The influence of simulated masticatory loading regimes on the bi-axial flexure strength and reliability of a Y-TZP dental ceramic

Summary 

Objectives

The purpose of the current study was to examine the influence of simulated masticatory loading regimes, to which all-ceramic crown or bridge restorations will routinely be subjected during their service-life, on the performance of a yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) dental ceramic.

Methods

Ten sets of 30 Y-TZP ceramic discs (13mm diameter, 1.48–1.54mm thickness) supplied by the manufacturer were randomly selected. Six groups were loaded for 2000cycles at 500N (383–420MPa), 700N (536–588MPa) and 800N (613–672MPa) with three groups maintained dry and the remaining three groups loaded while immersed in water at 37±1°C. A further two groups underwent extended simulated masticatory loading regimes at 80N (61–67MPa) for 10⁴ and 10⁵cycles under dry conditions. The mean bi-axial flexure strengths, standard deviations and associated Weibull moduli (m) were determined. The surface hardness was also determined using the Vickers hardness indentation technique.

Results

No significant difference (P>0.05) was identified in the bi-axial flexure strength of the simulated masticatory loading regimes and the control specimens loaded dry or wet. A significant increase in m was identified for the Y-TZP specimens following loading while immersed in water (8.6±1.6, 8.5±1.6 and 10.3±1.9) compared with the control (7.1±1.3). However, the extended loading regime to 10⁵cycles resulted in a significant reduction in the m of the Y-TZP specimens (5.3±1.0) compared with the control. Localised areas of increased surface hardness were identified to occur directly beneath the spherical indenter.

Conclusions

The occurrence of localised areas of increased surface hardness could be the result of either a transformation toughening mechanism or crushing and densification of the material beneath the indentor manifested as the formation of a surface layer of compressive stresses that counteracted the tensile field generated at the tip of a propagating crack which increased the Weibull modulus of the Y-TZP specimens. The reduced reliability of the Y-TZP specimens loaded to 80N for 10⁵cycles was associated with the accumulation of subcritical damage as a result of the extended nature of loading.

Keywords: Y-TZP, Zirconia, CAD/CAM, Weibull analysis, Bi-axial flexure strength, Simulated masticatory loading, Vickers hardness