## Effect of Crystal Size and Crystallinity on the Fracture Toughness and Strength of Stoichiometric Lithium Disilicate Glass-Ceramics

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Abstract
10-016 MARIZA VEIGA SENK SENK, M.V.(State University of Ponta Grossa); Zanotto, E.D.(Federal University of Sao Carlos); Serbena, F.C.(State University of Ponta Grossa); Mathias, I.(State University of Ponta Grossa); Cruz, C.(State University of Ponta Grossa); Glass-ceramics are advanced materials having homogeneous and complex micro or nanostructures, which coupled with absence of porosity often lead to superior combination of properties. Glass-ceramics have higher mechanical resistance than their parent glasses. The knowledge of the microstructural effect on the mechanical properties is of utmost importance. In this work, we investigated the variation of the fracture strength and toughness, ${K}_{IC}$, in an experimental (stoichiometric) lithium disilicate glass-ceramic as a function of the size of the precipitate and the crystalline volumetric fraction. The glass-ceramics were prepared by a double heat treatment for proper crystal nucleation and growth. Three crystal sizes were investigated: 7, 13 and 34 µm. The mechanical strength of the glass-ceramics increased with crystalline fraction and decreased only a little with increasing crystal size, as expected. The biaxial flexural strength of glassy samples was 117 ± 11 MPa, whereas for the samples with crystalline volume fraction of 98 % and crystal diameter of 7 µm it was 297 ± 9 MPa. Doubling the average crystal size did not significantly change the strength, i.e. (300 ± 30) MPa for crystals of 13 µm with crystallized volumetric fraction of approximately 94%. For 34 µm and with a 80% crystallized crystallized volume fraction, the strength was 230 ± 20 MPa. The fracture toughness clearly increased with the crystallized volumetric fraction and the larger the crystal diameter, the greater the value of ${K}_{IC}$. The highest fracture toughness was achieved for the samples with crystal diameter of 34 µm and 80 % or larger volume fraction, 3.1 ± 0.2 MPa.${m}^{0.5}$, an increase of 280 %, over that of the parent glass.
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