Internal Residual Stress Measurements in a Bioactive Glass-Ceramic Using Vickers Indentation

The residual stress distribution that arises in the glass matrix during cooling of a partially crystallized 17.2Na(2)O-32.1CaO-48.1SiO(2)-2.5P(2)O(5) (mol%) bioactive glass-ceramic was measured using the Vickers indentation method proposed by Zeng and Rowcliffe (ZR). The magnitude of the determined residual stress at the crystal/glass boundary was 1/4-1/3 of the values measured using X-ray diffraction (within the crystals) and calculated using Selsing`s model. A correction for the crack geometry factor, assuming a semi-elliptical shape, is proposed and then good agreement between experimental and theoretical values is found. Thus, if the actual crack geometry is taken into account, the indentation technique of ZR can be successfully used. In addition, a numerical model for the calculation of residual stresses that takes into account the hemispherical shape of the crystalline precipitates at a free surface was developed. The result is that near the sample surface, the radial component of the residual stress is increased by 70% in comparison with the residual stress calculated by Selsing`s model.CNPq/Brazil[151917/2006-0]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fapesp/Brazil[07/08179-9

Sinterability and mechanical properties of glass-ceramics in the system SiO2-Al2O3-MgO/ZnO

[EN] The sintering, crystallization and mechanical behavior of three glass compositions in the SiO-AlO-MgO/ZnO system were investigated. Partial substitution of MgO by ZnO causes a decrease in T, a slight increase in the crystallization temperature and improves sinterability. Cordierite and enstatite were crystallized in bulk glasses heat treated at 1000 °C. Addition of ZnO resulted in the crystallization of cordierite, willemite and gahnite for the heat treated bulk glasses and cristobalite, enstatite, gahnite and willemite for the sintered glass powders. The first phase to crystallize is the β-quartz solid solution at 850−900 °C for all compositions. The glass and glass-ceramic with ZnO had the lowest hardness and elastic modulus. There was no variation of fracture strength with composition. The glass-ceramic with ZnO had a similar wear resistance in comparison to the composition with higher MgO content.The authors acknowledge the financial support from CAPES PDSE (grant n°88881.132583/2016-01), FINEP, CNPq, Fundação Araucária, UGF from Brazil for funding laboratory infrastructure and are thankful to ICV-Madrid-Spain and C-LABMU/UEPG for the use of the characterization facilitiesPeer reviewe

Internal Residual Stresses In Partially Crystallized Photo-Thermo- Refractive Glass

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. © 2011 The American Ceramic Society

Internal Residual Stresses in Sintered and Commercial Low Expansion Li(2)O-Al(2)O(3)-SiO(2) Glass-Ceramics

We performed Synchrotron X-ray diffraction (XRD) analyses of internal residual stresses in monolithic samples of a newly developed Li(2)O-Al(2)O(3)-SiO(2) (LAS) glass-ceramic produced by sintering and in a commercial LAS glass-ceramic, CERAN (R), produced by the traditional crystal nucleation and growth treatments. The elastic constants were measured by instrumented indentation and a pulse-echo technique. The thermal expansion coefficient of virgilite was determined by high temperature XRD and dilatometry. The c-axis contracts with the increasing temperature whereas the a-axis does not vary significantly. Microcracking of the microstructure affects the thermal expansion coefficients measured by dilatometry and thermal expansion hysteresis is observed for the sintered glass-ceramic as well as for CERAN (R). The measured internal stress is quite low for both glass-ceramics and can be explained by theoretical modeling if the high volume fraction of the crystalline phase (virgilite) is considered. Using a modified Green model, the calculated critical (glass) island diameter for spontaneous cracking agreed with experimental observations. The experimental data collected also allowed the calculation of the critical crystal grain diameters for grain-boundary microcracking due to the anisotropy of thermal expansion of virgilite and for microcracking in the residual glass phase surrounding the virgilite particles. All these parameters are important for the successful microstructural design of sintered glass-ceramics.CNPq/Brazil[151917/2006-0]FAPESP/Brazil[07/08179-9]FAPESP/Brazil[05/53241-9]CNPqBrazilian Synchrotron Light Laboratory (LNLS)/MCT[XRD1-5824]Brazilian Synchrotron Light Laboratory (LNLS)/MCT[XRD1-6712

Thermo-Mechanical Stability and Gas-Tightness of Glass-Ceramics Joints for SOFC in the System MgO-BaO/SrO-B2O3-SiO2

The objective of this paper is to illustrate a variety of studies carried out to improve the quality of some particular glass-ceramic joining materials based on measured properties such as gas-tightness and mechanical resistance and demonstrate the feasibility of using the proposed materials for solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC) applications. First, the sealing conditions have been optimized for the two selected compositions in the system MgO-BaO/SrO-B2O3-SiO2. Once the joining materials have been optimized, the gas-tightness has been measured as a function of the glass-ceramic crystallization degree, its thermal cycling behavior and the influence of a reducing atmosphere on this property. The electrical resistance at high temperature has also been studied. Subsequently, the chemical compatibility of the joints steel/glass-ceramic has been evaluated by means of the analysis of the cross-sections using SEM and EDX. Furthermore, the mechanical and chemical stability of the joints has also been studied as a function of the crystallization degree, the resistance vs. thermal cycling and the influence of a reducing atmosphere. Finally, the mechanical resistance of the joints regarding flexural loading has been characterized employing a 4-point bending method both at room temperature and at relevant high temperatures varying the seal thickness. Overall, the results verify that the developed and tested materials are promising for long term stable SOFC and SOEC applications in advanced stack designs aiding prolonged lifetime under thermal-cyclic conditions