The present invention is directed to a kind of machinable glass ceramic which can be chemically toughened. The machinable and chemically toughenable glass ceramic, which comprises, as represented by weight percentage based on the following compositions, 25-75 wt % of SiO.sub.2, 6-30 wt % of Al.sub.2O.sub.3, 0.1-30 wt % of Na.sub.2O, 0-15 wt % of K.sub.2O, 0-30 wt % of B.sub.2O.sub.3, 4-35 wt % of MgO, 0-4 wt % of CaO, 1-20 wt % of F, 0-10 wt % of ZrO.sub.2, 0.1-10 wt % of P.sub.2O.sub.5, 0-1 wt % of CeO.sub.2 and 0-1 wt % of SnO.sub.2, wherein P.sub.2O.sub.5+Na.sub.2O>3 wt %, and Al.sub.2O.sub.3+Na.sub.2O+P.sub.2O.sub.5>17 wt %. Mica crystalline phase can be formed in the glass ceramic and the glass ceramic can be chemically toughened by one step, two steps or multiple steps with depth of K-ion layer of at least 15 m and surface compress stress of at least 300 MPa. The profile on depth of the ion exchange layer follows the complementary error function. Hardness can be improved by at least 20% after chemical toughening. The dimension deviation ratio is less than 0.06% by ion-exchanging.

The present invention is directed to a kind of machinable glass ceramic which can be chemically toughened. The machinable and chemically toughenable glass ceramic, which comprises, as represented by weight percentage based on the following compositions, 25-75 wt % of SiO.sub.2, 6-30 wt % of Al.sub.2O.sub.3, 0.1-30 wt % of Na.sub.2O, 0-15 wt % of K.sub.2O, 0-30 wt % of B.sub.2O.sub.3, 4-35 wt % of MgO, 0-4 wt % of CaO, 1-20 wt % of F, 0-10 wt % of ZrO.sub.2, 0.1-10 wt % of P.sub.2O.sub.5, 0-1 wt % of CeO.sub.2 and 0-1 wt % of SnO.sub.2, wherein P.sub.2O.sub.5+Na.sub.2O>3 wt %, and Al.sub.2O.sub.3+Na.sub.2O+P.sub.2O.sub.5>17 wt %. Mica crystalline phase can be formed in the glass ceramic and the glass ceramic can be chemically toughened by one step, two steps or multiple steps with depth of K-ion layer of at least 15 m and surface compress stress of at least 300 MPa. The profile on depth of the ion exchange layer follows the complementary error function. Hardness can be improved by at least 20% after chemical toughening. The dimension deviation ratio is less than 0.06% by ion-exchanging.

According to one embodiment, a method for thermally treating glass articles may include holding a glass article at a treatment temperature equal to an annealing temperature of the glass article 15 C. for a holding time greater than or equal to 5 minutes. Thereafter, the glass article may be cooled from the treatment temperature through a strain point of the glass article at a first cooling rate CR1 less than 0 C./min and greater than 20 C./min such that a density of the glass article is greater than or equal to 0.003 g/cc after cooling. The glass article is subsequently cooled from below the strain point at a second cooling rate CR.sub.2, wherein |CR.sub.2|>|CR.sub.1|.

According to one embodiment, a method for thermally treating glass articles may include holding a glass article at a treatment temperature equal to an annealing temperature of the glass article 15 C. for a holding time greater than or equal to 5 minutes. Thereafter, the glass article may be cooled from the treatment temperature through a strain point of the glass article at a first cooling rate CR1 less than 0 C./min and greater than 20 C./min such that a density of the glass article is greater than or equal to 0.003 g/cc after cooling. The glass article is subsequently cooled from below the strain point at a second cooling rate CR.sub.2, wherein |CR.sub.2|>|CR.sub.1|.

The present invention provides an on-line method for stabilizing surface compressive stress of chemically-tempered glass, which comprises the steps of: placing glass to be tempered together with a stabilizer in a tempering furnace containing a molten salt bath for glass tempering; and after reacting at a temperature for a period of time, removing the glass and the stabilizer from the tempering furnace. The stabilizer is capable of chemically reacting with impurity ions in the molten salt bath for glass tempering, to remove the impurity ions in the molten salt bath. Therefore, the presence of the stabilizer allows the impurity ion content in the molten salt bath for glass tempering to be stable without gradual accumulation.

The present invention provides an on-line method for stabilizing surface compressive stress of chemically-tempered glass, which comprises the steps of: placing glass to be tempered together with a stabilizer in a tempering furnace containing a molten salt bath for glass tempering; and after reacting at a temperature for a period of time, removing the glass and the stabilizer from the tempering furnace. The stabilizer is capable of chemically reacting with impurity ions in the molten salt bath for glass tempering, to remove the impurity ions in the molten salt bath. Therefore, the presence of the stabilizer allows the impurity ion content in the molten salt bath for glass tempering to be stable without gradual accumulation.

A window includes a base region and a compressive stress region disposed on the base region. The compressive stress region includes Li.sup.+, Na.sup.+, and K.sup.+ ions. The compressive stress region includes a first compressive stress portion in which a concentration of the K.sup.+ ions decreases, a concentration of Na.sup.+ ions increases, and a concentration of the Li.sup.+ ions increases, from a surface of the window toward the base region. A second compressive stress portion is adjacent to the first compressive stress portion. In the second compressive stress portion, the concentration of the Na.sup.+ ion decreases and the concentration of the Li.sup.+ ion increases, from the first compressive stress portion toward the base region. The window thereby has a high surface compressive stress value and impact resistance.

A glass article includes lithium aluminosilicate, includes a first surface, a second surface opposed to the first surface, a first compressive region extending from the first surface to a first compression depth, a second compressive region extending from the second surface to a second compression depth, and, a tensile region disposed between the first compression depth and the second compression depth, where a stress profile of the first compressive region has a first local minimum point at which the stress profile is convex downward and a first local maximum point at which the stress profile is convex upward, where a depth of the first local maximum point is greater than a depth of the first local minimum point, and where a stress of the first local maximum point is greater than a compressive stress of the first local minimum point.

A glass article includes lithium aluminosilicate, includes a first surface, a second surface opposed to the first surface, a first compressive region extending from the first surface to a first compression depth, a second compressive region extending from the second surface to a second compression depth, and, a tensile region disposed between the first compression depth and the second compression depth, where a stress profile of the first compressive region has a first local minimum point at which the stress profile is convex downward and a first local maximum point at which the stress profile is convex upward, where a depth of the first local maximum point is greater than a depth of the first local minimum point, and where a stress of the first local maximum point is greater than a compressive stress of the first local minimum point.

A window includes a base region and a compressive stress region disposed on the base region. The compressive stress region includes Li.sup.+, Na.sup.+, and K.sup.+ ions. The compressive stress region includes a first compressive stress portion in which a concentration of the K.sup.+ ions decreases, a concentration of Na.sup.+ ions increases, and a concentration of the Li.sup.+ ions increases, from a surface of the window toward the base region. A second compressive stress portion is adjacent to the first compressive stress portion. In the second compressive stress portion, the concentration of the Na.sup.+ ion decreases and the concentration of the Li.sup.+ ion increases, from the first compressive stress portion toward the base region. The window thereby has a high surface compressive stress value and impact resistance.