Abstract:

BACKGROUND: Because dental zirconia restorations require two layers ceramic material, chipping and delamination of veneering porocelain always occur in clinical application, multiple porcelain firing or heat treatment is always needed to adjust the shape and color, however, whether repeated firing could have some effects on the strength of zirconia/porcelain bilayered structure remains unclear.
OBJECTIVE: To study the mechanical property of zirconia/porcelain bilayered structure after repeated firing and to observe the microstructure of veneering porcelain and bonding interface of the bilayered complex.
METHODS: Bilayered three-point bending strength specimens of KAVO zirconia/Vita VM9 veneering porcelain were made according to ISO6872 standard, the thickness ratio of core to porcelain was 1:1. Specimens were subjected to 2, 4, 6, 8 times of dentine porcelain firing cycles. Three-point bending strength were tested, scanning electron microscopy was used to observe the microstructure characteristics of the interface and porcelain matrix. X-ray diffraction was used to analyze the crystal structure of veneering porcelain, and coefficient of thermal expansion was detected by thermal analyzer.
RESULTS AND CONCLUSION: Zirconia surface combined tightly with veneering porcelain interface after multiple firing up to 8 times, the crystal structure of leucite significantly changed at the interface and in the veneering porcelain, the porcelain but looked more porous and pores transferred to the veneer surface with the increasing firing times while the size of crystal phase were increased significantly, transferring to surface layer. The coefficient of thermal expansion was slightly increased, and then decreased at the eighth firing. The bonding strength of KAVO zirconia/Vita VM9 veneering porcelain was nearly identical after multiple dentine firing. There were significant morphological structure changes on the crystal phase in the veneering porcelain after multiple firing, however, three-point bending strength of zirconia/porcelain bilayered structure was not influenced.