Patent Publication Number: US-6215617-B1

Title: Support member for magnetic disk substrate

Description:
This is a division of application Ser. No. 08/609,136 filed Feb. 29, 1996, which application is hereby incorporated by reference in its entirety now U.S. Pat. No. 5,969,902. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a magnetic disk unit used as an external storage of computers and a support member for magnetic disk substrate used for this. 
     2. Description of the Prior Art 
     Hitherto, as shown in FIG. 5, the magnetic disk unit is fixed by inserting multiple pieces of magnetic disk substrate  12  and spacer  20  alternately into a hub  11  fixed to a rotary shaft  10 , and lastly pressing with a shim  30  and a clamp  40  and tightening with a screw  13 . And by allowing the magnetic head  14  to move on the surface of the magnetic disk substrate  12  free from contact while the magnetic disk substrate  12  is being rotated by rotation of the above rotary shaft  10 , the information is designed to be written to or read from the specified position of the magnetic disk substrate  12 . 
     In recent years, these magnetic disk units  50  are required to achieve an extremely small levitation rate for the distance between the magnetic head  14  and the magnetic disk substrate  12  as the densification of information increases, and for this purpose, the magnetic disk substrate  12  was formed with ceramics or glass which is highly rigid and is difficult to generate deformation, and the support member such as spacers  20  for fixing the magnetic disk substrate  12 , shims  30 , and clamps  40  were formed with ceramics or glass same as those in the case of magnetic disk substrate  12  in order to prevent deformation of the magnetic disk  12  caused by a thermal expansion difference and to reduce weight (Japanese Patent Publication No. Hei 5-80745 and Japanese Paten Application Laid Open No. Sho 61-148667). 
     For example, the spacer  20  for holding the magnetic disk substrate  12  to given intervals is made of alumina ceramics as shown in FIG.  8  and formed into a ring shape  21 . 
     On the other hand, in Japanese Patent Laid Open No. Sho 51-118408, support members such as spacers and shims are disclosed, which are designed to have a plurality of air grooves cut radially on the contact surface with the magnetic disk substrate  12  and form an air layer in the air groove in order to relieve stresses applied to the magnetic disk substrate  12  during clamping. 
     However, because ceramics or glass which comprises support members such as the spacer  20 , shim  30 , clamp  40 , etc. are fragile, there was a fear of generating chipping at the edge due to the stress at the time of clamping. This broken piece may damage the magnetic disk substrate  12  or break the magnetic head  14 , if it enters the clearance between the magnetic disk substrate  12  and the magnetic head  14  levitating on it. 
     On the other hand, with the support member designed to have air grooves cut on the contact surface and form air grooves in them, the greater the ratio of the air grooves accounting for the contact surface, the smaller is the contact surface with the magnetic disk substrate  12 , possibly resulting in strain in the magnetic disk substrate  12 . Consequently, it was unable to decrease the flatness of the magnetic disk substrate  12  at the time of clamping and it was unable to minimize the levitation rate of the magnetic head  14 . In particular, when the magnetic disk substrate  12  deforms in the shape of a letter V, the magnetic head  14  collides against the magnetic disk substrate  12  to result in breakage. 
     In recent years, it has been known that static electricity is charged in the magnetic disk substrate  12  when the information is read or written and noise is generated to destroy the recording contents, but charging of the magnetic disk substrate  12  was unable to be prevented because ceramics and glass are, in general, insulating materials. 
     Under these circumstances, in this invention, the support members such as shims, clamps, and spacers are formed with ceramics or glass and at the same time the ratio of the actual contact surface is designed to be 50-95% as well as the flatness of this contact surface is made to be 3 μm or less. 
     It is an object of this invention to provide a magnetic disk unit comprising one or a plurality of magnetic disk substrate arranged on a nearly cylindrical hub to be fixed to a rotating shaft via ring-form spacers and shims and held by clamps, the said magnetic disk substrate being made from ceramics or glass and held by spacers and shims whose actual contact area ratio with the magnetic disk substrate is designed to be 50-95%, wherein the flatness of the contact surface is designed to be 3 μm or less. 
     It is another object of this invention to provide a magnetic disk unit in which a through hole perpendicular to the contact surface of support member such as shims or clamps is drilled to dispose springs in the through hole or to fill the through hole with conductive materials or cover the surface with conductive materials in order to achieve conductivity between the top and the bottom of the contact surfaces and prevent charging of the magnetic disk. 
     This invention relates to a support member for magnetic disk substrate for holding the magnetic disk substrate to a specified position and a magnetic disk unit comprising one or a plurality magnetic disk substrate held to a hub by means of this. 
     Hitherto, as shown in FIG. 12, the magnetic disk unit was fixed by alternately inserting a plurality of magnetic disk substrate  115  and spacer  111  into a hub  114  fixed to a rotary shaft  113  and lastly pressing them with shims  110  and clamps  112  and tightening with screws  116 . And with the magnetic disk substrate  115  being rotated by the rotation of the rotary shaft  113 , the magnetic head  117  was allowed to move on the surface of the magnetic disk substrate  115  free from contact so that the information is written or read at a specified position of the magnetic disk substrate  115 . 
     In recent years, as the information density and the storage capacity increase, the magnetic disk unit  150  is required to further minimize the distance between the magnetic head  117  and the magnetic disk  115 , increase flatness of the magnetic disk substrate  115 , and improve smoothness of its surface, and, therefore, there is disclosed a magnetic disk substrate  115  using ceramics or glass, in which increased surface strength and improved surface smoothness are highly effectively achieved, and the support members such as spacers  111 , shims  110 , and clamps  112  for fixing and supporting the magnetic disk substrate  115  are formed with ceramics or glass for preventing deformation of the magnetic disk substrate  115  caused by thermal expansion difference as well as for reducing weight (Japanese Patent Publication No. Hei 5-80745, Japanese Non-examined Patent Publication No. Sho 61-148667). 
     However, because ceramics or glass composing the support member are, in general, insulating materials, it has recently been known that supporting the magnetic disk  115  with these support members charges the magnetic disk substrate  115 , generates noises in writing or reading the information, and possibly destroys the recording contents, and therefore, there is disclosed a method for preventing charging of the magnetic disk  115  by using the support member with the contact surface with the magnetic disk substrate  115  coated with metallic film such as aluminum, zinc, etc. (Japanese Non-examined Patent Publication No. Hei 2-226566). 
     However, because the support member with the contact surface coated with metallic film causes a large thermal expansion difference between the ceramics or glass constituting the substrate and the metallic film, there is a problem in which the flatness of the contact surface is impaired by the heat resulting from high-speed rotation. In addition, there is a problem of burr specific to metal. Consequently, because forming a magnetic disk unit  150  using this support member causes strain in the magnetic disk substrate  115  and impairs the parallelism between magnetic disk substrates  115 , it is unable to reduce the levitation rate of the magnetic head  117 , possibly causing the magnetic head  117  to come in contact with the magnetic disk substrate  115  and breaking the magnetic head  117 . 
     In addition, there is a fear of peeling of the metallic film due to the thermal expansion difference with the ceramics or glass forming the substrate, and as a result, there is a problem in which static electricity charged on the magnetic disk substrate  115  is unable to be released. 
     Furthermore, there is a problem in which when the inside and outside edges of the support member are sharp edges, the metallic film thickness coated to the edges become thinner, causing disconnection according to circumstances , and static electricity is unable to be released. 
     In addition, there is another problem in which since ceramics or glass forming the support member are fragile, sharp edges may cause chipping due to stress at the time of clamping and this broken piece entering the clearance between the magnetic disk substrate  115  and the magnetic head  117  levitating above the substrate may damage the magnetic disk substrate  115  or break the magnetic head  117 . 
     Consequently, in the magnetic disk unit  150  in which the magnetic disk substrate  115  is held with these support members, it is difficult to achieve sufficiently high densification and high capacity of the information and static electricity charged in the magnetic disk substrate  115  may generate noise in reading or writing the information and may destroy the recording contents. 
     In view of the foregoing problems, it is a main object of this invention to provide a magnetic disk support members made of ceramics or glass, such as shims, clamps, and spacers, comprising the inside and outside edges of the support members provided with 0.004-0.5-mm-wide taper or curvature and at least the contact surface with the magnetic disk substrate and the inner circumferential surface coated with conductive ceramic film 0.1-3 μm thick. 
     It is an object of this invention to provide a magnetic disk unit by holding one or a plurality of magnetic disk substrate comprising ceramics or glass to the hub fixed to a rotary shaft and comprising conductive material via a spacer and/or shim in a ring form made of ceramics or glass and with 0.04-0.5 mm taper or curvature provided on inner and outer edges and at least with the contact surface with the magnetic disk and the inner circumferential surfaces coated with 0.1-3 μm conductive hard film. 
     More specifically, this invention is coated with any one type of conductive hard films of TiC, TiN, ZrN, HfC, TaC, ZrC, WC, VC, NbC, TiB 2 , ITO (Indium Tin Oxide), and DLC (Diamond-like Carbon). 
     This invention relates to magnetic disk substrate support members such as spacers, shims, and clamps for holding the magnetic disk substrate to a specified position and a magnetic disk unit comprising glass magnetic disk substrates held by these support members. 
     Hitherto, as illustrated in FIG. 17, the magnetic disk unit was fixed by alternately inserting a plurality of magnetic disk substrate  215  and spacer  211  into a hub  214  fixed to a rotary shaft  213  and lastly pressing them with shims  210  and clamps  212  and tightening with screws  216 . And with the magnetic disk substrate  215  being rotated by the rotation of the rotary shaft  213 , the magnetic head  217  was allowed to move on the surface of the magnetic disk substrate  215  free from contact so that the information is written or read at a specified position of the magnetic disk substrate  215 . 
     In recent years, as the information density and the storage capacity increase, the magnetic disk unit  250  is required to further minimize the distance between the magnetic head  217  and the magnetic disk  215 , increase flatness of the magnetic disk substrate  215 , and improve smoothness of its surface, and, therefore, there is disclosed a magnetic disk substrate  215  using ceramics or glass, in which increased surface strength and improved surface smoothness are highly effectively achieved, and the support members such as spacers  211 , shims  210 , and clamps  212  for fixing and supporting the magnetic disk substrate  215  are formed with ceramics or glass for preventing deformation of the magnetic disk substrate  215  caused by thermal expansion difference as well as for reducing weight (Japanese Patent Publication No. Hei 5-80745, Japanese Non-examined Patent Publication No. Sho 61-148667). 
     However, because ceramics or glass composing the support member are, in general, insulating materials, it has recently been known that supporting the magnetic disk  215  with these support members charges the magnetic disk substrate  215 , generates noises in writing or reading the information, and possibly destroys the recording contents, and therefore, there is disclosed a method for preventing charging of the magnetic disk  215  by using the support member with the contact surface with the magnetic disk substrate  215  coated with metallic film such as aluminum, zinc, etc. (Japanese Non-examined Patent Publication No. Hei 2-226566). 
     However, because the support member with the contact surface coated with metallic film causes a large thermal expansion difference between the ceramics or glass constituting the substrate and the metallic film, there is a problem in which the flatness of the contact surface is impaired by the heat resulting from high-speed rotation. In addition, there is a problem of burr specific to metal. Consequently, because forming a magnetic disk unit  250  using this support member causes strain in the magnetic disk substrate  215  and impairs the parallelism between magnetic disk substrates  215 , it is unable to reduce the levitation rate of the magnetic head  217 , possibly causing the magnetic head  217  to come in contact with the magnetic disk substrate  215  and breaking the magnetic head  217 . 
     In addition, there is a fear of peeling of the metallic film due to the thermal expansion difference with the ceramics or glass forming the substrate, and as a result, there is a problem in which static electricity charged on the magnetic disk substrate  215  is unable to be released. 
     Furthermore, though the metallic film is coated in a thin film form to prevent damage to flatness of the contact surface, because a slight sliding occurs between the magnetic disk substrate  215  and the support member due to high-speed rotation, causing wear or peeling of the metallic film with small hardness, there is a problem that static electricity is unable to be released. 
     In addition, there is another problem in which there is about 2 to 5×10 −6 /°C. thermal expansion difference between the support member and the glass magnetic disk substrate even in the support member comprising conductive ceramics, generating strain in magnetic disk substrate  215  in a similar manner as in the case of the support member with the contact surface coated with metallic film or impairing the parallelism between magnetic disk substrates  215 . 
     In the magnetic disk unit  250  required for further increased density and larger capacity, there has not been obtained any unit  250  which satisfies the requirements for those using the above support members. 
     In view of the foregoing problems, in this invention, the magnetic disk substrate support members such as shims, spacers, and clamps are formed with conductive forsterite ceramics whose volume specific resistance is less than 10 7  Ω·cm. 
     This invention constructs a magnetic disk unit by inserting and fixing successively the above-mentioned support members and glass magnetic disk substrate into a hub comprising conductive materials. 
     This invention relates to a magnetic disk unit used as an external storage for computers and magnetic disk substrate support members used for the unit. 
     The magnetic disk unit used hitherto is designed to mount a plurality of magnetic disk substrates  315  and spacers  311  alternately to the hub  314  fixed to the rotary shaft  313 , as shown in FIG. 20, press shims  310  and clamps  312 , and tighten screws  316  for fixing. Rotating these magnetic disk substrates  315  by the rotation of the rotary shaft  313 , the magnetic head  317  moves on the surface of each magnetic disk substrate  315  without contact, and writes or reads the information at a specified position of each magnetic disk substrate  315 . 
     For the material of the magnetic disk substrate  315 , aluminum substrates or glass substrates are used, the surface of which magnetic film is formed. On the other hand, for the support materials such as shims  310 , spacer  311 , and clamps  312 , metallic materials such as aluminum or stainless steel are used. 
     In these magnetic disk units, in order to increase the recording density, the distance between the levitated magnetic head  317  and the magnetic disk substrate  315  should be minimized, and presently, this distance is required to achieve a levitation rate as small as 0.1 μm or less. Consequently, the present applicant has already proposed to use ceramic materials with high rigidity and small thermal expansion rate as a material for the magnetic disk substrate (for example, see Japanese Patent Publication No. Hei 3-64933, etc.). In addition to this, glass-coated ceramics, YAG, titanium, silicon, carbon, etc. have been proposed for the material of magnetic disk substrates  315 . 
     However, with conventional metal shims  310  and spacers  311 , and clamps  312 , the maximum flatness achieved for the surface in contact with the magnetic disk substrate  315  is at most 3 μm and at the same time because it is easy to deform when tightened due to low rigidity, bending is likely to occur in the magnetic disk substrate  315  at the time of tightening. Because when the magnetic disk substrate  315  bends, it is likely to collide against the magnetic head  317 , the levitation rate of the magnetic head  317  is unable to be reduced, giving rise to inconvenience that higher density recording is not possible. 
     When ceramic magnetic disk substrate  315  is used, if support members such as shims  310 , spacers  311 , and clamps  312  are made of aluminum or other metals, strain is generated in the magnetic disk substrate  315  or tightening becomes loose when high temperature occurs due to high-speed rotation during application because the difference between thermal expansion ratios of both is large. 
     Therefore, in a preferred embodiment of this invention, the magnetic disk substrate support members such as shims, spacers, and clamps are formed with ceramics or glass with thermal expansion ratio lower than 12×10 −6 °C., and at the same time the flatness of the surface in contact with magnetic disk substrate is designed to be 3 μm or lower. And combining this support member and a plurality of magnetic disk substrates, a magnetic disk unit is composed. 
     The support members referred to in this invention include spacers used for holding a plurality of magnetic disk substrate to specified intervals, and shims and clamps for mounting this magnetic disk substrate to the rotary shaft. 
     This invention relates to the magnetic disk unit used for external storage of computers and the support member for magnetic disk substrate used for the unit. 
     The magnetic disk unit used hitherto is designed to mount a plurality of magnetic disk substrates  415  and spacers  411  alternately to the hub  414  fixed to the rotary shaft  413 , as shown in FIG. 25, press shims  410  and clamps  412 , and tighten screws  416  for fixing. Rotating these magnetic disk substrates  415  by the rotation of the rotary shaft  413 , the magnetic head  417  moves on the surface of each magnetic disk substrate  415  without contact, and writes or reads the information at a specified position of each magnetic disk substrate  415 . 
     For the material of the magnetic disk substrate  415 , aluminum substrates or glass substrates are used, the surface of which magnetic film is formed. On the other hand, for the support materials such as shims  410 , spacers  411 , and clamps  412 , metallic materials such as aluminum or stainless steel are used (see for example Japanese Patent Publication No. Sho 61-278023). 
     In these magnetic disk units, in order to increase the recording density, the distance between the levitated magnetic head  417  and the magnetic disk substrate  415  should be minimized, and presently, this distance is required to achieve a levitation rate as small as 0.1 μm or less. Consequently, the present applicant has already proposed to use ceramic materials with high rigidity and small thermal expansion rate as a material for the magnetic disk substrate (for example, see Japanese Patent Publication No. Hei 3-64933, etc.). In addition to this, glass-coated ceramics, YAG, titanium, silicon, carbon, etc. have been proposed for the material of magnetic disk substrates  415 . 
     However, with conventional metal shims  410  and spacers  411 , and clamps  412 , the maximum flatness achieved for the surface in contact with the magnetic disk substrate  415  is at most 5 μm and at the same time because it is easy to deform when tightened due to low rigidity, bending is likely to occur in the magnetic disk substrate  415  at the time of tightening. Because when the magnetic disk substrate  415  bends, it is likely to collide against the magnetic head  417 , the levitation rate of the magnetic head  417  is unable to be reduced, giving rise to inconvenience that higher density recording is not possible. 
     When ceramic magnetic disk substrate  415  is used, if support members such as shims  410 , spacers  411 , and clamps  412  are made of aluminum or other metals, strain is generated in the magnetic disk substrate  415  or tightening becomes loose when high temperature occurs due to high-speed rotation during application because the difference between thermal expansion ratios of both is large. 
     Therefore, a proposal has been made to form the support members such as shims,  410 , spacers  411 , clamps  412 , etc. with ceramics, but in this case, hardness of the support member is excessively high and the magnetic film formed on the surface of the magnetic disk substrate  415  at the time of high-speed rotation is scraped away by ceramics, causing metallic powder. 
     In this invention, magnetic disk base support members such as shims, spacers, clamps are formed with ceramics with thermal expansion ratio 20×10 −6 °C., and the surface in contact with the magnetic disk substrate is coated with film of 450 kg/mm 2  or less, and the flatness of this contact surface is 5 μm or less. And combining these support members and magnetic substrates, a magnetic disk unit is composed. 
     The support members referred to in this invention include spacers used for holding a plurality of magnetic disk substrate to specified intervals, and shims and clamps for mounting this magnetic disk substrate to the rotary shaft. 
     According to this invention, because support members are formed with ceramics, a large rigidity can be achieved and the flatness of the contact surface with the magnetic disk can be improved, it is possible to maintain the magnetic disk to a high accuracy. Because the film with small hardness is provided on the support member surface, scraping of magnetic film on the magnetic disk surface can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing one example of support member related to this invention and (a) is a spacer and (b) a clamp; 
     FIG. 2 ( a ) to ( e ) are perspective views showing other embodiments of the spacer related to this invention; 
     FIG. 3 is a schematic sectional view showing the structure of the peripheral area of a support member related to this invention formed by uniaxial press former; 
     FIG. 4 is a longitudinal sectional view of spacer shown in FIG. 1 ( a ); 
     FIG. 5 is a longitudinal sectional view of magnetic disk unit related to this invention; 
     FIG. 6 is a schematic view showing interference fringes observed in magnetic disk substrates when the magnetic disk substrate is held with a spacer with varying actual contact area ratios; 
     FIG. 7 is a schematic illustration showing the strain rate of the magnetic disk substrate in the magnetic disk unit related to this invention; 
     FIG. 8 is a perspective view showing a spacer, one example of conventional support members; 
     FIG. 9 is a schematic illustration showing a spacer, one example of support members related to this invention, and (a) is a perspective view and (b) a sectional view; 
     FIG. 10 is a schematic illustration showing a shim, one example of support members related to this invention, and (a) is a perspective view and (b) a sectional view; 
     FIG. 11 is a schematic illustration showing a clamp, one example of support members related to this invention, and (a) is a perspective view and (b) a sectional view; 
     FIG. 12 is a longitudinal sectional view of a magnetic disk unit related to this invention; 
     FIG. 13 is an enlarged view showing the main section A of FIG. 9; 
     FIG. 14 is a schematic illustration showing a spacer, one example of support members related to this invention, and (a) is a perspective view and (b) a sectional view; 
     FIG. 15 is a schematic illustration showing a shim, one example of support members related to this invention, and (a) is a perspective view and (b) a sectional view; 
     FIG. 16 is a schematic illustration showing a clamp, one example of support members related to this invention, and (a) is a perspective view and (b) a sectional view; 
     FIG. 17 is a longitudinal sectional view of a magnetic disk unit related to this invention; 
     FIG. 18 is a schematic illustration showing a spacer, one example of support members for magnetic disk substrate related to this invention, and (a) is a perspective view and (b) a section taken on line X—X in (a); 
     FIG. 19 is a schematic illustration showing a clamp, one example of support members for magnetic disk substrate related to this invention, and (a) is a perspective view and (b) a section taken on line Y—Y in (a); 
     FIG. 20 is a sectional view of a magnetic disk unit related to this invention; 
     FIG. 21 is an enlarged sectional view comparing the surface condition of ceramic material to metal material; 
     FIG. 22 is a schematic illustration showing a bend of the magnetic disk substrate in the magnetic disk unit according to the invention; 
     FIG. 23 is a schematic illustration showing a spacer, one example of support members for magnetic disk substrate related to this invention, and (a) is a perspective view and (b) a section taken on line X—X in (a); 
     FIG. 24 is a schematic illustration showing a clamp, one example of support members for magnetic disk substrate related to this invention, and (a) is a perspective view and (b) a section taken on line Y—Y in (a); and 
     FIG. 25 is a sectional view of a magnetic disk unit according to this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now description will be made of embodiments according to this invention. 
     FIG. 1 ( a ) and ( b ) are perspective views showing magnetic disk substrate support members related to this invention, and (a) shows a spacer and (b) a clamp. 
     First of all, as shown in FIG. 1 (a), the spacer  20  is a ring  21  made of ceramics or glass, and to the inner and outer edges  23   a,    23   b,  the as-sintered surface C is formed for the portion to be chamfered to prevent chipping. 
     The contact surface  22  is finished to surface roughness of 0.2-2.0 μm in terms of center line average roughness (Ra) to prevent turning of the fixed magnetic disk substrate, and at the same time the flatness of the contact surface  22  is brought to 3 μm or less to prevent generation of strain in the magnetic disk substrate when clamped, and furthermore, the parallelism of top and bottom contact surfaces  22  is kept 5 μm in order to allow the magnetic disk substrate to provide a specified clearance. 
     The shim  30  is not illustrated, but is of the same profile as that of the spacer but is slightly thinner. 
     Next, as shown in FIG. 1 ( b ), the clamp  40  is a disk or plate type product  41  comprising either ceramics or glass. The contact surface  42  is finished to surface roughness of 0.2-2.0 μm in terms of center line average roughness (Ra) to prevent turning of the fixed magnetic disk substrate, and at the same time the flatness of the contact surface  42  is brought to 3 μm or less. At the center of the contact surface  42 , an indent portion  44  is provided for latching to the hub tip end section, and to the edge  43   a  of the indent section  44  and to the outer circumferential edge  43   b  of the plate-form product  41 , the as-sintered surface C is formed for the intended section  44 . 
     For other examples of support members such as spacers  20 , shims  30 , and clamps  40 , those with through-holes or grooves may be acceptable. 
     For example, the spacer  20  shown in FIG. 2 ( a ) has four through holes  22   a  drilled on the contact surface, and designing the spacer in this kind of construction is able to reduce the weight of the spacer  20 . And supporting the magnetic disk substrate  12  using this spacer  20  can greatly reduce the weight applied to the rotary shaft  10 , thereby allowing the magnetic disk substrate  12  to rotate at a high speed with a small torque. The through holes  22   a  drilled on the contact surface  22  may be acceptable if two or more holes are drilled at equal intervals. 
     The spacer  20  shown in FIG. 2 ( b ) has four through holes  22   a  drilled on the contact surface  22  as in the case of spacer  20  shown in FIG. 2 ( a ), and in addition, to the through hole  22   a,  a spring  25  comprising a conductive material such as metal is placed with both ends slightly protruding from each contact surface  22 . Consequently, if this spacer  20  is used to hold the magnetic disk substrate  12 , both ends of the spring  25  are able to house in the through-hole  22   a  because of elastic action, thereby preventing deformation of the magnetic disk substrate  12 , and furthermore, because both ends of the spring  25  come in contact with the magnetic disk substrate  12  arranged on top and bottom contact surfaces  22  of the spacer  20 , static electricity charged to the magnetic disk substrate  12  is allowed to escape, thereby preventing the recording contents from being destroyed. 
     The spring  25  may be designed to be inserted into the through hole  22   a  of the spacer when assembling the magnetic disk unit  50 , and for increased assembly operation efficiency, the spring  25  may be placed in the through hole  22   a  as well as affixed with adhesives, etc. 
     The spacer  20  shown in FIG. 2 ( c ) has conductive material  26  filled in the through hole  22   a  in place of the spring  25  so that continuity is achieved between the top and the bottom contact surfaces  22 . The whole surface of the conductive material  26  is not necessarily located on the same plane of the contact surface  22 , and part may be located on the same plane as that of each contact surface. For the conductive material  26 , aluminum, zinc, carbon and other metal or conductive resin may be used. 
     The spacer  20  shown in FIG. 2 ( d ) has the inner wall surface of the through hole  22   a  covered with conductive film  27  so that weight is reduced and continuity between the top and the bottom contact surfaces  22  can be achieved. The edge of the through hole  22   a  and others are chamfered to surface C and this is also covered with the conductive film  27 . 
     Consequently, the use of the spacer  20  shown in FIG. 2 ( c ) and ( d ) can also release static electricity charged in the magnetic disk. 
     The spacer  20  shown in FIG. 2 ( e ) has a plurality of air groove  22   b  radially formed, respectively, on the top and the bottom contact surfaces  22  and the adoption of this construction can not only reduce weight of the spacer  20  but also form an air layer at the air groove  22   b  to relieve stress applied to the magnetic disk substrate  12  at the time of clamping, thereby alleviating strain generated in the magnetic disk substrate  12 . 
     Now, the magnetic disk unit  50  with the magnetic disk substrate  12  supported by these spacers  20 , shims  30 , and clamps  40  is shown in FIG.  5 . 
     To the rotary shaft  10 , a metal hub  11  in a nearly cylindrical profile  14  equipped with a flange section  11   a  is fixed, and to the flange section  11   a  of the hub  11 , a multiplicity of magnetic disk substrate  12  and spacer  20  are inserted alternately, and at least are retained with shims  30  and clamps  40 , and with this configuration, the magnetic disk substrate  12  is placed and held between by the flange section  11   a  of the hub  11  and the clamp  40  and at the same time the magnetic disk substrate  12  is supported at specified intervals with the spacer  20  (shim  30 ), and in addition, by tightening the clamp  40  with the screw  13  to the hub  11 , the magnetic disk substrate  12  is designed to be fixed. While the magnetic disk substrate  12  is being rotated at a high speed via the rotary shaft  10 , using the magnetic head  14  levitated slightly away from the surface of the magnetic disk substrate  12 , information is written and read. 
     The magnetic disk substrate  12  equipped into the magnetic disk unit  50  is formed with ceramics such as alumina or glass, which is lightweight, highly rigid, and difficult to deform, to meet recent increased requirements for higher densification of information, and in the ceramic substrate, a glazed surface is formed on the surface and by forming the magnetic film on the glazed surface, the magnetic disk substrate  12  is able to be obtained, and in the glass substrate, by forming the magnetic film on the surface, the magnetic disk substrate  12  is able to be obtained. In addition, it is possible to use titanium, silicon, YAG, carbon, etc. for other substrate materials. 
     In particular, because it is possible to achieve the thermal expansion coefficient same as or approximate to that of the magnetic disk substrate  12  if the magnetic disk substrate  12  is supported with the support member according to this invention, the strain of the magnetic disk substrate  12  involved in the thermal expansion difference is canceled and the levitation rate of the magnetic head  14  can be best minimized, and the information recording density can be improved. 
     In the magnetic disk unit  50  shown in FIG. 5, the magnetic disk substrate  12  is supported by shims  30  intervening between the magnetic disk substrate  12  on the uppermost part and the clamp  40  but in addition to this, it may be designed to be supported by directly bringing the clamp  40  in contact with the magnetic disk substrate  12 , and in this case, the use of the clamp  40  shown in FIG. 1 ( b ) can achieve the accurate support of the magnetic disk substrate  12 . It may also be designed to remove the spacer  20  arranged between the flange section  11   a  of the hub  11  and the magnetic disk substrate  12  and support the magnetic disk substrate by bringing them in direct contact. Furthermore, in order to eliminate the thermal expansion difference with the magnetic disk substrate  12 , it is desirable to form the hub  11  with ceramics or glass. 
     Now, for the materials composing the support member such as spacers  20 , shims  30 , and clamps  40 , it is possible to use ceramics or glass shown in Table 1 whose thermal expansion coefficient is 20×10 −6 /°C., and more preferably 12×10 −6 /°C. or less. 
     Depending on the material of the magnetic disk substrate  12 , the material with approximate thermal expansion coefficient may be selected and applied from the materials of the support member. For example, when ceramic magnetic substrate  12  is used, ceramics with thermal expansion coefficient equivalent to or less than 10×10 −6 /°C. in Table 1 may be used as the support member, and similarly, when glass magnetic disk substrate  12  (thermal expansion coefficient: 8.0 to 9×10 −6 /°C.) is used, ceramics or glass such as forsterite, etc. whose thermal expansion coefficient is 8.0×10 −6 /°C. or higher in Table 1 is suitably used as support member. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Thermal expansion 
               
               
                 Material 
                 coefficient (× 10 −6 /° C.) 
               
               
                   
               
             
            
               
                 Alumina 
                 6.5-7.3 
               
               
                 Zirconia 
                 9.5-11.5 
               
               
                 Silicon carbide 
                 3.5-4.5 
               
               
                 Silicon nitride 
                 2.5-3.5 
               
               
                 Cermet 
                 7.0-8.0 
               
               
                 Forsterite 
                 8.0-12.0 
               
               
                 SiO 2  based glass 
                 8.5 
               
               
                 PbO—ZnO—B 2 O 3  based glass 
                 7.0-8.0 
               
               
                 PbO—B 2 O 3  based glass 
                 7.0-9.0 
               
               
                 Na 2 O—BaO—SiO 2  based glass 
                 8.5-11.0 
               
               
                 Na 2 O—Al 2 O 2 —B 2 O 3 —SiO 2  based glass 
                 7.0-10.0 
               
               
                 K 2 O—PbO—SiO 2  based glass 
                 8.0-9.5 
               
               
                   
               
            
           
         
       
     
     Now, the edges  23 ,  43  of the support member such as spacers  20 , shims  30 , and clamps  40  are chamfered to C surface, R surface, or tapered surface with greater taper angle than C surface to prevent chipping by stress at the time of clamping, but this invention is characterized by keeping this chamfered portion as-sintered surface. That is, if it is intended to chamfer the edge portions  23 ,  43  of the sintered support member, it takes extremely long time to process one support member and microcracks are generated in the polished chamfered surface, possibly generating chipping due to the stress at the time of clamping, but if the chamfered portion is kept as sintered, it possesses the original strength which the sintered products provides, and chipping due to stress at the time of clamping may be prevented. 
     The as-sintered chamfered surface referred to herein is defined as a surface which is sintered after the chamfered portion of a specified profile is formed beforehand by general forming process such as uniaxial press forming, isostatic press forming, or cast forming, and after sintering, no polishing nor grinding is provided for the chamfered portion. 
     For example, when the surface C is formed by a uniaxial press former such as a mechanical press, as shown in FIG. 3, the chamfered portion comprising the tapered surface T and the plane H is integrally formed, and then sintered, and the outer circumferential surface is polished to a specified size. In this event, the edge portions  23 ,  43  of the support member have the plane H removed and the surface C portion left. And this surface C becomes the as-sintered chamfered portion referred to herein. In the above example, a chamfered portion with the plane H removed is shown but it may have the plane  2  slightly left or furthermore, the support member may have a chamfered portion comprising both tapered surface T and plane H with the whole plane H completely left. Even this kind of support member will not generate chipping at the chamfered portion. 
     On the other hand, as the volume of the chamfered portion formed in the support member increases and the contact area of the contact surfaces  22 ,  42  in contact with the magnetic disk substrate  12  decreases, a large strain may be generated on the fixed magnetic disk substrate  12 . Even in the ratio of through holes  22   a  or air grooves  22   b  formed on the contact surface  22 ,  42  increases, there is no fear of generating strain in magnetic disk substrate  12 . 
     Consequently, in this invention, the ratio of actual contact area of the support member is set in the range of 50-95%, more preferably 70-95%. 
     The ratio of actual contact area referred to herein is the ratio of the contact surface  22 ,  42  after the chamfered portion, through hole  22   a,  and air groove  22   b  are formed on the edge portions  23 ,  43  to the overall area of the contact surface  22 ,  42  before the chamfered portion, through hole  22   a,  or air groove  22   b  are formed on the edge portions  23 ,  43  of the support member, and for example, when each size of the space  20  shown in FIG.  1 ( a ) are set as shown in FIG. 4, it is the value computed by the following equation. 
     Equation: 
     
       
         Ratio of actual contact area (%)=( R   2   −S   2 )/( P   2   −Q   2 )×100 
       
     
     If the ratio of actual contact area is less than 50%, the contact area with the magnetic disk substrate  12  is excessively so small that the magnetic disk substrate  12  deforms at the time of clamping and the levitation rate of the magnetic head  14  is unable to be minimized, and if the condition aggravates, the disk substrate  12  deforms in a letter V and the generation of this deformation causes the magnetic head  14  to come in contact with the substrate and break. 
     The higher the ratio of actual contact area, the higher is the flatness of the magnetic disk substrate  12 , but when it exceeds 95%, the chamfered rate becomes excessively small, and even if the chamfered portion is provided, chipping may result from the stress at the time of clamping. Consequently, it is important to set 95% for the upper limit of the ratio of actual contact area, and forming the chamfered portion on the edges  23 ,  43  in this range will not generate chipping due to stress at the time of clamping. 
     In the support member related to this invention, it is important to keep the flatness of the contact surfaces  22 ,  42  to 3 μm or less, preferably 1 μm or less, and more preferably 0.3 μm or less in order to prevent strain in the clamped magnetic disk substrate  12  as in the case of the ratio of actual contact surface. 
     In addition, it is important to achieve the moderately rough surface for the contact surfaces  22 ,  42 . 
     That is, if the surface roughness of the contact surfaces  22 ,  42  is less than 0.2 μm in terms of the center line average roughness (Ra), the contact surfaces  22 ,  42  are too soft to prevent slip of the magnetic disk substrate  12  rotating at high speed, and conversely, if it is greater than 2.0 μm in terms of the center line average roughness (Ra), large deformation is generated in the magnetic disk substrate  12 , and it is unable to bring the flatness to 2 μm or lower and at the same time, there is a possibility to damage the magnetic disk substrate  12 . 
     Consequently, it is recommended to provide surface roughness of 0.2 μm to 2.0 μm in terms of center line average roughness (Ra) to the contact surfaces  22 ,  42 . 
     In this way, because in this invention, the support member for holding the magnetic disk substrates  12  at given intervals is made of ceramics or glass, deformation of the magnetic disk substrate  12  arising from the thermal expansion difference can be prevented in a large scale, and because the ratio of the actual contact area is kept to 50-95% and at the same time the flatness is set to 3 μm or less, no V-letter deformation is generated in the magnetic disk substrate  12 , and even if any deformation is generated, the strain rate is minor and allows the substrate to smoothly deform, enabling stable writing and reading of the information. In addition, because the chamfered portion formed on the edges of the support member is an as-sintered surface, chipping generated by stress at the time of clamping can completely be prevented. 
     Consequently, if the magnetic disk substrate  12  is supported by the support member to form a magnetic disk unit, the flatness of the magnetic disk substrate  12  can be brought to 2 μm or less, and therefore, the levitation rate of the magnetic head can be kept to 0.1 μm or less, achieving high-density recording. 
     Now, the flatness and deformation degree of the magnetic disk substrate  12  when it is supported with these spacers  20  were measured with the optical interferometer while varying the ratio of the actual contact area of spacer  20 . 
     The spacer  20  used in this measurement was a ring  21 , 24 mm in outside diameter and 20 mm in inside diameter, and the surface C was formed for the as-sintered chamfered portion on the inner and outer edges  23 , and the flatness and deforming condition of the magnetic disk substrate  12  were measured with the ceramic magnetic disk substrate  12 , 65 mm in outside diameter supported with the spacer  20  as shown in FIG.  5 . 
     The value of the varied ratio of actual contact area and the flatness at that time are as per shown in Table 2, and the deforming degree of relevant conditions is shown as per FIG.  6 . 
     For the evaluation standard in this measurement, the magnetic disk substrates free from V-letter form deformation and with flatness 2 μm or less were regarded as excellent. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Ratio of actual 
                   
                   
               
               
                   
                 No. 
                 contact area (%) 
                 Flatness (μm) 
                 Rating 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 This 
                 1 
                 90 
                 0.6 
                 ∘ 
               
               
                 invention 
                 2 
                 80 
                 0.9 
                 ∘ 
               
               
                   
                 3 
                 70 
                 1.2 
                 ∘ 
               
               
                   
                 4 
                 60 
                 1.2 
                 ∘ 
               
               
                   
                 5 
                 50 
                 1.5 
                 ∘ 
               
               
                 Comparison 
                 6 
                 40 
                 2.1 
                 X 
               
               
                   
                 7 
                 30 
                 3.9 
                 X 
               
               
                   
               
            
           
         
       
     
     As clear from Table 2, because the ratio of actual contact area is less than 50% for sample No. 6 and 7, the flatness of the magnetic disk substrate  12  was deformed as greatly as 2.1-3.9 μm and it was unable to bring it down to 2 μm of less. As clear from (6) and (7) in FIG. 6, because long oval interference fringes were observed in a large quantity and their intervals were extremely small, localized pointed tips were formed on both ends on the major axis of the oval interference fringe and V-letter shape deformation was formed. 
     On the contrary, because in sample No. 1-5 related to this invention, the ratio of the actual contact area was 50% or more, it was possible to keep the flatness of the magnetic disk substrate  12  to be 1.5 μm or less and it was able to satisfy the standard value. As clear from (1)-(5) in FIG. 6, the intervals of interference fringes were large and the interference fringes were not long oval but smooth circle and free from any localized pointed tips. 
     In particular, with sample No. 1-3 with the ratio of actual contact area 70% or more, smooth circular interference fringes are observed, indicating that the substrate deforms in an extremely smooth conical shape. Consequently, if the magnetic head  14  is levitated on this magnetic disk substrate  12 , it can be located with a specified distance constantly maintained in a specified track, enabling stable and high-density recording. 
     Next, using a spacer  20  with the 90% ratio of actual contact area, flatness of the contact surface  22  was varied, and the rate of change at the peripheral area of the magnetic disk substrate  12  was measured when it was supported with this spacer  20 . As shown in FIG. 7, when the maximum deflection was generated by supporting the magnetic disk substrate  12  of radius R using the spacer  20  with the radius r and flatness F of the contact surface  22 , the camber angle θ of the magnetic disk substrate  12  is expressed by 
     
       
         θ=tan −1 (2 Fr/r   2   −F   2 ) 
       
     
     and if L=R−r, the rate of change Δg at the peripheral area of the magnetic disk substrate  12  is expressed as 
     
       
         Δg= L  tan θ F   
       
     
     
       
         =(2 Fr /( r   2   −F   2 )) L+F.   
       
     
     Now let r=11.53 mm and L=20.97 mm and change the value of flatness F, then we have Δg as shown in Table 3. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 No. 
                 Flatness (μm) 
                 Δg (μm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 This 
                 1 
                 0.1 
                 0.5 
               
               
                   
                 invention 
                 2 
                 0.2 
                 0.9 
               
               
                   
                   
                 3 
                 0.3 
                 1.3 
               
               
                   
                   
                 4 
                 1.0 
                 4.6 
               
               
                   
                 Comparison 
                 5 
                 3.0 
                 13.9 
               
               
                   
                   
                 6 
                 5.0 
                 23.1 
               
               
                   
                   
               
            
           
         
       
     
     As clear from Table 3, by setting the flatness F to 3 μm or less and preferably 1 μm or less, Δg can be extremely minimized. 
     As described above, this invention is designed to form the support member such as shims, clamps, and spacers with ceramics or glass and to set the ratio of actual contact area with the magnetic disk substrate to 50-95% and the flatness of the contact surface to 3 μm or less so that chipping generated at the edge due to stress at the time of clamping is prevented as well as the flatness of the magnetic disk substrate is improved and the levitation rate of the magnetic head is minimized. 
     In this invention, by composing the magnetic disk unit by supporting the magnetic disk substrate with spacers and shims comprising the support member, it is possible to increase the information recording density. In particular, if the magnetic disk unit is composed by combining the magnetic disk substrate made of ceramics or glass, deformation of the magnetic disk substrate is not generated at the time of clamping and the thermal expansion coefficient of the support member can be matched with that of magnetic disk substrate, and therefore, high-density recording of information is possible. 
     In addition, this invention can effectively release static electricity charged in the magnetic disk substrate and prevent the recording contents from being destroyed by drilling through holes perpendicular to the contact surface, placing springs comprising conductive materials inside the through holes, filling the through holes with the conductive materials or covering the inner wall surface of the through holes with the conductive film to achieve continuity between the top and the bottom contact surfaces, if support members such as the shims and clamps are formed with insulating ceramics or glass. 
     Now, description will be made on embodiments of this invention. 
     FIG. 9 to FIG. 11 are schematic illustration showing support members related to this invention, and FIG. 9 ( a ) is a perspective view showing a spacer  111  and (b) its sectional view, FIG. 10 ( a ) a perspective view showing a shim  110  and (b) its sectional view, and FIG. 11 ( a ) a perspective view showing a clamp  112 ′ and (b) its sectional view. First, the spacer  111  shown in FIG. 9 is a ring  212  made of ceramics or glass, and to the inner and outer edges  214   a,    214   b,  a taper surface (including surface C) or a curvature is formed. The contact surface  213  of this spacer  111  is finished to surface roughness of 0.1-2.0 μm in terms of center line average roughness (Ra) in order to prevent the fixed magnetic disk substrate from rolling as it rotates at high speed as well as the flatness of the contact surface  213  is finished to 3 μm or less to prevent deformation in the magnetic disk substrate when it is fixed, and in addition the parallelism of the top and bottom surfaces  213  is set to 5 μm or less so that the magnetic disk substrate is held to a specified position, And to the whole surface of the ring  212 , conductive hard film  215  with volume specific resistance of 1×10 7  Ω·cm or lower is covered in the film thickness ranging from 0.1 to 3 μm. 
     The shim  110  is designed to be of the same profile as the spacer  111  as shown in FIG. 10 but slightly thinner. 
     The clamp  112 ′ shown in FIG. 11 is a disk-shape plate  222 ′ made of ceramics or glass, and at one surface center, an indent section  227 ′ engaging to the top end part of the hub and a screw hole  228 ′ for fixing to the hub are provided, and to the inner and to the outer edges  224 ′ a  and  224 ′ b  of the plate  222 ′, the taper surface (including the face C) or curvature is formed. 
     The contact surface  223 ′ of this clamp  112 ′ is finished to surface roughness of 0.1-2.0μm in terms of center line average roughness (Ra) and the flatness of 3 μm or less to prevent deformation of the magnetic disk and to prevent rolling as in the case of the spacer  111 . To the whole surface of this plate-type clamp  222 ′, the conductive hard film  225 ′ with the volume specific resistance 1×10 7  Ω·cm or less is covered in the film thickness ranging from 0.1 to 3 μm. 
     In this way, since in this invention, the base material composing the support member such as shims  110 , spacers  111  and clamps  112 ′ is formed with ceramics or glass, the contact surface  203 ,  213 ,  223 ′ can be finished to remarkably high smoothness and excellent plane accuracy. Because in this invention, at least to the contact surface  203 ,  213 ,  223 ′ and the inner wall surface  206 ,  216 ,  226 ′, highly conductive hard film  205 ,  215 ,  225 ′ is covered in the film thickness as thin as 0.1-3 μm, the surface with excellent wear resistance can be obtained without impairing flatness and parallelism of the contact surface  203 ,  213 ,  223 ′. Moreover, the conductive hard film  205 ,  215   225 ′ provides the volume specific resistance 1×10 7  Ω·cm or less, static electricity charged to the magnetic disk substrate can be efficiently released, thereby preventing the recording contents from being destroyed. 
     For the conductive hard films  205 ,  115 ,  225 ′, those with thermal expansion coefficient in the range of 2-10×10 −6 /° C. are preferable, and because the use of those conductive hard film  205 ,  115 ,  225 ′ can approximate the thermal expansion coefficient to that of ceramics or glass (4.0-10.0×10 −6 /° C.) which form the basic material, no deformation will be generated in magnetic disk substrate due to the heat caused by high-speed rotation. 
     In addition, in this invention, taper or R formed in inner and outer edges of each support member can prevent chipping at the time of clamping as well as secure the film thickness of conductive hard films  205 ,  115 ,  225 ′ to be coated to the edges  204 ,  214 ,  224 ′ and prevent disconnection. 
     Now, the magnetic disk unit  150  with the glass magnetic disk substrate  115  with magnetic film provided on the surface supported by these support members is shown in FIG.  12 . 
     To the rotary shaft  113 , a metal hub  114  in a nearly cylindrical profile equipped with a flange section  114   a  is fixed, and to the flange section  114   a  of the hub  114 , a multiplicity of magnetic disk substrate  115  and spacer  111  shown in FIG. 9 are inserted alternately, and after lastly inserting the shim  110  shown in FIG. 9, they are retained with clamps  112  made of metal or ceramics such as alumina and the clamp  112  is tightened to the hub  114  with the screw  116  to fix the magnetic disk substrate  115 . 
     The magnetic disk unit  150  according to this invention does not generate any inconvenience as a result of a thermal expansion difference even if temperature rises during high-speed rotation because the thermal expansion coefficient of the magnetic disk substrate  115  is approximate to that of the support member (spacer  111 , shim  110 ). Consequently, the levitation rate of the magnetic head  117  with respect to the magnetic disk substrate  115  can be extremely reduced and the information recording density can be increased. Moreover, since the support member (spacer  111 , shim  110 ) possesses the electric conductivity, static electricity charged in the magnetic disk substrate  115  can be released via the metallic hub  114  and rotary shaft  113  and the recording contents can be prevented from being destroyed. 
     For the magnetic disk substrate  115 , in addition to glass substrate, the ceramic substrate on the surface of which a glass glazed layer is formed and the magnetic film is covered on the layer may be used. 
     In the magnetic disk unit  150  shown in FIG. 12, the magnetic disk substrates are supported between the top magnetic disk substrate  115  and the clamp  112  via the shim  110 , but for other embodiment, it may be designed to support the clamp  112  by directly bringing it in contact with the top magnetic disk substrate  115 , and in such event, the use of the clamp  112 ′ shown in FIG. 11 can support the magnetic disk substrate  115  with high accuracy as well as can effectively release static electricity charged to the top magnetic disk substrate  115 . 
     In addition, for another example of the magnetic disk unit  150 , the spacer  111  arranged between the flange section  114   a  of the hub  114  and the magnetic disk substrate  115  may be removed and the magnetic disk substrate may be brought in direct contact with the hub flange, and in such event, in order to eliminate the thermal expansion difference with the magnetic disk substrate  115 , a hub  114  made of ceramics or glass, on the surface of which the conductive ceramic film is covered, is recommended to use for increased effectiveness. 
     Now, for the base material constituting the support member such as the spacer  111 , shim  110  and clamp  112 , ceramics or glass with the thermal expansion coefficient 20×10 −6 /° C. or less and preferably 12×10 −6 /° C. can be used, and examples of ceramics include alumina ceramics, zirconia ceramics, silicon carbide ceramics, silicon nitride, Al 2 O 3 —TiC-based ceramics, forsterite ceramics, etc. 
     In particular, because these ceramics have the thermal expansion coefficient within the above-mentioned range as well as large specific rigidity, they hardly generate deformation at the time of clamping and can finish the contact surface  203 ,  231 ,  223 ′ to remarkable smooth surface with excellent flatness accuracy. 
     Depending on the material of magnetic disk substrate  115 , any ceramics with approximate thermal expansion coefficient may be chosen and applied from the material of the above-mentioned support members. For example, if the ceramic magnetic disk substrate  115  is used, ceramics with the thermal expansion coefficient 10×10 −6 /° C. or less should be used for support member, and similarly, when glass (thermal expansion coefficient 8.0-9×10 −6 /° C.) magnetic disk substrate  115  is used, forsterite ceramics or glass with thermal expansion coefficient 8.0×10 −6 /° C. or higher should be used for support member. 
     To the inner and outer edges  204 ,  214 ,  224 ′ of the substrate member such as the spacer  111 , shim  110  and clamp  112 , a taper (including surface C) or curvature is formed to prevent chipping due to stress at the time of clamping, but the excessively small taper or curvature generate disconnection at the edge  204 ,  214 ,  224 ′ to break continuity. 
     Consequently, in this invention, to the inner and outer edge  204 ,  214 ,  224 ′ of the support member, the taper or curvature 0.04 mm or wider is formed. 
     That is, if the taper or curvature width formed on the edge  204 ,  214 ,  224 ′ is smaller than 0.04 mm, since the edge  204 ,  214 ,  224 ′ of the support member is a sharp edge as described before, thickness of the conductive hard film  205 ,  215 ,  225 ′ coated to it decreases, possibly resulting in disconnection. However, because if the taper or curvature is 0.5 mm or wider, the contact area of the magnetic disk substrate  115  of the contact surface  203 ,  213 ,  223 ′ becomes small and strain is generated in the magnetic disk substrate  115  at the time of clamping, it is desirable to form the taper or curvature in the range of 0.04 to 0.5 mm for the inner and outer edge  204 ,  214 ,  224 ′ of the support member. 
     The taper or curvature width referred to in this invention is the width L from the end face of the edge  214   a  on the contact surface side  213  as shown in the enlarged view of the main section A in FIG. 13 when description is made with the spacer  111  taken as an example. 
     Of these support members, at least contact surfaces  203 ,  213 ,  223  must be finished to surface roughness 0.1-2.0 μm in terms of center line average roughness (Ra). This is because if the center line average roughness (Ra) of the contact surfaces  203 ,  213 ,  223 ′ is less than 0.1, the covered conductive hard film  205 ,  215 ,  225 ′ is unable to be closely affixed with sufficient anchor effects as well as the surface of the conducive hard film  205 ,  215 ,  225  becomes excessively smooth, generating slips in the magnetic disk substrate  115  as high speed rotation is carried out, and conversely, if the center line average roughness (Ra) of contact surface  203 ,  213 ,  223 ′ exceeds 2.0 μm, flatness of the magnetic disk substrate  115  will be impaired as well as damage the magnetic disk substrate  115 . 
     On the other hand, examples of conductive hard film  205 ,  215 ,  225 ′ for covering the support member include TiC, TiN, ZrN, HfC, TaC, ZrC, WC, VC, NbC, TiB 2 , ITO (Indium Tin Oxide), and DLC (Diamond-like Carbon), and any one of these films is preferable. 
     These conductive hard films  205 ,  215 ,  225 ′ has the volume specific resistance less than 1×10 −7 ·cm as shown in Table 4, the static electricity charged in the magnetic disk substrate  115  can be effectively removed. Moreover, because the thermal expansion coefficient is about 2-10×10 −6 /° C. which is equivalent to or approximate to that of the ceramics or glass composing the support member and the ceramics or glass composing the magnetic disk substrate  115 , conductive hard film  205 ,  215 ,  225 ′ is free from peeling or deformation due to heat as a result of high-speed rotation and the magnetic disk substrate  115  can be supported with extremely high flatness accuracy. Of the conductive hard films  205 ,  215 ,  225 ′, ITO (Indium Tin Oxide) means In  203  with Sn doped, and DLC (Diamond-like Carbon) means carbon with the amorphous structure. The DLC may have an amorphous structure containing Si with Zr or W doped. 
     However, it is important to provide the film thickness of the conductive hard film  205 ,  215 ,  225 ′ to be covered in the range from 0.1 to 3.0 μm. 
     This is because there is a fear of wear to be generated even in high-hardness conductive hard film  205 ,  215 ,  225 ′ if the film thickness is less than 0.1 μm, and conversely, if the film thickness becomes thicker than 3.0 μm, it is unable to keep the flatness of the contact surface  203 ,  213 ,  223 ′ to 3 μm or less. 
     The conductive hard film  205 ,  215 ,  225 ′ may be formed by the general film forming means such as PVD or CVD processes but when the conductive hard film  205 ,  215 ,  225 ′ is applied to the glassy support member, the use of the PVD process that enables low-temperature film forming is the most suitable because it will not impair smoothness of the support member. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Thermal expansion 
                 Volume specific 
                 Micro Vickers 
               
               
                 Conductive 
                 coefficient 
                 resistance 
                 hardness 
               
               
                 ceramic film 
                 (× 10 −6 /° C.) 
                 (× 10 −4  Ω · cm) 
                 (kg/mm 2 ) 
               
               
                   
               
             
            
               
                 TiC 
                 7.95 
                 1.05 
                 3170 
               
               
                 TiN 
                 9.35 
                  0.217 
                 2050 
               
               
                 HfC 
                 6.80 
                 1.95 
                 2530-3200 
               
               
                 TaC 
                 7.09 
                 0.3  
                 1720 
               
               
                 ZrC 
                 7.O1 
                 0.7  
                 2950 
               
               
                 VC 
                 7.25 
                 1.56 
                 2480 
               
               
                 NbC 
                 7.21 
                 0.74 
                 2170 
               
               
                 WC 
                 6.2  
                 0.12 
                 1716 
               
               
                 ZrN 
                 7.24 
                  0.136 
                 1670 
               
               
                 TiB2 
                 4.60 
                  0.12-0.284 
                 3370 
               
               
                 ITO 
                 — 
                 10.0  
                 — 
               
               
                 DLC 
                 3.0-5.0 
                 10 −5 -10 6    
                 3000-6000 
               
               
                   
               
            
           
         
       
     
     EXPERIMENTAL EXAMPLE 1 
     Now, a spacer  111  shown in FIG. 9 is prepared and adhesion and flatness of the conductive hard film  215  were measured with surface roughness of the contact surface  213  of the spacer  111  varied. 
     For the measuring method, the contact surface  213  of the spacer with roughness varied, respectively, was coated with 1-μm-thick TiN film and the flatness was measured, and at the same time, cellophane tape was affixed to the TiN film, and adhesion was measured by determining whether peeling of the film occurred when the cellophane tape was pulled. 
     In this experiment, the film which does not generate peeling and achieves the flatness 3 μm or less is regarded to be superior. 
     The surface roughness of the contact surface  213  and the results are shown in Table 5. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Centerline average 
                 Flatness when 1-μm 
                 Any 
                   
               
               
                   
                 roughness of contact 
                 Thick TiN film is 
                 sign of 
                 Overall 
               
               
                 No. 
                 surface (μm) 
                 coated (μm) 
                 peeling 
                 rating 
               
               
                   
               
             
            
               
                 1 
                 0.05 
                 1 
                 Present 
                 X 
               
               
                 2 
                 0.1 
                 1 
                 None 
                 ∘ 
               
               
                 3 
                 0.5 
                 1 
                 None 
                 ∘ 
               
               
                 4 
                 1.0 
                 1.5 
                 None 
                 ∘ 
               
               
                 5 
                 2.0 
                 3.0 
                 None 
                 ∘ 
               
               
                 6 
                 3.0 
                 3.8 
                 None 
                 X 
               
               
                   
               
            
           
         
       
     
     As clear from Table 5, for sample No. 1, the TiN film thickness is thin enough to achieve the flatness of 3 μm or less but because surface roughness is less than 0.1 μm in terms of center line average roughness (Ra), the TiN film was easily peeled. 
     Sample No. 6 provided the surface roughness as big as 3.0 μm in terms of center line average roughness (Ra) and did not cause peeling of the TiN film, but the surface roughness of the contact surface  213  was too coarse to achieve flatness 3 μm or less. 
     Conversely, sample No. 2-5 according to this invention provided surface roughness in the range of 0.1-2.0 μm in terms of center line average roughness (Ra) and did not generate any film peeling and were able to achieve flatness 3 μm or lower and satisfied the standard. 
     EXPERIMENTAL EXAMPLE 2 
     Next, the spacer  111  of FIG. 9 with varying thickness of conductive hard film  215  was prepared and flatness of the spacer  111  and adhesion of the conductive hard film  215  were measured. 
     The measuring method was same as that used for Experimental Example 1 in which the TiN film was covered to the contact surface  213  of the spacer  111  as the conductive hard film  215  and the flatness was assumed and at the same time cellophane tape was affixed to the TiN film and was pulled to determine any sign of peeling of the film. However, the surface roughness of the contact surface  212  was 0.2 μm in terms of center line average roughness (Ra) and the flatness was 1 μm. 
     In this experiment, the space which does not generate any film peeling and achieves the flatness 3 μm or less is regarded to be superior. 
     The surface roughness of the contact surface  215  and the results are shown in Table 6. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Centerline average 
                 Flatness when 1-μm 
                 Any 
                   
               
               
                   
                 roughness of 
                 Thick TiN film is 
                 sign of 
                 Overall 
               
               
                   
                 contact surface (μm) 
                 coated (μm) 
                 peeling 
                 rating 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 A 
                 0.05 
                 1 
                 Present 
                 x 
               
               
                 B 
                 0.1 
                 1 
                 None 
                 ∘ 
               
               
                 C 
                 0.5 
                 1.2 
                 None 
                 ∘ 
               
               
                 D 
                 1.0 
                 2.5 
                 None 
                 ∘ 
               
               
                 E 
                 2.0 
                 2.9 
                 None 
                 ∘ 
               
               
                 F 
                 3.0 
                 3.0 
                 None 
                 ∘ 
               
               
                 G 
                 4.0 
                 3.0 
                 None 
                 x 
               
               
                   
               
            
           
         
       
     
     As clear from Table 6, for sample A, the TiN film thickness was as thin as 0.05 μm and was able to achieve the flatness of 3 μm or less but satisfactory adhesion strength was unable to be obtained and peeling of TiN film occurred. 
     Sample G provided the film thickness as big as 4.0 μm with variations as well as flatness 3.6 μm and was unable to achieve flatness 3 μm or lower. 
     Conversely, sample B-F according to this invention provided film thickness in the range of 0.1-2.0 μm and did not generate any film peeling and were able to suppress flatness of support member to 3.0 μm or lower. 
     EXPERIMENTAL EXAMPLE 3 
     In addition, measurement was made on the continuity when the width L of surface C formed on the edge portion  214  of the spacer  111  was varied. 
     The surface roughness of the contact surface  213  was 0.2 μm in terms of center line average roughness (Ra) and the TiN film was covered to the whole surface of the spacer  111  in 1.0 μm thick for conductive hard film  215 . And the continuity from the contact surface  213  to the inner circumferential surface  216  was confirmed. 
     The relevant results are shown in Table 7. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Width L of 
                 Any sign ∘ 
               
               
                   
                 surface C (mm) 
                 continuity 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 0 
                 X 
               
               
                   
                 0.01 
                 X 
               
               
                   
                 0.02 
                 X 
               
               
                   
                 0.03 
                 X 
               
               
                   
                 0.04 
                 ◯ 
               
               
                   
                 0.05 
                 ◯ 
               
               
                   
                 0.06 
                 ◯ 
               
               
                   
                   
               
            
           
         
       
     
     As clear from Table 7, because the chamfering rate was small even if surface C treatment was performed on the edge portion  214  when the width L of surface C was 0.03 mm or less, the film thickness at this portion became thin and continuity was unable to be obtained, but continuity was obtained when the width L of surface C exceeded 0.04 mm. This indicates that the width L of surface C formed on the inner and outer edge portions  214  of the support member should be 0.04 mm or more. 
     In this experiment, an example in which surface C was formed on the edge portion  214  of the spacer  111  was shown but the same results were obtained with the taper or curvature. 
     As described above, this invention can efficiently release static electricity charged on the magnetic disk substrate and scarcely causes wear in the contact surface by providing the taper or curvature 0.04-0.5 mm wide on the inner and outer edges of the support members such as shims, clamps, and spacers composed with ceramics or glass and coating 0.1-3 μm thick conductive hard film on the contact surface with magnetic disk substrate and on the inner circumferential surface. Moreover, in this invention, since one type of TiC, TiN, ZrN, HfC, TaC, ZrC, WC, VC, NbC, TiB 2 , ITO, DLC is coated as the conductive hard film, it is able to achieve the same or approximate thermal expansion coefficients for support member and and magnetic disk substrate, generating no peeling of film due to heat resulting from high-speed rotation, and as a result, no strain is generated in the magnetic disk substrate, and therefore, the magnetic disk substrate can be supported with remarkably high flatness accuracy. 
     In this invention, since the magnetic disk unit is configured by supporting one or a plurality of magnetic disk substrate made of ceramics or glass via support members, which are formed in a ring made of ceramics or glass and are provided with 0.04-0.5 mm wide taper or curvature on the inner and outer edges and have at least the contact surface with the magnetic disk substrate and inner circumferential surface coated with conductive hard film 0.1-3 μm thick, to the hub fixed to a rotary shaft and made of conductive material, the levitation rate of the magnetic head with respect to the magnetic disk substrate can be reduced to a minimum, thereby enabling high-density recording (increased capacity) and efficient releasing of static electricity charged in the magnetic disk substrate via support members and the hub, and preventing recording contents from being destroyed. 
     Now, description will be made on embodiments of this invention. 
     FIG. 14 to FIG. 16 are schematic illustrations showing support members related to this invention, and FIG. 14 ( a ) is a perspective view showing a spacer  211  and (b) its sectional view, FIG. 15 ( a ) a perspective view showing a shim  210  and (b) its sectional view, FIG. 16 ( a ) a perspective view showing a clamp  212 ′ and (b) its sectional view. 
     First, the spacer  211  shown in FIG. 14 is a ring  312  made of forsterite ceramics with conductivity less than 10 7  Ω·cm in terms of volume specific resistance, and to the inner and outer edges  313   a,    313   b,  surface C or R is formed to prevent chipping. 
     The contact surface  313  is finished to surface roughness of 0.2-2.0 μm in terms of center line average roughness (Ra) in order to prevent the fixed magnetic disk substrate from rolling as it rotates at high speed as well as the flatness of the contact surface  313  is finished to 3 μm or less to prevent deformation in the magnetic disk substrate when it is fixed, and in addition the parallelism of the top and bottom surfaces  313  is set to 5 μm or less so that the magnetic disk substrate is held to specified intervals. The shim  210  is designed to be of the same profile as the spacer  211  as shown in FIG. 15 but slightly thinner. 
     The clamp  212 ′ shown in FIG. 16 is a disk-shape plate  322 ′ made of forsterite ceramics, and the contact surface  323 ′ is finished to surface roughness of 0.2-2.0 μm in terms of center line average roughness (Ra) and the flatness of 3 μm or less to prevent deformation of the magnetic disk and to prevent rolling as in the case of the spacer  211 . And at the center of the contact surface  323 ′, and indent  324 ′ for engaging with the hub tip end portion and a screw hole  325 ′ for fixing to the hub are provided, and on the inner and outer edges  323 ′ a,    323 ′ b  of the plate-type clamp  322 ′, surface C or R is formed. 
     In this way, since in this invention, the base material composing the support member such as shims  210 , spacers  211  and clamps  212 ′ is formed with forsterite ceramics with volume specific resistance less than 10 7  Ω·cm, static electricity charged in magnetic disk substrate can be efficiently released. In addition, because the forsterite ceramics composing the support member provides the same or approximate thermal expansion coefficient (8.0-10.0×10 −6 /° C.) as that of ceramics or glass magnetic disk substrate, it can prevent deformation of magnetic disk substrate as a result of thermal expansion difference and can support the magnetic disk substrate with remarkable high accuracy. 
     Now, the magnetic disk unit  250  with the glass magnetic disk substrate  215  with magnetic film provided on the surface supported by these support members is shown in FIG.  17 . 
     To the rotary shaft  213 , a metal hub  214  in a nearly cylindrical profile  14  equipped with a flange section  214   a  is fixed, and to the flange section  214   a  of the hub  214 , a multiplicity of magnetic disk substrate  215  and spacer  211  are inserted alternately, and after inserting the shim  210  shown in FIG. 15, they are retained with clamps  212  made of metal or ceramics such as alumina and the clamp  212  is tightened to the hub  214  with the screw  216  to fix the magnetic disk substrate  215 . 
     The magnetic disk unit  250  according to this invention does not generate any inconvenience as a result of a thermal expansion difference even if temperature rises during high-speed rotation because the thermal expansion coefficient of the magnetic disk substrate  215  is same or approximate to that of the support member (spacer  211 , shim  210 ). Consequently, the levitation rate of the magnetic head  217  with respect to the magnetic disk substrate  215  can be extremely reduced and the information recording density can be increased. Moreover, since the support member (spacer  211 , shim  21 ) possesses the electric conductivity, static electricity charged in the magnetic disk substrate  215  can be released via the metallic hub  214  and rotary shaft  213  and the recording contents can be prevented from being destroyed. 
     For the magnetic disk substrate  215 , those in which a glass glazed layer is formed on the ceramic substrate surface, on which the magnetic film is covered may be used in addition to glass substrate. 
     In the magnetic disk unit  250  shown in FIG. 17, the magnetic disk substrate  215  is supported by shims  210  intervening between the magnetic disk substrate  215  on the uppermost part and the clamp  212  but in addition to this, it may be designed to be supported by directly bringing the clamp  212  in contact with the magnetic disk substrate  215 , and in this case, the use of the clamp  212 ′ shown in FIG. 16 can achieve the accurate support of the magnetic disk substrate  215  as well as can effectively release static electricity charged in the uppermost magnetic disk substrate  215 . 
     For other example of the magnetic disk unit  250 , it may also be designed to remove the spacer  211  arranged between the flange section  214   a  of the hub  214  and the magnetic disk substrate  25  and support the magnetic disk substrate by bringing them in direct contact, and in this case, in order to eliminate the thermal expansion difference with the magnetic disk substrate  12 , it is desirable to form the hub  214  with conductive ceramics, in particular, forsterite ceramics with electric conductivity. 
     By the way, the conductive forsterite ceramics which constitutes the support members such as spacer  211 , shim  210  and clamp  212 ′ is primarily composed of forsterite (2MgO.SiO 2 ) and has a metallic compound added as a conducting material, and in particular, those with iron-based compounds such as iron oxide (FeO, Fe 2 O 3 ), triiron tetra oxide (Fe 3 O 4 ), iron hydroxide oxide (FeO(OH)), etc. added as metallic compound are preferable. 
     That is, the inventors of this invention have earnestly made researches on the materials which has the conductivity less than 10 7 Ω·cm in terms of volume specific resistance and provides same or approximate thermal expansion coefficient of glass magnetic disk substrate  215 , and as a result, have found that adding iron-based compounds such as iron oxide, triiron tetraoxide, iron hydroxide oxide, etc. to a certain range can provide electric conductivity with scarcely impairing the mechanical properties. In addition, forsterite ceramics containing iron compounds is difficult to generate chipping as compared to insulating forsterite ceramics and can reduce the volume of surface C or R at the edges ( 303   a ,  303   b,    313   a,    313   b,    323   a ′,  323   b ′). Consequently, since the ratio of the contact area of the contact surface ( 303 ,  313 ,  323 ′) with the magnetic disk substrate  215  can be increased, the deformation of the magnetic disk substrate  215  can be suppressed. 
     The preferable dosage of the iron compound is 20-60 wt % to the total volume. This is because when the dosage is less than 20 wt %, the volume specific resistance exceeds 10 7  Ω·cm and high insulation performance results, and it becomes unable to prevent charging of magnetic disk substrate, and conversely if the dosage is greater than 60 wt %, mechanical properties (Young&#39;s modulus, bending strength) are greatly reduced, and the flatness of the contact surfaces  303 ,  313 ,  323 ′ of each support member is unable to be maintained and at the same time, the thermal expansion coefficient lowers, increasing the thermal expansion difference with the glass magnetic disk substrate  215  and generating deformation in magnetic disk substrate  215 . 
     In the meanwhile, to fabricate the support member according to this invention, first of all, the ceramic raw material with MgO and SiO 2  mixed at a ratio of 50 to 50 is mixed and ground in a wet condition and dried to produce granules with MgO and SiO 2  finely and homogeneously dispersed, and then it is sintered to form ceramic particles with the crystal structure comprising 2MgO.SiO 2  and/or MgSiO 3 . 
     Then, to 40-80 wt % of this ceramic particle, at least one or more types of iron oxide (FeO, Fe 2 O 3 ), triiron tetra oxide (Fe 3 O 4 ), iron hydroxide oxide (FeO(OH)) are added in the range of 20-60 wt % to the total together with a binder, and after blended and dried, the mixture is formed into a compact in a ring, cylindrical, or other desired shape by a mechanical press, and the compact is sintered for 1-2 hours at sintering temperature of 1200-1300° C. in atmosphere, and the support member according to this invention can be obtained. 
     In this ceramics, it is preferable that for the forsterite component, crystals of 2MaO.SiO 2  and/or MgSiO 3  exist and for iron-based compound components, crystals of one or more types of MgFe 2 O 3 , Fe 3 O 4  exist. The existence of these crystals means that the peak of each crystal is determined by X-ray refraction analysis. 
     In the forsterite ceramics, impurities such as Ti0 2 , Ca0, etc. may be included in the range of 15 wt % or less with respect to the total. 
     The support members comprising conductive forsterite ceramics obtained in this way provides the volume specific resistance less than 10 7  Ω·cm, the thermal expansion coefficient in the range of 9-11×10 −6 /° C. Young&#39;s modulus of 100-140 GPa, and bending strength of 10-14 kg/mm 2 . 
     As described above, because the support member according to this invention can have the thermal expansion coefficient equivalent to or approximate to that of the magnetic disk substrate and provides electric conductivity, it does not cause any inconvenience related to the thermal expansion difference even if it acquires high temperature at the time of high-speed rotation and can release static electricity charged in the magnetic disk substrate  215 . Moreover, if the magnetic disk unit according to this invention in which the glass magnetic disk substrates are supported using the above-mentioned support members is used, the levitation rate of the magnetic head  217  with respect to the magnetic disk substrate can be remarkably reduced and high density recording can be achieved and at the same time, destruction of recording contents resulting from magnetic disk  215  being charged can be prevented. 
     Experimental Example 
     Now, of the forsterite composing the support member according to this invention, the volume specific resistance, Young&#39;s modulus, bending strength, and thermal expansion coefficient were measured when the dosage of iron-based compounds (iron oxide (FE0, Fe 2 0 3 ), triiron tetra oxide (Fe 3 0 4 )) was varied. 
     For the evaluation standard of this experiment, support members with volume specific resistance less than 10 7  Ω·cm, Young&#39;s modulus of 100 GPa or higher, bending strength of 10 kg/mm 2  or higher, and the thermal expansion coefficient in the range of 9-11×10 −6 /° C. were regarded as superior. 
     The results are shown in Table 8 to 10, respectively. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 8 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Volume 
                   
                   
                 Thermal 
               
               
                   
                   
                 Sintering 
                 specific 
                 Young&#39;s 
                 Bending 
                 expansion 
               
               
                   
                 Dosage (wt %) 
                 conditions 
                 resistance 
                 modulus 
                 strength 
                 coefficient 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 Forsterite 
                 FeO 
                 Temperature 
                 Hours 
                 (Ω · cm) 
                 (GPa) 
                 (kg/mm 2 ) 
                 (1/° C.) 
               
               
                   
               
               
                 1 
                 *100  
                  0 
                 1250 
                 2 
                 9 × 10 11   
                 150 
                 15.0 
                 10 × 10 −6   
               
               
                 2 
                 *85 
                 15 
                 1230 
                 2 
                 9 × 10 7    
                 140 
                 15.0 
                 10 × 10 −6   
               
               
                 3 
                  80 
                 20 
                 1230 
                 2 
                 1 × 10 5    
                 140 
                 13.0 
                 10 × 10 −6   
               
               
                 4 
                  75 
                 25 
                 1230 
                 2 
                 2 × 10 4    
                 140 
                 13.0 
                 10 × 10 −6   
               
               
                 5 
                  65 
                 35 
                 1230 
                 2 
                 2 × 10 4    
                 130 
                 12.0 
                 10 × 10 −6   
               
               
                 6 
                  55 
                 45 
                 1240 
                 2 
                 5 × 10 3    
                 135 
                 11.0 
                 10 × 10 −6   
               
               
                 7 
                  45 
                 55 
                 1250 
                 2 
                 3 × 10 3    
                 130 
                 10.0 
                 10 × 10 −5   
               
               
                 8 
                  40 
                 60 
                 1250 
                 2 
                 1 × 10 5    
                 100 
                 10.0 
                  9 × 10 −6   
               
               
                 9 
                 *35 
                 65 
                 1250 
                 2 
                 1 × 10 5    
                  80 
                  5.0 
                  3 × 10 −6   
               
               
                   
               
               
                 *shows valued outside the range of this invention.  
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 9 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Volume 
                   
                   
                 Thermal 
               
               
                   
                   
                 Sintering 
                 specific 
                 Young&#39;s 
                 Bending 
                 expansion 
               
               
                   
                 Dosage (wt %) 
                 conditions 
                 resistance 
                 modulus 
                 strength 
                 coefficient 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 Forsterite 
                 FeO 
                 Temperature 
                 Hours 
                 (Ω · cm) 
                 (GPa) 
                 (kg/mm 2 ) 
                 (1/° C.) 
               
               
                   
               
               
                 11 
                 *85 
                 15 
                 1230 
                 2 
                 9 × 10 11   
                 150 
                 15.0 
                 10 × 10 −6   
               
               
                 12 
                  80 
                 20 
                 1235 
                 2 
                 9 × 10 6    
                 140 
                 13.0 
                 10 × 10 −6   
               
               
                 13 
                  75 
                 25 
                 1230 
                 2 
                 1 × 10 5    
                 130 
                 13.0 
                 10 × 10 −6   
               
               
                 14 
                  65 
                 35 
                 1230 
                 2 
                 2 × 10 4    
                 120 
                 11.0 
                 10 × 10 −6   
               
               
                 15 
                  55 
                 45 
                 1240 
                 2 
                 2 × 10 4    
                 120 
                 10.0 
                 10 × 10 −6   
               
               
                 16 
                  45 
                 55 
                 1250 
                 2 
                 1 × 10 5    
                 100 
                 10.0 
                 10 × 10 −6   
               
               
                 17 
                  40 
                 60 
                 1250 
                 2 
                 7 × 10 5    
                 100 
                 10.0 
                  9 × 10 −6   
               
               
                 18 
                 *35 
                 65 
                 1260 
                 2 
                 1 × 10 5    
                  90 
                  5.0 
                  5 × 10 −6   
               
               
                   
               
               
                 *shows valued outside the range of this invention.  
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 10 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Volume 
                   
                   
                 Thermal 
               
               
                   
                   
                 Sintering 
                 specific 
                 Young&#39;s 
                 Bending 
                 expansion 
               
               
                   
                 Dosage (wt %) 
                 conditions 
                 resistance 
                 modulus 
                 strength 
                 coefficient 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 Forsterite 
                 FeO 
                 Temperature 
                 Hours 
                 (Ω · cm) 
                 (GPa) 
                 (kg/mm 2 ) 
                 (1/° C.) 
               
               
                   
               
               
                 21 
                 *85 
                 15 
                 1240 
                 2 
                 5 × 10 11   
                 150 
                 14.0 
                 10 × 10 −6   
               
               
                 22 
                  80 
                 20 
                 1240 
                 2 
                 2 × 10 6    
                 140 
                 13.5 
                 10 × 10 −6   
               
               
                 23 
                  75 
                 25 
                 1250 
                 2 
                 1 × 10 6    
                 130 
                 13.5 
                 10 × 10 −6   
               
               
                 24 
                  65 
                 35 
                 1250 
                 2 
                 1 × 10 6    
                 120 
                 13.0 
                 10 × 10 −6   
               
               
                 25 
                  60 
                 40 
                 1250 
                 2 
                 7 × 10 5    
                 120 
                 12.0 
                 10 × 10 −6   
               
               
                 26 
                  55 
                 45 
                 1260 
                 2 
                 2 × 10 5    
                 115 
                 12.0 
                 10 × 10 −6   
               
               
                 27 
                  50 
                 50 
                 1260 
                 2 
                 1 × 10 5    
                 100 
                 10.0 
                 10 × 10 −6   
               
               
                   
               
               
                 *shows valued outside the range of this invention.  
               
            
           
         
       
     
     As a result, because in sample No. 1, 2, 11, 21, the dosage of iron-based compounds (iron oxide (Fe0, Fe 2 0 3 ), triiron tetra oxide (Fe 3 O 4 )) is less than 20 wt %, the volume specific resistance became 10 7  Ω·cm or higher and it was unable to provide electric conductivity necessary for support members. 
     In sample No. 9, 18, because the dosage of iron-based compounds (iron oxide (Fe0, Fe 2 0 3 ) was more than 60 wt %, the volume specific resistance become less than 10 7  Ω·cm but Young&#39;s modulus, bending strength, and thermal expansion coefficient did not satisfy the standard values. 
     Conversely, in sample No. 3-8, 12-17, and 22-27, because the dosage of iron-based compounds (iron oxide (Fe0, Fe 2 0 3 ), triiron tetra oxide (Fe 3 O 4 )) is within the range of 20-60 wt %, the volume specific resistance was less than 10 7  Ω·cm, Young&#39;s modulus 100 GPa or higher, bending strength 10 kg/mm 2  or higher, and thermal expansion coefficient within the range of 9-11×10 −6 /° C., satisfying the standard values. 
     As described above, this invention can bring the thermal expansion coefficient of the support members such as shims, clamps, and spacers to the same or approximate level to that of magnetic disk substrate by forming the support members with forsterite ceramics with electric conductivity less than 10 7  Ω·cm in terms of volume specific resistance, thereby eliminating inconvenience caused by thermal expansion coefficient difference even when the support members acquire high temperature during high-speed rotation, and at the same time efficiently releasing static electricity charged in the magnetic disk substrate. 
     As described above, the magnetic disk unit is configured by inserting and fixing glass magnetic disk substrates successively to the hub made of conductive material using the support member, and it is therefore able to extremely minimize the levitation rate of the magnetic head with respect to the magnetic disk substrate, and high-density recording (increased capacity) can be achieved and at the same time static electricity charged in the magnetic disk substrate can be efficiently released via the support member and hub, and destruction of recording contents can be prevented. 
     Now description will be made on embodiments according to this invention. 
     First of all, a spacer, one example of the support member of this invention is shown in FIG.  18 . This spacer  311  is formed in a ring shape made of ceramics, and the contact surface  311   a  with the top and bottom magnetic disk substrates is finished to flatness 3 μm or less and a smooth surface 2.0 μm or less, and the parallelism of top and bottom contact surfaces  311   a  is kept 5 μm or lower. Each edge  311   b  is chamfered to surface C or R. 
     The shim, one example of support members of this invention is not illustrated, but is of the same profile as that of the spacer but is slightly thinner. 
     Next, the clamp, another example of support member is shown in FIG.  19 . This clamp  312  is a plate type clamp made of ceramics and the contact surface  312   a  is finished to the flatness 3 μm or lower and the smooth surface with roughness of 2.0 μm or lower. The outer circumference edge  312   b  is chamfered to surface C or R and a screw hole  312   b  is provided for tightening. In addition, a stepped area  312   d  is provided for being engaged with the hub at the time of installation. 
     Now, the magnetic disk unit using these shims  310 , spacers  311 , and clamps  312  is shown in FIG.  20 . To the flange portion  314   a  formed at the hub  314  connected to the rotary shaft  313 , ceramic spacer  311  and magnetic disk substrate  315  are arranged alternately, and the top ends of these are retained with shims  310  and clamps  312  and tightened with screws  316 , thereby tightening 2 to 8 pieces of magnetic disk substrate  315  are fixed as specified intervals. And with the magnetic head  317  levitating on the surface of the magnetic disk substrate  315  with a minimum distance kept while rotating the hub  314  and each of the magnetic disk substrates  315  by the rotary shaft  313 , information is written into and read from the specified position. 
     For the magnetic disk substrate  315 , in general, the aluminum substrate is used, but magnetic disk substrate with a glazed surface formed on the surface of the ceramics such as alumina and the magnetic film equipped on the glazed surface or the magnetic disk substrate completely formed with the glass substrate and the magnetic film equipped on the surface may be used. In addition, it is possible to use titanium, silicon, YAG, carbon, etc. for other substrate materials. 
     Because shims  310 , spacers  311 , and clamps  312 , support members of this invention are made of high-rigidity ceramics, they are free from deformation at the time of clamping, and because the contact surface  311   a,    312   a  is finished in the surface with flatness 3 μm or lower, each magnetic disk substrate  315  can be supported at a remarkably high accuracy. 
     In addition, if ceramic disk substrate made of ceramics or glass is used for the magnetic disk substrate  315 , the thermal expansion ratio to shims  310 , spacers  311 , and clamps  312  become approximate one another, and no inconvenience resulting from thermal expansion difference will be generated even if they become high temperature during high-speed rotation. Consequently, it is possible to dramatically reduce the levitation rate of the magnetic head  317  with respect to the magnetic disk substrate and to increase the information recording-density. 
     In the example of FIG. 20, the clamp  312  is designed to support the magnetic disk substrate  315  via the shim  310  but in addition to this, it may be designed to support the magnetic disk substrate by directly coming in contact with the top magnetic disk substrate  315 . It may also be designed to bring the hub  314  in contact with the magnetic disk substrate  315  to support. In such event, it is desirable to form the hub  314  with ceramics or glass. 
     Now, for the materials composing the support member such as shim  310 , spacer  311 , and clamp  312 , it is possible to use ceramics or glass whose thermal expansion coefficient is 20×10 −6 /° C., and more preferably 12×10 −6 /° C. or less, but for ceramics, as the characteristics are shown in Table 11, alumina ceramics, zirconia ceramics, silicon carbide ceramics, silicon nitride ceramics, alumina-titanium carbide based ceramics, barium titanate, cermet, forsterite ceramics, and others can be used. 
     Alumina ceramics in Table 11 is a sintered material which contains 90 wt % Al 2 0 3  and the remainder comprising Si0 2 , Mg0, Ca0, etc. The conductive alumina ceramics is a sintered material which contains 70-80 wt % Al 2 0 3  and the remainder comprising 10-20 wt % Ti0 2  as a conductivity provider, and the conductive alumina ceramics sintered in the oxidizing atmosphere provides 10 8  Ω·cm volume specific resistance and that sintered in the reducing atmosphere provides 10 3 -10 6  Ω·cm volume specific resistance. 
     Zirconia ceramics primarily consists of Zr0 2  and is partially stabilized zirconia ceramics in which the tetragonal phase is 80 mol % or higher by containing stabilizers such as Y 2 0 3 , Ca0, Mg0, etc. 
     In addition, silicon carbide ceramics contains more than 90 wt % SiC and the remainder comprising carbon (C) and boric acid (B), or Al 2 0 3 , Y 2 0 3 , etc., while silicon nitride ceramics contains more than 90 wt % Si 3 N 4  and the remainder comprising Al 2 0 2 , Y 3 0 3 , etc. 
     The alumina-titanium carbide based ceramics is a sintered material primarily composed of 20-80 wt % Al 2 0 3  and 80-20 wt % TiC and provides high hardness and electric conductivity. In addition, barium titanate is primarily composed of 10-20 mol % Ba0 and 90-80 mol % Ti0 2 , and contains at least one type of metallic oxide chosen from Al, Si, Zr, Nb, and Sr at a ratio of 0.01-4.0 parts in weight to 100 parts in weight of the main component, and is fired in the reducing atmosphere. 
     Cermet is a composite sintered material comprising a ceramic component which forms a hard phase and a metal component which forms a binder phase, and in particular cermet composed of 10-90 wt % TiC and 5-90 wt % TiN, and containing a 5a group metal carbide as an additive and iron group metal as a binder phase is used. 
     In addition, forsterite ceramics is a sintered material primarily composed of 2 Mg0·Si0 2  and because the Vickers hardness is as low as 1000 kg/mm 2  or lower, forsterite ceramics can be used to prevent damage to magnetic disk substrate  315 . 
     Comparison of the properties of these ceramics with aluminum (metal) used as a comparison example indicates that as clear from Table 11, all these ceramics are difficult to deform at the time of tightening because of their high Young&#39;s modulus 13000 kg/mm 2  or higher and also difficult to generate inconvenience at the time of high temperature because their thermal expansion ratio is as small as 12×10 −6 /° C. 
     Furthermore, since materials such as conductive alumina ceramics, silicon carbide ceramics, alumina-titanium carbide ceramics, barium titanate, cermet, and the like provide the electric conductivity with the volume specific resistance 10 6  Ω·cm or lower, they can allow static electricity in magnetic disk substrate  315  to escape. The silicon nitride ceramics does not possess electric conductivity but by allowing it to contain a conductivity provider such as Ti to contain, the volume specific resistance can be lowered to 10 6  Ω·cm or less, and it is more suited to use these conductive silicon nitride ceramics. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 11 
               
               
                   
                   
               
               
                   
                   
                 Young&#39; 
                 Vickers 
                 Thermal 
                 Volume specific 
               
               
                   
                 Specific 
                 modulus 
                 hardness 
                 conductivity 
                 resistance 
               
               
                   
                 gravity 
                 (kg/mm 2 ) 
                 (kg/mm 2 ) 
                 × 10 −6 /° C. 
                 Ω · cm 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Alumina 
                 2.7 
                 7200 
                 65 
                 24.2 
                 10 −8   
               
               
                 Alumina ceramics 
                 3.5-4.0 
                 25000-40000 
                 1200-1800 
                 6.5-8.0 
                 10 13 -10 15   
               
               
                 Conducive alumina 
                 3.5-4.0 
                 25000-40000 
                 1000-1200 
                 6.5-8.0 
                 10 3 -10 8   
               
               
                 Zriconia ceramics 
                 5.5-6.0 
                 20000-26000 
                 1200-1400 
                  9.5-11.5 
                 10 13 -10 15   
               
               
                 Silicon carbide 
                 2.8-3.5 
                 35000-45000 
                 2300-2500 
                 3.5-4.5 
                 10 2 -10 6   
               
               
                 ceramics 
               
               
                 Silicon nitride 
                 2.8-3.5 
                 30000-35000 
                 1400-1600 
                 2.5-3.5 
                 10 13 -10 15   
               
               
                 ceramics 
               
               
                 Alumina-titanium 
                 4.0-4.5 
                 40000-45000 
                 1850-1950 
                 6.5-8.0 
                 10 −2 -10 −1   
               
               
                 carbide ceramics 
               
               
                 Barium titanate 
                 4.0-4.5 
                 17000-20000 
                 1500-1900 
                  8.0-10.0 
                 10 2 -10 6   
               
               
                 Cermet 
                 5.5-7.5 
                 40000-50000 
                 1400-1800 
                 7.0-8.0 
                 10 −4 -10 −1   
               
               
                 Forsterite ceramics 
                 2.7-3.1 
                 13000-16000 
                  700-1000 
                  8.0-12.0 
                 10 14 - 
               
               
                   
               
            
           
         
       
     
     For glass as the material composing the support member of this invention, general sheet glass whose characteristics are shown in Table 12 or various glasses shown in Table 13 may be used. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 12 
               
               
                   
                   
               
               
                   
                   
                 SiO 2   
                 70-73 
                   
               
               
                   
                   
                 Al 2 O 3   
                 1.0-1.8 
               
               
                   
                   
                 Fe 2 O 3   
                 0.08-0.14 
               
               
                   
                   
                 CaO 
                  7-12 
               
               
                   
                 Composition 
                 MgO 
                 1.0-4.5 
               
               
                   
                 (%) 
                 R 2 O 
                 13-15 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Specific gravity 
                 about 2.5 
               
               
                   
                 Bending strength 
                 about 5 kgf/mm 2   
               
               
                   
                 Young&#39;s modulus 
                 7300 kgf/mm 2   
               
               
                   
                 Poison&#39;s ratio 
                 0.23 
               
            
           
           
               
               
               
               
            
               
                   
                 Hardness 
                 Moh&#39;s 
                 about 6.5 
               
               
                   
                   
                 Vickers 
                 548 kgf/mm 2   
               
            
           
           
               
               
               
            
               
                   
                 Compressive strength 
                 60-120 kgf/mm 2   
               
               
                   
                 Specific heat 
                 0.13 cal/g° C. (0-50° C.) 
               
               
                   
                 Thermal conductivity 
                 0.02 cal/cm sec ° C. (0° C.) 
               
               
                   
                 Thermal expansion coefficient 
                 8.5 × 10 −6 /° C. (20-350° C.) 
               
               
                   
                 Softening point 
                 720-730° C. 
               
               
                   
                 Refraction index 
                 about 1.52 
               
               
                   
                 Reflectance 
                 about 4% 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 13 
               
               
                   
               
               
                   
                 Thermal expansion 
               
               
                 Glass system 
                 coefficient (× 10 −6 /° C.) 
               
               
                   
               
             
            
               
                 PbO · ZnO · B 2 O 2  system 
                 7.0-8.0 
               
               
                 PbO · B 2 O 3  system 
                 7.0-9.0 
               
               
                 Na 2 O · BaO · SiO 2  system 
                 8.5-11.0 
               
               
                 Na 2 O · Al 2 O 3  · BaO · SiO 2  system 
                 7.0-10.0 
               
               
                 PbO · B 2 O 3  · SiO 2  system 
                 7.5-9.5 
               
               
                 Na 2 O · B 2 O 3 · ZnO system   
                 7.0-8.0 
               
               
                 K 2 O · PbO · SiO 2  system 
                 8.0-9.5 
               
               
                 Na 2 O · K 2 O · PbO · SiO 2  system 
                 8.0-10.0 
               
               
                 K 2 O · BaO · SiO 2  system 
                 9.0-10.0 
               
               
                   
               
            
           
         
       
     
     And the material with the approximate thermal expansion ratio should be used in accord with the material of the magnetic disk substrate  315 . For example, if ceramic magnetic disk substrate  315  is used, ceramics with thermal expansion ratio 10×10 −6 /° C. or lower in Table 11 should be used as support member, and similarly, when glass (thermal expansion ratio 8.0-9.0×10 −6 /° C.) magnetic disk substrate is used, it is most suitable that ceramics such as forsterite, etc. with thermal expansion ratio 8×10 −6 /° C. or higher in Table 12 and 13 should be used as support member. 
     In the support member of this invention, it is important to keep the flatness of contact surface  311   a,    312   a to 3 μm or lower and preferably 1 μm or lower and more suitably 0.3 μm or lower, and providing the contact surface  311   a,    312   a  with this kind of excellent flatness enables accurate positioning of each magnetic disk substrate  315  as well as further high-density recording. In addition, with the same reasons, the parallelism between the top and the bottom contact surfaces  311   a,    312   a  should be 5 μm or lower and preferably 3 μm or lower. 
     For example, in order to achieve the 3-μm or lower parallelism between the contact surfaces  311   a,    312   a  of the support member comprising the said ceramic materials, using a double-end grinding machine or polishing machine, the ceramic material should be ground or polished with diamond tools or diamond abrasive grains, and with the ceramic material with high rigidity, it is possible to achieve the flatness and parallelism 3 μm or lower, and preferably 1 μm or lower, and more suitably 0.3 μm or lower. In addition, the surface roughness (Ra) of the ceramic material can be made 2.0 μm or lower and as its surface condition is shown in comparison with that of the metallic materials in FIG. 21, since the surface of the ceramic material is free from any protrusion and only dent exists, the surface has no detrimental effect on the magnetic disk substrate  315  in contact with this surface. 
     Now, the flatness of the contact surface  311   a  of the spacer  311  was varied and the displacement at the peripheral area of the magnetic disk substrate  315  when supported with this spacer  311  was determined. As shown in FIG. 22, when the maximum deflection occurs with the magnetic disk substrate  315  with radius R supported using the spacer  311  with flatness F of the contact surface  311   a  in radius r, the camber angle of the magnetic substrate is expressed by 
     
       
         θ=tam −1  (2 Fr/r   2   −F   2 ) 
       
     
     and if L=R−r, the displacement rate Δg at the peripheral area of the magnetic disk substrate is expressed as 
     
       
         Δ g=L  tan θ+ F = (2 Fr/ ( r 2− F 2))  L+F.   
       
     
     Now let r=11.53 mm and L=20.97 mm and change the value of flatness F, then we have Δg as shown in Table 14. As clear from Table 14, Δg can be remarkably minimized by bringing the flatness F to 3 μm or lower and preferably 1 μm or lower. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 14 
               
               
                   
                   
               
               
                   
                 Flatness F 
                 Δg 
               
               
                   
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 5 
                 23.1 
               
               
                   
                 3 
                 13.9 
               
               
                   
                 1 
                 4.6 
               
               
                   
                 0.1 
                 1.3 
               
               
                   
                   
               
            
           
         
       
     
     In this way, according to this invention, by forming the support member such as shims, spacers, and clamps with ceramics or glass with thermal expansion ratio 20×10 −6 /° C. or lower and preferably 12×10 −6 /° C. or lower and designing the flatness of the surface in contact with the magnetic disk substrate to be 3 μm or lower, the magnetic disk substrate can be supported at high accuracy and the levitation rate of the magnetic head can be reduced to 0.1 μm or lower, and higher density recording is enabled. 
     If this substrate member is combined with ceramics or glass magnetic substrate to form a magnetic disk unit, the thermal expansion ratio of the support member coincides with that of the magnetic disk substrate, and therefore, it is possible to provide a magnetic disk unit with various features including such that deformation of the magnetic disk substrate or loose tightening can be prevented even if high temperature is acquired during application. 
     Now description will be made on embodiments according to this invention. 
     First of all, a spacer, one example of the support member of this invention is shown in FIG.  23 . This spacer  411  is formed in a ring shape made of ceramics, the surface of which the film  421  with hardness  450  kg/mm 2  or lower is equipped, and the contact surface  411   a  with the top and bottom magnetic disk substrates is finished to flatness 5 μm or less and a smooth surface 2.0 μm or less in terms of surface roughness (Ra), and the parallelism of top and bottom contact surfaces  411   a  is kept 5 μm or lower. Each edge  411   b  is chamfered to surface C or R. 
     The shim, one example of support members of this invention is not illustrated, but is of the same profile as that of the spacer  411  but is slightly thinner. 
     Next, the clamp, another example of support member is shown in FIG.  24 . This clamp  412  is a plate type clamp made of ceramics, the bottom surface of which is equipped with film with hardness 450 kg/mm 2  or lower, and the contact surface  412   a  is finished to the flatness 3 μm or lower, and the smooth surface with roughness of 2.0 μm or lower. The outer circumference edge  412   b  is chamfered to surface C or R and a screw hole  412   b  is provided for tightening. In addition, a stepped area  412   d  is provided for being engaged with the hub at the time of installation. 
     Now, the magnetic disk unit using these shims  410 , spacers  411 , and clamps  412  is shown in FIG.  25 . To the flange portion  414   a  formed at the hub  414  connected to the rotary shaft  413 , ceramic spacer  411  and magnetic disk substrate  415  are arranged alternately, and the top ends of these are retained with shims  410  and clamps  412  and tightened with screws  416 , thereby tightening 2 to 8 pieces of magnetic disk substrate  415  are fixed as specified intervals. And with the magnetic head  417  levitating on the surface of the magnetic disk substrate  415  with a minimum distance kept while rotating the hub  414  and each of the magnetic disk substrates  415  by the rotary shaft  413 , information is written into and read from the specified position. 
     For the magnetic disk substrate  415 , in general, the aluminum substrate is used, but magnetic disk substrate with a glazed surface formed on the surface of the ceramics such as alumina and the magnetic film equipped on the glazed surface, or the magnetic disk substrate completely formed with the glass substrate and the magnetic film equipped on the surface may be used. In addition, it is possible to use titanium, silicon, YAG, carbon, etc. for other substrate materials. 
     Because shims  410 , spacers  411 , and clamps  412 , support members of this invention are made of high-rigidity ceramics, they are free from deformation at the time of clamping, and because the contact surface  411   a,    412   a  is finished to the surface with flatness 5 μm or lower, each magnetic disk substrate  415  can be supported at a remarkably high accuracy. Furthermore, because the shims  410 , spacers  411 , and clamps  412 , the support members of this invention, are equipped with comparatively soft film  421 ,  422  with hardness 450 kg/mm 2  or lower on the surface, scraping off of magnetic film on the surface of the magnetic disk substrate  415  can be prevented. 
     In addition, if ceramic disk substrate made of ceramics or glass is used for the magnetic disk substrate  415 , the thermal expansion ratio to shims  410 , spacers  411 , and clamps  412  become approximate one another, and no inconvenience resulting from thermal expansion difference will be generated even if they become high temperature during high-speed rotation. Consequently, it is possible to dramatically reduce the levitation rate of the magnetic head  417  with respect to the magnetic disk substrate and to increase the information recording density. 
     In the example of FIG. 25, the clamp  412  is designed to support the magnetic disk substrate  415  via the shim  410  but in addition to this, it may be designed to support the magnetic disk substrate by directly coming in contact with the top magnetic disk substrate  415 . It may also be designed to bring the hub  414  in contact with the magnetic disk substrate  415  to support. In such event, it is desirable to form the hub  414  with ceramics or glass. 
     Now, for the material of the said film  421 ,  422 , metal material such as cobalt, nickel, chromium, aluminum, silver, platinum, copper, ferritic stainless steel, and the like or synthetic resin or other materials are used, and should be covered on the support member surface by plating or other methods. 
     Table 15 shows Vickers hardness of various metal materials and the results of investigation for any sign of metallic powder generated by scraping off of the magnetic film when they are used as film  412 . The results clearly indicate that metallic power is not generated and scraping off of the magnetic film can be prevented if the Vickers hardness of the film  421  is set to 450 kg/mm 2  or lower and preferably 200 kg/mm 2  or lower. These metal materials provide electric conductivity, providing another effect of easily releasing static electricity generated. 
     In addition, with respect to the thickness of film  421 ,  422 , if it is thinner than 0.3 μm, the effects of reducing the surface hardness are poor, while if it is thicker than 5 μm, it becomes difficult to achieve the flatness of contact surface  411   a,    412   a  to 5 μm or lower. In actuality, as shown in Table 16, when measurements were taken on the flatness of the contact surface  411   a  with the thickness of film  421  varied, it was difficult to lower the flatness when the film  421  was thicker than 5 μm. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 15 
               
               
                   
               
               
                   
                   
                 Vickers hardness 
                 Any sign of metallic 
               
               
                 No. 
                 Material 
                 (kg/mm 2)   
                 powder generation 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 *1 
                 Alumina 
                 1200-1800 
                 Present 
               
               
                 2 
                 Cobalt 
                  147 
                 None 
               
               
                 3 
                 Nickel 
                  90-100 
                 None 
               
               
                 4 
                 Chromium 
                  130 
                 None 
               
               
                 5 
                 Aluminum 
                  15-65 
                 None 
               
               
                 6 
                 Silver 
                   30 
                 None 
               
               
                 7 
                 Platinum 
                   40 
                 None 
               
               
                 8 
                 Copper 
                   35-40 
                 None 
               
               
                 9 
                 Ferritic stainless steel 
                  170-200 
                 None 
               
               
                 10 
                 Austenitic stainless steel 
                  410 
                 None 
               
               
                   
               
               
                 *indicates a comparison example.  
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 16 
               
               
                   
                   
               
               
                   
                 Film thickness 
                 Flatness 
               
               
                   
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 0.9 
               
               
                   
                 2 
                 0.9 
               
               
                   
                 3 
                 1.2 
               
               
                   
                 4 
                 2.1 
               
               
                   
                 5 
                 3.0 
               
               
                   
                 6 
                 4.8 
               
               
                   
                   
               
            
           
         
       
     
     For the materials composing the support member such as shim  410 , spacer  411 , and clamp  412 , it is possible to use ceramics or glass whose thermal expansion coefficient is 20×10 −6 /° C., and more preferably 12×10 −6 /° C. or less, but for ceramics, as the characteristics are shown in Table 17, alumina ceramics, zirconia ceramics, silicon carbide ceramics, silicon nitride ceramics, alumina-titanium carbide based ceramics, barium titanate, cermet, forsterite ceramics, and others can be used. 
     Alumina ceramics in Table 17 is a sintered material which contains 90 wt % Al 2 0 3  and the remainder comprising Si0 2 , Mg0, Ca0, etc. The conductive alumina ceramics is a sintered material which contains 70-80 wt % Al 2 O 3  and the remainder comprising 10-20 wt % Ti0 2  as a conductivity provider, and the conductive alumina ceramics sintered in the oxidizing atmosphere provides 10 8  Ω·cm volume specific resistance and that sintered in the reducing atmosphere provides 10 3 -10 6  Ω·cm volume specific resistance. 
     Zirconia ceramics primarily consists of Zr0 2  and is partially stabilized zirconia ceramics in which the tetragonal phase is 80 mol % or higher by containing stabilizers such as Y 2 0 3 , Ca0, Mg0, etc. 
     In addition, silicon carbide ceramics contains more than 90 wt % SiC and the remainder comprising carbon (C) and boric acid (B), or Al 2 0 3 , Y 2 0 3 , etc., while silicon nitride ceramics contains more than 90 wt % Si3N4 and the remainder comprising Al 2 0 3 , Y 2 0 3 , etc. 
     The alumina-titanium carbide based ceramics is a sintered material primarily composed of 20-80 wt % Al 2 0 3  and 80-20 wt % TiC and provides high hardness and electric conductivity. In addition, barium titanate is primarily composed of 10-20 mol % BaO and 90-80 mol % Ti0 2 , and contains at least one type of metallic oxide chosen from Al, Si, Zr, Nb, and Sr at a ratio of 0.01-4.0 parts in weight to 100 parts in weight of the main component, and is fired in the reducing atmosphere. 
     Cermet is a composite sintered material comprising a ceramic component which forms a hard phase and a metal component which forms a binder phase, and in particular cermet composed of 10-90 wt % TiC and 5-90 wt % TiN, and containing a 5a group metal carbide as an additive and iron group metal as a binder phase is used. 
     In addition, forsterite ceramics is a sintered material primarily composed of 2 Mg0·Si0 2  and because the Vickers hardness is as low as 1000 kg/mm 2  or lower, forsterite ceramics can be used to prevent damage to magnetic disk substrate  415 . 
     Comparison of the properties of these ceramics with aluminum (metal) used as a comparison example indicates that as clear from Table 11, all these ceramics are difficult to deform at the time of tightening because of their high Young&#39;s modulus 13000 kg/mm 2  or higher and also difficult to generate inconvenience at the time of high temperature because their thermal expansion ratio is as small as 12×10 −6 /° C. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 17 
               
               
                   
                   
               
               
                   
                   
                 Young&#39; 
                 Vickers 
                 Thermal 
                 Volume specific 
               
               
                   
                 Specific 
                 modulus 
                 hardness 
                 conductivity 
                 resistance 
               
               
                   
                 gravity 
                 (kg/mm 2 ) 
                 (kg/mm 2 ) 
                 × 10 −6 /° C. 
                 Ω · cm 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Alumina 
                 2.7 
                 7200 
                 65 
                 24.2 
                 10 −8   
               
               
                 Alumina ceramics 
                 3.5-4.0 
                 25000-40000 
                 1200-1800 
                 6.5-8.0 
                 10 13 -10 15   
               
               
                 Conducive alumina 
                 3.5-4.0 
                 25000-40000 
                 1000-1200 
                 6.5-8.0 
                 10 3 -10 8   
               
               
                 Zriconia ceramics 
                 5.5-6.0 
                 20000-26000 
                 1200-1400 
                  9.5-11.5 
                 10 13 -10 15   
               
               
                 Silicon carbide 
                 2.8-3.5 
                 35000-45000 
                 2300-2500 
                 3.5-4.5 
                 10 2 -10 6   
               
               
                 ceramics 
               
               
                 Silicon nitride 
                 2.8-3.5 
                 30000-35000 
                 1400-1600 
                 2.5-3.5 
                 10 13 -10 15   
               
               
                 ceramics 
               
               
                 Alumina-titanium 
                 4.0-4.5 
                 40000-45000 
                 1850-1950 
                 6.5-8.0 
                 10 −2 -10 −1   
               
               
                 carbide ceramics 
               
               
                 Barium titanate 
                 4.0-4.5 
                 17000-20000 
                 1500-1900 
                  8.0-10.0 
                 10 2 -10 6   
               
               
                 Cermet 
                 5.5-7.5 
                 40000-50000 
                 1400-1800 
                 7.0-8.0 
                 10 −4 -10 −1   
               
               
                 Forsterite ceramics 
                 2.7-3.1 
                 13000-16000 
                  700-1000 
                  8.0-12.0 
                 10 14 - 
               
               
                   
               
            
           
         
       
     
     And the material with the approximate thermal expansion ratio should be used in accord with the material of the magnetic disk substrate  415 . For example, if ceramic magnetic disk substrate  415  is used, ceramics with thermal expansion ratio 10×10 −6 /° C. or lower in Table 17 should be used as support member, and similarly, when glass (thermal expansion ratio 8.0-9.0×10 −6 /° C.) magnetic disk substrate is used, it is most suitable that ceramics such as forsterite, etc. with thermal expansion ratio 8×10 −6 /° C. or higher in Table 15 should be used as support member. 
     In the support member of this invention, it is important to keep the flatness of contact surface  411   a,    412   a  to 5 μm or lower and preferably 1 μm or lower and more suitably 0.3 μm or lower, and providing the contact surface  411   a,    412   a  with this kind of excellent flatness enables accurate positioning of each magnetic disk substrate  415  as well as further high-density recording. In addition, with the same reasons, the parellelism between the top and the bottom contact surfaces  411   a,    412   a  should be 5 μm or lower and preferably 3 μm or lower. 
     For example, in order to achieve the 5-μm or lower parallelism between the contact surfaces  411   a,    412   a  of the support member comprising the said ceramic materials, using a double-end grinding machine or polishing machine, the ceramic material should be ground or polished with diamond tools or diamond abrasive grains, and with the ceramic material with high rigidity, it is possible to achieve the flatness and parallelism 4 μm or lower and preferably 1 μm or lower, and more suitably 0.3 μm or lower. And if film  412 ,  422  is formed 0.3-5 μm or lower in thickness on this, both the flatness and parallelism of the surface can be brought to 5 μm or lower. 
     In this way, according to this invention, since the support member such as shims, spacers, and clamps is formed with ceramics with thermal expansion ration 20×10 −6 /° C. or lower and preferably 12×10 −6 /° C. or lower and the film with Vickers hardness 450 kg/mm 2  or lower is equipped on the contact surface with the magnetic disk substrate, and the flatness of this contact surface is designed to be 5 μm or lower, the magnetic disk substrate can be supported at high accuracy and the levitation rate of the magnetic head can be reduced to 0.1 μm or lower, thereby enabling still higher density recording. 
     This invention also can provide magnetic disk units with various features such that because the surface hardness of the support member is low, there is no fear of the support member to scrape off the magnetic film of the magnetic disk substrate and the generation of metallic powder can be prevented.