Aluminum alloy substrate for magnetic disks and method for manufacturing the same

Abstract

To provide an aluminum alloy substrate for a magnetic disk, in which generation of microscopic defects is reduced without significantly changing manufacturing steps, and a manufacturing method thereof.SOLUTION: An aluminum alloy substrate for a magnetic disk has two or more Ni-P plating layers continuously laminated. The two or more Ni-P plating layers respectively have the same component composition. In addition, the Ni-P plating firm is formed using an electroless Ni-P plating solution containing the same concentration of P.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an aluminum alloy substrate for a magnetic disk and a method for manufacturing the same, and more particularly to an aluminum alloy substrate for a magnetic disk in which the generation of extremely minute defects is reduced and a method for manufacturing the same.

Background Art

[0002] Aluminum alloy magnetic disk substrates used in storage devices of computers and data centers have good plating properties and excellent mechanical properties and workability. A general aluminum alloy magnetic disk is manufactured by first producing an annular aluminum alloy substrate, plating the aluminum alloy substrate, and then attaching a magnetic material to the surface of the aluminum alloy substrate provided with a plating film.

[0003] As an aluminum alloy magnetic disk substrate, for example, an aluminum alloy substrate having an alloy composition of the JIS 5086 series is widely used. Also, for the purpose of improving or reducing the occurrence of defects due to plating, an aluminum alloy substrate in which the contents of Fe, Si, etc. in the JIS 5086 series aluminum alloy are limited to reduce the generation of intermetallic compounds in the matrix, or an aluminum alloy substrate in which Cu and Zn, which are optional components in the JIS 5086 series aluminum alloy, are deliberately added, etc. are also used.

[0004] An aluminum alloy magnetic disk made from an aluminum alloy such as the JIS 5086 series alloy as a raw material is manufactured, for example, by the following manufacturing process. First, an aluminum alloy adjusted to a desired alloy composition is cast, and after homogenizing the ingot, hot rolling is performed. Next, cold rolling is performed to produce a rolled material having a thickness required for a magnetic disk. This rolled material can also be annealed during the cold rolling process if necessary. Next, this rolled material is punched into an annular shape, and further, in order to remove distortions generated during the manufacturing process, etc., the annular aluminum alloy plates are laminated and pressure annealing is performed while applying pressure from both sides to flatten them. Through such a process, an annular aluminum alloy substrate is produced.

[0005] The annular aluminum alloy substrate produced in this manner is subjected to pretreatment including cutting, grinding, degreasing, etching, and zincate treatment (Zn substitution). Next, as a base treatment, Ni-P, a hard nonmagnetic metal, is electrolessly plated to form a plating film on the surface of the aluminum alloy substrate. Furthermore, after polishing the surface of the plating film, a magnetic material is added and the magnetic material is deposited onto the surface of the plating film by sputtering. In this way, an aluminum alloy magnetic disk is manufactured.

[0006] Incidentally, in recent years, with the development of cloud services, the construction of new data centers and the replacement of existing data centers with high-capacity HDDs have been actively pursued. Given this situation, increasing the capacity of HDDs has become indispensable. Increasing the number of magnetic disks installed is an effective way to increase the capacity of HDDs. However, since there is an upper limit to the number of disks that can be installed, further increases in capacity require increasing the storage capacity per magnetic disk. On the other hand, for example, if there are dents or defects on the surface of the Ni-P plating film applied to an aluminum alloy substrate, data must be read and written while excluding the area around the defect, and as a result, the storage capacity per magnetic disk decreases in proportion to the number of defects. Therefore, reducing defects on the surface of the Ni-P plating film is essential for increasing storage capacity.

[0007] Surface defects in Ni-P plating films can be caused by pores resulting from the detachment of intermetallic compounds from the aluminum alloy substrate, or by pores caused by the dissolution of the aluminum alloy substrate due to local galvanic reactions between the aluminum alloy substrate and the intermetallic compounds. The occurrence of these pores can be suppressed by reducing the content of Fe and Si, which can form intermetallic compounds in the aluminum alloy.

[0008] However, with the recent increase in capacity, countermeasures against extremely small defects (hereinafter also referred to as "micro-defects"), which had not been considered a problem until now, are required. These micro-defects have an extremely large aspect ratio and are characterized by extending from the surface of the Ni-P plating film into the recessed areas of the aluminum alloy substrate. However, since reducing the Fe and Si content in the aluminum alloy substrate is not expected to have much effect on reducing micro-defects, a completely different approach is needed to solve the problem.

[0009] For example, Patent Documents 1 to 3 disclose a technique for reducing surface defects by forming two Ni-P plating layers on an aluminum alloy substrate. However, in these techniques, since Ni-P layers with different properties are formed on the aluminum alloy substrate, plating solutions with different compositions are required. Furthermore, the technique described in Patent Document 2 requires the interposition of an intermediate layer between the first and second Ni-P plating layers, which necessitates significant process changes and the addition of processes. Moreover, because an intermediate layer with a different composition or made of a different metal is provided, there is a possibility that the plating film may deform due to differences in physical properties, such as due to rapid temperature changes, and that the plating film may peel off as a result. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Patent No. 4285222 [Patent Document 2] Japanese Patent Publication No. 2011-134419 [Patent Document 3] Japanese Patent Publication No. 2013-218765 [Overview of the project] [Problems that the invention aims to solve]

[0011] This invention provides an aluminum alloy substrate for magnetic disks and a method for manufacturing the same, which reduces the occurrence of minute defects without significantly altering the manufacturing process. [Means for solving the problem]

[0012] The inventors of this invention have diligently researched the mechanism of generation of minute defects. As a result, they have clarified that minute defects are caused by the continued dissolution of the aluminum alloy substrate during the electroless Ni-P plating reaction. In other words, if the electroless Ni-P plating reaction can be stopped, the dissolution of the aluminum alloy substrate will also stop, the minute defects will disappear midway through the process, and will not appear on the surface of the aluminum alloy substrate. In the case of electroplating, it is possible to stop the plating reaction by stopping the current, but in electroless plating, since it utilizes a chemical reaction in the solution, it is not easy to stop the reaction in the solution. For example, although the electroless Ni-P plating reaction can be stopped or extremely slowed down by lowering the temperature of the solution, it is necessary to move from high temperature to low temperature and then return to high temperature, which requires a great deal of time and cost.

[0013] Therefore, the inventors have discovered a method in which the aluminum alloy substrate is removed from the plating solution during electroless Ni-P plating to stop the reaction in the solution, and then immersed in the same plating solution again to resume electroless Ni-P plating. With this method, it is only necessary to remove the aluminum alloy substrate from the plating solution once, and since the same plating solution is used, it is possible to reduce minute defects without increasing the processing time or making significant changes to the process. In other words, the inventors have found that minute defects can be reduced by performing electroless Ni-P plating with the same composition in at least two stages, and have completed the present invention.

[0014] An embodiment of the present invention is an aluminum alloy substrate for magnetic disks, in which two or more Ni-P plating layers are continuously laminated, and each of the two or more Ni-P plating layers is a Ni-P plated film formed using an electroless Ni-P plating solution having the same component composition and containing the same concentration of P.

[0015] According to one embodiment of the present invention, an aluminum alloy substrate for a magnetic disk has a P concentration in the Ni-P plating film that is in the range of 10 mass% to 14 mass%.

[0016] According to one embodiment of the present invention, an aluminum alloy substrate for magnetic disks, under conditions in which the aluminum alloy substrate is immersed in 20-30 vol% nitric acid at 30-50°C, the time required for at least one of the Ni-P plating films to peel off is 20% or more longer than the time required for the peel off of a single Ni-P plating layer in an aluminum alloy substrate on which a single Ni-P plating layer having the same thickness as the total thickness of the Ni-P plating film and the same component composition as the Ni-P plating film is provided on the surface.

[0017] A method for manufacturing an aluminum alloy substrate for a magnetic disk according to an embodiment of the present invention includes: a first plating step of immersing an aluminum alloy substrate for plating in an electroless Ni-P plating solution; a reaction stopping step of removing the aluminum alloy substrate for plating that has been immersed in the electroless Ni-P plating solution and stopping the chemical reaction; and a second plating step of immersing the aluminum alloy substrate for plating after the reaction stopping step back into the electroless Ni-P plating solution to apply a Ni-P plating film of a predetermined thickness.

[0018] According to a method for manufacturing an aluminum alloy substrate for a magnetic disk according to one embodiment of the present invention, the reaction cessation step includes an exposure step in which the aluminum alloy substrate for plating, which is immersed in the electroless Ni-P plating solution, is removed and exposed to the air for 10 seconds or more but less than 300 seconds.

[0019] According to a method for manufacturing an aluminum alloy substrate for a magnetic disk according to one embodiment of the present invention, the reaction stopping step includes an immersion step of removing the aluminum alloy substrate for plating that is immersed in the electroless Ni-P plating solution and immersing it in pure water for 1 second or more.

[0020] According to the method for manufacturing an aluminum alloy substrate for a magnetic disk according to an embodiment of the present invention, the reaction stop step includes an intermediate treatment step of taking out the aluminum alloy substrate for plating immersed in the electroless Ni-P plating solution, exposing it in the atmosphere for 10 seconds or more and less than 300 seconds, and immersing it in pure water for 1 second or more.

Effects of the Invention

[0021] According to the present invention, it is possible to provide an aluminum alloy substrate for a magnetic disk with reduced generation of extremely minute defects and a method for manufacturing the same without significantly changing the manufacturing process.

Embodiments for Carrying Out the Invention

[0022] Hereinafter, the present invention will be described in detail based on embodiments. The present invention is to perform electroless Ni-P plating using a plating solution having the same composition in at least two steps, that is, by continuously laminating two or more Ni-P plating layers on an aluminum alloy substrate using an electroless Ni-P plating solution having the same component composition and the same concentration, it is possible to reduce the generation of extremely minute defects occurring in the aluminum alloy substrate for a magnetic disk. Hereinafter, the mechanism by which defects occur on the surface of the plating film, and further, the aluminum alloy substrate for a magnetic disk according to the present embodiment (hereinafter, also simply referred to as "aluminum alloy substrate") and its manufacturing method will be described in detail.

[0023] 1. Mechanism of Defect Generation 1-1. Regarding Extremely Minute Defects Defects occurring on an aluminum alloy substrate exist in various forms such as simple depressions and protrusions. However, the most difficult-to-address defects are extremely minute defects. Simple depressions and protrusions can be improved by polishing the surface of the aluminum alloy substrate to make it smooth. However, extremely minute defects have a very large aspect ratio that extends from the surface of the aluminum substrate to the surface of the Ni-P plating film. Therefore, once extremely minute defects occur, they cannot be removed. That is, even if the surface of the Ni-P plating film is polished when extremely minute defects occur, the extremely minute defects cannot be removed because they continuously extend in the thickness direction of the Ni-P plating film up to the aluminum alloy substrate. Thus, when extremely minute defects occur on the aluminum alloy substrate, the surrounding area cannot be used as a storage area of the magnetic disk. As a result, the storage capacity per magnetic disk decreases. Furthermore, if the number of extremely minute defects is large, there is a possibility that the magnetic disk cannot be used. Therefore, suppressing the occurrence of extremely minute defects is extremely important.

[0024] 1-2. Generation mechanism of depressions and protrusions as defects on the surface of the plating film Defects such as dents and protrusions that occur on aluminum alloy substrates are related to the intermetallic compounds in the aluminum alloy substrate. The dissolution of the aluminum alloy substrate is caused by a galvanic reaction between the aluminum alloy substrate and the intermetallic compounds during the process from pretreatment to electroless Ni-P plating. Al-Fe and Al-Si intermetallic compounds present on the surface of the aluminum alloy substrate exhibit a higher potential than the aluminum alloy substrate. That is, a local galvanic cell is formed with the intermetallic compound as the cathode site and the surrounding aluminum alloy substrate as the anode site. There are two types of defects on the plating surface that occur due to the local galvanic reaction. The first is a dent defect that occurs on the surface of the plating film when the local galvanic reaction during the pretreatment process causes the aluminum alloy substrate around the intermetallic compound to dissolve, and the intermetallic compound falls off, forming a large hole in the surface of the aluminum alloy substrate. This hole is not filled even during the subsequent electroless Ni-P plating. The second type of defect is a protruding defect on the surface of the plating film, which occurs when intermetallic compounds are detached, forming large holes on the surface of the aluminum alloy substrate. During subsequent pretreatment, a zincate film preferentially deposits on the edges of these holes, and further, electroless Ni-P plating also preferentially deposits. The size of the recessed and protruding defects is 1 μm or larger and 0.5 μm or larger, respectively, and these defects can be removed by polishing the surface of the plating film as described above.

[0025] 1-3. Mechanism of generation of minute defects The minute defects that occur on the aluminum alloy substrate extend from the surface of the plating film to the aluminum alloy substrate. This means that the reaction that forms minute defects continued from the beginning to the end of the electroless Ni-P plating reaction. More specifically, if the aluminum alloy substrate is exposed during the electroless Ni-P plating process, local galvanic reactions cause the aluminum alloy substrate around the intermetallic compound to dissolve, resulting in continuous localized gas generation. This creates minute plating defects with a large aspect ratio that extend from the aluminum alloy substrate to the surface of the Ni-P plating film. In addition to gas generation, pathways formed when aluminum ions generated as the aluminum alloy substrate dissolves diffuse into the plating solution can also become minute defects. The size of these minute defects is less than 1 μm and they have a straw-like shape. However, because these minute defects extend from the surface of the Ni-P plating film to the aluminum alloy substrate, they cannot be removed by polishing the surface of the plating film. In other words, minute defects are defects that occur in the early stages of the electroless Ni-P plating reaction and cannot be removed later.

[0026] 2. Aluminum alloy substrate The aluminum alloy substrate according to this embodiment comprises two or more Ni-P plating layers. The two or more Ni-P plating layers are stacked in a continuous manner, and each of the two or more Ni-P plating layers is a Ni-P plating film formed using an electroless Ni-P plating solution having the same component composition and containing the same concentration of P. Here, having the same component composition means that the types and amounts of components such as Ni and P contained in the electroless Ni-P plating solution used to form each Ni-P plating film are the same, and if other additives are included, they are included in the same amount, and if other additives are not included, they are not included in the same amount. Furthermore, containing the same concentration of P means that the concentration of P in the electroless Ni-P plating solution used to form each Ni-P plating film is the same.

[0027] To continuously layer Ni-P plating films formed using electroless Ni-P plating solutions with the same component composition, these Ni-P plating films are formed by performing electroless Ni-P plating in at least two stages using a plating solution of the same composition. Specifically, the layering of two or more Ni-P plating layers is achieved, as described later, by first performing Ni-P plating on an aluminum alloy substrate, then removing the aluminum alloy substrate from the plating solution, and then performing Ni-P plating again using the same plating solution. Furthermore, by repeating this process, it is possible to layer three or more Ni-P plating layers on an aluminum alloy substrate. As a result, in the second and subsequent electroless Ni-P plating processes, the reaction in the exposed areas of the aluminum alloy substrate is stopped, thereby suppressing the progress of localized galvanic reactions and reducing the occurrence of minute defects.

[0028] Thus, the aluminum alloy substrate according to this embodiment can be manufactured using existing equipment and only by modifying the manufacturing method, without requiring new production equipment. Furthermore, since no other intermediate layers are interposed between each Ni-P plating layer, significant process changes and additions are not required. Therefore, it is possible to reduce minute defects that occur on the surface of the plating film without increasing manufacturing costs. In addition, by reducing the occurrence of minute defects, it is possible to increase the storage capacity per magnetic disk. Furthermore, it may be possible to relax the upper limit on the Fe and Si content in the aluminum alloy, thus contributing to cost reduction.

[0029] Aluminum alloy substrates are manufactured using aluminum alloy materials. The aluminum alloys used as materials for the aluminum alloy materials are preferably Al-Mg alloys such as JIS 5086 series alloys, Al-Fe alloys such as 8000 series alloys, and especially Al-Mg alloys such as JIS-A5086P alloys.

[0030] Al-Mg alloys are aluminum alloys that contain Mg as an essential element. Examples of such Al-Mg alloys include the aluminum alloy specified in JIS-A5086P (an aluminum alloy containing 3.5% to 4.5% by mass of Mg, 0% to 0.50% by mass of Fe, 0.40% or less by mass of Si, 0.20% to 0.70% by mass of Mn, 0.05% to 0.25% by mass of Cr, 0% to 0.10% by mass of Cu, 0% to 0.15% by mass of Ti, and 0% to 0.25% by mass of Zn, with the remainder being Al and unavoidable impurities).

[0031] The thickness of the aluminum alloy substrate is not particularly limited, but from the viewpoint of thinning the magnetic disk substrate, it is preferably 0.7 mm or less, and more preferably 0.5 mm or less. In particular, when the thickness of the aluminum alloy substrate is 0.5 mm or less, i.e., when it is an aluminum alloy substrate for thin magnetic disks, it exhibits high impact resistance and reliability. Even when the thickness of the aluminum alloy substrate exceeds 0.5 mm, it can still exhibit high impact resistance and reliability, but when the aluminum alloy substrate is thicker, the deformation when an impact is applied is small, and it is not necessarily required that all the physical properties of the aluminum alloy substrate according to this embodiment be satisfied. Furthermore, the lower limit of the thickness of the aluminum alloy substrate is preferably 0.1 mm or more from the viewpoint of strength and manufacturing difficulty.

[0032] The surface of the aluminum alloy substrate is provided with two or more Ni-P plating layers, and each Ni-P plating layer is a Ni-P plating film. Such Ni-P plating films are formed by electroless Ni-P plating on the aluminum alloy substrate. The concentration of P in the Ni-P plating film is preferably in the range of 10 mass% to 14 mass% and more preferably in the range of 12 mass% to 14 mass%.

[0033] The total thickness of the Ni-P plating film is preferably 5 μm or more, and more preferably 8 μm or more. Furthermore, the upper limit of the total thickness of the Ni-P plating film is preferably 30 μm or less, and more preferably 15 μm or less, within a range that does not affect the thinning of the aluminum alloy substrate. In other words, the total thickness of two or more Ni-P plating layers formed on the aluminum alloy substrate is preferably 5 μm or more and 30 μm or less, and more preferably 8 μm or more and 15 μm or less.

[0034] 3. Method for manufacturing aluminum alloy substrates for magnetic disks The method for manufacturing an aluminum alloy substrate for magnetic disks according to this embodiment will be described below.

[0035] 3-1. Fabrication of aluminum alloy sheets First, the molten aluminum alloy is prepared to fall within a specified alloy composition range. Next, the prepared molten aluminum alloy is cast according to a conventional method such as semi-continuous casting (DC casting). The cooling rate during casting is preferably in the range of 0.1 to 1000°C / s. The cast aluminum alloy ingot is subjected to homogenization treatment as needed. The conditions for the homogenization treatment are not particularly limited; for example, a single-stage heat treatment at 500°C or higher for 0.5 hours or more can be performed. Furthermore, there is no particular upper limit to the heating temperature during the homogenization treatment, but since there is a risk of melting of the aluminum alloy if it exceeds 650°C, the upper limit should be 650°C.

[0036] Aluminum alloy ingots, whether homogenized or not, are processed into sheets by hot rolling. In the hot rolling process, if homogenization is performed, the starting temperature for hot rolling is preferably 300 to 550°C, and the ending temperature is preferably less than 380°C, and more preferably 300°C or lower. The lower limit of the ending temperature for hot rolling is not particularly limited, but to prevent defects such as edge cracking, the lower limit is preferably 200°C. On the other hand, if homogenization is not performed, the starting temperature for hot rolling is preferably 380°C or lower, and more preferably 350°C or lower. The ending temperature for hot rolling is not particularly limited, but to prevent defects such as edge cracking, the lower limit is preferably 100°C.

[0037] After hot rolling is complete, the product is finished to the required thickness by cold rolling. The cold rolling conditions are not particularly limited and can be determined according to the required strength and thickness of the product, with a rolling ratio of 20-90% being preferable. Furthermore, to ensure cold rolling workability, an annealing treatment may be performed at, for example, 280-450°C for 0-10 hours before or during cold rolling. In this manner, an aluminum alloy sheet is manufactured.

[0038] 3-2. Fabrication of aluminum alloy substrates for plating Next, an aluminum alloy substrate for plating is manufactured using the fabricated aluminum alloy sheet. First, the aluminum alloy sheet is punched into a ring shape using a press or the like to produce a ring-shaped aluminum alloy sheet. Then, this ring-shaped aluminum alloy sheet is pressure annealed to produce a flattened disk blank. The flattened disk blank is then subjected to machining processes in this order, including cutting, grinding, and, for example, a heat treatment to relieve stress at 300-400°C for 5-15 minutes, to produce a substrate for magnetic disks. Next, this substrate for magnetic disks is subjected to pre-plating treatments including degreasing, etching, and zincate treatment to produce an aluminum alloy substrate for plating.

[0039] For degreasing, it is preferable to use a degreasing solution, such as a commercially available AD-68F (manufactured by Uemura Kogyo Co., Ltd.) degreasing solution, and perform degreasing at a temperature of 40-70°C, a processing time of 3-10 minutes, and a concentration of 200-800 mL / L. For etching, it is preferable to use an etching solution, such as a commercially available AD-107F (manufactured by Uemura Kogyo Co., Ltd.) etching solution, and perform etching at a temperature of 50-75°C, a processing time of 0.5-5 minutes, and a concentration of 20-100 mL / L. For zincate treatment, it is preferable to use a zincate treatment solution, such as a commercially available AD-301F-3X (manufactured by Uemura Kogyo Co., Ltd.) zincate treatment solution, and perform etching at a temperature of 10-35°C, a processing time of 0.1-5 minutes, and a concentration of 100-500 mL / L. In addition, a normal desmatt treatment may be performed between the etching treatment and the zincate treatment.

[0040] 3-3. Fabrication of aluminum alloy substrates for magnetic disks The method for manufacturing an aluminum alloy substrate for magnetic disks according to this embodiment includes: (a) a first plating step of immersing an aluminum alloy substrate for plating in an electroless Ni-P plating solution; (b) a reaction stopping step of removing the aluminum alloy substrate for plating from the electroless Ni-P plating solution and stopping the chemical reaction; and (c) a second plating step of immersing the aluminum alloy substrate for plating after the reaction stopping step back into the electroless Ni-P plating solution to apply a Ni-P plating film of a predetermined thickness. That is, when performing the under-plating treatment on the manufactured aluminum alloy substrate for plating, electroless Ni-P plating of the same composition is performed in at least two stages. Specifically, when performing two stages of electroless Ni-P plating, the aluminum alloy substrate is manufactured in the following three manner.

[0041] The method for manufacturing an aluminum alloy substrate according to the first embodiment includes: (a-1) a first plating step of immersing an aluminum alloy substrate for plating in an electroless Ni-P plating solution; (b-1) an exposure step of removing the aluminum alloy substrate for plating that has been immersed in the electroless Ni-P plating solution and exposing it to the air for 10 seconds or more but less than 300 seconds; and (c-1) a second plating step of immersing the aluminum alloy substrate for plating after the exposure step back into the electroless Ni-P plating solution to apply a Ni-P plating film of a predetermined thickness. The same plating solution is used for each electroless Ni-P plating treatment in steps (a-1) and (c-1). For this electroless Ni-P plating process, it is preferable to use an electroless Ni-P plating solution, such as a commercially available Nimden HDX (manufactured by Uemura Kogyo Co., Ltd.), and perform the plating under the following conditions: temperature 80-95°C, processing time 90-180 minutes, Ni concentration 3-10 g / L, sodium hypophosphite concentration 25-40 g / L (P concentration 0.73-1.12 mass%). In this case, the total plating time (immersion time) with the electroless Ni-P plating solution should be 90-180 minutes. Furthermore, in step (b-1), the reaction of the aluminum substrate can be stopped by exposure to air for 10 seconds to less than 300 seconds. In particular, by setting the upper limit of the exposure time to less than 300 seconds, oxidation inside the pores formed on the surface of the first layer of plating is suppressed, and the pores are more easily filled when the second layer of plating is applied, thus preventing peeling of the plating film. Therefore, the upper limit of the exposure time is preferably 120 seconds or less, and more preferably 60 seconds or less. Furthermore, when laminating three or more Ni-P plating layers on an aluminum alloy substrate, steps (b-1) and (c-1) may be repeated.

[0042] The method for manufacturing an aluminum alloy substrate according to the second embodiment includes: (a-2) a first plating step of immersing an aluminum alloy substrate for plating in an electroless Ni-P plating solution; (b-2) an immersion step of removing the aluminum alloy substrate for plating from the electroless Ni-P plating solution and immersing it in pure water for 1 second or more; and (c-2) a second plating step of immersing the aluminum alloy substrate for plating after the immersion step back into the electroless Ni-P plating solution to apply a Ni-P plating film of a predetermined thickness. The same plating solution is used for each electroless Ni-P plating treatment in steps (a-2) and (c-2). For this electroless Ni-P plating process, it is preferable to use an electroless Ni-P plating solution, such as a commercially available Nimden HDX (manufactured by Uemura Kogyo Co., Ltd.), as in the first embodiment, and to perform the plating under the following conditions: temperature 80-95°C, processing time 90-180 minutes, Ni concentration 3-10 g / L, sodium hypophosphite concentration 25-40 g / L (P concentration 0.73-1.12 mass%). In this case, the total plating time (immersion time) with the electroless Ni-P plating solution should be 90-180 minutes. In step (b-2), the reaction of the aluminum substrate can be stopped by immersing it in pure water for 1 second or more. On the other hand, the upper limit of the immersion time is preferably 60 seconds or less, and more preferably 30 seconds or less, from the viewpoint of oxidation of the plating surface and decrease in substrate temperature. Furthermore, when laminating three or more Ni-P plating layers on an aluminum alloy substrate, steps (b-2) and (c-2) may be repeated.

[0043] The method for manufacturing an aluminum alloy substrate according to the third embodiment includes an intermediate processing step (b-3) that combines the above-described steps (b-1) and (b-2). Specifically, it includes (a-3) a first plating step of immersing an aluminum alloy substrate for plating in an electroless Ni-P plating solution, (b-3) an intermediate processing step of removing the aluminum alloy substrate for plating that has been immersed in the electroless Ni-P plating solution, exposing it to air for 10 seconds or more but less than 300 seconds, and immersing it in pure water for 1 second or more, and (c-3) a second plating step of immersing the aluminum alloy substrate for plating after the intermediate process again in the electroless Ni-P plating solution to impart a Ni-P plating film of a predetermined thickness. The same plating solution is used for each electroless Ni-P plating process in steps (a-3) and (c-3). For this electroless Ni-P plating process, it is preferable to use an electroless Ni-P plating solution, such as a commercially available Nimden HDX (manufactured by Uemura Kogyo Co., Ltd.), as in the first and second embodiments, and to perform the plating under the following conditions: temperature 80-95°C, processing time 90-180 minutes, Ni concentration 3-10 g / L, and sodium hypophosphite concentration 25-40 g / L (P concentration 0.73-1.12 mass%). In this case, the total plating time (immersion time) with the electroless Ni-P plating solution should be 90-180 minutes. Furthermore, in step (b-3), the exposure condition of exposure to air for 10 seconds or more but less than 300 seconds is the same as the conditions in the exposure step of the first embodiment described above, and the immersion condition of immersion in pure water for 1 second or more is the same as the conditions in the immersion step of the second embodiment described above. Also, in step (b-3), the order of the exposure step and the immersion step is not limited, and either step may be performed first. Furthermore, when laminating three or more Ni-P plating layers on an aluminum alloy substrate, steps (b-3) and (c-3) may be repeated.

[0044] In the method for manufacturing an aluminum alloy substrate according to this embodiment, step (b) (reaction cessation step) includes the above-described (b-1) exposure step, (b-2) immersion step, or (b-3) intermediate processing step. In step (b) described above, the aluminum alloy substrate for plating is physically separated from the electroless Ni-P plating solution by removing it from the electroless Ni-P plating solution, thereby stopping the local galvanic reaction caused by the generation of minute defects. The method for removing the aluminum alloy substrate for plating is not particularly limited as long as the aluminum alloy substrate for plating can be lifted out of the electroless Ni-P plating solution, and no special operation is required on the Ni-P plating solution side. Furthermore, the minute defects that were partially formed in step (a) due to the cessation of the local galvanic reaction on the aluminum alloy substrate are filled (sealed) when electroless Ni-P plating is started again in step (c), as a further Ni-P plating film (second plating film) is formed on the surface of the Ni-P plating film (first plating film) formed in step (a). As a result, the occurrence of minute defects on the aluminum alloy substrate can be reduced.

[0045] In step (b), there is no particular limit to the number of times the aluminum alloy substrate for plating is removed, but a single removal is sufficient to stop the local galvanic reaction. Furthermore, if the aluminum alloy substrate for plating is removed just before the completion of the first plating process in step (a), even if a reduction in minute defects is achieved, there is a possibility that the sealed minute defects may reappear on the surface when the surface of the Ni-P plating film on the aluminum alloy substrate is polished in a subsequent process. For this reason, it is preferable to remove the aluminum alloy substrate for plating once when the remaining film thickness relative to the required plating thickness is 2 μm. In addition, after the second plating process in step (c), finishing polishing with a cloth or the like may be performed as needed.

[0046] 4. Evaluation method for extremely small defects When an aluminum alloy substrate coated with electroless Ni-P plating is immersed in nitric acid, the Ni-P plating film dissolves uniformly from the surface. Because the dissolution proceeds uniformly, the irregularities on the surface of the Ni-P plating film also dissolve along their shape. As mentioned above, since minute defects extend from the surface of the Ni-P plating film to the aluminum alloy substrate, when a Ni-P plating film containing minute defects is immersed in nitric acid, the nitric acid penetrates into the interior of the minute defects, dissolving the sides and bottom surfaces of the minute defects as well. In other words, as the immersion time in nitric acid progresses, the bottom surface of the minute defects reaches the interface with the aluminum alloy substrate. In addition, a zincate film composed of Zn and Fe remains at the interface between the Ni-P plating film and the aluminum alloy substrate. Therefore, when nitric acid penetrates into the interior of the minute defects, the dissolution of this zincate film progresses, causing the Ni-P plating film to peel off. The more minute defects there are, the more starting points there are for delamination. Therefore, the time it takes for the Ni-P plating film to begin delamination corresponds to the number of minute defects present. In other words, the longer the time it takes for the Ni-P plating film to delaminate, the fewer minute defects are present in the Ni-P plating film, and the more the occurrence of minute defects is reduced.

[0047] Based on these properties of nitric acid, in the aluminum alloy substrate according to this embodiment, when evaluating the occurrence of the above-mentioned minute defects, it is preferable that the time required for at least one Ni-P plating film to peel off under the condition of immersing the aluminum alloy substrate in 20-30 vol% nitric acid at 30-50°C is 20% or more longer than the time required for a single Ni-P plating layer as a comparative plating layer to peel off in an aluminum alloy substrate on which a single Ni-P plating layer having the same thickness as the total thickness of the Ni-P plating film and the same component composition as the Ni-P plating film is provided on the surface, i.e., a single Ni-P plating layer as a comparative plating layer. For example, if the time required for a single Ni-P plating layer in the comparative plating layer to peel off is 100 seconds, then in the aluminum alloy substrate according to this embodiment, if the time required for at least one Ni-P plating film to peel off is 120 seconds or more, it can be evaluated that the occurrence of minute defects has been reduced.

[0048] 5. Magnetic disks In the aluminum alloy substrate according to this embodiment, a magnetic disk can be manufactured by attaching a magnetic material to the surface of a Ni-P plating film by sputtering. Such a magnetic disk comprises the aluminum alloy substrate according to this embodiment and a magnetic material layer provided on the aluminum alloy substrate. Furthermore, a protective layer made of a carbon-based material such as diamond-like carbon may be further formed on the magnetic material layer by CVD. In addition, a lubricating layer, which is a lubricating oil, may be applied on the protective layer. Such a magnetic disk can be used, for example, as a magnetic disk for energy-assisted HDDs such as heat-assisted magnetic recording and microwave-assisted recording.

[0049] Although the aluminum alloy substrate for magnetic disks and its manufacturing method according to this embodiment have been described above, the present invention is not limited to the above embodiments, and various modifications and changes are possible based on the technical concept of the present invention. [Examples]

[0050] The present invention will be described in more detail below based on examples, but the present invention is not limited thereto.

[0051] [Example 1] <Fabrication of aluminum alloy substrates> An aluminum alloy molten metal, whose composition was adjusted to be a JIS-A5086P alloy (Al-Mg alloy), was cast using the DC casting method to produce an ingot. Both sides of the obtained ingot were machined to a thickness of 15 mm, and a homogenization treatment was performed at 520°C for 1 hour. Next, hot rolling was performed at a starting temperature of 460°C and an ending temperature of 340°C to produce a hot-rolled sheet with a thickness of 3.0 mm. The hot-rolled sheet was then cold-rolled (rolling rate 67%) without intermediate annealing to produce a final rolled sheet with a thickness of 0.52 mm. The aluminum alloy sheet thus obtained was punched out into an annular shape with an outer diameter of 96 mm and an inner diameter of 24 mm to produce an annular aluminum alloy sheet.

[0052] A disc blank was fabricated from the annular aluminum alloy plate obtained as described above by pressurized flattening annealing at 300°C for 3 hours under a pressure of 1.5 MPa. The end faces of this disc blank were machined to an outer diameter of 95 mm and an inner diameter of 25 mm. Next, the surface of the disc blank was ground down to a thickness of 10 μm, and then subjected to a stress-relieving heat treatment at 350°C for 10 minutes. After that, it was degreased at 60°C for 5 minutes using AD-68F (manufactured by Uemura Kogyo) degreasing solution, etched at 65°C for 3 minutes using AD-107F (manufactured by Uemura Kogyo) etching solution, and then dematted for 50 seconds with a 30% HNO3 aqueous solution (room temperature).

[0053] Subsequently, the substrate was treated with AD-301F (manufactured by Uemura Kogyo) zincate treatment solution for 50 seconds. After the zincate treatment, the zincate layer was removed with a 30% HNO3 aqueous solution (room temperature) for 60 seconds, and then treated again with AD-301F (manufactured by Uemura Kogyo) zincate treatment solution for 60 seconds to prepare an aluminum alloy substrate for plating. After the second zincate treatment, the prepared aluminum alloy substrate for plating was immersed in an electroless Ni-P plating solution (Nimden HDX (manufactured by Uemura Kogyo)) to form a Ni-P plating film with a P concentration of 12 mass% on both sides of the aluminum alloy substrate for plating (first plating treatment). After 60 minutes, the aluminum alloy substrate for plating was removed from the electroless Ni-P plating solution and exposed to air for 30 seconds as an intermediate treatment step. Then, the aluminum alloy substrate for plating was immersed again in the same electroless Ni-P plating solution for 60 minutes (second plating treatment). Next, a finishing polish was performed using a cloth (polishing amount 3 μm) to fabricate an aluminum alloy substrate in which two Ni-P plating layers were continuously laminated.

[0054] [Example 2] An aluminum alloy substrate was prepared in the same manner as in Example 1, except that in the intermediate processing step, the aluminum alloy substrate for plating was immersed in pure water for 30 seconds instead of being exposed to air for 30 seconds.

[0055] [Example 3] An aluminum alloy substrate was prepared in the same manner as in Example 1, except that in the intermediate processing step, the aluminum alloy substrate for plating was exposed to air for 30 seconds and then immersed in pure water for another 30 seconds.

[0056] [Comparative Example 1] An aluminum alloy substrate was prepared in the same manner as in Example 1, except that the immersion time in the electroless Ni-P plating solution was set to 120 minutes in the first plating process, and subsequent exposure to air and the second plating process were omitted.

[0057] [Comparative Example 2] An aluminum alloy substrate was fabricated in the same manner as in Example 1, except that the exposure time to air was set to 5 minutes during the intermediate processing.

[0058] <Evaluation: Extremely minute defect> Each fabricated aluminum alloy substrate was immersed in a 30% HNO3 aqueous solution at 50°C, and the time until the plating began to peel off (immersion time) was measured. The results are shown in Table 1.

[0059] [Table 1]

[0060] As shown in Table 1, the time required to remove the plating film from the aluminum alloy substrate of Comparative Example 1 was 400 seconds, while the times required to remove the plating film from the aluminum alloy substrates of Examples 1 to 3 were 490 seconds, 483 seconds, and 531 seconds, respectively. In other words, in Example 1, the time required to remove the plating film was more than 20% longer than in Comparative Example 1, where the time required to remove a single Ni-P plating layer was measured, indicating a reduction in the occurrence of minute defects. Furthermore, in Comparative Example 2, the exposure to air was long (5 minutes), resulting in a significantly shorter time until the plating film was removed. [Industrial applicability]

[0061] The aluminum alloy substrate for magnetic disks and its manufacturing method according to the present invention can reduce minute defects without significantly altering the current manufacturing process. This makes it possible to increase the storage capacity per magnetic disk without increasing manufacturing costs, and furthermore, to improve productivity.

Claims

1. An aluminum alloy substrate for magnetic disks having two or more Ni-P plating layers structurally stacked in a continuous manner, An aluminum alloy substrate for magnetic disks, characterized in that each of the two or more Ni-P plating layers is a Ni-P plating layer formed using an electroless Ni-P plating solution having the same component composition and containing the same concentration of P.

2. The aluminum alloy substrate for magnetic disks according to claim 1, wherein the concentration of P in the Ni-P plating layer is in the range of 10 mass% or more and 14 mass% or less.

3. The aluminum alloy substrate for magnetic disks according to claim 1 or 2, wherein, under conditions of immersion of the aluminum alloy substrate in 20-30 vol% nitric acid at 30-50°C, the time required for at least one of the Ni-P plating layers to peel off is 20% or more longer than the time required for one Ni-P plating layer to peel off in an aluminum alloy substrate having a single Ni-P plating layer on its surface having the same thickness as the total thickness of the Ni-P plating layer and the same component composition as the Ni-P plating layer.

4. A first plating process involves immersing an aluminum alloy substrate for plating in an electroless Ni-P plating solution, A reaction stopping step is performed to remove the aluminum alloy substrate for plating that is immersed in the electroless Ni-P plating solution and to stop the chemical reaction. A second plating process involves immersing the aluminum alloy substrate for plating, after the reaction cessation step, again in the electroless Ni-P plating solution to apply a Ni-P plating layer of a predetermined thickness. A method for manufacturing an aluminum alloy substrate for a magnetic disk according to any one of claims 1 to 3.

5. The method for manufacturing an aluminum alloy substrate for a magnetic disk according to claim 4, wherein the reaction cessation step includes an exposure step of removing the aluminum alloy substrate for plating, which is immersed in the electroless Ni-P plating solution, and exposing it to the air for 10 seconds or more but less than 300 seconds.

6. The method for manufacturing an aluminum alloy substrate for a magnetic disk according to claim 4, wherein the reaction cessation step includes an immersion step of removing the aluminum alloy substrate for plating that is immersed in the electroless Ni-P plating solution and immersing it in pure water for 1 second or more.

7. The method for manufacturing an aluminum alloy substrate for a magnetic disk according to claim 4, wherein the reaction cessation step includes an intermediate processing step of removing the aluminum alloy substrate for plating that is immersed in the electroless Ni-P plating solution, exposing it to air for 10 seconds or more but less than 300 seconds, and immersing it in pure water for 1 second or more.

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