Patent Publication Number: US-2022212446-A1

Title: Glass laminates having determined stress profiles and methods of making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/837,411 filed on Apr. 23, 2019 the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present specification generally relates to glass articles and, more particularly, to laminated glass articles including a plurality of glass layers and methods for forming the same. 
     TECHNICAL BACKGROUND 
     Glass articles can be used in a wide variety of products, including, for example, auto-glazing, architectural panels, appliances, and cover glasses (e.g., for touch-screen devices such as smartphones, tablets, laptop computers, and monitors). Relatively large flaws can be introduced into the surfaces of the glass articles during use. For example, when a smartphone is dropped onto a rough surface, such as asphalt, the local indention caused by contact with sharp features of the rough surface can cause flaws as deep as about 300 μm in the surface of the cover glass. 
     Accordingly, a need exists for glass articles with improved resistance to breakage caused by deep flaws, improved mechanical reliability, and improved drop performance. 
     SUMMARY 
     According to various aspects disclosed herein, a laminated glass article comprises a core layer comprising a core glass composition having an average core coefficient of thermal expansion (CTE core ) and a clad layer directly adjacent to the core layer and comprising a clad glass composition having an average clad coefficient of thermal expansion (CTE clad ) that is less than the CTE core  such that the clad layer is in compression and the core layer is in tension. A compressive stress of the clad layer increases with increasing distance from the outer surface of the clad layer, transitions to a minimum tensile stress as a step-change at an interface region between the core layer and the clad layer, and a magnitude of tensile stress increases continuously from the step-change to a maximum tensile stress in the core layer. 
     Another aspect includes the laminated glass article of the previous aspect, wherein a surface compressive stress of the clad layer is at least 200 MPa. 
     Another aspect includes the laminated glass article of any of the previous aspects, wherein a surface compressive stress of the clad layer is at least 250 MPa. 
     Another aspect includes the laminated glass article of any of the previous aspects, wherein the laminated glass article comprises a depth of compression of up to 50 μm. 
     According to other aspects disclosed herein, a method of preparing a glass article comprises laminating at least one core layer and at least one clad layer to form a laminated glass article, the at least one core layer comprising a core glass composition having an average core coefficient of thermal expansion (CTE core ) and the at least one clad layer comprising a clad glass composition having an average clad coefficient of thermal expansion (CTE clad ) that is less than the CTE core  such that the at least one clad layer is in compression and the at least one core layer is in tension, heating the laminated glass article to a first temperature that is from 50° C. to 200° C. greater than a glass transition temperature T g  of the laminated glass article, equilibrating the laminated glass article at the first temperature for a predetermined period of time, and quenching the laminated glass article to a second temperature below a strain point of the laminated glass article. 
     Another aspect includes the method of the previous aspect, wherein the first temperature is greater than or equal to 750° C. and less than or equal to 900° C. 
     Another aspect includes the method of any of the previous aspects, wherein the second temperature is 25° C.±10° C. 
     Another aspect includes the method of any of the previous aspects, further comprising subjecting the laminated glass article to an ion exchange treatment. 
     Another aspect includes the method of any of the previous aspects, wherein after quenching, a compressive stress of the clad layer increases with increasing distance from the outer surface of the clad layer, transitions to a minimum tensile stress as a step-change at an interface region between the core layer and the clad layer, and a magnitude of tensile stress increases continuously from the step-change to a maximum tensile stress in the core layer. 
     Another aspect includes the method of any of the previous aspects, wherein after quenching, a surface compressive stress of the clad layer is at least 200 MPa. 
     According to aspects disclosed herein, a laminated glass article comprises a core layer comprising a core glass composition having an average core coefficient of thermal expansion (CTE core ) and a clad layer directly adjacent to the core layer and comprising a clad glass composition and having an average clad coefficient of thermal expansion (CTE clad ) that is less than the CTE core  such that the clad layer is in compression and the core layer is in tension. A compressive stress of the clad layer decreases with increasing distance from the outer surface of the clad layer, transitions to a minimum tensile stress as a step-change at an interface region between the core layer and the clad layer, and a tensile stress increases continuously from the step-change to a maximum tensile stress in the core layer. 
     Another aspect includes the glass article of the previous aspect, wherein a surface compressive stress of the clad layer is at least 500 MPa. 
     Another aspect includes the glass article of any of the previous two aspects, wherein the minimum tensile stress is at least 50 MPa. 
     Another aspect includes the glass article of any of the previous three aspects, wherein the compressive stress decreases with increasing distance from the outer surface of the clad layer in according to a non-linear relationship. 
     Another aspect includes the glass article of any of the previous four aspects, wherein the tensile stress increases to a maximum tensile stress according to a non-linear relationship. 
     According to other aspects disclosed herein, a method of preparing a glass article comprises laminating at least one core layer and at least one clad layer to form a laminated glass article, the at least one core layer comprising an ion-exchangeable core glass composition having an average core coefficient of thermal expansion (CTE core ) and the at least one clad layer comprising an ion-exchangeable clad glass composition having an average clad coefficient of thermal expansion (CTE clad ) that is less than the CTE core  such that the at least one clad layer is in compression and the at least one core layer is in tension; and contacting the laminated glass article with an ion exchange bath comprising a first ion source and a second ion source. After the contacting, the laminated glass article has a depth of compression (DOC) of greater than or equal to 50 μm. 
     Another aspect includes the method of the previous aspect, wherein after the contacting, the laminated glass article has a DOC of greater than or equal to 70 μm. 
     Another aspect includes the method of any of the previous two aspects, wherein after the contacting, the laminated glass article has a DOC of greater than or equal to 100 μm. 
     Another aspect includes the method of any of the previous three aspects, wherein after the contacting, the laminated glass article has a DOC of greater than or equal to 200 μm. 
     Another aspect includes the method of any of the previous four aspects, wherein the ion exchange bath is a first ion exchange bath, the method further comprising contacting the laminated glass article with a second ion exchange bath comprising at least one additional ion source. Alternatively, the aspect may include the method of any of the previous four aspects, wherein contacting the laminated glass article with an ion exchange bath comprises contacting the laminated glass article with a first ion exchange bath comprising the first ion source and subsequently contacting the laminated glass article with a second ion exchange bath comprising the second ion source. 
     Another aspect includes the method of any of the previous five aspects, wherein the first ion source comprises a source of sodium ions, and the second ion source comprises a source of potassium ions. 
     Another aspect includes the method of any of the previous seven aspects, further comprising, prior to contacting the laminated glass article with the ion exchange bath: heating the laminated glass article to a first temperature that is from 50° C. to 200° C. greater than a glass transition temperature T g  of the laminated glass article; equilibrating the laminated glass article at the first temperature for a predetermined period of time; and quenching the laminated glass article to a second temperature. 
     Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both 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 are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a glass article in accordance with one or more embodiments shown and described herein; 
         FIG. 2  is a cross-sectional view of an overflow distributor that can be used to form a glass article in accordance with one or more embodiments shown and described herein; 
         FIG. 3  is a plot of the stress profiles of an example embodiment and a comparative example in which stress is plotted on the y-axis and depth within the glass article is plotted on the x-axis; 
         FIG. 4  is a plot of the retained strength profile of an example embodiment and three comparative examples in which retained strength is plotted on the y-axis and flaw size is plotted on the x-axis; 
         FIG. 5  is a plot of the residual stress of an example embodiment and three comparative examples in which the residual stress is plotted on the y-axis and the depth is plotted on the x-axis; 
         FIG. 6  is a schematic cross-sectional view of a ring-on-ring apparatus; 
         FIG. 7  is a plot of the results of the abraded ring-on-ring test for Example 1 and Comparative Examples A, B, D, and 1 in which the load to failure is plotted on the y-axis and the abrasion pressure is plotted on the x-axis; 
         FIG. 8  is a plot of the residual stress of a laminated glass article having a CTE mismatch between layers of the glass article and the residual stress of a single layer of glass that was chemically strengthened in which the residual stress is plotted on the y-axis and the depth is plotted on the x-axis; 
         FIG. 9  is a plot of various examples of stress profiles formed from chemical strengthening and combined stress profiles formed a combination of mechanical strengthening and chemical strengthening in which the residual stress is plotted on the y-axis and the depth is plotted on the x-axis; 
         FIG. 10  is a plot of various examples of retained strength profiles formed from chemical strengthening and combined retained strength profiles formed a combination of mechanical strengthening and chemical strengthening in which the retained strength is plotted on the y-axis and the depth is plotted on the x-axis; 
         FIG. 11  is a plot of the residual stress of various example embodiments and comparative examples in which the residual stress is plotted on the y-axis and the depth is plotted on the x-axis; and 
         FIG. 12  is a plot of the retained strength of various example embodiments and comparative examples in which the retained strength is plotted on the y-axis and the depth is plotted on the x-axis. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. 
     Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification. 
     As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” component includes aspects having two or more such components, unless the context clearly indicates otherwise. Also, the word “or” when used without a preceding “either” (or other similar language indicating that “or” is unequivocally meant to be exclusive—e.g., only one of x or y, etc.) shall be interpreted to be inclusive (e.g., “x or y” means one or both x or y). 
     The term “and/or” shall also be interpreted to be inclusive (e.g., “x and/or y” means one or both x or y). In situations where “and/or” or “or” are used as a conjunction for a group of three or more items, the group should be interpreted to include one item alone, all the items together, or any combination or number of the items. Moreover, terms used in the specification and claims such as have, having, include, and including should be construed to be synonymous with the terms comprise and comprising. 
     Unless otherwise indicated, all numbers or expressions, such as those expressing dimensions, physical characteristics, and the like, used in the specification (other than the claims) are understood to be modified in all instances by the term “approximately.” At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the claims, each numerical parameter recited in the specification or claims which is modified by the term “approximately” should be construed in light of the number of recited significant digits and by applying ordinary rounding techniques. 
     All disclosed ranges are to be understood to encompass and provide support for claims that recite any and all subranges or any and all individual values subsumed by each range. For example, a stated range of 1 to 10 should be considered to include and provide support for claims that recite any and all subranges or individual values that are between and/or inclusive of the minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less (e.g., 5.5 to 10, 2.34 to 3.56, and so forth) or any values from 1 to 10 (e.g., 3, 5.8, 9.9994, and so forth). 
     All disclosed numerical values are to be understood as being variable from 0-100% in either direction and thus provide support for claims that recite such values or any and all ranges or subranges that can be formed by such values. For example, a stated numerical value of 8 should be understood to vary from 0 to 16 (100% in either direction) and provide support for claims that recite the range itself (e.g., 0 to 16), any subrange within the range (e.g., 2 to 12.5) or any individual value within that range (e.g., 15.2). 
     The drawings shall be interpreted as illustrating one or more embodiments that are drawn to scale and/or one or more embodiments that are not drawn to scale. This means the drawings can be interpreted, for example, as showing: (a) everything drawn to scale, (b) nothing drawn to scale, or (c) one or more features drawn to scale and one or more features not drawn to scale. Accordingly, the drawings can serve to provide support to recite the sizes, proportions, and/or other dimensions of any of the illustrated features either alone or relative to each other. Furthermore, all such sizes, proportions, and/or other dimensions are to be understood as being variable from 0-100% in either direction and thus provide support for claims that recite such values or any and all ranges or subranges that can be formed by such values. 
     The terms recited in the claims should be given their ordinary and customary meaning as determined by reference to relevant entries in widely used general dictionaries and/or relevant technical dictionaries, commonly understood meanings by those in the art, etc., with the understanding that the broadest meaning imparted by any one or combination of these sources should be given to the claim terms (e.g., two or more relevant dictionary entries should be combined to provide the broadest meaning of the combination of entries, etc.) subject only to the following exceptions: (a) if a term is used in a manner that is more expansive than its ordinary and customary meaning, the term should be given its ordinary and customary meaning plus the additional expansive meaning, or (b) if a term has been explicitly defined to have a different meaning by reciting the term followed by the phrase “as used in this document shall mean” or similar language (e.g., “this term means,” “this term is defined as,” “for the purposes of this disclosure this term shall mean,” etc.). References to specific examples, use of “i.e.,” use of the word “invention,” etc., are not meant to invoke exception (b) or otherwise restrict the scope of the recited claim terms. Other than situations where exception (b) applies, nothing contained in this document should be considered a disclaimer or disavowal of claim scope. 
     As used herein, the term “average coefficient of thermal expansion,” or “average CTE,” refers to the average coefficient of linear thermal expansion of a given material or layer between 0° C. and 300° C. As used herein, the term “coefficient of thermal expansion,” or “CTE,” refers to the average coefficient of thermal expansion unless otherwise indicated. 
     In various embodiments, one or more properties of the glass article are improved by use of a post-forming fictivation process. As used herein, “fictivation” refers to imposing a specified fictive temperature or thermal history on a glass through appropriate heat treatment. As used herein, the term “fictive temperature” refers to the temperature which reflects the structural contribution to the enthalpy of the glass. The fictive temperature of a glass may be determined by calorimetric methods, as described by Xiaoju Guo et al. in “Unified approach for determining the enthalpic fictive temperature of glasses with arbitrary thermal history,” (Journal of Non-Crystalline Solids  357  (2011) pp. 3230-3236), the contents of which are incorporated herein by reference in their entirety. In the glasses described herein, the fictive temperature is from 50° C. to 200° C. greater than the glass transition temperature (Tg) of the glass article. 
     Glass Laminates 
     In various embodiments, a glass article comprises at least a first layer and a second layer. For example, the first layer comprises a core layer, and the second layer comprises one or more clad layers adjacent to the core layer. The first layer and/or the second layer are glass layers comprising a glass material, a ceramic material, a glass-ceramic material, or a combination thereof. In some embodiments, the first layer and/or the second layer are transparent glass layers. 
     The core layer has an average core CTE (core CTE or CTE core ), and the clad layer has an average clad CTE (clad CTE or CTE clad ). For example, the core layer is formed from a first glass composition with the core CTE and the clad layer is formed from a second glass composition with the clad CTE. The core CTE is greater than the clad CTE such that the core layer is in tension and the clad layer is in compression, prior to any heat treatment or ion exchange. In some embodiments, a stress profile of the glass laminate comprises a compressive stress peak disposed within the clad layer (e.g., between an outer surface of the clad layer and an inner surface of the clad layer). Additionally, or alternatively, the stress profile of the glass laminate may comprise a continuously increasing region disposed within the clad layer (e.g., the compressive stress increases between the outer surface of the clad layer and the depth of compression, excluding any compressive stress spike). In some embodiments, a compressive stress of the clad layer increases with increasing depth within the glass article to the interface between the clad layer and the core layer. In still other embodiments, a stress profile of the glass laminate comprises a compressive stress peak at the surface of the glass laminate and a compressive stress that continuously decreases with increasing depth within the glass article to the interface between the clad layer and the core layer. 
       FIG. 1  is a cross-sectional view of one embodiment of a glass article  100 . The glass article  100  is a laminated sheet including a plurality of glass layers. In embodiments, the laminated sheet may be substantially planar, as shown in  FIG. 1 , or the laminated sheet may be non-planar. The glass article  100  comprises a core layer  102  disposed between a first clad layer  104  and a second clad layer  106 . In some embodiments, the first clad layer  104  and the second clad layer  106  are exterior layers relative to the core layer  102 , as shown in  FIG. 1 . For example, an outer surface  108  of the first clad layer  104  is an outer surface of the glass article  100  and/or an outer surface  100  of the second clad layer  106  is an outer surface of the glass article  100 . In other embodiments, the first clad layer  104  and/or the second clad layer  106  are intermediate layers disposed between the core layer  102  and an exterior layer (not shown). 
     The core layer  102  comprises a first major surface and a second major surface opposite the first major surface. In some embodiments, the first clad layer  104  is fused to the first major surface of the core layer  102 . Additionally or alternatively, the second clad layer  106  is fused to the second major surface of the core layer  102 . In such embodiments, an interface  112  between the first clad layer  104  and the core layer  102  and/or an interface  114  between the second clad layer  106  and the core layer  102  are free of any bonding material such as, for example, an adhesive, a coating layer, or any non-glass material added or configured to adhere the respective clad layers to the core layer. Thus, the first clad layer  104  and/or the second clad layer  106  are fused directly to the core layer  102  and/or are directly adjacent to the core layer  102 . In some embodiments, the glass article  100  includes one or more intermediate layers disposed between the core layer and the first clad layer and/or between the core layer and the second clad layer. For example, the intermediate layers may comprise intermediate glass layers and/or diffusion layers formed at the interface of the core layer and the clad layer. The diffusion layer can comprise a blended region comprising components of each layer adjacent to the diffusion layer (e.g., a blended region between two directly adjacent glass layers). In some embodiments, the glass article  100  comprises a glass-glass laminate in which the interfaces between directly adjacent glass layers are glass-glass interfaces. 
     In some embodiments, the core layer  102  comprises a first glass composition and first and/or second clad layers  104 ,  106  comprise a second glass composition that is different than the first glass composition. The first glass composition and the second glass composition are different from each other prior to chemically strengthening the glass article  100  as described herein. For example, in the embodiment shown in  FIG. 1 , the core layer  102  comprises the first glass composition and each of the first clad layer  104  and the second clad layer  106  comprises the second glass composition. In other embodiments, the first clad layer comprises the second glass composition and the second clad layer comprises a third glass composition that is different than the first glass composition and/or the second glass composition. 
     The glass article can be formed using a suitable process such as, for example, a fusion draw, down draw, slot draw, up draw, or float process. In some embodiments, the glass article  100  is formed using a fusion draw process.  FIG. 2  is a cross-sectional view of an example embodiment of overflow distributor  200  that can be used to form a glass article such as, for example, the glass article  100 . 
     The overflow distributor  200  can be configured as described in U.S. Pat. No. 4,214,886, which is incorporated by reference in its entirety. For example, the fusion draw apparatus  200  includes a lower overflow distributor  220  and an upper overflow distributor  240  positioned above the lower overflow distributor  220 . The lower overflow distributor  220  includes a trough  222 . A first glass composition  224  is melted and fed into the trough  222  in a viscous state. The first glass composition  224  forms the core layer  102  of the glass article  100  as further described below. The upper overflow distributor  240  includes a trough  242 . A second glass composition  244  is melted and fed into the trough  242  in a viscous state. The second glass composition  244  forms first and second clad layers  104 ,  106  of the glass article  100  as further described below. 
     The first glass composition  224  overflows trough  222  and flows down opposing outer forming surfaces  226  and  228  of the lower overflow distributor  220 . The outer forming surfaces  226  and  228  converge at a draw line  230 . The separate streams of the first glass composition  224  flowing down respective outer forming surfaces  226  and  228  of the lower overflow distributor  220  converge at the draw line  230  where they are fused together to form the core layer  102  of the glass article  100 . 
     The second glass composition  224  overflows the trough  242  and flows down opposing outer forming surfaces  246  and  248  of the upper overflow distributor  240 . The second glass composition  244  is deflected outward by the upper overflow distributor  240  such that the second glass composition  244  flows around the lower overflow distributor  220  and contacts the first glass composition  224  flowing over the outer forming surfaces  226  and  228  of the lower overflow distributor  220 . The separate streams of the second glass composition  244  are fused to the respective separate streams of the first glass composition  224  flowing down the respective outer forming surfaces  226  and  288  of the lower overflow distributor  220 . Upon convergence of the streams of the first glass composition  224  at the draw line  230 , the second glass composition  244  forms first and second clad layers  104 ,  106  of the glass article  100 . 
     In some embodiments, the first glass composition  224  of the core layer  102  in the viscous state is contacted with the second glass composition  244  of the first and second clad layers  104 ,  106  in the viscous state to form the laminated sheet. In some of such embodiments, the laminated sheet is part of a glass ribbon traveling away from the draw line  230  of the lower overflow distributor  220 , as shown in  FIG. 2 . The glass ribbon can be drawn away from the lower overflow distributor  220  by a suitable means including, for example, gravity and/or pulling rollers. The glass ribbon cools as it travels away from the lower overflow distributor  220 . The glass ribbon is severed to separate the laminated sheet therefrom. Thus, the laminated sheet is cut from the glass ribbon. The glass ribbon can be severed using a suitable technique such as, for example, scoring, bending, thermally shocking, and/or laser cutting. In some embodiments, the glass article  100  comprises the laminated sheet as shown in  FIG. 1 . In other embodiments, the laminated sheet can be processed further (e.g., by cutting or molding) to form the glass article. 
     Although the glass article  100  is shown in  FIG. 1  as including three layers, other embodiments are contemplated. For example, the glass article may have two, four, or more layers. Glass articles including two layers can be formed using two overflow distributors positioned such that the two layers are joined while traveling away from the respective draw lines of the overflow distributors or by using a single overflow distributor with a divided trough such that two glass compositions flow over opposing outer forming surfaces of the overflow distributor and converge at the draw line of the overflow distributor. Glass articles including four layers can be formed using additional overflow distributors and/or using overflow distributors with divided troughs. Thus, a glass article having a predetermined number of layers can be formed by modifying the overflow distributor accordingly. 
     Although the glass article  100  is shown in  FIG. 1  as comprising a laminated sheet, other forms are contemplated. For example, the glass article may be in the form of a laminated tube comprising multiple tubular layers (e.g., formed by one or more annular orifices), and a partial cross-section of the laminated tube may have a laminated structure similar to that shown in  FIG. 1 . In other embodiments, the glass article may be a shaped glass article, such as may be formed by shaping or molding a laminated sheet. 
     In some embodiments, the glass article  100  has a thickness of at least about 0.05 mm, at least about 0.1 mm, at least about 0.2 mm, or at least about 0.3 mm Additionally or alternatively, the glass article  100  has a thickness of less than about 3 mm, less than about 2 mm, less than about 1.5 mm, less than about 1 mm, less than about 0.7 mm, or less than about 0.6 mm. In some embodiments, a ratio of a thickness of the core layer  102  to a thickness of the glass article  100  is at least about 0.5, at least about 0.7, at least about 0.8, at least about 0.85, at least about 0.9, or at least about 0.95. Additionally or alternatively, the ratio of the thickness of the core layer to the thickness of the glass article  100  is less than about 0.95, less than about 0.93, less than about 0.9, less than about 0.87, or less than about 0.85. In some embodiments, a thickness of the second layer (e.g., each of the first clad layer  104  and the second clad layer  106 ) is from about 0.01 mm to about 0.6 mm. 
     In some embodiments, the first glass composition and/or the second glass composition have a liquidus viscosity suitable for forming the glass article  100  using a fusion draw process as described herein. For example, the first glass composition of the core layer  102  may have a liquidus viscosity of at least about 100 kP, at least about 200 kP, or at least about 300 kP. Additionally or alternatively, the first glass composition comprises a liquidus viscosity of less than about 3000 kP, less than about 2500 kP, less than about 1000 kP, or less than about 800 kP. The second glass composition of the first and/or second clad layers  104  and  106  may have a liquidus viscosity of at least about 50 kP, at least about 100 kP, or at least about 200 kP. Additionally or alternatively, the second glass composition comprises a liquidus viscosity of less than about 3000 kP, less than about 2500 kP, less than about 1000 kP, or less than about 800 kP. The first glass composition can aid in carrying the second glass composition over the overflow distributor to form the second layer. Accordingly, the second glass composition can have a liquidus viscosity that is lower than generally considered suitable for forming a single layer sheet using a fusion draw process. 
     In various embodiments described herein, the glass article  100  is strengthened by a combination of mechanical strengthening and thermal tempering, or fictivation, and/or ion exchange. For example, the glass article  100  may have a CTE mismatch as described herein, and may be thermally tempered, or fictivated, to further improve the stress profile against deep flaws and improve Knoop and Vickers scratch performance. As another example, the glass article  100  may have a CTE mismatch as described herein, and may be ion exchanged to improve the stress profile against deep flaws. 
     Mechanical Strengthening 
     In various embodiments, the glass article  100  is mechanically strengthened. For example, the second glass composition forming the first and/or second clad layers  104 ,  106  may have a different CTE than the first glass composition forming the core layer  102 . In particular, in some embodiments, the first and second clad layers  104 ,  106  may be formed from a glass composition having a lower CTE than the glass composition of the core layer  102 . The CTE mismatch (i.e., the difference between the CTE of the first and second clad layers  104 ,  106  and the CTE of the core layer  102 ) results in the formation of compressive stress in the clad layers and tensile stress in the core layer upon cooling of the glass article  100 . In various embodiments, each of the first and second clad layers, independently, can have a higher CTE, a lower CTE, or substantially the same CTE as the core layer. Surface compressive stresses tend to suppress existing surface flaws from developing into cracks. Higher CTE mismatch results in higher surface compression in the clad layer. Additionally, a thicker clad layer results in a deeper depth of compression (DOC). However, such higher surface compressive stress and deeper DOC also result in increasing tensile stress in the core layer. Accordingly, the various factors should be balance with one another as described herein. 
     In embodiments, the CTE of the core layer  102  and the CTE of the first and/or second clad layers  104 ,  106  differ by at least about 5×10 −7 ° C. −1 , at least about 15×10 −7 ° C. −1 , at least about 25×10 −7 ° C. −1 , or at least about 30×10 −7 ° C. −1 . Additionally or alternatively, the CTE of the core layer  102  and the CTE of the first and/or second clad layers  104 ,  106  differ by less than about 100×10 −7 ° C. −1 , less than about 75×10 −7 ° C. −1 , less than about 50×10 −7 ° C. −1 , less than about 40×10 −7 ° C. −1 , less than about 30×10 −7 ° C. −1 , less than about 20×10 −7 ° C. −1 , or less than about 10×10 −7 ° C. −1 . For example, in some embodiments, the CTE of the core layer and the CTE of the first and/or second clad layers differ by about 5×10 7 ° C. −1  to about 30×10 −7 ° C. −1 , or about 5×10 7 ° C. −1  to about 20×10 −7 ° C. −1 . In some embodiments, the second glass composition of the first and/or second clad layer has a CTE of less than about 66×10 −7 ° C. −1 , less than about 55×10 −7 ° C. −1 , less than about 50×10 −7 ° C. −1 , less than about 40×10 −7 ° C. −1 , or less than about 35×10 −7 ° C. −1 . Additionally or alternatively, the second glass composition of the first and/or second clad layer has a CTE of at least about 10×10 −7 ° C. −1 , at least about 15×10 −7 ° C. −1 , at least about 25×10 −7 ° C. −1 , or at least about 30×10 −7 ° C. −1 . The first glass composition of the core layer may have a CTE of at least about 40×10 −7 ° C. −1 , at least about 50×10 −7 ° C. −1 , at least about 55×10 −7 ° C. −1 , at least about 65×10 −7 ° C. −1 , at least about 70×10 −7 ° C. −1 , at least about 80×10 −7 ° C. −1 , or at least about 90×10 −7 ° C. −1 . Additionally or alternatively, the first glass composition of the core layer may have a CTE of less than about 120×10 −7 ° C. −1 , less than about 110×10 −7 ° C. −1 , less than about 100×10 −7 ° C. −1 , less than about 90×10 −7 ° C. −1 , less than about 75×10 −7 ° C. −1 , or less than about 70×10 −7 ° C. −1 . 
     Thermal Tempering 
     In various embodiments described herein, the glass article  100  is thermally tempered, or fictivated. In embodiments, the glass article  100  is heated to a first temperature that is from 50° C. to 200° C. greater than the glass transition temperature (T g ) of the clad layer, followed by equilibrating the glass article at the first temperature for a predetermined time period, and then quickly quenching the glass article to a second temperature below the lower strain point of the core layer or the clad layer. In some embodiments, the glass article is heated to a first temperature that is from 50° C. to 200° C. greater than the glass transition temperature (T g ) of the clad layer followed by equilibration at the first temperature and rapid quenching to a second temperature below the lower strain point of the clad layer or the core layer. In embodiments in which the first and second clad layers  104 ,  106  have different T g s, the first temperature is greater than the higher T g . In some embodiments, the first temperature is higher than the T g  of the clad layer and less than the T g  of the core layer. In other embodiments, the first temperature is higher than the T g  of the clad layer and the T g  of the core layer. In some embodiments, the first temperature is higher than the thickness weighted average T g  of the clad layer and the core layer. In some embodiments, the glass article is heated to a first temperature that is greater than or equal to 750° C. and less than or equal to 900° C., or greater than or equal to 775° C. and less than or equal to 875° C., or even greater than or equal to 790° C. and less than or equal to 860° C. In some embodiments, the second temperature is less than the strain point of the clad layer and less than the strain point of the core layer. In other embodiments, the second temperature is less than the strain point of the clad layer and higher than the strain point of the core layer. In some embodiments, the second temperature is less than the thickness weighted average strain point of the clad layer and the core layer. In some embodiments, the glass is fast cooled from the first temperature to a second temperature which is about room temperature (25° C.±10° C.). 
       FIG. 3  is a graphical illustration comparing an example mechanical stress profile  302  generated by CTE mismatch alone and an example combined thermal mechanical stress profile  304  generated by thermal tempering and CTE mismatch. As shown in  FIG. 3 , the positive stress corresponds to a compressive stress while negative stresses correspond to a tensile stress. For the mechanical stress profile  302  and the combined thermal mechanical stress profile  304 , the heat transfer coefficient was 0.03. Each of the glass articles was laminated with a core/clad ratio of 4.0 and a clad thickness of 55 μm. The stress profiles are represented by the stress as a function of depth within the glass article  100 . The depth within the glass article  100 , given as the distance from an outer surface of the glass article  100 , is plotted on the x-axis and the stress is plotted on the y-axis. 
     The stress profile of a glass article can be measured using any suitable technique including, for example, using a birefringence based measurement technique or a refracted near-field (RNF) technique. For example, stress measurements can be performed according to ASTM C1422 and ATSM C1279. The stress profile comprises the stress in the glass article  100  as a function of depth within the glass article. The depth within the glass article  100 , given as the distance from an outer surface of the glass article, is plotted on the x-axis, and the stress is plotted on the y-axis. The depth within the glass article may be referred to herein as depth of compression (DOC). Compressive stress is shown on the positive x-axis, and tensile stress is shown on the negative y-axis. However, the values of compressive and tensile stresses described herein refer to the absolute values of the stresses, or the magnitude of the stress. Thus, tensile stresses are given herein as positive values as opposed to negative values. 
     Referring to the mechanical stress profile  302 , the compressive region (e.g., the clad layer) has a thickness of about 50 μm and a first compressive stress of about 150 MPa. Mechanical stress  302  is a step function. Thus, the compressive stress increases continuously from the surface compressive stress throughout the compressive region, and the stress transitions to the maximum tensile stress as a step-change at an interface region between the clad layer and the core layer). 
     Referring to the combined thermal mechanical stress profile  304 , the compressive region extends to a depth of layer (DOL) of about 50 μm and has a surface compressive stress of at least 200 MPa. In various embodiments, the surface compressive stress is greater than 250 MPa. The compressive stress increases continuously with increasing distance from the outer surface of the clad layer from the surface compressive stress throughout the clad layer, transitions to the minimum tensile stress as a step-change at an interface region between the core layer and the clad layer, and a magnitude of tensile stress increases continuously from the step-change to the maximum tensile stress within the core layer. Thus, in contrast to the mechanical stress profile  302 , the combined thermal mechanical stress profile  304  has a region of continuously increasing tensile stress in the core layer to the mid-point of the core layer. 
     Moreover, as can be seen in the combined thermal mechanical stress profile  304 , the thermal tempering introduces a parabolic shape to the profile, particularly in the core layer, which may sometimes be referred to as the tensile region. Such a shape in the stress profile indicates that a flaw extending into the core layer will not peak until deeper into the depth of the glass article. 
     It will be recognized that  FIG. 3  illustrates only a portion of each of the stress profiles through a portion of the thickness of the glass article (e.g., through one clad layer and a portion of the core layer). For a symmetrical glass article, the stress profile through the remaining portion of the thickness of the glass article is a mirror image of the illustrated portions of the stress profiles in  FIG. 3 . 
     The retained strength of a glass article can be determined based on the stress profile of the glass article. For example, the retained strength is determined by forming a flaw extending from a surface of the glass article to a specified depth and then determining the strength of the glass article after formation of the flaw. The strength is flexural strength of the glass article determined using, for example, a ring-on-ring test method (e.g., as described in ASTM C1499-09), a ball-on-ring test method, a three-point bend test method, a four-point bend test method, or another suitable method or technique. Such a retained strength determination can be conducted using a fracture mechanics simulation based on the stress profile of the glass article. 
       FIG. 4  is a graphical illustration comparing example retained strength profiles corresponding to stress profiles generated by mechanical strengthening alone, thermal tempering alone, and a combination of mechanical strengthening and thermal tempering. The retained strength profiles are represented by the retained strength as a function of flaw size. The flaw size, given as the distance from an outer surface of the glass article to the point to which the flaw extends, is plotted on the x-axis, and the retained strength is plotted on the y-axis. 
     The retained strength profiles  402 ,  404 ,  406  and  408  were generated using a fracture mechanics simulation. The mechanical retained strength profile  402  was based on a laminated glass article including CTE mismatch (e.g., mechanically strengthened). The thermal retained strength glass profiles  404  and  406  were based on single layer glass articles subjected to thermally tempering (e.g., thermally tempered). The combined retained strength profile  408  was based on a laminated glass article with CTE mismatch subjected to thermal tempering (e.g., mechanically strengthened and thermally tempered). 
     As shown in  FIG. 4 , each of the strength profiles has a relatively high retained strength (e.g., at least about 250 MPa) near the outer surface of the glass article, which can aid in avoiding breakage of the glass article as a result of relatively shallow falls (e.g., less than about 10 μm). However, the combined retained strength profile  408  maintains a higher retained strength than the mechanical retained strength profile  402  deeper into the glass article (e.g., to a distance further from the surface of the clad layer). For example, the retained strength of the combined retained strength profile  408  is higher than that of mechanical retained strength profiles  402  for flaw sizes from about 5 μm to about 140 μm, which can aid in avoiding breakage of the glass article as a result of relatively deep flaws. Moreover, the retained strength of the combined retained strength profile  408  remains comparable with that of the thermal retained strength glass profiles  404  and  406  for flaw sizes greater than about 40 μm, with a significant improvement in strength for flaw sizes from about 25 μm to about 40 μm. Thus, improved resistance to breakage resulting from such flaw sizes translates into improved drop performance for a cover glass having a retained strength profile similar to the combined retained strength profile  408 . 
       FIG. 5  depicts a residual stress model for the profiles depicted in  FIG. 4 . The residual stress profiles  502 ,  504 ,  506  and  508  were generated using a fracture mechanics simulation. The mechanical residual stress profile  502  was based on a laminated glass article including CTE mismatch. The thermal residual stress glass profiles  504  and  506  were based on single layer glass articles subjected to thermally tempering. The combined residual stress profile  508  was based on a laminated glass article with CTE mismatch subjected to thermal tempering. 
     As shown in  FIG. 5 , the combined residual stress profile  508  has a relatively constant amount of compressive stress for a depth of compression (DOC) of up to about 50 μm, which corresponds to the interface region between the clad layer and the core layer. The compressive stress in the combined residual stress profile  508  is significantly greater than the residual compressive stress in the comparative residual stress profiles for depths of at least 25 μm to 50 μm. Additionally, the compressive stress remains relatively constant through the DOC to the interface region, which may prevent surface flaws from developing into cracks. At the interface region between the clad layer and the core layer, the stress transitions from compressive stress in a step-change to a minimum tensile stress, which increases smoothly and continuously from the step-change through the core layer. 
     In addition to providing improved strength and stress profiles, in various embodiments, the combined mechanical and fictivation of the glass article can provide improved resistance to sharp contact damage events, as evidenced by the Vickers scratch threshold and indentation test performance and Knoop scratch test performance. 
     The Vickers indentation threshold measurements described herein are performed by applying and then removing an indentation load to the glass surface at a rate of 0.2 mm/min. The maximum indentation load is held for 10 seconds. The indentation threshold is defined at the indentation load at which 50% of 10 indents exhibit any number of radial/median cracks emanating from the corners of the indent impression. The maximum load is increased until the threshold is met for a given glass composition. All indentation measurements are performed at room temperature in 50% relative humidity. 
     The Vickers scratch threshold refers to the load at which lateral cracking is first observed in a glass article in response to scratching the surface of the glass article with a Vickers indenter at increasing load. The test procedure is similar to that used to determine Knoop scratch threshold, except that the Vickers indenter is substituted for the Knoop diamond. The lateral cracking is evidenced by sustained cracks in the glass article that are greater than twice the width of the original scratch or groove formed by the Vickers indenter. 
     Knoop scratch thresholds (KST) described herein are determined using a Knoop diamond indenter. The scratch threshold is determined by first identifying the load range for lateral crack onset. Once the load range is identified, a series of 5 mm long scratches under increasing constant load with three or more scratches per load are generated at a speed of 4 mm/s in order to identify the Knoop scratch threshold. Lateral cracks are defined as sustained cracks that are larger than twice the width of the groove. 
     Table 1 provides the change (Δ) in minimum and maximum Vickers scratch threshold, Vickers indentation threshold, and Knoop scratch threshold values for an example glass article having a thickness of 0.55 mm that was mechanically and thermally tempered (Example 1) as compared to an identical glass article that was mechanically strengthened. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Δ Minimum and Maximum Values 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Δ minimum 
                 Δ maximum 
               
               
                   
                   
               
               
                   
                 Vickers 
                 1 N 
                   2 N 
               
               
                   
                 Indentation 
                   
                   
               
               
                   
                 (kg) 
                   
                   
               
               
                   
                 Knoop 
                 1 N 
                 −1 N 
               
               
                   
                 Scratch 
                   
                   
               
               
                   
                 (N) 
                   
                   
               
               
                   
                 Vickers 
                 2 N 
                   3 N 
               
               
                   
                 Scratch 
                   
                   
               
               
                   
                 (N) 
               
               
                   
                   
               
            
           
         
       
     
     As shown by the data provided in Table 1, the combination of thermal tempering and mechanical strengthening of the glass article can result in an increased minimum scratch/indentation fracture resistance, as well as an increased maximum both Vickers scratch and Vickers indentation values. 
     In one or more embodiments, the glass articles described herein further exhibit improved surface strength when subjected to abraded ring-on-ring (AROR) testing. The strength of a material is defined as the stress at which fracture occurs. The AROR test is a surface strength measurement for testing flat glass specimens, and ASTM C1499-09(2013), entitled “Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature,” serves as the basis for the AROR test methodology described herein. The contents of ASMT C1499-09 are incorporated herein by reference in their entirety. In embodiments, the glass specimen is abraded prior to ring-on-ring testing with 90 grit silicon carbide (SiC) particles that are delivered to the glass sample using the method and apparatus described in Annex A2, entitled “Abrasion Procedures,” of ASMT C158-02(2012), entitled “Standard Test Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture). The contents of ASTM C158-02 and the contents of Annex 2 in particular are incorporated herein by reference in their entirety. 
     Prior to ring-on-ring testing, a surface of the glass article is abraded as described in ASTM C158-02, Annex 2, to normalize and/or control the surface defect condition of the sample using the apparatus shown in Figure A2.1 of ASTM C158-02. The abrasive material is sandblasted onto the surface of the glass article at a predetermined pressure. After air flow is established, 5 cm 3  of abrasive material is dumped into a funnel and the sample is sandblasted for 5 seconds after introduction of the abrasive material. 
     For the ring-on-ring test, a glass article having at least one abraded surface is placed between two concentric rings of differing size, as shown in  FIG. 6 , to determine equibiaxial flexural strength (i.e., the maximum stress that a material is capable of sustaining when subjected to flexure between two concentric rings). In the abraded ring-on-ring configuration  600 , the abraded glass article  610  is supported by a support ring  620  having a diameter D2. A force F is applied by a load cell (not shown) to the surface of the glass article by a loading ring  630  having a diameter D1. 
     The ratio of the diameters of the loading ring and support ring D1/D2 may be in a range from about 0.2 to about 0.5. In some embodiments, D1/D2 is about 0.5. Loading and support rings  630 ,  620  should be aligned concentrically to within 0.5% of support ring diameter D2. The load cell used for testing should be accurate to within ±1% at any load within a selected range. In some embodiments, testing is carried out at a temperature of 23±2° C. and a relative humidity of 40±10%. 
     For fixture design, the radius r of the producing surface of the loading ring  630 , h/2≤r≤3h/2, where h is the thickness of the glass article  610 . Loading and support rings  630 ,  620  are typically made of hardened steel with hardness HRc&gt;40. ROR fixtures are commercially available. 
     The intended failure mechanism for the ROR test is to observe fracture of the glass article  610  originating from the surface  630   a  within the loading ring  630 . Failures that occur outside of this region—i.e., between the loading rings  630  and support rings  620 —are omitted from data analysis. Due to the thinness and high strength of the glass article  610 , however, large deflections that exceed ½ of the specimen thickness h are sometimes observed. It is therefore not uncommon to observe a high percentage of failures originating from underneath the loading ring  630 . Stress cannot be accurately calculated without knowledge of stress development both inside and under the ring (collected via strain gauge analysis) and the origin of failure in each specimen. AROR testing therefore focuses on peak load at failure as the measured response. 
     The strength of the glass article depends on the presence of surface flaws. However, the likelihood of a flaw of a given size being present cannot be precisely predicted, as the strength of glass is statistical in nature. A probability distribution can therefore generally be used as a statistical representation of the data obtained. 
       FIG. 7  depicts a graph of the load to failure (on the y-axis) as a function of the abrasion pressure (on the x-axis) for Example 1 and Comparative Examples A, B, C, and D. Table 2 provides a description of Example 1 and Comparative Examples A-D. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Description of Example 1 and 
               
               
                 Comparative Examples A-D 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Thickness 
                   
               
               
                   
                 Sample 
                 (mm) 
                 Condition 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 
                 0.55 
                 Laminated; 
               
               
                   
                 1 
                   
                 thermal 
               
               
                   
                   
                   
                 tempered 
               
               
                   
                 Comp. 
                 0.3 
                 Laminated; 
               
               
                   
                 Ex. A 
                   
                 NIX 
               
               
                   
                 Comp. 
                 0.3 
                 Laminated; 
               
               
                   
                 Ex. B 
                   
                 IOX 
               
               
                   
                 Comp. 
                 0.55 
                 Laminated; 
               
               
                   
                 Ex. C 
                   
                 IOX 
               
               
                   
                 Comp. 
                 0.5 
                 IOX; single 
               
               
                   
                 Ex. D 
                   
                 layer 
               
               
                   
                   
               
            
           
         
       
     
     As shown in  FIG. 7 , for pressures above about 5 psi, Example 1 exhibits a substantially constant retained strength until about 35 psi. Accordingly, the glass article that is mechanically and thermally strengthened is relatively insensitive to the depth of damage, whereas the Comparative Examples exhibit greater changes in strength over the same interval.  FIG. 7  also demonstrates that the fictivated laminate had exceptional performance, exhibiting almost twice the retained strength at 25 psi than Comparative Example D. 
     In various embodiments, the second layer (e.g., the first clad layer  104  and/or the second clad layer  106 ) comprise a relatively low CTE, ion-exchangeable glass composition, and the first layer (e.g., the core layer  102 ) comprises a relatively high CTE ion-exchangeable glass composition. Exemplary glass compositions that may be suitable for use in the second layer include those described in U.S. Patent Application Publication No. 2014/00141217 and U.S. Patent Application Publication No. 2015/0030827, each of which is incorporated herein by reference in its entirety. Exemplary glass compositions that may be suitable for use in the first layer include those described in U.S. Patent Application Publication No. 2014/00141217 and U.S. Patent Application Publication No. 2015/0037552, each of which is incorporated herein by reference in its entirety. 
     Chemical Strengthening 
     In various embodiments described herein, the glass article  100  may be strengthened by a combination of mechanical strengthening and chemical strengthening as an alternative to, or in addition to, thermal tempering. For example, the glass article  100  may have a CTE mismatch as described herein, and may be chemically strengthened to further increase the compressive stress through at least the clad layer as compared to lamination or CTE mismatch alone. 
     In various embodiments described herein, after being mechanically strengthened as described in detail above, the glass article  100  is chemically strengthened. For example, the glass article  100  may be subjected to a dual ion exchange treatment to increase the compressive stress in a region of the glass article at an outer surface of the glass article (e.g., an outer portion of the clad layer). In some embodiments, the ion exchange treatment comprises applying an ion exchange medium to one or more surfaces of the glass article  100  or otherwise contacting the laminated glass article  100  with an ion exchange medium (e.g., an ion exchange bath). The ion exchange medium may be a solution, a paste, a gel, or another suitable medium comprising one or more ion sources or sources of larger ions to be exchanged with smaller ions in the glass network (e.g., the glass network of the second layer). The terms “larger ions” and “smaller ions” are relative terms, meaning that the larger ions are relatively large compared to the smaller ions and the smaller ions are relatively small compared to the larger ions. Thus, the larger ions have a larger ionic radius than the smaller ions, and the smaller ions have a smaller ionic radius than the larger ions. In some embodiments, the second layer of the glass article  100  comprises an alkali aluminosilicate glass. Thus, the smaller ions in the surface layer of the glass and the larger ions in the ion exchange medium may be monovalent alkali metal cations (e.g., Li + , Na + , and/or K + ). Alternatively, monovalent cations in the glass article  100  may be replaced with monovalent cations other than alkali metal cations (e.g., Ag +  or the like). In some embodiments, the second layer of the glass article  100  comprises an alkaline earth aluminosilicate glass. In some embodiments, the ion exchange medium comprises a molten salt solution, and the ion exchange treatment comprises immersing the laminated glass article in a molten salt bath comprising larger ions to be exchanged with smaller ions (e.g., Na + , Li + , Ca 2+ , and/or Mg 2+ ) in the glass matrix. In some embodiments, the molten salt bath comprises a salt (e.g., a nitrate, a sulfate, and/or a chloride) of the larger ions. For example, the molten salt bath may include molten KNO 3 , molten NaNO 3 , or a combination thereof. Additionally or alternatively, the temperature of the molten salt bath may be from about 380° C. to about 450° C., and an immersion time is from about 2 hours to about 16 hours. 
     In various embodiments described herein, the ion exchange medium includes at least two ion sources (e.g., at least two cations). In some embodiments, a single ion exchange medium may include both ion sources, while in other embodiments, a first ion exchange medium may include a first ion source while a second ion exchange medium may include a second ion source. For example, in some embodiments, the ion exchange medium may include Na +  and K +  ions (or salts thereof), which may be used to adjust the stress at different points in the stress profile. In other embodiments, the glass article may be contacted with a first ion exchange medium that includes Na+ ions and then may be contacted with a second ion exchange medium that includes K+ ions. By replacing smaller ions in the glass matrix with larger ions at the surface of the glass article  100 , the compressive stress of the second layer is increased at the outer surface of the glass article. For example, during the ion exchange treatment, the larger ions from the ion exchange medium diffuse into an outer portion of the second layer of the glass article  100  and the smaller ions from the glass matrix diffuse out of the outer portion of the second layer of the glass article. Thus, the outer portion of the second layer comprises an exchanged region of the glass article. The increased concentration of the larger ions in the ion exchanged region causes crowding of the glass network and increases the compressive stress of the glass article  100  in the ion exchanged region. In some embodiments, subjecting the glass article  100  to the ion exchange treatment increases a surface compressive stress at the outer surface of the glass article (e.g., from an initial surface compressive stress generated by the CTE mismatch) to a final compressive stress value. For example, the final compressive stress value is at least about 200 MPa, at least about 300 MPa, at least about 400 MPa, at least about 500 MPa, at least about 600 MPa, at least about 700 MPa, at least about 800 MPa, at least about 900 MPa, or at least about 1,000 MPa. Additionally or alternatively, the final compressive stress value is less than about 1300 MPa, less than about 1200 MPa, less than about 1000 MPa, less than about 900 MPa, or less than about 800 MPa. 
     Moreover, in various embodiments, the glass article may have a depth of compression (DOC) of greater than or equal to 50 μm. For example, the glass article may have a DOC of greater than or equal to 70 μm, greater than or equal to 100 μm, or even greater than or equal to 200 μm. The DOC may be achieved by ion exchange in addition to increasing a thickness of the clad layer. 
       FIG. 8  is a graphical illustration comparing an example mechanical stress profile  802  generated by CTE mismatch alone and an example chemical stress profile  804  generated by chemical strengthening alone. The stress profiles are represented by the stress as a function of depth within the glass article  100 . The depth within the glass article  100 , given as the distance from an outer surface of the glass article  100 , is plotted on the x-axis and the stress is plotted on the y-axis. 
     Referring to the mechanical stress profile  802 , the compressive region (e.g., the clad layer) has a thickness (e.g., a DOC) of about 50 μm and a first compressive stress of about 150 MPa. Mechanical stress  302  is a step function. Thus, the compressive stress remains substantially constant from the surface through the clad layer, and transitions to the maximum tensile stress as a step-change at an interface region between the clad layer and the core layer. 
     Referring to the chemical stress profile  804 , the compressive region extends to a depth of compression (DOC) of about 80 μm and has a surface compressive stress of about 900 MPa. The stress transitions continuously from the surface compressive stress at the outer surface of the compressive region to the maximum tensile stress within the tensile region. Thus, in contrast to the mechanical stress profile  802 , the chemical stress profile  804  does not have a region of constant compressive stress or a step-change between the compressive stress region and the tensile region. 
     In various embodiments, the glass article  100  is strengthened by a combination of mechanical strengthening and chemical strengthening. For example, the glass article  100  having a CTE mismatch as described herein (e.g., a glass laminate) is chemically strengthened to further increase the compressive at the outer surface of the compressive layer.  FIG. 9  is a graphical illustration of various examples of stress profiles formed from chemical strengthening and combined stress profiles formed a combination of mechanical strengthening and chemical strengthening. 
     It will be recognized that  FIG. 9  illustrates only a portion of each of the stress profiles through a portion of the thickness of the glass article (e.g., through one clad layer and a portion of the core layer). For a symmetrical glass article (e.g., a core layer positioned between two clad layers having identical thickness and glass composition), the stress profiles through the remaining portion of the thickness of the glass article is a mirror image of the illustrated portions of the stress profiles in  FIG. 9 . In the examples shown in  FIG. 9 , the stress profile  902  and the stress profile  904  correspond to profiles for glass that was chemically strengthened using a dual ion exchange treatment as described herein. The stress profile  906  and the stress profile  908  are stress profiles for glass articles that were mechanically strengthened and chemically strengthened using a dual ion exchange treatment. In particular, the glass article that has the stress profile  908  has a lower CTE difference between laminate layers, a lower peak CS, and a shallower depth of layer for the ion exchange and step-change. Stress profile  902  corresponds to the dual ion exchanged used for the glass laminate of stress profile  906  and stress profile  904  corresponds to the dual ion exchange used for the glass laminate of stress profile  908 . 
     The compressive stress in each of stress profiles  906  and  908  decreases continuously from a first compressive stress of about 1000 MPa and 800 MPa, respectively, with increasing distance from the outer surface of the clad layer and then transitions to the minimum tensile stress as a step-change at an interface region between the core layer and the clad layer. A magnitude of tensile stress increases continuously from the step-change to the maximum tensile stress in the core layer. In contrast, the compressive stress in each of the stress profiles  902  and  904  decreases continuously with increasing distance from the outer surface of the clad layer and smoothly transitions to a maximum tensile stress in the core layer. 
     In some embodiments, the glass article  100  is chemically strengthened to increase the compressive stress throughout the clad layer. Thus, the chemical strengthening is performed in such a manner that nearly the entire thickness of the clad layer is chemically strengthened and the stress is increased throughout the compressive layer as compared to a non-laminated glass exposed to the same ion exchange treatment. 
     The retained strength of a glass article can be determined based on the stress profile of the glass article. For example, the retained strength is determined by forming a flaw extending from a surface of the glass article to a specified depth and then determining the strength of the glass article after formation of the flaw. 
       FIG. 10  is a graphical illustration comparing example retained strength profiles corresponding to stress profiles generated by chemical strengthening alone and a combination of mechanical strengthening and chemical strengthening. The retained strength profiles are represented by the retained strength as a function of flaw size. The flaw size, given as the distance from an outer surface of the glass article to the point to which the flaw extends, is plotted on the x-axis, and the retained strength is plotted on the y-axis. 
     Chemical retained strength profiles  1002  and  1004  were generated using a fracture mechanics simulation based on the chemical stress profiles  902  and  904  shown in  FIG. 9 , respectively. Combined retained strength profiles  1006  and  1008  were generated using a fracture mechanics simulation based on the combined stress profiles  906  and  908  shown in  FIG. 9 , respectively. 
     As shown in  FIG. 10 , each of the strength profiles has a relatively high retained strength (e.g., at least about 200 MPa) near the outer surface of the glass article, which can aid in avoiding breakage of the glass article as a result of relatively shallow falls (e.g., less than about 10 μm). However, the combined retained strength profiles  1006  and  1008  maintain a higher retained strength than the chemical retained strength profiles  1002  and  1004  deeper into the glass article. For example, the retained strength of the combined retained strength profiles  1006  and  1008  is higher than that of chemical retained strength profiles  1002  and  1003  for flaw sizes from about 5 μm to about 90 μm, which can aid in avoiding breakage of the glass article as a result of relatively deep flaws. Thus, improved resistance to breakage resulting from such flaw sizes translates into improved drop performance for a cover glass having a retained strength profile similar to the combined retained strength profile  1006  and/or the combined retained strength profile  1008 . Moreover, the improved resistance to breakage resulting from large flaws can be achieved by the combined retained strength profile  1006  and/or the combined retained strength profile  1008  without substantially increasing the maximum tensile stress of the tensile region as compared to the retained strength profile  1002  and/or the retained strength profile  1004 . For example, increasing the compressive stress relatively deep into the compressive region (e.g., such as by increasing the DOC) can help to maintain the area under the compressive portion of the stress profile curve, which is proportional to the maximum tensile stress in the tensile region, relatively low while also providing protection against breakage caused by relatively deep flaws. Thus, the maximum tensile stress can be maintained below the frangibility limit. Additionally or alternatively, the thickness of the compressive region is sufficiently large to maintain relatively high compressive stress deep into the glass article (e.g., to achieve improved resistance to breakage resulting from large flaws) without increasing the maximum tensile stress to an unacceptable level (e.g., above the frangibility limit). The frangibility limit can be determined, for example, as described in U.S. Patent Application Publication No. 2010/0035038, which is incorporated herein by reference in its entirety. 
     The high surface compression can help to prevent flaws from propagating within the depth of the glass article. Additionally, the continued high compression level, although decreasing with increasing depth, meets the flaw through a greater depth of the glass article described herein. Such an increase in compressive stress can help to arrest the propagation of the flaw. Thus, the increased compression can provide improved resistance to flaw propagation compared to a glass article strengthened by mechanical strengthening alone. Moreover, because the clad layer is under compressive stress from the CTE mismatch prior to any chemical strengthening, the glass article can have a higher surface compressive stress compared to a glass article strengthened by ion exchange alone. Thus, by combining mechanical strengthening and dual ion exchange, the benefits of the compressive peak can be achieved without sacrificing the relatively high surface compressive stress that can help to prevent formation of surface flaws. 
     If a flaw does propagate beyond surface, the compressive stress extending relatively deep into the glass article can help to prevent failure of the glass article as a result of the flaw (e.g., by preventing the flaw from reaching the core layer that is in tension). Thus, the presence of the increased compression through a greater depth of the glass article (e.g., provided by mechanical strengthening) can provide improved resistance to failure compared to a glass article strengthened by chemical strengthening alone and having a rapidly decreasing compressive stress deeper into the glass article. Accordingly, the stress profile generated by the combination of mechanical strengthening and dual ion exchange strengthening as described herein can enable improved performance of the glass article compared to conventional strengthening techniques. 
     In some embodiments, the position of the compressive peak can be tailored to form a glass article with a determined stress profile adapted for a particular application. For example, the time and/or the temperature for the ion exchange region can be increased such that the ion exchanged region of the second layer extends deeper into the glass article. Thus, the position of the compressive peak can be shifted deeper into the glass article. Alternatively, the time and/or temperature of the ion exchange treatment can be decreased such that the ion exchanged region of the second layer extends shallower into the glass article. Thus, the position of the compressive peak can be shifted shallower into the glass article. Positioning the compressive peak deeper within the glass article can help to improve the reliability of the glass article, represented by retained strength, compared to positioning the compressive peak shallower within the glass article, for the same central tension. However, positioning the compressive peak shallower within the glass article can help to improve the strength of the glass article compared to positioning the compressive peak deeper within the glass article, for the same central tension. Thus, the compressive peak can be positioned to balance strength and reliability. 
     Moreover, in various embodiments, the thickness of the clad layer may be adjusted to provide for a varying depth of compression (DOC) as well as improved retained strength for flaws of various ranges.  FIG. 11  depicts various stress profiles that have been adjusted to provide various depths of compression. In  FIG. 11 , stress profile  1102  corresponds to a single glass sheet that was ion exchanged. Accordingly, stress profile  1102  has a parabolic shape and a compressive stress spike at the surface. In other words, the compressive stress peaks at the surface of the glass article, and decreases continuously, smoothly transitioning from compression to tension and to a maximum tension at the middle of the glass article. Stress profiles  1104  and  1106  correspond to laminated glass articles with 200 μm and 250 μm thick clad layers, respectively. Each glass article was ion exchanged. Accordingly, each stress profile has a compressive stress spike at the surface of the glass article, and the compressive stress decreases continuously with increasing distance from the surface, then transitions to a minimum tensile stress as a step-change at an interface region between the core layer and the clad layer, and the magnitude of tensile stress increases continuously from the step-change to a maximum tensile stress. By comparing stress profile  1104  to stress profile  1106 , it can be seen that the increased thickness of the clad layer increases the depth of compression from about 200 μm to about 250 μm. 
     Stress profiles  1108  and  1110  correspond to single layer glass articles that were ion-exchanged. As shown in  FIG. 11 , stress profile  1108  is parabolic in shape, having a compressive stress that decreases with increasing distance from an outer surface of the clad layer and smoothly transitions to a tensile stress. In contrast, stress profile  1110  includes a compressive stress spike at the outer surface of the clad layer, and the compressive stress decreases continuously with increasing distance from the outer surface of the clad layer, transitioning smoothly to a tensile stress. Stress profile  1112  corresponds to a glass laminate having a clad layer with a thickness of 300 μm, which drives the depth of compression deeper into the thickness of the glass. Additionally, in contrast to stress profiles  1104  and  1106 , stress profile  1112  does not have a compressive stress spike at the surface of the clad layer. Accordingly, the compressive stress decreases with increasing distance from the outer surface of the clad layer, transitioned to a minimum tensile stress as a step-change at an interface region between the core layer and the clad layer, and a magnitude of tensile stress increases continuously from the step-change to a maximum tensile stress in the core layer. 
       FIG. 12  depicts the retained strength profiles that correspond to the stress profiles depicted in  FIG. 11 . As can be seen, the lamination of the glass articles coupled with ion-exchange treatment increases the strength for most flaw sizes, and increasing the depth of compression further increases the strength deeper into the glass article. In particular, strength profile  1212  exhibits a retained strength of greater than 300 MPa to a flaw size of nearly 275 μm, whereas strength profiles  1204  and  1206  exhibit similar strengths to flaw sizes of about 200 μm and 250 μm respectively. For single layer glass sheets, the strength profiles  1202 ,  1208 , and  1210  have a continuously decreasing strength that drops below about 200 MPa at or before a depth of 200 μm. Accordingly, the retained strength can be adjusted by adjusting the DOC to address flaws of various sizes. 
     The glass articles described herein can be used for a variety of applications including, for example, for cover glass or glass backplane applications in consumer or commercial electronic devices including, for example, LCD and LED displays, computer monitors, and automated teller machines (ATMs); for touch screen or touch sensor applications, for portable electronic devices including, for example, mobile telephones, personal media players, and tablet computers; for integrated circuit applications including, for example, semiconductor wafers; for photovoltaic applications; for architectural glass applications; for automotive or vehicular glass applications; or for commercial or household appliance applications. In various embodiments, a consumer electronic device (e.g., smartphones, tablet computers, personal computers, ultrabooks, televisions, and cameras), an architectural glass, and/or an automotive glass comprises a glass article as described herein. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.