Ion-Exchange Strengthening of High Average Power Phosphate Laser Glass

Thermal stress induced fracture limits the power output of high average power glass laser slabs1. The fracture of laser glass is initiated by the onset of unstable propagation of subsurface microcracks in which local tensile stress exceeds the critical level. The production of subsurface flaws is inevitable for a standard high precision fabrication process. Applying a compressive stress surface layer can be used to counterbalance the thermal tensile stress imposed upon the surface during operation. We have developed an ion-exchange process submerging the phosphate glass into a mixed alkali nitrate salt melt. An ionic size differential induces a compressive layer of tens of microns. The process avoids water corrosion and preserves the finished optical surface quality during ion-exchange. Furthermore, we have optimized the process taking into account network relaxation as well as ion-exchanged concentration profile. To characterize the surface compressive stress, we have employed a biaxial flexure bending test. We have found that a finished HAP-4 laser glass with a tensile breaking strength of 200MPa can be strengthened to twice its original strength, namely 400MPa. The process has been scaled up to production in which large laser glass slabs or most other phosphate glass can be treated routinely. We have strengthened polished HAP-4 slabs(42cm × 14cm × 1cm) to up to twice their original thermal shock resistance without deteriorating their optical surface quality.