Effect Of Zirconia Content On Properties Of An Experimental Lithium Disilicate Glass-ceramic

Glass ceramics is a kind of all ceramic materials，which is obtained bycontrolled crystallization of base glass matrix after heat treatment. It iscomposed of crystal and glass phases. Owing to the glass component,glass-ceramics have the potential to match the translucent characteristic ofnatural tooth. Therefore, glass ceramics are considered as the suitable materialfor dental restorative application such as inlays, single crowns and veneers. Inthe previous research, an experimental lithium disilicate glass ceramic （ELDC）,located in the SiO₂-Li₂O-Al₂O₃–K₂O–P₂O₅system, was developed with highflexural strength and suitable optical properties. The aims of this study were toanalyze the influence of adding a second phase material （zirconium） into ELDCon the properties of the zirconia-lithium disilicate glass-ceramic composite（ZLDCC）, such as flexural strength and fracture toughness, before and afterheat-pressing treatment. The influence of different ZrO₂content on phaseformation and properties of ZLDCC was analyzed. The authors also investigated the effect of zirconium on the translucency of the ZLDCCmulti-component system..This study was composed of three parts:Part one was to investigate the mechanical properties and microstructureof ZLDCC. The influence of zirconium on the properties of ELDC and therelated mechanism were also discussed. Zirconium oxide with different massfraction （0%，1%，2%，3%，4%） was added into the ELDC system to formZLDCC. The flexural strength and fracture toughness of the specimens weremeasured and the microstructure was evaluated by scanning electronmicrograph （SEM）. Crystalline composition of ZLDCC was also determinedusing X-ray diffraction （XRD）. The results showed that the flexural strengthand fracture toughness of ZLDCC increased as ZrO₂content increased. TheZLDCC specimens with4%ZrO₂showed the highest flexural strength andfracture toughness. But significant statistical difference was found betweengroup0%and4%. SEM observations showed the crystals formed an anisotropyand interlocking microstructure. The addition of ZrO₂had no effect on thephase composition of ceramic.Part two was to determine the mechanical properties of ZLDCC afterdental heat-pressing treatment. The effect of ZrO₂addition and heat-pressingtreatment on the properties of ZLDCC was also discussed. The compositeswere made from ELDC by adding zirconium oxide with different massfraction（0%，1%，2%，3%，4%）. After cold isostatic pressing and crystallizationheat treatment, all specimens were subjected to dental heat-pressing treatment.After that, the flexural strength and fracture toughness of the specimens were measured and the microstructure was evaluated by SEM. Crystallinecomposition of ZLDCC was also determined using XRD. The results showedthat the fracture toughness of ZLDCC ceramics increased as ZrO₂contentincreased. On the contrast, the flexural strength of composite ceramic decreasedas ZrO₂content increased. The highest fracture toughness of ZLDCC wasrecorded for specimens with4%ZrO₂. Significant statistical difference wasfound among groups. The phase composition of ZLDCC did not change afterheat-pressing, in which lithium silicate presented as the main phase. In addition,a small amount of lithium phosphate crystal was observed. The SEMobservations showed that the crystal size grew after heat-pressing procedureand the crystal showed some kind of orientation following the pressingdirection.Part three was to evaluate the effect of zirconium on the translucency ofZLDCC. The powder of ZLDCC with different ZrO₂contents （0%，1%，2%，3%，4%） were prepared and heat treated. Half specimens of each group weresubjected to heat-pressing procedure. L^*a^*b^*values of all specimens weremeasured using a spectrophotometer and TP values were calculated. The resultsshow that the TP values of ZLDCC ceramic firstly increased as the ZrO₂content increased to1%, and after that value decreased. Therefore, thetranslucency of ZLDCC ceramic could be improved by the addition of ZrO₂with certain content.In this study, the feasibility of improving the mechanical properties ofELDC by adding ZrO₂was verified. The phase transformation of zirconiacrystal can be used to improve the fracture toughness of ZLDCC. The addition of zirconia did not affect the crystal phase composition of glass ceramic. Thetranslucency of ELDC can be improved within a certain content of zirconiaaddition.