TECHNICAL FIELD

The present invention relates to a chemically strengthened glass.

BACKGROUND ART

In a flat panel display device such as a digital camera, a cellular phone, or a portable information terminal PDA (Personal Digital Assistants), a thin sheet-like cover glass is disposed on a front surface of a display so as to occupy a wider area than an image display part thereof in order to protect the display and enhance the appearance thereof. Glass is high in theoretical strength. However, once the glass gets scratched, the strength of the glass is remarkably lowered. Accordingly, a chemically strengthened glass having a compressive stress layer formed in its glass surface by ion exchange or the like has been used as a cover glass requiring strength.

In accordance with a request to make the flat panel display device lighter and thinner, it is also requested to make the cover glass itself thinner. Accordingly, the cover glass is required to have higher strength in both of its surface and its end surface in order to satisfy the object.

In order to improve the surface strength of the chemically strengthened glass, it is required to increase the depth of the compressive stress layer. Conventionally, the depth of the compressive stress layer is increased by a chemical strengthening treatment performed at a low temperature and for a long time to thereby improve the surface strength (for example, Patent Literature 1).

In addition, it has been known that in the case where the hydrogen concentration profile in the surface layer of a chemically strengthened glass is adjusted within a specific range and a property of the glass surface is not lower than a specific value, the surface strength of the glass can be improved dramatically without polishing the glass surface or etching the glass surface with hydrofluoric acid after chemical strengthening (Patent Literature 2).

Furthermore, as for the strength of glass, it has been known that the strength of the glass is lowered by the existence of hydrogen (moisture) in the glass (Non-Patent Literatures 1 and 2).

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-2016-084282Patent Literature 2: JP-A-2015-166312

Non-Patent Literature

Non-Patent Literature 1: S. ITO et al., “Crack Blunting of High-Silica Glass”, Journal of the American Ceramic Society, Vol. 65, No. 8, (1982), 368-371Non-Patent Literature 2: Won-Taek Han et al., “Effect of residual water in silica glass on static fatigue”, Journal of Non-Crystalline Solids, 127, (1991) 97-104

SUMMARY OF INVENTION

Technical Problem

According to the method described in Patent Literature 1, owing to the chemical strengthening treatment performed at a high temperature of 400° C. or more and for a short time, productivity can be improved, and the depth of the compressive stress layer can be increased. However, there is a problem that the surface strength is lowered. Although the surface strength is intended to be improved by polishing after the chemical strengthening treatment, there is a concern that the glass surface may get scratched by the polishing to thereby lower the strength. Furthermore, there is another concern that warpage of the glass may be increased by the polishing.

Furthermore, according to the glass described in Patent Literature 2, the hydrogen concentration profile in the surface layer of the chemically strengthened glass is adjusted within a specific range, and the surface roughness is not lower than a specific value. Thus, a glass sheet whose surface strength is improved can be provided. However, in the case where glass contains phosphorus, the phosphorus contained in the glass composition has a high affinity to water electrically. In addition, glass having a high phosphorus content has a poor water resistance so that the glass is easily eroded by water. Thus, the hydrogen concentration profile is affected by the phosphorus. Therefore, there is a problem that the surface strength is not sufficiently improved within the range of the hydrogen concentration profile disclosed in Patent Literature 2.

Therefore, an object of the present invention is to provide a chemically strengthened glass in which even when a glass having a glass composition containing phosphorus affecting the improvement of strength is chemically strengthened at a high temperature and for a long time, a sufficient depth can be secured in a compressive stress layer of a surface layer, so that a sufficient surface strength can be obtained and lowering of the surface strength of the glass can be effectively suppressed without polishing after the chemical strengthening.

Solution to Problem

As a result of earnest examinations, the present inventors have found that in a chemically strengthened glass having a specific glass composition containing phosphorus, a surface roughness (Ra) is set to be not less than a specific value, a depth of a compressive stress layer in a surface layer of the chemically strengthened glass is set to be not less than a specific value, and an average hydrogen concentration at a specific depth from the outermost surface of the chemically strengthened glass is set within a specific range, so that the surface strength of the glass can be improved dramatically and the reliability of the surface strength can be improved, without polishing the surface of the chemically strengthened glass or etching the surface of the chemically strengthened glass with hydrofluoric acid after chemical strengthening treatment performed at a high temperature and for a long time. Thus, the present inventors have completed the present invention.

That is, the present invention is configured as follows.<1> A chemically strengthened glass including a compressive stress layer that is formed in a surface layer of the glass by an ion exchange method, in which:the glass has a surface roughness (Ra) of 0.50 nm or more;the glass has a depth of the compressive stress layer of 55 μm or more;the glass has a composition including, in mole percentage on an oxide basis, 55.5 to 76.5% of SiO2, 5 to 20% of Al2O3, 8 to 25% of Na2O and 0.1% or more of P2O5;a hydrogen concentration Y1 in a region of a depth X from an outermost surface of the chemically strengthened glass satisfies Expression (I) below at X1=0.10 to 0.25 (μm);a hydrogen concentration Y2 in a region of the depth X from the outermost surface of the chemically strengthened glass satisfies Expression (II) below at X2=0.25 to 0.4 (μm); anda surface strength F (N) measured on the conditions below by a ball-on-ring test satisfies a relation of F≥1200×t2with respect to a sheet thickness t (mm) of a sheet of the glass:
Y1=a1X1+b1(I),in which:Y1 is the hydrogen concentration (mol/L, measured as H2O);X1 is the depth from the outermost surface of the glass (μm);a1is −1.700 to −0.500; andb1is 0.100 to 0.460,
Y2=a2X2+b2(II),in which:Y2 is the hydrogen concentration (mol/L, measured as H2O);X2 is the depth from the outermost surface of the glass (μm);a2is −0.200 to −0.010; andb2is 0.050 to 0.150,ball-on-ring test conditions:

A glass sheet having a sheet thickness t (mm) is disposed on a stainless steel ring having a diameter of 30 mm and including a contact portion rounded with a curvature radius of 2.5 mm, and a steel ball having a diameter of 10 mm is brought into contact with the glass sheet. In this state, the ball is loaded at a center of the ring under a static load conditions. A breaking load (in units of N) at which the glass is broken is regarded as a BOR strength, and an average value of 20 times measurements of the BOR strength is regarded as the surface strength F. Here, the case where a breaking start point of the glass is 2 mm or more away from a loading point of the ball is excluded from data for calculating the average value.<2> The chemically strengthened glass according to the configuration <1>, having an average hydrogen concentration c1of 0.050 to 0.180 mol/L, in a region of the depth X1=0.10 to 0.25 μm from the outermost surface of the chemically strengthened glass, and having an average hydrogen concentration c2of 0.020 to 0.080 mol/L, in a region of the depth X2=0.25 to 0.4 μm.

<3> The chemically strengthened glass according to the configuration <1> or <2>, having no polishing flaws in the surface thereof.<4> The chemically strengthened glass according to any one of the configurations <1> through <3>, including 1% or more of an alkaline earth metal RO, in which RO designates MgO+CaO+SrO+BaO.<5> The chemically strengthened glass according to any one of the configurations <1> through <4>, having a surface compressive stress of 100 MPa or more.<6> The chemically strengthened glass according to any one of the configurations <1> through <5>, having an internal tensile stress of 72 MPa or less.<7> The chemically strengthened glass according to any one of the configurations <1> through <6>, in which the depth of the compressive stress layer is 55 μm or more.

Advantageous Effects of Invention

According to the chemically strengthened glass having a specific glass composition according to the present invention, a depth of a compressive stress layer is made not less than a specific value, a surface roughness (Ra) is made not less than a specific value, and a hydrogen concentration profile in a surface layer of the glass is set within a specific range, so that it is possible to remarkably improve the surface strength of the glass and improve reliability of the surface strength, even after a chemical strengthening treatment performed at a high temperature and for a long time and without polishing the glass surface after the chemical strengthening.

