Impact of different pretreatments and aging procedures on the flexural strength and phase structure of zirconia ceramics

Abstract

OBJECTIVE
To test the impact of zirconia pretreatment and aging on flexural strength and phase structure.
METHODS
For flexural strength measurements, 180 3Y-TZP$_{0.25}$ specimens were fabricated and pretreated: (i) air-abraded (105-μm alumina, 0.25MPa), (ii) air-abraded (50-μm alumina, 0.25MPa), (iii) air-abraded (30-μm silica-coated alumina, 0.28MPa) (iv) non-pretreated. Each pretreated group (n=15) was aged: (a) hydrothermal (134°C, 0.23MPa, 2h) (b) in a mastication simulator (1,200,000×, 5/55°C) and (c) not aged. The fractured specimens were stored dry for 5 years (23°C) for analysis of phase transformation. Additionally, specimens were fabricated from 3Y-TZP$_{0.25}$ (n=12) and 3Y-TZP$_{0.05}$ (n=8), pretreated (i, ii, iii, iv), and hydrothermally aged. Each air-abrasion method was alternated using 0.05, 0.25 and 0.4MPa pressure. The phase transformation was examined by Raman spectroscopy and surface topography by scanning electron microscope. Data were analyzed using univariate ANOVA with the Scheffé post hoc test and partial-eta-squared (ƞ$_{p}$²) (α=0.05).
RESULTS
The highest impact on flexural strength was exerted by the pretreatment (η$_{P}$²=0.261, p<0.001), followed by interactions between pretreatment and aging (η$_{P}$²=0.077, p=0.033). Non-pretreated and non-aged specimens showed the lowest monoclinic percentage. Hydrothermal aging and 5 years of storage at room temperature increased the monolithic percentage of 3Y-TZP$_{0.25}$. The highest phase transformation was observed in groups air-abraded with 105-μm alumina particles. Increasing pressure during the air-abrading process increased the content of the monoclinic phase in zirconia surfaces.
SIGNIFICANCE
Air-abrasion with 30-μm silica-coated alumina powder can be recommended for pretreatment of 3Y-TZP$_{0.25}$ and 3Y-TZP$_{0.05}$. For air-abrasion using alumina powder lower pressure should be used.