In the Ionex™ process, glass is treated in a molten salt (usually KNO3) causing “ion exchange” of small Na+ ions from the glass with larger K+ ions from the salt. The “stuffing” effect of the large K+ ions forces the surface into a compression, producing a closure stress on cracks. Since glass fails in tension only generally originating at a surface, an externally applied tensile stress must first overcome the surface compression before the occurrence of crack growth. It is also more difficult for new cracks to form during handling because of increased abrasion resistance. This means that glass is effectively stronger; the strength is increased in relation to the magnitude of compressive stress at the crack tip. Since the treatment is carried out at temperatures just below the glass transition temperature, the product does not suffer any significant deformation or optical distortion. In Saxon’s Ion-Klad™ process, glass is subjected to longer process. This allows minimum strengths to be increased by creating a greater depth of surface compression (case depth), going beyond the deeper flaws.
Figure 1: Large K+ exchange sites with host Na+ ions forcing surface into compression, creating a closure stress on cracks, there by increasing the strength and making it more difficult for new cracks to form. Furthermore, the surface compression all but eliminates problems of “delayed failure”; i.e. time-dependent weakening under load.
Why Ion Exchange?
Ion exchange has several advantages over conventional thermal tempering.
- Strengths are significantly higher (as high as 150,000 psi. depending on glass type and surface conditions).
- Thin samples and complex shapes are easily handled.
- A uniform compression case depth is produced.
- Treatment temperatures are below the strain point, so the sample geometry remains constant, and there is no optical distortion.
Despite these advantages, ion exchange can not be applied in all cases, and in others may be too expensive. For example:
- Glasses with low alkali content do not strengthen well.
- Relative to thermal tempering, the case depth in chemical strengthening is small – generally less than 50 microns (0.002″), except in a few specially designed glass compositions. An example of a specially designed glass composition is the aircraft windshield where protection from birds in flight requires case depths of 200 – 300 microns (0.008″ – 0.012″).
- Treatment times on the order of 2 – 100 hours may be required. Long treatment times may improve the reliability by increasing case depth at greatly increased cost.