Surface tensile layer generation during thermal annealing of phosphate glass

Surface tensile layers (up to 20 MPa) are generated during thermal annealing of metaphosphate glass in ambient air. The surface tensile stress is caused by changes in properties of the phosphate glass, particularly the glass transition temperature and the coefficient of thermal expansion, due to the glass reaction with water vapor and diffusion of OH groups into the region just below the surface (<2 mm). An increase in the OH content of the glass from about 80 to 5000 ppmw causes a decrease in the glass transition temperature from 468 to 405°C and an increase in the linear thermal expansion coefficient from 12.9 to 14.2 × 10 ⁻⁶ K ⁻¹ . A mechanism for the generation of the tensile surface layer is proposed and simulated experimentally by fusion-bonding at 500°C two samples of the same composition but with OH contents that differ by up to 50×. The stresses in the annealed fusion-bonded samples are determined from measured stress birefringence. Peak tensile stresses calculated by finite element analysis agree with measured values to within 30%.