Microstructure and mechanical properties of nucleant-free Li2O-CaO-SiO2 glass-ceramics

An attractive characteristic of the Li2O-CaO-SiO2 glass-forming system is the possibility of obtaining internally nucleated crystal phases - CaSiO3 and Li2SiO3 - having elongated morphologies that can potentially lead to tough glass-ceramics. Another great advantage of this system is that it does not need a nucleating agent to crystallize internally. In this research work, three glasses with systematic compositional variations within the range of 30-50 mol% CaSiO3/70-50 mol% Li2SiO3 were prepared, heat-treated and their microstructures and mechanical properties evaluated. Heating cycles were applied in two-stage treatments. The temperatures were selected based on previous DSC analyses. The crystallized samples were characterized by XRD and SEM to examine the nature, size and morphology of their crystals. The effect of microstructure on the Vickers hardness (Hv) and indentation fracture toughness (Kc) of the glass-ceramics was determined. Elastic modulus (E), biaxial strength (σm) and fracture toughness (KDTIC) were determined from nanoindentation, ball-on-three-balls and double torsion tests, respectively. The level of internal residual stresses was evaluated by X-rays diffraction. The best mechanical properties were exhibited by a composition containing 44 mol% CaSiO3 heat-treated at 498 °C for 24 h for nucleation and at 700 °C for 2 h for crystal growth. This composition resulted in a glass-ceramic with the following microstructural features and properties: average Li2SiO3 (LS) crystal size of 8.5 μm, wollastonite (CS) phase surrounding the LS crystals with approximately 50% LS crystallized volume fraction, KDTIC = 2.3 ± 0.5 MPa m1/2, σm = 270 ± 20 MPa, HV = 8.4 ± 0.7 GPa and E = 146 ± 8 GPa. These are exciting mechanical properties for glass-ceramics intended for load bearing applications.

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