Anisotropic residual stresses in partially crystallized Li2O±2SiO2 glass-ceramics

Levels of residual stresses in partially crystallized Li2O±2SiO2 glass-ceramics were calculated for several crystallographic planes. The stresses were also determined by an X-ray di€raction method (XRD) using synchrotron radiation. The linear thermal expansion coecient (ahkl) of a fully crystallized, stress-free, powdered sample, was measured in situ using a hot-stage XRD technique. The thermal expansion coecient depends on the crystallographic (hkl) direction. For some direction, the thermal expansion coecient of the crystalline phase is greater than that of the glass matrix, while for other direction the inverse is observed. The average thermal expansion coecient calculated from the unit cell expansion with temperature aucˆ (10.1 ‹ 0.5) ́ 10ÿ6°Cÿ1 was close to that obtained by a dilatometric method for a polycrystalline sample áacñˆ (10.8 ‹ 0.5) ́ 10ÿ6°Cÿ1. The theoretical residual stresses were calculated by the Selsing model using experimental data for the elastic constants combined with the thermal expansion coecient for each crystallographic plane. Experimental determinations of residual stresses were made using an XRD technique for six di€erent sets of (hkl) planes in two partially crystallized specimens. The type of experimental residual stresses (tensile or compressive) for each crystallographic plane was in agreement with the predicted type. Regarding the stress magnitude, the experimental results vary from ÿ58/ÿ117 MPa (compressive) to 60/149 MPa (tensile), whereas the calculated magnitudes vary from ÿ192 MPa (compressive) to 16/102 MPa (tensile). These magnitudes were con®rmed by microRaman spectroscopy. The di€erences between the theoretical and experimental stress magnitudes are within the limits of experimental errors. Therefore, the Selsing model can give a reasonable estimate of residual stresses for real (anisotropic) materials. Ó 1999 Elsevier Science B.V. All rights reserved.