JP2015181077A - Magnetic disk titanium plate and manufacturing method of the same and magnetic disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic disk titanium plate manufacturing method that is suitable to be used for a magnetic disk of a heat-assisted magnetic recording method, and can easily form a magnetic disk titanium plate of hardness equal to or more than Hv450, and a hardened modification layer of hardness equal to or more than Hv450 on a surface side of the titanium-made disk substrate without using expensive devices, and to provide the magnetic disk.SOLUTION: A magnetic disk titanium plate is composed of a disk substrate made up of pure titanium or a titanium alloy, and a hardened modification layer that is formed on a surface side of the disk substrate, in which oxygen density in the modification layer is equal to or 9 atom% and equal to or less than 30 atom%.

Description

  The present invention relates to a magnetic disk titanium plate used as a medium for high-density recording and reproduction in a computer or the like, a method of manufacturing the magnetic disk titanium plate, and a magnetic disk.

  Conventionally, an aluminum alloy is often used for a disk substrate of a medium for high-density recording and reproduction such as a computer. Since this aluminum alloy is softer than other metals, it is used as a magnetic disk by providing a coating with a thickness of about 10 μm by a hard nickel-phosphorous plating on its surface. In the manufacturing process of the magnetic disk, a magnetic film is further provided on the surface of the nickel-phosphorus film by sputtering, and the substrate temperature when the magnetic film was formed by this conventional method was about 300 ° C. .

  There is a growing need for further increase in recording density of magnetic disks, and recently, a heat-assisted magnetic recording method has attracted particular attention as a technique for increasing the recording density. A heat treatment temperature when a magnetic film suitable for the heat-assisted magnetic recording method is provided on the surface of the magnetic disk is about 500 to 600 ° C., and a conventional magnetic disk in which a nickel-phosphorus coating is provided on a disk substrate made of an aluminum alloy However, the heat resistance is insufficient and it is not suitable for the heat-assisted magnetic recording system.

  In order to cope with the heat-assisted magnetic recording method, it has been attempted to use a titanium plate having excellent high-temperature strength for the disk substrate. However, titanium has an original hardness of Hv 350 or less and is harder than the nickel-phosphorus coating. Since it is low, it cannot be used as it is. In this specification, the hardness of the disk substrate is sometimes simply expressed as hardness, but all indicate Vickers hardness.

  For this reason, an attempt has been made to provide a hardened layer on the surface side of the titanium plate. According to Patent Documents 1 and 2, the surface of the disk substrate made of pure titanium is hardened by heat treatment under atmosphere control. According to Patent Document 3, a material in which an oxide film is formed on the surface side of a disk substrate made of pure titanium by electrolytic polishing or anodization is made of pure titanium by a sputtering method. A material in which a hard film is formed on the surface side of a disk substrate has been proposed.

  However, the hardness of the modified layer formed by the methods described in Patent Documents 1 and 2 is about Hv 300 to 400, and the hardness of the nickel-phosphorus coating provided on the surface of the disk substrate made of a conventional aluminum alloy. Compared with about Hv600, which is apparently low in hardness, it can be said to be insufficient in practice.

  In the technique described in Patent Document 3, the oxygen concentration of the disk substrate is increased to increase the strength and hardness of the oxide film, but the oxygen concentration is in the range of JIS 2 to 4 types of pure titanium and the hardness of the material. Is only Hv 150 to 250, which is not far from about Hv 600, which is the hardness of the nickel-phosphorus coating described above.

  On the other hand, when forming the modified layer hardened by the sputtering method as described in Patent Document 4, it seems that it is possible to obtain a modified layer having sufficient hardness, but the sputtering method is adopted. In order to achieve this, an expensive device is required, and since processing for each part is required, the cost becomes very high, and this is a technique that remains a major problem to be improved in this respect.

Japanese Patent Laid-Open No. 1-112521 JP-A-5-28456 JP-A-6-12661 Japanese Unexamined Patent Publication No. Hei 4-98616

  The present invention has been made in order to solve the above-described conventional problems, and is excellent in heat resistance when a magnetic film is provided on the surface thereof. Therefore, the present invention is suitable for use in a magnetic disk of a heat-assisted magnetic recording system, and its hardness. Without using an expensive apparatus, a titanium plate for a magnetic disk having a level of Hv450 or higher that has no practical problem, and a hardened modified layer having a hardness of Hv450 or higher on the surface side of a titanium disk substrate. It is an object of the present invention to provide a method of manufacturing a magnetic disk titanium plate that can be easily formed, and a magnetic disk having a recording medium layer formed on the surface of a modified layer of the magnetic disk titanium plate. is there.

  The titanium plate for a magnetic disk of the present invention is composed only of a disk substrate made of pure titanium or a titanium alloy having a modified layer hardened by solid solution of oxygen formed on the surface side. The titanium plate for a magnetic disk, wherein the oxygen concentration is 9 atomic% or more and 30 atomic% or less.

  Moreover, it is preferable that the oxygen concentration in the said modified layer is 9 atomic% or more and 20 atomic% or less.

  The method for manufacturing a titanium plate for a magnetic disk according to the present invention includes a step of heat-treating the disk substrate at 850 ° C. or higher when forming the modified layer on the surface side of the base material of the disk substrate. It is a manufacturing method of the titanium plate for discs.

  Moreover, it is preferable that the said heat processing is performed in the inert gas whose oxygen concentration is 20 ppm or less by mass ratio.

  Moreover, it is preferable to remove the scale layer formed on the surface of the modified layer after the heat treatment.

  The magnetic disk of the present invention is a magnetic disk characterized in that a recording medium layer is formed on the surface of the modified layer of the titanium plate for magnetic disk.

  The titanium plate for a magnetic disk of the present invention can withstand a high temperature of 500 to 600 ° C. when a magnetic film is formed on the surface, and can be used for a magnetic disk of a thermally assisted magnetic recording system capable of achieving a high recording density. It is suitable for use and has a hardness of Hv450 or higher at a level that does not cause a problem when it comes into contact with the magnetic head.

  According to the method for manufacturing a titanium plate for a magnetic disk of the present invention, a hardened modified layer having a hardness of Hv450 or more can be easily formed on the surface side of a titanium disk substrate without using an expensive apparatus. Can do.

It is a graph which shows the relationship between the depth from the surface (just under a scale layer) of the test bodies 1-3, and Vickers hardness. 4 is a drawing-substituting photograph showing a cross-sectional observation result of a metallographic structure with an optical microscope near the surface of the test body 1. 3 is a drawing-substituting photograph showing a cross-sectional observation result of a metallographic structure with an optical microscope near the surface of a specimen 2. FIG. 5 is a drawing-substituting photograph showing a cross-sectional observation result of a metallographic structure with an optical microscope near the surface of a specimen 3. It is a graph which shows the relationship between the Vickers hardness of the hardened modified layer, and the oxygen concentration in the hardened modified layer. It is a graph which shows the relationship between the depth from the surface (directly under a scale layer) of the test bodies 3 and 4, and Vickers hardness. 4 is a drawing-substituting photograph showing a cross-sectional observation result of a metallographic structure with a scanning electron microscope near the surface of a test body 3. 3 is a drawing-substituting photograph showing a cross-sectional observation result of a metallographic structure with a scanning electron microscope in the vicinity of the surface of a specimen 4.

  Aluminum alloys are widely used as materials for magnetic disk substrates, but in recent years, there is a need for higher recording density for magnetic disks, and heat-assisted magnetic recording methods are attracting attention as a technology for achieving higher recording densities. . However, a conventionally used magnetic disk in which a nickel-phosphorus coating is separately provided on the surface of a disk substrate made of an aluminum alloy has a problem in that it has insufficient heat resistance and is not suitable for a heat-assisted magnetic recording system.

  Therefore, the present inventors tried to use a titanium plate having excellent high-temperature strength for the disk substrate in order to cope with the heat-assisted magnetic recording system. However, since the hardness of the titanium plate is as low as Hv 350 or less and cannot be used as it is as a disk substrate, pay attention to forming a hardened modified layer on the surface side of the titanium plate. We examined by research.

  As a result, a hardened modified layer hardened by oxygen solid solution can be formed on the surface side of the base material of the disk substrate by heat-treating a disk substrate made of pure titanium or a titanium alloy at a high temperature of 850 ° C. or higher. In addition, it has been found that when the oxygen concentration in the modified layer is 9 atomic% or more, the modified layer has a hardness of Hv450 or more.

  Also, depending on the atmosphere (oxygen concentration) during the heat treatment, a scale layer made of titanium oxide may be formed so as to cover the surface of the hardened modified layer. Remove it.

  Hereinafter, the present invention will be described in more detail based on embodiments shown in the accompanying drawings.

  The titanium plate for a magnetic disk of the present invention is composed only of a disk substrate made of pure titanium or a titanium alloy having a hardened modified layer formed on the surface side. First, the disk substrate will be described in detail, and the modified layer will be described in detail separately.

<Disc board>
As described above, the disk substrate is made of pure titanium or a titanium alloy. The material of this disk substrate is, for example, produced by a conventionally known manufacturing method or the like in which pure titanium or a titanium alloy is melted and cast to form an ingot, hot rolled, and then cold rolled. . The material of the disk substrate is preferably annealed, but regardless of the finished state, for example, “annealing + pickling finish”, “vacuum heat treatment finish”, “bright annealing finish”, etc. are adopted. be able to.

  Although the disk substrate is made of pure titanium or a titanium alloy, it is not limited to pure titanium or a titanium alloy having a specific composition. However, considering the ease of cold rolling that can be cold rolled at a total rolling rate of 35% or more without intermediate annealing, and the viewpoint of securing the subsequent workability, the mass ratio is O: 1500 ppm or less, Fe: It is preferably 1500 ppm or less, C: 800 ppm or less, N: 300 ppm or less, and H: 130 ppm or less, with the balance being Ti and inevitable impurities. For example, a JIS type 1 cold rolled plate can be used.

  Of the additive elements, O is more preferably 1000 ppm or less, and Fe is more preferably 1000 ppm or less. Further, the lower limit of the addition amount of the additive element is not particularly defined, but for example, O: 100 ppm, Fe: 100 ppm, C: 50 ppm, N: 30 ppm, H: 10 ppm.

  Further, the disk substrate may be formed with a modified layer in a state of a plate material before being punched into a disk shape, or a modified layer is formed after punching a normal titanium plate material into a disk shape. It may be a thing.

<Hardened modified layer>
In general, a coating such as a nickel-phosphorus coating is separately provided on the surface of the magnetic disk. The reason why the film is provided on the surface of the magnetic disk is that the magnetic head may come into contact with the disk surface during information recording. However, if the surface is not hard enough, the disk surface will be dented or wrinkled when the magnetic head is in contact. This is because it may occur. Such dents and wrinkles are not preferable because information cannot be recorded, which hinders high recording density.

  The hardness of the nickel-phosphorous film is about Hv 600, but providing the nickel-phosphorous film on the surface of the disk substrate is not suitable for the heat-assisted magnetic recording system in terms of heat resistance. Therefore, the feature of the present invention is that the surface side of the disk substrate itself is cured to a hardness equivalent to that of the nickel-phosphorus coating, that is, the hardened modified material having the same hardness as that of the nickel-phosphorus coating on the surface side of the disk substrate. Specifically, the hardness of the modified layer is Hv450 or higher, and in order to obtain this hardness, the oxygen concentration in the modified layer should be 9 atomic% or higher. good.

  If the hardness of the hardened modified layer is less than Hv450, as described above, there is a possibility that dents and wrinkles are generated on the disk surface when the magnetic head is contacted. The preferred hardness is Hv 480 or more, and the more preferred hardness is Hv 500 or more. On the other hand, from the viewpoint of the oxygen concentration in the modified layer, the preferable oxygen concentration is 9.5 atomic% or more, and the more preferable oxygen concentration is 10.0 atomic% or more.

  On the other hand, the upper limit of the hardness of the hardened modified layer is not particularly limited from the viewpoint of the performance of the magnetic disk, but if the modified layer is too hard, the precision when the disk surface is mirror finished It takes too much time during polishing, and if it becomes too hard, it becomes brittle, making it difficult to obtain a mirror surface even by precision polishing. Accordingly, the oxygen concentration in the modified layer is 30 atomic% or less, preferably 20 atomic% or less.

  In the present invention, the thickness of the hardened modified layer is not particularly limited, but is preferably 15 μm or more, and more preferably 20 μm or more. When manufacturing a magnetic disk, there is a process of mirror-polishing the surface, and it is assumed that the thickness of the modified layer is slightly reduced by polishing. Therefore, the modified layer has a certain thickness including the polishing allowance. is necessary. In particular, there is no need to define the upper limit of the thickness of the modified layer, but since the modified layer is obtained by heat treatment, the upper limit of the actual thickness is considered to be about 100 μm.

  The thickness and hardness of the hardened modified layer can be determined by performing cross-section processing using a sample and measuring the cross-section Vickers hardness with a micro Vickers hardness tester. The cross section processing is performed by embedding the sample in a resin, cutting the sample together with the resin to obtain a cross section sample, and further polishing the cross section. Thereafter, the hardness is measured sequentially from the outermost surface of the sample in the depth direction, and the position when reaching Hv450 is set as the lowermost surface of the modified layer, and the thickness of the modified layer is obtained. In the case where a scale layer is formed on the sample, the thickness of the modified layer is measured starting from directly below the scale layer.

  Further, the oxygen concentration in the hardened modified layer can be measured by a composition analysis by a scanning electron microscope / energy dispersive X-ray analysis (SEM / EDX) from a cross section. In the present invention, the acceleration voltage at the time of measurement is 15 kV, carbon, nitrogen, oxygen, and titanium are measured to determine their respective compositions (atomic%), thereby defining the oxygen concentration in the modified layer.

  In addition, when performing the structure observation from the cross section and the composition analysis in the hardened modified layer, the preparation and observation of the cross section sample are performed by a general method. For example, after the collected sample is embedded in a resin, the whole resin is cut, the cross section is processed to be flat by polishing, and then a cross-sectional sample is prepared by etching using a mixed solution of hydrofluoric acid and nitric acid. . Tissue observation is performed with an optical microscope, but observation with an electron microscope having better resolution may be used in combination.

<Manufacturing method>
The titanium plate for a magnetic disk of the present invention is produced by a conventionally known manufacturing method in which titanium or a titanium alloy is melted and cast to form an ingot, hot-rolled and then cold-rolled. In addition, the manufacturing process includes a process of heat-treating the disk substrate at 850 ° C. or higher in order to form a hardened modified layer on the surface side of the disk substrate obtained through the above-described steps. Further, it may include a step of polishing and smoothing the surface of the disk substrate before the heat treatment, and a step of cutting the disk substrate into a predetermined size before or after the heat treatment.

  The hardened modified layer is formed when oxygen in the atmosphere diffuses and penetrates from the surface of the disk substrate into the disk substrate during heat treatment, so that oxygen is dissolved in the surface layer of the disk substrate.

  The temperature is the main governing factor of the oxygen dissolution rate when the hardened modified layer is formed. If the temperature during the heat treatment is lower than 850 ° C., the oxygen dissolution rate on the surface of the disk substrate becomes extremely slow, so that the formed modified layer can obtain the necessary hardness as a hardened modified layer. It becomes difficult, and it takes a long time to form a hardened modified layer.

  For this reason, the heat treatment temperature for forming a hardened modified layer having an oxygen concentration of 9 atomic% or higher and a hardness of Hv 450 or higher is 850 ° C. or higher. Preferably it is 860 degreeC or more, More preferably, you may be 870 degreeC or more. On the other hand, the upper limit of the heat treatment temperature is not particularly specified. However, if the temperature during the heat treatment is too high, a very hard and brittle layer exceeding Hv1000 is formed, which is undesirable because it affects the yield. Therefore, the upper limit of the heat treatment temperature is about 1200 ° C. It is preferable that

  As described above, the formation of the hardened modified layer on the surface side of the disk substrate proceeds by oxygen solid solution, but if the oxygen concentration in the heat treatment atmosphere is too high, the modified layer by oxygen solid solution is used. As a result, a porous scale layer is formed on the surface of the modified layer, resulting in a decrease in yield. Therefore, the oxygen concentration in the atmosphere is preferably low, and the heat treatment is preferably performed in an inert gas having an oxygen concentration of 20 ppm or less by mass ratio. From the viewpoint of low oxygen concentration, heat treatment in a reduced pressure atmosphere such as under vacuum is assumed.

  Thus, depending on the oxygen concentration during the heat treatment, a porous and brittle scale layer is formed on the surface of the modified layer that has been hardened after the heat treatment. Since this scale layer is an unnecessary layer, it is preferably removed. A method for removing the scale layer is not particularly defined, but a suitable formulation may be selected as appropriate, for example, removal by physical polishing, removal by chemical etching, or the like. Further, the removal of the scale layer is performed after an appropriate criterion is determined by prior examination, for example, until a metallic color is observed in the color tone, or until the Vickers hardness from the surface is Hv 1000 or less. It is recommended.

<Production of magnetic disk>
Next, a method for producing a magnetic disk using the titanium plate for magnetic disk of the present invention will be briefly described.

  First, a magnetic disk titanium plate having a hardened modified layer formed on the surface is appropriately punched into a disk shape, such as a blank material with an outer diameter of 2.5 inches, and the resulting magnetic disk The surface is flattened by performing strain relief annealing while applying a load in a state where a plurality of sheets are stacked. Next, processing for adjusting the end face shape and dimensions of the magnetic disk is performed, and finally, polishing such as mirror polishing for smoothing the surface is performed. After the above steps, a recording medium layer is formed on the surface of the modified layer to complete the magnetic disk. In the case of a magnetic disk made of a conventional aluminum alloy, a step for forming a nickel-phosphorus coating is required, but this step can be omitted in the present invention.

  Moreover, after blanking a normal titanium plate into a blank material shape, a blank material whose surface is hardened by performing a process of forming a hardened modified layer on the surface is prepared, and then a planarization process and a shape are performed. A magnetic disk may be manufactured by a process of trimming, mirror polishing, and recording medium layer formation.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

<Example 1>
(Preparation of test specimen)
For the preparation of the test body, a JIS type 1 titanium plate was used. The chemical composition of the titanium plate material is, by mass ratio, O: 450 ppm, Fe: 250 ppm, N: 40 ppm, with the balance being Ti and inevitable impurities. The plate thickness is 1.0 mm and the size is 20 × 50 mm. This titanium plate was obtained by subjecting a titanium raw material to a conventionally known melting step, hot rolling step, cold rolling step, and pickling step.

This titanium plate (disk substrate) was heat-treated in a nitrogen gas atmosphere containing 2 ppm of oxygen. The heat treatment temperature was three conditions of 800 ° C., 900 ° C., and 1000 ° C., and the holding time was 60 minutes for each temperature condition. The heat treatment was performed after the inside of the furnace was replaced with nitrogen gas using an atmospheric heat treatment furnace. The obtained specimens are as follows.
Specimen 1: Heat treatment in nitrogen gas at 800 ° C. for 60 minutes (comparative example)
Test specimen 2: heat treatment in nitrogen gas at 900 ° C. for 60 minutes (invention example)
Specimen 3: Heat treatment at 1000 ° C. for 60 minutes in nitrogen gas (invention example)

(Evaluation of hardness and thickness of the hardened modified layer)
Measurement of the hardness and thickness of the hardened modified layer formed on the surface side of the specimen after heat treatment is performed by embedding each specimen and then cutting the specimen together with the resin to perform cross-section processing. Then, the cross section is smoothed by polishing (rough polishing, buff polishing), and then the micro Vickers hardness is measured from the surface of the specimen toward the depth direction, and the hardness and thickness of the modified layer are evaluated. It was.

  Specifically, the measurement of micro Vickers hardness is performed using a micro Vickers hardness measurement apparatus (MVK-G3 manufactured by Akashi). When a scale layer is formed on the outermost surface, the micro Vickers hardness is directly below the base layer (base material). The outermost surface) was used as the starting point, the interval was 10 μm in the depth direction, the load was 10 g, and the holding time was 15 seconds.

(Evaluation of the composition of the hardened modified layer)
After measuring the Vickers hardness of the cross section with a micro Vickers hardness tester, the base material region showing a hardness of about Hv200, which is the original hardness of the titanium material, and the hardened modification showing a hardness of Hv450 or more A compositional analysis of the layer region was performed. In addition, since the test body 1 of the comparative example does not have a hardened modified layer region of Hv450 or higher, the composition analysis was performed only on the test bodies 2 and 3 of the invention example.

  Specifically, the composition analysis of the hardened modified layer and the base material region is performed using a scanning electron microscope / energy dispersive X-ray analysis (SEM / EDX) apparatus, and the acceleration voltage is set to 15 kV. The composition (atomic%) of nitrogen, oxygen, and titanium was measured, and the oxygen concentration (atomic%) in each region was determined. Further, in the test bodies 2 and 3, the oxygen concentration in the depth direction was measured from directly below the scale, and the relationship between the two was determined from the hardness data and the measured oxygen concentration.

(Observation of cross-sectional structure of hardened modified layer)
After cross-section processing and hardness measurement of the test body, the cross-section of the test body was buffed to finish it smoothly, and the polished surface was etched with 1% by mass hydrofluoric acid / 10% by mass nitric acid aqueous solution, and then optical The cross-sectional structure was observed with a microscope at a magnification of 500 times.

(Test results)
FIG. 1 shows the relationship between the depth from the surface (immediately below the scale layer) and Vickers hardness of specimens 1 to 3 that were heat-treated at three different temperatures, and FIG. FIG. 3 shows a cross-sectional observation result by an optical microscope near the surface of the test specimen 2, and FIG. 4 shows a cross-sectional observation result by an optical microscope near the surface of the test specimen 3, respectively.

  According to FIG. 1, the hardness of the specimen 1 is about Hv300 even in the hardest region, and no hardened modified layer having a hardness of Hv450 or higher is formed. On the other hand, in the specimen 2, the region having a hardness of Hv450 or higher is a region within about 30 μm from the surface, and it can be determined that a hardened modified layer having a hardness of Hv450 or higher is formed in this region. Moreover, in the test body 3, the area | region whose hardness is Hv450 or more is an area | region within about 80 micrometers from the surface, and it can be judged that the hardened modified layer whose hardness is Hv450 or more is formed in this area | region.

  Although the thickness of the hardened modified layer is not specified in the present invention, both the hardness and thickness of the test body 2 that was heat-treated at 900 ° C. and the test body 3 that was heat-treated at 1000 ° C. A sufficiently hardened modified layer is formed on the surface. On the other hand, it can be determined that the specimen 1 that has been heat-treated at 800 ° C. has a hardness of about Hv300 at a position of 10 μm from the surface, but a hardened modified layer is not formed.

  In addition, the hardness and hardness of the hardened modified layer were obtained by using the test bodies 2 and 3 that obtained the result that the hardened modified layer satisfying the requirements of the present invention was formed on the surface. FIG. 5 shows the result of determining the relationship of the oxygen concentration in the modified layer (described as a hardened layer). As is clear from FIG. 5, there is a clear correlation between the hardness and the oxygen concentration, and the oxygen concentration is about 9 atomic% or higher and the hardness is Hv 450 or higher.

<Example 2>
Unlike the first example, the same JIS type 1 titanium plate material as in Example 1 was used, and the titanium plate material (disk substrate) was heat-treated in an air atmosphere. The heat treatment temperature was 1000 ° C. and the holding time was 60 minutes. The obtained specimens are as follows.
Specimen 4: Heat treatment in the atmosphere at 1000 ° C. for 60 minutes (Invention example)

  Using this test body 4, evaluation of the hardness and thickness of the hardened modified layer and observation of the cross-sectional structure of the hardened modified layer were performed in the same manner as in Example 1.

  In order to confirm the influence of different heat treatment atmospheres in a nitrogen gas atmosphere and an air atmosphere, the test results of the test body 3 and the test body 3 that were heat-treated at the same heat treatment temperature and holding time as the test body 4 were compared. . FIG. 6 shows the relationship between the depth from the surface of specimens 3 and 4 (just below the scale layer) and the Vickers hardness, and FIG. 7 shows the result of cross-sectional observation by the scanning electron microscope near the surface of specimen 3. 8 shows cross-sectional observation results by the scanning electron microscope in the vicinity of the surface of the specimen 4. The cross-sectional observation image in FIG. 7 and the cross-sectional observation image in FIG. 4 are cross-sectional observation images of the same specimen 3, but the magnification is different by 200 times.

  According to FIG. 6, it can be seen that both the specimens 3 and 4 have a hardened modified layer having sufficient hardness and thickness on the surface, and the formation of the hardened modified layer itself is the atmosphere. It turns out that it is hardly influenced.

  On the other hand, when the cross-sectional observation images of FIGS. 7 and 8 are compared, it can be seen that a considerably thicker scale layer is formed in the specimen 4 that has been heat-treated in the atmosphere than in the specimen 3. Therefore, it can be understood that a lower oxygen concentration during the heat treatment is preferable.

Claims (6)

It consists only of a disk substrate made of pure titanium or a titanium alloy in which a modified layer hardened by oxygen solid solution is formed on the surface side,
A titanium plate for a magnetic disk, wherein an oxygen concentration in the modified layer is 9 atomic% or more and 30 atomic% or less.   The titanium plate for a magnetic disk according to claim 1, wherein the oxygen concentration in the modified layer is 9 atomic% or more and 20 atomic% or less. It is a manufacturing method of the titanium plate for magnetic discs of Claim 1 or 2, Comprising:
A method of manufacturing a titanium plate for a magnetic disk, comprising the step of heat-treating the disk substrate at 850 ° C. or higher when forming the modified layer on the surface side of the base material of the disk substrate.   4. The method for producing a titanium plate for a magnetic disk according to claim 3, wherein the heat treatment is performed in an inert gas having an oxygen concentration of 20 ppm or less by mass ratio.   5. The method for producing a titanium plate for a magnetic disk according to claim 3, wherein the scale layer formed on the surface of the modified layer is removed after the heat treatment.   A magnetic disk, wherein a recording medium layer is formed on a surface of the modified layer of the titanium plate for a magnetic disk according to claim 1.