Crystallized glass for information recording medium, crystallized glass substrate, and information recording medium using the crystallized glass substrate

Enclosed are crystallized glasses suitable for a substrate for an information recording medium such as a magnetic disc, optical disc, or optomagnetic disc, a substrate for information recording medium using such a crystallized glass substrate, and an information recording medium using such a substrate for information recording medium. The crystallized glasses are capable of providing a glass substrate having a high Young's modulus, as well as excellent mechanical strength, surface flatness, and heat resistance and having an excellent surface smoothness upon polishing. Glass substrate having an excellent surface smoothness using such a crystallized glass are also disclosed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a crystallized glass suitable for a substrate for
 an information recording medium such as a magnetic disc, optical disc, or
 optomagnetic disc, a substrate for information recording medium using such
 a crystallized glass substrate, and an information recording medium using
 such a substrate for information recording medium. More particularly, this
 invention relates to a crystallized glass capable of providing a glass
 substrate having a high Young's modulus, as well as excellent mechanical
 strength, surface flatness, and heat resistance and having an excellent
 surface smoothness upon polishing and to a glass substrate having an
 excellent surface smoothness using such a crystallized glass.
 2. Description of Related Art
 Major components of a magnetic recording apparatus in, e.g., a computer,
 includes a magnetic recording medium and a magnetic head for magnetic
 recording and reproducing. As a magnetic recording medium, known are a
 flexible disc and a hard disc. Aluminum alloy, among materials, has been
 used for the substrate material for hard disc. The floating amount of the
 magnetic head is significantly reduced in accordance with recent trends
 that hard disc drives for note type personal computer are made smaller and
 that the magnetic recording is made with higher density. A very high
 precision, according to those trends, is required for surface smoothness
 on the magnetic disc substrate. In the case of an aluminum alloy, however,
 it is difficult to manufacture a flat surface with a precision of a
 certain degree or higher, because the polished surface may be plastically
 deformed due to a low hardness even if a polishing material having a high
 precision and a machine tool are used for polishing. Even if
 nickel-phosphorus plating is made on a surface of the aluminum alloy, the
 surface roughness Ra cannot be controlled at five angstroms or less.
 According to the developments of the hard disc drives that become smaller
 and thinner, there are strong demands for making thinner the magnetic disc
 substrate. The aluminum alloy, however, has low strength and rigidity, and
 therefore, it is difficult to make the disc thin while the hard disc drive
 maintains certain strength as required from the specification for the
 drive.
 To solve such problems, a glass substrate for magnetic disc claiming high
 strength, high rigidity, high impact resistance, and high surface
 smoothness has been developed. Chemically reinforced glass substrates
 whose substrate surface is reinforced with an ion exchange method and
 crystallized substrates subjecting to a crystallization process, inter
 alia, are known well.
 As a glass substrate reinforced by ion exchange, e.g., a glass disclosed in
 Japanese Unexamined Patent Publication No. 1-239,036 has been known. This
 ion exchange reinforced glass substrate is made of a glass including, by
 percent by weight, SiO.sub.2 of 50-65%, Al.sub.2 O.sub.3 of 0.5 to 14%,
 R.sub.2 O (wherein R denotes alkali metal ion) of 10 to 32%, ZnO of 1 to
 15%, and Ba.sub.2 O.sub.3 of 1.1 to 14% where the glass is reinforced by
 forming compression stress layers on a surface of the glass substrate by
 an ion exchange method with alkali ions, and the above publication
 discloses such a glass substrate for magnetic disc.
 As a crystallized glass, e.g., there is a disclosure in Japanese Patent
 Publication No. 2,516,553. This crystallized glass includes, by weight,
 SiO.sub.2 of 65 to 83%, Li.sub.2 O of 8 to 13%, K.sub.2 O of 0 to 7%, MgO
 of 0.5 to 5.5%, ZnO of 0 to 5%, PbO of 0 to 5% (provided that MgO+ZnO+PbO
 is of 0.5 to 5% by weight), P.sub.2 O.sub.5 of 1 to 4%, Al.sub.2 O.sub.3
 of 0 to 7%, and As.sub.2 O.sub.3 +Sb.sub.s O.sub.3 of 0 to 2% and is a
 crystallized glass for magnetic disc including fine crystal particles of
 LiO.sub.2.cndot.2SiO.sub.2 as a primary crystal.
 A crystallized glass is disclosed also in Japanese Unexamined Patent
 Publication No. 7-291,660. The crystallized glass is obtained by melting a
 glass composed of, by percent by weight, SiO.sub.2 of 38 to 50%, Al.sub.2
 O.sub.3 of 13 to 30%, MgO of 10 to 20% provided that, by weight ratio,
 Al.sub.2 O.sub.3 /MgO is 1.2 to 2.3, B.sub.2 O.sub.3 of 0 to 5%, CaO of 0
 to 5%, BaO of 0 to 5%, SrO of 0 to 5%, ZnO of 0.5 to 7.5%, TiO.sub.2 of 4
 to 15%, ZrO.sub.2 of 0 to 5%, and As.sub.s O.sub.3 and/or Sb.sub.2 O.sub.3
 of 0 to 2% and thermally treating it after molding, and is a cordierite
 based crystallized glass having a feature that the glass contains as a
 primary crystal a cordierite based crystal. The above publication also
 discloses a substrate for magnetic disc made of a crystallized glass.
 Another crystallized glass is also disclosed in Japanese Unexamined Patent
 Publication No. 9-77,531. This crystallized glass is a glass ceramic
 product having a Young's modules in a range from about 14.times.10.sup.6
 to about 24.times.10.sup.6 psi (96 to 165 Gpa) and a breakdown tenacity
 more than 1.0 Mpa.cndot.ml/2. The crystallized glass is constituted of a
 crystal phase conglomerate mainly made of a spinel type crystal uniformly
 sized and evenly dispersed in the residual glass matrix phase including
 rich silicon. The glass is substantially made of, by percent by weight,
 SiO.sub.2 of 35 to 60%, Al.sub.2 O.sub.3 of 20 to 35%, MgO of 0 to 25%,
 ZnO of 0 to 25%, TiO.sub.2 of 0 to 20%, ZrO.sub.2 of 0 to 10%, Li.sub.2 O
 of 0 to 2%, NiO of 0 to 8%, wherein the total of MgO and ZnO is at least
 10%, and may contain an arbitrary component selected up to 5% from a group
 constituted of BaO, CaO, PbO, SrO, P.sub.2 O.sub.5, B.sub.2 O.sub.3 and
 Ga.sub.2 O.sub.3, and R.sub.2 O of 0 to 5% selected from a group
 constituted of Na.sub.2 O, K.sub.2 O, Rb.sub.s O, and Cs.sub.s O, and a
 transitional metal oxide of 0 to 8%. The glass may be a glass ceramic
 having a composition in which the total amount of TiO.sub.2 +ZrO.sub.2
 +NiO is of 5% or more in the case where Al.sub.2 O.sub.3 is contained only
 in an amount of about 25% or less, and the above publication also
 discloses a substrate for magnetic disc made of a glass ceramic.
 However, in accordance with recent trends that the hard discs are made
 smaller and thinner and that the recording is made with a higher density,
 flying height of the magnetic head is lower, and the disc is rotated at a
 higher speed, so that strength, Young's modulus, and surface smoothness of
 the disc substrate material are further severely needed. Particularly, the
 surface smoothness and surface flatness of the substrate material are
 strictly on demands due to trends for higher density information recording
 on 3.5 inch hard discs for personal computers and servers, and the disc
 has to be rotated at 10,000 rpm or more in corresponding to higher speed
 of data processing. While the rigidity of the substrate material is
 subject to a further strict standard, it is apparent that the conventional
 aluminum substrate is already limiting itself. As far as demands for disc
 drives having a higher capacity and higher speed are necessary from now
 on, a substrate material for magnetic recording medium is required
 doubtlessly to have high Young's modulus, high strength, excellent surface
 flatness, good impact resistance, and so on.
 The necessity for high Young's modulus can be illustrated based on the
 following facts. That is, according to recent trends for HDDs (hard disc
 drives) which are made smaller with high capacity and high operation
 speed, future substrates for magnetic recording medium may have the
 thickness of 0.635 mm, currently 0.8 mm in the case of 3.5 inches and of
 0.43 mm or 0.38 mm, currently 0.635 mm in the case of 2.5 inches, and the
 rotation speed of the substrate may predictably be made higher to 140,000
 rpm from current 100,000 rpm as the maximum speed. Such a substrate for
 magnetic recording medium may tend to sustain more looseness and
 undulation and warp as the substrate for magnetic recording medium becomes
 thinner, and the stress (force exerted to the disc based on an air
 pressure created from rotation) that the substrate receives predictably
 becomes larger as the substrate spins with a higher speed. Based on a
 dynamics theory, flexion W of a disc which receives load P per unit area
 is denoted as follows:
 ##EQU1##
 wherein: a represents the outer diameter of the disc; h represents the
 thickness of the substrate; E represents the Young's modulus of the disc
 material. Only gravity is exerted to the disc at a still state. The warp
 is indicated as, where the specific gravity of the disc material is
 represented by d,
 ##EQU2##
 Herein, G represents the specific modulus of elasticity (=Young's
 modulus/specific gravity) of the disc material. Meanwhile, in the case
 that the gravity component can be neglected upon balancing centrifugal
 components in a rotational state of the disc, the force exerted to the
 disc can be deemed as air pressure based on the rotation. Such an air
 pressure is a function relating to the rotation speed of the disc, and it
 can be said generally as proportional to square of the speed. Accordingly,
 where the disc spins at a high speed the warp W can be represented as
 follows:
 ##EQU3##
 According to this consequence, it turns out that a substrate material
 having a high Young's modulus is required to suppress the vibration in the
 substrate that is subject to high speed spinning. From a calculation done
 by the inventors, the specific modulus of elasticity of the substrate
 material is required to be at least 37 MNm/kg or higher if the substrate
 thickness is reduced to 0.43 mm from 0.635 mm in the case of 2.5 inch
 substrates and to 0.635 mm from 0.8 mm in the case of 3.5 inch substrates.
 If the rotation speed of the 3.5 inch high end substrate is made faster
 from current 7,200 rpm to future 10,000 rpm, the aluminum substrate having
 a Young's modulus of around 70 Gpa cannot correspond to it, and a new
 substrate material having a Young's modulus of at least 110 Gpa or higher
 is required. Because the substrate has not only a higher rigidity but also
 a higher impact resistance and strength as the substrate material has
 higher specific modulus of elasticity and higher Young's modulus, the
 market of the hard disc drive strongly seeks a glass material having a
 higher modulus of elasticity and a higher Young's modulus.
 The chemically reinforced glass as disclosed above in Japanese Unexamined
 Patent Publication No. 1-239,036, however, has a Young's modulus of about
 80 Gpa, and it is apparent that such a glass cannot response to strict
 demands on upcoming hard discs. The conventional glass for ion exchange
 reinforced substrate has alkali ions in a large amount introduced into the
 glass for ion exchange, and therefore, the reinforced glass has a low
 Young's modulus (90Gpa) as well as a low rigidity, so that the glass
 cannot correspond to substrates for 3.5 inch high end disc or thinner
 disc. A large amount of alkali component can be contained in a glass
 subjecting to a chemical reinforcement by the ion exchange. Therefore, if
 the glass is used for a long time under a high temperature and moisture
 environment, alkali ions may be deposited from pin holes in the magnetic
 film, thin portions of the magnetic film such as vicinities of the
 magnetic film, or exposed portions of the glass and may disadvantageously
 induce corrosions and deterioration of the magnetic film. During the
 manufacturing process for the magnetic recording medium, a prescribed
 thermal process can be used for improving characteristics such as coercive
 force of a magnetic layer after the magnetic layer is formed on the glass
 substrate. With such a conventional ion exchange reinforced glass,
 however, the glass transition temperature is at most around 500.degree.
 C., and since the glass lacks heat resistance, there also raises a problem
 that the glass cannot obtain a high coercive force.
 The conventional crystallized glass as disclosed above in Japanese Patent
 Publication No. 2,516,553 has a little better property than the above
 chemically reinforced glass substrate in terms of the Young's modulus and
 the heat resistance. However, it has the surface roughness of 10 angstroms
 or higher, poor surface smoothness, and a limitation against lower flying
 of the magnetic head. Therefore, there raises a problem that the glass may
 not correspond to high density trends of the magnetic recording.
 Furthermore, the Young's modulus is at most about 90 to 100 Gpa, so that
 such a glass cannot be used for substrates for 3.5 inch high end disc or
 thinner disc.
 The crystallized glass disclosed in Japanese Unexamined Patent Publication
 No. 7-291,660 has a Young's modulus of at most 100 to 130 Gpa, which is
 inadequate for use. Moreover, the glass has a limited surface smoothness
 in which the mean roughness of central line (Ra) is about only eight
 angstroms and lacks smoothness. In addition, the glass has a high liquid
 phase temperature of about 1400.degree. C., and therefore, the glass has a
 disadvantage that the glass is hardly subject to a high temperature
 melting and high temperature molding.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide a crystallized glass suitable
 for a substrate for information recording medium such as a magnetic disc
 having high Young's modulus, strength, and heat resistance as well as
 excellent surface smoothness and surface uniformity, which can be produced
 in an inexpensive way with a relatively low liquid phase temperature, in
 consideration of future demands on such a substrate for magnetic recording
 medium with thinner size, high strength, high heat resistance, high impact
 resistance, and so on.
 It is another object of the invention to provide a substrate for
 information recording medium such as a magnetic disc or the like and an
 information recording medium such as a magnetic disc using this substrate,
 made of the above crystallized glass in having high Young's modulus,
 strength, and heat resistance as well as excellent surface smoothness and
 surface uniformity, which can be produced in an inexpensive way with a
 relatively low liquid phase temperature.
 The foregoing objects are accomplished by providing a crystallized glass
 substrate for information recording medium having a composition including
 SiO.sub.2 of 35 to 65 mol %, Al.sub.2 O.sub.3 of 5 to 25 mol %, MgO of 10
 to 40 mol %, TiO.sub.2 of 5 to 15 mol %, and Y.sub.2 O.sub.3 of 0.8 to 10
 mol %. The inventors have discovered that where a SiO.sub.2 --Al.sub.2
 O.sub.3 --MgO based glass with a TiO.sub.2 component as a nucleus forming
 agent contains Y.sub.2 O.sub.3 as a necessary element, a good crystallized
 glass is obtainable which is suitable for substrate for information
 recording medium having a high Young's modulus of 120 Gpa or higher and a
 good surface smoothness. The inventors completed the above crystallized
 glass.
 In another aspect of the invention, a crystallized glass substrate for
 information recording medium has a composition including SiO.sub.2 of 35
 to 65 mol %, Al.sub.2 O.sub.3 of 5 to 25 mol %, MgO of 10 to 40 mol %, and
 TiO.sub.2 of 5 to 15 mol %, in which a mole ratio (Al.sub.2 O.sub.3 /MgO)
 is less than 0.5. The inventors have discovered that where a SiO.sub.2
 --Al.sub.2 O.sub.3 --MgO based glass with a TiO.sub.2 component as a
 nucleus forming agent is controlled to have a mole ratio (Al.sub.2 O.sub.3
 /MgO) less than 0.5, a good crystallized glass is obtainable which is
 suitable for substrate for information recording medium having a high
 Young's modulus of 140 Gpa or higher and a good surface smoothness, as
 well as a relatively low liquid phase temperature. The inventors then
 completed the above crystallized glass.
 In yet another aspect of the invention, a crystallized glass substrate for
 information recording medium has a composition including SiO.sub.2 of 35
 to 65 mol %, Al.sub.2 O.sub.3 of 5 to 25 mol %, MgO of 10 to 40 mol %,
 TiO.sub.2 of 5 to 15 mol %, and Li.sub.2 O of 0.2 to 10 mol %, wherein the
 substrate has a primary crystal phase made of a quasi-stable quartz solid
 solution and an enstatite, and wherein the mean grain size of the crystal
 grains is 1 micron or less. The inventors also have discovered that where
 a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO based original glass with TiO.sub.2
 component as a nucleus forming agent contains Li.sub.2 O as a necessary
 element, and where the primary crystal phase is made of a quasi-stable
 quartz solid solution having one or more components selected from
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2,
 MgO.Al.sub.2 O.sub.3. 4SiO.sub.2 or a mixture composition made of
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2 and an enstatite having a composition of
 MgO.SiO.sub.2 and (Mg.Al)SiO.sub.3, the crystallized glass is formed with
 a very high Young's modulus of 130 Gpa or higher in having a very smooth
 surface upon fining the mean grain size of the crystal grain at 1 micron
 or smaller. The inventors thereby completed the above crystallized glass.
 In a further aspect of the invention, a crystallized glass for information
 recording disc has a composition including SiO.sub.2 of 42 to 65 mol %,
 Al.sub.2 O.sub.3 of 0 to 15 mol %, MgO of 5 to 30 mol %, Y.sub.2 O.sub.3
 of 0.5 to 8 mol %, and Li.sub.2 O greater than 10 mol % but equal to or
 less than 25 mol %, wherein the glass has a primary crystal phase made of
 a quasi-stable quartz solid solution and an enstatite.
 Moreover, in a still another aspect of the invention, a crystallized glass
 for information recording disc has a composition including SiO.sub.2 of 35
 to 55 mol %, Al.sub.2 O.sub.3 equal to or more than 0 mol % and less than
 5 mol %, MgO of 25 to 45 mol %, Y.sub.2 O.sub.3 of 0.5 to 8 mol %,
 ZrO.sub.2 of 0 to 10 mol %, and TiO.sub.2 of 0 to 12 mol %, providing that
 ZrO.sub.2 +TiO.sub.2 is of 4.5 mol % to 18 mol %, wherein the glass has a
 primary crystal phase made of a quasi-stable quartz solid solution and/or
 an enstatite. The inventors discovered that where an original glass in
 which a MgO--SiO.sub.2 based glass contains Y.sub.2 O.sub.3, TiO.sub.2,
 ZrO.sub.2, and Al.sub.2 O.sub.3 as necessary components is thermally
 treated in a proper temperature range, the crystallized glass on which
 fine crystal grains or particles made of quartz solid solution, enstatite,
 and the like are deposited has a high Young's modulus of 140 to 200 Gpa,
 as well as excellent mechanical strength, surface smoothness, surface
 flatness, and heat resistance, and can be molded easily. The crystallized
 glass substrate thus obtained can be polished easily and has an excellent
 chemical property as a magnetic disc substrate. The inventors thereby
 completed the above crystallized glass.
 This invention also concerns information recording medium having a
 recording layer formed on a substrate for information recording medium
 made of the crystallized glass thus produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In this description, a symbol "%" means "mol % or mole percentage" unless
 specifically indicated.
 First Embodiment [crystallized glass]
 Each component constituting a glass for a crystallized glass (First
 Embodiment) constituting a substrate for information recording medium is
 described below.
 SiO.sub.2 is a glass material having a meshed structure and also serves as
 a structural component for, as major deposited crystals, a quasi-stable
 quartz solid solution such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2,
 MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2 and
 an enstatite having a component of MgO.SiO.sub.2 and enstatite solid
 solution with components of (Mg.Al)SiO.sub.3. Where SiO.sub.2 is contained
 in an amount less than 35%, the melting glass becomes so unstable, thereby
 making molding at a high temperature hard and crystal depositions
 difficult. Furthermore, where SiO.sub.2 is contained in an amount less
 than 35%, the residual glass matrix phase may suffer from impaired
 chemical resistance, and the glass tends to suffer from a worse heat
 resistance. On the other hand, where SiO.sub.2 is contained in an amount
 more than 65%, the quasi-stable quartz solid solution and the enstatite as
 the primary crystal phase tend to be not readily deposited, and the
 Young's modulus of the glass tends to rapidly become small. Therefore, a
 proper contained amount of SiO.sub.2 is in a range of 35 to 65% in
 consideration of deposited crystal species, deposited amounts, chemical
 resistance, heat resistance, molding property, and productivity. From a
 viewpoint to obtain a crystallized glass having more preferable property,
 the contained amount of SiO.sub.2 is preferably in a range of 37 to 60%.
 Al.sub.2 O.sub.3 is an intermediate oxide of a glass and is a structural
 component of, as major crystal species, a quasi-stable quartz solid
 solution such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2
 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2. Introduction of
 Al.sub.2 O.sub.3 promotes deposition of the quasi-stable quartz solid
 solution and contributes to improvements of surface hardness of the glass.
 However, if the contained amount of Al.sub.2 O.sub.3 is less than 5%, the
 high Young's modulus crystal as described above may not be deposited well,
 and the glass matrix phase may suffer from impaired chemical resistance
 while the substrate material may lose the required strength. On the other
 hand, when the contained amount of Al.sub.2 O.sub.3 exceeds 25 mol %, the
 high Young's modulus crystal phase such as an enstatite may be not readily
 deposited, and the glass may not be melted easily due to a high melting
 temperature while losing its transparency and easiness for molding.
 Therefore, in consideration of solubility of the glass, molding property
 at high temperature, deposited crystal species, and so on, the contained
 amount of Al.sub.2 O.sub.3 is in a range of 5 to 25% and preferably in a
 range of 7 to 22%.
 MgO is a modification component of the glass, has a crystal structure of
 the enstatite or quasi-stable quartz solid solution and also serves as a
 main component of the crystal species having a composition such as
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, or
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2. Where the contained amount of MgO is less
 than 10%, the crystal as described above may not be deposited well, and
 the glass tends to lose the transparency and may be subject to a higher
 melting temperature, while a temperature span suitable for glass molding
 operation tends to be narrowed. On the other hand, if the contained amount
 of MgO exceeds 40%, the high temperature viscosity of the glass suddenly
 falls to render the glass thermally unstable, thereby impairing
 productivity, as well as lowering the Young's modulus and the durability.
 Therefore, in consideration of productivity, chemical resistance, high
 temperature viscosity, strength, and so on of the glass, the contained
 amount of MgO is in a range of 10 to 40% and preferably in a range of 12
 to 38%.
 TiO.sub.2 is a nucleus forming agent for crystal phase deposition of the
 enstatite crystal phase and the crystal phase of 2MgO.2Al.sub.2
 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2
 O.sub.3.4SiO.sub.2 having a crystal structure of a quasi-stable quartz
 solid solution. TiO.sub.2 also has an effect to suppress loss of glass
 transparency where the contained amount of SiO.sub.2 is less. It is to be
 noted that where the contained amount of TiO.sub.2 is less than 5%, a
 uniform crystallized glass may not be produced easily due to surface
 crystallization on the glass where TiO.sub.2 does not serve adequately as
 the nucleus forming agent for primary crystal. On the other hand, where
 the contained amount of TiO.sub.2 exceeds 15%, the glass may suffer from
 divided phases due to too lowered high temperature viscosity or from loss
 of transparency, thereby making the productivity of the glass extremely
 impaired. Consequently, in consideration of productivity, chemical
 resistance, high temperature viscosity, crystal nucleus production, and so
 on of the glass, the contained amount of TiO.sub.2 is in a range of 5 to
 15% and preferably in a range of 5.5 to 14%.
 For the crystallized glass according to the invention, Y.sub.2 O.sub.3
 works importantly. As described in embodiments below, introduction of
 Y.sub.2 O.sub.3 of 2%, for example, increases the Young's modulus of the
 crystallized glass by about 10 Gpa, so that the liquid temperature can be
 reduced by about 50 to 100.degree. C. That is, introduction of Y.sub.2
 O.sub.3 in a small amount significantly improves characteristics and
 productivity of glass. However, if the contained amount of Y.sub.2 O.sub.3
 is less than 0.8%, such effects of Y.sub.2 O.sub.3 are not obtainable
 adequately. The Y.sub.2 O.sub.3 has power to suppress the growth of the
 primary crystal contained in the glass. Therefore, if the contained amount
 of Y.sub.2 O.sub.3 is too much, a surface crystallization occurs during a
 thermal treatment for crystallizing the glass, so that the aimed glass may
 not be produced. From this viewpoint, a proper contained amount of Y.sub.2
 O.sub.3 is equal to and less than 10%. Particularly, the contained amount
 of Y.sub.2 O.sub.3 is preferably equal to or less than 8%.
 As components other than the above, one or more oxides of alkali metals and
 alkali earth metals, such as Li.sub.2 O, Na.sub.2 O, K.sub.2 O, CaO, SrO,
 BaO, ZnO, NiO of 0 to 10 mol % and B.sub.2 O.sub.3, P.sub.2 O.sub.5,
 R.sub.2 O.sub.3 R is rare earth metal ions except Y), ZrO.sub.2,
 CeO.sub.2, N.sub.2 O.sub.5 (N is Nb or Ta) of 0 to 5 mol % can be
 contained as far as the above oxides do not impair the characteristics
 belonging to the crystallized glass.
 As an anti-bubbling agent, As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 can be
 contained for malting the glass uniform. In accordance with the high
 temperature viscosity varied in association with the glass composition,
 addition of either or both of As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 to the
 glass in an appropriate amount creates a glass with further uniformity. If
 the addition amount of the anti-bubbling agent is too much, the specific
 gravity of the glass may increase to lower the Young's modulus, and a
 platinum crucible may inflict damages due to agent's reaction with the
 crucible for melting. It is therefore proper that the adding amount of the
 anti-bubbling agent is equal to or less than 2%, and preferably equal to
 or less than 1.5%.
 Impurities in a raw material other than the above fundamental components,
 e.g., Cl, F, SO.sub.3, and the like, which serve as a glass clarifier may
 be contained as far as the impurities are in an amount less than 1%, which
 do not impair the characteristics belonging to the crystallized glass.
 The primary crystal phase of the crystallized glass according to the
 invention can be, e.g., an enstatite crystal phase having a composition of
 MgO.SiO.sub.2 and enstatite solid solution with components of
 (Mg.Al)SiO.sub.3 and a quasi-stable quartz solid solution having one or
 more compositions selected from a group of 2MgO.2Al.sub.2
 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2
 O.sub.3.4SiO.sub.2, or particularly, .beta.-quartz solid solution. It is
 to be noted that the enstatite crystal phase includes clinoenstatite,
 protoenstatite, and one or more kinds of enstatites. With the crystallized
 glass of the invention, crystals other than the above such as spinel,
 mullite, 2MgO.SiO.sub.2, MgO.SiO.sub.2, can be contained. The mean crystal
 size contained in the invented crystallized glass is preferably 3 microns
 or less, more preferably 1 micron or less, and further preferably 0.5
 micron or less. If the mean of the crystal size exceeds 1 micron, it not
 only reduces the mechanical strength of the glass but also impairs surface
 roughness of the glass upon induction of defects of the crystal during
 polishing.
 Second Embodiment [crystallized glass]
 Each component constituting a glass for the invented crystallized glass
 (Second Embodiment) is described below.
 SiO.sub.2 is a glass material having a meshed structure and also serves as
 a structural component for, as major deposited crystals, a quasi-stable
 quartz solid solution such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2,
 MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2 and
 an enstatite having a component of MgO.SiO.sub.2 and enstatite solid
 solution with components of (Mg.Al)SiO.sub.3. Where SiO.sub.2 is contained
 in an amount less than 35%, the melting glass becomes so unstable, thereby
 making molding at a high temperature hard and crystal depositions
 difficult. Furthermore, where SiO.sub.2 is contained in an amount less
 than 35%, the residual glass matrix phase may suffer from impaired
 chemical resistance, and the glass tends to suffer from a worse heat
 resistance. On the other hand, where SiO.sub.2 is contained in an amount
 more than 65%, the quasi-stable quartz solid solution and the enstatite as
 the primary crystal phase tend to be not readily deposited, and the
 Young's modulus of the glass tends to rapidly become small. Therefore, a
 proper contained amount of SiO.sub.2 is in a range of 35 to 65% in
 consideration of deposited crystal species, deposited amounts, chemical
 resistance, heat resistance, molding property, and productivity. From a
 viewpoint to obtain a crystallized glass having more preferable property,
 the contained amount of SiO.sub.2 is preferably in a range of 40 to 60%.
 Al.sub.2 O.sub.3 is an intermediate oxide of a glass and is a structural
 component of, as major crystal species, a quasi-stable quartz solid
 solution such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2
 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2. Introduction of
 Al.sub.2 O.sub.3 promotes deposition of the quasi-stable quartz solid
 solution and contributes to improvements of surface hardness of the glass.
 However, if the contained amount of Al.sub.2 O.sub.3 is less than 5%, the
 high Young's modulus crystal as described above may not be deposited well,
 and the glass matrix phase may suffer from impaired chemical resistance
 while the substrate material may lose the required strength. On the other
 hand, when the contained amount of Al.sub.2 O.sub.3 exceeds 25 mol %, an
 enstatite as a primary crystal phase may be not readily deposited, and the
 glass may not be melted easily due to a high melting temperature while
 losing its transparency and easiness or molding. Therefore, in
 consideration of solubility of the glass, molding property at high
 temperature, deposited crystal species, and so on, the contained amount of
 Al.sub.2 O.sub.3 is in a range of 5 to 25% and preferably in a range of 7
 to 22%.
 MgO is a modification component of the glass, has a crystal structure of
 the enstatite having a composition of MgOSiO.sub.2 and enstatite solid
 solution with components of (Mg.Al)SiO.sub.3 or a quasi-stable quartz
 solid solution, and also serves as a main component of the crystal species
 having a composition such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2,
 MgO.Al.sub.2 O.sub.3.3SiO.sub.2, or MgO.Al.sub.2 O.sub.3.4SiO.sub.2. Where
 the contained amount of MgO is less than 10%, the crystal as described
 above may not be deposited well, and the glass tends to lose the
 transparency and may be subject to a higher melting temperature, while a
 temperature span suitable for glass molding operation tends to be
 narrowed. On the other hand, if the contained amount of MgO exceeds 40%,
 the high temperature viscosity of the glass suddenly falls to render the
 glass thermally unstable, thereby impairing productivity, as well as
 lowering the Young's modulus and the durability. Therefore, in
 consideration of productivity, chemical resistance, high temperature
 viscosity, strength, and so on of the glass, the contained amount of MgO
 is in a range of 10 to 40% and preferably in a range of 12 to 38%.
 It is to be noted that the contained amounts of MgO and Al.sub.2 O.sub.3
 are adjusted so that the mole ratio (Al.sub.2 O.sub.3 /MgO) is less than
 0.5. If the mole ratio (Al.sub.2 O.sub.3 /MgO) is equal to or more than
 0.5, the Young's modulus of the crystallized glass tends to drop suddenly.
 Where the ratio Al.sub.2 O.sub.3 /MgO is set less than 0.5, a crystallized
 glass having a high Young's modulus of 150 GPa or higher can be obtained.
 It is preferable to set the ratio Al.sub.2 O.sub.3 /MgO less than 0.45.
 However, if the mole ratio of Al.sub.2 O.sub.3 /MgO is too small, the high
 temperature viscosity of the glass may be made lower, and therefore, the
 ratio is properly 0.2 or higher, and more preferably 0.25 or higher.
 TiO.sub.2 is a nucleus forming agent for crystal phase deposition of the
 enstatite crystal having a composition of MgO.SiO.sub.2 and enstatite
 solid solution with components of (Mg.Al)SiO.sub.3 and the crystal phase
 of 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2 having a crystal structure of a
 quasi-stable quartz solid solution. TiO.sub.2 also has an effect to
 suppress loss of glass transparency where the contained amount of
 SiO.sub.2 is less. It is to be noted that where the contained amount of
 TiO.sub.2 is less than 5%, a uniform crystallized glass may not be
 produced easily due to surface crystallization on the glass where
 TiO.sub.2 does not serve adequately as the nucleus forming agent for
 primary crystal. On the other hand, where the contained amount of
 TiO.sub.2 exceeds 15%, the glass may suffer from divided phases due to too
 lowered high temperature viscosity or from loss of transparency, thereby
 making the productivity of the glass extremely impaired. Consequently, in
 consideration of productivity, chemical resistance, high temperature
 viscosity, crystal nucleus production, and so on of the glass, the
 contained amount of TiO.sub.2 is in a range of 5 to 15% and preferably in
 a range of 5.5 to 14%.
 For the crystallized glass according to the invention, Y.sub.2 O.sub.3 is
 not a necessary component, but, as described in embodiments below,
 introduction of Y.sub.2 O.sub.3 of 2%, for example, increases the Young's
 modulus of the crystallized glass by about 10 Gpa, so that the liquid
 temperature can be reduced by about 50 to 100.degree. C. That is,
 introduction of Y.sub.2 O.sub.3 in a small amount significantly improves
 characteristics and productivity of glass, and Y.sub.2 O.sub.3 can be
 effective as far as the contained amount of Y.sub.2 O.sub.3 is 0.8% or
 more. However, the Y.sub.2 O.sub.3 has power to suppress the growth of the
 primary crystal contained in the glass. Therefore, if the contained amount
 of Y.sub.2 O.sub.3 is too much, a surface crystallization occurs during a
 thermal treatment for crystallizing the glass, so that the aimed glass may
 not be produced. From this viewpoint, a proper contained amount of Y.sub.2
 O.sub.3 is equal to and less than 10%. Particularly, the contained amount
 of Y.sub.2 O.sub.3 is preferably equal to or less than 8%.
 As components other than the above, one or more oxides of alkali metals and
 alkali earth metals, such as Li.sub.2 O, Na.sub.2 O, K.sub.2 O, CaO, SrO,
 BaO, ZnO, NiO of 0 to 10 mol % and B.sub.2 O.sub.3, P.sub.2 O.sub.5,
 R.sub.2 O.sub.3 (R is rare earth metal ions except Y), ZrO.sub.2,
 CeO.sub.2, N.sub.2 O.sub.5 (N is Nb or Ta) of 0 to 5 mol % can be
 contained as far as the above oxides do not impair the characteristics
 belonging to the crystallized glass.
 As an anti-bubbling agent, As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 can be
 contained for making the glass uniform. In accordance with the high
 temperature viscosity varied in association with the glass composition,
 addition of either or both of As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 to the
 glass in an appropriate amount creates a glass with further uniformity. If
 the addition amount of the anti-bubbling agent is too much, the specific
 gravity of the glass may increase to lower the Young's modulus, and a
 platinum crucible may inflict damages due to agent's reaction with the
 crucible for melting. It is therefore proper that the adding amount of the
 anti-bubbling agent is equal to or less than 2%, and preferably equal to
 or less than 1.5%.
 Impurities in a raw material other than the above fundamental components,
 e.g., Cl, F, SO.sub.3, and the like, which serve as a glass clarifier may
 be contained as far as the impurities are in an amount less than 1%, which
 do not impair the characteristics belonging to the crystallized glass.
 The primary crystal phase of the crystallized glass according to the
 invention can be, e.g., an enstatite having a composition of MgO.SiO.sub.2
 and enstatite solid solution with components of (Mg.Al)SiO.sub.3 and a
 quasi-stable quartz solid solution having one or more compositions
 selected from a group of 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2
 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2, or particularly,
 .beta.-quartz solid solution. It is to be noted that the enstatite crystal
 phase includes clinoenstatite, protoenstatite, and one or more kinds of
 enstatites. With the crystallized glass of the invention, crystals other
 than the above such as spinel, mullite, 2MgO.SiO.sub.2, MgO.SiO.sub.2, can
 be contained. The mean crystal size contained in the invented crystallized
 glass is preferably 3 microns or less, more preferably 1 micron or less,
 and further preferably 0.5 micron or less. If the mean of the crystal size
 exceeds 1 micron, it not only reduces the mechanical strength of the glass
 but also impairs surface roughness of the glass upon induction of defects
 of the crystal during polishing.
 Third Embodiment [crystallized glass]
 Each component constituting a glass for the invented crystallized glass
 (Third Embodiment) is described below.
 SiO.sub.2 is a glass material having a meshed structure and also serves as
 a structural component for, as major deposited crystals, a quasi-stable
 quartz solid solution such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2,
 MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2 and
 an enstatite having a component of MgO.SiO.sub.2 and enstatite solid
 solution with components of (Mg.Al)SiO.sub.3. Where SiO.sub.2 is contained
 in an amount less than 35%, the melting glass becomes so unstable, thereby
 making molding at a high temperature hard and crystal depositions
 difficult. Furthermore, where SiO.sub.2 is contained in an amount less
 than 35%, the residual glass matrix phase may suffer from impaired
 chemical resistance, and the glass tends to suffer from a worse heat
 resistance. On the other hand, where SiO.sub.2 is contained in an amount
 more than 65%, the quasi-stable quartz solid solution and the enstatite as
 the primary crystal phase tend to be not readily deposited, and the
 Young's modulus of the glass tends to rapidly become small. Therefore, a
 proper contained amount of SiO.sub.2 is in a range of 35 to 65% in
 consideration of deposited crystal species, deposited amounts, chemical
 resistance, heat resistance, molding property, and productivity. From a
 viewpoint to obtain a crystallized glass having more preferable property,
 the contained amount of SiO.sub.2 is preferably in a range of 37 to 60%.
 Al.sub.2 O.sub.3 is an intermediate oxide of a glass and is a structural
 component of, as major crystal species, a quasi-stable quartz solid
 solution such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2
 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2. Introduction of
 Al.sub.2 O.sub.3 promotes deposition of the quasi-stable quartz solid
 solution and contributes to improvements of surface hardness of the glass.
 However, if the contained amount of Al.sub.2 O.sub.3 is less than 5%, the
 high Young's modulus crystal as described above may not be deposited well,
 and the glass matrix phase may suffer from impaired chemical resistance
 while the substrate material may lose the required strength. On the other
 hand, when the contained amount of Al.sub.2 O.sub.3 exceeds 25 mol %, an
 enstatite as a primary crystal phase may be not readily deposited, and the
 glass may not be melted easily due to a high melting temperature while
 losing its transparency and easiness for molding. Therefore, in
 consideration of solubility of the glass, molding property at high
 temperature, deposited crystal species, and so on, the contained amount of
 Al.sub.2 O.sub.3 is in a range of 5 to 25% and preferably in a range of 7
 to 22%.
 MgO is a modification component of the glass, has a crystal structure of
 the enstatite having a composition of MgO.SiO.sub.2 and enstatite solid
 solution with components of (Mg.Al)SiO.sub.3 or a quasi-stable quartz
 solid solution, and also serves as a main component of the crystal species
 having a composition such as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2,
 MgO.Al.sub.2 O.sub.3.3SiO.sub.2, or MgO.Al.sub.2 O.sub.3.4SiO.sub.2. Where
 the contained amount of MgO is less than 10%, the crystal as described
 above may not be deposited well, and the glass tends to lose the
 transparency and may be subject to a higher melting temperature, while a
 temperature span suitable for glass molding operation tends to be
 narrowed. On the other hand, if the contained amount of MgO exceeds 40%,
 the high temperature viscosity of the glass suddenly falls to render the
 glass thermally unstable, thereby impairing productivity, as well as
 lowering the Young's modulus and the durability. Therefore, in
 consideration of productivity, chemical resistance, high temperature
 viscosity, strength, and so on of the glass, the contained amount of MgO
 is in a range of 10 to 40% and preferably in a range of 12 to 37%.
 It is to be noted that the contained amounts of MgO and Al.sub.2 O.sub.3
 are adjusted so that the mole ratio (Al.sub.2 O.sub.3 /MgO) is less than
 0.9. If the mole ratio (Al.sub.2 O.sub.3 /MgO) is equal to or more than
 0.9, the Young's modulus of the crystallized glass tends to drop suddenly.
 Where the ratio Al.sub.2 O.sub.3 /MgO is set less than 0.9, a crystallized
 glass having a high Young's modulus of 150 GPa or higher can be obtained.
 It is preferable to set the ratio Al.sub.2 O.sub.3 /MgO less than 0.5,
 more preferably the ratio Al.sub.2 O.sub.3 /MgO less than 0.45. However,
 if the mole ratio of Al.sub.2 O.sub.3 /MgO is too small, the high
 temperature viscosity of the glass may be made lower, and therefore, the
 ratio is properly 0.2 or higher, and more preferably 0.25 or higher.
 TiO.sub.2 is a nucleus forming agent for crystal phase deposition of the
 enstatite crystal having a composition of MgO.SiO.sub.2 and enstatite
 solid solution with components of (Mg.Al)SiO.sub.3 and the crystal phase
 of 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2 having a crystal structure of a
 quasi-stable quartz solid solution. TiO.sub.2 also has an effect to
 suppress loss of glass transparency where the contained amount of
 SiO.sub.2 is less. It is to be noted that where the contained amount of
 TiO.sub.2 is less than 5%, a uniform crystallized glass may not be
 produced easily due to surface crystallization on the glass where
 TiO.sub.2 does not serve adequately as the nucleus forming agent for
 primary crystal. On the other hand, where the contained amount of
 TiO.sub.2 exceeds 15%, the glass may suffer from divided phases due to too
 lowered high temperature viscosity or from loss of transparency, thereby
 making the productivity of the glass extremely impaired. Consequently, in
 consideration of productivity, chemical resistance, high temperature
 viscosity, crystal nucleus production, and so on of the glass, the
 contained amount of TiO.sub.2 is in a range of 5 to 15% and preferably in
 a range of 5.5 to 14%.
 For the crystallized glass according to the invention, Li.sub.2 O is a
 component to reduce the liquid phase temperature of the glass and to
 promote deposition of further fine crystal grains. For example, if
 Li.sub.2 O of about 2% is introduced into a MgO--Al.sub.2 O.sub.3
 --SiO.sub.2 --TiO.sub.2 glass, the crystal grain size becomes about a half
 or below in comparison with a glass with no addition of Li.sub.2 O while
 the Young's modulus of the glass is almost unchanged. Another remarkable
 role of Li.sub.2 O is to lower the liquid temperature of the glass, and
 the liquid temperature of the glass can be reduced by about 50.degree. C.
 upon introduction of Li.sub.2 O of about 2%. However, if the contained
 amount of Li.sub.2 O is too much, the glass remarkably shows divided
 phases and may suffer from worse productivity due to such as a larger
 tendency of the transparency loss. Therefore, a proper contained amount of
 Li.sub.2 O is in a range of 0.2 to 10%, and a contained amount of Li.sub.2
 O is more preferably in a range of 0.5 to 8%.
 For the crystallized glass according to the invention, Y.sub.2 O.sub.3 is
 not a necessary component, but, as described in embodiments below,
 introduction of Y.sub.2 O.sub.3 of 2%, for example, increases the Young's
 modulus of the crystallized glass by about 10 Gpa, so that the liquid
 temperature can be reduced by about 50 to 100.degree. C. That is,
 introduction of Y.sub.2 O.sub.3 in a small amount significantly improves
 characteristics and productivity of glass, and Y.sub.2 O.sub.3 can be
 effective as far as the contained amount of Y.sub.2 O.sub.3 is 0.8% or
 more. However, the Y.sub.2 O.sub.3 has power to suppress the growth of the
 primary crystal contained in the glass. Therefore, if the contained amount
 of Y.sub.2 O.sub.3 is too much, a surface crystallization occurs during a
 thermal treatment for crystallizing the glass, so that the aimed glass may
 not be produced. From this viewpoint, a proper contained amount of Y.sub.2
 O.sub.3 is equal to and less than 10%. Particularly, the contained amount
 of Y.sub.2 O.sub.3 is preferably equal to or less than 8%.
 As components other than the above, the crystallized glass of the invention
 can contain, as arbitrary components, Na.sub.2 O of 0 to 10 mol %, K.sub.2
 O of 0 to 10 mol %, CaO of 0 to 10 mol %, Sro of 0 to 10 mol %, BaO of 0
 to 10 mol %, ZnO of 0 to 10 mol %, NiO of 0 to 10 mol %, R.sub.2 O.sub.3
 of 0 to 5 mol % (wherein R is B ions or rare earth metal ions), CeO.sub.2
 of 0 to 5 mol %, ZrO.sub.2 of 0 to 5 mol %, N.sub.2 O.sub.5 of 0 to 5 mol
 % (wherein N is P ions, Nb ions, and Ta ions), As.sub.2 O.sub.3 of 0 to 2
 mol %, and Sb.sub.2 O.sub.3 of 0 to 2 mol %.
 Where the glass contains oxide components of alkali or alkali earth metal
 such as Na.sub.2 O, K.sub.2 O, CaO, SrO, BaO, ZnO, and NiO, the high
 temperature viscosity of the glass can be adjusted in suppressing the
 tendency of transparency loss and unifying the crystal grains. For
 example, if the above component is introduced into the crystallized glass
 of the invention in an amount of 2 to 5%, the Young's modulus is reduced
 more or less, but the introduction improves the glass productivity and
 unifies the size of the crystal grains, and other characteristics of the
 glass can be improved. In total consideration of respective glass features
 such as Young's modulus, productivity, surface smoothness of the
 crystallized glass, strength, and the like, the contained amounts of the
 oxide components of alkali metals and alkali earth metals such as Na.sub.2
 O, K.sub.2 O, CaO, SrO, BaO, ZnO, and NiO, is preferably 10% or less, more
 preferably 7% or less.
 Where B.sub.2 O.sub.3 or P.sub.2 O.sub.5 is contained in the invented
 crystallized glass, the loss of transparency can be suppressed mainly in a
 molding temperature range of the glass. It is to be noted that because
 those components reduce the Young's modulus of the glass significantly, a
 desirable contained amount is 5% or less. In consideration of the
 productivity of the glass, it is further desirable that the contained
 amount is 4% or less.
 Where R.sub.2 O.sub.3 (R is rare earth metal ions except Y), CeO.sub.2, or
 N.sub.2 O.sub.5 (N is Nb or Ta) is contained in the invented glass, the
 glass may have better thermal stability, productivity, and Young's
 modulus. Since those components also operate to suppress crystal grain
 growth, those components allow the glass to be produced with excellent
 surface smoothness. However, any of those oxides is expensive, and if
 introduced too much, those makes the liquid temperature of the glass worse
 and the specific gravity of the glass increase abruptly. Therefore, it is
 proper to set that the contained amount is 5% or less. The introduction
 amount of R.sub.2 O.sub.3 is properly set 5% or less, more preferably 4%
 or less, in consideration of productivity, specific gravity, liquid phase
 temperature of the glass.
 As an anti-bubbling agent, As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 can be
 contained in the invented glass for making the glass uniform. In
 accordance with the high temperature viscosity varied in association with
 the glass composition, addition of either or both of As.sub.2 O.sub.3 and
 Sb.sub.2 O.sub.3 to the glass in an appropriate amount creates a glass
 with further uniformity. If the addition amount of the anti-bubbling agent
 is too much, the specific gravity of the glass may increase to lower the
 Young's modulus, and a platinum crucible may inflict damages due to
 agent's reaction with the crucible for melting. It is therefore proper
 that the adding amount of the anti-bubbling agent is equal to or less than
 2%, and preferably equal to or less than 1.5%.
 Impurities in a raw material other than the above fundamental components,
 e.g., Cl, F, SO.sub.3, and the like, which serve as a glass clarifier may
 be contained as far as the impurities are in an amount less than 1%, which
 do not impair the characteristics belonging to the crystallized glass.
 The primary crystal phase of the crystallized glass according to the
 invention can be, e.g., an enstatite having a composition of MgO.SiO.sub.2
 and enstatite solid solution with components of (Mg.Al)SiO.sub.3 and a
 quasi-stable quartz solid solution having one or more compositions
 selected from a group of 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2
 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2, or particularly,
 .beta.-quartz solid solution. It is to be noted that the enstatite crystal
 phase includes clinoenstatite, protoenstatite, and one or more kinds of
 enstatites. With the crystallized glass of the invention, crystals other
 than the above, such as spinel, mullite, 2MgO.SiO.sub.2, MgO.SiO.sub.2,
 and Mg-Al-titanate can be contained.
 The mean crystal size of the enstatite and quasi-stable quartz solid
 solution contained in the invented crystallized glass is preferably 1
 micron or less. Where the mean crystal size of the crystal grains is set 1
 micron or less, the crystallized glass can be formed with excellent
 strength and surface smoothness. If the mean crystal grain size exceeds 1
 micron, it not only reduces the mechanical strength of the glass but also
 impairs surface roughness of the glass upon induction of defects of the
 crystal during polishing. The mean grain size of the crystal grain is
 preferably 0.5 micron or less.
 Fourth Embodiment [crystallized glass]
 Components constituting the invented crystallized glass (Fourth Embodiment)
 are indicated on an oxide basis as well as the original glass. The reason
 that the component ranges of the original glass are restricted in a way as
 described above is as follows.
 SiO.sub.2 is a glass material having a meshed structure and also serves as
 a structural component for, as major deposited crystals, a quasi-stable
 quartz solid solution and an enstatite. SiO.sub.2 also serves as a
 structural component of .beta.-spodumene though not a primary crystal.
 Where SiO.sub.2 is contained in an amount less than 42%, the melting glass
 becomes so unstable, thereby making molding at a high temperature hard and
 depositions of the above crystal difficult as the primary crystal.
 Furthermore, where SiO.sub.2 is contained in an amount less than 42%, the
 residual glass matrix phase may suffer from impaired chemical resistance,
 and the glass tends to suffer from a worse heat resistance. On the other
 hand, where SiO.sub.2 is contained in an amount more than 65%, the Young's
 modulus of the glass tends to rapidly become small. Therefore, a proper
 contained amount of SiO.sub.2 is in a range of 42 to 65% in consideration
 of deposited crystal species, deposited amounts, Young's modulus, chemical
 resistance, heat resistance, molding property, and productivity. A
 preferable lower limit is 45% or higher, more preferably, 48% or higher,
 and a preferable upper limit is 62% or lower, more preferably, 60% or
 lower.
 Al.sub.2 O.sub.3 is an intermediate oxide of a glass and is a structural
 component of, as major crystal species, a quasi-stable quartz solid
 solution. Introduction of Al.sub.2 O.sub.3 promotes deposition of the
 quasi-stable quartz solid solution crystal and contributes to improvements
 of surface hardness of the glass. However, if the contained amount of
 Al.sub.2 O.sub.3 exceeds 15%, the glass may not be molded well while
 becoming not readily melting due to a high melting temperature and a
 liquid phase temperature. Therefore, the contained amount of Al.sub.2
 O.sub.3 is set equal to or less than 15%. In consideration of solubility
 of the glass, molding property at high temperature, deposited crystal
 species, and so on, the contained amount of Al.sub.2 O.sub.3 is in a range
 of 0 to 15%. The lower limit is preferably 1% or higher, more preferably
 2% or higher, and the upper limit is preferably 10% or lower, more
 preferably 7% or lower. The total amount of SiO.sub.2 and Al.sub.2 O.sub.3
 is preferably 50% or higher, more preferably 55% or higher, from a
 viewpoint to give a high temperature viscosity allowing the glass to be
 molded.
 MgO is a very important component having effects for producing the quartz
 solid solution and the enstatite from a thermal treatment of the original
 glass with SiO.sub.2 and for maintaining transparency in improving the
 hardness and heat resistance. However, if the contained amount of MgO is
 less than 5%, the above effects cannot be obtained. As the contained
 amount of MgO is less, the glass more tends to lose its transparency and
 to increase the melting temperature, so that the contained amount of MgO
 is set 5% or higher. On the other hand, if the contained amount of MgO
 exceeds 30%, the liquid phase temperature of the glass suddenly becomes
 higher, thereby impairing productivity, as well as furnishing property.
 Therefore, the contained amount of MgO is set 30% or less. In
 consideration of productivity, melting property, mechanical strength, and
 the like of the glass, the contained amount of MgO is in a range of 5 to
 30%. The lower limit is preferably 7% or higher, more preferably 10% or
 higher, and the upper limit is preferably 25% or lower, more preferably
 20% or lower.
 The crystallized glass includes Y.sub.2 O.sub.3. Introduction of Y.sub.2
 O.sub.3 of at least 0.5% increases the Young's modulus of the crystallized
 glass by about 5 Gpa, so that the liquid temperature can be reduced by
 about 50.degree. C. Furthermore, introduction of Y.sub.2 O.sub.3 of at
 least 0.5% improves thermal stability of the glass. Thus, introduction of
 Y.sub.2 O.sub.3 in a small amount significantly improves characteristics
 and productivity of glass. However, since the Y.sub.2 O.sub.3 has power to
 suppress the growth of the primary crystal contained in the glass, if the
 contained amount of Y.sub.2 O.sub.3 is too much, a surface crystallization
 occurs during a thermal treatment for crystallizing the glass, so that the
 crystallized glass may not be produced with the aimed surface smoothness.
 Therefore, the contained amount of Y.sub.2 O.sub.3 is set 8% or less. The
 lower limit of the contained amount of Y.sub.2 O.sub.3 is preferably 0.5%
 or higher, more preferably 1% or higher, and the upper limit of the
 contained amount of Y.sub.2 O.sub.3 is preferably 5% or lower, more
 preferably 3% or lower.
 Li.sub.2 O is a component having effects for producing crystals of the
 quartz solid solution and the .beta.-spodumene solid solution from a
 thermal treatment of the original glass with SiO.sub.2 and for reducing
 the liquid phase temperature and crystallization treatment temperature of
 the glass. If the contained amount of Li.sub.2 O is 10% or less, the above
 effects cannot be obtained. Furthermore, if the contained amount of
 Li.sub.2 O is 10% or less, the glass has a higher melting temperature, so
 that the glass disc molding has a narrower working temperature span.
 Therefore, it is appropriate to set the contained amount of Li.sub.2 O as
 exceeding 10%. On the other hand, if the contained amount of Li.sub.2 O
 exceeds 25%, the glass becomes very unstable, and the obtained
 crystallized glass tends to have a greatly low Young's modulus. The
 contained amount of Li.sub.2 O is therefore set 25% or less. In
 consideration of productivity, melting property, mechanical strength, and
 the like of the glass, the lower limit of the contained amount of Li.sub.2
 O is preferably 10.5% or higher, more preferably 11% or higher, and the
 upper limit of the contained amount of Li.sub.2 O is preferably 22% or
 lower, more preferably 20% or lower.
 The crystallized glass according to the invention includes, as a primary
 crystal phase, either or both of a quartz solid solution and an enstatite.
 The crystal phase of the quartz solid solution is a quasi-stable quartz
 solid solution having one or more compositions selected from a group of
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2, or particularly, .beta.-quartz solid
 solution. The crystal phase of the enstatite is an enstatite crystal phase
 having a composition of MgO.SiO.sub.2 and enstatite solid solution with
 components of (Mg.Al)SiO.sub.3. It is to be noted that the enstatite
 crystal phase includes clinoenstatite, protoenstatite, and one or more
 kinds of enstatites. With the crystallized glass of the invention,
 crystals other than the above, such as .beta.-spodumene solid solution can
 be contained as crystal phases.
 The crystal contained in the invented crystallized glass preferably has
 crystal grain sizes capable of forming a polished surface whose surface
 roughness Ra on the invented crystallized glass is in a range of 0.1 to
 0.9 nm, and more preferably, crystal grain sizes capable of forming a
 polished surface whose surface roughness Ra is in a range of 0.1 to 0.5
 nm. Where the crystal grain sizes of the crystal phase contained in the
 crystallized glass meet the above range, the glass can provide an
 information recording disc having excellent surface smoothness.
 With the crystallized glass of the invention, it is preferable to select
 the component of the glass so that the liquid phase temperature of the
 original glass is 1200.degree. C. or less. More preferably, it is selected
 so that the liquid phase temperature of the original glass is 1150.degree.
 C. or less. Where the original glass has a lower liquid phase temperature,
 the crystallized glass substrate can be produced easily. That is, because
 no very high temperature is required for the steps of melting, molding,
 and the like of the raw material done at the manufacturing process for the
 glass substrate, the glass can be advantageously produced with ease where
 wide variations are available for choices of melting furnaces and
 materials for forming molding.
 TiO.sub.2, ZrO.sub.2, and P.sub.2 O.sub.5 operate as forming agents for
 crystal nucleus and promote deposition of fine crystal gains such as the
 quartz solid solution and the enstatite. Those also serve as components to
 give the glass thermal stability where the contained amount of SiO.sub.2
 is relatively small. Accordingly, the crystallized glass of the invention
 preferably contains at least one kind of TiO.sub.2, ZrO.sub.2, and P.sub.2
 O.sub.5. If the total contained amount of TiO.sub.2, ZrO.sub.2, and
 P.sub.2 O.sub.5 is 5% or less at that time, an effect for nucleus forming
 agents for primary crystal may not be obtained adequately, so that surface
 crystallization occurs on the glass, and so that the uniform crystallized
 glass tends to be hard to be obtained. Therefore, the total contained
 amount of TiO.sub.2, ZrO.sub.2, and P.sub.2 O.sub.5 is preferably 5% or
 more. On the other hand, if the total contained amount of TiO.sub.2,
 ZrO.sub.2, and P.sub.2 O.sub.5 is preferably 18% or more, the glass may
 suffer from divided phases due to too lowered high temperature viscosity
 or from loss of transparency, thereby making the productivity of the glass
 extremely impaired. Therefore, the total contained amount of TiO.sub.2,
 ZrO.sub.2, and P.sub.2 O.sub.5 is preferably 18% or less. Consequently, in
 consideration of productivity, chemical resistance, high temperature
 viscosity, crystal nucleus production, and so on of the glass, the total
 contained amount of TiO.sub.2, ZrO.sub.2, and P.sub.2 O.sub.5 is in a
 range of 5 to 18%. The lower limit of the total contained amount of
 TiO.sub.2, ZrO.sub.2, and P.sub.2 O.sub.5 is preferably 6% or higher, more
 preferably 7% or higher, and the upper limit is preferably 15% or lower,
 more preferably 13% or lower
 The oxide components of alkali or alkali earth metal such as Na.sub.2 O,
 K.sub.2 O, CaO, SrO, BaO, ZnO, and NiO can adjust mainly the high
 temperature viscosity of the glass, suppressing the tendency of
 transparency loss and unifying the crystal grains. If at least one of the
 above components is added to the glass, the Young's modulus may be reduced
 more or less, but this addition improves the glass productivity and
 unifies the size of the crystal grains, and other characteristics of the
 glass can be improved. In consideration of respective glass features such
 as Young's modulus, productivity, surface smoothness of the crystallized
 glass, strength, and the like, the contained amounts of Na.sub.2 O is in a
 range of 0 to 10%; the contained amount of K.sub.2 O is in a range of 0 to
 10%; and the contained amount of Na.sub.2 O and K.sub.2 O is preferably
 10% or less. More preferably, the contained amount of Na.sub.2 O, K.sub.2
 O, and Na.sub.2 O+K.sub.2 O is 8% or less. Similarly, the contained amount
 of CaO is in a range of 0 to 10%; the contained amount of SrO is in a
 range of 0 to 10%; the contained amount of BaO is in a range of 0 to 10%;
 the contained amount of ZnO is in a range of 0 to 10%; the contained
 amount of NiO is in a range of 0 to 10%; the contained amount of CaO, SrO,
 BaO, ZnO, and NiO is preferably 10% or less. Moreover, the contained
 amount of CaO, SrO, BaO, ZnO, and NiO and the total of CaO, SrO, BaO, ZnO,
 and NiO is preferably 8% or less.
 The crystallized glass according to the invention, as far as does not lose
 any characteristics of the glass, in addition to the above components, can
 contain some rare earth metal oxide component such as B.sub.2 O.sub.3,
 Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 and La.sub.2 O.sub.3. However, those
 components significantly reduce the Young's modulus of the glass.
 Therefore, the contained amount of B.sub.2 O.sub.3 is in a range of 0 to
 5%; the contained amount of R.sub.2 O.sub.3 is in a range of 0 to 5%
 (wherein R is rare earth metal ions (e.g., Nd.sup.3+, Pr.sup.3+,
 Pm.sup.3+, Sm.sup.3+, Eu.sup.3+, Gd.sup.3+, Tb.sup.3+, Dy.sup.3+,
 Ho.sup.3+, Er.sup.3+, Tm.sup.3+, Yb.sup.3+)); the contained amount of
 CeO.sub.2 is in a range of 0 to 5%; the contained amount of N.sub.2
 O.sub.5 is in a range of 0 to 5% (wherein N is Nb or Ta); and the total of
 B.sub.2 O.sub.3, R.sub.2 O.sub.3, CeO.sub.2, and N.sub.2 O.sub.5 is
 preferably equal to or less than 5 mol %. In consideration of productivity
 of the glass, the contained amount of each component and the contained
 amount of the total are preferably set 4% or less.
 As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 are components to be added as
 anti-bubbling agents, to make uniform the glass as a raw material for
 crystallized glass. Addition of either or both of As.sub.2 O.sub.3 and
 Sb.sub.2 O.sub.3 to the glass in an appropriate amount according to the
 high temperature viscosity of the respective glasses, creates a glass with
 further uniformity. If the addition amount of the anti-bubbling agent is
 too much, the specific gravity of the glass may increase to lower the
 Young's modulus, and a platinum crucible may inflict damages due to
 agent's reaction with the crucible for melting. Therefore, the contained
 amount of As.sub.2 O.sub.3 is in a range of 0 to 2%; the contained amount
 of Sb.sub.2 O.sub.3 is in a range of 0 to 2%; the total of As.sub.2
 O.sub.3 and Sb.sub.2 O.sub.3 is preferably equal to or less than 2 mol %.
 Particularly, the contained amount of As.sub.2 O.sub.3, Sb.sub.2 O.sub.3
 and the total of As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 are preferably set
 1.5% or less.
 Fifth Embodiment [crystallized glass]
 Components constituting the invented crystallized glass (Fifth Embodiment)
 are indicated on an oxide basis as well as the original glass. The reason
 that the component ranges of the original glass are restricted in a way as
 described above is as follows.
 SiO.sub.2 is a glass material having a meshed structure and also serves as
 a structural component for, as major deposited crystals, an enstatite and
 a quasi-stable quartz solid solution. Where SiO.sub.2 is contained in an
 amount less than 35%, the melting glass becomes so unstable, thereby
 making molding at a high temperature hard and depositions of the above
 crystal difficult. Furthermore, where SiO.sub.2 is contained in an amount
 less than 35%, the residual glass matrix phase may suffer from impaired
 chemical resistance, and the glass may suffer from worse heat resistance.
 On the other hand, where SiO.sub.2 is contained in an amount more than
 55%, the Young's modulus of the glass tends to rapidly become small.
 Therefore, in consideration of deposited crystal species, deposited
 amounts, chemical resistance, heat resistance, molding property, and
 productivity, a lower limit of the contained amount of SiO.sub.2 is 35%
 while a preferable upper limit is 55%. A preferable lower limit is 37% or
 higher, more preferably, 40% or higher, and a preferable upper limit is
 54% or lower, more preferably, 53% or lower.
 Al.sub.2 O.sub.3 is an intermediate oxide of a glass and is a structural
 component of, as major crystal species, a quasi-stable quartz solid
 solution. Introduction of Al.sub.2 O.sub.3 promotes deposition of the
 quasi-stable quartz solid solution crystal and contributes to improvements
 of surface hardness of the glass. However, if the contained amount of
 Al.sub.2 O.sub.3 exceeds 5%, the glass may not be molded well while
 becoming not readily melting due to a high melting temperature and a
 liquid phase temperature. Therefore, the contained amount of Al.sub.2
 O.sub.3 is set equal to or less than 5%. In consideration of solubility of
 the glass, molding property at high temperature, deposited crystal
 species, and so on, the lower limit of the contained amount of Al.sub.2
 O.sub.3 is preferably 0%, more preferably 1%. The upper limit of the
 contained amount of Al.sub.2 O.sub.3 is preferably 4.5%, more preferably
 4%.
 It is to be noted that although Al.sub.2 O.sub.3 may not be contained in
 the glass, the total contained amount of SiO.sub.2 and Al.sub.2 O.sub.3 is
 preferably 40 mol % or higher from a viewpoint to give the glass adequate
 chemical resistance and thermal stability enabling mass-production.
 Therefore, in the case that Al.sub.2 O.sub.3 is not contained, the
 contained amount of SiO.sub.2 is 40 mol % or more. The total contained
 amount of SiO.sub.2 and Al.sub.2 O.sub.3 is preferably 42 mol % or higher.
 MgO is a component having effects for producing the enstatite crystal from
 a thermal treatment of the original glass with SiO.sub.2 and for
 maintaining transparency in improving the hardness and heat resistance. If
 the contained amount of MgO is less than 25%, the above effects cannot be
 obtained. Therefore, the contained amount of MgO is set 25% or higher. On
 the other hand, if the contained amount of MgO exceeds 45 mol %, the high
 temperature viscosity of the glass suddenly falls to make the glass
 thermally unstable, thereby impairing productivity, as well as furnishing
 property. Therefore, the contained amount of MgO is set 45% or less. In
 consideration of productivity, chemical resistance, high temperature
 viscosity, strength, and the like of the glass, regarding the contained
 amount of MgO, the lower limit is 25% while the upper limit is 45%. The
 lower limit is preferably 28%, more preferably 32%, and the upper limit is
 preferably 43%, more preferably 42%.
 The crystallized glass includes Y.sub.2 O.sub.3. Introduction of Y.sub.2
 O.sub.3 of at least 0.5% increases the Young's modulus of the crystallized
 glass by about 5 Gpa, so that the liquid temperature can be reduced by
 about 50.degree. C. Furthermore, introduction of Y.sub.2 O.sub.3 of at
 least 0.5% improves thermal stability of the glass. Thus, introduction of
 Y.sub.2 O.sub.3 in a small amount significantly improves characteristics
 and productivity of glass. However, since the Y.sub.2 O.sub.3 has power to
 suppress the growth of the primary crystal contained in the glass, if the
 contained amount of Y.sub.2 O.sub.3 is too much, a surface crystallization
 occurs during a thermal treatment for crystallizing the glass, so that the
 crystallized glass may not be produced with the aimed surface smoothness.
 Therefore, the contained amount of Y.sub.2 O.sub.3 is set 8% or less. The
 lower limit of the contained amount of Y.sub.2 O.sub.3 is preferably 0.5%
 or higher, more preferably 1% or higher, and the upper limit of the
 contained amount of Y.sub.2 O.sub.3 is preferably 5% or lower, more
 preferably 3% or lower.
 TiO.sub.2 and ZrO.sub.2 are necessary components for depositions of the
 crystal grains such as the quartz solid solution and the enstatite as
 forming agents for crystal nucleus, and also have an effect to give the
 glass a thermal stability where the contained amount of SiO.sub.2 is less.
 Where the total contained amount of TiO.sub.2 and ZrO.sub.2 is less than
 4.5%, a uniform crystallized glass may not be produced easily due to
 surface crystallization on the glass, because TiO.sub.2 and ZrO.sub.2 does
 not serve adequately as the nucleus forming agent for primary crystal.
 Accordingly, the total contained amount of TiO.sub.2 and ZrO.sub.2 is 4.5%
 or higher. If the contained amount of TiO.sub.2 and ZrO.sub.2 exceeds 18%,
 the glass may suffer from divided phases due to too lowered high
 temperature viscosity or from loss of transparency, thereby making the
 productivity of the glass extremely impaired. Accordingly, the total
 contained amount of TiO.sub.2 and ZrO.sub.2 is 18% or lower. In
 consideration of productivity, chemical resistance, high temperature
 viscosity, crystal nucleus production, and so on of the glass, the lower
 limit of the total contained amount of TiO.sub.2 and ZrO.sub.2 is 4.5%,
 and the upper limit is 18%. The lower limit of the total contained amount
 of TiO.sub.2 and ZrO.sub.2 is preferably 5%, and the upper limit is
 preferably 15%. It is to be noted that it is appropriate that the
 contained amount of ZrO.sub.2 is in a range of 0 to 10 mol % and that the
 contained amount of TiO.sub.2 is in a range of 0 to 12 mol %, in
 consideration of the high temperature melting property and thermal
 stability of the glass.
 The crystallized glass according to the invention includes, as a primary
 crystal phase, either or both of a quartz solid solution and an enstatite.
 The crystal phase of the quartz solid solution is a quasi-stable quartz
 solid solution having one or more compositions selected from a group of
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2, or particularly, .beta.-quartz solid
 solution. The crystal phase of the enstatite is an enstatite crystal phase
 having a composition of MgO.SiO.sub.2 and enstatite solid solution with
 components of (Mg.Al)SiO.sub.3. It is to be noted that the enstatite
 crystal phase includes clinoenstatite, protoenstatite, and one or more
 kinds of enstatites.
 The crystal contained in the invented crystallized glass preferably has
 crystal grain sizes capable of forming a polished surface whose surface
 roughness Ra on the invented crystallized glass is in a range of 0.1 to
 0.9 nm, and more preferably, crystal grain sizes capable of forming a
 polished surface whose surface roughness Ra is in a range of 0.1 to 0.5
 nm. Where the crystal grain sizes of the crystal phase contained in the
 crystallized glass meet the above range, the glass can provide an
 information recording disc having excellent surface smoothness.
 Alkali metal oxides such as Li.sub.2 O, Na.sub.2 O, and K.sub.2 O are
 additive components to decrease the liquid phase temperature of the glass
 and to make deposition of further fine crystal grains. For example, if
 Li.sub.2 O of about 2% is introduced into a MgO--Al.sub.2 O.sub.3
 --SiO.sub.2 --TiO.sub.2 glass, the crystal grain size becomes about a half
 or below in comparison with a glass with no addition of Li.sub.2 O while
 the Young's modulus of the glass is almost unchanged. Another remarkable
 role of Li.sub.2 O, Na.sub.2 O, and K.sub.2 O is to lower the liquid
 temperature of the glass, and the liquid temperature of the glass can be
 reduced by about 50.degree. C. upon introduction of the alkali metal
 oxides of about 2%. However, if the contained amount of alkali components
 such as Li.sub.2 O is too much, the glass may suffer from worse
 productivity due to such as a lower Young's modulus of the glass and a
 larger tendency of the transparency loss. Therefore, a total introduction
 amount of Li.sub.2 O, Na.sub.2 O, and K.sub.2 O is 5% or less. The total
 introduction amount of Li.sub.2 O, Na.sub.2 O, and K.sub.2 O is more
 preferably 4% or less.
 The oxide components of alkali earth metal such as CaO, SrO, BaO, ZnO, and
 NiO can adjust mainly the high temperature viscosity of the glass,
 suppressing the tendency of transparency loss and unifying the crystal
 grains. If at least one of the above components is added to the glass, the
 Young's modulus may be reduced more or less, but this addition improves
 the glass productivity and unifies the size of the crystal grains, and
 other characteristics of the glass can be improved. In consideration of
 respective glass features such as Young's modulus, productivity, surface
 smoothness of the crystallized glass, strength, and the like, the
 contained amount of CaO is in a range of 0 to 10%; the contained amount of
 SrO is in a range of 0 to 10%; the contained amount of BaO is in a range
 of 0 to 10%; the contained amount of ZnO is in a range of 0 to 10%; the
 contained amount of NiO is in a range of 0 to 10%; the contained amount of
 CaO, SrO, BaO, ZnO, and NiO is preferably 10% or less. Moreover, the
 contained amount of CaO, SrO, BaO, ZnO, and NiO and the total of CaO, SrO,
 BaO, ZnO, and NiO is preferably 8% or less.
 The crystallized glass according to the invention, as far as does not lose
 any characteristics of the glass, in addition to the above components, can
 contain some rare earth metal oxide component such as B.sub.2 O.sub.3,
 P.sub.2 O.sub.5, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 and La.sub.2 O.sub.3.
 However, those components significantly reduce the Young's modulus of the
 glass. Therefore, the contained amount of B.sub.2 O.sub.3 is in a range of
 0 to 5%; the contained amount of P.sub.2 O.sub.5 is in a range of 0 to 5%;
 the contained amount of R.sub.2 O.sub.3 is in a range of 0 to 5% (wherein
 R is rare earth metal ions (e.g., Nd.sup.3+, Pr.sup.3+, Pm.sup.3+,
 Sm.sup.3+, Eu.sup.3+, Gd.sup.3+, Tb.sup.3+, Dy.sup.3+, Ho.sup.3+,
 Er.sup.3+, Tm.sup.3+, Yb.sup.3+)); the contained amount of CeO.sub.2 is in
 a range of 0 to 5%; the contained amount of N.sub.2 O.sub.5 is in a range
 of 0 to 5% (wherein N is Nb or Ta); and the total of B.sub.2 O.sub.3,
 P.sub.2 O.sub.5, R.sub.2 O.sub.3, CeO.sub.2, and N.sub.2 O.sub.5 is
 preferably equal to less than 5 mol %. In consideration of productivity of
 the glass, the contained amount of each component and the contained amount
 of the total are preferably set 4% or less.
 As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 are components to be added as
 anti-bubbling agents, to make uniform the glass as a raw material for
 crystallized glass. Addition of either or both of As.sub.2 O.sub.3 and
 Sb.sub.2 O.sub.3 to the glass in an appropriate amount according to the
 high temperature viscosity of the respective glasses, creates a glass with
 further uniformity. If the addition amount of the anti-bubbling agent is
 too much, the specific gravity of the glass may increase to lower the
 Young's modulus, and a platinum crucible may inflict damages due to
 agent's reaction with the crucible for melting. Therefore, the contained
 amount of As.sub.2 O.sub.3 is in a range of 0 to 2%; the contained amount
 of Sb.sub.2 O.sub.3 is in a range of 0 to 2%; the total of As.sub.2
 O.sub.3 and Sb.sub.2 O.sub.3 is preferably equal to or less than 2 mol %.
 Particularly, the contained amount of As.sub.2 O.sub.3, Sb.sub.2 O.sub.3
 and the total of As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3 are preferably set
 1.5% or less.
 Crystallized Glass and Manufacturing Method for Substrate
 The crystallized glass and substrate according to the invention can be
 manufactured using known manufacturing methods for glass. For example, in
 a high temperature melting method, a glass raw material of a prescribed
 blend is melt in air or an inert gas atmosphere; the glass is unified by
 bubbling, adding anti-bubbling agents, or stirring; the glass is molded
 into glass plates by a method such as a known pressing method and a down
 drawing molding method; the glass is then subject to furnishing such as
 grinding or polishing, to produce glass molded articles having desired
 sizes and shapes. When the final products are substrates, the glass molded
 articles can be formed in consideration of the shape of the substrates.
 The obtained glass molded articles are then subject to a thermal treatment
 for crystallization. There is no special limitation on the thermal
 treatment method, and it can be selected in accordance with the contained
 amount of the crystallization promoting agent, the glass transition
 temperature, the peak temperature for crystallization. It is preferable,
 from a viewpoint to make smaller the crystal, that the crystal is grown at
 a raised temperature 850 to 1150.degree. C. after the glass is thermally
 treated at a relatively low temperature (e.g., 700 to 850.degree. C.) at
 the initial stage to generate many crystal nucleuses. To manufacture the
 crystallized glass of the invention, systematic changes of the schedules
 for the thermal treatments and glass compositions allow the deposited
 crystal sizes and crystal amounts to be controlled, thereby capable of
 adjusting widely the property of the crystallized glasses. In this
 invention, the manufacturing process for crystallization can be controlled
 easily because the permissive temperature range for thermal treatment for
 production of crystal nucleuses and thermal treatment for crystal growth
 for forming crystallized glasses having the same Young's modulus, the same
 crystal grain size, and the same homogeneity of crystallization has a
 temperature span of 30.degree. C. or more.
 With the crystallized glass according to the invention, the glass has a
 crystal structure of the enstatite crystal phase having the composition of
 MgO.SiO.sub.2 and enstatite solid solution with components of
 (Mg.Al)SiO.sub.3 from the thermal treatment or a quartz solid solution,
 and it is appropriate to use a thermal condition for depositing, as a
 primary crystal, at least one kind among crystals having compositions such
 as 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2 O.sub.3.3SiO.sub.2, and
 MgO.Al.sub.2 O.sub.3.4SiO.sub.2. It is to be noted that although
 2MgO.SiO.sub.2, spinel, mullite, or other crystals, as primary crystals
 other than the above, can be deposited, it is desirable to set a condition
 for depositing, as the primary crystal phase, the enstatite crystal phase
 (the enstatite crystal phase includes clinoenstatite, protoenstatite, and
 one or more kinds of enstatites) and a quasi-stable quartz solid solution,
 or particularly, .beta.-quartz solid solution. The treatment temperature
 for crystallization is kept at a low temperature as much as possible as a
 such condition. For example, it is appropriate to set the condition at
 1150.degree. C. or less. It is also appropriate to set the treatment
 temperature for nucleus production at a temperature 30 to 60.degree. C.
 higher than the glass transition temperature.
 The molded articles of the crystallized glass to which the thermal
 treatment is already finished can be polished when necessary, and there is
 no restriction on the polishing method. The glass molded articles can be
 polished by known methods in use of synthetic hone particles such as
 artificial or synthetic diamonds, silicon carbides, aluminum oxides, and
 boron carbides, and natural hone particles such as natural diamonds,
 cerium oxides, and the like. The substrate for information recording
 medium according to the invention made of the invented crystallized glass
 can be obtained by forming the molded articles into the shapes of the
 substrates in use of the above method.
 The substrate made from the invented crystallized glass preferably has a
 surface smoothness in which the mean roughness Ra measured by an AFM is 20
 angstroms or less. Particularly, where the crystallized glass of the
 invention is used for a magnetic disc substrate, the mean roughness Ra on
 the surface greatly affects the recording density of the magnetic disc. If
 the surface roughness exceeds 20 angstroms, a high recording density may
 not be achieved. The surface roughness of the substrate made from the
 invented crystallized glass is preferably 15 angstroms or less in
 consideration of a high recording density of the magnetic disc, more
 preferably, 10 angstroms or less.
 The substrate made of the crystallized glass according to the invention
 including, as primary crystals, at least one of quartz solid solution
 crystals selected from 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, MgO.Al.sub.2
 O.sub.3.3SiO.sub.2, and MgO.Al.sub.2 O.sub.3.4SiO.sub.2 and/or the
 enstatite crystal phase is useful for magnetic disc substrates because of
 the high strength, high rigidity, high Young's modulus with excellent
 chemical resistance and heat resistance. Since the crystallized glass of
 the invention is free of alkali or of low alkali, the magnetic film can be
 kept in the best state by reducing greatly corrosions between the
 substrate and the magnetic film even where used for the magnetic disc
 substrate.
 The magnetic disc substrate made of the crystallized glass of the invention
 can satisfy all conditions required for magnetic disc substrate such as
 surface smoothness, flatness, strength, rigidity, chemical resistance, and
 heat resistance. The substrate has a Young's modulus about twice in
 comparison with a conventional crystallized glass (Li.sub.2 O--Si.sub.2 O
 crystallized glass), so that the substrate can suppress in a smaller
 amount the warp occurring due to high speed rotation of the disc, and so
 that it is suitable for a substrate material for realizing high TPI hard
 discs.
 Since the crystallized glass according to the invention has good property
 of heat resistance, surface smoothness, chemical resistance, optical
 characteristics, and mechanical strength, it can be used for substrates
 for information recording medium, glass substrates for opto-magnetic disc,
 glass substrates for opto-electronics such as optical discs, heat
 resistance substrates for low temperature polysilicon liquid crystal
 display as expected as LCD of the next generation, and glass substrates
 for electrical or electronic parts.
 Description for Magnetic Disc
 An information recording medium according to the invention has a feature
 having the invented substrate and a recording layer formed on the
 substrate. Hereinafter, a magnetic disc (hard disc) is described in which
 at least a magnetic layer is formed on a major surface of the substrate
 made of the crystallized glass of the invention.
 As layers other than the magnetic layer, exemplified are, in terms of
 functions, an undercoat layer, a protection layer, a lubrication layer, an
 undulation control layer, and the like, which are formed when necessary.
 Those layers are formed in use of various thin film formation
 technologies. The material for the magnetic layer is not limited
 specifically. As such a magnetic layer, for example, Co based, ferrite
 based, and iron-rare earth based magnetic layers are exemplified. The
 magnetic layer can be for either of horizontal magnetic recording and
 vertical magnetic recording.
 As a magnetic layer, for example, magnetic thin films such as CoPt, CoCr,
 CoNi, CoNiCr, CoCrTa, CoPtCr, coNiCrPt, CoNiCrTa, CoCrPtTa, CoCrPtSiO, in
 which Co is used as the main component, are exemplified. The magnetic
 layer can be divided with non-magnetic layers to form multilayer structure
 for aiming noise reduction.
 The undercoat layer for the magnetic layer is selected according to the
 magnetic layer. As an undercoat layer, undercoat layers made of at least
 one material selected from non-magnetic metals such as Cr, Mo, Ta, Ti, W,
 V, B, and Al, or oxide, nitride, carbide, or the like of those metals. In
 the case of the magnetic layer made of a Co as a major component, Cr
 solely or Cr alloy is preferably used in terms of improvements for
 magnetic property. The undercoat layer is not limited to a single layer
 and can have a multilayer structure in which the same or different layers
 are accumulated. For example, an undercoat layer having multiple layers
 such as Al/Cr/CrMo, Al/Cr/Cr, or the like is exemplified.
 An undulation control layer may be formed between the substrate and the
 magnetic layer or on a top of the magnetic layer to prevent the magnetic
 head and the magnetic disc from absorbing to each other. By forming the
 undulation control layer, the surface roughness on the magnetic disc is
 properly adjusted, thereby preventing the magnetic head and the magnetic
 disc from absorbing to each other, and thereby providing a magnetic disc
 highly reliable. Several materials for undulation control layers and
 forming methods are known, and not limited to those. For example, as a
 material for undulation control layer, undercoat layers made of at least
 one metal selected from a group of Al, Ag, Ti, Nb, Ta, Bi, Si, Zr, Cr, Cu,
 Au, Sn, Pb, Sb, Ge, Mg, and the like, an alloy made from those, oxides,
 nitrides, and carbides of those are exemplified. From a viewpoint for easy
 formation, it is desirable to use a metal having a main component of Al
 such as Al sole, Al alloy, Al oxide, and Al nitride.
 In view of head extension, the surface roughness on the undulation control
 layer is preferably set Rmax=50 to 300 angstroms. A more desirable range
 is Rmax=100 to 200 angstroms. If Rmax is only 50 angstroms, the magnetic
 head surface is close to a flat, and thereby the magnetic head and the
 magnetic disc are absorbed to each other to inflict damages on the
 magnetic head and the magnetic disc or to unfavorably cause a head clash
 due to absorbing. If Rmax exceeds 300 angstroms, grind height becomes too
 large, thereby causing unfavorably reduction of recording density.
 It is to be noted that certain undulation can be applied on the glass
 substrate surface without forming any undulation control layer by etching
 or laser beam radiation, as a texturing process.
 As a protection layer, for example, Cr film, Cr alloy film, carbon film,
 zirconium film, silica film, and the like can be exemplified. Those
 protection films can be formed successively in an in-line sputtering
 apparatus or the like together with the undercoat layer, the magnetic
 layer, and the like. Those protection layers can be formed as a single
 layer or may have a multilayer structure made of the same or different
 layers.
 On the protection layer, or in lieu of the protection layer, another
 protection layer can be formed. For example, tetraalkoxysilane may be
 applied on the protection layer upon diluting with a solvent based on an
 alcohol in dispersing colloidal silica fine particles and may be sintered
 to form an silicon oxide film. In such a case, it serves for both
 functions of the protection film and the undulation control layer.
 Although various proposals are made as a lubrication layer, as a general
 rule, a perfluoropolyether serving as a liquid lubricant is diluted with
 fluorine based solvent, and is coated on the medium surface by a dipping
 method, a spin coating method, or a spray method, thereby forming the
 layer in thermally treating it when necessary.
 EXAMPLES
 Although this invention is described in detail by exemplifying the
 following Examples, this invention is not limited to those Examples.
 Examples 1-1 to 3-19
 Glass compositions of Examples 1-1 to 1-16 are shown in Tables 1, 2 by mol
 %. Glass compositions of Examples 2-1 to 2-18 are shown in Tables 3, 4 by
 mol %. Glass compositions of Examples 3-1 to 3-19 are shown in Tables 5, 6
 by mol %. As a starting raw material for melting such a glass, SiO.sub.2,
 Al.sub.2 O.sub.3, Al(OH).sub.3, MgO, CaCO.sub.3, Y.sub.2 O.sub.3,
 TiO.sub.2, ZrO.sub.2, Li.sub.2 CO.sub.3, and the like were used in a
 prescribed blend shown in Tables 1, 2 where measured to be 250 to 300 g to
 form a ready-mixed batch upon adequate mixing, and then, the materials
 were introduced in a platinum crucible to melt the glass for four to five
 hours in open air in stirring at 1550.degree. C. After melting, the glass
 liquid was casted into a carbon made mold in a size of
 180.times.15.times.25 mm and was subsequently placed in an anneal furnace
 right after the glass liquid was slowly cooled to the glass transition
 temperature, and it was further cooled to room temperature in the furnace
 upon one hour annealing in a temperature span of the glass transition
 point. The obtained glass did not show any deposited crystal observable by
 means of a microscope.
 The glass in the size of 180.times.15.times.25 mm was placed in a thermal
 furnace after polished to sizes of 100.times.10.times.10 mm,
 10.times.10.times.20 mm, and 10.times.1.times.20 mm. The glass was heated
 to a first thermal treatment temperature shown in Tables 1, 2 at a
 temperature increase rate of 1 to 5.degree. C. per minute, and was subject
 to the first thermal treatment while kept at the temperature for two to
 ten hours. The glass was then heated to a second thermal treatment
 temperature shown in Tables 1, 2 at a temperature increase rate of 2 to
 10.degree. C. per minute from the first thermal treatment temperature
 right after the first thermal treatment ends, and was kept at the
 temperature for one to five hours, and a crystallized glass was produced
 by cooling the glass to room temperature in the furnace. The obtained
 crystallized glass was polished to have a length of 95 mm as a sample for
 measurements of Young's modulus and specific gravity. The Young's modulus
 was measured by an ultrasound method using a 95.times.10.times.10 mm
 sample. Data measured thus are shown in Tables 1 to 6 together with the
 glass compositions.
 For the purpose of comparison, Comparative Example 1 for an ion exchange
 glass substrate disclosed in Japanese Unexamined Patent Publication No.
 1-239,036, and Comparative Example 2 for a glass substrate disclosed in
 U.S. Pat. No. 2,516,553 are shown with their compositions and
 characteristics in Table 7.
 TABLE 1
 Crystallized glass compositions and property of
 Examples
 Oxide 1-1 1-2 1-3 1-4 1-5
 1-6 1-7 1-8
 SiO.sub.2 50.00 49.87 48.00 50.00 50.00
 47.00 47.00 47.00
 Al.sub.2 O.sub.3 17.50 17.70 17.50 19.00 19.00
 15.00 17.50 20.00
 MgO 20.50 21.03 22.50 16.00 19.00
 26.00 23.50 21.00
 Y.sub.2 O.sub.3 2.00 2.00 2.00 2.50 2.50
 2.00 2.00 2.00
 TiO.sub.2 10.00 9.40 10.00 9.50 9.50
 10.00 10.00 10.00
 ZnO 3.00
 1st thermal 800.degree. C. 810.degree. C. 810.degree. C. 805.degree. C.
 810.degree. C. 800.degree. C. 800.degree. C. 810.degree. C.
 treatment
 temp.
 1st thermal 4 hours 4 hours 4 hours 4 hours 4 hours 4
 hours 4 hours 4 hours
 treatment
 hour
 Temp. 5.degree. C./min 1.degree. C./min 1.degree. C./min 5.degree.
 C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min
 increase
 rate
 2nd thermal 1000.degree. C. 1000.degree. C. 1000.degree. C. 1000.degree.
 C. 1000.degree. C. 1000.degree. C. 1000.degree. C. 1000.degree. C.
 treatment
 temp.
 2nd thermal 4 hours 4 hours 4 hours 4 hours 4 hours 4
 hours 4 hours 4 hours
 treatment
 hour
 Primary A, B, A, B, A, B, A, B, A, B,
 A, B, A, B, A, B,
 crystal C, D C, D C, D C, D C, D
 C, D C, D C, D
 phase
 Secondary E, F E, F E, F E, F E, F
 E, F E, F E, F
 crystal
 phase
 Young's 135.6 131.9 138.4 131.9 137.4
 153.8 147.5 139.7
 modulus
 Poisson's 0.232 0.246 0.247 0.227 0.258
 0.243 0.244 0.239
 ratio
 Crystal species: A: 2MgO.2Al.sub.2 O.sub.3.4SiO.sub.2 (quartz solid
 solution), B: MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), C:
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), D: enstatite,
 E: forsterite, F: Mg--Al-titanate
 TABLE 2
 Crystallized glass compositions and property of
 Examples
 Oxide 1-9 1-10 1-11 1-12 1-13
 1-14 1-15 1-16
 SiO.sub.2 47.00 47.00 55.00 52.50 50.00
 46.00 44.00 52.00
 Al.sub.2 O.sub.3 17.50 17.50 14.44 15.53 16.63
 18.38 19.25 12.50
 MgO 21.50 19.50 18.56 19.97 21.37
 23.62 24.75 23.50
 Y.sub.2 O.sub.3 2.00 2.00 2.00 2.00 2.00
 2.00 2.00 2.00
 TiO.sub.2 12.00 14.00 10.00 10.00 10.00
 10.00 10.00 10.00
 ZnO
 1st thermal 800.degree. C. 795.degree. C. 800.degree. C. 803.degree. C.
 805.degree. C. 805.degree. C. 805.degree. C. 790.degree. C.
 treatment
 temp.
 1st thermal 4 hours 4 hours 4 hours 4 hours 4 hours 4
 hours 4 hours 4 hours
 treatment
 hour
 Temp. 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree.
 C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min
 increase
 rate
 2nd thermal 1000.degree. C. 1000.degree. C. 1000.degree. C. 1000.degree.
 C. 1000.degree. C. 1000.degree. C. 1000.degree. C. 1000.degree. C.
 treatment
 temp.
 2nd thermal 4 hours 4 hours 4 hours 4 hours 4 hours 4
 hours 4 hours 4 hours
 treatment
 hour
 Primary A, B, A, B, A, B, A, B, A, B,
 A, B, A, B, A, B,
 crystal C, D C, D C, D C, D C, D
 C, D C, D C, D
 phase
 Secondary E, F E, F E, F E, F E, F
 E, F E, F E, F
 crystal
 phase
 Young's 140.1 132.9 123.2 131.5 133.7
 143.7 154.1 134.7
 modulus
 Poisson's 0.22 0.22 0.21 0.21 0.237
 0.24 0.231 0.221
 ratio
 Crystal species: A: 2MgO.2Al.sub.2 O.sub.3.4SiO.sub.2 (quartz solid
 solution), B: MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), C:
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), D: enstatite,
 E: forsterite, F: Mg--Al-titanate
 TABLE 3
 Crystallized glass compositions and property
 of Examples
 Oxide 2-1 2-2 2-3 2-4 2-5
 2-6 2-7 2-8 2-9
 SiO.sub.2 47.00 41.00 43.00 45.00 49.00
 47.00 47.00 47.00 47.00
 Al.sub.2 O.sub.3 12.50 12.50 12.50 12.50 12.50
 12.50 12.50 12.50 12.50
 MgO 28.50 34.50 32.50 30.50 26.50
 27.50 30.50 26.50 26.50
 Y.sub.2 O.sub.3 2.00 2.00 2.00 2.00 2.00
 3.00 2.00 2.00
 TiO.sub.2 10.00 10.00 10.00 10.00 10.00
 10.00 10.00 9.50 9.50
 La.sub.2 O.sub.3
 ZnO
 BaO
 2.50
 SrO
 2.50
 NiO
 Al.sub.2 O.sub.3 /MgO 0.44 0.36 0.38 0.41 0.47
 0.45 0.47 0.41 0.41
 1st thermal 800.degree. C. 790.degree. C. 790.degree. C. 790.degree. C.
 790.degree. C. 790.degree. C. 780.degree. C. 792.degree. C. 792.degree. C.
 treatment
 temp.
 1st thermal 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Temp. 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min
 increase rate
 2nd thermal 1000.degree. C. 1000.degree. C. 1000.degree. C.
 1000.degree. C. 1000.degree. C. 1000.degree. C. 1000.degree. C.
 1000.degree. C. 1000.degree. C.
 treatment
 temp.
 2nd thermal 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Primary A, B, A, B, A, B, A, B, A, B,
 A, B, A, B, A, B, A, B,
 crystal phase C, D C, D C, D C, D C, D
 C, D C, D C, D C, D
 Secondary E, F E, F E, F E, F E, F
 E, F E, F E, F E, F
 crystal phase
 Young's 157.3 179.1 170 163.4 149
 152.4 149.8 145.0 142.6
 modulus
 Poisson's 0.237 0.245 0.245 0.241 0.233
 0.233 0.248 0.246 0.244
 ratio
 Crystal species: A: 2MgO.2Al.sub.2 O.sub.3.4SiO.sub.2 (quartz solid
 solution), B: MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), C:
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), D: enstatite,
 E: forsterite, F: Mg--Al-titanate
 TABLE 4
 Crystallized glass compositions and property
 of Examples
 Oxide 2-10 2-11 2-12 2-13 2-14
 2-15 2-16 2-17 2-18
 SiO.sub.2 47.00 47.00 47.00 47.00 47.00
 47.00 47.00 39.00 39.00
 Al.sub.2 O.sub.3 12.50 12.50 12.50 12.50 12.50
 12.50 12.50 12.50 12.50
 MgO 26.50 26.50 26.50 28.50 28.50
 28.50 28.50 36.50 38.50
 Y.sub.2 O.sub.3 2.00 2.00
 2.00 2.00
 TiO.sub.2 9.50 9.50 10.00 10.00 10.00
 10.00 10.00 10.00 10.00
 La.sub.2 O.sub.3 2.00
 ZnO 2.50
 BaO
 SrO
 NiO 2.50
 CeO.sub.2 2.00
 Nb.sub.2 O.sub.3 2.00
 Ta.sub.2 O.sub.5
 2.00
 ZrO.sub.2
 2.00
 Al.sub.2 O.sub.3 /MgO 0.41 0.41 0.41 0.44 0.44
 0.44 0.44 0.34 0.34
 1st thermal 780.degree. C. 785.degree. C. 780.degree. C. 774.degree. C.
 760.degree. C. 795.degree. C. 780.degree. C. 790.degree. C. 785.degree. C.
 treatment
 temp.
 1st thermal 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Temp. 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min
 increase rate
 2nd thermal 1000.degree. C. 1000.degree. C. 1000.degree. C.
 1000.degree. C. 1000.degree. C. 1000.degree. C. 1000.degree. C.
 970.degree. C. 1000.degree. C.
 treatment
 temp.
 2nd thermal 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Primary A, B, A, B, A, B, A, B, A, B,
 A, B, A, B, A, B, A, B,
 crystal phase C, D C, D C, D C, D C, D
 C, D C, D C, D C, D
 Secondary E, F E, F E, F E, F E, F
 E, F E, F E, F E, F
 crystal phase
 Young's 155.5 153.3 156.1 151.5 152
 132.9 147.8 187.9 198
 modulus
 Poisson's 0.235 0.23 0.23 0.233 0.223
 0.221 0.227 0.24 0.242
 ratio
 Crystal species: A: 2MgO.2Al.sub.2 O.sub.3.4SiO.sub.2 (quartz solid
 solution), B: MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), C:
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), D: enstatite,
 E: forsterite, F: Mg--Al-titanate
 TABLE 5
 Crystallized glass compositions and property
 of Examples
 Oxide 3-1 3-2 3-3 3-4 3-5 3-6
 3-7 3-8 3-9 3-10
 SiO.sub.2 48.50 48.50 48.00 48.00 48.00 48.00
 48.00 47.00 47.00 50.00
 Al.sub.2 O.sub.3 17.50 17.50 17.50 17.50 17.50
 17.50 15.00 12.50 15.00 15.00
 MgO 20.00 17.50 21.50 20.50 22.50 20.00
 20.50 26.50 24.00 21.00
 Y.sub.2 O.sub.3 2.00 2.00 2.00 2.00
 2.00 2.00 2.00 2.00 2.00
 TiO.sub.2 9.50 9.50 8.50 9.50 9.50 7.50
 9.50 9.50 9.50 9.50
 La.sub.2 O.sub.3 2.50 5.00 2.50 2.50 2.50
 5.00 5.00 2.50 2.50 2.50
 1st thermal 770.degree. C. 750.degree. C. 765.degree. C. 765.degree. C.
 765.degree. C. 735.degree. C. 735.degree. C. 750.degree. C. 755.degree. C.
 750.degree. C.
 treatment
 temp.
 1st thermal 4 hours 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Temp. 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree.
 C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min
 increase
 rate
 2nd thermal 950.degree. C. 950.degree. C. 950.degree. C. 950.degree. C.
 950.degree. C. 950.degree. C. 950.degree. C. 950.degree. C. 950.degree. C.
 950.degree. C.
 treatment
 temp.
 2nd thermal 4 hours 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Primary A, B, A, B, A, B, A, B, A, B, A, B,
 A, B, A, B, A, B, A, B,
 crystal C, D C, D C, D C, D C, D C, D
 C, D C, D C, D C, D
 phase
 Secondary E, F E, F E, F E, F E, F E, F
 E, F E, F E, F E, F
 crystal
 phase
 Young's 138.9 132.6 141.1 139.9 137.6 135.9
 135.2 149.4 145.4 139.4
 modulus
 Poisson's 0.24 0.246 0.244 0.243 0.243 0.246
 0.248 0.245 0.245 0.244
 ratio
 Crystal species: A: 2MgO.2Al.sub.2 O.sub.3.4SiO.sub.2 (quartz solid
 solution), B: MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), C:
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), D: enstatite,
 E: forsterite, F: Mg--Al-titanate
 TABLE 6
 Crystallized glass compositions and property
 of Examples
 Oxide 3-11 3-12 3-13 3-14 3-15
 3-16 3-17 3-18 3-19
 SiO.sub.2 50.00 47.00 41.00 43.00 45.00
 47.00 43.00 41.00 45.00
 Al.sub.2 O.sub.3 15.00 12.50 12.50 12.50 12.50
 12.50 12.50 12.50 12.50
 MgO 21.00 27.50 36.50 32.50 30.50
 28.50 32.50 34.50 30.50
 Y.sub.2 O.sub.3 2.00 2.00 2.00 2.00 2.00
 2.00 2.00 2.00 2.00
 TiO.sub.2 9.50 9.50 10.00 10.00 10.00
 10.00 9.00 9.00 9.00
 La.sub.2 O.sub.3 2.50 2.50 2.00 2.00 2.00
 2.00 1.00 1.00 1.00
 1st thermal 755.degree. C. 750.degree. C. 750.degree. C. 750.degree. C.
 750.degree. C. 750.degree. C. 765.degree. C. 765.degree. C. 770.degree. C.
 treatment
 temp.
 1st thermal 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Temp. 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min 5.degree. C./min 5.degree. C./min
 5.degree. C./min 5.degree. C./min
 increase
 rate
 2nd thermal 950.degree. C. 950.degree. C. 950.degree. C. 950.degree. C.
 950.degree. C. 950.degree. C. 950.degree. C. 950.degree. C. 950.degree. C.
 treatment
 temp.
 2nd thermal 4 hours 4 hours 4 hours 4 hours 4 hours
 4 hours 4 hours 4 hours 4 hours
 treatment
 hour
 Primary A, B, A, B, A, B, A, B, A, B,
 A, B, A, B, A, B, A, B,
 crystal C, D C, D C, D C, D C, D
 C, D C, D C, D C, D
 phase
 Secondary E, F E, F E, F E, F E, F
 E, F E, F E, F E, F
 crystal
 phase
 Young's 135.7 149.5 170.1 163.2 156.8
 150.4 169.4 180 160.7
 modulus
 Poisson's 0.245 0.245 0.248 0.246 0.244
 0.243 0.248 0.241 0.248
 ratio
 Crystal species: A: 2MgO.2Al.sub.2 O.sub.3.4SiO.sub.2 (quartz solid
 solution), B: MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), C:
 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 (quartz solid solution), D: enstatite,
 E: forsterite, F: Mg--Al-titanate
 TABLE 7
 Comparative Examples
 1 2
 Chemically Commercially available TS-
 reinforced glass 10 crystallized glass
 Oxide Japanese Unexamined Patent U.S.Pat. No. 2,516,553
 Publication No. 1-239036
 SiO.sub.2 73.0
 Al.sub.2 O.sub.3 0.6
 CaO 7.0
 Na.sub.2 O 9.0
 K.sub.2 O 9.0
 ZnO 2.0
 As.sub.2 O.sub.3 0.2
 Young's 79 90-100
 modulus
 (GPa)
 Surface 12 10-35
 roughness
 Ra (nm)
 As apparent from the consequences in Tables 1, 2, the crystallized glass of
 the invention in Examples 1-1 to 1-16 has higher strength characteristics
 such as the Young's modulus (120 GPa or more) and specific modulus of
 elasticity (in a range of 40-60 MNm/kg). Therefore, if those glasses are
 used as a substrate for information recording medium such as magnetic
 recording medium, the substrate is hardly subject to warps or deviations
 even where spun at a high speed and also can correspond to further thinner
 substrates. The surface roughness (Ra) of the crystallized glass can be
 polished to 5 angstroms or less by an ordinary optical glass polishing
 method using abrasives such as synthetic diamonds, silicon carbides,
 calcium oxides, iron oxides, and cerium oxides. Therefore, the substrate
 can be obtained with excellent flatness, and the substrate can be useful
 as a glass substrate for magnetic recording medium for aiming low flying
 of the magnetic head.
 FIGS. 1, 2 show photographs of an atomic force microscope showing the
 crystallized glass obtained in Example 1-16. As shown in pictures, the
 surface smoothness of the crystallized glass of Example 1-16 can be
 polished to 4 angstroms or less by a polishing method for ordinary optical
 glass using cerium oxides. Where the liquid temperature of Example 1-16
 was measured, it was 1320.degree. C., and it indicated easiness for
 molding.
 As apparent from the consequences in Tables 3, 4, the crystallized glass of
 the invention in Examples 2-1 to 2-18 has higher strength characteristics
 such as the Young's modulus (140 GPa or more) and specific modulus of
 elasticity (in a range of 40-60 MNm/kg). Therefore, if those glasses are
 used as a substrate for information recording medium such as magnetic
 recording medium, the substrate is hardly subject to warps or deviations
 even where spun at a high speed and also can correspond to further thinner
 substrates. The surface roughness (Ra) of the crystallized glass can be
 polished to 5 angstroms or less by an ordinary optical glass polishing
 method using abrasives such as synthetic diamonds, silicon carbides,
 calcium oxides, iron oxides, and cerium oxides. Therefore, the substrate
 can be obtained with excellent flatness, and the substrate can be useful
 as a glass substrate for magnetic recording medium for aiming low flying
 of the magnetic head.
 FIGS. 3, 4 show photographs of an atomic force microscope showing the
 crystallized glass obtained in Example 2-5. As shown in pictures, the
 surface smoothness of the crystallized glass of Example 1-16 can be
 polished to 4.5 angstroms or less by a polishing method for ordinary
 optical glass using cerium oxides. Where the liquid temperature of Example
 2-1 was measured, it was 1292.degree. C., and it indicated easiness for
 molding.
 As apparent from the consequences in Tables 5, 6, the crystallized glass of
 the invention in Examples 3-1 to 3-19 has higher strength characteristics
 such as the Young's modulus (130 GPa or more) and specific modulus of
 elasticity (in a range of 40-60 MNm/kg). Therefore, if those glasses are
 used as a substrate for magnetic recording medium, the substrate is hardly
 subject to warps or deviations even where spun at a high speed and also
 can correspond to further thinner substrates. The surface roughness (Ra)
 of the crystallized glass can be polished to 5 angstroms or less by an
 ordinary optical glass polishing method using abrasives such as synthetic
 diamonds, silicon carbides, calcium oxides, iron oxides, and cerium
 oxides. Therefore, the substrate can be obtained with excellent flatness,
 and the substrate can be useful as a glass substrate for magnetic
 recording medium for aiming low flying of the magnetic head.
 Moreover, where the liquid temperature of Example 3-13 was measured, it was
 1280.degree. C., and it indicated adequate easiness for molding.
 The surface roughness was measured by surface observations in use of an
 atomic force microscope (AFM). The arithmetic mean roughness was
 calculated in an area of 2.times.2 micron for 3 to 5 spots on each sample
 surface. Although the surface roughness is different depending on
 polishing conditions and thermal treatment conditions as a matter of
 course, FIGS. 5, 6 show AFM photographs after the crystallized glass of
 Example 3-4 thermally treated under the thermal condition shown in Table 5
 was polished by a polishing process for optical glass. As shown in FIG. 6,
 it turned out that the crystal grain of the invented crystallized glass is
 further smaller than one micron. The surface roughness of Example 3-1 is
 about 4 angstroms, small, so that the glass can adequately correspond to
 demands for surface smoothness for magnetic disc of the next generation. A
 crystallized glass can be produced with more excellent surface smoothness
 if the thermal treatment condition and polishing condition are made most
 appropriate.
 To the contrary, the chemically reinforced glass substrate of Comparative
 Example 1 has excellent surface smoothness and flatness, but it has
 limited strength characteristics such as heat resistance and Young's
 modulus, which is inferior in comparison with the crystallized glass of
 the invention. Accordingly, where a magnetic recording medium is
 manufactured, a thermal treatment may not be done adequately to the
 magnetic layer for obtaining a high coercive force, so that magnetic
 recording medium cannot be obtained with a high coercive force. The glass
 of Comparative Example 1 may sustain corrosions occurring between the
 substrate and the magnetic film because the glass contains alkali in a
 large amount, and the magnetic film may be damaged.
 The crystallized glass substrate of Comparative Example 2 is inferior to
 the glass of the invention in terms of Young's modulus, specific modulus
 of elasticity, and smoothness. Particularly, since the smoothness of the
 substrate is deteriorated by existence of large crystal grains, it is
 difficult to render a high density recording.
 Examples 4-1 to 4-8
 Glass compositions of Examples 4-1 to 4-8 are shown in Table 8 by mol %. As
 a starting raw material for melting such a glass, SiO.sub.2, Al.sub.2
 O.sub.3, Al(OH).sub.3, MgO, CaCO.sub.3, Y.sub.2 O.sub.3, TiO.sub.2,
 ZrO.sub.2, Li.sub.2 CO.sub.3, and the like were used in a prescribed blend
 shown in Table 1 where measured to be 250 to 300 g to form a ready-mixed
 batch upon adequate mixing, and then, the materials were introduced in a
 platinum crucible to melt the glass for four to five hours in open air in
 stirring at 1550.degree. C. After melting, the glass liquid was casted
 into a carbon made mold in a size of 180.times.15.times.25 mm and was
 subsequently placed in an anneal furnace right after the glass liquid was
 slowly cooled to the glass transition temperature, and it was further
 cooled to room temperature in the furnace upon one hour annealing in a
 temperature span of the glass transition point. The obtained glass did not
 show any deposited crystal observable by means of a microscope.
 The glass in the size of 180.times.15.times.25 mm was placed in a thermal
 furnace after polished to sizes of 100.times.10.times.10 mm,
 10.times.10.times.20 mm, and 10.times.1.times.20 mm. The glass was heated
 to a first thermal treatment temperature (nucleus formation temperature)
 shown in Table 1 at a temperature increase rate of 3 to 10.degree. C. per
 minute, and was subject to the first thermal treatment while kept at the
 temperature for two to fifteen hours. The glass was then heated to a
 second thermal treatment temperature (crystallization temperature) shown
 in Table 1 at a temperature increase rate of 1 to 20.degree. C. per minute
 from the first thermal treatment temperature right after the first thermal
 treatment ends, and was kept at the temperature for one to eight hours,
 and a crystallized glass was produced by cooling the glass to room
 temperature in the furnace.
 The obtained crystallized glass was polished to have a length of 95 mm as a
 sample for measurements of Young's modulus and specific gravity. The
 sample used for the Young's modulus measurement was further cut and
 precisely polished in a size of 25 mm.times.2 mm.times.15 mm for a sample
 for measuring surface roughness. The Young's modulus was measured by an
 ultrasound method using a 95.times.10.times.10 mm sample. Data measured
 thus are shown in Table 8 together with the glass compositions.
 TABLE 8
 Crystallized glass compositions and property of Examples
 Glass
 component
 (mol %) 4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8
 SiO.sub.2 56.00 58.00 56.00 58.00 55.00 55.00 55.00 50.00
 Al.sub.2 O.sub.3 5.00 2.50 2.50 4.50 4.00 4.00 5.00 5.00
 MgO 15.00 14.00 15.00 14.00 15.00 12.00 15.00 20.00
 Y.sub.2 O.sub.3 1.00 1.00 1.00 1.00 0.50 0.50 1.00 2.00
 TiO.sub.2 8.50 8.50 8.50 8.50 8.50 8.50 5.00 8.50
 ZrO.sub.2 2.00 2.00 2.00 2.00 2.00 2.00 2.00
 Li.sub.2 O 12.50 14.00 15.00 14.00 15.00 18.00 15.00 12.50
 K.sub.2 O 1.00
 P.sub.2 O 1.00
 Nucleus 635 620 625 620 670 610 650 750
 formation
 temperature
 (.degree. C.)
 Nucleus 4 4 4 4 4 4 4 4
 formation
 time (hr)
 Crystallization 800 750 730 800 800 800 800 950
 temperature
 (.degree. C.)
 Crystallization 4 4 4 4 4 4 4 4
 time (hr)
 Liquid phase 1150 1110 1150 1150 1150 1150 1150 1150
 temperature
 (.degree. C.)
 Surface 0.3 0.4 0.3 0.3 0.3 0.3 0.4 0.3
 roughness Ra
 (nm)
 Young's 121 130 133 117 125.2 123.4 131 145
 modulus (Gpa)
 The surface roughness was measured by surface observations in use of an
 atomic force microscope (AFM). The arithmetic mean roughness was
 calculated in an area of 5.times.5 micron for 3 to 5 spots on each sample
 surface. Although the surface roughness is different depending on
 polishing conditions and thermal treatment conditions as a matter of
 course, FIG. 7 shows AFM photographs after the crystallized glass of
 Example 4 thermally treated under the thermal condition shown in Table 1
 was polished by a polishing process for optical glass. The surface
 roughness of Example 4 is about 0.3 nm, small, so that the glass can
 adequately correspond to demands for surface smoothness for magnetic disc
 of the next generation. A crystallized glass can be produced with more
 excellent surface smoothness if the thermal treatment condition and
 polishing condition are made most appropriate.
 As apparent from Table 8, the glass substrate of the invention (Examples
 4-1 to 4-8) has a higher Young's modulus (in a range of 115 to 150 GPa).
 Therefore, if those glasses are used as a substrate for magnetic recording
 medium, the substrate is hardly subject to warps or deviations even where
 spun at a high speed and also can correspond to further thinner
 substrates. The surface roughness (Ra) of the crystallized glass can be
 polished to 5 angstroms (0.5 nm) or less, and therefore, the substrate can
 be obtained with excellent flatness and can be useful as a glass substrate
 for magnetic recording medium in aiming low flying of the magnetic head.
 Examples 5-1 to 5-6
 Glass compositions of Examples 5-1 to 5-6 are shown in Table 9 by mol %. As
 a starting raw material for melting such a glass, SiO.sub.2, Al.sub.2
 O.sub.3, Al(OH).sub.3, MgO, CaCO.sub.3, Y.sub.2 O.sub.3, TiO.sub.2,
 ZrO.sub.2, Li.sub.2 CO.sub.3, and the like were used in a prescribed blend
 shown in Table 1 where measured to be 250 to 300 g to form a ready-mixed
 batch upon adequate mixing, and then, the materials were introduced in a
 platinum crucible to melt the glass for four to five hours in open air in
 stirring at 1550.degree. C. After melting, the glass liquid was casted
 into a carbon made mold in a size of 180.times.15.times.25 mm and was
 subsequently placed in an anneal furnace right after the glass liquid was
 slowly cooled to the glass transition temperature, and it was further
 cooled to room temperature in the furnace upon one hour annealing in a
 temperature span of the glass transition point. The obtained glass did not
 show any deposited crystal observable by means of a microscope.
 The glass in the size of 180.times.15.times.25 mm was placed in a thermal
 furnace after polished to sizes of 100.times.10.times.10 mm,
 10.times.10.times.20 mm, and 10.times.1.times.20 mm. The glass was heated
 to a first thermal treatment temperature (nucleus formation temperature)
 shown in Table 1 at a temperature increase rate of 3 to 10.degree. C. per
 minute, and was subject to the first thermal treatment while kept at the
 temperature for two to fifteen hours. The glass was then heated to a
 second thermal treatment temperature (crystallization temperature) shown
 in Table 1 at a temperature increase rate of 3 to 10.degree. C. per minute
 from the first thermal treatment temperature right after the first thermal
 treatment ends, and was kept at the temperature for one to five hours, and
 a crystallized glass was produced by cooling the glass to room temperature
 in the furnace.
 The obtained crystallized glass was polished to have a length of 95 mm as a
 sample for measurements of Young's modulus and specific gravity. The
 sample used for the Young's modulus measurement was further cut and
 precisely polished in a size of 25 mm.times.2 mm.times.15 mm for a sample
 for measuring surface roughness. The Young's modulus was measured by an
 ultrasound method using a 95.times.10.times.10 mm sample. Data measured
 thus are shown in Table 9 together with the glass compositions.
 TABLE 9
 Crystallized glass compositions and property of Examples
 Glass
 component
 (mol %) 5-1 5-2 5-3 5-4 5-5 5-6
 SiO.sub.2 52.00 48.00 52.00 48.00 47.00 42.00
 Al.sub.2 O.sub.3 4.00 4.00 2.00 4.00
 MgO 34.80 34.80 36.50 42.00 42.00 40.00
 Y.sub.2 O.sub.3 1.00 1.00 1.00 1.00 2.20 5.00
 TiO.sub.2 4.00 6.80 6.50
 ZrO.sub.2 5.70 5.70 6.00 5.00 3.50
 Li.sub.2 O 2.50 2.50 2.50
 Na.sub.2 O 1.00 2.00
 K.sub.2 O 1.00 1.00
 Nucleus 760 750 765 815 725 730
 formation
 temperature
 (.degree. C.)
 Nucleus 4 4 4 4 8 8
 formation
 time (hr)
 Crystallization 1000 1000 1000 1000 950 950
 temperature
 (.degree. C.)
 Crystallization 4 4 4 4 4 4
 time (hr)
 Surface 0.5 0.3 0.3 0.5 0.3 0.3
 roughness Ra
 (nm)
 Young's 145 150 145 150 162 185
 modulus (Gpa)
 The surface roughness was measured by surface observations in use of an
 atomic force microscope (AFM). The arithmetic mean roughness was
 calculated in an area of 5.times.5 micron for 3 to 5 spots on each sample
 surface. Although the surface roughness is different depending on
 polishing conditions and thermal treatment conditions as a matter of
 course, FIG. 8 shows AFM photographs after the crystallized glass of
 Example 5 thermally treated under the thermal condition shown in Table 1
 was polished by a polishing process for optical glass. The surface
 roughness of Example 5 is about 0.5 nm, small, so that the glass can
 adequately correspond to demands for surface smoothness for magnetic disc
 of the next generation. A crystallized glass can be produced with more
 excellent surface smoothness if the thermal treatment condition and
 polishing condition are made most appropriate.
 As apparent from Table 9, the glass substrate of Examples of the invention
 has a higher Young's modulus (in a range of 140 to 200 GPa). Therefore, if
 those glasses are used as a substrate for magnetic recording medium, the
 substrate is hardly subject to warps or deviations even where spun at a
 high speed and also can correspond to further thinner substrates. The
 surface roughness (Ra) of the crystallized glass can be polished to 0.5 nm
 or less, and therefore, the substrate can be obtained with excellent
 flatness and can be useful as a glass substrate for magnetic recording
 medium in aiming low flying of the magnetic head.
 The crystallized glass according to First to Third Embodiments can be
 molded easily, has a high Young's modulus of 110 GPa or above, a high heat
 resistance of 900.degree. C. or above, an excellent surface smoothness
 (surface roughness Ra is less than 20 angstroms), can be used as a
 substrate material for high rigidity and high strength and suitably as a
 material for electronic parts.
 The substrate made of the crystallized glass according to First to Third
 Embodiments of the invention, since the material has an excellent heat
 resistance, can render a necessary thermal treatment for improving
 property of a magnetic film without deforming the substrate, can achieve
 low flying of a magnetic head, or namely, a high density recording because
 of the excellent flatness, and can make thinner magnetic discs and higher
 rotations because the glass has high Young's modulus, specific modulus of
 elasticity, and strength, as well as can advantageously avoid breakdown of
 a magnetic disc.
 The crystallized glass according to First to Third Embodiments of the
 invention, since obtained in a relatively stable manner and can be
 produced easily with a business scale, can be expected greatly as an
 inexpensive substrate glass for magnetic recording medium for the next
 generation.
 The crystallized glass according to Fourth and Fifth Embodiments of the
 invention, can be molded easily, has a high Young's modulus of 110 GPa or
 more and a high heat resistance against about 700.degree. C., and has an
 excellent surface smoothness (surface roughness Ra is in a range of 0.1 to
 0.9 nm). Therefore, the glass can provide a substrate material having a
 high rigidity and a high strength and materials for electronic parts. The
 magnetic disc using the crystallized glass according to Fourth and Fifth
 Embodiments of the invention can render a necessary thermal treatment for
 improving property of a magnetic film without deforming the substrate.
 Furthermore, the magnetic disc using the crystallized glass according to
 Fourth and Fifth Embodiments of the invention can achieve low flying of a
 magnetic head, or namely, a high density recording because of the
 excellent flatness, and can make thinner magnetic discs and higher
 rotations because the glass has high Young's modulus, specific modulus of
 elasticity, and strength, as well as can advantageously avoid breakdown of
 a magnetic disc.
 The melting condition of the original glass has a temperature range of 1350
 to 1450.degree. C. for two to five hours to make the glass clarified and
 unified, so that the glass has good melting property and can be produced
 easily with a business scale, and so that the glass can be greatly
 expected as a substrate glass for inexpensive magnetic recording medium of
 the next generation.
 The information recording medium according to the invention has a high
 Young's modulus, and uses a crystallized glass substrate having an
 excellent surface smoothness. Therefore, the medium can render vibrations
 less even during a high speed spin of the substrate, and particularly, it
 is suitable for hard disc drives of high performance such as for servers
 or the like.
 The foregoing description of preferred embodiments of the invention has
 been presented for purposes of illustration and description, and is not
 intended to be exhaustive or to limit the invention to the precise form
 disclosed. The description was selected to best explain the principles of
 the invention and their practical application to enable others skilled in
 the art to best utilize the invention in various embodiments and various
 modifications as are suited to the particular use contemplated. It is
 intended that the scope of the invention should not be limited by the
 specification, but be defined claims set forth below.