1、Harry P. Papanagiotou,Steven M. Morgano, Russell A. GiordanoIn vitro evaluation of low-temperature aging effects and finishing procedureson the flexural strength and structural stability of Y-TZP dental ceramics.J Prosthet Dent 2006;96:154-64.
Abstract：Statement of problem. Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic material has been reported to possess exceptionally high flexural strength; however, the strength of Y-TZP ceramics can be affected by surface treatments performed by dental laboratory technicians and by intraoral conditions.Purpose. This study investigated the influence of low-temperature-degradation (LTD) treatment, airborneparticle abrasion, and polishing on the flexural strength and structural stability of a Y-TZP ceramic material.Material and methods. By sectioning zirconia ceramic blocks (Vita In-Ceram YZ blocks) into 253432-mm bars, 310 specimens were fabricated and divided into 9 groups: Group C, as-sintered (control); group B24h, boiled in water for 24 hours; group B7d, boiled in water for 7 days; group H6h, stored in humidified air at
2508C for 6 hours; group H24h, stored in humidified air at 2508C for 24 hours; group H7d, stored in humidified air at 2508C for 7 days; group P, polished; group A, airborne-particle abraded; and group AB, airborne-particle abraded and boiled for 7 days. The flexural strengths (MPa) of all specimens were determined by using a 3-point bending test. The variability was analyzed by using Weibull statistics. Pairwise differences among the 9 groups were evaluated by using confidence intervals (95%) for scale and shape parameters. From every block used, 8 specimens were obtained and every specimen was marked according to the location of its origin from the original block. Surfaces of the specimens were evaluated by using scanning electron microscopy (SEM) at various magnifications. X-ray diffraction analysis was performed to identify possible tetragonal-to-monoclinic phase transformations. Energy dispersive spectroscopy (EDS) was used to obtain information on the chemical composition. Results. Aging or ‘‘finishing’’ treatments had no significant negative effects on flexural strengths. Mean flexural strengths ranged from 796.7 to 950.2 MPa. Group A exhibited the highest mean flexural strength (950.2 MPa). Weibull modulus values ranged between 5.6 and 9.3. Tetragonal-to-monoclinic phase transformation was detected for a specimen boiled for 24 hours. More tetragonal-to-monoclinic phase transformation was detected for specimens boiled for 7 days and stored in humidified air at 2508C for 7 days. The SEM examination of fractured surfaces revealed sintering defects, and EDS analysis showed less yttria concentration on Y-TZP specimens that were boiled for 7 days. Conclusion. Within the limitations of this study, the results suggested that the LTD procedures and the polishing treatment used did not reduce the flexural strengths of zirconia bars. Airborne-particle abrasion increased the flexural strength of specimens.

2.Andr´e R. Studart∗, Frank Filser, Peter Kocher, Ludwig J. Gauckler. Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges .dental materials 2 3 ( 2 0 0 7 ) 106–114
Abstrct:Objectives. The aim of this study was to evaluate the cyclic fatigue behavior of zirconia (3YTZP) in water and derive guidelines for the design of zirconia-based dental bridges with extended lifetime. Methods. The subcritical crack growth parameters under aqueous and cyclic loading conditions were determined from Weibull distributions of the initial mechanical strength and of the lifetime of TZP specimens. The strength and lifetime datawere obtained using a specially designed bending machine under simple and oscillatory loading conditions, respectively. Results. The TZP components submitted to cyclic loading inwater exhibited subcritical crack propagation at stress levels significantly (∼50%) lower than the critical stress intensity factor (KIC = 5.6MPam1/2). In spite of this susceptibility to subcritical crack growth, calculations based on the fatigue parameters and on the stress applied on the prosthesis indicate that posterior bridges with zirconia frameworks can exhibit lifetimes longer than 20 years if the
diameter of the bridge connector is properly designed. Significance. This in vitro study indicates that partially stabilized zirconia (3Y-TZP) can withstand the severe cyclic loading and wet conditions typically applied in the molar region of the mouth and is therefore an appropriate material for the fabrication of all-ceramic multiunit posterior bridges.

3.A.R. Studart, F. Filser, P. Kocher, L.J. Gauckler. In vitro lifetime of dental ceramics under cyclic loading in water.Biomaterials 28 (2007) 2695–2705
Abstract:All-ceramic dental restorations exhibit enhanced esthetics and biocompatibility as compared to traditional metal-based prosthesis. However, long-term fatigue and subcritical crack growth in the presence of water and cyclic loading can decrease the strength of ceramic components over time. We investigated the cyclic fatigue in water of three dental materials currently used as frameworks in all-ceramic restorations: a 3 mol%-yttria partially stabilized zirconia (3Y-TZP, Cercon, Degudent GmbH), a Al2O3–ZrO2–Glass composite (Inceram-Zirconia, Vita Zahnfabrik GmbH) and a Li2O 2SiO2 glass ceramic (Empress 2, Ivoclar Vivadent AG). Fatigue and fast fracture tests were performed to determine the Weibull distribution of lifetime and initial mechanical strength for each framework component. In spite of its noticeable susceptibility to fatigue in water, the 3Y-TZP material was found to be particularly suitable for the preparation of posterior all-ceramic bridges due to its high initial mechanical strength. Guidelines are provided for the selection of materials and the design of all-ceramic posterior bridges exhibiting lifetime longer than 20 years under severe wet and cyclic loading
conditions.

4.Berit I Ardlin.Transformation-toughened zirconia for dental inlays,crowns and bridges:chemical stability and effect of low-temperature aging on flecural strength and surface structure. Dental materials 18(2002)590-595
Abstract:OBJECTIVES: One concern, for transformation-toughened zirconia (Y-TZP) is their liability to low-temperature aging with accompanying alterations of properties such as strength. The loss in strength is attributed to the transformation of tetragonal grains to monoclinic. The transformation is related to loading of the dental inlays, crowns and bridges (reconstructions), temperature and time of exposure to surrounding media (aging) and the manufacturing process of Y-TZP. The purpose of this study was to determine chemical stability and effect of aging (4% acetic acid at 80 degrees C for 168 h) on flexural strength, surface and crystalline structures of two shades, P0 and P17, of a Y-TZP ceramic used for dental reconstructions. The hypotheses to be tested were that both shades of the dental Y-TZP ceramic have high flexural strength and chemical stability compared to other dental ceramics, and that the strength, surface and crystal structures of the ceramic were not affected by aging. METHODS: Forty specimens of Y-TZP, 20 of the shade P0 and 20 of the shade P17 were ground and polished. Ten specimens of each shade were exposed to low-temperature aging. The flexural strength of all 40 specimens was registered. Surfaces of the specimens were evaluated by using scanning electron microscopy (SEM), X-ray diffractometry and roughness recorder. The chemical solubility in 4% acetic acid was recorded by weight loss, and SEM was used to evaluate the surfaces of Y-TZP and dental feldspathic porcelain samples immersed in 8% SnF. Wilcoxon Signed Rank test was used for statistical analysis. RESULTS: As expected, the two shades, P0 and P17, of the studied dental Y-TZP had high strengths that were not affected by aging, and high chemical stability in the tested solutions. Contrary to what was assumed the crystal and surface structures of P0 and P17 were affected. Transformation from tetragonal to monoclinic structures occurred and small elevations on the ceramic surfaces were observed after aging. SIGNIFICANCE: The transformed-toughened Y-TZP ceramic used for dental inlays, crowns and bridges has high flexural strength compared to other dental ceramics and is chemically stable. The chemical solubility is well below the limit values of ISO 6872:1995 for dental ceramics