INTRODUCTION
It’s always a challenge of developing better materials to replace our damaged teeth, which are the strongest tissue in our body. Glass ceramic (GC) material is one of the dominant materials which have been used for dental restoration.[1] One of the well-known glass-ceramics is lithium disilicate based GC, which has balanced strength and translucency, but the strength should be improved.[1-2] Zirconia full ceramics have better flexural strength and toughness, but the translucency is that promising.[1-2] The question is if we can find a material that could combine the translucency of GC and the strength of zirconia ceramics. Our previous study has shown a new GC with zirconia in silica matrix, which has good translucency and high flexural strength and toughness.[3-4] It seems a good combination of GC and ceramics for dental applications. In this study, we tried to find the effect of zirconia concentration and microstructure on the appearance and aiming mechanical strength.
 
METHODS
The GC raw powders were synthesized by a sol-gel process. All GC samples have been sintered by SPS. The crystal phase was analyzed by XRD. The flexural strength was analyzed by a universal mechanical tester. The toughness was tested by the indentation method. SEM was used to observe the surface and cross-section. The microstructure was done by TEM. A TEM tomography was done to study its 3D structure. APT (Atomic probe tomography) was used to analyze the chemical compositions and atomic structure at a 3D level.
 
RESULTS AND DISCUSSION
The content of zirconia in the silica matrix was studied. It varied from 30% to 80% in molar ratio.  We could see that the flexural strength and toughness of the GCs could be enhanced when the zirconia content increased, but not always. The results showed 65% of zirconia gave the highest values. The translucency of GCs was good, and increased when the zirconia content decreased. The growth of zirconia crystal size and the phase compositions have been studied when the zirconia content and the heat treatment changed. The phase transformation is size-driven. 3D TEM tomography showed zirconia nanocrystals aligned in a certain area and formed nanofibers on a 3D scale. APT analysis reveals the distribution of yttrium and the chemical composition of the zirconia/silica heterophase interface in the Y doped GCs. A special core−shell structure, with a thin Zr/Si interfacial layer as a shell and a ZrO2 solid solution as a core, was confirmed.
 
CONCLUSION
The zirconia concentration and the microstructure of the zirconia/silica GCs determine the translucency and mechanical strength. There is a threshold of zirconia concentration regarding the mechanical strength. The regionally ordered structure contributes to the excellent combination of translucency and flexural strength of the GCs.
 
 
REFERENCES
Fu L. et al, Materials, 13: 1049, 2020
Zarone F. et al, BMC Oral Health, 19: 134, 2019
Fu, L. et al, J. Eur. Ceram. Soc., 37: 4067–4081, 2017
Fu, L. et al, J. Eur. Ceram. Soc. 36: 3487–3494, 2016