Abstract:
A glass composition is disclosed which has a high modulus of elasticity (Young&#39;s modulus) and a high rigidity (modulus of elasticity/specific gravity) and is capable of being effectively inhibited from bending or vibrating. Also disclosed is a substrate for information recording media, which comprises the glass composition. The glass composition comprises the following components in terms of mol %: 40 to 60% SiO 2 , 8 to 25% Al 2 O 3 , 2 to 20% Li 2 O, 0 to 5% Na 2 O; provided that the content of R 2 O (R 2 O=Li 2 O+Na 2 O) is from 2 to 20%, 0 to 10% TiO 2 , 0 to 10% ZrO 2 , 5 to 25% MgO, 0 to 25% CaO and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from more than 15 to 40%.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a high-modulus glass composition. More particularly, this invention relates to a glass composition suitable for use as a substrate for information recording media, which is required to have excellent surface smoothness and a high modulus. The present invention further relates to a substrate for information recording media, which comprises the glass composition, and to an information recording medium. 
     BACKGROUND OF THE INVENTION 
     Information recording devices such as magnetic disks are always required to have a larger recording capacity and to attain a reduction in access time such as disk rotational delay. One possible means for satisfying the latter requirement is to heighten the rotational speed of a medium. 
     However, the media comprising a substrate currently in use are weighed down by themselves and resonate considerably at an increased rotational speed. Eventually, the surface of such a medium comes into contact with the head to cause an error or crushing. It is therefore impossible to narrow the gap between the magnetic disk head and the recording medium to or below a certain level, and this constitutes a serious obstacle to an increase in recording capacity. 
     For reducing the bending of a substrate medium and diminishing the resonance of the medium being rotated, it is necessary to heighten both the modulus of elasticity (Young&#39;s modulus) of the substrate of the medium and the rigidity thereof which is the value obtained by dividing the modulus of elasticity by the specific gravity. 
     The aluminum alloy which has been most commonly used as the substrates of magnetic disks has a modulus of elasticity of 71 GPa and a rigidity of 26 GPa·cm 3 /g. This conventional substrate material, having such properties, hardly copes with the trend toward higher rotational speeds of 10,000 rpm and above. In addition, it has become necessary to increase the thickness of substrates made of the above material, although this goes against the current trend toward thickness reduction in disk substrates for device miniaturization. 
     In contrast, substrates made of a tempered glass are superior to the aluminum substrate in both modulus of elasticity and specific gravity. For example, a glass substrate obtained by subjecting a commercially available soda-lime glass to ion exchange in a molten potassium salt is on the market. This substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa·cm 3 /g. 
     Also known besides the above one is a glass substrate obtained by tempering commercially available Corning 0317. Although this substrate has a modulus of elasticity of 72 GPa and a rigidity of 29 GPa·cm 3 /g, these properties are still insufficient. 
     A high-rigidity substrate for information recording media which is made of a material other than tempered glasses is on the market. This substrate comprises a crystallized glass having a modulus of elasticity of 90 GPa and a rigidity of 38 GPa·cm 3 /g. However, this substrate, after polishing, inevitably has residual crystal grains projecting from the surface because of the nature of the production process in which crystals are precipitated inside. Namely, this crystallized-glass substrate has a drawback that it is inferior in surface smoothness to the substrates made of a tempered glass. 
     Consequently, in view of the expected future trend toward even higher rotational speeds in information recording devices and smaller thickness in disk substrates, there is a desire for a glass composition which has further improved properties, i.e., which has a high Young&#39;s modulus and a high rigidity, can be easily tempered, and gives a substrate having high surface smoothness through polishing. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a glass composition which has a high modulus of elasticity (Young&#39;s modulus) and a high rigidity (modulus of elasticity/specific gravity) and is capable of being effectively inhibited from bending or vibrating when used as the substrate of an information recording medium. 
     Another object of the present invention is to provide a substrate for information recording media, which comprises the glass composition. 
     The present invention has been achieved in view of the above-described problems of prior art techniques and the above-described requirements. 
     The present invention provides a glass composition comprising the following components in terms of mol %: 40 to 60% SiO 2 , 8 to 25% Al 2 O 3 , 2 to 20% Li 2 O, 0 to 5% Na 2 O; provided that the content of R 2 O (R 2 O=Li 2 O+Na 2 O) is from 2 to 20%, 0 to 10% TiO 2 , 0 to 10% ZrO 2 , 5 to 25% MgO, 0 to 25% CaO and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from more than 15 to 40%. 
     The glass composition preferably comprises the following components in terms of mol %: 40 to 55% SiO 2 , 10 to less than 20% Al 2 O 3 , 2 to 10% Li 2 O, 5 to 5% Na 2 O; provided that the content of R 2 O (R 2 O=Li 2 O+Na 2 O) is from 2 to 10%, 0 to 10% TiO 2 , 0 to 10% ZrO 2 , 2 to 25% MgO, 0 to 25% CaO, and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from more than 15 to 30%. 
     The glass composition preferably has a rigidity as defined by (Young&#39;s modulus)/(specific gravity) of 35 GPa·cm 3 /g or higher and a modulus of elasticity as represented by Young&#39;s modulus of 95 GPa or higher. 
     Furthermore, the glass composition is preferably one which has undergone an ion exchange treatment in at least one molten salt containing ions of potassium, sodium, or both. 
     The present invention further provides a substrate for information recording media, which comprises the above-described glass composition which has undergone the ion exchange treatment. 
     This invention still further provides an information recording medium containing the substrate. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The reasons for limitations of the components of the high-rigidity high-modulus glass composition of the present invention are explained below. Hereinafter, unless otherwise indicated, all percents are by mole. 
     SiO 2  is the main component constituting the glass. If the proportion of SiO 2  is lower than 40%, the glass has impaired chemical durability. On the other hand, if the proportion thereof exceeds 60%, the desired modulus of elasticity is not obtained. Consequently, the proportion of SiO 2  should be from 40 to 60%, and is preferably from 40 to 55%. 
     Al 2 O 3  is an ingredient which improves the modulus of elasticity and rigidity of the glass and increases the depth of a compression stress layer formed by ion exchange. Al 2 O 3  further serves to improve the water resistance of the glass. If the proportion of Al 2 O 3  is lower than 8%, these effects are insufficient. On the other hand, if the proportion thereof exceeds 25%, the results are an increased viscosity, an increase in liquidus temperature which is severer than the viscosity increase, and impaired meltability. Consequently, the proportion of Al 2 O 3  should be from 8 to 25%, and is preferably from 10 to less than 10%. 
     Li 2 O, which is an ingredient to be replaced in ion exchange, serves to improve the modulus of elasticity and rigidity of the glass and to lower the melting temperature of the glass to thereby enhance its meltability. If the proportion of Li 2 O is less than 2%, the rigidity is insufficient. On the other hand if the proportion thereof exceeds 20%, the substrate has impaired weatherability and impaired acid resistance. Consequently, the proportion of Li 2 O should be from 2 to 20%, and is preferably 2 to 10%. 
     Na 2 O, which is an ingredient to be replaced in ion exchange, serves to lower the melting temperature and the liquidus temperature to thereby enhance meltability. If the proportion of Na 2 O exceeds 5%, the modulus of elasticity is decreased, and weatherability and acid resistance are impaired. Consequently, the proportion of Na 2 O should be 5% or less. 
     If the total amount of Li 2 O and Na 2 O (R 2 O) is less than 2%, ion exchange cannot be conducted, and meltability is insufficient. On the other hand, if the total amount thereof exceeds 20%, the modulus of elasticity and rigidity are decreased. Consequently, the total amount of R 2 O is preferably from 2 to 20%. 
     TiO 2  is an ingredient which improves the modulus of elasticity, rigidity, and weatherability of the glass. However, if the proportion thereof exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of TiO 2  should be 10% or lower. 
     ZrO 2  is an ingredient which improves the modulus of elasticity, rigidity, and weatherability of the glass. However, if the proportion of ZrO 2  exceeds 10%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of ZrO 2  should be 10% or lower. 
     MgO is an ingredient which heightens the modulus of elasticity, rigidity, and meltability of the glass. However, if the proportion of MgO is less than 5%, its effect is small. On the other hand, if the proportion thereof exceeds 25%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of MgO is preferably from 5 to 25%. 
     CaO is an ingredient which heightens the modulus of elasticity, rigidity, and meltability of the glass. However, if the proportion of CaO exceeds 25%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the proportion of CaO is preferably 25% or lower. 
     SrO is an ingredient which heightens the meltability of the glass. However, if the glass contains SrO in a large amount, it disadvantageously has an increased specific gravity. Consequently, the proportion of SrO should be 6% or lower. 
     If the total amount of MgO, CaO, and SrO (i.e., the amount of RO) is 15% or lower, the glass is insufficient in modulus of elasticity, rigidity, and meltability. If the total amount thereof exceeds 40%, the glass has an elevated liquidus temperature and impaired devitrification resistance. Consequently, the total amount of RO is preferably from more than 15 to 40%, and more preferably from more than 15 to 30%. 
     Besides the ingredients described above, other ingredients may be added in a total amount of up to 3% for the purposes of coloring, melt clarification, etc. Examples of such optional ingredients include As 2 O 3 , Sb 2 O 3 , SO 3 , SnO 2 , Fe 2 O 3 , CoO, Cl, and F. 
     This glass composition, which contains Li 2 O and Na 2 O, can be easily made to have an increased fracture strength by immersing the composition in at least one molten salt containing ions of potassium, sodium, or both at a temperature not higher than the distortion point of the glass composition to thereby interchange these ions and thus generate a compression stress on the surfaces of the composition. 
     When this glass composition is used as a substrate for information recording media, this substrate is less apt to bend or suffer resonant vibration because it has a higher modulus of elasticity and a higher rigidity than conventional substrates. Therefore, the recording medium employing this glass composition is especially suitable for use in recording apparatuses of the high rotational speed type. 
    
    
     The present invention will be explained below in more detail by reference to the following Examples. However, the invention should not be construed as being limited to these Examples. 
     EXAMPLES 1 TO 10 
     Ten compositions as examples of the glass composition of the present invention are shown in Table 1 together with properties thereof. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Example 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
                   
               
             
          
           
               
                 Component 
                 SiO 2   
                 41.8 
                 42.0 
                 45.0 
                 40.0 
                 47.0 
                 54.0 
               
               
                 (mol %) 
                 Al 2 O 3   
                 9.5 
                 9.0 
                 19.0 
                 22.0 
                 13.0 
                 17.0 
               
               
                   
                 Li 2 O 
                 16.8 
                 18.0 
                 7.0 
                 2.0 
                 14.0 
                 7.0 
               
               
                   
                 Na 2 O 
                 0.0 
                 1.0 
                 0.0 
                 0.0 
                 3.0 
                 0.0 
               
               
                   
                 K 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 MgO 
                 12.5 
                 17.0 
                 13.0 
                 18.0 
                 8.0 
                 11.0 
               
               
                   
                 CaO 
                 18.0 
                 8.0 
                 12.0 
                 14.0 
                 10.0 
                 7.0 
               
               
                   
                 SrO 
                 0.0 
                 1.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 ZrO 2   
                 0.0 
                 3.0 
                 4.0 
                 2.0 
                 5.0 
                 4.0 
               
               
                   
                 TiO 2   
                 0.0 
                 1.0 
                 0.0 
                 2.0 
                 0.0 
                 0.0 
               
               
                   
                 SnO 2   
                 1.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 As 2 O 3   
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
             
          
           
               
                 Specific gravity 
                 2.77 
                 2.74 
                 2.65 
                 2.64 
                 2.74 
                 2.67 
               
               
                 (g/cm 3 ) 
               
               
                 Modulus of elasticity 
                 108 
                 111 
                 105 
                 103 
                 107 
                 100 
               
               
                 (GPa) 
               
               
                 Rigidity 
                 39 
                 41 
                 40 
                 39 
                 39 
                 37 
               
               
                 (GPa · cm 3 /g) 
               
               
                   
               
             
          
           
               
                   
                 Example 
                 Comparative Example 
               
             
          
           
               
                   
                 7 
                 8 
                 9 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
             
          
           
               
                 Component 
                 SiO 2   
                 48.5 
                 55.5 
                 59.0 
                 65.0 
                 45.0 
                 71.6 
                 67.0 
               
               
                 (mol %) 
                 Al 2 O 3   
                 14.0 
                 14.0 
                 8.0 
                 6.0 
                 21.0 
                 0.9 
                 10.3 
               
               
                   
                 Li 2 O 
                 10.0 
                 8.0 
                 4.0 
                 2.0 
                 5.0 
                 0.0 
                 0.0 
               
               
                   
                 Na 2 O 
                 4.0 
                 2.0 
                 1.0 
                 5.0 
                 10.0 
                 12.7 
                 13.0 
               
               
                   
                 K 2 O 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.5 
                 2.3 
               
               
                   
                 MgO 
                 6.5 
                 12.0 
                 14.0 
                 3.0 
                 6.0 
                 6.0 
                 5.1 
               
               
                   
                 CaO 
                 6.0 
                 2.5 
                 2.0 
                 11.0 
                 10.0 
                 8.4 
                 0.5 
               
               
                   
                 SrO 
                 3.0 
                 1.0 
                 0.0 
                 0.0 
                 3.0 
                 0.0 
                 0.0 
               
               
                   
                 ZrO 2   
                 5.0 
                 5.0 
                 8.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 TiO 2   
                 3.0 
                 0.0 
                 4.0 
                 8.0 
                 0.0 
                 0.0 
                 0.6 
               
               
                   
                 SnO 2   
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 As 2 O 3   
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 1.2 
               
             
          
           
               
                 Specific gravity 
                 2.86 
                 2.82 
                 2.92 
                 2.74 
                 2.65 
                 2.50 
                 2.46 
               
               
                 (g/cm 3 ) 
               
               
                 Modulus of Elasticity 
                 104 
                 102 
                 107 
                 89 
                 90 
                 72 
                 72 
               
               
                 (GPa) 
               
               
                 Rigidity 
                 36 
                 36 
                 36 
                 32 
                 34 
                 29 
                 29 
               
               
                 (GPa · cm 3 /g) 
               
               
                   
               
             
          
         
       
     
     An explanation is first given on the Examples. Common raw materials for glass including silica, alumina, lithium carbonate, sodium carbonate, basic magnesium carbonate, calcium carbonate, potassium carbonate, strontium carbonate, titania, and zirconia were mixed together to prepare batches as base materials for substrate glasses so that these batches gave compositions respectively having the components shown in Table 1. Each batch prepared was held at 1,550° C. for 4 hours with a platinum crucible and then poured on an iron plate. This glass was held at 650° C. for 30 minutes in an electric furnace. Thereafter, the furnace was switched off to allow the glass to cool to room temperature. Thus, sample glasses were obtained. 
     The specific gravity, modulus of elasticity (Young&#39;s modulus), and rigidity (Young&#39;s modulus/specific gravity) of each of the sample glasses were measured or calculated in the following manners. The results obtained are shown in Table 1 above. 
     Modulus of elasticity was determined by the following method. 
     A sample glass was cut into a piece and each side thereof was subjected to mirror polishing to obtain a platy sample having dimensions of 5×30×30 mm. Each sample was examined for density by the Archimedes method. Furthermore, the modulus of elasticity of each sample was calculated by the ultrasonic method using a sing-around oscillator. 
     The glasses of Examples 1 to 9 according to the present invention each had a Young&#39;s modulus exceeding 95 GPa and a rigidity of 35 GPa·cm 3 /g or higher. 
     The glasses of Examples 1 to 9 were then immersed for 1 hour in a melt of an 80:20 salt mixture of KNO 3  and NaNO 3  heated at 380° C. to conduct tempering. Thereafter, each tempered glass was examined with a polarizing microscope to determine the thickness of the resulting compression stress layer. 
     In each of the glasses of Examples 1 to 9, a compression stress layer having a thickness of 50 μm or larger had been formed. The components of these glasses thus proved to be suitable for tempering. 
     APPLICATION EXAMPLE 
     The sample glass of Example 1 described above was cut into a ring form having an outer diameter of 95 mm and an inner diameter of 25 mm. This disk was ground and polished and subsequently subjected to tempering under the same conditions as the above and then to mirror polishing (surface roughness R a ≦1 nm; JIS B 0601-1994) to regulate the thickness thereof to 1.0 mm. Thus, a substrate for magnetic recording media was obtained. 
     Using the thus-produced substrate for magnetic recording media, a magnetic disk medium was produced in the following manner. 
     Chromium, Co—Cr—Ta, and carbon were deposited on the substrate as an undercoat layer, recording layer, and protective layer, respectively, by sputtering. A lubricating layer was further formed thereon to obtain a magnetic disk medium. 
     The medium thus obtained was set in a closed type magnetic-disk drive and continuously rotated at each of 10,000 rpm and 12,000 rpm. In either case, the medium was found to be free from troubles such as head crushing caused by substrate vibration, because the glass substrate had a high Young&#39;s modulus and a high rigidity. 
     COMPARATIVE EXAMPLES 1 TO 4 
     An explanation is given below on Comparative Examples 1 to 4. The compositions of Comparative Examples 1 to 4 are outside the scope of the present invention. In particular, the composition of Comparative Example 3 is a general soda-lime glass composition. 
     Sample glasses were prepared in the same manner as in the Examples, except the following. In Comparative Example 4, the batch prepared was held at 1,600° C. for 16 hours with a platinum crucible and then poured on an iron plate. This glass was held at 650° C. for 30 minutes in an electric furnace, which was then switched off to allow the glass to cool to room temperature to obtain a sample glass. 
     The specific gravity, modulus of elasticity (Young&#39;s modulus), rigidity (Young&#39;s modulus/specific gravity), and compression stress layer thickness of each of these sample glasses were measured or calculated in the same manners as in the Examples The results obtained are shown in Table 1. 
     However, tempering was conducted in the following manners, as different from that in Examples 1 to 9. The glass of Comparative Example 3 was immersed for 3 hours in a molten salt of KNO 3  heated at 380° C. to conduct tempering. The glass of Comparative Example 2 was immersed for 16 hours in a molten salt of KNO 3  heated at 440° C. to conduct tempering. A section of each of these samples was examined with a polarizing microscope in the same manner as in the Examples to determine the thickness of the resulting compression stress layer. 
     The glasses of Comparative Examples 1 to 4 each had a Young&#39;s modulus of 90 GPa or lower and a rigidity of lower than 35 GPa·cm 3 /g. 
     Further, the compression stress layer in each of the glasses of Comparative Examples 3 and 4 had a thickness of 11 μm and 39 μm, respectively, which were far smaller than 50 μm or more in Examples 1 to 9, despite the higher melt temperature and the longer immersion periods than in the Examples. 
     As described above in detail, the following effects are brought about by the present invention. 
     According to the present invention, a glass composition having a higher rigidity and a higher modulus of elasticity than conventional glasses can be obtained. 
     According to one preferred embodiment of the present invention, a high-rigidity high-modulus glass composition which can be easily produced and has satisfactory devitrification resistance and weatherability can be obtained because the range of the glass component has been narrowed. 
     According to another preferred embodiment of the present invention, a limited high-rigidity high-modulus glass composition is provided. This glass composition can have a higher rigidity and a higher modulus of elasticity than the conventional glasses and conventional aluminum alloy, which have conventionally been used as a substrate for magnetic disk. 
     According to still another preferred embodiment of the present invention, tempering is easy as in general soda-lime glasses, making it possible to form a surface compression stress layer on the glass composition to a larger depth. 
     According to another embodiment of the present invention, a glass substrate suitable for use as the substrate of an information recording medium can be obtained which comprises the above glass composition and has a high rigidity, a high modulus of elasticity, and a high fracture strength. 
     According to further embodiment of the present invention, a recording medium employing the above glass substrate having a high rigidity, high modulus of elasticity, and high fracture strength can be obtained. This recording medium can be rotated at a higher speed, bends less, and is less apt to suffer resonant substrate vibration. Hence, the gap between the magnetic disk head and the recording medium can be narrowed, making it possible to attain an increase in storage capacity and a reduction in access time. Therefore, the recording medium employing the glass composition is especially suitable for use in recording apparatuses of the high rotational speed type. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.