Chemical Toughening
Glasses generally break because tension acting on a surface flaw exceeds the engineering strength of the glass. Silicate glasses are strong in compression but weak in tension. By introducing compression stress into the structure, the tensile strength of the material is increased. At Phoenix this is done by chemical tempering (via ion exchange).
During chemical tempering, the glass is submerged into a molten salt bath containing ions larger than those present within itself. Due to a concentration gradient of the ions, mass transport must take place. As the larger ion diffuses from the molten salt into the surface, it replaces the smaller ion from the glass. The larger ion squeezing into surface introduces compressive stress in the glass's surface, resulting from the layer on the surface having a different composition from the underlying bulk glass.
The diffusion rate increases exponentially with temperature, until the glass transition temperature is reached, and surface compression results of up to 1000 MPa are achieved. We can chemically temper flat and moulded glass. This process has up to 4 times the mechanical and thermal strength of annealed glass without the optical and physical distortion usually associated with thermal tempering. .
