Aging of 3Y-TZP dental zirconia and yttrium depletion
Introduction
To date, stabilized zirconia is a well-established biomaterial used in dentistry. Zirconia used in dental applications is usually stabilized with 3 mol% Y2O3 (3Y-TZP) [1], [2], [3]. The use of Y2O3 was first suggested with the addition of 2–3mol% Y2O3 resulting in materials with only the desired tetragonal crystallographic phase (t-ZrO2), called tetragonal zirconia polycrystals (TZP) [4]. The 3Y-TZP zirconia presents advanced mechanical properties as well as chemical, thermal and dimensional stability, compared to other ceramic systems.
Under stress, a spontaneous transformation takes place, from the high-energy state of the t-ZrO2, which is present at temperatures between 1170 °C and 2370 °C, to the lowest energy state of the monoclinic (m-ZrO2) phase, present at room temperature. This t → m transformation, known as a “transformation toughening mechanism” [5], is followed by a 3–4% volume expansion as a result of the larger volume occupied by the monoclinic phase, compared to the tetragonal. This expansion is considered to protect the zirconia ceramics from failure by restricting the growth and propagation of any surface microcracks inside the bulk of the material [5], [6].
The crucial complication of Y-TZP ceramics occurs when the transformation proceeds in the presence of moisture and is known as aging or low-thermal degradation, (LTD) first reported by Kobayashi et al. [7]. The transformation is initiated at isolated grains on the surface by a stress corrosion type mechanism and when the entire surface is degraded, the degradation develops into the bulk of the material by micro and macro cracking [8], reducing its mechanical properties [9].
Up to date, investigations have attempted to explain this phenomenon and the following mechanisms have been proposed. Chevalier and his co-authors in 2009 suggested that O2− from the segregation of water plays the main role for the filling of oxygen vacancies present in the structure and leads to the destabilization of the tetragonal zirconia [10]. Sato and Shimada [11] and Yoshimura et al. [12] also proposed that water reacts with the ZrO bond and due to the movement of OH at the surface and in the lattice, YOH and ZrOH bonds are formed that trigger the t → m transformation. Lange et al. [13] postulated that water reacts with Y2O3 to form clusters of small (20–50 nm) α-Y(OH)3 crystallites and induce depletion of the stabilizer. It was assumed that this reaction reproduces a monoclinic core (depleted of Y2O3) on the surface of an exposed tetragonal grain. Finally, they concluded that the above mechanism is promoted firstly because Y(OH)3 is more resistant than Y2O3 in a hydrothermal environment and secondly because yttrium is less active in the degraded zirconia structure [13].
The understanding of the aging process and its influence is crucial for the future of zirconia as an acceptable dental material. Therefore, the present was performed to investigate the surface alterations on a 3Y-TZP dental ceramic, during the process of in-vitro aging, and to further explore the yttrium depletion mechanism that occurs upon interaction with water. It was hypothesized that the in-vitro aging of of 3Y-TZP zirconia specimens for 5 h and 10 h results in significant yttrium depletion from the superficial layers of the material’s structure that facilitates the t → m transformation.
Section snippets
Manufacturing of the specimens
A total of 15 specimens were utilized derived from a 3Y-TZP zirconia ceramic (IPS e-max ZirCAD, Ivoclar-Vivadent). Specimens with final dimensions (after sintering) 8 × 2 mm were prepared from zirconia blocks via CAD/CAM technology. Sintering was performed using the Vita zircomat T furnace, according to the manufacturer’s instructions.
Aging of zirconia specimens
For the aging of the zirconia specimens, the model proposed by [10] was utilized, with aging time set at 5 and 10 h in an autoclave (KavoKlave 2100, KavoDental,
X-ray diffraction analysis (XRD)
X-ray diffraction patterns for all groups are presented in Fig. 1. The untreated materials presented the characteristic reflections of the t-ZrO2 and no reflection attributed to the m-ZrO2. On the contrary, the XRD patterns of the 5 h and 10 h aged specimens presented the characteristic peaks of the m-ZrO2. The monoclinic content was calculated as 12.7% after 5 h of aging and as 22.6% after 10 h of aging.
The characteristics of the unit cells of the specimens before and after aging are presented in
Discussion
The LTD phenomenon of the 3Y-TZP zirconia ceramics is still a matter of debate, as of what mechanism governs the aging process. The results of the present study clearly suggest that yttrium is removed from the lattice and that yttrium depletion takes place during the t → m transformation through aging in water vapor, confirming the research hypothesis. EDS analysis, the gradual reduction in luminescence as a result of oxygen vacancies reduction, as well as the Y/Zr % atomic concentration ratio
Acknowledgments
There have been no financial supports for this work and the whole financing was made by own resources. The authors wish to acknowledge Prof C. Dendrinou-Samara from the Laboratory of Inorganic Chemistry in Aristotle University of Thessaloniki for the courtesy of the fluorescence microscope and the PhD candidate Miss K. Giannousi for her assistance in obtaining the fluorescence images.
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