Surface tensile layer generation during thermal annealing of phosphate glass

Surface tensile layers (up to 20 MPa) are generated during thermal annealin g of metaphosphate glass in ambient air. The surface tensile stress is caus ed 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). Pin increase in the OH content of the glas s from about 80 to 5000 ppmw causes a decrease in the glass transition temp erature from 468 to 405 degrees C and an increase in the linear thermal exp ansion coefficient from 12.9 to 14.2 x 10(-6) K-1. A mechanism for the gene ration of the tensile surface layer is proposed and simulated experimentall y by fusion-bonding at 500 degrees C two samples of the same composition bu t with OH contents that differ by up to 50x, The stresses in the annealed f usion-bonded samples are determined from measured stress birefringence, Pea k tensile stresses calculated by finite element analysis agree with measure d values to within 30%. (C) 2000 Elsevier Science B.V. All rights reserved.