Effect of differences in coefficient of thermal expansion of veneer and Y-TZP ceramics on interface phase transformation

Abstract

Statement of problem: The stability of veneering ceramics requires further investigation.

Purpose: The purpose of this study was to investigate, with Raman spectroscopy, the occurrence and extent of the tetragonal to monoclinic phase transformation in yttria partially stabilized tetragonal polycrystalline zirconia (Y-TZP) ceramic at the veneer-framework interface.

Materials and methods: Three different Y-TZP blanks Cercon base, ZENO TEC, and Zerion were used. The specimens were sintered according to the recommendations of the manufacturers (1350°C Cercon base, 1450°C ZENO TEC, and 1500°C Zerion for 2 hours). Three veneer ceramics with different coefficients of thermal expansion, Cercon ceram kiss, Zirox, and VITA VM9 were used to veneer the Y-TZP frameworks. For the investigation of the temperature gradient on the phase transformation of Y-TZP ceramic, some specimens were fractured, with a universal testing machine along the long axis of the coping.

Results: No evidence was found for the presence of the tetragonal to monoclinic phase transformation for nonabraded and nonveneered specimens. The tetragonal to monoclinic phase transformation was observed at the veneer-framework interface for all veneered specimens and was dependent on the sintering temperature of the framework. The highest volume fractions of the monoclinic phase at the veneer-framework interface amounted to 0.57 (Cercon base), 0.69 (ZENO TEC), and 0.72 for the Zerion framework. The tetragonal to monoclinic phase transformation was not homogenous along the veneer-framework interface and depended on the distance from the cross section of the veneer-framework interface. The greatest tetragonal to monoclinic transformation was observed at the interface and disappeared with increasing distance from it. For the veneered Cercon base specimens, the volume fraction of the monoclinic phase decreased from 0.53 to 0.13 over a distance of 20 μm. No phase transformation was observed at a distance that exceeded 20 μm from the cross section of the veneer-framework interface.

Conclusion: This study demonstrated that the firing process of veneer ceramic and the difference in the coefficients of thermal expansion between the ceramic framework and the veneer ceramic significantly influenced the stability of the tetragonal phase of Y-TZP ceramic at the veneer-framework interface.