Thermoformed cover glass for an electronic device

A method for forming a cover sheet for an electronic device comprising:

• positioning a layer of a first glass material in a mold, the first glass material having a first Young's modulus;
• positioning a layer of a second glass material adjacent to the layer of the first glass material;
• the second glass material having a second Young's modulus less than the first Young's modulus;
• heating the layer of the first glass material and the layer of the second glass material;
• applying, with the mold, pressure to the layer of the first glass material and the layer of the second glass material to chemically bond the layer of the first glass material to the layer of the second glass material and form a unitary cover sheet having a contoured shape, the applying the pressure occurring while:
• the layer of the first glass material is at a first temperature greater than a glass transition temperature and less than a melting point of the first glass material;
• and
• the layer of the second glass material is at a second temperature greater than a glass transition temperature and less than a melting point of the second glass material;
• cooling the unitary cover sheet to a third temperature below the glass transition temperature of each of the first glass material and the second glass material, the operation of cooling the unitary cover sheet comprising:
• cooling a first region of an interior surface of the unitary cover sheet by cooling a first zone of the mold at a first rate, thereby producing a first density within the first region of the interior surface, the interior surface defined by the layer of the second glass material;
• and
• cooling a second region of the interior surface of the unitary cover sheet by cooling a second zone of the mold at a second rate, less than the first rate, thereby producing a second density, greater than the first density, within the second region of the interior surface;
• and
• chemically strengthening the unitary cover sheet to form:
• a first compressive stress layer along an exterior surface of the unitary cover sheet defined by the layer of the first glass material;
• and
• a second compressive stress layer along the interior surface of the unitary cover sheet.

A method for forming a cover sheet for an electronic device, comprising:

• positioning a first glass sheet having a first Young's modulus with respect to a second glass sheet having a second Young's modulus less than the first Young's modulus;
• applying heat to the first glass sheet and to the second glass sheet;
• using a mold to apply pressure to a surface of the first glass sheet and to a surface of the second glass sheet to:
• form a unitary sheet by bonding the first glass sheet to the second glass sheet, the bonding including chemically bonding the first glass sheet to the second glass sheet;
• and
• conform the unitary sheet to a contoured shape to form a contoured cover sheet, the first glass sheet defining an outer surface and the second glass sheet defining an inner surface of the contoured cover sheet;
• cooling the contoured cover sheet, the operation of cooling the contoured cover sheet comprising:
• cooling a first region of the outer surface of the contoured cover sheet by cooling a first zone of the mold at a first rate, thereby producing a first density within the first region of the outer surface;
• and
• cooling a second region of the outer surface of the contoured cover sheet by cooling a second zone of the mold at a second rate, less than the first rate, thereby producing a second density, greater than the first density, at the second region of the outer surface;
• and
• chemically strengthening the contoured cover sheet to form a first compressive stress layer along the outer surface and a second compressive stress layer along the inner surface.