
f: 1.53 μg · cm ⁻² · min ⁻¹
Example 12-5
(Simulation) pH value of polishing liquid: 3
Polishing liquid temperature: 90 ° C
Elution rate of glass component: 5.8 × 10 ⁻² μg · cm ⁻² · min ⁻¹
f: 0.08 μg · cm ⁻² · min ⁻¹

About the glass substrate produced using the glass of Example 4, both the main surfaces of a glass substrate are mirror-polished using the double-side polish apparatus 20 shown in FIG. As an abrasive grain, the average primary particle diameter is 30
Nanometer silica particles are used, and hydrochloric acid having a pH value of 3 is used as a dispersant. The content rate (concentration) of silica particles in the polishing liquid is 10% by mass, and the temperature (T) of the polishing liquid is 90 ° C. Using a polishing pad made of a soft foamed urethane resin, mirror polishing is performed until the arithmetic average roughness (Ra) of both main surfaces of the glass substrate is 0.4 nm or less and the maximum peak height (Rp) is 2 nm or less.
About the glass substrate produced using the glass of Example 4, both main surfaces of a glass substrate are mirror-polished in the same procedure as the above. However, the pH value and temperature (T) of the polishing liquid are the conditions of Examples 4-1 to 4-4. In the glass substrate after mirror polishing, the surface roughness of both main surfaces deteriorates, the arithmetic average roughness (Ra) exceeds 0.4 nm, and the maximum peak height (Rp) exceeds 2 nm.
About the glass substrate produced using the glass of Example 12, the arithmetic mean roughness (Ra) of the two main surfaces is 0.4 nm or less, the maximum peak height ( Mirror polishing is performed until Rp) is 2 nm or less. The pH value and temperature (T) of the polishing liquid are the conditions of Example 12-5.
About the glass substrate produced using the glass of Example 12, both main surfaces of a glass substrate are mirror-polished in the same procedure as the above. However, the pH value and temperature (T) of the polishing liquid are the conditions of Examples 12-1 to 12-4. In the glass substrate after mirror polishing, the surface roughness of both main surfaces deteriorates, the arithmetic average roughness (Ra) exceeds 0.4 nm, and the maximum peak height (Rp) exceeds 2 nm.

  The alkali-free glass of the present invention is suitable as a substrate glass for a magnetic recording medium.

DESCRIPTION OF SYMBOLS 10 Glass substrate 20 Double-side polish apparatus 30 Carrier 40 Upper side polishing pad 50 Lower side polishing pad 201 Upper surface plate 202 Lower surface plate 203 Sungear

Claims (7)

Young's modulus is 90 GPa or more,
In mass% display based on oxide,
SiO ₂ 52 ~65,
Al ₂ O _{3 over} 23.5-30
MgO 2.5-20,
CaO 2-30
Containing
Non- alkali glass having SiO ₂ + Al ₂ O ₃ of 77 to 90. The alkali-free glass for a magnetic recording medium according to claim 1, wherein the specific elastic modulus is 35 GPa · cm ³ / g or more.   The alkali-free glass for magnetic recording media according to claim 1, wherein the glass transition point is 740 ° C. or higher. The magnetic recording medium for the alkali-free glass according to any one of claims 1 to 3 temperature T ₂ at which the viscosity η is 10 ² poise is 1730 ° C. or less. The magnetic recording medium for the alkali-free glass according to any one of claims 1 to 4 the temperature T ₄ at which the viscosity η is 10 ⁴ poise is 1370 ° C. or less.   The glass substrate for magnetic recording media using the alkali free glass for magnetic recording media in any one of Claims 1-5. The alkali-free glass for magnetic recording media according to any one of claims 1 to 5, further comprising a step of mirror polishing the main surface of the glass substrate using a polishing pad and a polishing solution containing polishing abrasive grains having a pH of less than 7. A method for producing a glass substrate for a magnetic disk using
In the mirror polishing step, mirror polishing is performed under the condition that the elution rate f (μg · cm ⁻² · min ⁻¹ ) of the glass component derived from the following formula (1) satisfies the following formula (2). A method for producing a glass substrate for a magnetic disk.
(1)
(In the formula (1), f ₀ = 7.93 × 10 ⁶ , and α and β are α = 0.0121 × w−1 when the SiO ₂ content w of the alkali-free glass is 60% by mass or more. .46, β = −0.0868 × w + 4.38, and when the SiO ₂ content w of the alkali-free glass is less than 60% by mass, α = 0.0562 × w−4.07, β = −0. (381 × w + 21.8, pH is the pH value of the polishing liquid, and γ = 5.81 × 10 ³ )
1 × 10 ⁻³ ≦ f ≦ 1 (2)