Magnetic disk substrate, method for measuring thermal diffusivity of magnetic disk substrate, method for inspecting magnetic disk substrate, method for manufacturing magnetic disk substrate, and magnetic disk

Abstract

To provide an aluminum alloy substrate for a magnetic disk that can exhibit excellent flatness even after processing into a magnetic disk, a magnetic disk based thereon, methods for manufacturing and inspecting the aluminum alloy substrate for a magnetic disk, and related methods for measuring thermal diffusivity.SOLUTION: A magnetic disk substrate includes a blank substrate made of an aluminum alloy and a Ni-P plating layer formed on both surfaces of the blank substrate, with a thermal diffusivity of 53×10-6 m2 / s or less through the thickness direction. A magnetic disk further includes a magnetic film on the Ni-P plating layer. A thermal diffusivity measurement method involves heating from one surface of the magnetic disk substrate after polishing and smoothing the surface of the Ni-P plating layer by irradiating it with a laser at a constant period α, measuring a phase lag θ of a temperature change period α on the other surface, and calculating the thermal diffusivity through the thickness direction from the phase lag θ.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a magnetic disk substrate, a method for measuring the thermal diffusivity of a magnetic disk substrate, a method for inspecting a magnetic disk substrate, a method for manufacturing a magnetic disk substrate, a magnetic disk, a method for measuring the thermal diffusivity of a magnetic disk, and a method for manufacturing a magnetic disk. [Background technology]

[0002] In recent years, there has been an increasing demand for larger capacity and higher density hard disks. To achieve this, the number of magnetic disks mounted in storage devices is increasing, and this has led to a demand for thinner magnetic disk substrates. Furthermore, to achieve higher density, the magnetic region on magnetic disks is becoming increasingly smaller, and the distance between the magnetic head and magnetic disk in hard disk drives is also becoming narrower.

[0003] Thinner substrates tend to lack rigidity, making them prone to deformation such as waviness during high-speed rotation. Waviness is a major factor in collisions between magnetic disks and between a magnetic disk and a magnetic head, and so it is necessary to suppress it. From a similar perspective, it is desirable for magnetic disks to be flat. In particular, considering the narrow spacing between magnetic heads and magnetic disks in recent years, magnetic disk substrates are required to have a high degree of surface smoothness and flatness to prevent head crashes.

[0004] Several studies have been conducted to date to reduce physical errors such as head crashes in hard disks. For example, Patent Document 1 describes a magnetic disk substrate having a flatness below a predetermined value and suppressing change in flatness before and after heat treatment. Patent Document 2 discloses a technology for suppressing disk flutter by specifying the size, Young's modulus, density, and height of the protrusions around the indentations in the plating film of the aluminum alloy substrate within specific ranges. Patent Document 3 describes that an aluminum alloy substrate for magnetic disks in which the metal structure of the core material has second-phase particles of a specific size and the linear expansion coefficients of the core material and skin material are specified can suppress disk flutter and achieve excellent flatness and surface smoothness. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6684865 [Patent Document 2] Japanese Patent Publication No. 2020-107383 [Patent Document 3] Patent No. 6588508 Summary of the Invention [Problem to be solved by the invention]

[0006] Although Patent Document 1 describes a magnetic disk substrate with good flatness, it does not describe any specific means for suppressing deterioration of flatness. Therefore, even if the magnetic disk substrate disclosed in Patent Document 1 has good flatness itself, when a magnetic disk is manufactured through a process such as forming a magnetic layer, the smoothness and flatness may deteriorate.

[0007] The technology described in Patent Document 2 limits the size of the substrate to a specific range, and does not necessarily achieve the goal of increasing the capacity of hard disks. Patent Document 2 also only discloses plating with a Ni-P alloy or the like as a means of flattening an aluminum alloy substrate. As will be shown in the Examples and Comparative Examples described below, even aluminum alloy substrates plated with Ni-P may suffer from poor flatness when magnetic disks are manufactured through processes such as forming a magnetic layer. The same is true for aluminum alloy substrates disclosed in Patent Document 3, in which the metal structure and linear expansion coefficient of the core material are specified, and the excellent flatness that is currently required cannot necessarily be consistently achieved.

[0008] In order to solve the above-mentioned problems, the present invention aims to provide a magnetic disk substrate that can exhibit excellent flatness when a magnetic disk is manufactured through a process such as forming a magnetic layer, a method for measuring the thermal diffusivity of a magnetic disk substrate, a method for inspecting a magnetic disk substrate, a method for manufacturing a magnetic disk substrate, a magnetic disk, a method for measuring the thermal diffusivity of a magnetic disk, and a method for manufacturing a magnetic disk. [Means for solving the problem]

[0009] As a result of extensive research, the inventors discovered that among magnetic disk substrates having a Ni-P plating layer on the surface of a blank substrate made of an aluminum alloy, if the substrate has a thermal diffusivity below a specific value, the magnetic disk produced through processes such as forming a magnetic layer will have good flatness, leading to the completion of the present invention.

[0010] In order to achieve the above object, the gist of the present invention is as follows. (1) A magnetic disk substrate comprising a blank substrate made of an aluminum alloy and a Ni-P plating layer formed on both surfaces of the blank substrate, Thermal diffusivity through the thickness is 53 x 10 -6 m 2 / s or less magnetic disk substrate. (2) The magnetic disk substrate according to claim 1, wherein the aluminum alloy is an alloy containing magnesium. (3) A magnetic disk further comprising a magnetic film on the Ni-P plating layer constituting the magnetic disk substrate of (1) or (2) above. (4) A method for measuring the thermal diffusivity of a magnetic disk substrate having Ni-P plating layers on both surfaces of a blank substrate made of an aluminum alloy, comprising: A method for measuring the thermal diffusivity of a magnetic disk substrate, which comprises polishing and smoothing the surface of the Ni-P plating layer, irradiating one of the two surfaces of the magnetic disk substrate with a laser at a constant period α to heat it, measuring the phase lag θ of the temperature change period α from the temperature change of the other surface, and calculating the thermal diffusivity of the magnetic disk substrate in the thickness direction from the measured value of the phase lag θ. (5) A method for measuring the thermal diffusivity of a magnetic disk further comprising a magnetic film on the Ni-P plating layer constituting the magnetic disk substrate of (1) or (2), comprising: A method for measuring the thermal diffusivity of a magnetic disk, comprising: irradiating one of both surfaces of the magnetic disk with a laser at a constant period α to heat it; measuring the phase lag θ of the temperature change period α from the temperature change of the other surface; and calculating the thermal diffusivity in the thickness direction of the magnetic disk from the measured value of the phase lag θ. (6) A method for inspecting a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and a Ni-P plating layer formed on both surfaces of the blank substrate, the method comprising: calculating the thermal diffusivity according to the thermal diffusivity measurement method of (4) above; and determining whether the thermal diffusivity is 53×10 -6 m 2 An inspection method for magnetic disk substrates that accepts magnetic disk substrates with a speed of 1 / s or less. (7) A method for manufacturing a magnetic disk substrate, comprising the steps of: grinding a blank substrate made of an aluminum alloy; a step of forming Ni-P plating layers on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate; a step of polishing the magnetic disk substrate; and A method for manufacturing a magnetic disk substrate, comprising the step of calculating the thermal diffusivity of the magnetic disk substrate by the thermal diffusivity measurement method of (4) above. (8) A method for manufacturing a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and a Ni-P plating layer formed on both surfaces of the blank substrate, the method comprising: The thermal diffusivity of the magnetic disk substrate in the thickness direction is 53×10 -6 m 2 selecting the blank substrate and the Ni-P plating layer so that the thermal conductivity is 1 / s or less; grinding the blank substrate; and a step of forming the Ni-P plating layer on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate; and a step of polishing the magnetic disk substrate. A method for manufacturing a magnetic disk substrate, comprising: (9) A method for manufacturing a magnetic disk, comprising the steps of: grinding a blank substrate made of an aluminum alloy; a step of forming Ni-P plating layers on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate; and a step of polishing the magnetic disk substrate. forming a magnetic layer on both surfaces of the magnetic disk substrate to form a magnetic disk; A method for manufacturing a magnetic disk, comprising the steps of: irradiating one of both surfaces of the magnetic disk with a laser at a constant period α to heat it; measuring a phase delay θ of the temperature change period α from the temperature change of the other surface; and calculating the thermal diffusivity in the thickness direction of the magnetic disk from the measured value of the phase delay θ. [Effects of the Invention]

[0011] According to the present invention, there are provided a magnetic disk substrate that can exhibit excellent flatness when a magnetic disk is manufactured through a process such as forming a magnetic layer, and a magnetic disk with excellent flatness. Also provided are a method for manufacturing or inspecting such a magnetic disk substrate, a method for measuring the thermal diffusivity of a magnetic disk substrate that can be used in those methods, and a method for measuring the thermal diffusivity of a magnetic disk and a method for manufacturing a magnetic disk. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a flow diagram for explaining temperature changes at various locations when a magnetic disk is manufactured from a magnetic disk substrate through processes such as forming a magnetic layer. [Figure 2] FIG. 1 is a flow diagram for explaining the mechanism by which flatness deteriorates when a magnetic disk is manufactured from a magnetic disk substrate through a process of forming a magnetic layer, etc. [Figure 3] 1A to 1C are diagrams for explaining a thermal diffusivity measuring method and an inspection method according to the present invention. [Figure 4] 1 is a flow chart showing an example of a manufacturing process for an aluminum alloy substrate for a magnetic disk according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The magnetic disk substrate and the like according to the present invention will be described in detail below based on typical embodiments, but the present invention is not limited to these embodiments.

[0014] <Magnetic disk substrate> The first embodiment of the present invention is a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and Ni-P plating layers formed on both surfaces thereof, and the thermal diffusivity in the thickness direction is 53×10 -6 m 2 / s or less, it is a substrate for magnetic disks.

[0015] Here, the term "blank substrate" refers to a substrate processed into a doughnut-shaped disk that does not have a magnetic film or an underlying surface treatment layer. In this embodiment, the blank substrate is made of an aluminum alloy and has a Ni-P plating layer on its surface. The Ni-P plating layer has excellent adhesion to the aluminum alloy, moderate hardness that makes it less likely to deform when contacted with a magnetic head, excellent nonmagnetic stability and film thickness uniformity, and the advantage of being less likely to develop surface defects or residual stress. Therefore, it is often used for aluminum alloy substrates for magnetic disks.

[0016] <Thermal diffusivity> The magnetic disk substrate of this embodiment further has a thermal diffusivity of 53×10 -6 m 2 / s or less. The inventors have found that a magnetic disk substrate with such a thermal diffusivity is unlikely to suffer deterioration in flatness even when heated during, for example, the magnetic film formation process in manufacturing a magnetic disk. Note that "thermal diffusivity" is a physical property value that indicates the ease with which heat is transmitted, and the smaller this value, the more difficult it is for heat to be transmitted.

[0017] The thermal diffusivity generally indicates the thermal diffusivity in the thickness direction and / or circumferential direction of a magnetic disk. As will be described in detail later, it is possible to measure the thermal diffusivity of a magnetic disk, including the components of the magnetic disk (blank substrate, layers on the blank substrate), in both the thickness direction and circumferential direction. From the viewpoint of ease of measurement, it is preferable to indicate the thermal diffusivity in the thickness direction.

[0018] Although the present invention is not limited to any particular theory, it is thought that the reason why the magnetic disk substrate of this embodiment exhibits excellent flatness even after processing into a magnetic disk is that temperature is not easily transmitted to various parts of the substrate, particularly the blank substrate part, and as a result, expansion and contraction are less likely to occur during processing, for example, during sputter heating.

[0019] FIG. 1 and FIG. 2 are flowcharts for explaining the temperature change of each part and the mechanism of deterioration of flatness when manufacturing a magnetic disk through steps such as forming a magnetic layer. In the step of forming a magnetic film, the substrate 1 for the magnetic disk is heated by preheating before forming the magnetic film, and the temperature (T1) increases from the initial T0. However, prior to the temperature T2 of the blank substrate 2, the temperature T3 of the plating layer 3 on the surface side temporarily rises (b: T2 < T3). Here, if the thermal diffusivity of the substrate 1 is small, the temperature T2 of the blank substrate 2 does not rise much. Therefore, deformation of the blank substrate 2 and the resulting change in flatness are unlikely to occur (FIG. 1c).

[0020] On the contrary, if the thermal diffusivity of the substrate 1 is large, the temperature of the blank substrate 2 can also rise in response to the temperature rise of the plating layer 3. The blank substrate 2 will be rapidly heated. When the blank substrate is rapidly heated, its flatness tends to deteriorate. It is considered that when the thermal diffusivity is large, heat is easily transferred to the blank substrate 2 and deformation is likely to occur (FIG. 2c).

[0021] In the case of a substrate for a magnetic disk with a thermal diffusivity in the thickness direction of 53×10 -6 m 2 / s or less as in this embodiment, as shown in FIG. 1, the high temperature of the plating layer is difficult to transfer to the inside, and the temperature rise of the blank substrate part and the accompanying expansion and deformation are suppressed. As a result, it is presumed that the flatness is unlikely to deteriorate even after heating the magnetic disk.

[0022] In order to exhibit good flatness, the upper limit value of the thermal diffusivity in the thickness direction of the substrate for the magnetic disk may be 55×10 -6 m 2 / s, but basically it is 53×10 -6 m 2 / s, and further 50×10 -6 m 2 / s, 47×10 -6 m 2 / s, 45×10 -6 m 2 / s, 44×10 -6 m 2 / s, 43×10<00m 2 / s, 42 × 10 -6 m 2 / s, 41 × 10 -6 m 2 / s, 40 × 10 -6 m 2 / s, 35 × 10 -6 m 2 / s, 30 × 10 -6 m 2 / s, 25 × 10 -6 m 2 / s, 20 × 10 -6 m 2 / s, 10 × 10 -6 m 2 / s, 5.0 × 10 -6 m 2 / s, 1.0 × 10 -6 m 2 / s etc. can also be used.

[0023] The lower limit of the thermal diffusivity is not limited, but is 0×10 -6 m 2 / s, as well as 0.5×10 -6 m 2 / s, 1.0 × 10 -6 m 2 / s, 5.0 × 10 -6 m 2 / s, 10 × 10 -6 m 2 / s, 15 × 10 -6 m 2 / s, 20 × 10 -6 m 2 / s, 25 × 10 -6 m 2 / s, 30 × 10 -6 m 2 / s, 35 × 10 -6 m 2 For example, depending on the purpose and material of the magnetic disk substrate, the thermal diffusivity in the thickness direction may be set to 0.5×10 -6 ~53×10 -6 m 2 / s, 1.0 × 10 -6 ~50×10 -6 m 2 / s, 5.0 × 10 -6 ~45×10 -6 m 2 / s, 10 × 10 -6 ~40×10 -6 m 2 It may be within the range of / s etc.

[0024] <Measurement of thermal diffusivity> In this embodiment, the thermal diffusivity of the magnetic disk substrate can be measured, for example, by a method such as that shown in Figure 3. A heating laser oscillator 4 is used to irradiate one surface of the magnetic disk substrate 1 with a laser beam having a period α, and the period α of the temperature change on the other surface is measured with a detector 5 to calculate the phase lag θ. It is known that the phase lag θ is proportional to the -1 / 2 power of the thermal diffusivity, and the thermal diffusivity can be determined from the thickness of the magnetic disk substrate and the phase lag θ. The proportionality constant can be calculated by measuring a sample with a known thermal diffusivity using the same device. Details of the thermal diffusivity measurement method will be described later, and the following describes the components of the magnetic disk substrate of the first embodiment.

[0025] <Blank board> In the magnetic disk substrate of this embodiment, the blank substrate is made of an aluminum alloy. While there are no particular limitations on its size, the effects of the present invention are most pronounced in thin magnetic disk substrates having a thickness of, for example, 0.6 mm or less, particularly 0.5 mm or less, particularly less than 0.5 mm, and even 0.4 mm or less. Such thin substrates tend to have low rigidity, and if their thermal diffusivity is high, heating during magnetic film formation can easily cause deterioration in smoothness and flatness. For the same reason, the effects of the present invention are most pronounced in magnetic disk substrates having an outer diameter of 95 mm or more. There is no particular upper limit on the outer diameter, but considering the size of a typical hard disk drive, it can be, for example, 97 mm or less. There is also no particular limit on the lower limit on the thickness, but considering the rigidity of the magnetic disk, it can be, for example, approximately 0.30 mm or more.

[0026] (aluminum alloy) Substrates made of aluminum alloys are suitable for magnetic disks because they are less likely to develop defects, have good mechanical properties and processability, and are low-cost. In this embodiment, there are no particular limitations on the type of aluminum alloy used to construct the blank substrate, and various known alloys can be used. For example, conventionally used alloys containing elements such as magnesium (Mg), copper (Cu), zinc (Zn), and chromium (Cr) may also be used. These alloys have the advantages of being low-cost, high-strength, and less likely to develop defects. They may also contain elements such as iron (Fe), manganese (Mn), and nickel (Ni), which can improve rigidity. Such highly rigid substrates are advantageous in suppressing fluttering. Additionally, aluminum alloys containing elements such as silicon (Si) are also useful because they generally have high hardness and strength.

[0027] The aluminum alloy is preferably an alloy containing a metal selected from magnesium, nickel, and manganese, particularly an alloy containing magnesium; or an alloy containing silicon and / or iron, such as an Al-Mg-Ni-based, Al-Mg-Mn-based, Al-Mg-based, or Al-Si-Fe-based alloy. Among these, an Al-Mg-based alloy is preferred. Substrates made of such alloys tend to have lower thermal diffusivities.

[0028] Furthermore, by using an A5000 series (Al-Mg-based) alloy or an A8000 series alloy, especially A5086, defects are less likely to occur in the substrate and sufficient mechanical properties can be imparted. Specific examples of the composition of these aluminum alloys include 2.0-4.5% Mg, 0.60% or less Fe, 0.40% or less Si, 0.20-1.0% Mn, 0-0.40% Cr, 0.50% or less Cu, and 0.50% or less Zn, with the balance being Al and unavoidable impurities.

[0029] Alternatively, an Al-Si-Cu-Ni-Mg alloy may be used. For example, an alloy containing Cu: 0.8 to 1.3%, Si: 11.0 to 13.0%, Mg: 0.7 to 1.3%, Zn: 0 to 0.15%, Fe: 0 to 0.8%, Mn: 0 to 0.15%, Ni: 0.8 to 1.5%, Ti: 0 to 0.20%, Sn: 0 to 0.05%, Cr: 0 to 0.10%, with the balance being aluminum and inevitable impurities can be used.

[0030] Furthermore, as examples of other specific compositions of the aluminum alloy, there are cases where it contains Mg: 1.0 to 6.5%, Cu: 0 to 0.070%, Zn: 0 to 0.60%, Fe: 0 to 0.50%, Si: 0 to 0.50%, Cr: 0 to 0.20%, Mn: 0 to 0.50%, Zr: 0 to 0.20%, Be: 0 to 0.0020%, with the balance being aluminum and inevitable impurities. Also, components other than those mentioned above may be contained, for example, 0.1% or less for each element and 0.3% or less in total. In the above compositions, all “%” means “mass %”.

[0031] <Ni-P plating layer> The Ni-P plating layer is a plating layer of an alloy containing nickel and phosphorus, and is widely used for the aluminum alloy substrate for magnetic disks as described above. The Ni-P plating layer not only has excellent adhesion to the aluminum alloy, but also has appropriate hardness, so it is difficult to be deformed by contact with the magnetic head, and it also has excellent non-magnetic stability and film thickness uniformity, and moreover, it has the advantage of being difficult to generate surface defects and residual stress, and can also play a role in flattening the surface of the substrate to suppress surface defects.

[0032] In the substrate for magnetic disks of the present embodiment, there is no particular limitation on the type of the Ni-P plating layer. For example, it may be amorphous or contain a crystal phase, and it may be formed by any of the electroplating method and the electroless plating method. There is also no limitation on the content rates of nickel and phosphorus, and it may be an alloy containing other elements such as tungsten, molybdenum, chromium, etc.

[0033] From the viewpoint of further reducing the thermal diffusivity of the magnetic disk substrate, the Ni-P plating layer is preferably amorphous and electrolessly plated. It also preferably has a nickel content of 80 to 95% by mass and a phosphorus content of 5 to 20% by mass. For example, the Ni-P plating layer may be a medium-phosphorus type with a phosphorus content of 5 to 10% by mass, or a high-phosphorus type with a phosphorus content of 10 to 13% by mass. Depending on the purpose, a low-phosphorus type Ni-P plating layer with a phosphorus content of less than 5% by mass may also be used.

[0034] There are also no particular limitations on the thickness of the Ni-P plating layer. For example, the thickness of one side of the plating layer can be approximately 100 nm to 100 μm. Here, if the thickness of one side of the Ni-P plating layer is approximately 1 to 50 μm, particularly approximately 3 to 25 μm, or 0.1 to 10%, particularly approximately 1 to 5% of the thickness of the blank substrate, the magnetic disk substrate can have a particularly low thermal diffusivity and better flatness.

[0035] <Method for manufacturing magnetic disk substrate> The magnetic disk substrate as described above can be manufactured, for example, according to the flow shown in Figure 4. Figure 4 is a flow diagram showing an example of a manufacturing process for an aluminum alloy substrate for a magnetic disk according to the present invention. The manufacturing method for a magnetic disk substrate will be described in more detail below based on the flow shown in Figure 4.

[0036] In Figure 4, the steps of preparing the aluminum alloy components (step S101), casting the aluminum alloy (step S102), homogenizing treatment (step S103), hot rolling (step S104), and cold rolling (step S105) are steps for producing an aluminum alloy material by melting and casting, and then turning it into an aluminum alloy plate. Next, a disk blank made of the aluminum alloy is produced by blanking and pressure flattening (step S106). The produced disk blank is then subjected to pretreatment such as cutting and grinding (step S107) to produce a circular aluminum alloy plate.

[0037] The blank substrate thus prepared is subjected to a zincate treatment (step S108) and a Ni-P plating treatment (step S109), such as electroless Ni-P plating, to produce an aluminum alloy substrate for magnetic disks. The produced aluminum alloy substrate for magnetic disks (aluminum alloy blank substrate) is then subjected to rough polishing (step S110) and precision polishing (step S111) to polish and smooth both surfaces of the Ni-P plating layer, followed by attachment of a magnetic material (step S113), to produce a magnetic disk. After step S111 or step S113, the substrate is subjected to thermal diffusivity measurement (steps S112 and S114). From the viewpoint of production efficiency, it is preferable to measure thermal diffusivity after precision polishing (step S111). The following describes each of these steps in detail.

[0038] First, a molten aluminum alloy material having the above-mentioned composition is prepared by heating and melting it according to a conventional method (step S101). Next, the prepared molten aluminum alloy material is cast by a semi-continuous casting (DC casting) method, a continuous casting (CC casting) method, or the like to cast the aluminum alloy material (step S102). Vertical semi-continuous casting is particularly preferred. The production conditions for the aluminum alloy material in the DC casting method and the CC casting method are as follows:

[0039] In DC casting, the molten metal poured through the spout loses heat through the bottom block, the water-cooled mold walls, and cooling water that is sprayed directly onto the outer periphery of the ingot, causing it to solidify and be drawn downward as an aluminum alloy ingot.

[0040] On the other hand, in the CC casting method, molten metal is supplied through a casting nozzle between a pair of rolls (or a belt caster or block caster), and thin aluminum alloy plates are directly cast by extracting heat from the rolls.

[0041] The main difference between DC casting and CC casting is the cooling rate during casting. CC casting has a faster cooling rate, which means that the size of the second phase particles is smaller than in DC casting.

[0042] The DC cast aluminum alloy ingot is subjected to a homogenization treatment as needed (step S103). When the homogenization treatment is performed, the heat treatment is preferably performed at 280 to 620°C for 0.5 to 30 hours, and more preferably at 300 to 620°C for 1 to 24 hours. If the heating temperature during the homogenization treatment is less than 280°C or the heating time is less than 0.5 hours, the homogenization treatment may be insufficient, resulting in a large variation in the loss factor among the aluminum alloy sheets. If the heating temperature during the homogenization treatment exceeds 620°C, melting may occur in the aluminum alloy ingot. If the heating time during the homogenization treatment exceeds 30 hours, the effect saturates, and no further significant improvement can be obtained.

[0043] Next, the aluminum alloy ingot (DC casting), which may or may not have been subjected to homogenization treatment as necessary, is hot rolled to form a plate material (step S104). The conditions for hot rolling are not particularly limited, but the hot rolling start temperature is preferably 250 to 600°C, and the hot rolling end temperature is preferably 230 to 450°C.

[0044] Next, the hot-rolled rolled sheet or the cast sheet cast by CC casting is cold-rolled to produce an aluminum alloy sheet having a thickness of, for example, about 0.30 to 0.60 mm (step S105). The conditions for cold rolling are not particularly limited and may be determined depending on the required product sheet strength and sheet thickness (thickness dimension), and the rolling reduction is preferably 10 to 95%.

[0045] It is preferable to perform annealing before or during cold rolling to ensure cold rolling workability. The temperature during annealing is preferably 250 to 500°C, particularly 300 to 450°C. By performing annealing under these conditions, deformation is less likely to occur even during long-term use, and good flatness can be maintained. More specific annealing conditions include, for example, holding at 300 to 450°C for 0.1 to 10 hours in batch heating, or holding at 400 to 500°C for 0 to 60 seconds in continuous heating. Here, a holding time of 0 seconds means that cooling is performed immediately after the desired holding temperature is reached.

[0046] The aluminum alloy plate obtained by cold rolling is then punched into an annular shape to obtain an annular aluminum alloy plate. The annular aluminum alloy plate is preferably made into a disk blank by blanking and pressure flattening treatment (step S106). The blanking and pressure flattening treatment (also called "pressure annealing") is carried out at a temperature equal to or higher than the recrystallization temperature of the aluminum alloy, under a pressure of 30 to 60 kg / cm. 2 For example, a flattened blank can be produced by holding the blank in air at a temperature of 250 to 500°C, particularly 300 to 400°C, for 0.5 to 10 hours, particularly 1 to 5 hours.

[0047] Prior to the subsequent zincate treatment, the disk blank is subjected to cutting and grinding (step S107) and, if necessary, heat treatment. It is also preferable to degrease and etch the surface of the disk blank. Degreasing can be performed using, for example, a commercially available degreasing solution such as AD-68F (manufactured by Uemura Kogyo Co., Ltd.) at a concentration of 200 to 800 mL / L, at a temperature of 40 to 70°C, and for a treatment time of 3 to 10 minutes. Etching can also be performed by acid etching using, for example, a commercially available etching solution such as AD-107F (manufactured by Uemura Kogyo Co., Ltd.) at a concentration of 20 to 100 mL / L, at a temperature of 50 to 75°C, and for a treatment time of 0.5 to 5 minutes.

[0048] Next, the disk blank surface that has been degreased and etched as desired is subjected to a zincate treatment (Zn substitution treatment) (step S108). In the zincate treatment, a zincate film is formed on the disk blank surface. A commercially available zincate treatment solution can be used for the zincate treatment, and it is preferably performed under conditions of a concentration of 100 to 500 mL / L, a temperature of 10 to 35°C, and a treatment time of 0.1 to 5 minutes. The zincate treatment is performed at least once, but may be performed two or more times. By performing the zincate treatment multiple times, fine Zn can be precipitated to form a uniform zincate film.

[0049] When zincate treatment is performed two or more times, Zn stripping treatment may be performed between treatments. Zn stripping treatment is preferably performed using an HNO3 solution at a concentration of 10 to 60%, at a temperature of 15 to 40°C, and for a treatment time of 10 to 120 seconds (for this reason, it is also called "nitric acid stripping treatment"). Furthermore, it is preferable that the second and subsequent zincate treatments be performed under the same conditions as the first zincate treatment.

[0050] Furthermore, the zincate-treated disk blank surface is subjected to, for example, electroless Ni-P plating (step S109) as a surface treatment for magnetic material attachment. The electroless Ni-P plating step is preferably carried out using a commercially available plating solution, such as Nimuden (registered trademark) HDX manufactured by Uemura Kogyo Co., Ltd., under conditions of Ni concentration: 3 to 10 g / L, temperature: 80 to 95°C, and treatment time: 30 to 180 minutes.

[0051] Next, the plated surface after the electroless Ni-P plating process is subjected to the following polishing process (steps S110 to S111).

[0052] <Polishing process> In the polishing process of the magnetic disk substrate, it is preferable to carry out polishing in multiple stages by adjusting the diameter of the polishing abrasive grains. Generally, it is preferable to carry out rough polishing and precision polishing using a double-sided simultaneous polishing machine, and the magnetic disk substrate of this embodiment can also be polished using a commercially available batch-type double-sided simultaneous polishing machine. It is preferable to carry out dummy polishing before rough polishing to control the surface of the polishing pad.

[0053] (double-sided polishing machine) A double-side simultaneous polisher typically comprises upper and lower cast iron platens, a carrier for holding multiple substrates between the upper and lower plates, and polishing pads attached to the substrate contact surfaces of the upper and lower plates. During the polishing process, multiple substrates are typically held between the upper and lower plates by the carrier, and each substrate is clamped between the upper and lower plates at a predetermined processing pressure. The substrates are then clamped together from above and below by the polishing pads. Next, the upper and lower plates are rotated in opposite directions while a predetermined amount of polishing liquid is supplied between the polishing pads and each substrate. During this process, the carrier is also rotated by a sun gear, causing the substrates to perform planetary motion. As a result, the substrates slide across the surfaces of the polishing pads, polishing both surfaces simultaneously.

[0054] There are no particular limitations on the type or details of the structure of the double-sided polishing machine, and any commercially available machine may be used. Here, the thickness of the carrier that holds the substrate in the double-sided polishing machine is preferably 0.05 to 0.15 mm smaller than the thickness of the substrate, and more preferably 0.07 to 0.12 mm smaller. Such a carrier can further improve the flatness of the magnetic disk substrate. In particular, it is preferable that the thickness of the carrier in the polishing machine used in the rough polishing step be 0.05 to 0.15 mm smaller than the thickness of the substrate.

[0055] (rough polishing) The rough polishing method is not particularly limited, and can be performed using, for example, a polishing solution containing alumina with a particle size of 0.1 to 1.0 μm and a polishing pad made of hard or soft polyurethane. However, the rough polishing conditions are not limited to these and can be selected from known polishing conditions. For example, abrasive grains of desired particle size, such as silica, cerium oxide, zirconium oxide, SiC, or diamond, may be used instead of the above-mentioned alumina. Note that "hard" refers to a hardness (Asker C) of 85 or more measured according to the measurement method specified in the Society of Rubber Industry of Japan Standard (compliant standard: SRIS0101), and "soft" refers to a hardness of 60 to 80.

[0056] Specific rough polishing conditions are influenced by the material of the substrate used and the steps leading up to rough polishing (e.g., the composition of the aluminum alloy and steps S101 to S109 in the substrate manufacturing process), and are difficult to determine uniquely. Furthermore, the rough polishing conditions are not limited to specific conditions. For example, the rough polishing conditions may be a polishing time of 2 to 5 minutes, a polishing platen rotation speed of 10 to 35 rpm, a sun gear rotation speed of 5 to 15 rpm, a polishing solution supply rate of 1000 to 5000 mL / min, and a processing pressure of 20 to 250 g / cm. 2 , preferably 20 to 150 g / cm 2 , more preferably 60 to 120 g / cm 2 The polishing amount can be set to 2.5 to 3.5 μm.

[0057] (Dummy polishing) During the polishing process, it is preferable to perform dummy polishing prior to the rough polishing described above to control the surface of the polishing pad. Generally, the dummy polishing process uses a dummy substrate and is performed under the same conditions as the rough polishing process, preferably under the same conditions. There is no particular limitation on the dummy substrate used, but it is preferable to use a blank substrate of the same type as the blank substrate to be used as the product, particularly a blank substrate manufactured under the same conditions as the blank substrate to be used as the product. In the dummy polishing process of this embodiment, for example, the polishing pad surface can be controlled by polishing until the thickness of the Ni-P plating layer reaches the desired value.

[0058] This dummy polishing allows the surface of the polishing pad used in the rough polishing process to be adjusted to a suitable state. Note that dummy polishing is an optional process and may be omitted if the polishing pad surface is adjusted and controlled. For example, dummy polishing can be performed prior to the start of a rough polishing lot, and then the adjusted polishing pad can be used to rough polish blank substrates for multiple batches.

[0059] (Precision polishing) The precision polishing method is not particularly limited, and various known methods can be used. For example, a polishing solution containing colloidal silica with a particle size of approximately 0.01 to 0.10 μm and a soft polishing pad can be used. Of course, the precision polishing conditions are not limited to these. Abrasive grains of desired particle size, such as cerium oxide, zirconium oxide, colloidal silica, SiC, and diamond, can also be used. Furthermore, by such treatment, the main surface of the substrate is polished to a mirror finish, producing a magnetic disk substrate. After polishing, the substrate is preferably washed with a neutral detergent, pure water, IPA, or the like.

[0060] The specific conditions for precision polishing are also difficult to determine uniquely because they are affected by the material of the substrate used and the steps up to rough polishing, and are not limited to specific conditions. For example, the polishing time is 2 to 5 minutes, the rotation speed of the polishing plate is 10 to 35 rpm, the rotation speed of the sun gear is 5 to 15 rpm, the polishing liquid supply rate is 1000 to 5000 mL / min, and the processing pressure is, for example, 10 to 200 g / cm. 2 , preferably 20 to 120 g / cm 2 , especially 50-100g / cm 2 The polishing amount can be set to 1.0 to 1.5 μm.

[0061] The magnetic disk substrate thus subjected to the polishing treatment is subjected to the thermal diffusivity measurement as described above (step S112 or step S114), thereby completing the manufacture of the magnetic disk substrate.

[0062] The present invention is based on the discovery that the change in flatness of a magnetic disk or a magnetic disk substrate does not depend on the thermal diffusivity of each of these components (magnetic disk substrate, Ni-P plating layer, magnetic layer, etc.) in a manner that can be predicted, but rather has the thermal diffusivity of the magnetic disk or magnetic disk substrate. Therefore, from the perspective of further reducing the thermal diffusivity of the magnetic disk substrate or improving the flatness, it is preferable to include a step in each manufacturing process of the magnetic disk substrate, for example, in step S101, where the composition is examined and changed, and further include a step in steps S108 and S109 where various plating conditions (type, composition, thickness, etc.) are selected.

[0063] The above embodiment is a method for manufacturing a magnetic disk substrate including a blank substrate made of an aluminum alloy and Ni-P plating layers formed on both surfaces of the blank substrate, and the magnetic disk substrate has a thermal diffusivity of 53×10 in the thickness direction. -6 m 2 The present invention encompasses a method for manufacturing a magnetic disk substrate, which includes the steps of selecting a blank substrate and a Ni-P plating layer so that the surface roughness is 1 / s or less, grinding the blank substrate, forming Ni-P plating layers on both surfaces of the blank substrate after grinding to form a magnetic disk substrate, and polishing the magnetic disk substrate.

[0064] <Magnetic disk> A magnetic material is attached to the magnetic disk substrate manufactured as described above (step S113), and by stacking as desired, a magnetic disk such as a hard disk can be manufactured. If thermal diffusivity measurement is not performed in the step of S112, it is performed in S114 after S113. The magnetic material can be attached by, for example, sputtering.

[0065] As a second embodiment, the present invention encompasses a magnetic disk further comprising a magnetic film on the Ni-P plating layer constituting the magnetic disk substrate of the first embodiment. The magnetic disk of this embodiment has excellent surface flatness and smoothness, making it less susceptible to problems such as head crash. Therefore, it is suitable as a magnetic disk for high-capacity hard disk drives.

[0066] ≪Measurement method≫ In the following, various measurement methods and the like that are important in manufacturing and inspecting magnetic disk substrates and magnetic disks with good flatness as described above will be explained.

[0067] <Thermal diffusivity measurement method> The thermal diffusivity of a magnetic disk substrate can be measured by the above-mentioned method shown in Fig. 3, etc. The present invention, as a third embodiment, encompasses a method for measuring the thermal diffusivity of a magnetic disk substrate having Ni-P plating layers on both surfaces of a blank substrate made of an aluminum alloy, in which the surface of the Ni-P plating layer is polished and smoothed, and one of the two surfaces of the magnetic disk substrate is heated by irradiating it with a laser at a constant period α, and the phase lag θ of the temperature change period α is measured from the temperature change of the other surface, and the thermal diffusivity of the magnetic disk substrate in the thickness direction is calculated from the measured value of the phase lag θ.

[0068] The present invention also includes, as a fourth embodiment, a method for measuring the thermal diffusivity of a magnetic disk further comprising a magnetic film on the Ni-P plating layer constituting the magnetic disk substrate, the method comprising: irradiating one of both surfaces of the magnetic disk with a laser at a constant period α to heat it; measuring a phase lag θ of the temperature change period α from the temperature change of the other surface; and calculating the thermal diffusivity of the magnetic disk in the thickness direction from the measured value of the phase lag θ.

[0069] Here, it is preferable to apply a black spray to both surfaces of the magnetic disk substrate or the magnetic disk in advance. It is also desirable to cut the sample into a shape and / or size that is easy to measure in advance. To calculate the thermal diffusivity in the thickness direction, for example, as shown in Figure 3, a laser oscillator (e.g., a heating laser oscillator 4), a measurement unit for the magnetic disk substrate 1, and a detector 5 are arranged in a straight line in the thickness direction of the magnetic disk substrate, and the phase delay θ of the detected temperature change 5a relative to the oscillated laser wave 4a is measured while maintaining their relative positions.

[0070] In this embodiment, the thermal diffusivity of the magnetic disk or substrate is generally measured in the thickness direction. However, if it is difficult to measure in the thickness direction for some reason, such as the sample shape, the thermal diffusivity in the circumferential direction can be used instead. For example, if the thermal diffusivity in the circumferential direction is 55×10 -6 m 2 / s or less, especially 53 × 10 -6 m 2 It may be considered whether the value is less than / s.

[0071] Here, when calculating the thermal diffusivity in the circumferential direction, for example, a laser oscillator (e.g., a heating laser oscillator), a measurement unit for the magnetic disk substrate, and a detector are arranged parallel to the thickness direction of the magnetic disk substrate, and a measurement is performed while maintaining these relative positions.Then, while keeping the laser irradiated onto the same measurement unit, the detector is moved perpendicular to the thickness direction and the operation of measuring the phase delay θ is repeated multiple times, and the thermal diffusivity in the circumferential direction can be calculated by finding the average value of the obtained phase delay θ.

[0072] (Evaluation of smoothness) The smoothness of a magnetic disk substrate or a magnetic disk can be evaluated by flatness such as the PV value. The PV value is the difference between the highest point (P) and the lowest point (V) when measuring the height (unevenness) of a main surface, and can be measured using, for example, an optical inspection device.

[0073] <Inspection method for magnetic disk substrates> The thermal diffusivity measurement method described above can be applied to the inspection of magnetic disk substrates. For example, by performing the magnetic material attachment step (step S113) only on blank substrates that have given good results in the thermal diffusivity measurement (step S112) or on blank substrates from the same lot that gave good results, unnecessary steps can be eliminated.

[0074] As a fifth embodiment, the present invention provides a method for inspecting a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and Ni-P plating layers formed on both surfaces of the blank substrate, the method comprising: calculating a thermal diffusivity according to the thermal diffusivity measurement method of the third embodiment; and determining whether the thermal diffusivity is 53×10 -6 m 2 This includes an inspection method that allows magnetic disk substrates with a speed of 1 / s or less to be accepted as acceptable products.

[0075] <Method of manufacturing magnetic disk substrate> The present invention also encompasses, as a sixth embodiment, a method for manufacturing a magnetic disk substrate, including the steps of grinding a blank substrate made of an aluminum alloy, forming Ni-P plating layers on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate, polishing the magnetic disk substrate, and calculating the thermal diffusivity of the magnetic disk substrate by the thermal diffusivity measurement method of the third embodiment. Each step in this embodiment may be carried out in accordance with the above-described conditions for manufacturing an aluminum alloy substrate.

[0076] <Magnetic Disk Manufacturing Method> The present invention further encompasses, as a seventh embodiment, a method for manufacturing a magnetic disk, comprising the steps of: grinding a blank substrate made of an aluminum alloy; forming Ni-P plating layers on both surfaces of the blank substrate after grinding to form a magnetic disk substrate; polishing the magnetic disk substrate; forming magnetic layers on both surfaces of the magnetic disk substrate to form a magnetic disk; and irradiating one of the both surfaces of the magnetic disk with a laser at a constant period α to heat it, measuring a phase lag θ of the temperature change period α from the temperature change of the other surface, and calculating the thermal diffusivity of the magnetic disk in the thickness direction from the measured value of the phase lag θ.

[0077] In this embodiment, the step of calculating the thermal diffusivity can be performed in the same manner as in the thermal diffusivity measurement method of the fourth embodiment. Also in this embodiment, each step may be performed in accordance with the above-mentioned conditions for manufacturing the aluminum alloy substrate. This embodiment may also include a step of polishing the magnetic disk prior to measuring the thermal diffusivity.

[0078] <Application to other types of substrates> The thermal diffusivity measuring method according to the third embodiment of the present invention can be applied to magnetic disk substrates other than aluminum alloy substrates having a Ni-P plating layer, such as aluminum alloy substrates having other components, and even glass substrates. The same applies to the inspection method according to the fourth embodiment of the present invention. Below, we will explain an example in which the above embodiment is applied to glass substrates, which are often used as magnetic disk substrates along with aluminum alloys.

[0079] <Glass substrate> Glass substrates are suitable for magnetic disk substrates because they are less likely to develop defects, have good mechanical properties and processability, and are also less susceptible to plastic deformation. There are no particular limitations on the material of the glass substrate to which the method of the above embodiment can be applied, and glass ceramics such as amorphous glass and crystallized glass can be used. From the viewpoints of substrate flatness, formability, and processability, it is preferable to use amorphous glass.

[0080] The composition of the material constituting the glass substrate is not particularly limited, and examples include aluminosilicate glass (aluminosilicate glass), soda-lime glass, soda-aluminosilicate glass, aluminoborosilicate glass, borosilicate glass (borosilicate glass), and even physically strengthened glass or chemically strengthened glass that has been subjected to a process such as air cooling or liquid cooling. Among these, aluminosilicate glass, particularly amorphous aluminosilicate glass, is preferred. Substrates made of such materials are excellent in terms of flatness and strength, and can also have good long-term reliability.

[0081] Known aluminosilicate glass contains, for example, 55-75% SiO2 as the main component, 0.7-25% Al2O3, 0.01-6% Li2O, 0.7-12% Na2O, 0-8% K2O, 0-7% MgO, 0-10% CaO, 0-10% ZrO2, and 0-1% TiO2, and substrates made of such materials can be used in this embodiment as well. Note that in the compositions above and below, all "%" means "% by mass."

[0082] <Glass substrate manufacturing method> In manufacturing a glass substrate, first, a glass plate of a predetermined thickness is prepared (steps S201 and S202). For example, a molten glass material having a desired component composition is prepared by heating and melting it according to a conventional method (step S201). Next, the prepared molten glass material is formed into a glass plate by a known manufacturing method such as a float method, a downdraw method, a direct press method, a redraw method, or a phasing method (step S202). Here, it is preferable to use a redraw method in which a base glass plate manufactured by a float method or the like is heated and softened, and then stretched to a desired thickness, because this method allows for the relatively easy manufacture of glass plates with small thickness variations.

[0083] Next, a circular glass substrate is formed from the glass plate obtained in step S202 by a coring process (step S203). It is preferable to polish the inner and outer peripheral end faces by cutting and grinding (step S204). The obtained glass blank may also be subjected to an annealing process.

[0084] Next, in step S205, the formed annular plate is optionally lapped to adjust the plate thickness. Depending on the thickness of the glass substrate obtained in the steps up to step S204, lapping may be omitted and the process may proceed to the polishing step described below. For example, glass plates manufactured by the redraw method generally have small thickness variations, so the lapping step S205 may not be performed. When the glass plate is manufactured by the float method or direct press method, lapping is preferably performed. The lapping step can be performed using, for example, a batch-type double-sided polisher using diamond pellets.

[0085] Next, the molded glass substrates are optionally subjected to a lapping step (step S205) using diamond pellets or the like. Subsequently, or after step S204, the glass substrates are clamped together from above and below with polishing pads, and multiple glass substrates are simultaneously polished using, for example, cerium oxide abrasive grains (step S206). If desired, a chemical strengthening treatment (step S207) is performed, followed by precision polishing using, for example, colloidal silica abrasive grains (step S208).

[0086] Rough polishing of a glass substrate can be performed using, for example, a polishing solution containing cerium oxide with a particle size of 0.1 to 1.0 μm and a hard polishing pad with a hardness of 86 to 88. When roughly polishing a glass substrate, the rotation speed of the polishing table is 10 to 35 rpm, the rotation speed of the sun gear is 5 to 15 rpm, the polishing solution supply rate is 1000 to 5000 mL / min, and the processing pressure is 20 to 250 g / cm. 2 , preferably 20 to 150 g / cm 2 , more preferably 60 to 120 g / cm 2 The polishing time is preferably 2 to 10 minutes.

[0087] Precision polishing of glass substrates can be carried out using a polishing solution containing colloidal silica with a particle size of, for example, about 0.01 to 0.10 μm, particularly about 10 to 50 nm, and a soft polishing pad made of urethane foam or the like with a hardness of 75 to 77. In this case, the rotation speed of the polishing platen is 10 to 35 rpm, the rotation speed of the sun gear is 5 to 15 rpm, the polishing solution supply rate is 1000 to 5000 mL / min, and the processing pressure is, for example, 10 to 200 g / cm. 2 , especially 20~120g / cm 2 , especially 50-100g / cm 2 The polishing time is preferably 2 to 12 minutes.

[0088] The polished glass blank substrate thus obtained is subjected to thermal diffusivity measurement according to the third embodiment or inspection according to the fifth embodiment (step S209), and then a magnetic disk is manufactured by a magnetic material attachment step (step S210).

[0089] In addition, in the inspection of the glass blank substrate, the thermal diffusivity is, for example, 53 × 10 -6 m 2 / s or less, especially 50 × 10 -6 m 2 / s or less may be accepted as acceptable products, or products with a smaller thermal diffusivity, specifically 10 × 10 -6 m 2 / s or less, 1×10 -6 m 2 / s or less, 50×10 -8 m 2 / s or less, e.g., 10×10 -8 ~10×10 -6 m 2 / s, especially 30 × 10 -8 ~1×10 -6 m 2 Products with a thermal diffusivity of about / s may be considered acceptable.

[0090] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, but includes all aspects encompassed by the concept of the present invention and the scope of the claims, and can be modified in various ways within the scope of the present invention. [Example]

[0091] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0092] [Example 1] The aluminum alloy was subjected to the above process to prepare a magnetic disk substrate having a NiP plating layer, and the thermal diffusivity was measured.

[0093] (Magnetic disk substrate fabrication) An aluminum alloy (Al-Mg-Mn alloy-1) was melted according to a standard method and DC cast (vertical semi-continuous casting) to obtain a slab with a width of 1310 mm and a thickness of 500 mm. Each of the four sides of the slab was chamfered by 10 mm. After homogenization at 540°C for 6 hours, the slab was hot-rolled to a hot-rolled sheet, starting at 540°C and finishing at 340°C. This hot-rolled sheet was cold-rolled to obtain a cold-rolled sheet. This cold-rolled sheet was punched into a ring shape with an inner diameter of 24 mm and an outer diameter of 98 mm. The cold-rolled sheet was then subjected to pressure annealing at 320°C for 3 hours in air under a load of 30 MPa using a continuous annealing furnace, followed by pressure flattening. A disk blank was thus obtained.

[0094] Furthermore, the inner and outer peripheries of the disk blank were machined to obtain a circular disk blank with an inner diameter of 25 mm and an outer diameter of 97 mm. At the same time, the inner and outer periphery end faces were chamfered. The disk blank after this machining was surface ground. Both sides of this disk blank were subjected to degreasing, etching, first zincating, Zn stripping, and second zincating treatments according to conventional methods. Furthermore, pure water washing was performed between each treatment from the degreasing treatment to the second zincating treatment.

[0095] Thereafter, both sides of the disk blank were subjected to electroless Ni-P plating treatment according to the usual method, and the Ni-P plated disk blank was then set in a double-sided simultaneous polishing machine and subjected to a rough polishing process and a precision polishing process to produce an aluminum alloy substrate with a plate thickness of 0.50 mm.

[0096] (thermal diffusivity measurement) One of the precision-polished substrates prepared as described above was cut into a roughly triangular shape with an arc of approximately 50 mm, and the entire surface was coated with a black spray (DRY GRAPHITE FILM LUBRICANT manufactured by Nippon Senpaku Kogyo Co., Ltd.). Next, the thermal diffusivity through the thickness direction was measured at 25°C using a thermowave analyzer manufactured by Peter Corporation, with a laser frequency of 20 to 160 Hz and a laser output of 5.6 mW.

[0097] (Measurement of flatness change) The flatness (PV) of one of the precision-polished substrates was measured using a Zygo Mesa optical inspection system. In addition, one of the substrates prepared under the same conditions was subjected to a pre-sputtering preheating process at a temperature of 170°C for 60 seconds to simulate the magnetic film formation process. The flatness (PV) of the heated sample was measured in the same manner as above, and the difference from the PV of the unheated sample was calculated as the change in flatness.

[0098] For reference, the thermal diffusivity of the sample before degreasing (G sub) and the sample after sputter heating was also measured in the same manner as above. The measurement results are shown in Table 1 below.

[0099] [Examples 2 to 4, Comparative Example 1] The same operations as in Example 1 were carried out, except that instead of Al-Mg-Mn alloy-1, Al-Mg-Ni alloy-1 (Example 2), Al-Mg-Mn alloy-2 (Example 3), Al-Mg alloy-1 (Example 4), Al-Mg-Ni alloy-2 (Example 5), or Al-Mg alloy-2 (Comparative Example 1) shown in Table 1 was used as the aluminum alloy. The measurement results are shown in Table 1.

[0100] [Table 1]

[0101] According to the present invention, a Ni-P plating layer is provided and the thermal diffusivity is 53×10 -6 m 2 In the magnetic disk substrates of Examples 1 to 5, the thermal diffusivity was 53×10 -6 m 2 The change in flatness before and after heating was small compared to the magnetic disk substrate of Comparative Example 1, which had a heating rate of over / s. It was clear that the magnetic disk substrate according to the present invention does not experience a significant deterioration in flatness even when subjected to a heat treatment under the same conditions as in the magnetic film formation process.

[0102] The results of Comparative Example 1 do not necessarily indicate that the Al-Mg alloy used was inappropriate. In fact, in Example 4, which used an Al-Mg alloy of similar composition, the change in flatness before and after heating was small. As indicated by the thermal diffusivity of the G sub-substrate in each Example, the physical properties of a magnetic disk substrate with a Ni-P plating layer, such as thermal diffusivity, vary depending not only on the composition of the aluminum alloy constituting the blank substrate, but also on the composition of the Ni-P plating layer, the combination of the two, and the polishing conditions of the blank substrate surface and the magnetic disk substrate surface. Therefore, the blank substrate in Comparative Example 1 could also be used as a magnetic disk substrate in which deterioration of flatness due to heat treatment is suppressed, depending on the composition, polishing condition, and thickness of the Ni-P plating layer to be combined.

[0103] The results of these examples and comparative examples also demonstrate the importance of considering not only the material of the blank substrate but also the thermal diffusivity of the substrate after plating and polishing, or of the magnetic disk, when selecting a magnetic disk substrate that is less likely to deteriorate in flatness. [Explanation of symbols]

[0104] 1. Magnetic disk substrate 2 blank boards 3 Ni-P plating layer 4. Heating laser oscillator 4a Oscillated laser wave 5. Detector 5a Detected temperature change

Claims

1. A magnetic disk substrate comprising a blank substrate made of an aluminum alloy and a Ni-P plating layer formed on both surfaces of the blank substrate, Thermal diffusivity in the thickness direction is 53 x 10 －６ m ２ / s or less.

2. 2. The magnetic disk substrate according to claim 1, wherein the aluminum alloy is an alloy containing magnesium.

3. 3. A magnetic disk further comprising a magnetic film on the Ni--P plating layer constituting the magnetic disk substrate according to claim 1.

4. A method for measuring the thermal diffusivity of a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and having Ni—P plating layers on both surfaces thereof, comprising: A method for measuring the thermal diffusivity of a magnetic disk substrate, which comprises polishing and smoothing the surface of the Ni-P plating layer, irradiating one of the two surfaces of the magnetic disk substrate with a laser at a constant period α to heat it, measuring a phase lag θ of the temperature change period α from the temperature change of the other surface, and calculating the thermal diffusivity of the magnetic disk substrate in the thickness direction from the measured value of the phase lag θ.

5. A method for measuring the thermal diffusivity of a magnetic disk further comprising a magnetic film on the Ni—P plating layer constituting the magnetic disk substrate according to claim 1 or 2, comprising: A method for measuring the thermal diffusivity of a magnetic disk, comprising: irradiating one of both surfaces of the magnetic disk with a laser at a constant period α to heat it; measuring the phase lag θ of the temperature change period α from the temperature change of the other surface; and calculating the thermal diffusivity in the thickness direction of the magnetic disk from the measured value of the phase lag θ.

6. A method for inspecting a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and a Ni-P plating layer formed on both surfaces of the blank substrate, the method comprising: calculating a thermal diffusivity according to the thermal diffusivity measurement method of claim 4; and determining whether the thermal diffusivity is 53×10 －６ m ２ A magnetic disk substrate inspection method in which magnetic disk substrates with a density of 1 / s or less are accepted as acceptable products.

7. A method for manufacturing a magnetic disk substrate, comprising: grinding a blank substrate made of an aluminum alloy; a step of forming a Ni—P plating layer on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate; a step of polishing the magnetic disk substrate; and 5. A method for manufacturing a magnetic disk substrate, comprising the step of calculating the thermal diffusivity of the magnetic disk substrate by the thermal diffusivity measuring method according to claim 4.

8. A method for manufacturing a magnetic disk substrate comprising a blank substrate made of an aluminum alloy and a Ni—P plating layer formed on both surfaces of the blank substrate, comprising: The thermal diffusivity of the magnetic disk substrate in the thickness direction is 53×10 －６ m ２ selecting the blank substrate and the Ni—P plating layer so that the grinding the blank substrate; and a step of forming the Ni-P plating layer on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate; and a step of polishing the magnetic disk substrate. A method for manufacturing a magnetic disk substrate, comprising:

9. A method for manufacturing a magnetic disk, comprising: grinding a blank substrate made of an aluminum alloy; a step of forming a Ni—P plating layer on both surfaces of the blank substrate after grinding to obtain a magnetic disk substrate; and a step of polishing the magnetic disk substrate. forming a magnetic layer on both surfaces of the magnetic disk substrate to form a magnetic disk; A method for manufacturing a magnetic disk, comprising the steps of: irradiating one of both surfaces of the magnetic disk with a laser at a constant period α to heat it; measuring a phase delay θ of the temperature change period α from the temperature change of the other surface; and calculating the thermal diffusivity in the thickness direction of the magnetic disk from the measured value of the phase delay θ.

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