Design of mechanically advantaged glasses with hydration-induced stress profiles

The glass composition design work leading to the discovery of highly crack resistant glasses exhibiting hydration-induced stress profiles is described. Initial hydration studies on ternary aluminosilicate glasses show the importance of potassium for facilitating hydration. Further modification of the glass composition through the incorporation of P₂O₅ increased the hydration rate such that a specimen with a 29-µm hydration depth was prepared by holding in an 85°C 85% relative humidity chamber for 65 days. Not only did this glass have a high Vickers indentation crack resistance of >20 kgf, but the sample also displayed considerable stored energy at failure. This indication of a stress profile was subsequently measured and a compressive stress (CS) of 400 MPa with a compressive depth of layer of 29 µm was found. The initially long process times were shortened using pressurized steam vessels. When held at 250°C and .3 MPa, samples can be prepared with surface CSs >300 MPa and compressive depths >30 µm in less than 8 h.