Two types of glass strengthening (tempering) are known in the art. The most popular is thermal tempering, which involves heat and quenching. The heat treatment used in thermal tempering is typically over 580 degrees C., and mostly over 600 degrees C. The second type of glass strengthening is known as chemical strengthening (or chemical tempering).
Chemically strengthened glass is a type of glass that has increased strength as a result of a chemical process that is performed after the base glass is made. When broken, it still shatters in long pointed splinters. For this reason, it is often laminated when used in safety glass applications. However, chemically strengthened glass is typically much stronger (e.g., six to eight times stronger) than non-chemically strengthened float glass. The chemical strengthening process by an ion-exchange is often a process by which the metal ions (for example, Na ions) having a smaller radius contained in glass are replaced by ions (for example K ions) having a larger radius contained in the glass to generate a compressive stress layer on a glass surface, and improve the glass strength. A chemical strengthening method is to replace alkali metal ions (typically Li ions and/or Na ions) having a smaller ion radius existing on the glass plate surface with alkali ions (e.g., Na ions and/or K ions for Li ions, or K ions for Na ions) having a larger ion radius by ion exchange at temperatures lower than or equal to a glass transition point. Example chemical strengthening processes are described in, for example and without limitation, U.S. Patent Document Nos. 2014/0302330, 2013/0101798, and application Ser. No. 13/137,696. the disclosures of which are all incorporated herein by reference.
The glass is typically chemically strengthened by a surface finishing process. For instance, glass may be submersed in a bath containing a potassium salt (typically potassium nitrate) at a high temperature such as around 300° C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution. These potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. This replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension.
Another example chemical strengthening process is a multi-stage process in which the glass is first immersed in a sodium nitrate bath at a high temperature such as around 430-450° C., which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article. In any event, chemical strengthening results in a strengthening similar to toughened glass. However, the process does not use extreme variations of temperature (e.g., temperatures over 580 or 600 degrees C. are not needed) and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern.
Conventional float glass often has a composition as follows,
Ingredientwt. %SiO271.18%A12O31.02%Na2O13.52%K2O0.24%CaO8.76%MgO3.99%Fe2O3 (total iron)0.09%SO30.20%
This conventional float glass, before any chemical strengthening, has a strain temperature (log η=14.5) of 519 degrees C., a cool time (secs) of 100.99, a USPXXIII (ml) of 6.14, a glass molar volume (cm3) of 23.67, a Young's modulus (GPa) of 72.2, a shear modulus (GPa) of 29.6, a non-bridging oxygen (NBO) of 17.81, and a %NBO of 29.54.
Unfortunately, it has been found that the above-identified conventional float glass compositions is not well suited for chemical strengthening. Following chemical strengthening, the “depth of layer” in the glass indicates the depth of the compressive stress layer in the glass after the ion exchange of the chemical tempering process. The deeper the layer, the stronger the glass. Thus, a higher “depth of layer” is desirable following chemical strengthening because it provides for better impact resistance and better scratch resistance in chemically strengthened glass (compared to a thinner layer). The “depth of layer” for the above-identified conventional float glass following chemical strengthening is undesirably small/thin, and thus does not provide for a high degree of impact and scratch resistance which may be desirable in certain instances. For example, following a four hour ion-exchange chemical strengthening process at 435 degrees C. which resulted in maximum compressive stress (MPa) of 710.6 MPa, the “depth of layer” of the above-identified conventional float glass was only 10.5 μm. This thickness of the compressive stress layer (“depth of layer”) is undesirable low/thin.
In view of the above, it is apparent that there exists a need in the art for a glass composition which may achieve a larger (deeper) “depth of layer” upon chemical strengthening, and thus improved impact and/or scratch resistance.