Foldable substrates, foldable apparatus, and methods of making

A method of making a foldable substrate comprising:

• chemically strengthening a substrate in a first molten salt bath maintained at from 380° C. to 480° C. for a first period of time from 20 minutes to 8 hours to form an intermediate substrate, the substrate being chemically strengthened comprises:
• an initial first major surface and an initial second major surface opposite the first major surface present in a first portion and a second portion;
• and
• a central portion positioned between an initial first portion and an initial second portion, the central portion comprising an existing central thickness defined between an existing first central surface area and an existing second central surface area opposite the existing first central surface area, the existing first central surface area is recessed from the existing first major surface by an existing first distance;
• disposing a diffusion layer over the first central surface area and the second central surface area of the intermediate substrate;
• immersing the intermediate substrate having the diffusion layer disposed thereon in a second molten salt bath maintained at from 380° C. to 480° C. for a second period of time from 1 minute to 3 hours;
• and then
• removing the diffusion layer to form the foldable substrate,
• wherein the foldable substrate is a glass-based substrate comprising:
• the first portion comprising a substrate thickness, a first compressive stress region extending to a first depth of compression from a first major surface, a second compressive stress region extending to a second depth of compression from a second major surface;
• the second portion comprising the substrate thickness, a third compressive stress region extending to a third depth of compression from the first major surface, a fourth compressive stress region extending to a fourth depth of compression from the second major surface;
• and
• the central portion positioned between the first portion and the second portion, the central portion comprising a central thickness defined between a first central surface area and a second central surface area opposite the first central surface area, a first central compressive stress region extending to a first central depth of compression from the first central surface area, a second central compressive stress region extending to a second central depth of compression from a second central surface area, the first central surface area is recessed from the first major surface by a first distance, and the central thickness is less than the substrate thickness.

A method of making a foldable substrate comprising:

• disposing a diffusion layer over the first central surface area and the second central surface area of an existing substrate;
• chemically strengthening the existing substrate having the diffusion layer disposed thereof in a first molten salt bath maintained at from 380° C. to 480° C. for a first period of time from 1 minute to 30 minutes to form an intermediate substrate, the substrate being chemically strengthened comprises:
• an initial first major surface and an initial second major surface opposite the first major surface present in a first portion and a second portion;
• and
• a central portion positioned between an initial first portion and an initial second portion, the central portion comprising an existing central thickness defined between an existing first central surface area and an existing second central surface area opposite the existing first central surface area, the existing first central surface area is recessed from the existing first major surface by an existing first distance;
• removing the diffusion layer from the intermediate substrate;
• immersing the intermediate substrate, after the diffusion layer is removed, in a second molten salt bath maintained at from 380° C. to 480° C. for a second period of time from 20 minutes to 4 hours to form the foldable substrate,
• wherein the foldable substrate is a glass-based substrate comprising:
• the first portion comprising a substrate thickness, a first compressive stress region extending to a first depth of compression from a first major surface, a second compressive stress region extending to a second depth of compression from a second major surface;
• the second portion comprising the substrate thickness, a third compressive stress region extending to a third depth of compression from the first major surface, a fourth compressive stress region extending to a fourth depth of compression from the second major surface;
• and
• the central portion positioned between the first portion and the second portion, the central portion comprising a central thickness defined between a first central surface area and a second central surface area opposite the first central surface area, a first central compressive stress region extending to a first central depth of compression from the first central surface area, a second central compressive stress region extending to a second central depth of compression from a second central surface area, the first central surface area is recessed from the first major surface by a first distance, and the central thickness is less than the substrate thickness.