WO2020231693A1

WO2020231693A1 - Glass substrate for magnetic recording and method of making

Info

Publication number: WO2020231693A1
Application number: PCT/US2020/031606
Authority: WO
Inventors: Ming-Huang Huang; Robert George MANLEY; Rajesh Vaddi; Nikolay Zhelev ZHELEV; Bin Zhu
Original assignee: Corning Incorporated
Priority date: 2019-05-13
Filing date: 2020-05-06
Publication date: 2020-11-19
Also published as: TW202106649A

Abstract

A process for preparing a glass substrate for an information recording media is provided. The process can include providing a laminated glass article having at least one outer cladding layer and an inner core layer. A first outer cladding layer can be in contact with and adjacent to a first surface of the inner core layer. A second outer cladding layer, if present, can be contact with and adjacent to a second surface of the inner core layer. The process can include reducing the thickness the at least one cladding layer to obtain a glass substrate having at least one flat surfaces that are suitable for a recording process. A glass substrate prepared according to the process is also provided. A hard disk drive that includes the glass substrate is also provided.

Description

GUASS SUBSTRATE FOR MAGNETIC RECORDING

AND METHOD OF MAKING

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No. 62/846,923, filed on May 13, 2019, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

[0001] The data storage capacity of magnetic devices, such as a hard disk drive, has increased substantially in recent years. Heat-assisted magnetic recording (HAMR) is an emerging technology that temporarily heats the disk material during writing, which makes it more receptive to magnetic effects and allows writing to much smaller regions. One of the key recording coatings for HAMR is a highly ordered granular film with high perpendicular magnetic anisotropy. A high perpendicular magnetic anisotropy requires the substrate to be heated to 600 °C or higher to obtain a good uniformity of grain size and distribution and a good control of randomness of positions, texture, and orientation. The high deposition temperature used for HAMR media coatings precludes the use of conventional substrates, such as those made of aluminum. Instead, glass is a preferred substrate for HAMR coatings. A smooth, flat glass surface is required for the perpendicular magnetic recording technology.

SUMMARY

[0002] In various embodiments, a process for preparing a glass substrate for an information recording media is provided. The process can include providing a laminated glass article having at least one outer cladding layer and an inner core layer. A first outer cladding layer can be in contact with and adjacent to a first surface of the inner core layer. A second outer cladding layer, if present, can be contact with and adjacent to a second surface of the inner core layer. The process can include reducing the thickness of the at least one the outer cladding layer to obtain a glass substrate having one or two flat surfaces that are suitable for a recording process (e g., HAMR). [0003] In various embodiments, a glass substrate prepared according to the process is provided. In various embodiments, a hard disk drive comprising the glass substrate is provided.

[0004] The foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings, which are to be considered together, are included to provide a further understanding of the features and advantages of the various embodiments disclosed in, or rendered obvious by, the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 A schematically depicts a cross section of a laminated glass article, in accordance with some embodiments described herein. FIG. IB schematically depicts the laminated glass article of FIG. 1A with noticeable surface thickness variation on its opposing outer cladding surfaces. FIG. 1C schematically depicts a cross section of a laminated glass article with noticeable surface thickness variation on its outer cladding surface, in accordance with some embodiments described herein.

[0006] FIG. 2 schematically depicts an embodiment of a fusion draw process for making the laminated glass article of FIG. 1A.

[0007] FIG. 3 A schematically depicts a cross section of a laminated glass substrate, in accordance with some embodiments described herein. FIG. 3B schematically depicts a cross section of a laminated glass substrate, in accordance with some embodiments described herein. FIG. 3C schematically depicts a cross section of a laminated glass substrate, in accordance with some embodiments described herein. FIG. 3D schematically depicts a cross section of a laminated glass substrate, in accordance with some embodiments described herein.

DETAILED DESCRIPTION

[0008] The description of the embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this disclosure. The drawing figures are not necessarily to scale and certain features of the various embodiments may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. In this description, relative terms such as“horizontal,”“vertical,” “up,”“down,”“top,”“bottom,” as well as derivatives thereof (e.g.,“horizontally,”

“downwardly,”“upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus“outwardly,”“longitudinal” versus“lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as“connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both moveable or rigid attachments or relationships, unless expressly described otherwise, and includes terms such as “directly” coupled, secured, etc. The term“operatively coupled” is such an attachment, coupling, or connection that allows the pertinent structures to operate as intended by virtue of that relationship.

[0009] The laminated glass article 100 shown in FIG. 1 A may be formed by a fusion lamination process, such as the process described in U.S. Patent No. 4,214,886, which is incorporated herein by reference in its entirety. As shown in FIG. 2, for example, the laminate fusion draw apparatus 200 for forming a laminated glass article includes an upper isopipe 202 that is positioned over a lower isopipe 204. The upper isopipe 202 includes a trough 210 into which a molten glass cladding composition 206 is fed from a melter (not shown). Similarly, the lower isopipe 204 includes a trough 212 into which a molten glass core composition 208 is fed from a melter (not shown).

[0010] As the molten glass core composition 208 fills the trough 212, it overflows the trough 212 and flows over the outer forming surfaces 216 and 218 of the lower isopipe 204. The outer forming surfaces 216 and 218 of the lower isopipe 204 converge at a root 220.

Accordingly, the molten glass core composition 208 flowing over the outer forming surfaces 216 and 218 rejoins at the root 220 of the lower isopipe 204, thereby forming the glass core layer 102 of the laminated glass article 100. [0011] Simultaneously, the molten glass cladding composition 206 overflows the trough 210 formed in the upper isopipe 202 and flows over the outer forming surfaces 222 and 224 of the upper isopipe 202. The molten glass cladding composition 206 is outwardly deflected by the upper isopipe 202 such that the molten glass cladding composition 206 flows around the lower isopipe 204 and contacts the molten glass core composition 208 flowing over the outer forming surfaces 216 and 218 of the lower isopipe, fusing to the molten glass core composition 208 and forming glass cladding layers 104a and 104b around the glass core layer 102 of the laminated glass article 100.

[0012] The laminate fusion draw apparatus 200 produces laminated glass in such a manner as to provide substantially uniform thickness of the glass layers in the newly formed laminated glass article 100. Alternatively, the viscosity of the molten glass cladding

composition 206, the tilt angle of trough 210, and the flow rate can be adjusted to differentiate the thickness of the individual cladding layers 104a and 104b independently of the other. For example, in some embodiments the tilt angle of trough 210 can be large enough so that the molten glass composition flows over one of its sides and the resulting laminated glass article has a single cladding layer (i.e., 104a or 104b) in contact with the core 102, as shown in FIG. 1C.

[0013] Molten glass compositions, such as, for example, the chemical compositions described in U.S. Pat. Appl. Nos. US 2016/0114564 and US 2018/0312422, which are incorporated herein by reference in their entireties, may be used in fusion forming processes. Other compositions are contemplated. The properties of the molten glass compositions (e.g., the liquidus viscosities, temperatures, and the like) can be controlled to make them well suited for use with fusion forming processes, such as the fusion down draw process or the fusion lamination process. Specific examples of glass compositions for the core layer include, e.g., Lotus NXT glass (Coming) and the like. Specific examples of glass compositions for the cladding layers include, e.g., Eagle XG glass (Corning), Gorilla glass (Corning), and the like.

[0014] Glass produced using a fusion forming process, such as the up-draw or down- draw fusion processes, generally has a low incidence of surface flaws. However, thickness variation and glass warp can result from airflow and surface temperature variations in proximity to the fusion draw apparatus. Those defects mostly affect the surface of the glass. For laminated glass formed by fusion draw process, in which the outer cladding is formed at the same time as the inner core, the core will be shielded from the airflow and surface temperature modulations and will have reduced thickness variation and warping compared to the exterior surface of the cladding. For example, the laminated glass article 100 in FIG. IB has noticeable surface thickness variation on the outer main surfaces of the cladding layers 104a and 104b, but no noticeable thickness variation or warping on the main surfaces of the inner core layer 102

[0015] To increase the recording density of a hard disk drive, the distance between the magnetic recording head and the disk surface, defined as flight height, must be as narrow as possible. Generally, a flying height of about 10 nm or less is required. Therefore, the flatness and smoothness of a glass substrate is important to prevent the recording head from contacting the magnetic disk, leading to damage of the head. Flatness refers to the total thickness variation (TTV) of the substrate. Generally, a TTV of less than about 1 micron is required. Smoothness, or roughness, refers to the surface texture, and is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If the deviations are large, the surface is rough. If the deviations are small, then the surface is smooth. The profile (line) roughness parameter (Ra) is commonly used to calculate a roughness value. Generally, Ra is required to be less than 0.2 nm.

[0016] In some embodiments, the laminated glass article 100 is prepared from a combination of different chemical compositions. In such embodiments, the glass chemical composition for the cladding layers 104a and 104b has different properties compared to the glass chemical composition for the core 102. In some embodiments, the core and cladding glass compositions have different material hardness, different thermal conductivity, and/or different chemical durability. Thermal conductivity is a calculated value equal to the product of the thermal diffusivity multiplied by specific heat multiplied by density of the glass. Thermal conductivity can be improved by tuning the core-cladding glass composition of the laminated glass article 100. In some embodiments, the glass core 102 comprises a composition having higher thermal conductivity. In some embodiments, an alkali-free glass composition can be used in the cladding layers 104a and 104b to prevent impurity diffusion from the core if the cladding layers are not completely removed. [0017] Chemical durability, or corrosion resistance, is measured as weight loss per surface area (mg/cm²) after immersion in a corrosive environment. The rate of weight loss is based on the time required to obtain the weight loss, at a certain temperature. Chemical durability is highly dependent upon testing conditions. Other properties, including mechanical and electrical properties, viscosity, refractive index, friction coefficient, strength, impact resistance, scratch resistance, etc., will also depend on the glass compositions and the treatments to the laminated glass article 100 components.

[0018] As disclosed herein, a laminated glass article 100 in which the core 102 and the cladding layers 104a and 104b were prepared from different glass compositions is advantageous for the production of a flat, smooth substrate for information recording media. For example, in some embodiments, the exterior, main surfaces of the cladding layer 104a, 104b, or both (i.e., the surface that is not adjacent to the core 102, and not the peripheral side surfaces) can be treated to remove defects, such as warping or thickness variation, and otherwise improve the final surface quality. In such embodiments, the at least one of the cladding layer can be polished or materially reduced to ensure that the resulting laminated glass substrate (101) has at least one flat and smooth surface for an information recording media. FIGS. 3A-3D, for example, show various embodiments of the laminated glass substrate 101, which is the laminated glass article 100 after a surface treatment to the cladding layer 104a, 104b, or both.

[0019] As shown in FIG. 3A, for example, in some embodiments the cladding layer 104b is partially reduced (with respect to the exterior surface of cladding layer 104b in the laminated glass article 100). In other embodiments, as shown in FIG. 3B, the cladding layer 104b is completely reduced to the adjacent surface of the core 102. In some embodiments, as shown in FIG. 3C, both of the cladding layers 104a and 104b are partially reduced. In some embodiments, as shown in FIG. 3D, the at least one cladding layer (104a and/or 104b) is completely reduced to the adjacent surface of the core 102, such that the glass core 102 remains. The reference number 106 in each of the FIGS. 3A-3D refers to the portion of the cladding layer(s) of the laminated article 100 that was removed to produce the laminated glass substrate 101a, 101b, 101c, or lOld.

[0020] In some embodiments, the glass compositions are selected so the cladding layers 104a and 104b are relatively softer than the core 102. In some embodiments, the glass compositions are selected so the core 102 is more corrosive resistant (chemically durable) than the cladding layers 104a and 104b. In such embodiments, the glass in the cladding layer is easier to reduce (either mechanically, chemically, or both) than the glass in the core 102. Accordingly, in some embodiments, because the core glass is harder, more corrosive resistant, or both, the core layer 102 serves as a backstop during the cladding reduction process. Further, in some embodiments, because the core 102 is shielded by the cladding from the airflow and surface temperature modulations in the laminate fusion process, the core 102 offers a substantially flat and smooth surface having minimal surface defects. In such embodiments, the hardness and elastic properties, thermal conductivity, and/or the chemical durability, of the cladding and core glass compositions are precisely controlled by tuning the glass compositions and the thickness ratio of the layers to ensure that a flat and smooth surface of the glass core 102 emerges after the at least one cladding layer is completely removed.

[0021] Any suitable process can be used in the cladding reduction phase. In some embodiments, for example, a chemical etching process is used to reduce the cladding layers 104a and 104b. In such embodiments, the compositions for the core 102 and the cladding layers 104a and 104b is precisely controlled to establish a differential etch selectivity based on their relative corrosive resistance. Additionally, the thickness of the core 102 and of the cladding layers 104a and 104b is adjustable to precisely control the rate of material reduction. In such embodiments, the core 102 serves as a stop during the etch process. For example, the core 102 comprises a glass composition that is substantially resistant to the etchant, but the glass composition making up the cladding layer(s) is not resistant to the same etchant. After selectively etching away some or all of the glass in the cladding layer, the core glass, or the core glass and remaining cladding, respectively, is obtained with a smooth, flat surface for information recording media.

[0022] In some embodiments, the etching step involves wet chemistry (e.g., solutions comprising acids, bases, fluorides, etc.). In some embodiments, the etching step involves dry chemistry (e.g., a plasma deposited via chemical vapor deposition).

[0023] In some embodiments, an acid etch surface treatment is used. This etching process comprises contacting one or both exterior surfaces of the laminated glass article 100 with an acidic glass etching medium, which can be readily tailored to the specific glass composition and applied to both planar and complex surface geometries. Etching the laminated glass article 100 can alter the size or geometry of its surface flaws, and/or reduce the size and number of surface flaws, while at the same time have a minimal effect on the general topography of the treated surface. Further, in some embodiments, acid etching has been found to be effective to reduce strength variability, even in glass having a low incidence of surface flaws, including an up-drawn or down-drawn (e.g., fusion-drawn) glass sheet is conventionally thought to be largely free of surface flaws. In some embodiments, as the acid etches away the cladding layers 104a and 104b, the newly obtained surface is polished, smooth, and flat.

[0024] A variety of etchant chemicals, concentrations, and treatment times can be used to achieve a desirable level of surface treatment. For example, chemicals that are useful for the acid etching process include a fluoride-containing aqueous media having at least one active glass etching compound, such as, but not limited to, HF, combinations of HF with one or more of HC1, HNO3 and H2SO4, ammonium bifluoride, sodium bifluoride and other suitable compounds. Maintaining adequate control over the thickness of the removed glass 106 by etching in HF- containing solutions can be facilitated if the concentration of HF is closely controlled. Exemplary methods for etching glass layers is described in International Application No. PCT/US13/43561, filed May 31, 2013, the entirety of which is incorporated herein by reference.

[0025] In some embodiments, surface etching treatments of limited duration are required. In particular, the step of contacting a surface of the cladding layer of the laminated glass article 100 with an etching medium can be carried out for a period of time not exceeding that required for effective removal of about 2 to about 80 pm of surface glass 106, including all ranges and sub-ranges therebetween. For example, in some embodiments the step of contacting a surface of the cladding layer of the laminated glass article 100 with an etching medium can be carried out for a period of time not exceeding that required for effective removal of about 5 to about 50 pm or about 10 to about 30 pm of surface glass 106. In other embodiments, for example, about 3 pm, 10 pm, 15 pm, 20 pm, 40 pm, etc. is removed. The actual etching time required to limit glass removal in any particular case can depend upon the composition and temperature of the etching medium as well as the composition of the cladding layer. For example, in an etching composition comprising HF, the rate of etching will depend on the silica content in the glass composition. In such embodiments, a greater the ratio of oxygen to silicon, the slower the etching rate.

[0026] In some embodiments, each of the cladding layers 104a and 104b independently have a thickness of about 1 pm to about 100 pm, including all ranges and sub-ranges

therebetween. For example, in some embodiments, the thickness is about 3 pm to about 75 pm, or about 5 pm to about 50 pm. In some embodiments, the acid etching process is designed to remove not more than about 50 pm of surface glass, not more than about 30 pm of surface glass, not more than 15 pm of surface glass, not more than 5 pm of surface glass, not more than 2 pm of surface glass, etc. In some embodiments, about 20 pm to about 50 pm of glass cladding is removed, including all ranges and sub-ranges therebetween. For example, in some embodiments, about 30 pm to about 50 pm of glass cladding is removed.

[0027] In some embodiments, lapping and/or grinding is used to reduce the at least one cladding layer. In such embodiments, the compositions for the core 102 and the cladding layers 104a and 104b is precisely controlled to establish a differential hardness selectivity. The laminated glass with softer cladding layers and a harder core allows the cladding to be removed faster than the core layer 102. The hardness and elastic properties can be precisely controlled by tuning the core-cladding glass compositions and the core-cladding thickness ratio. The effects of mechanical polishing are dependent on, for example, the size and hardness of the glass, and the stiffness and abrasiveness of the grinding pad. In some embodiments, the lapping and/or grinding process is combined with chemical polishing, a process known as Chemical Mechanical Polishing (CMP). In such embodiments, the combined processes can provide a flat and smooth surface for information recording media. Generally, in some embodiments, etching is more preferable than the lapping/grinding processes because etching yields an ultra-smooth glass surface without exposing the glass to harsh polishing and grinding processes.

[0028] With respect to the reduction phase, the material removal rate of 106 depends on certain factors, such as the applied pressure and relative velocity, the thickness of the layer to be removed (106), the time, etc. In tribology, the volume of the reduced material in sliding or abrasive wear conditions is inversely proportional to the hardness of the material, as shown in formula (I) below, in which V is the volume removed, L is the load on the sample, S is the aelative sliding distance, H is the hardness of the material concerned, and k_w is the wear coefficient.

[0029] In formula (I), the hardness ( ) of the material concerned refers to a value obtained using standard testing protocols (e.g., Mohs Hardness Test, Vickers Hardness, impact resistance, microindentation).

[0030] In some embodiments, the core 102 is comprised of a first glass composition and the at least one cladding layer (104a and/or 104b) is comprised of a second glass composition, and the first glass composition and the second glass composition are different chemical compositions. In such embodiments, the first glass composition has a first hardness value, and the second glass composition has a second hardness value. In some embodiments, the first hardness value is greater than the second hardness value. For example, in such embodiments, the hardness differential is about 3% to about 50%, including all ranges and sub-ranges

therebetween. For example, in some embodiments, the hardness differential is at least 3%, at least 5%, at least 10%, or at least 20%. In some embodiments, the first hardness value is about 3% to about 50% greater than the second hardness value. In some embodiments, the first hardness value is about 5%, about 10%, about 15%, about 20%, about 30%, etc. greater than the second hardness value.

[0031] Thermal conductivity is a useful design parameter for the rate of heat transfer through a material, where heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. Thermal conductivity (2) can be calculated by multiplying a material’s thermal diffusivity by its density (r ) and by its specific heat capacity (l = a· r·c Thermal diffusivity (or) measures the rate of heat transfer from a hot end of a material to its cold end. Laser flash analysis can be used to measure the thermal diffusivity of glass compositions. The specific heat of a material is the amount of energy required to raise the temperature of one unit amount of a material by one unit of temperature. Specific heat capacity (c_p) can be measured using differential scanning calorimetry (DSC).

[0032] In some embodiments, the core 102 is comprised of a first glass composition and the at least one cladding layer (104a and/or 104b) is comprised of a second glass composition, and the first glass composition and the second glass composition are different chemical compositions. In such embodiments, the first glass composition has a first thermal conductivity, and the second glass composition has a second thermal conductivity. In some embodiments, the value of the first thermal conductivity is greater than the value of the second thermal conductivity. For example, in some embodiments, the thermal conductivity differential is about 3% to about 50%, including all ranges and sub-ranges therebetween. For example, in some embodiments, the thermal conductivity differential is at least 3%, at least 5%, at least 10%, or at least 20%. In some embodiments, the first thermal conductivity value is about 3% to about 50% greater than the second thermal conductivity value. In some embodiments, the first thermal conductivity value is about 5%, about 10%, about 15%, about 20%, about 30%, etc. greater than the second thermal conductivity value.

[0033] According to the various embodiments described herein, the glass substrate is a disk configured for a hard disk drive having at least one surface that is suitable for the deposition of an information recording media (e.g., a coating or thin film, such as highly ordered L10 FePtX-Y nano-granular film). After the recording media is deposited on the glass substrate, information is recorded on the media using a magnetic recording head, which is spaced apart from the surface of the disk by about 10 nm or less (flying height). According to the various embodiments described herein the glass substrate has at least one surface that is substantially flat, having a total thickness variation less than about 1 to 5 microns, including all ranges and sub-ranges therebetween, which is less than the flying height and therefore suitable for the recording process.

[0034] According to various embodiments described herein, the glass substrate with a suitable surface for the recording process is obtained from a laminated glass article having two cladding layers around a core layer 102. One or both of the cladding layers are partially or completely reduced to obtain the flat surface. When one cladding layer is completely reduced, then both the core 102 and the opposing cladding layer remain. When both cladding layers are completely reduced, then the core 102 remains. In either case, the reduction of the cladding layer produces a substantially flat surface that is suitable for a substrate and the recording process.

[0035] According to various embodiments described herein, the laminated glass article includes at least one cladding layer (104a and/or 104b) comprising a first glass composition and a core layer 102 comprising a second glass composition that is a different chemical composition than the first glass composition. The first and second glass compositions are suitable for producing a laminated glass article via a laminate fusion process, and the first and second glass compositions have different material hardness, different thermal conductivity, and/or different chemical durability. Accordingly, when the laminated class article is subjected to a reduction process, such as acid etching, the thickness of the cladding layer(s) will reduce at a faster rate than that of the core 102. The laminated glass article is formed by fusion process, which forms the outer cladding at the same time as the inner core 102, the core is shielded by the cladding layers from the airflow and surface temperature modulations that can cause thickness variation and warping.

[0036] According to various embodiments described herein, the core layer 102 will have two opposing main surfaces (a first and second surface) that are substantially flat and suitable for a substrate and the recording process. The glass composition of the cladding layers 104a and 104b and the core 102 are selected based on differential properties, such as hardness, thermal conductivity, and chemical durability. In such embodiments, the cladding layers 104a and 104b are reduced at a faster rate than the core 102, which yields a glass substrate having two substantially flat surfaces that are suitable for a substrate and the recording process.

[0037] The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this application. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this application. All such modifications are intended to be encompassed within the below claims.

[0038] Although the subject matter has been described in terms of exemplary

embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments, which may be made by those skilled in the art.

Claims

CLAIMS What is claimed is:

1. A process for preparing a glass substrate for an information recording media, comprising:

providing a laminated glass article comprising a first outer cladding layer and an inner core layer, wherein the first outer cladding layer is in contact with and adjacent to a first surface of the core layer; and

reducing a thickness of the first cladding layer to obtain the glass substrate with at least one flat surface.

2. The process of claim 1 , wherein the laminated glass article comprises a second outer cladding layer that is in contact with and adjacent to a second surface of the core layer.

3. The process of claim 2, wherein the core layer comprises a first glass composition and the first and second outer cladding layers comprise a second glass composition different than the first glass composition.

4. The process of claim 2, wherein the laminated glass article is prepared using a fusion process.

5. The process of claim 1 , wherein the at least one flat surface has a total thickness variation less than about 1 to 5 microns.

6. The process of claim 1 , wherein the first outer cladding layer is reduced such that the at least one flat surface is the first surface of the core layer.

7. The process of claim 3, wherein a first hardness value of the first glass composition is greater than a second hardness value of the second glass composition.

8. The process of claim 7, wherein the first hardness value is about 5% to 10% greater than the second hardness value.

9. The process of claim 1 , wherein the laminated glass article is about 0.3 to 1.0 millimeters thick and the reduced thickness of the at least one cladding layer is about 30 to 50 microns.

10. The process of claim 3, wherein a first thermal conductivity value of the first glass composition is greater than a second thermal conductivity value of the second glass composition.

11. The process of claim 10, wherein the first thermal conductivity value is about 5% to 10% greater than the second thermal conductivity value.

12. The process of claim 3, wherein a first rate of weight loss of the first glass composition is slower than a second rate of weight loss of the second glass composition.

13. The process of claim 1, wherein the reducing step comprises lapping, grinding, etching, or polishing.

14. The process of claim 12, wherein the reducing step comprises an acidic etching solution.

15. The process of claim 1, wherein the at least one flat surface has a roughness parameter Ra of about 0.2 to 1 nm.

16. The process of claim 1, wherein the at least one flat surface has a roughness parameter Ra of less than 0.2 nm.

17. The process of claim 2, wherein the thickness of each of the first and second outer cladding layers is reduced to obtain the glass substrate.

18. A glass substrate prepared according to the process in claim 1.

19. The glass substrate according to claim 18, wherein the at least one flat surface has a total thickness variation of about 1 to 5 microns.

20. A hard disk drive comprising the glass substrate of claim 18.