Effect of the temperature on the mechanical properties and translucency of lithium silicate dental glass-ceramic

Fracture resistance and translucency are among the most important properties for the use of lithium silicate glass ceramics as dental prostheses, both being greatly influenced by the temperature of the crystallization heat treatment. In this work, the effect of short time heat treatments at different temperatures on the lithium metasilicate (Li2SiO3) -> lithium disilicate (Li2Si2O5) phase transformation and resulting properties was investigated. Commercial Li2SiO3 samples were heat treated under vacuum for 7 min, at 820 degrees C, 830 degrees C and 840 degrees C, to induce the phase transformation into Li2Si2O5. The heat treated samples were characterized by their phase composition (XRD), microstructure, mechanical properties (Vickers Hardness, fracture toughness, bending strength and elastic parameters) and translucency. The results indicate that 814 degrees C is the onset temperature for the crystallization of lithium disilicate (Li2Si2O5). A progressive conversion of Li2SiO3 into Li2Si2O5, besides the reduction of the amorphous phase content is observed with increasing temperature. Furthermore, the increasing amount of the Li2Si2O5 phase and the concomitant reduction of the amount of amorphous phase resulted in a slight reduction of the hardness from 6.13 +/- 0.22 to 5.76 +/- 0.17 GPa, a slight increase in fracture toughness from 1.30 +/- 0.02 to 1.60 +/- 0.03 MPa m(1/2) and in a significant improvement of the bending strength from 166 +/- 22 to 303 +/- 59 MPa. The improvement in mechanical properties is associated with the reduction of amorphous phase and increasing amount of the Li2Si2O5 phase, which is composed of elongated grains with high aspect ratio. However, translucency diminished 40% comparing samples treated at 820 degrees C or 840 degrees C, which is attributed to the reduction of the amount of intergranular amorphous phase.