This disclosure relates to glass-based articles exhibiting improved damage resistance, including improved fracture resistance, and more particularly to glass and glass ceramic articles exhibiting a non-zero metal oxide concentration gradient or concentration that varies along a substantial portion of the thickness. 
Glass-based articles often experience severe impacts that can introduce large flaws into a surface of such articles.  Such flaws can extend to depths of up to about 200 micrometers (microns, or μm) from the surface.  Traditionally, thermally tempered glass has been used to prevent failures caused by the introduction of such flaws into the glass because thermally tempered glass often exhibits large compressive stress (CS) layers (e.g., approximately 21% of the total thickness of the glass), which can prevent the flaws from propagating further into the glass and thus, can prevent failure.  An example of a stress profile generated by thermal tempering is shown in FIG.  1.  In FIG.  1, the thermally treated glass article 100 includes a first surface 101, a thickness t1, and a surface CS 110.  The thermally treated glass article 100 exhibits a CS that decreases from the first surface 101 to a depth of compression (DOC) 130, as defined herein, at which depth the stress changes from compressive to tensile stress and reaches a maximum central tension (CT) 120. 
Thermal tempering is currently limited to thick glass-based articles (i.e., glass-based articles having a thickness t1 of about 3 millimeters or greater) because, to achieve the thermal strengthening and the desired residual stresses, a sufficient thermal gradient must be formed between the core of such articles and the surface.  Such thick articles are undesirable or not practical in many applications such as display (e.g., consumer electronics, including mobile phones, tablets, computers, navigation systems, and the like), architecture (e.g., windows, shower panels, countertops etc.),  transportation (e.g., automotive, trains, aircraft, sea craft, etc.),  appliance, or any application that requires superior fracture resistance but thin and light-weight articles. 
Although chemical strengthening is not limited by the thickness of the glass-based article in the same manner as thermally tempering, known chemically strengthened glass-based articles do not exhibit the stress profile of thermally tempered glass-based articles.  An example of a stress profile generated by chemical strengthening (e.g., by an ion exchange process), is shown in FIG.  2.  In FIG.  2, the chemically strengthened glass-based article 200 includes a first surface 201, a thickness t2 and a surface CS 210.  The glass-based article 200 exhibits a CS that decreases from the first surface 201 to a DOC 230, as defined herein, at which depth the stress changes from compressive to tensile stress and reaches a maximum CT 220.  As shown in FIG.  2, such profiles exhibit a substantially flat CT region or CT region with a constant or near constant tensile stress along at least a portion of the CT region.  Often, known chemically strengthened glass-based articles exhibit a lower maximum CT value, as compared to the maximum central value shown in FIG.  1. 
Accordingly, there is a need for thin glass-based articles that exhibit improved fracture resistance.