Magnetic disk substrate and method of manufacturing the same

Abstract: A magnetic disk substrate comprising 0.5 wt% to 1.0 wt% of Mo, 0.03 wt% to 0.5 wt% of O + 2N + 0.75C, and a balance of Ti. The alloy plate having a com­ position of above range is cold-rolled at a rolling ratio of 30% or more to form a magnetic disk substrate material, and the material is thermal-flattened under the following condition: 500 ≦ T ≦ -(150/11)·t + 7850/11 1 ≦ t where T represents a thermal-flattening temperature (°C), and t represents a thermal-flattening time (houre), thereby obtaining a magnetic disk substrate having the average grain size of 30 µm or less.

Description: This invention concerns a magnetic disk substrate, and in particular, a magnetic disk substrate with excellent heat resistance which can be made very thin, and which is suitable for the manufacture of magnetic disks of high recording density. Magnetic disk, used as a recording medium for a computer, comprises a substrate and a magnetic film formed on the substrate. The substrate is required to have the following properties: (a) Its surface properties after precision machining, precision grinding or precision polishing must be satisfactory so that magnetic heads can run smoothly over the disk, and such that stable magnetic properties are obtained with few magnetic errors. (b) There must be no protuberances or holes in its surface which could lead to defects in the magnetic medium formed on the substrate. (c) It should have sufficient strength and rigidity to withstand high speed rotation when the disk is used, and precision machining during manufacture. (d) It must be able to withstand the heat applied when forming the magnetic medium. (e) It must be lightweight and non-magnetic. (f) It must have some degree of corrosion resistance. Conventionally, Aℓ-Mg or other aluminum alloys are used as substrate materials satisfying these conditions. Recently, there has been a trend in magnetic disks toward higher recording densities and greater compactness, so the following conditions must also be met: (A) The magnetic medium must have improved magnetic properties, for example it must be possible to form a medium with high coercive force. (B) The magnetic medium must be made thinner. (C) The flying height of the magnetic heads must be lessened. (D) The gap length of the magnetic heads must be reduced. (E) Techniques for positioning the magnetic heads must be improved. (F) The substrate must be made more compact and thinner. In order to satisfy these requirements, various provisions are conventionally adopted. A magnetic medium of high recording density is formed by sputtering (Kyuzo Nakamura, "Kinzoku" (Metals), Nov. 1986), or the substrate is made resistant to the heat applied during sputtering. To lessen the flying height of the magnetic heads, the aluminum alloy disk is given a Ni-P plating to cover inclusions. In this way, high level surface properties are obtained with little surface roughness and undulations (Masahiro Saito, "Jitsumu Hyoumen Gijutsu" (Practical Surface Technology), Vol. 35, No. 6, 1988). Further, alloy compositions with few inclusions that could lead to electrical and magnetic errors are used for the substrate (Unexamined Published Japanese Patent No. 63-216953), and attempts are made to reduce the thickness of the substrate itself. To satisfy the above conditions (a) - (f), new materials are constantly being developed to replace aluminum alloys. Examples of these are glass (Hiroyoshi Ishizaki, "Kougyou Zairyo" (Engineering Materials), Vol. 35, No. 5), ceramics (Takeshi Matsumoto, "Jisei Zairyo Kenkyuucai" (Magnetic Material Research Group), "Denshi Zairyo Hyomen Shori Gijutsu" (Electronic Material Surface Treatment Technology), July 1987, and titanium (Published Unexamined Japanese Patent Applications Nos. 52-105804, 59-151335 and 63-142521). Aℓ-Mg or other aluminum alloys conventionally used as magnetic disk substrates however have the disadvantage that they do not have sufficient heat resistance to withstand the substrate temperature rise produced by sputtering, which is carried out to form a magnetic medium with a high coercive force and low thinness required for high recording density. Further, although the composition of this material was adjusted, it still contained essentially a large number of inclusions and high level surface properties could not be obtained. It is therefore impossible to achieve the reduction of the head flying height required for high recording density, and achieve a sufficient reduction of electrical errors. To obtain high level surface properties, the surface is plated with Ni-P as described above, but the method has the disadvantage that the plating yield is fairly low. Further, the Ni-P plating tends to crystallize and retain its magnetism at the fairly low temperature of about 350°C, and peel off. Further, if the aluminum alloy substrate is made thin according to demand, sufficient strength and rigidity cannot be guaranteed to enable it to withstand the high speed disk rotation (for example 3600 rpm) required for a short access time. Further, if glass or ceramics were used as substrate materials, sufficient heat resistance and strength are obtained, but both materials are brittle and are therefore not very reliable. In the case of glass, moreover, gas discharges occurred due to the high temperature applied for sputtering, and elemental impurities in the glass diffused into the magnetic medium, whereby the magnetic properties of the medium are adversely affected. Ceramic substrates, on the other hand, contain a large number of pores, and sufficient surface properties cannot be obtained. Glass and ceramics are therefore still unsatisfactory as substrate materials. As mentioned above, various proposals have been made regarding titanium substrates. In said Published Patent Application No. 52-105804, for example, a magnetic disk substrate is proposed based on titanium or titanium alloy with an oxide or nitride layer formed on the surface. This layer is formed by oxidizing or nitriding the surface, thereby increasing surface hardness and improving machinability so as to obtain satisfactory surface properties. Further, Published Unexamined Japanese Patent Application No. 59-151335 proposes a magnetic disk substrate of α type titanium alloy containing no less than 80% of hcp phase in terms of volume fraction, and of strength no less than 589 MPa (60 kgf/mm²). Further, ordinary pure titanium and α type titanium alloys unavoidably contain β stabilizing elements such as Fe and V to the extent of 0.1 weight %. These β stabilizing elements easily segregate in the melting, blooming and rolling steps, and easily give rise to pitting due to segregation during polishing. If it is attempted to obtain high level surface properties, therefore, the yield declines. Further, the rate of oxidation and nitriding is different in the α and β phases (segregation band), and it is difficult to make oxidizing or nitriding uniformly. The yield of substrates coated with an oxide or nitride layer is therefore lower than that of substrates consisting only of titanium, and manufacturing costs are higher. It is stipulated that the total content of elemental impurities in ordinary titanium or α type titanium should not exceed 0.3 weight%. This level of impurities is much lower than that in aluminum alloys, however it is still not sufficiently low to eliminate all the errors due to inclusions, and to obtain high level surface properties. At present, therefore, a satisfactory magnetic disk substrate cannot be obtained even if titanium is used as the principal material. In consideration of the above problem, an object is to provide a magnetic disk substrate with excellent heat resistance that can be made thin, is free from pitting that is responsible for magnetic errors, can be manufactured in a high yield, and has high-level surface properties and to provide a method of manufacturing the substrate. According to the present invention, there is provided a magnetic disk substrate as defined in claim 1. The substrate comprises 0.5 wt% to 1.0 wt% of Mo, 0.03 wt% to 0.5 wt% of O + 2N + 0.75C, and a balance of Ti. In this case, it is preferable that the average grain size is no greater than 30 µm. According to the present invention, there is provided a method of manufacturing a magnetic disk substrate as defined in claim 3. An alloy plate comprising 0.5 to 1.0 wt% of Mo, 0.03 to 0.5 wt% of O + 2N + 0.75C fall and a balance of Ti is cold-rolled at a rolling ratio of 30% or more to form a magnetic disk substrate material, and the material is flattened under the following condition: 500 ≦ T ≦ -(150/11)·t + 7850/11 1 ≦ t ≦ 15 where T represents a thermal-flattening temperature (°C), and t represents a thermal-flattening time (hour). The present inventors made extensive studies and found that surface properties of the magnetic disk substrate mainly depended on the component and composition of the substrate material and the excellent surface properties could be obtained using Ti as the matrix of the substrate material and controlling gaseous components in the Ti and additive components. They also found that the further excellent surface properties could be obtained by making the average grain size of the disk substrate no greater than 30 µm and that, in order to manufacture the magnetic disk having fine crystal grains, a cold-rolling ratio and a thermal-flattening condition had to be suitably controlled. The present invention has thus been made from the above findings. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a graph showing a relationship between a thermal-flattening condition and flatness of a substrate after thermal-flattening; and Fig. 2 is a graph showing a relationship between the thermal-flattening condition and an average grain size of the substrate after thermal-flattening. The present invention will be described in detail below. A magnetic disk substrate according to the present invention comprising 0.5 to 1.0 wt% of Mo, 0.03 to 0.5 wt% of O + 2N + 0.75C fall within the and the balance of Ti. The melting point of titanium is as high as about 1,650°C. If titanium is used as a magnetic disk substrate material, the substrate can be given satisfactory heat resistance, and it is not deformed even at 300 to 400°C which is the temperature at which sputtering is performed to form the magnetic medium. Furthermore, the tensile strength of titanium is approximately 294 MPa (30 kgf/mm²), its modulus of longitudinal elasticity is approximately 98 GPa (10,000 kgf/mm²), that is, its strength and rigidity are high. Even if it is thin, it can fully withstand the centrifugal force produced at high rotation speeds of 3,600 rpm or over. Further, since the matrix of the substrate is made of only titanium, it can easily be manufactured in small thicknesses, and there is no peeling at inter-layer interfaces unlike in Ni-P plated Al alloy substrates. It is therefore preferable to use titanium as the substrate material. In the present invention, Mo, is added to the titanium matrix. Mo has an effect for making the grains fine, when Mo is added to the titanium matrix. The addition of these elements therefore can make the surface properties excellent. However, when a content of Mo is less than 0.5 wt%, the effect is insufficient. When the content exceeds 1.0 wt%, these elements are easily segregated to cause pitting during polishing. Therefore, the content of Mo is defined to fall within the range of 0.5 to 1.0 wt%. O, N and C may be solid-dissolved in the titanium matrix, and substrate strength is improved by a solid-solution hardening. These elements are therefore effective in permitting the thickness of the titanium substrate to be made smaller. Although these elements can improve strength, they also impair workability, and have an adverse effect on yield in manufacture of blanks before the mirror finish step. The contents of O, N and C added are therefore limited to a specific range. If O, N and C are contained in the matrix independently, the desired strength is not obtained if their contents are less than 0.03 wt%, 0.015 wt% and 0.04 wt%, respectively. It is preferable that their contents are respectively O ≧ 0.03 wt%, N ≧ 0.015 wt% and C ≧ 0.04 wt%. If the contents of these elements are excessive, workability declines. More specifically, if O, N and C exceed 0.5 wt%, 0.25 wt%, and 0.67 wt%, respectively, cold workability is degraded, and it is preferable that O ≦ 0.5 wt%, N ≦ 0.25 wt% and C ≦ 0.67 wt%. As all these elements exhibit similar actions, it is necessary to specify their total content. Further, their composition ratio is given as O : N : C = 1 : 2 : 0.75. The total content is therefore specified by the condition that 0.03 wt% ≦ O + 2N + 0.75C ≦ 0.5 wt%. The substrate material is a polycrystalline material, and the crystals on its surface have various orientations. In the mirror finish step, the speed of precision polishing and precision grinding is different depending on the crystal orientation, and differences in level occur among the crystals. The finer the grain size, the smaller the difference in the orientations of neighboring grains and the smaller the differences in level. Conventionally, in a cold-rolled sheet of an α type titanium alloy, the grain size was about 50 µm on average, but better substrate surface properties can be obtained by making the average no greater than 30 µm. More specifically, if the average grain size is greater than 30 µm, the difference in level assumes a relatively large value of 0.04 µm to 0.08 µm, but by making the grain size no greater than 30 µm, the difference in level can be made less than 0.04 µm and satisfactory surface properties can be obtained. It is therefore preferable that the average grain size is not greater than 30 µm. As described above, the grain size can be made fine by properly adding Mo. To make the average grain size smaller and no greater than 30 µm, the manufacturing conditions must be properly specified. The titanium magnetic disk substrate can be generally obtained by the following manner. An ingot having predetermined components and a predetermined composition is break down-forged, thermal-rolled and then cold-rolled, and the resultant cold-rolled sheet is punched out in a ring shape to form a substrate material. This substrate material is thermal-flattened, and its surface is polished to form a mirror surface. The grain size of the substrate depends on the draft in cold rolling and the thermal-flattening temperature and time in the above steps. Therefore, in order to obtain a substrate having an average grain size of no greater than 30 µm, the draft in cold rolling and the thermal-flattening temperature and time must be specified within predetermined ranges. According to the present invention, the titanium alloy plate having the composition within the above composition range is cold-rolled and distorted at a rolling ratio of 30% or more to control the structure during thermal-flattening. The disk substrate material formed by punching out the cold-rolled sheet is thermal-flattened in the following conditions to cause recrystalization and is flattened under the condition satisfying the following inequalities: 500 ≦ T ≦ -(150/11)·t + 7850/11 and 1 ≦ t ≦ 15, where a thermal-flattening temperature is represented by T°C and a thermal-flattening time is represented by t hours. Thus, the average grain size can be no greater than 30 µm without deteriorating flatness required as a magnetic disk substrate. Note that the substrate is preferably thermal-flattened while being clamped between two bases each having high flatness (the flatness of no greater than 100 µm). When thermal-flattening is performed at a temperature of less than 500°C for less than 1 hour, distortion of the material cannot be effectively removed and is left after thermal flattening, thereby failing to obtain excellent flatness. Although the flatness of the magnetic disk substrate becomes excellent by increasing the thermal-flattening temperature and prolonging the thermal-flattening time (the flatness is close to the flatnesses of the bases when thermal-flattening is performed by clamping the substrate between the bases), the grains are enlarged when the increases in temperature and time are exceeded. That is, even if the conditions of 500 ≦ T and 1 ≦ t ≦ 15 are satisfied, when thermal-flattening is performed using temperature and time which do not satisfy the condition of T ≦ -(150/11)·t + 7850/11, the average grain size exceeds 30 µm. At a rolling ratio of less than 30%, it is difficult to leave distortion enough to maintain structural control during thermal-flattening in the cold-rolled sheet. If a rare-earth element such as Y is added to the substrate material in a small amount to improve machinability, the effect of the present invention is not lost. As described above, Mo has an effect for making grains fine. Generally, β-stablizing elements have such an effect of Mo, such as the elements Ni, Cr, Co, or Fe. Examples The present invention will be described below in detail by way of its examples. Ingots (composition numbers 1 to 9) having the compositions shown in Table 1 were manufactured by VAR melting, and were break down-forged at 1,000°C to form slabs of thickness of 20 mm. Final hot rolling was performed on the slabs at 870°C so as to obtain a hot-rolled sheet of thickness of 6 mm. The oxide layer on this thermal-rolled sheet was removed, the sheet was ground-cut into a plate thickness of 5 mm, and cold rolling was then performed on the plate so as to obtain a cold-rolled plate of thickness of 0.6 mm. The strength and cracks in the cold-rolled plate was estimated. This cold-rolled plate was punched out to form ring plates (the outer diameter of 95 mm, the inner diameter of 25 mm, and the thickness of 0.6 mm), and the ring plates were thermal-flattening at a temperature of 600°C for 6 hours. Then, the surfaces of the disks were ground by # 400, # 800, # 1,500, and # 4,000 grindstones (# is JIS (Japanese Industrial Standard) mesh number) in that order finally given an alumina finishing polish so as to produce magnetic disk substrates. 50 magnetic disk substrates were prepared from each of the above compositions. Pits, grain sizes, and surface properties (Ra) of these magnetic disk substrates were estimated. Table 1 shows the estimated results with the compositions of the substrates. Note that, in Table 1, the composition numbers 1 to 5 were within the composition range of the present invention, and the composition numbers 6 to 9 were comparative examples which fell outside the range of the present invention. The terms "cracks" refers to the presence or absence of cracks of 1 mm or more (the absence is indicated by symbol o and the presence is indicated by symbol ×) produced in the process of rolling. The term "number of substrates with pitting" refers to the number of disks in which pits were observed when the 50 disks were observed by a differential interference microscope at a magnification of 100x, 60 fields, respectively. In the column concerning surface properties (Ra), a roughness curve was calculated by eliminating low frequency components from the outline curve of the sectional surface obtained when the object to be measured was cut along a flat surface perpendicular to the surface to be measured. The length L of this curve in the direction of the surface was extracted, and the roughness curve was represented by y = f(x) with the center line of the extracted part on the x axis, and the direction of vertical magnification on the y axis. The value of Ra was then given in µm units by the following expression (1): As shown in Table 1, the disks represented by the composition numbers 1 to 5 within the range of the present invention had sufficient strength, no cracks and no pits. The disks had excellent surface properties of Ra = 0.02 µm. The disk of the composition number 6 having a small amount of Mo had a large average grain size and a poor surface property of Ra = 0.06 µm. In the disk of the composition number 7 having a large amount of Mo, pits were produced. In addition, the disk of the composition number 8 having a small content of O + 2N + 0.75C had small yield strength. In the disk of the composition number 9 having a large content of O + 2N + 0.75C, cracks were produced. That is, it was assured that if the composition of the magnetic disk substrate was within the range of the present invention, a magnetic disk substrate with excellent properties could be obtained. The substrate of the composition number 5 having a composition within the range of the present invention was examined by changes in conditions of the thermal-flattening to obtain grain sizes, surface properties, and flatness in each of the conditions. The results are shown in Table 2, and Figs. 1 and 2. Table 2 shows grain sizes and surface properties in each thermal-flattening condition. Fig. 1 shows the relationship between the thermal-flattening condition and the flatness, and Fig. 2 shows the relationship between the thermal-flattening condition and the average grain size. Referring to Fig. 1, symbol o indicates disks each having average flatness of less than 100 µm, and symbol  represents disks each having average flatness of 100 µm or more. Referring to Fig. 2, symbol o represents disks each having an average grain size of 30 µm or less, and symbol  represents disks having an average grain size of more than 30 µm. Note that the above flatness indicates a maximum displacement value at a length of 100 mm, and the flatness of a practical magnetic disk substrate is required to be less than 100 µm. As can be apparent from Table 2, the following results can be confirmed. When the average grain size was less than 30 µm, all the surface properties Ra were desirably 0.02 µm or less. When the average grain size exceeds 30 µm, Ra was more than 0.02 µm. As is apparent from Fig. 1, the following result could be confirmed. When a thermal-flattening temperature was less than 500°C or a thermal-flattening time was less than 1 hour, the average flatness was 100 µm or more which was not a sufficient value. In addition, as shown in Fig. 2, the average grain size exceeded 30 µm in an area where the thermal-flattening temperature was high and the thermal-flattening time was long. A boundary line of the average grain size of 30 µm was a line indicates by the alternate long and short dashed line in Fig. 2. Assuming that a flattening temperature was represented by T (°C) and the time was represented by t (hour), the boundary line was expressed by the following equation: T = -(150/11)·t + 7850/11 Therefore, in the area of T ≦ -(150/11)·t + 7850/11, the average grain size of less than 30 µm could be obtained. In an area where the condition was not satisfied, the average grain size was increased. In the area wherein the average grain size was more than 30 µm, since Ra was more than 0.02 µm as described above, it was assured that an increase in roughness of the grain size was a cause for degradation of the surface properties. An influence of a cold-rolling condition in the surface properties of the substrate will be described below. The ingot of the composition number 5 was break down-forged, hot-rolled and grind under the same condition as described above to be formed into a sheet having a thickness of 5 mm, and the sheet was cold-rolled in the condition illustrated in Table 3. Ring plates (outer diameter of 95 mm and inner diameter of 25 mm) were formed by punching out the cold-rolled sheet to form disk substrate materials. The materials were thermal-flattened at a temperature of 600°C for 6 hours and polished under the same conditions as described above to obtain 5 magnetic disk substrate samples for each of cold rolling conditions. In the samples formed in the above described manner, their grain sizes and surface properties (Ra) were estimated. Table 3 shows the cold rolling conditions and properties of the samples. As described in Table 3, in the cold rolling ratio of 30% or more, it was confirmed to obtain an average grain size of 30 µm or less and excellent surface properties. As described above, it was confirmed that when the substrate composition was within the range of the present invention, a magnetic disk substrate having excellent surface properties could be obtained, and that when the average grain size was controlled to be 30 µm or less, more excellent surface properties could be obtained. In addition, an alloy plate having a composition within the range of the present invention can be cold-rolled at 30% or more and thermal-flattened under a condition which satisfies 500 ≦ T ≦ -(150/11)·t + (7850/11) and 1 ≦ t ≦ 15, where a thermal-flattened temperature is T°C and a thermal-flattening time is t hour, thereby controlling the average grain size to be 30 µm or less. Therefore, more excellent surface properties can be obtained. Note that a magnetic disk substrate of the present invention can also be employed to a high-density recording medium such as an Fe oxide thin film or a barium ferrite thin film requiring a high substrate temperature.

Claims: A magnetic disk substrate comprising 0.5 wt% to 1.0 wt% of Mo, 0.03 wt% to 0.5 wt% of O + 2N + 0.75C, and a balance of Ti. A magnetic substrate according to claim 1, characterized in that an average grain size is no more than 30 µm. A method of manufacturing a magnetic disk substrate comprising the steps of forming a magnetic disk substrate material by cold-rolling an alloy plate at a rolling ratio of no less than 30%, the alloy plate comprising 0.5 wt% to 1.0 wt% of Mo, 0.03 wt% to 0.5 wt% of O + 2N + 0.75C, and a balance of Ti, and thermal-flattening the magnetic substrate material under a condition defined below: 500 ≦ T ≦ -(150/11)·t + 7,850/11 1 ≦ t ≦ 15 where T represents a thermal-flattening temperature (°C), and t represents a thermal-flattening time (hour).

Inventors: FUKAI HIDEAKI C O PATENT LICEN, MINAKAWA KUNINORI C O PATENT L, SUENAGA HIROYOSHI C O PATENT L, FUKAI, HIDEAKI, C/O PATENT & LICENSE DEPT., MINAKAWA, KUNINORI, C/O PATENT & LICENSE DEPT., SUENAGA, HIROYOSHI, C/O PATENT & LICENSE DEPT., Fukai, Hideaki, c/o Patent &amp; License Dept., Minakawa, Kuninori, c/o Patent &amp; License Dept., Suenaga, Hiroyoshi, c/o Patent &amp; License Dept.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1990
Main classification: G11B5
IPC codes: G11B5