High modulus glass-ceramics containing fine grained spinel-type crystals

Abstract: This invention is directed at glass-ceramic articles, and methods for producing such articles, exhibiting Young's modulus between about 14 to 24 x 10⁶ psi and a fracture toughness in excess of 1.0 MPa√in. The crystal phase assemblage is comprised predominately of generally uniformly-sized spinel-type crystals which are uniformly dispersed within a highly siliceous residual glass matrix phase. The composition of these glass-ceramic articles is consists essentially, expressed in terms of weight percent on the oxide basis, of 35-60% SiO₂, 20-35% Al₂O₃, 0-25% MgO, 0-25% ZnO, at least about 10% MgO+ZnO, 0-20% TiO₂, 0-10% ZrO₂, 0-2% Li₂O and 0-8% NiO. However, if the Al₂O₃ constituent is present in an amount less than about 25%, the TiO₂+ZrO₂+NiO amount should be at least 5%. The composition may also contain up to 5% of optional constituents selected from the group consisting of BaO, CaO, PbO, SrO, P₂O₅, B₂O₃, Ga₂O₃, 0-5% R₂O and 0-8% of a transition metal oxide; the R₂O selected from the group consisting of Na₂O, K₂O, Rb₂O and Cs₂O.

Description: FIELD OF THE INVENTION This invention relates to a low alkali glass-ceramic exhibiting a high elastic modulus and having a crystal phase assemblage comprised predominately of uniformly-sized and uniformly distributed ultra-fine grained spinel-type crystals, and a method for producing the glass-ceramic article. BACKGROUND OF THE INVENTION Glass-ceramic articles have been marketed commercially for over 30 years. The initial disclosure of such articles is found in U.S. Pat. No. 2,920,961 (Stookey) wherein it discloses the preparation of glass-ceramic articles through the heat treatment of precursor glass bodies. As explained therein, such articles are prepared in three general steps: (1) a glass forming batch, customarily containing a nucleating agent, is melted; (2) that melt is simultaneously cooled to a temperature below the glass's transformation range and a glass article is shaped therefrom; and, (3) that glass article is heat treated at temperatures above the annealing point of the glass and, frequently, above the softening point of the glass for a sufficient length of time to cause the glass to crystallize in situ. The heat treatment can be scheduled in such a manner as to control the size and, in some instances, the identity of the crystals developed. Therefore, in sum, the crystallization present in a glass-ceramic article can be the result of both the base composition of the precursor glass and the heat treatment the glass body is subjected to. Glass-ceramic articles containing a spinel-type crystal phase are well known in the art; the original disclosure occurring in U.S. Pat. No. 3,268,315 (Stookey), and later developments described U.S. Patents 3,873,329, 3,936,287, and 3,681,102. Other compositionally similar patents generally disclose the existence of a spinel-type crystal phase include U.S. Patents 3,585,054, 3,681,102, 3,962,514, 4,059,454, 4,867,750, 5,028,567, 5,079194, and U.K. Patent 1,544,779. Journal of Non-Crystalline Solids, vol. 91, no. 3, 1987, pages 333-350 describes a glass-ceramic article with a composition (T-51:42.7 SiO 2 , 28.9 Al 2 O 3 , 11.4 MgO, 10.0 TiO 2 , 6.8 ZrO 2 and 0.05 Cr 2 O 3 in wt %), with spinel (MgAl 2 O 4 ) as main crystal phase and quartz and zirconia as accessory crystal phases as well as a heat treatment schedule (nucleation at 750°C, crystallisation at 950° and up to 300°C/hr heating rate). DE-A-28 05 764 describes a glass-ceramic article with the following composition recited in example 4: 35.5 SiO 2 , 33.8 Al 2 O 3 , 13.3 MgO, 11.6 TiO 2 , 5.9 ZnO in wt %, with spinel (MgAl 2 O 4 ) as main crystal phase and rutile as an accessory crystal phase as well as a heat treatment schedule (nucleation at 800°C, crystallisation at 1000°C). US-A-4,687,750 describes a glass-ceramic article with a composition essentially free of Li 2 O, Na 2 O, CaO, SrO, and BaO and consisting essentially, by weight, of about 1.5 - 5 % R 2 O, consisting of 0 - 3 % K 2 O and/or 0 - 3 % Rb 2 O and/or 0 - 3 % Cs 2 O, 13 - 22 % ZnO, 16 - 28 % Al 2 O 3 , 45 - 65 % SiO 2 , and 1.5 - 8 % TiO 2 with fine grained (10 - 20 nm) gahnite (ZnAl 2 O 4 ) as virtually the sole crystal phase as well as a heat treatment schedule (nucleation at 800°C, crystallisation at 1000°C and 300°C/hr heating rate). US-A-3,252,778 describes a ceramic body which is formed by melting a glass-forming composition consiting essentially, by weight, of about 40 - 68 % SiO 2 , 8 - 32 % Al 2 O 3 , 7 - 14 % TiO 2 , and 8.5 - 23 % MgO, as well as a heat treatment schedule with a nucleation at 820°C, crystallisation at 990 - 1000°C and heating rates of 60 - 420°C/hr). Spinel is not indicated to be the predominant crystal phase. Materials Research Bulletin, vol. 8, no. 9, 1973, pages 1073 - 1078 describes in table 1, examples 1, 2 and 6 glass-ceramics including MgO, Al 2 O 3 , SiO 2 , CaO and TiO 2 as well as a heat treatment schedule with a nucleation at 740°C, a crystallisation at 1100°C and a heating rate of 180°C/hr. Spinel is not indicated to be the predominant crystal phase. Although all of the aforementioned disclose the formation of glass-ceramic articles which contain spinel crystals, the instant invention can be differentiated from these references for a number of reasons because the exact compositional range, carefully delineated and disclosed herein, is not taught in any of the references; thus, the resulting phase assemblage and the corresponding physical properties are significantly different. SUMMARY OF THE INVENTION Accordingly, the present invention is directed towards the production of glass-ceramic articles exhibiting Young's modulus between about 96.53 - 165.48 x 10 3 MPa (about 14 to 24 x 10 6 psi) and a fracture toughness in excess of 1.0 MPa.m 1/2 , wherein the crystal phase assemblage is comprised predominately of generally uniformly-sized spinel crystals which are uniformly dispersed within a highly siliceous residual glass matrix phase, the spinel crystals are entirely of the aluminate variety, entirely of the titanate variety or a mixture of both aluminate and titanate crystals, the aluminate spinels consisting of either MgAl 2 O 4 or a solid solution of (Mg,Zn) Al 2 O 4 , the titanate spinels consisting of either Mg 2 TiO 4 or a solid solution represented by (Mg,Zn) 2 TiO 4 , the crystals exhibiting dimensions finer than about 100 nm (about 1,000 Å) in diameter, the article having an overall composition consisting essentially of, expressed in terms of weight % on the oxide basis, 40-50 % SiO 2 , 22-30 % Al 2 O 3 , 7-18 % MgO, 0-12 % ZnO, 7-13 % TiO 2 , at least 13 % MgO+ZnO, 0-4 % ZrO 2 , 0-5 % NiO, at least 8 % TiO 2 +ZrO 2 with the proviso that, if the Al 2 O 3 content is less than about 25 %, the TiO 2 +ZrO 2 +NiO content is at least 5 %, and up to 5 % of optional constituents selected from the group consisting of BaO, CaO, PbO, SrO, P 2 O 5 , B 2 O 3 , Ga 2 O 3 , 0-5 % R 2 O wherein R 2 O is selected from the group consisting of Na 2 O, K 2 O, Rb 2 O and Cs 2 O and 0-8 % of a transition metal oxide. In addition, glass-ceramic articles may be produced in which the so-formed crystals exhibit a dimension of equal to or finer than about 30 nm (about 300Å) in diameter; specifically, those contemplated for use as rigid disk substrate for use in magnetic memory disk devices. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is replica electron micrograph (REM) illustrating the highly uniform, very fine grained microstructure, comprising spinel crystals exhibited by the inventive glass-ceramic designated as Example 20. FIG. 2 is an atomic force micrograph (AFM) illustrating the surface smoothness exhibited by the highly uniform, very fine grained spinel crystal-containing microstructure of the Example 6 glass-ceramic. FIG. 3 is an atomic force micrograph, 3-D view of FIG 2., again illustrating the surface smoothness exhibited by the microstructure of the Example 6 glass-ceramic. FIG 4 is a graph of the cross-sectional profile of the FIG. 2 AFM, illustrating the smoothness of the surface and the uniformity of the microstructure of the inventive glass-ceramic designated as Example 6. DETAILED DESCRIPTION OF THE INVENTION As indicated above, glass-ceramics have been known in the art for at least thirty years. In general, glass-ceramics consist of randomly oriented crystals dispersed within a matrix of residual glass and can be produced through controlled internal nucleation and crystallization of a precursor glass body. Hence, a glass forming batch of the desired composition is melted and that melt is cooled and simultaneously formed into a glass shape of a predetermined configuration utilizing conventional glass forming techniques. Glass forming batches for the inventive low alkali spinel crystal-containing glass-ceramic precursor glasses are easily melted and the ensuing viscous melts can be shaped into articles of widely varying geometries. The composition of precursor glass, and ultimately the glass-ceramic, consists essentially, expressed in terms of weight percent on the oxide basis, of about 40-50% SiO 2 , 22-30% Al 2 O 3 , 7-18% MgO, 0-12% ZnO, 7-13% TiO 2., at least 13% MgO+ZnO, 0-4% ZrO 2 , 0-5% NiO, and at least 8% TiO 2 +ZrO 2 . If the Al 2 O 3 constituent is present in an amount less than about 25%, the TiO 2 +ZrO 2 +NiO amount should be at least 5%. The composition may also include up to 5% of optional constituents selected from the group consisting of BaO, CaO, PbO, SrO, P 2 O 5 , B 2 O 3 , Ga 2 O 3 , 0-5% R 2 O, and 0-8% of a transition metal oxide. The R 2 O amount is selected from the group consisting of Na 2 O, K 2 O, Rb 2 O, Cs 2 O while, preferably, the transition metal oxide is selected from the group consisting of the oxides of Cu, Co, Fe, Mn, Cr, Sn and V; if the oxides of Cr, Co, Mn, Cu present they should not exceed about 1% of the glass composition. Once the glass shape was formed, it was heat treated in accordance with a pre-designed schedule to induce nucleation and the growth of crystals on the nuclei; and thereafter, where necessary, the crystallized article was finished to precise dimensions and desired surface texture. Specifically, the glass article was heated to a temperature range of about 750-850°C at a temperature rate of approximately 300°C/hour and thereafter maintained at the temperature for a time sufficient to cause the article to obtain a high degree of nucleation. Once nucleated, the glass article was then heated to a temperature range about 900-1150°C at a temperature rate of approximately 300°C/hour. The glass article was thereafter maintained at that temperature for a time sufficient to cause the article to crystallize in situ forming a crystallized glass article which exhibits a crystal phase assemblage comprised predominately of spinel crystals. More specifically, the glass-ceramic articles formed will exhibit a crystal phase assemblage comprised predominately of uniformly-sized spinel crystals exhibiting ultra-fine grains. Predominately, as used herein, indicates that the spinel crystals are present in a greater amount than any of the other accessory crystal phases which may be exhibited by the inventive glass-ceramic. Preferably, the spinel crystals comprise about 25%, or greater, by weight, of the glass-ceramic article. The so-formed spinel crystals exhibit dimensions finer than about 100 nm (1000Å) in diameter, and preferably equal to, or finer than about 30 nm (300Å) in diameter. Furthermore, the spinel crystals are uniformly distributed within a residual siliceous glass matrix phase. Depending on the precursor glass chosen, the resultant spinel crystals which are present in the crystal phase assemblage may be entirely of the aluminate variety, as represented by (Mg,Zn)Al 2 O 4 , entirely of the titanate variety, as represented by (Mg/Zn) 2 TiO 4 or a mixture of both, as their is only limited solid solution between the aluminate and titanate spinels. It should be noted that the crystal phase assemblage may also include, in addition to the requisite predominate spinel crystals, those accessory phases selected from the group consisting of MgTi 2 O 5 , quartz solid solution, an (Mg,Zn) petalite-type phase, enstatite, sapphirine, mullite, rutile, cordierite, tetragonal zirconia and/or ZrTiO 4 , transition metal spinels and mixtures thereof. Regardless of the crystal phase assemblage obtained in addition to the spinel crystals, these crystals, like the aforementioned spinel crystals are uniform in size and are uniformly dispersed within the highly siliceous residual glass phase. Both the MgO and ZnO constituents are key spinel crystal formers, therefore, it is imperative that either one or the other, or both, are present in the precursor glass, and ultimately the glass-ceramic. Inclusion of high amounts of MgO by weight, tends to yield glass-ceramics exhibiting a high moduli, i.e, a modulus equal to, or exceeding 117.215 x 10 3 MPa (17X10 6 psi.) Specifically, the highest modulus glass-ceramics are obtained with a higher MgO/ZnO ratio, although respectably high modulus values can be achieved with Zn-rich values. On the other hand, the inclusion of high amounts of ZnO, 5-10%, tends to minimize the crystallization of other phases. For instance, the presence of high amounts of ZnO can be particularly important if a single phase/spinel crystal-containing glass-ceramic is required. Otherwise, without the inclusion of ZnO, the Mg-Ti spinels are generally accompanied by phases such as MgTi 2 O 5 , quartz and/or (Mg,Zn) petalite-type phases, and sometimes cordierite. Similar to the effect of ZnO, the inclusion of NiO, usually as a substitute for either MgO or ZnO, also aids in the minimizing the number of phases. Typically, the NiO combines with a portion of the SiO 2 to form Ni 2 SiO 4 spinel (solid solution), while at the same time lowering the tendency toward quartz crystallization. In addition, the NiO also acts as a auxiliary nucleating agent. Nucleation efficiency is enhanced by the presence of components such as TiO 2 , ZrO 2 and NiO, and also high levels of the Al 2 O 3 . If the Al 2 O 3 constituent is present in an amount is less than about 25%, it becomes necessary to include in the composition at least 5% by weight of either TiO 2 , ZrO 2 or NiO. In addition to contributing efficient nucleation, the TiO 2 constituent also may enter the spinel phase itself Also, amounts of Cr 2 O 3 up to its solubility limit of approximately 1%, may be added to further enhance the nucleation efficiency. If a glass-ceramic exhibiting a higher spinel crystal content and increased elastic modulus is desired, the composition of the precursor glass, and the ultimate glass-ceramic, should be comprised of increased amounts of the spinel forming constituents MgO or ZnO and Al 2 O 3. . Specifically, increased amounts of the MgO, ZnO and Al 2 O 3 coupled with lowered amounts of SiO 2 result in a glass-ceramic with a higher spinel content and an increased elastic modulus; however one skilled in the art should recognize that a decrease in the glass stability of the glass-ceramic precursor glass would be a likely tradeoff. Silica typically does not enter the spinel phase, unless NiO is present in the glass, rather, the silica typically provides the matrix of residual glass which is uniformly distributed with the crystal phase assemblage. Additionally, the silica may combine with alumina and any excess MgO, ZnO and/or Li 2 O to form quartz solid solution, (Mg,Zn) petalite-type phases, enstatite or cordierite. These phases may be very fine grained themselves and, if so, they would likely provide extra strength and toughness to the materials over that obtained with spinel crystals and glass alone. If the inventive glass-ceramic exhibits an accessory phase of quartz solid solution, it is typically present as a solid solution between SiO 2 , LiAlO 2 , MgAl 2 O 4 , and ZnAl 2 O 4 ; however, the SiO 2 constituent is usually predominating. The addition of small amounts of Li 2 O can induce quartz solid solution crystals, if desired in place of siliceous residual glass. In other words, it should be fairly obvious that numerous compositions within the claimed inventive compositions may be used to produce different microstructures comprising a variety of crystal phase assemblages. However, it should be reiterated that the inventive glass-ceramics, regardless of accessory crystal phases exhibited, possess a number of common attributes: (1) the requisite spinel-type crystals in whatever form, titanate or aluminate, are present as the predominate crystal phase; (2) whatever crystal phase assemblage is obtained possesses spinel crystals which exhibit a fine crystal or grain size diameter which is less than about 100 nm (1000Å), and preferably less than about 30 nm (300 Å), and, (3) the crystals are uniformly distributed within a highly siliceous residual glass phase. As a result of the so-formed uniform microstructures possessing ultra-fine crystal or grain sizes, extremely smooth polished surfaces may be produced from the inventive glass-ceramics without excessive polishing. Potentially, these nanocrystalline microstructure-containing glass-ceramics can exhibit polished surfaces possessing average roughness measurements on the order of about 0.5 nm (5Å) or less, as measured by atomic force microscopy (AFM) techniques. One potential application for these high modulus glass-ceramics exhibiting this type of possible surface roughness is as ultra-smooth, high-performance rigid disk substrates for use in magnetic memory storage devices. In addition, it should be noted that the strength and toughness exhibited by these inventive nanocrystalline glass-ceramics exhibits are greater than those which might otherwise be expected from such ultra-fine microstructures. While not intending to be limited by theory, this unexpected strength and toughness in excess of 1.0 Mpa·m ½ ., is thought to be a result of a phenomenon known as dispersion strengthening. Spinel crystals have both a higher density and a higher thermal expansion than the surrounding residual glass. As a result, during the cooling of the glass-ceramic after heat-treatment, the glassy phase surrounding such crystals produces tangential compression around the crystal, while radial tension exists between the crystals. Thus, any fracture will be deflected around each crystal. The deflection path around each tiny crystal is not long, but there are millions of them, resulting in an extremely tortuous fracture causing high energy of fracture. This fracture toughness increases according to the relation K lc =(2Eγ) ½ , where E is the Young's modulus and γ the energy of fracture; both E and γ are relatively high in these inventive glass-ceramics and thus provide an unexpectedly high fracture toughness. EXAMPLES Table I specifically records a number of precursor glass compositions, expressed in terms of weight percent on the oxide basis, which, when crystallized, illustrate the glass-ceramics operable in the present invention. The actual batch ingredients for the glasses can comprise any materials, either the oxides or other compounds, which, upon being melted together, are converted into the desired glass forming melt containing the proper oxide proportions, e.g., high purity sand is conveniently employed as the source of SiO 2 . The batch materials were thoroughly mixed together in order to secure a homogeneous melt, and subsequently placed into silica and/or platinum crucibles. The crucibles were placed into a furnace and the glass batch was then melted and maintained at temperatures ranging from 1500-1650°C for times ranging from about 6-16 hours. The melts were thereafter poured into steel molds to yield glass slabs having dimensions of approximately 4"X8"X½" (∼20X107X1¼ cm). Subsequently, those slabs were transferred immediately to an annealer operating at about 650-750°C. Samples were held at this temperature for about 1-3 hours and subsequently cooled overnight. After the glass slabs were removed from the annealer, glass tabs of approximately 1"X2"X½" (2½X5X1¼cm) were cut from the slabs. These glass tabs were then subjected to one of the heat treatments reported in Table III, i.e. the glass tabs were crystallized in situ to form glass-ceramic tabs. The tabs were then measured for the properties reported herein. Additionally, if so desired, these glass-ceramic tabs could be ground, lapped and polished using standard techniques in order to generate surface characteristics which would allow the surface and the microstructure to be studied using standard analytical techniques. Included among such analytical techniques are scanning electron microscopy (SEM), replica electron microscopy (REM), atomic force microscopy (AFM), etc., In addition, Table I reports the crystalline phases (Phases), as identified through X-ray diffraction, developed as a result of the heat treatment of the glass examples; the predominate phase present (pred) and the accessory phases (acces.). As can be determined from a review of the Table, all of the examples reported therein contain a crystal phase assemblage which is comprised predominately of the requisite spinel crystals (S). Additionally, the table reports that the other accessory phases which are present, including MgTi 2 O 5 (M), quartz solid solution (Q), (Mg,Zn) petalite-type phase (P), enstatite (E) mullite (Mu), tetragonal zirconia (Z) and ZrTiO 4 (Zr). Table II reports the heat treatment schedule the precursor glass examples were subjected to (H. T.), the visual description (Appear.), the elastic or Young's modulus (E), the bulk density (B.D.), the coefficient of thermal expansion (C.T.E.), the Knoop hardness (KHN),the fracture toughness (KIC) and the modulus of rupture of an abraded sample (MOR). It will be recognized that, whereas the above description is drawn to laboratory practice, the glasses operable in the invention described herein can be melted in large scale/commercial melting tanks and formed into desired shapes using conventional glass melting techniques and forming processes. It is only necessary that the compositions be fired at sufficiently high temperatures and for a sufficient length of time to produce a homogeneous melt, and thereafter the melt is cooled and simultaneously shaped into a glass body which is customarily annealed. Moreover, it should be recognized that various heat-treatment cycles and compositions may be used to produce different microstructures or crystal phase assemblages. However, as Table I reveals, all of the Examples produced contain the requisite spinel crystals in whatever form, titanate or aluminate, as one of the major crystal phases. The Examples marked with * are "Reference Examples" not according to the invention. Designation Heat Treatment Schedule Designation Heat Treatment Schedule A R.T. to 800°C at 300°C/hour E R.T. to 810°C at 300°C/hour Hold at 800° C for 2 hours Hold at 810° C for 4 hours Raise to 1000°C at 300°C/hour Raise to 965° C at 300°C/hour Hold at 1000°C for 4 hours Hold at 965°C for 4 hours Cool to R.T. at furnace rate Cool to R.T. at furnace rate B R.T. to 800°C at 300°C/hour F R.T. to 800°C at 300°C/hour Hold at 800° C for 2 hours Hold at 800° C for 2 hours Raise to 975° C at 300°C/hour Raise to 950° C at 300°C/hour Hold at 975°C for 4 hours Hold at 950°C for 4 hours Cool to R.T. at furnace rate Cool to R.T. at furnace rate C R.T. to 800°C at 300°C/hour G R.T. to 800°C at 300°C/hour Hold at 800° C for 2 hours Hold at 800° C for 1 hours Raise to 950° C at 300°C/hour Raise to 1000° C at 300°C/hour Hold at 950°C for 1 hour Hold at 1000°C for 2 hours Cool to R.T. at furnace rate Cool to R.T. at furnace rate D R.T. to 850°C at 300°C/hour Hold at 850° C for 2 hours Raise to 1000° C at 300°C/hour Hold at 1000°C for 4 hours Cool to R.T. at furnace rate FIG. 1 is a replica electron micrograph, taken at magnification of 50,000X, of a polished section of the inventive glass-ceramic article reported above as Example 20. The bar at the bottom of the micrograph represents 1 micron. In order to enhance the contrast between the crystals and the residual glass, the sample, which appears in the micrograph, was sufficiently polished using standard techniques and acid etched. Specifically, the sample was acid-etched for 1 minute using an etchant solution comprised of a combination of ½% HF and 1⅛% HCl. As a result of this acid etching, the so-formed crystals, a mixture of titanate spinel, aluminate spinel, Mg-dititanate (MgTi 2 O), β-quartz and a (Mg,Zn) petalite-type phase in the instant example, are not as readily dissolved by the acid as the residual glass, and thus protrude above the matrix glass. As can be easily observed in the micrograph, the crystals are generally uniform in size and exhibit grain sizes of less than 10 nm (100Å). In addition, it is also apparent from the micrographs, that the crystals which make up the crystal phase assemblage are uniformly dispersed within the residual glass. However, as earlier indicated, other compositions within this claimed scope may result in different crystal phase assemblage combinations and thus potentially different surface textures than that which appears in this micrograph. FIG. 2 is an atomic force micrograph illustrating the surface roughness/crystal phase microstructure of the Example 6 glass-ceramic. The sample was sufficiently polished utilized standard techniques in order to generate a sample for measuring the surface texture. The scale of the micrograph is 2X2 µm. The image statistics/roughness analysis results revealed by this micrograph are a R a of 0.53 nm (5.3 Å), i.e, an extremely smooth polished surface. Examination of the micrograph supports the earlier observation that the crystals which make up the crystal phase assemblage for this particular example are uniformly dispersed within the glass matrix and exhibit ultra fine grain sizes of less than about 100Å. It should be noted that, as opposed to the REM micrograph above, the crystals are represented by the indentions in this AFM. FIG 3 is an atomic force micrograph, 3-D view of FIG 2, illustrating the surface roughness/crystal phase microstructure of the inventive glass-ceramic. Again the sample was sufficiently polished to generate a sample for measuring the surface texture. The X and Y dimensions of this micrograph are 2µm and 2µm, respectively. Once again, as can be observed from this micrograph, the resulting surface, following polishing, is quite uniformly textured and extremely smooth. As was earlier indicated, this is a result of the uniform distribution of the spinel crystals and accessory crystals within the residual siliceous glass phase. Additionally, the examination reveals that the surface texture (roughness) corresponds with the internal microstructure, with the spinel crystals and accessory crystals protruding above the mean surface topology. i.e., the harder spinel crystals and accessory crystals protrude above the residual glass following polishing, thus resulting in a uniformly textured surface. FIG 4 is a graph illustrating the cross-sectional profile, taken along line A-A of FIG. 3, illustrating the surface roughness of the inventive glass-ceramic designated as Example 5. This figure lends support to the observation revealed in FIG. 3 that the surface which forms as a result of polishing, is uniformly textured and extremely smooth. The harder crystals which are uniformly distributed throughout the surface and protrude above the residual siliceous glass matrix are represented by the peaks in the profile which rise above the 0 nm mark. It should be noted that, the properties which the inventive glass ceramic material disclosed herein possess render it eminently suitable for use in a magnetic memory storage device comprised of a head pad and rigid information disk. Specifically, for use in the rigid information disk as the rigid disc substrate which possesses a layer of magnetic media on the its surface. In other words, the substrate would be comprised of the inventive glass-ceramic material exhibiting the crystal phase assemblage comprised predominately of spinel crystals. Based on its overall combination of properties, i.e., an elastic modulus of 129.21x10 3 MPa (18.74x10 6 psi), a fracture toughness of 1.3 MPa·m ½ and a Knoop Hardness number of 760 KHN/100 and a MOR(abraded) of 96.53 MPa (14,000 psi), example 6 is deemed to be the most preferred composition.

Claims: A glass-ceramic article exhibiting Young's modulus between about 96.53 - 165.48 x 10 3 MPa (about 14 to 24 x 10 6 psi) and a fracture toughness in excess of 1.0 MPa.m 1/2 , wherein the crystal phase assemblage is comprised predominately of generally uniformly-sized spinel crystals which are uniformly dispersed within a highly siliceous residual glass matrix phase, the spinel crystals are entirely of the aluminate variety, entirely of the titanate variety or a mixture of both aluminate and titanate crystals, the aluminate spinels consisting of either MgAl 2 O 4 or a solid solution of (Mg,Zn) Al 2 O 4 , the titanate spinels consisting of either Mg 2 TiO 4 or a solid solution represented by (Mg,Zn) 2 TiO 4 , the crystals exhibiting dimensions finer than about 100 nm (about 1,000 Å) in diameter, the article having an overall composition consisting essentially of, expressed in terms of weight % on the oxide basis, 40-50 % SiO 2 , 22-30 % Al 2 O 3 , 7-18 % MgO, 0-12 % ZnO, 7-13 % TiO 2 , at least 13 % MgO+ZnO, 0-4 % ZrO 2 , 0-5 % NiO, at least 8 % TiO 2 +ZrO 2 with the proviso that, if the Al 2 O 3 content is less than about 25 %, the TiO 2 +ZrO 2 + NiO content is at least 5 %, and up to 5 % of optional constituents selected from the group consisting of BaO, CaO, PbO, SrO, P 2 O 5 , B 2 O 3 , Ga 2 O 3 , 0-5 % R 2 O wherein R 2 O is selected from the group consisting of Na 2 O, K 2 O, Rb 2 O and Cs 2 O and 0-8 % of a transition metal oxide. The glass-ceramic according to claim 1 wherein the transition metal oxide is selected from the group consisting of the oxides of Cu, Co, Fe, Mn, Cr, Sn and V, the oxides of Cu, Co, Mn and Cr not exceeding about 1%. A glass-ceramic article according to claim 1 wherein the spinel crystals exhibit dimensions finer than about 30 nm (about 300Å) in diameter. A glass-ceramic article according to claim 1 wherein the crystal phase assemblage contains an accessory crystal phase selected from the group consisting of MgTi 2 O 5 , quartz solid solution, (Mg,Zn) petalite-type phases, enstatite, sapphirine mullite, rutile, cordierite, tetragonal zirconia, ZrTiO 4 , transition metal spinels and mixtures thereof. A method for making a glass-ceramic article according to any one of claims 1-4, which comprises the steps of melting a batch for said composition; and, making an article from said batch, heating the glass article to a temperature range of about 750-850°C at a temperature rate of approximately 300°C/hour; and, maintaining the temperature for a time sufficient to cause the article to obtain a high degree of nucleation; and, heating the glass article to a temperature range about 900-1150°C at a temperature rate of approximately 300°C/hour; and, maintaining the temperature for a time sufficient to cause the article to crystallize in situ forming a crystallized glass article which exhibits crystal phase assemblage comprised predominately of generally uniformly-sized spinel-type crystals as described in claim 1 which are uniformly dispersed within a highly siliceous residual glass matrix phase. Use of the glass-ceramic of any one of claims 1-4 as the substrate in a magnetic memory storage device comprised of a head pad and rigid information disk, the disk comprising said substrate having a layer of magnetic media on the surface thereof. Use of the glass-ceramic according to any one of claims 1-4, as the substrate in a rigid information disk for a magnetic memory storage device.

Inventors: BEALL GEORGE HALSEY, PINCKNEY LINDA RUTH, BEALL, GEORGE HALSEY, PINCKNEY, LINDA RUTH
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1995
Main classification: C03C10
IPC codes: C03B27, G11B5, C03C10