REV.CHIM.(Bucharest)♦70♦No. 3 ♦2019 http://www.revistadechimie.ro 863 Studies About Degradation of Zirconia in Artificial Saliva SABINA DAVID 1 , IOAN SARBU 1 , MIHAI COSMIN COTRUT 2 , RALUCA MONICA COMANEANU 3 *, MIREL STOIAN 3 , DAN NICOLAE PATROI 3 1 University of Medicine and Pharmacy Carol Davila, Faculty of Dental Medicine, 17-23 Calea Plevnei, 010221, Bucharest, Romania 2 Politehnica University of Bucharest, Materials Science and Engineering Faculty, 313 Splaiul Independentei, 060042, Bucharest, Romania 3 Titu Maiorescu University of Bucharest, Faculty of Dental Medicine, 67A Ghe. Petrascu, 031593, Bucharest, Romania To study zirconia degradation, we used samples with 16.5 mm diameter and 3 mm thick, milled from IPS e.max Zir CAD disks and subjected to sintering. Samples were immersed in Fusayama Meyer artificial saliva, removed after one day, 3 days, 7 days, 14 days and 21 days and examined from the point of view of surface morphology and elemental composition with an electronic scanning microscope. By SEM examining of samples prior to immersion in artificial saliva we found that the material contained strictly the chemical elements specified by the manufacturer. SEM examination and mass evaluations performed after immersion in artificial saliva demonstrate that the material did not suffer degradation processes, the minimum differences in the measurements being within the analytical balance weighing range. Keywords: zirconia, artificial saliva, degradation * email: monica_tarcolea@yahoo.co.uk All authors have equal contribution to the study and the publication The low-temperature degradation of zirconia, also known as aging, is produced by a slow surface transformation of metastable tetragonal crystals in a stable monoclinic structure in the presence of water or vapors of water. [1] A certain degree of transformation in this sense is beneficial for improving the physical properties of the material. However, there is a narrow boundary between improving and destroying mechanical properties, because accelerating the aging process leads to deterioration of properties [2]. The transformation initially starts with a corrosion mechanism, inside the granules on the surface of the material, by the isolated increase in granule volume, pushing neighboring granules, producing microfishers that allow penetration of water and phase destabilization [3]. Experimental observations have shown that degradation occurs most rapidly at temperatures between 200 and 300 °C and is time dependent [1]. The aging process depends on a number of features of the microstructure, such as [1]: - porosity, - residual stress, - size of the granules - the stabilizer content of the processed material. The critical granule’s size reported in the literature [1] is between 0.2 and 1 µm, depending on the Y 2 O 3 content. The presence of granules larger than 1 µm suffers a more pronounced transformation from the tetragonal to the monoclinic phase, associated with a significant decrease in resistance, while the granules smaller than 0.4 µm did not show a significant change in phase content or resistance [1]. It has also been found that at a granule size below 0.2 µm, tetragonal-monoclinic transformation is not possible which can lead to a reduction in tear strength [1]. An in vitro study [4] which investigating the effect of aging on zirconia used in oral rehabilitation has found that, although the aging process reduces the mechanical characteristics of zirconia, the decrease occurs within clinically acceptable values. To assess the degree of degradation of ceramics in different solutions, it is not possible to use electrochemical techniques, it is preferable to immerse the samples under similar conditions to the environment in which the materials are used. Experimental part To study zirconia degradation, we used samples with 16.5 mm diameter and 3 mm thick, milled from IPS e.max Zir CAD disks and subjected to sintering. Samples were immersed in Fusayama Meyer artificial saliva (0.4 gl -1 NaCl, 0.9 gl -1 KCl, 1 gl -1 uree, 0.69 gl -1 NaH 2 PO 4 , 0.795 gl -1 CaCl*2H 2 O) at pH 5.2 maintained at 37 ± 0.1 °C by a Memmert IF55 incubator. Zirconia samples were examined before immersion in artificial saliva from the point of view of surface morphology and elemental composition with an electronic scanning microscope (SEM) equipped with an energy dispersion spectrometer - EDS (Phenom ProX model, manufacturer from PhenomWorld, Netherlands). Before the immersion tests were performed, the samples were sonicated for 20 min in acetone to be degreased and cleaned with impurities and then rinsed with ultrapure water (ASTM I type). We used 5 discs to track their mass in pre-established time ranges: one day, 3 days, 7 days, 14 days and 21 days. For this purpose we used a Kern ALT 100-5AM balance with an accuracy of 0.01 mg. Samples were removed from artificial saliva at predetermined ranges, cleaned with deionized water, dried for one hour in a Memmert UF55 oven, and then stored 72 Fig. 1.Experimental samples - zirconia discs