Magnetic disk substrate and method for manufacturing the same
This application is a continuation-in-part of Ser. No. 08/248,757, filed May 25, 1994, now U.S. Pat. No. 5,391,522, which is a continuation of Ser. No. 08/092,551, filed Jul. 16, 1993, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a magnetic disk substrate made of a glass-ceramic having a crystal structure with improved surface characteristics after polishing, a magnetic disk formed by subjecting the magnetic disk substrate to a film forming process and a method for manufacturing the magnetic disk substrate. 
     There has been a rapid progress in developments in magnetic disks demand for which has recently increased remarkably as external media of large scale computers and personal computers. Characteristics required generally for a magnetic disk substrate are as follows: 
     (1) In CSS (contact start and stop) characteristics of a magnetic disk, stiction tends to occur between the head and the magnetic disk due to increase in contact resistance caused by high speed rotation of the magnetic disk, if the magnetic disk has a smooth surface with surface roughness (Ra) below 15 Å. The magnetic disk therefore is required to have surface roughness of 15 Å or more. Moreover, if the magnetic disk has an excessively rough surface having surface roughness exceeding 50 Å, damage to the head or the recording media tends to occur. The surface roughness of the magnetic disk substrate therefore should not exceed 50 Å. 
     (2) Since the amount of lifting of the head tends to decrease to the order of 0.1 μm to 0.05 μm owing to improvement of recording density of the magnetic disk, the surface of the disk must be flat and relatively smooth. 
     (3) The material of the magnetic disk substrate must not include anisotropic crystal or other defects and its structure must be dense, homogeneous and fine. 
     (4) The material must have sufficient mechanical strength and hardness for standing a high speed rotation and contact with the head. 
     (5) The material must not contain Na 2  O ingredient in principle because the Na 2  O ingredient causes dispersion of sodium ion during the film forming process with resulting deterioration in characteristics of the film. 
     (6) The material must have chemical durability against rinsing and etching with various chemicals. 
     Aluminum alloy has been conventionally used as a material of magnetic disk substrate. The alluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the alluminum alloy substrate is not sufficient in flatness and surface roughness and cannot cope with the recent requirement for high density recording necessitated by increase in the amount of information. 
     As a material for overcoming this problem of the alluminum alloy substrate, known in the art is a glass substrate for magnetic disks made of chemically tempered glass. This glass substrate, however, has the following disadvantages: 
     (1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the disk substrate. 
     (2) For improving the CSS characteristics, the substrate must be subjected to mechanical or chemical processing for making texture. This prevents mass production of the product at a competitive cost. 
     (3) Since the Na 2  O ingredient is included as an essential ingredient in the glass, the film forming characteristic of the glass is deteriorated with the result that a surface coating processing becomes necessary. It has recently been put into practice in the field of chemically tempered glass and crystallized glass-ceramic to make the surface of the glass rough during the polishing process for overcoming the problem of the texture processing for improving the CSS characteristics. This technique however is not sufficient either for realizing a stable mass production of the product at a competitive cost. 
     Aside from the alluminum alloy substrate and chemically tempered glass substrate, known also in the art are glass-ceramics which satisfy some of the above described requirements. For example, the SiO 2  --Al 2  O 3  --Li 2  O system crystallized glass-ceramic described in the Japanese Patent Application Laid-open No. Sho 60-229234 contains beta-quartz solid solution or beta-spodumene solid solution and has a grain diameter of crystal ranging from about 0.1 μm to about 1.0 μm. For another example, the SiO 2  --Li 2  O system glass-ceramic described in the Japanese Patent Application Laid-open No. Sho 62-72547 contains Li 2  O.2SiO 2  and Li 2  O.SiO 2  as the main crystalline phase. The Li 2  O.2SiO 2  phase has a rod-like crystal having a grain diameter of crystal in the range from about 0.3 μm to about 1.5 μm and the Li 2  O.SiO 2  phase has a granular crystal having a grain diameter of crystal in the range from about 0.3 μm to about 0.5 μm. U.S. Pat. No. 3,231,456 discloses a SiO 2  --Li 2  O--P 2  O 5  --MgO system glass-ceramic added with CuO and SnO ingredients in which Li 2  O.2SiO 2  is grown as a main crystal phase and alpha-quartz may be grown as a sub crystal phase. Since, however, these glass-ceramics require a heat treatment for crystallization in a high temperature range from 850° C. to 1050° C., they cannot grow a globular grain strucuture of aggregated particles as the grain structure of alpha-quartz as the glass-ceramic made according to the present invention as will be discussed in detail later. Besides, in U.S. Pat. No. 3,231,456, the crystal phase and crystal structure are discussed for providing a ceramic material which is suitable for use as a bonding material. 
     U.S. Pat. No. 3,977,857 discloses a glass-ceramic of a SiO 2  --Li 2  O--MgO--P 2  O 5  --(Na 2  O+K 2  O) system which, as a metal bonding glass-ceramic, is suitable for use as an electric insulating material which can be bonded directly to a metal member. The specification of this U.S. patent teaches that the main crystal phase of the glass-ceramic obtained is Li 2  O.2SiO 2  and does not teach anything about the globular grain structure of aggregated particles as the grain structure of alpha-quartz as in the present invention. 
     Japanese Patent Application Laid-open No. Sho 63-210039 discloses a glass-ceramic of a SiO 2  --Li 2  O--MgO--P 2  O 5  system which is suitable for use as a magnetic head substrate. The specification of this publication teaches that the main crystal phases of the glass-ceramic obtained are Li 2  O.2SiO 2  and alpha-cristobalite and does not teach anything about the globular grain structure of aggregated particles as the grain structure of alpha-quartz as in the present invention. Besides, the nucleating temperature is 550° C.-800° C. and the MgO ingredient is added for the purpose of restricting variation in a coefficient of thermal expansion by coexistence with the P 2  O 5  ingredient. 
     In short, crystal phases growing in the prior art Li 2  O--SiO 2  system glass-ceramics (i.e., Japanese Patent Application Laid-open No. Sho 62-72547, U.S. Pat. Nos. 3,231,456, 3,977,857 and Japanese Patent Application Laid-open No. 63-210039) are Li 2  O.2SiO 2  which constitutes the main crystal phase and a small amount of SiO 2  (alpha-cristobalite or alpha-quartz) which constitutes the sub crystal phase. Therefore, in these prior art glass-ceramics, the Li 2  O.2SiO 2  crystal phase plays a major role and the sub crystal phase of alpha-quartz or alpha-cristobalite does not. These prior art glass-ceramics cannot provide the surface roughness of 15 Å to 50 Å which is necessary for the CSS characteristics of the magnetic disk as the surface roughness inherent in the glass-ceramics after polishing. These prior art glass-ceramics require some kind of texture processing for making the surface of the glass-ceramics rough after the polishing process for improving the CSS characteristics which are essential for the material of a magnetic disk substrate. 
     It is, therefore, an object of the invention to provide a magnetic disk substrates made of a glass-ceramic which has eliminated the disadvantages of the prior art products and has excellent surface characteristics after polishing by controlling the crystal structure and crystal grain of a crystal produced. It is also an object of the invention to provide a magnetic disk made by subjecting this magnetic disk substrate to a film forming processing. It is also an object of the invention to provide a method for manufacturing such a magnetic disk substrate. 
     SUMMARY OF THE INVENTION 
     Accumulated studies and experiments made by the inventors of the present invention for achieving the above described object of the invention have resulted in the finding, which has led to the present invention, that a glass-ceramic obtained by subjecting to heat treatment, within a specific temperature range, a base glass containing, as an essential ingredient, a MgO ingredient in the SiO 2  --Li 2  O--P 2  O 5  system has, as its crystal phases, a microstructure in which globular grains each being made of aggregated particles of alpha-quartz (alpha-SiO 2 ) have randomly grown in a uniformly grown crystal phase of lithium disilicate (Li 2  O.2SiO 2 ), that, since this microstructure consists of the lithium disilicate phase which is mechanically and chemically instable and the alpha-quartz phase of the globular grains each being made of aggregated particles which is mechanically and chemically stable, this microstructure provides a rough surface which is produced by mechanical and chemical actions performed during the polishing process and that a magnetic disk substrate having excellent surface characteristics after polishing is obtainable by controlling the size of the globular grains of aggregated particles of alpha-quartz. 
     A magnetic disk substrate achieving the above described object of the invention is made of a glass-ceramic having crystal phases of lithium disilicate (Li 2  O.2SiO 2 ) and alpha-quartz (SiO 2 ) characterized in that said crystal phase of alpha-quartz is formed of grown crystal grains each having a globular grain structure made of aggregated particles, each globular grain having a size within a range from 0.3 μm to 3 μm and that surface roughness (Ra) of a polished surface of the magnetic disk substrate is within a range from 15 Å to 50 Å. 
     In one aspect of the invention, this magnetic disk substrate is formed by subjecting to heat treatment a base glass which consists of in weight percent: 
     
         ______________________________________                                    
SiO.sub.2              65-83%                                             
Li.sub.2 O             8-13%                                              
K.sub.2 O              0-7%                                               
Mgo + ZnO + PbO        0.5-5.5%                                           
in which MgO           0.5-5.5%                                           
ZnO                    0-5%                                               
PbO                    0-5%                                               
P.sub.2 O.sub.5        1-4%                                               
Al.sub.2 O.sub.5       0-7%                                               
As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3                                       
                       0-2%                                               
______________________________________                                    
 
    
     In another aspect of the invention, there is provided a magnetic disk consisting essentially of the above described substrate having a coating of magnetic media formed on the surface thereof. 
     In another aspect of the invention, there is provided a method for manufacturing the above described substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The file of this patent contains at least one drawing executed in color. Copies of this patent with colored drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. 
     FIG. 1A is a graph which shows shows the surface roughness of the glass ceramic of Example 4 in Table 1, after polishing. 
     FIG. 1B is a graph which shows the surface roughness of the glass ceramic of Comparative Example 1 in Table 1. 
     FIG. 1C is a graph which shows the surface roughness of the glass ceramic of Comparative Example 2 in Table 1. 
     FIG. 2A is a photomicrograph of a scanning electron microscope image of the glass ceramic of Example 4 after hydrofluoric acid etching at 10,000 times magnification. 
     FIG. 2B is a photomicrograph of a scanning electron microscope image of the glass ceramic of Example 4 after hydrofluoric acid etching at 50,000 times magnification. 
     FIG. 2C is a photomicrograph of a scanning electron microscope image of the glass ceramic of Comparative Example 1. 
     FIG. 2D is a photomicrograph of a scanning electron microscope image of the glass ceramic of Comparative Example 2 at 10,000 times magnification. 
     FIG. 2E is a photomicrograph of a scanning electron microscope image of the glass ceramic of Comparative Example 2 at 50,000 times magnification. 
     FIG. 3 is a graph which shows the relation between the temperature of crystallization and the surface roughness of the aggregated particles of alpha quartz in the glass ceramic of Example 5. 
     FIG. 4 is a three-dimensional image obtained from an atomic force microscope of the surface glass ceramic of Example 5. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While various compostions may be used as a base glass composition for producing the magnetic disk substrate of the present invention having the above described characteristic feature, one preferred composition range of a glass-ceramic used for producing a magnetic disk substrate of the present invention will be described below. This composition is expressed on the basis of composition of oxides as in its base glass. The above described content ranges of the respective ingredients have been selected for the reasons stated below. 
     The SiO 2  ingredient is an important ingredient which produces alpha-quartz (SiO 2 ) and lithium disilicate (Li 2  O.2SiO 2 ) as crystal phases by heat treating the base glass. If the amount of this ingredient is below 65%, the crystal produced in the glass-ceramic is instable and its texture tends to become too rough if the amount of this ingredient exceeds 83%, difficulty arises in melting the base glass. As a result of experiments, it has been found that a particularly preferable range of this ingredient is from 70% to 82%. 
     The Li 2  O ingredient is also an important ingredient which produces lithium disilicate (Li 2  O.2SiO 2 ) crystal as a crystal phase by heat treating the base glass. If the amount of this ingredient is below 8%, growth of this crystal becomes difficult and melting of the base glass becomes also difficult. If the amount of this ingredient exceeds 13%, the grown crystal of the glass-ceramic becomes instable and its texture tends to become too rough and, besides, chemical durability and hardness of the glass-ceramic are deteriorated. It has been found that a particularly preferable range of this ingredient is from 8% to 12% and the most preferable range is from 8% to 11%. 
     The K 2  O ingredient is an ingredient which improves melting property of the base glass and can be included up to 7%. A particularly preferable range of this ingredient is from 1% to 6%. 
     The MgO ingredient is an important ingredient which has been found in the present invention to cause a globular crystal grain of aggregated particles of alpha-quartz (SiO 2 ) as the crystal phase to grow randomly in the entire glass. If the amount of this ingredient is below 0.5%, the above described effect cannot be obtained whereas if the amount of this ingredient exceeds 5.5%, growth of a desired crystal becomes difficult. A particularly preferable range is from 1% to 5%. 
     The ZnO and PbO ingredients may also be added because they have a function similar to that of MgO. If the amount of the ZnO or PbO ingredient exceeds 5%, growth of a desired crystal becomes difficult. A particularly preferable range of ZnO is from 0.2% to 5%. 
     For the same reason, the total amount of the MgO, ZnO and PbO ingredients should be within the range from 0.5 to 5.5%. 
     The P 2  O 5  ingredient is an essential ingredient as nucleating agent for the glass-ceramic. If the amount of this ingredient is below 1%, a desired crystal cannot be produced whereas if the amount of this ingredient exceeds 4%, the grown crystal of the glass-ceramic becomes instable and too rough and, besides, its stability against devitrification is deteriorated. A particularly preferable range of this ingredient is from 1% to 3%. 
     The Al 2  O 3  ingredient is an effective ingredient for improving chemical durability of the crystallized glass-ceramic. If the amount of this ingredient exceeds 7%, the melting property of the base glass is deteriorated and, as a result, the rate of crystal growth of alpha-quartz (alpha-SiO 2 ) as the main crystal phase is reduced. A particularly preferable range of this ingredient is from 1% to 6%. 
     The As 2  O 3  and/or Sb 2  O 3  ingredients may be added as refining agents in melting the base glass. It will suffice if the total amount of one or both of these ingredients up to 2% is added to the glass. 
     In addition to the above described ingredients, one or more of B 2  O 3 , CaO, SrO, BaO, TiO 2  and ZrO 2  ingredients may be added in a small amount within the range in which desired properties of the glass-ceramic according to the invention will not be impaired. 
     For manufacturing the magnetic disk substrate according to the invention, the base glass having the above described composition is molten and after being subjected to heat forming and/or cold forming, is heat treated for producing a crystal nucleus under a temperature within a range from 450° C. to 540° C. and is further heat treated for crystallization under a temperature within a range from 700° C. to 840° C. 
     If the temperature of nucleation is below 450° C., nucleation caused by phase separation due to the P 2  O 5  ingredient is insufficient whereas if the temperature of nucleation exceeds 540° C., microcrystals of Li 2  O.SiO 2  which grow as the crystal nucleus fail to grow uniformly and also become gross crystal nucleus and, for this reason, such gross crystal nucleus of lithium monosilicate prevents aggregation of small particles of alpha quartz which subsequently grow and causes them to scatter and grow as monospherical grains of alpha-quartz instead of globular grains each being made of aggregated particles. 
     The temperature of crystallization, associated with the effect caused by the MgO ingredient, performs a very important function for controlling the size of the globular grain of aggregated particles of alpha-quartz (alpha-SiO 2 ). If the temperature of crystallization is below 700° C., the crystal of alpha-quartz will not grow sufficiently whereas if the temperature of crystallization exceeds 840° C., it becomes difficult to maintain the globular grain structure of aggregated particles with resulting failure in obtaining the above described advantage of the invention. The glass-ceramic thus heat treated is then subjected to the lapping process and finally polishing process provide a magnetic disk substrate and the surface roughness (Ra) after final polishing comes within the range from 15 Å to 50 Å. 
     EXAMPLES 
     Preferred examples of the glass-ceramic according to the invention will now be described. Table 1 shows examples (No. 1 to No. 13) of the glass-ceramic for a magnetic disk substrate according to the invention and comparative examples of the prior art SiO 2  --Li 2  O--Al 2  O 3  --P 2  O 5  system glass-ceramic (Japanese Patent Application Laid-open No. Sho 62-72547, Comparative Example 1), the SiO 2  --Li 2  O--MgO--P 2  O 5  system glass-ceramic (U.S. Pat. No. 3,231,456, Comparative Example 2) and the SiO 2  --Li 2  O--MgO--P 2  O 5  system glass-ceramic (Japanese Patent Application Laid-open No. Sho 63-210039, Comparative Example 3) together with results of measurements of the temperatures of the heat treatment, the time of the heat treatment, the diameter of grown crystal grain of alpha-quartz, main crystal phases and surface roughness (Ra) after polishing. In the Table, α-Q represents alpha-quartz(α-SiO 2 ) and α-Cri represents alpha-cristobalite (α-SiO 2 ). 
     
                                           TABLE 1                                 
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(weight %)                                                                
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                 Examples No.                                             
                 1     2     3     4     5     6                          
__________________________________________________________________________
SiO.sub.2        67.5  69.0  69.5  73.0  76.7  75.5                       
Li.sub.2 O       10.0  9.0   9.0   8.0   10.5  10.0                       
Al.sub.2 O.sub.3 6.0   5.0   5.5   5.0   3.8   4.5                        
MgO              2.5   3.5   1.0   4.0   2.5   1.0                        
ZnO              1.0   0.5   0.5   0.5   0.5                              
PbO              1.5   1.5   4.0               2.0                        
K.sub.2 O        6.0   7.0   6.0   4.0   3.5   4.5                        
P.sub.2 O.sub.5  2.0   1.5   2.0   4.0   2.0   2.0                        
As.sub.2 O.sub.3 0.5   0.5         0.5   0.5   0.5                        
Sb.sub.2 O.sub.3             0.5                                          
Others           CaO 1.0                                                  
                       BaO 1.5                                            
                             TiO.sub.2 2.0                                
                                   SrO 1.0                                
                 B.sub.2 O.sub.3 2.0                                      
                       ZrO.sub.2 1.0                                      
Temperature of nucleation (°C.)                                    
                 480   450   450   540   480   500                        
Time of nucleation (Hr)                                                   
                 2     2     2     5     5     1                          
Temperature of crystallization (°C.)                               
                 750   760   770   780   750   750                        
Time of crystallization (Hr)                                              
                 2     3     2     3     3     2                          
Crystal phase    α-Q                                                
                       α-Q                                          
                             α-Q                                    
                                   α-Q                              
                                         α-Q                        
                                               α-Q                  
                 Li.sub.2 O.2SiO.sub.2                                    
                       Li.sub.2 O.2SiO.sub.2                              
                             Li.sub.2 O.2SiO.sub.2                        
                                   Li.sub.2 O.2SiO.sub.2                  
                                         Li.sub.2 O.2SiO.sub.2            
                                               Li.sub.2 O.2SiO.sub.2      
Diameter of α-Q of aggregated                                       
                 0.7   0.6   0.9   1.3   1.2   0.8                        
particles (μm)                                                         
Surface roughness Ra (Å)                                              
                 16    15    18    35    32    18                         
__________________________________________________________________________
                 Examples No.                                             
                 7     8     9     10    11    12                         
__________________________________________________________________________
SiO.sub.2        76.0  76.5  78.5  82.0  76.7  75.7                       
Li.sub.2 O       10.5  10.5  9.0   11.0  10.5  11.0                       
Al.sub.2 O.sub.3 4.0   3.5   2.0   1.3   3.8   3.8                        
MgO              2.5   1.0   2.5   1.0   2.5   3.0                        
ZnO              0.5   2.0   1.0         0.5   0.5                        
PbO                          1.0                                          
K.sub.2 O        4.0   4.0   4.0   2.5   4.0   4.3                        
P.sub.2 O.sub.5  2.0   2.0   1.5   1.7   2.0   2.0                        
As.sub.2 O.sub.3 0.5   0.5   0.5   0.5   0.5   0.5                        
Sb.sub.2 O.sub.3                                                          
Others                                                                    
Temperature of nucleation (°C.)                                    
                 500   540   500   540   540   540                        
Time of nucleation (Hr)                                                   
                 5     5     5     5     5     5                          
Temperature of crystallization (°C.)                               
                 780   780   780   840   780   770                        
Time of crystallization (Hr)                                              
                 2     5     2     5     2     2                          
Crystal phase    α-Q                                                
                       α-Q                                          
                             α-Q                                    
                                   α-Q                              
                                         α-Q                        
                                               α-Q                  
                 Li.sub.2 O.2SiO.sub.2                                    
                       Li.sub.2 O.2SiO.sub.2                              
                             Li.sub.2 O.2SiO.sub.2                        
                                   Li.sub.2 O.2SiO.sub.2                  
                                         Li.sub.2 O.2SiO.sub.2            
                                               Li.sub.2 O.2SiO.sub.2      
Diameter of α-Q of aggregated                                       
                 1.2   1.5   1.4   1.3   1.0   1.1                        
particles (μm)                                                         
Surface roughness Ra (Å)                                              
                 30    40    36    33    20    22                         
__________________________________________________________________________
                       Examples                                           
                             Comparative Examples                         
                       No.                                                
                       13    1       2       3                            
__________________________________________________________________________
SiO.sub.2              75.7  71.0    80.1    79.6                         
Li.sub.2 O             11.0  12.0    10.7    8.3                          
Al.sub.2 O.sub.3       3.3   9.6             3.0                          
MgO                    3.0           3.6     1.5                          
ZnO                    0.5                                                
PbO                          0.9                                          
K.sub.2 O              4.0                   3.5                          
P.sub.2 O.sub.5        2.0   1.9     2.6     2.9                          
As.sub.2 O.sub.3       0.5                   0.3                          
Sb.sub.2 O.sub.3                                                          
Others                       Na.sub.2 O 3.8                               
                                     CuO 2.0 Na.sub.2 O 1.0               
                             CaO 0.4 SnO 1.0                              
                             SrO 0.4                                      
Temperature of nucleation (°C.)                                    
                       540   500     550     570                          
Time of nucleation (Hr)                                                   
                       5     5       5       1                            
Temperature of crystallization (°C.)                               
                       740   840     850     810                          
Time of crystallization (Hr)                                              
                       6     2       2       2                            
Crystal phase          α-Q                                          
                             Li.sub.2 O.2SiO.sub.2                        
                                     Li.sub.2 O.2SiO.sub.2                
                                             Li.sub.2 O.2SiO.sub.2        
                       Li.sub.2 O.2SiO.sub.2                              
                             α-cri                                  
                                     α-Q                            
                                             α-cri                  
Diameter of α-Q of aggregated                                       
                       1.5   No aggregated                                
                                     No aggregated                        
                                             No aggregated                
particles (μm)            particles                                    
                                     particles                            
                                             particles                    
Surface roughness Ra (Å)                                              
                       35    11      10      10                           
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     In the accompanying drawings, FIG. 1A shows the surface roughness (Ra) after polishing of the glass-ceramic of Example 4 in Table 1, FIG. 1B that of the glass-ceramic of the Comparative Example No. 1 in Table 1, and FIG. 1C that of the glass-ceramic of the Comparative Example No. 2 in Table 1 for comparison. FIGS. 2A and 2B show SEM (scanning electron microscope) images of the crystal structure after HF etching of the glass-ceramic of Example 4 and FIG. 2C that of the glass-ceramic of the Comparative Example No. 1 and FIGS. 2D and 2E those of the glass-ceramic of the Comparative Example No. 2, respectively for comparison. FIG. 3 is a graph showing relationship between the temperature of crystallization and the surface roughness (Ra) of the aggregated particles of alpha-quartz in the glass-ceramic of Example No. 5. FIG. 4 is a three-dimensional image by AFM (atomic force microscope) of the glass-ceramic of the Example No. 5 having the surface roughness (Ra) of 32 Å after polishing. It will be seen from FIG. 4 that the globular crystal grains each made of aggregated particles of alpha-quartz form minute projections scattered over the entire surface of the magnetic disk substrate. 
     For manufacturing the glass-ceramics of the above described examples, materials including oxides, carbonates and nitrates are mixed and molten in a conventional melting apparatus at a temperature within the range from about 1350° C. to about 1500° C. The molten glass is stirred to homogenize it and thereafter formed into a desired shape and annealed to provide a formed glass. Then, this formed glass is subjected to heat treatment to produce the crystal nucleus under a temperature within the range from 450° C. to 540° C. for about one to five hours and then is subjected to further heat treatment for crystallization under a temperature within the range from 700° C. to 840° C. for about one to five hours to produce the desired glass-ceramic. For producing a magnetic disk substrate having the surface roughness (Ra) from 15 Å to 50 Å, the above described glass-ceramic is lapped for about 10 to 20 minutes with grains having average grain diameters ranging from 9 μm to 12 μm and then is finally polished for about 30 to 40 minutes with cerium oxide having average grain diameters ranging from 1 μm to 2 μm. 
     The glass-ceramic substrate is subjected to a film forming process which is known per se and thereby is formed into a high recording density magnetic disk. More specifically, the glass-ceramic substrate is heated in a vacuum state and subsequently is coated, by a sputtering process, with an intermediate layer of cromium, a magnetic layer of cobalt alloy and a protective layer of carbon and then is coated with a lubricating material layer on the surface of the protective layer to provide a magnetic disk. 
     As shown in Table 1, the glass-ceramic of Example No. 4 has a desired surface roughness but the glass-ceramics of the Comparative Examples Nos. 1 to 3 have no such desired surface roughness. As will be apparent from FIGS. 1A, 1B and 1C, the glass-ceramic of Example No. 4 has a desired surface roughness but the glass-ceramic of the Comparative Examples Nos. 1 and 2 do not. As will be apparent from the SEM images of FIGS. 2A, 2B and 2C, the alpha-quartz crystal grains of the glass-ceramic of Example No. 4 exhibit the globular grain structure of aggregated particles and are projecting from the surface of the glass-ceramic whereas the crystal grains of the Comparative Example No. 1 exhibit the acicular or rod-like grain structure and, accordingly, the desired surface roughness cannot be obtained. Similarly, the crystal grains of the Comparative Example No. 3 exhibit the acicular or rod-like grain structure and the desired surface roughness cannot be obtained. Further, as shown in FIGS. 2D and 2E, the crystal grains of the Comparative Example No. 2 exhibit a non-aggregated mono-spherical structure in which a desired surface roughness cannot be obtained and the grain diamter cannot be controlled freely as in the applicant&#39;s invention as will be described more fully later. As to the grain diameter of the glass-ceramic according to the invention, the grain diameter can be controlled to the range from 0.3 μm to 3 μm by varying heat treatment conditions. 
     Difference between the globular grain structure made of aggregated particles of alpha-quartz according to the applicant&#39;s invention and the non-aggregated mono-spherical grain structure of alpha-quartz of Comparative Example No. 2 will be described more fully below. 
     As will be apparent from the SEM images of the glass-ceramic of Example No. 4 shown in FIG. 2B, each of alpha-quartz grains of the present invention has a globular structure which is composed of many minute particles aggregated into a globe. These alpha-quartz grains are randomly scattered in the fine particles of lithium disilicate (Li 2  O.2SiO 2 ) as shown in FIG. 2A. This state has been found quite beneficial for controlling the surface roughness (Ra) after polishing as will be described below. In contrast, as will be seen from the scanning microscopic photographs of FIGS. 2D and 2E, it has been confirmed by X-ray diffraction analysis that in the glass-ceramic of Comparative Example No. 2, the lithium disilicate (Li 2  O.2SiO 2 ) crystal grains and the alpha-quartz (alpha-SiO 2 ) crystal grains grow but distinction in the state of growing between these two different crystal grains is not so clear as in the glass-ceramic of the present invention and, besides, these two crystal grains do not assume definite shapes as in the glass-ceramic of the present invention. 
     The surface roughness control characteristics of the magnetic disk substrate made according to the invention are obtained by the specific crystal structure of the glass-ceramic in which the globular crystal grains each being formed of aggregated small particles of alpha-quartz which is inherently stronger than lithium disilicate physically and chemically are randomly scattered in the much smaller crystal grains of lithium disilicate. This specific structure causes the lithium disilicate grains to be ground more quickly than the alpha quartz grains in the polishing process and, as a result, the alpha-quartz grains come to project relatively prominently over the surface of the lithium disilicate grains as the polishing process progresses whereby surface roughness (Ra) of a relatively large value can be obtained. It will be clearly seen from the three-dimensional image by AFM shown in FIG. 4 that the globular grains of aggregated small particles of physically and chemically strong alpha-quartz project randomly over the entire polished surface. 
     An additional advantage of the globular grain structure formed of aggregated particles of alpha-quartz in the glass-ceramic which constitutes the magnetic disk substrate of the invention is that the grain diameter can be variably controlled by changing the temperature of crystallization to such a degree that any desired grain diameter can be achieved. This is because growth of the crystal grain of alpha-quartz in the present invention depends largely upon the degree of aggregation of small particles and this aggregation depends upon the temperature of crystallization employed. Such fine control of the grain diameter can never be achieved in the prior art glass-ceramic of the Comparative Example No. 2 in which the crystal grains are mono-spherical and do not assume definite shapes, though both the lithium disilicate and alpha-quartz crystal phases are produced in the glass-ceramic. 
     In summing up, in one preferred embodiment of the magnetic disk substrate according to the invention is obtained by heat treating a SiO 2  --Li 2  O--P 2  O 5  --MgO system glass of a specific composition range and, therefore, the crystal phase thereof consists of alpha-quartz (alpha-SiO 2 ) and the lithium disilicate (Li 2  O.2SiO 2 ). The grown crystal grain of alpha-quartz has a globular grain structure formed of aggregated particles which facilitates control of the grain size by heat treatment and achievement of a desired surface roughness of the polished surface whereby the glass-ceramic can be utilized as an excellent magnetic disk substrate. 
     According to the method for manufacturing the magnetic disk substrate according to the invention comprising the steps of melting and forming the above described base glass and then subjecting the formed glass to heat treatment for producing a crystal nucleus under a temperature within the range from 450° C. and 540° C. and then to a further heat treatment for crystallization under a temperature within a range from 700° C. to 840° C., the glass-ceramic which has the globular grains formed of aggregated particles and thereby has excellent characteristics can be manufactured at good yield.