Effect of preheating and isothermal holding time on the crystallization, densification and properties of a sintered lithium silicate glass-ceramic


The objective of this work was to study the effect of different heating-up procedures and isothermal treatment times on crystallization, densification and properties of a sintered non-stoichiometric lithium silicate glassceramic. Glass samples containing SiO2, Li2O and K2O as main constituents, besides minor amounts of other oxides such as Al2O3, P2O5 and ZrO2, were melted at 1500 degrees C - 60 min and subsequently fragmented (<25 mu m) into a fine powder. The obtained glass particles were characterized by scanning electron microscopy SEM, X-ray diffraction, XRD and differential thermal analysis, DSC. The thermal analysis indicated the presence of two exothermal peaks, the first at approximately 640 degrees C, characteristic of the crystallization of lithium metasilicate, Li2SiO3, and the second close to 820 degrees C, corresponding to the crystallization of lithium dissilicate, Li2Si2O5. In order to study the crystallization behavior the glass powder was uniaxially compacted and investigated by dilatometry. The samples were sintered under different conditions: Group 1: sintering at 840 degrees C with isothermal holding times of 5, 15, 60 and 180 min; Group 2: preheating to 660 degrees C with an isothermal plateau for 720 min and further heating up to 840 degrees C with identical holding times of 5, 15, 60 and 180 min. The sintered glassceramics were characterized by XRD, Rietveld refinement, SEM, relative density, hardness, fracture toughness and flexural strength. The results showed that the samples of group 1, directly heated up to 840 degrees C, achieved a relative density ranging between 60 and 70% and a hardness and fracture toughness lower than the second group that was subjected to a differentiated preheating process. The sintered samples for less than 60 min, presented Li2SiO3 and Li2Si2O5 as crystal phases and a microstructure formed by low aspect ratio crystals, besides apparent porosity. The samples sintered at 840 degrees C for 180 min, independently of the heating up procedure, presented only Li2Si2O5 as crystal phase with elongated grains with an aspect ratio higher than 2 and relative densities in the order of 84 to 91%. The hardness ranged in the order of 5.6 to 6 GPa and the average fracture toughness was 1.5 MPa m1/2. The materials subjected to the preheating process and treated at 840 degrees C for 180 min exhibited a flexural strength of 160 MPa, while those directly heated to 840 degrees C and treated for 180 min showed a flexural strength of only 75 MPa.



subject category

Materials Science


Monteiro, RH; Alves, MFRP; Strecker, K; dos Santos, C; Elias, CN



The authors thank FAPERJ (grant no. E26-201.476/2014 and E26-202.811-2018) and CNPq (grant n. 311.119/2017-4 and 301.683/2017-14) for financial support and to ProtMat Materiais Avancados for supplying the raw-materials.

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