1

Introduction

Zirconia ceramics are increasingly used in dental restorations as they attain good esthetics, excellent biocompatibility and low plaque accumulation combined with remarkable strength properties, higher than many other ceramic materials. For dental applications, tetragonal zirconia polycrystals are commonly stabilized with 3 mol% yttria (3Y-TZP) . The stabilization of the tetragonal crystallographic phase at room temperature is the reason for the high strength of Y-TZP zirconia dental ceramics. However transformation of the tetragonal phase ( t ), which is its high-energy state, to the monoclinic ( m ) phase that is its lowest energy state (‘natural’ equilibrium state) tends to spontaneously take place in regions of concentrated stress, in particular at flaws at the surface or within the lattice. This t → m transformation is associated with a net 3–4% volume expansion due to the larger volume occupied by the monoclinic phase compared to the tetragonal . This positive change in volume can cause the closure of any surface micorcracks restricting them to propagate into the bulk of the material, thus it has been reported as a toughening mechanism that protects zirconia ceramics from failure. However, a severe transformation to the monoclinic phase could generate stresses that can cause surface uplifts and fragmentation of the material .

The main problem associated to the Y-TZP zirconia ceramics is their sensitivity to low temperature degradation (LTD) . Kobayashi et al. were the first to observe this degradation and reported on the aging phenomenon of zirconia ceramics at 150–400 °C. Low temperature aging occurs within the temperature range 65–500 °C, with the maximum rate at 250 °C and is related to a surface transformation of the metastable t -phase to the stable m -phase, especially in the presence of water or water vapor . Transformation initiates at isolated grains on the surface by a stress corrosion type mechanism and once the whole surface is degraded, the degradation propagates into the bulk material through micro- and macro-cracking causing the diminishing of the mechanical properties. The interaction of water with zirconia that leads to the formation of Zr OH bonds due to oxygen vacancy occupation has been proposed as a mechanism for the t → m transformation. This generates tensile hydrostatic stresses in the grains and modifies the oxygen configuration around the Zr ions, which lead to destabilization of the tetragonal phase. According to Chevalier et al. “the large shear strains and displacements accompanying the transformation can also create cracks along the grain boundaries that in turn allow the moisture to penetrate further into the material and the process is repeated as moisture ingress continues”. Although this mechanism is very slow at oral temperatures other contributing factors such as constant humidity resulting from saliva, temperature and pH changes due to various drinks and repeated high occlusal loads due to mastication, can accelerate the aging process, and reduce the mechanical properties of the materials . A second mechanism, the destabilization mechanism, is based on the reaction of OH ⁻ ions with yttria, leading to the formation of Y(OH) ₃ or Y(O)OH. Reduction of the yttrium content leads to the destabilization of the t ZrO ₂ and its transformation to the m -phase .

Aging environments used in the studies of low-temperature degradation in zirconia ceramics vary from the relatively mild, aging in humid air (for periods in excess of 3 years) , to the much more aggressive autoclave conditions . The kinetics of the moisture-induced transformation , seem to be well fitted with the standard Mehl–Avrami–Johnson equations for the nucleation and growth process. Chevalier stated that aging of zirconia with 1 h of autoclave treatment at 134 °C and 2 bar pressure results in a significant t → m transformation that has theoretically the same effect as 3–4 years in vivo . Based on these calculations, the ISO standard (13356:2008) imposes that the wt.% of the monoclinic phase should not exceed the maximum of 25% after an accelerated test conducted for 5 h at 134 °C and 2 bar pressure . Although currently accelerated tests are the only basis for extrapolating an estimation of the transformation rate and, hence, of the product lifetime, a substantial controversy concerning the strict correlation of the in vitro accelerating test duration to a specific product lifetime has been recently arisen . Lughi and Sergo recently reported that no safe lifetime predictions can be made on the reliability of Y-TZP ceramics at low temperature degradation from accelerated tests and that in vivo predictions based on the calculated activation energy are too risky, as actual clinical conditions may be even worse due to the additional effect of stress. However the performance of zirconia ceramics after accelerating tests may reveal useful information about the microstructure and the properties of a variety of Y-TZP ceramics that may affect their prognosis. Gaillard et al. found that hydrothermal degradation at 131 °C in water vapor for 1–60 h induced t → m phase transformation and microcracking under the surface that were associated with a decrease in hardness and Young’s modulus of the degraded surface.

The reliability of strength of zirconia ceramics has been discussed in terms of probability of failure and Weibull modulus ( m ), parameters of Weibull statistical analysis suggested in ISO 6872 . The Weibull analysis interprets with higher precision the flexural strength values of brittle materials received through a three or four point bending test . A higher Weibull modulus corresponds to a more homogeneous flaw distribution and suggests that the defects inside the material are uniform and evenly distributed throughout the entire volume, resulting in higher structural reliability and lower failure probability .

The aim of the present work was to investigate the effect of in vitro aging on the flexural strength of two cold isostatic pressed zirconia core ceramics for all-ceramic restorations. The null hypothesis was that the accelerated in vitro aging of 5 and 10 h would not significantly affect their flexural strength. The second hypothesis was that there would be no difference in the flexural strength between the two ceramics either before or after 5 and 10 h aging.

2

Materials and methods

Two commercially available zirconia ceramics (IPS e.max ^® ZirCAD (Ivoclar-Vivaden, Schaan, Liechtenstein)) and (ZENO Zr (Wieland Dental + Technik GmbH & Co. KG, Germany)), were used in this study. A total of 120 bar-shaped specimens (25 mm × 4 mm × 2 mm) were milled from different zirconia ceramic blocks using the Zeno ^® Tec System. After sintering to full density according to manufacturer’s instructions, one third of the specimens (20 from each ceramic) were used as controls, while from the remaining 80 ones, 40 were aged in vitro for 5 h (20 from each ceramic) and 40 for 10 h (20 from each ceramic) at 121 °C and 2 bar in an autoclave (Kavo autoclave sterilizer, KavoDental, Germany). The experimental groups and the associated abbreviations are presented in Table 1 . Three randomly selected specimens from each group were mirror polished with diamond suspensions of decreasing size (30, 6, 3 μm) for FTIR, Raman and XRD analysis.