Effects of multiple firings on the low-temperature degradation of dental yttria-stabilized tetragonal zirconia

Statement of problem The clinical failure of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) substrates may be attributed to their susceptibility to low-temperature degradation (LTD). The acceleration of LTD by multiple veneering firings of Y-TZP remains poorly understood. Purpose The purpose of this in vitro study was to evaluate the effects of different numbers of veneering firing cycles on the LTD of Y-TZP. Material and methods Ninety bar-shaped specimens were cut from a machinable Y-TZP block and fully sintered. The Y-TZP bars were divided into 3 groups; each group received 1, 3, or 5 veneering porcelain firing cycles. Specimens from each firing cycle (n=10) were then subjected to hydrothermal aging in an autoclave sterilizer for 10 or 20 hours to accelerate the LTD of Y-TZP. Specimens without hydrothermal aging served as the control. XRD was used to characterize phase transformations. The flexural strength and surface Vickers hardness of the 9 subgroups were investigated, and scanning electron microscopy was used to characterize the fractured surface morphology. Results Hydrothermal aging increased the monoclinic phase, although increased hydrothermal aging time provided no additional effects. Veneering firings did not enhance the influence of hydrothermal aging on phase transformation. No statistical significance was found among the 9 groups in terms of flexural strength or surface Vickers hardness (P>.05). Conclusions Additional veneering porcelain firing cycles do not accelerate the LTD of Y-TZP.