Internal Residual Stresses in Partially Crystallized Photo‐Thermo‐Refractive Glass

Abstract

Photo-thermo-refractive glass containing nanosized NaF crystals embedded in the glassy matrix shows a significant shift of X-ray diffraction (XRD) lines to lower angles resulting from large residual (tensile) stresses within the crystals. This is thus an excellent ''model'' system to test residual stresses models in glass-ceramics and composites because: the estimated stresses are high-about 1 GPa-the precipitates are nearly spherical, the NaF crystals structure is cubic and their volume fraction is quite small, which eliminates overlap between the stress fields of neighbor crystals. Samples treated at a sufficiently high temperature to develop larger (micrometer size) crystals revealed microcracking of the glassy matrix around the crystals, which partially relieved the residual stresses and decreased the shift of the XRD peaks. The experimental results for the magnitude of the residual stresses and the critical crystal diameter for microcracking agree with theoretical values calculated by the Selsing and the Davidge & Green models, respectively. These results suggest that these two models can be used for stress estimates and as a first approach for the design of tough glass-ceramics. R. Bermejo-contributing editorThis work was financially supported by CNPq/Brazil-process no. 151917/2006-0, FAP-ESP/Brazil-contract no. 07/08179-9, the Brazilian Synchrotron Light Laboratory/MCT research proposals XRD2-5322 and XRD1-5824 and DARPA/ADHELS program.

Residual stress effect on the fracture toughness of lithium disilicate glass‐ceramics

“…21,44,51 For tensile residual stresses in the precipitate, the radial component is tensile, and the tangential components are compressive. 23,24 Another important observation in the microstructure of the different GCs is the size of the LS2 crystals. Table 11 shows the smallest crystals, of approximately 1 μm in length, for the HH samples.…”

“…These may optimize the mechanical properties of GCs, allowing not only the expansion of the LS2 GCs application, but also for all other types of GCs that have similar microstructures. 9,[21][22][23][24] Several aspects of the microstructure affect the mechanical properties of GCs and warrant a more in-depth investigation: the nature of the crystallized phases, the crystallized volume fraction, the crystal size and shape, and the level and type (compression or tension) of internal residual stresses. These will be focused on here.…”

Internal residual stresses in glass-ceramics: A review

“…(a) The XRD pattern of PTR glass, a UV-exposed sample, heat treated for 450°C-1 h/520°C-2 h, and the corresponding NaF nano-precipitates (indicated by the arrows); (b) a bright-field TEM image of an exposed PTR glass, heat treated for 483°C-1 h/515°C-1 h; (c) the XRD pattern of PTR glass, a UV-exposed sample, heat treated for 450°C-1 h/650°C-20 min; and (d) a scanning electron microscopy image of the fractured surface of a PTR sample, unexposed, heat treated at 450°C-1 h/650°C-20 min, showing a fractured NaF precipitate and cracks in the surrounding matrix indicated by the arrows. Reproduced with modifications from Serbena et al [46]. studied.…”

“…The spontaneous radius for microcracking in PTR glass-ceramics has been studied by Serbena et al [46]. Two different heat treatments produced two types of precipitates: NaF cuboidal nanocrystals (shown in Fig.…”

“…Residual stresses in a photo-thermo-refractive (PTR) glassceramic with a very low volume fraction crystallized (b1%) were measured by Serbena et al [46] using synchrotron XRD. The crystallized phase was an NaF nanocrystal, and due to its cubic structure, it is isotropic and suitable for investigating Selsing's model with no influence of crystal anisotropy.…”