WO2008150355A1

WO2008150355A1 - Thermally-bonded glass-ceramic/glass laminates, their use in armor applications and methods of making same

Info

Publication number: WO2008150355A1
Application number: PCT/US2008/006297
Authority: WO
Inventors: Linda R. Pinckney; Steven A. Tietje; Jian-Zhi J. Zhang
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 2007-05-21
Filing date: 2008-05-16
Publication date: 2008-12-11
Anticipated expiration: 2009-11-21
Also published as: IL202211A0; MX2009012640A; JP2011510832A; CN101679131A; EP2148845A1; BRPI0811914A2

Abstract

The invention is directed to direct bonding of a glass-ceramic to one or a plurality glass layers to form a transparent laminate, thereby eliminating one or more of the polymer interlayers between the glass-ceramic and glass layer, and between the individual glass layers of the transparent laminate when more than one glass layer is present. The direct bonding eliminates or minimizes any deleterious effects of temperature and humidity that can occur when polymer interlayers are used to effect glass-ceramic/glass and/or glass/glass bonding in transparent laminates. The bonding is carried out in ambient atmosphere at a temperature between the softening and strain points of the 'softer' or 'lower softening point' material, typically the glass material, and is carried out without the use a polymer interlayer or adhesive, or the application of a voltage. The glass-ceramic/glass laminates can be used in transparent armor applications, particularly when combined with a spall catcher layer that is bonded to the glass layer furthest from the glass-ceramic layer.

Description

THERMALLY-BONDED GLASS-CERAMIC/GLASS LAMINATES, THEIR USE IN ARMOR APPLICATIONS AND METHOD OF MAKING SAME

[0001] This application claims the benefit of priority under 35 U.S.C. § 1 19(e) of U.S. Provisional Application Serial No. 11/804,856 filed on May 21, 2007.

Field

[0002] The invention is directed to a transparent glass-ceramic/glass laminate that can be used for a variety of applications including for ballistic protection. In particular, the invention is directed to a glass-ceramic/glass laminate in which glass-ceramic and glass layers are directly thermally bonded to one another without the use of a polymer or adhesive interlayer or the application of a voltage during the bonding process. The glass-ceramic/glass laminate can be used to provide ballistic protection while remaining transparent in both the visual and infrared (night vision) electromagnetic spectral ranges.

Background

[0003] Transparent armor is a material that is designed to provide ballistic protection while remaining optically transparent. This type of material can be used in combat and non-combat situations (e.g. riot control) for windows (both vehicles and buildings), protective visors, and protective covers for equipment such as sensing devices among other uses. While there may be specific requirements for each particular use, there are requirements that are common to most systems or devices. Specifically, the primary requirement for a transparent armor is that it not only defeat a specific threat, but that it also have the ability to receive multiple hits without distorting vision in the area surrounding the hit. Additional requirements, which may have to be optimized, depend on the particular application in which the armor will be used. These additional requirements include weight, space efficiency and cost-versus-performance. While many problems with transparent armor can be cured by increasing the thickness of the armor, this solution is not desirable because it increases the weight that has to be carried by a person or vehicle, increases distortion and thus reduces transparency due to the thickness of the material, and, in vehicles, is impractical due to space limitations. [0004] Transparent materials that are used for ballistic protection (armor) include (1) conventional glasses, for example, soda-lime and borosilicate glass which are typically manufactured using the float process; (2) crystalline materials such as aluminum oxy- nitride (ALON), spinel, and sapphire; and (3) glass-ceramic materials (GC). In a typical transparent armor system, one or more sheets of a glass material are laminated into a composite layered structure and a thick polymer material is employed as the backing or "spall catcher." For example, Figure 1 illustrates a common layered transparent armor composite 10. The incoming projectile is in the direction of arrow 12 and the projectile hits the strike face which is the first of four glass layers 14 (arrows 14 shown to the first and fourth of the four layers). Polymer interlayers or adhesives 16 are positioned between the exemplary four glass layers. The last layer 18 is the "backing layer" or "spall catcher," typically a polymeric material such as polycarbonate. A polymer/adhesive layer 16 is also placed between the spall catcher and the last glass layer. In one variation, a hard ceramic material may be used as the strike surface to improve ballistic performance against more potent threats such as armor piercing bullets. Unless thick glass is available, the polymer sheets are laminated between the multiple glass layers to build up the thickness required to defeat the designated threat. These polymer layers can vary in thickness and consist of either a soft or a hard material.

[0005] The polymer sheets and interlayer adhesive materials of Figure 1 are chosen from materials in use in present day armor technology. In addition to ballistics performance, some of the considerations include:

(1) index of refraction matching with the glass or ceramic,

(2) ability to accommodate its thermal expansion mismatch with the glass/ceramic ceramics, and

(3) environmental performance in the armor system in the range of potential use environments.

This last requirement concerning environments performance has in particular proven to be problematic, given the wide potential range of temperatures and humidities experienced by an armor system. The temperature range specified for an armor system, for example, encompasses -40°F to +140°F (-40⁰C to +60°C), and there is discussion about increasing the upper temperature requirement. Not only is the thermal expansion mismatch between the glass and the polymer interlayers exacerbated by this nearly 200°F (~110°C) range, but the inherently high dn/dT and propensity for heat and solar damage of the polymer leads to non-uniform discoloration, cracking, and delamination of the interlayers and resulting in loss of transparency/visibility. In addition, when a polymer interlayer between two hard, glass-based sheets exceed certain thickness, e.g., 0.015" (-0.38 mm), the laminate suffers from loss of ballistic performance due to the polymer's lack of rigid support to the glass layer in front of it, resulting in tensile loading and failure of the glass sheet under impact. The use of a thinner polymer interlayer would alleviate the problem, but this is both not practical and is difficult to carry out due to non-flatness of the glass sheets in typical commercial materials. In an ideal design using polymer interlayers, which has not yet been achieved, one would like to minimize the use of polymer interlayer as much as possible.

[0006] In typical transparent armor systems, which are composites having layers of ballistic materials such as glass and polymeric materials, the bonding is typically done using adhesive materials. A review of the technical literature, patent publications and issued patents did not discover any references to the use of thermal bonding, also known as fusion bonding, of a glass and glass-ceramic, for an armor system. (Commonly used synonyms for the word "bonding" include joining and sealing). See "Ceramic Armor Materials By Design," James W. McCauley et al., Eds., Ceramic Transactions, Vol. 134 (2002)]. Articles on glass bonding methods include: (1) low- temperature silicate-based sol gel bonding (S.D. Conzone et al., "Low temperature bonding ofZerodur and SiC>₂for optical device manufacture '; Inorganic Materials III [conference], A Marker et al., Eds., Proc. SPIE Vol.4452 (2001), pages 107-114); (2) anodic bonding (see J. Wei et al., "Glass-to-glass anodic bonding process and electrostatic forces^'", Thin Solid Films VoIs. 462-463 (2004), pages 487-491); and (3) methods of frit-bonding (see C. Hudecek, "Sealing Glasses", Engineered Materials Handbook. Vol. 4, Ceramics and Glasses (ASM International, 1991), pages 1069-1073). The low-temperature sol-gel bonding proposed by Conzone is for joining of near-zero expansion precision optical components, for which precise dimensional stability is required. The bonding requires clean, polished surfaces for each substrate and the applications cited by Conzone were joint assemblies for mirror blanks and microlithographic stages. [0007] Anodic bonding is commonly used for wafer bonding, particularly for glass-to- metal wafers and glass-to-semiconductor (silicon) wafers , but has also been described for glass-to-glass bonding of sodium-containing glasses such as Pyrex^® [see Wei above]. In this method, an external voltage is applied and the substrates subjected to a temperature range compatible with microelectronic processing. The electrostatic force generated by the bonding voltage is the driving force for bring the two substrates into intimate contact and bonding, and is requires that the surfaces be smooth and flat. (Wei et al. used borosilicate wafers with Ra of less than 1.5 run and flatness better than 500 μm.)

[0008] Frit-sealing has been used for joining glass to metals, metals to metals, ceramics to ceramics, and various combinations thereof as described by Hudecek, including glass-to-glass sealing for precise optical components [H. A. Miska, "Aerospace and military applications, " Engineered Materials Handbook, Vol. 4, Ceramics and Glasses (ASM International, 1991), pages 1016-1020]. This technique requires temperatures high enough to effect softening and flow of the glass frit. Related techniques are used to fuse specialized glass sheets for various display and solar module applications. B. G. Song et al., "Development ofin-site laser vacuum annealing and sealing processes for an application to field emission displays," IEEE (2001), pages 219-220).

[0009] Considering all the disadvantages of the present composite armor systems, and particularly the problems crated by the use of polymeric adhesive materials to bond the various layer of on the armor composite together, further improvements in composite armor is desirable, The present invention discloses methods by which a glass-ceramic material can be thermally bonded to a glass material, thereby eliminating the use of a polymeric material between the layers of glass-ceramic and glass, and further discloses the product made using the method. The method disclosed herein is particularly use for transparent armor systems where degradation of the polymeric interlayers can cause discoloration and delamination.

Summary

[0010] The invention is directed to a transparent laminate having a glass-ceramic layer and at least one glass layer or a plurality of glass layers in which the glass-ceramic layer and an adjacent glass layer are directly thermally bonded to one another thereby eliminating one or more of the polymer interlayers and minimizing any deleterious effects of temperature and humidity, and is further directed to a method for preparing the transparent laminate. In addition, when a plurality of glass layers are used in making the laminate, the glass layers are also thermally bonded to one another without the use of a polymeric or adhesive interlayer between the glass layers or the application of a voltage during the bonding process. The transparent laminate of the invention is particularly suitable for providing ballistic protection and is suitable for use in transparent armor application. The bonding is carried out in ambient atmosphere or under vacuum at a temperature between the softening and strain points of the "softer" or "lower softening point" material (that is, the glass). The glass-ceramic/glass laminate can be combined with a spall catcher material to form a transparent armor laminate. In addition to ballistic protection in a combat situation, the glass-ceramic/glass laminates according to the invention can be used in non-combat situations to provide protection to buildings, vehicles and persons in situations such as hurricanes and tornados where high winds create flying objects that could penetrate ordinary windows or personal protective equipment.

[0011] The transparent glass-ceramic/glass laminate, when used in armor applications, has a glass-ceramic strike face layer and one or a plurality of glass layers, the glass-ceramic layer and the glass layers being thermally bonded to each other without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process. An incoming projectile first strikes the glass-ceramic layer. The transparent glass-ceramic/glass laminate according to invention can further have a spall-catcher layer bonded to the glass layer furthest from the glass-ceramic layer, the spall-catcher layer being a transparent polymeric material; for example, a materials selected from the group of polycarbonates, acrylates and methacrylates and other transparent polymeric materials known in the art.

[0012] A transparent glass-ceramic laminate, whether for use as transparent armor or for other applications, can be made by a method having at least the steps of: providing a sheet or layer of a transparent glass-ceramic material; providing a sheet or layer of a transparent glass material; placing the sheet or layer of the glass material on top of the glass-ceramic material in a manner such that the glass sheet or layer is in contact with the glass-ceramic material; heating the glass-ceramic layer and the glass layer in a furnace to a temperature between the softening point of the glass and the annealing point of the glass for a time sufficient for the glass-ceramic and the glass layer to thermally bond together; and cooling the bonded materials to ambient temperature to thereby yield a transparent armor laminate having a glass-ceramic layer and one or a plurality of glass layers in which each of the glass layers is thermally bonded to at least one adjacent layer.

In an additional embodiment of the foregoing a plurality of glass sheets can be provided and placed on top of the glass-ceramic such that one of the glass sheets is in contact with the glass-ceramic and each of the plurality of glass sheets is in contact with adjacent glass sheets. The glass sheets are bonded to one anther during the process of bonding the glass-ceramic sheet to the glass sheet adjacent to it. Further, the bonding between the glass-ceramic layer and the glass layer, or between glass layers, can be carried out either in ambient atmosphere or under vacuum.

[0013] The thermal bonding of the method is carried out without the use of a polymeric or adhesive interlay er or the application of a voltage during the bonding process. Providing the sheet of a glass-ceramic material and one or a plurality of sheets of a glass material means providing a glass-ceramic material and a glass material in which the difference between the coefficient of thermal expansion between the two materials is 2.5 ppm/°C or less. An example of such material, without limitation, is a glass-ceramic in which the crystalline phase is spinel. The

include bonding a transparent spall-catcher layer to the outermost glass layer; the bonding being carried out using a polymer interlayer or an adhesive. Examples of spall-catcher material include, without limitation, transparent polycarbonate, acrylates, methacrylates and other transparent polymer materials known in the art.

Brief Description of the Drawings

[0014] Figure 1 illustrates a convention transparent armor laminate having polymer interlayers between the glass and spall-catcher layers [0015] Figure 2 illustrates a transparent armor laminate according to the invention in which the glass-ceramic and glass layers are thermally bonded together and the spall catcher is bonded to the last glass layer either thermally or by means of a polymer interlayer.

[0016] Figure 3 illustrates the thermal bonding between a 1 mm thick glass-ceramic strike plate and a 1 cm thick borosilicate glass layer.

[0017] Figure 4 illustrates the thermal cracking that results during cooiing when there is significant thermal expansion mismatch between a glass-ceramic strike plate (CTE =

3.5 ppm) and a soda-lime glass plate (CTE = 9.0 ppm).

[0018] Figure 5 is an illustration of a laminate using a frit or frit material to bond glass-ceramic-to-glass and glass-to-glass in a glass-ceramic/glass laminate having a plurality of glass layers.

[0019] Figure 6 is an illustration of a shaped frit object that can be used to bond glass-ceramic-to-glass and glass-to-glass while forming a sealed gap between the layers being bonded.

[0020] Figure 7 is an illustration of a laminate system having two stacks of laminates bonded together.

[0021] Figure 8 is an additional illustration of a laminate system having two stacks of laminates bonded together.

Detailed Description

[0022] As used herein the terms "ambient temperature" and "ambient pressure" mean the temperature and/or pressure of the furnace's surrounding environment. The term "layer" is used to describe a material having a length L, a width W and a thickness or depth T, and T is smaller than L or W. The terms "area" and "face" mean L x W and further mean (a) the face or area which an incoming projectile strikes and/or (b) the face or area that is bonded to another layer's (or stack's) face by means described herein to form the laminate. All layers are bonded face-to-face. Lastly, in all Figures element 12 represented by the broad arrow is the incoming projectile and the first face or area it strikes is known in the art as the "strike-face."

[0023] The present invention relates to the use of thermally-bonded glass-ceramic-to- glass composite laminates; and also to a method that can be used to thermally bond a glass-ceramic to glass as well bond glass-to-glass. The transparent laminate is suitable for use in any application where projection is desired from projectiles of any type. The glass and glass-ceramic material used to make a composite armor according to the invention must have closely matched coefficients of thermal expansion. For example, a composite armor can be made by thermally bonding Code 9665 spinel glass-ceramic (Corning Incorporated) bonded to commercially-available borosilicate glasses such as Pyrex (Corning Incorporated) and similar borosilicate glasses known in the art. Thermal bonding allows for the elimination of one or more polymer interiayers, which are prone to problems such as delamination or non-uniform discoloration at high or low temperatures. Applications may include armor systems for ground vehicles and aircraft as well as for personal protective devices.

[0024] In a typical transparent armor configuration, one or more sheets of a glass material are laminated into a composite layered structure with a polymer material as backing or "spall catcher". Unless thick glass is available, relatively thin polymer sheets are laminated between multiple glass plies to build up the thickness required to defeat the designated threat. These polymer layers can vary in thickness and can be either soft or hard. In addition to ballistics performance, some of the considerations for choice of adhesive include (a) index of refraction matching with the glass or ceramic, ability to accommodate its thermal expansion mismatch with the glass/ceramic ceramics, and consistent performance of the armor system over a range of environmental conditions.

[0025] This last requirement, however, has proven to be particularly problematic, given the wide potential range of temperatures and humidities experienced by an armor system. The specified temperature range, for example, encompasses -40°F to +140⁰F (-40⁰C to +60⁰C), and there is some discussion about increasing the upper temperature requirement. The thermal expansion mismatch between the glass and the polymer interlayer is exacerbated by this nearly 200⁰F (~110⁰C) range, as is the propensity for heat and solar radiation damage and the inherently high dn/dT of the polymer; leading to non-uniform discoloration, cracking, and delamination of the interiayers and resulting in a gradual or catastrophic loss of transparency/visibility. Furthermore, when a polymer interlayer between two hard, glass-based sheets exceed certain thickness, e.g., 0.015 inch (-0.38 mm), the laminate suffers from loss of ballistic performance due to the polymer's lack of rigid support to the glass layer in front of it, resulting in tensile loading and failure of the glass sheet under impact. To use a thinner polymer interlay er would alleviate the problem, but is practically difficult due to non-flatness of the glass sheets in typical commercial materials. In an ideal design, one would like to minimize the use of polymer interlay er as much as possible. This is a significant problem for the military. Figure 1 illustrates a conventionally designed transparent armor laminate having a strike face 14 [diagonal hatched], a plurality of glass layers 15, a spall catcher 18 layer, and a plurality of polymer interlay ers between the iayers represented by broad black lines 16.

[0026] The use of a thermally-bonded glass-ceramic/glass laminate can eliminate the transparency/visibility problems that arise by eliminating the polymer layer between the glass-ceramic strike face and its glass backing layer, and any additional glass layers that may be present. Figure 2 illustrates the thermally bonded transparent armor laminate according to the invention. As illustrated in Figure 2 the laminate has a glass-ceramic strike face 24 (vertical hatching), a plurality of glass layers as represented by numeral 25 and a spall catcher material 28. The glass-ceramic and glass layers are directly thermally bonded, without use of an adhesive or polymer interlayer, as taught herein. The spall catcher, which is typically a polycarbonate or acrylic material, is bonded using an adhesive polymer interlayer. The optical properties of the transparent composite (laminate) of the invention meet visible transparency as well as near IR transparency requirements for most military armor systems. No stringent surface preparation is required. The invention combines a glass-ceramic strike face with backing by glass interlayer(s) provides superior ballistics performance over all-glass or all-glass-ceramic configurations, offers several important attributes that include:

(1) The ability to achieve ballistics performance equivalent to that of glass, with lower thickness, thereby providing critically-needed lower weight for armor systems.

(2) The ability to achieve superior ballistics performance with the same laminate thickness used for current transparent armor.

(3) The ability to provide these attributes with the added advantage of eliminating at least one polymer interlayer, with its tendency toward nonuniform discoloration, delamination, and cracking at extreme high or low temperatures This concept present by the preseat invention may also be combined with glass-to-glass bonding as well in order to eliminate all polymer interfacial layers.

[0027] Glass-ceramics are microcrystalline solids produced by the controlled devitrification of glass. Glasses are melted, fabricated to shape, and then converted by heat treatment to a predominantly crystalline ceramic. The basis of controlled crystallization lies in efficient internal nucleation, which aliows development of fine, randomly oriented grains without voids, micro-cracks, or other porosity. Like glass and ceramics, glass-ceramics are brittle materials which exhibit elastic behavior up to the strain that yields breakage. However, because of the nature of the crystalline microstructure, mechanical properties including strength, elasticity, fracture toughness, and abrasion resistance are higher in glass-ceramics than in glass. The presence of uniformly distributed crystals throughout a glass-ceramic causes deflection and blunting of cracks, thereby providing enhanced resistance to fracture propagation.

[0028] The present invention relates to the use of thermally-bonded glass-ceramic-to- glass composite laminates for use in armor systems. The glass and glass-ceramic must have closely matched coefficients of thermal expansion. Typically the glass-ceramic layer has a coefficient of thermal expansion in the range of 1-9 ppm/°C (1-9 x 10^"6/°C). An example, without limitation, of such a system is Code 9664 spinel glass-ceramic bonded to commercially-available borosilicate glasses such as Pyrex^® (softening point of ~820°C, anneal temperature ~565°C) or Borofloat^® (Schott Glass, softening point of ~820°C, anneal temperature ~560°C) . Both of these borosilicate materials have thermal expansion coefficients of —3.5 ppm over the temperature range of 25°-500°C. Thermal bonding permits the elimination of one or more polymer interlayers, which are prone to problems such as delamination or non-uniform discoloration at high or low temperatures.

[0029] Bonding is carried out in ambient atmosphere (air) at a temperature between the softening and annealing temperatures of the glass sheet. Figure 3 illustrates the results obtained after bonding a 1 mm thick glass-ceramic circular wafer (Corning code 9665) to a 1 cm thick borosilicate glass sheet. [The shapes were selected so that the two materials could be better discerned in the figure. The bonded glass-ceramic/glass laminate is placed on a mat to illustrate the clarity of the bonded material.] For the example illustrated in Figure 3, a glass -ceramic wafer or block was placed in a small electric furnace. The sheet of borosilicate glass was suspended by its corners on small refractory blocks, slightly above (~2mm) the surface of the glass ceramic. The furnace was then heated at a rate of 300°C/hour to 765°C, held at temperature for 4 hours. Subsequently the furnace's heaters were turned off and the sample cooled to ambient temperature at the furnace's natural cooling rate. During this process, the glass "softens" onto and bonds with the glass-ceramic, and the resulting composite or laminate cools to room temperature as described. The resulting bond between the glass-ceramic and the borosilicate glass is strong, and the transparency of the glass-ceramic and the glass are preserved resulting in a transparent glass-ceramic/glass laminate.

[0030] While the foregoing example illustrated the bonding of a single glass-ceramic wafer or sheet to a single borosilicate glass wafer or sheet; a complete, interlayer-free glass-ceramic/glass laminate can also be formed using a glass-ceramic as the strike face which is backed by a plurality of glass layers using the same method as described above for bonding the glass-ceramic and a single glass sheet. For example, a "glass-ceramic/n-glass" laminate can be formed where "n" is an integer equal to or greater than 1. Typically in glass-ceramic/glass laminates "n" will have a value in the range of 1-6 depending on the thickness and/or weight of the individual glass layers. The bonding of the glass-ceramic to a glass layer and the bonding of the "n" glass layers to themselves can be carried out in one step as by layering one glass layer on top of another, or the glass layers can be bonded to one another sequentially.

[0031] The bonding between the glass-ceramic layer and a glass layer, or between one glass layer and another glass layer, can be carried out either in ambient atmosphere or with the application of vacuum. For example, without limitation, a glass-ceramic layer and a glass layer can be placed in a vacuum oven, the oven evacuated to a selected pressure, the glass-ceramic and glass layers are heated to a selected temperature between the softening temperature and the annealing temperature of the lower melting material (typically the glass), held at the selected temperature under vacuum for a time in the range of 1 -4 hours to bond the glass layer to the glass-ceramic layer, and then cooled to ambient temperature. After cooling to ambient temperature air is admitted into the vacuum oven. The selected pressure at which the bonding is carried of is less than 50 mm Hg and preferable less than 1 mm Hg. The glass-ceramic/glass laminate may then simultaneously or subsequently have further glass layers bonded to the glass layer of the glass-ceramic/glass laminate either under vacuum or in the ambient atmosphere as described herein. Cooling is carried at the furnace's natural cooling rate or at a rate of 25°C/hour or less as desired. While generally the glass-ceramic/glass laminates are removed from the furnace at ambient temperature, they can also be removed at any temperature below 1 OU⁰C and allowed to cooi more rapidly to room (ambient) temperature without damage to the laminate. If the bonding was carried out under vacuum, dry air can be slowly admitted to aid in cooling while simultaneously bringing the furnace from its vacuum condition back to ambient atmospheric pressure.

[0032] The key to successful thermal bonding of a glass-ceramic and a glass is that the two have similar thermal expansion characteristics. Figure 4 illustrates a configuration in which the Code 9965 glass-ceramic (CTE -3.5 ppm) is bonded to a soda- lime glass (CTE = 9.0 ppm). [Note: The pieces of Figure 4 are placed on a mat for demonstration purposes.] While the glass-ceramic and the soda-lime glass did bond, (as evidenced by visibility of the circular outline of the original glass-wafer), the high thermal expansion difference between the two materials resulted in severe cracking during the cool-down process.

[0033] Bonding is carried out in an ambient atmosphere at a temperature between the softening and annealing temperatures of the glass sheet. Figure 32 illustrates the bonding a Corning code 9665 glass-ceramic to a borosilicate glass. In Figure 3 a 1 mm thick glass-ceramic wafer is bonded to a 1 cm thick glass wafer. The bonding was carried out by suspending the borosilicate glass by its corners on small refractory blocks slightly (~2 mm) above the surface of the glass-ceramic. The furnace is then heated at a rate of approximately 300°C/hour to 765⁰C and held at this temperature for a time in the range of 1-4 hours. Subsequently, the furnace's heaters are turned off and the samples are cooled at the furnace's natural cooling rate, or alternatively are cooled to the room (ambient) temperature at a rate of less than 25°C/hour. During this process, the glass "softens" onto and bonds with the glass-ceramic, and the resulting composite or laminate cools to room temperature as described. The resulting bond between the glass-ceramic and the borosilicate glass is strong, and the transparency of the glass-ceramic and the glass are preserved resulting in a transparent glass-ceramic/glass laminate. While the foregoing example illustrated the bonding of a single glass-ceramic wafer or sheet to a single borosilicate glass wafer or sheet, the same process can be used to form a glass-ceramic/glass laminate having a plurality of glass sheets. For example a glass-ceramic/glass/n-glass laminate where "n" is an integer equal to or greater than 1. Typically in glass-ceramic/glass laminates "n" will have a value in the range of 1-6 depending on the thickness and/or weight of the individual glass layers.

[0034] In an alternative method the glass-ceramic material and one or a plurality of sheets of the glass material are placed in contact with one another and are placed into or mechanically moved into a furnace where they are heated to a temperature between the softening point and the annealing point of the lower melting material, typically the glass, at a rate in the range of 350-400°C/hour, held at the selected temperature between the softening point and the annealing point of the lower melting material for a time in the range of 1-6 hours, and then cooled to room (ambient temperature) at a rate of 25°C or less. Once the glass-ceramic/glass laminate is formed, in a further step a spall-catcher material is bonded to the glass-ceramic/glass laminate using a transparent polymer interlayer or adhesive.

[0035] The key to successful thermal bonding of a glass-ceramic and a glass material is that the glass-ceramic and the glass have similar thermal expansion characteristics. Figure 4 illustrates an example in which the glass-ceramic and the glass have different coefficients of thermal expansion. In Figure 4 code 9665 glass (CTE = 3.5 ppm) is bonded to a to a soda-lime glass (CTE = 9.0 ppm). While the glass-ceramic and the soda-lime glass did indeed bond together (as evidenced by the visibility of the circular outline of the original glass-ceramic wafer), the high thermal expansion difference between the two materials caused severe cracking during the cool-down process. Generally, the difference in the coefficient of thermal expansion between the glass-ceramic and the glass should as small as possible and generally the difference should be less than approximately 2.5 ppm/°C regardless of the choice of glass-ceramic material and glass material. In one embodiment the difference should be in the range of 2 ppm/°C or less. For example, if the glass-ceramic has a CTE = 4 ppm/°C, the glass material should have a CTE in the range of 2-6 ppm/°C. In another embodiment the CTE difference should 1 ppm or less. [0036] The invention is also directed to a laminate system that has plurality of laminate stacks that are bonded together using a bonding agent. A single laminate stack having a glass-ceramic layer and one or a plurality or glass layer (with or without the presence of a spall-catcher layer bonded to the last glass layer) as described above is most useful for situations where it is expected that a single projectile will strike the laminate. While such system also affords protection in situations where multiple projectiles may strike the laminate, improving multi-strike protection is highly desirable. This can be accomplished using a laminate system having a plurality of laminate stacks. In all such multiple-stack systems a first stack (stack A) is provided that has a glass-ceramic layers and one or a plurality of glass layers in which the glass-ceramic layer is thermally bonded, without the use of a voltage during the bonding process, to one glass layer and, when a plurality of glass layers are used, each glass layer is thermally bonded, without the application of any voltage during the bonding process, to the glass layers adjacent to it. Stack A can be bonded to one of a plurality of additional stacks (B, C, . . ., etc.) using bonding materials including, without limitation, a polymer interlayer, an adhesive, a frit paste material or a shaped frit material. The additional stacks B, C, . . . etc., can individually have the same configuration as Stack A or they can individually have a configuration that is different from that of Stack A. For example, one or more of the B, C, . . . etc. stacks can be a glass-ceramic/glass or a glass/glass stack in which the layers of the stack are bonded together using a polymer interlayer or adhesive; or the layers can be thermally bonded, without the use of a polymer interlayer or adhesive as taught herein; or the layers can be bonded by a combination of the foregoing. Finally, for particular applications such as armor applications, a spall catcher layer can optionally be added as the last layer to the stack furthest from the face receiving the incoming projectile, and the spall catcher is bonded as described elsewhere herein. Figures 7 and 8 represent, without limitation, two stack combinations of may possible combinations according to the invention.

[0037] Figure 7 illustrates a two stack system 200, A + B, in which the stacks A and B are bonded using a polymer interlayer, an adhesive, a frit paste or a shaped frit material 230. Stack A has a glass-ceramic layer 210 thermally bonded to a single glass layer 220 as taught herein, and stack B has two glass layers 240 thermally bonded to one another. The glass of glass layers 240 may be the same as or different from the glass of layer 220. Figure 7 also illustrates the presence of the optional spall-catcher ^yυ which is bonded to stack B outer layer 240 using a polymer interlayer of adhesive.

[0038] Figure 8 illustrates a two stack system 300, A' + B', in which the stacks A' + B' are bonded using a polymer interlayer, an adhesive, a frit paste or a shaped frit material 330. Stack A' and B' each has a glass-ceramic layer 310 and two glass layers 320, with the layers 310/320/320 being thermally bonded to one another as taught herein. Figure 8 also illustrates the presence of the optional spall-catcher 390 which is bonded to stack B outer layer 240 using a polymer interlayer of adhesive. In another embodiment of Figure 8 (not illustrated), stack B' can be replaced by stack B as illustrated in Figure 7. The foregoing Figures 7 and 8 are used for illustration purposes and are not to be construed as limiting the invention. Examples, without limitation, of addition combinations would be to add additional glass/glass or glass-ceramic/glass stacks having one or a plurality of glass layers. Further, the stacks can be bonded to one another such that the bonding results in a gap between the stacks as is described below. This gap can be filled with a fluid; for example air or other gas, or an index healing fluid or gel as described below.

[0039] In a further embodiment of the invention the glass-ceramic layer can be replaced by a first glass layer comprising a plurality of glass layers thermally bonded to one another as taught herein. This thermally bonded first glass layer can then be bonded to an additional one or plurality glass layers using a polymer interlayer, an adhesive, thermal bonding without the use of any voltage during the thermal bonding process, a frit paste or a shaped frit material to yield, excluding the bonding matrials such as polymer interlayer, adhesive, etc., if any are present, an all-glass system. Replacing the glass-ceramic layer with the thermally bonded first glass layer as described results in an all-glass transparent system that is lower cost than a glass-ceramic/glass system. Depending on the use of the all-glass system the amount protection it affords may be adequate for preventing penetration by a projectile. For example, it may be sufficient for use as building windows in a situation where the principal danger is from flying debris or shrapnel resulting from natural or man-made events. In transparent armor applications an optional transparent spall-catcher material layer is bonded to the glass layer furthest from the thermally bonded first glass layer. The "additional one or plurality glass layers" can be bonded to one another using a polymer interlayer, an adhesive, thermal bonding without the use of any voltage during the thermal bonding process, a frit paste or a shaped frit material. The spall-catcher layer can be bonded to the glass layer furthest from the first glass layer using a polymer interlayer or an adhesive

[0040] In addition to the thermal bonding laminates and methods described above, a glass-ceramic/glass laminate can be prepared by thermal bonding of a glass-ceramic layer and glass layer using an amorphous glass or a devitrified (glass-ceramic) seal or frit (also called herein "frit bonding") to thereby eliminate one or more polymer interlayers and minimize the deleterious effects that temperature and humidity have on the polymer interlayers. The frit material can be applied either as a paste or as a shaped frit material such as is further described herein.

[0041] Frit bonding is a thermal process that can be used to eliminate one or more of the polymer interlayers that may be present in a glass-ceramic/glass laminate having a glass-ceramic layer and one or a plurality of glass layers. When a plurality of glass layer are used to form the laminate a first glass layer is frit bonded to the glass-ceramic layer and the remainder of the glass layers are frit bonded to one another and the first glass layer. Frit bonding has been used to join glass to metals, glass-ceramics to glass and glass to glass, and various combinations of the foregoing. For example, devitrifying (crystallizing) glass seals were used to bond the funnel and panel of the cathode ray tubes that were a key component of the first television sets. The chemistry of the bonding materials can be designed to provide desired properties; for example, high or low thermal expansion for a specific bonding application. The use of frits or frit materials and frit bonding techniques for various applications are described, for example, in U.S. Patent Nos. 3,951,669, 5,281,560 and 6,998,776, and PCT International Publication WO 2006/044383.

[0042] The frit bonding technique requires processing temperatures high enough to affect the softening and flow of the amorphous glass or glass-ceramic frit. Many early glass frits were developed to be compatible with low processing temperatures (-450 °C) h d compositions in the lead-zinc-borate family (these were commonly called "solder glasses"). More recently, lead-free frits in the tin-zinc-phosphate family were developed for similar low temperature applications (see US 5,281,560). The bulk thermal expansions of these seals can be tailored by the addition of appropriate fillers. However, there are situations where when processing temperatures of even ~450 ⁰C are impractical or even impossible. One example of the latter is in the preparation of hermetic seals for OLED (organic light emitting diode) devices where exposure to elevated temperatures will degrade the organic material. These problems can be alleviated by spot-sealing, such as laser-sealing a suitably-doped glass frit around the edges of the parts to be joined [e.g. U.S. 6,998,776 and WO 2006/044383]. In cases such as these, the frit is doped with an element, such as a transition metal element, that absorbs electromagnetic radiation at a particular wavelength (laser, IR, etc.), such that the frit softens and forms a strong bond when exposed to the appropriate wavelength. Related techniques are used to fuse glass sheets for various display and solar module applications [B. G. Song et al., Development ofin-site laser vacuum annealing and sealing processes for an application to field emission displays, Proceedings of the 14^th International Vacuum Microelectronics Conference, IEEE (2001), pp 219-220].

[0043] The frit bonding techniques can also assure reliable sealing between a glass-ceramic and a glass sheet, or between a first glass and a second glass sheet, to form a liquid-tight gap between the sheets. This liquid-tight seal allow for the introduction of special fluids having optical functions into the gap. One particular function is a "self-healing feature wherein an index-matched fluid is used to fill cracks in a glass and/or glass-ceramic layer to improve transparency in the event the glass-ceramic or a glass ceramic layer is impacted by a projectile. In addition, the fluid can be used to disperse the front shock wave that results from the impact or a projectile. The controlled gap size and the environmental robustness of seal material will allow the fluid in the gap to be an invisible part of a transparent laminate during normal use. Once the transparent laminate develops cracks after projectile impact, the fluid, under pressure (from the shock wave produced by a projectile) as well as meniscus forces, will be able to fill the cracks and "optically repair" the transparent laminate. In the case of armor applications this feature allows the user to see through the transparent laminate for a period of time needed for post-impact reaction. The viscosity of the fluid is selected to balance the speed of crack filling against the potential loss of fluid in major damage areas. In typical ballistic applications the time that the window should stay clear so that the user can escape a danger situation is typically in range of minutes to several hours; for example, 5 minutes to 4 hours. [0044] The sealing is carried out using processes known to those skilled in the art. For example, in order to bond plate A to plate B, an appropriate glass frit paste is deposited on one or both of the plates and the plates are stacked together. The assembly is then heated to a temperature, for example in an oven, above the softening temperature of the frit. Alternatively, a paste of a glass frit that is doped with a suitable element is deposited on one or both of the giass plates and the plates stacked together. The assembly, or only the fritted part of the assembly, is then irradiated with electromagnetic radiation of the appropriate wavelength to be absorbed by the doped frit, causing local heating and softening of the glass frit. The process takes place in ambient atmosphere and no special cleaning is required. The resulting bond is strong and the transparency preserved. Alternative methods of frit sealing, of course, also may be envisioned. It is key for successful bonding that the glass-ceramic and glass have similar expansion characteristics. Figure 5 is a side view of a representative frit-bonded glass-ceramic/glass laminate having a glass-ceramic strike face or layer 110 and two glass layers 140 in which the layers are bonded using either a shaped frit 120 (see Figure 6) or a frit paste 130. Figure 6 is a 2-dimensional top view of a 3-dimensional shaped frit 120 which has a length L, a width W, a thickness T (represented by the double-headed arrow) and a depth D (not shown in the illustration). The length and width are selected can be any value suitable for the application. The thickness T can be any value suitable for the application and is typically in the range of 5-15 mm, and the depth (D) is can be any value suitable for the application and is typically in the range of 2-10 mm. When the frit 120 is bonded between two laminate layers a gap or volume (not numbered or illustrated in Figure 5 or 6) is defined. This gap or volume can be with a fluid or gel including air, nitrogen or other selected gas or mixture of gases, or a transparent gel or fluid; for example, an index matching fluid and/or an index matching fluid for self-healing and/or dissipating a projectile's frontal shock wave as discussed above.

[0045] A completely polymer interlayer-free glass-ceramic/glass laminate can also be envisioned for configurations with a glass-ceramic strike face backed by a plurality of glass layers. In this case, frit sealing can be employed between two identical glass layers as well. The bonds between the glass -ceramic layer and its adjacent glass layer, and between the glass layers of the plurality of glass layers (glass-to-glass bonding) can be processed (sintered) simultaneously. For armor application a spall catcher layer can be bonded to the glass layer furthest from the glass-ceramic layers of a glass-ceramic/glass laminate have one or a plurality of glass layers.

[0046] Thus, in a particular embodiment the invention describes a thermally bonded transparent glass-ceramic/glass laminate having: a glass-ceramic layer, one or a plurality of glass layers and one or a plurality of frit layers for bonding the glass-ceramic layer to one glass layer and/or, when a plurality of glass layers are present, for bonding the glass layers of said plurality of glass layer to one another; the frit layer being thermally bonded to said glass-ceramic and glass layers without the use of a polymer interlayer or adhesive or the application of any voltage during the bonding process; wherein the frit layer is selected from the group consisting of a shaped frit and a frit paste, and when said frit layer is a shaped frit layer, after bonding the frit layer to glass-ceramic layer and the glass layer or to two glass layers, the frit layer together with the layers to which it is bonded define a volume. This volume may be filled as described above.

[0047] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

We claim:

1. A transparent glass-ceramic/glass laminate, said laminate comprising a glass-ceramic layer and at least one glass layer, said glass-ceramic layer and said glass layer being thermally bonded to one another without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process.

2. The transparent glass-ceramic/glass laminate according to claim 1, wherein the difference between the coefficient of thermal expansion of the glass-ceramic layer and the coefficient of thermal expansion of the glass layer is in the range of 2.5 ppm/°C or less.

3. The transparent glass-ceramic/glass laminate acording to claim 1, wherein the difference between the coefficient of thermal expansion of the glass-ceramic and the coefficient of thermal expansion of the glass is in the range of 1 ppm/°C or less.

4. The transparent glass-ceramic/glass laminate according to claim 1, wherein the glass-ceramic layer is a glass-ceramic of spinel crystals.

5. The transparent glass-ceramic/glass laminate according to claim 1, wherein the glass-ceramic layer has a coefficient of thermal expansion in the range of 1-9 ppm/°C.

6. The transparent glass-ceramic/glass laminate according to claim 1, wherein the laminate further comprises a spall-catcher layer bonded to the glass layer furthest from the glass-ceramic layer, said spall-catcher layer being a transparent polymeric material selected from the group consisting of polycarbonates, acrylates and methacrylates.

7. A transparent glass-ceramic/glass armor laminate, said laminate comprising a glass-ceramic strike face layer and one or a plurality of glass layers, said glass-ceramic layer and said glass layers being thermally bonded to each other without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process, and a transparent spall-catcher layer bonded to the glass layer furthest from the glass-ceramic layer,

8. The transparent glass-ceramic/glass armor laminate according to claim 7, wherein the difference between the coefficient of thermal expansion of the glass-ceramic and the coefficient of thermal expansion of the glass is in the range of 2.5 ppm/°C or less.

9. The transparent glass-ceramic/glass armor laminate acording to claim 7, wherein the difference between the coefficient of thermal expansion of the glass-ceramic and the coefficient of thermal expansion of the glass is in the range of 1 ppm/°C or less.

10. The transparent glass-ceramic/glass armor laminate according to claim 7, wherein the glass-ceramic layer is a glass-ceramic of spinel crystals.

11. The transparent glass-ceramic/glass armor laminate according to claim 7, wherein the glass-ceramic layer has a coefficient of thermal expansion in the range of l-9 ppm/°C.

12. The transparent glass-ceramic/glass armor laminate according to claim 7, wherein the spall-catcher layer is selected from the group consisting of polycarbonates, acrylates and methacrylates

13. A method for making a transparent glass-ceramic/glass, said method comprising the steps of: providing a sheet of a transparent glass-ceramic material; providing at least one sheet of a transparent glass material; placing the sheet of the glass material on top of the glass-ceramic material in a manner such that the glass sheet is in contact with the glass-ceramic material; placing the glass-ceramic sheet and the glass sheet in a furnace; heating the glass-ceramic sheet and the glass sheet in a furnace to a temperature between the softening point of the glass and the annealing point of the glass for a time sufficient for the glass-ceramic and the glass sheets to thermally bond together to form a glass-ceramic/glass laminate having a glass-ceramic layer and a glass layer; and cooling the bonded layers to ambient temperature to thereby yield a transparent laminate having a glass-ceramic layer and a glass layer thermally bonded to one another; wherein said thermal bonding is carried out without the use of a polymeric or adhesive interlayer between the layers or the application of a voltage.

14. The method according to claim 13, wherein providing at least one glass sheet means providing a plurality of glass sheets, said plurality of said glass sheets being in contact with one another and one of said glass sheets being in contact with the glass-ceramic sheet; and the glass-ceramic and glass sheets being bonded together to form a bonded glass-ceramic/glass laminate by placing the sheets in the furnace, heating the sheets as indicated to thereby thermally bond the sheets together, and cooling the bonded sheet to form a glass-ceramic/glass laminate having a glass-ceramic layer and a plurality of glass layers in which the layers are thermally bonded to adjacent layers.

15. The method according to claim 13, wherein the difference between the coefficient of thermal expansion of the glass-ceramic and the glass material is 2.5 ppm/°C or less.

16. The method according to claim 13, wherein the difference between the coefficient of thermal expansion of the glass-ceramic and the glass material is 1 ppm/°C or less.

17. The method according to claim 13, wherein said method is carried out in said furnace at atmospheric pressure.

18. The method according to claim 13, wherein said method is carried out under vacuum.

19. The method according to claim 13, wherein said method includes the further step of bonding a spall-catcher material to the outermost glass layer of the formed and cooled laminate, said bonding being carried out by the use of a polymer interlayer of an adhesive.

20. A transparent glass-ceramic/glass laminate system, said laminate system comprising at least two stacks of laminated materials, a first stack having a glass-ceramic strike-face layer and one or a plurality of glass layers, said glass-ceramic layer being thermally bonded to a glass layer without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process, and when a plurality of glass layers are present, each of said glass layers is thermally bonded to adjacent glass layers without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process; and each stack of the remainder of the at least two stacks of laminated materials is selected independently from the group consisting of laminates having:

(a) a glass-ceramic layer and one or a plurality of glass layers and

(b) a plurality of glass layers, and said glass-ceramic layer is thermally bonded to a glass layer without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process, and when a plurality of glass layers are present, each of said glass layers is thermally bonded to adjacent glass layers without the use of a polymer interlayer or adhesive between the layers or the application of any voltage during the bonding process; where in the stacks are bonded together using one selected from the group consisting of a polymer interlayer, an adhesive, a frit paste and a shaped frit material.