Comparison of subcritical growth parameters of a Y-TZP obtained via cyclic or dynamic fatigue tests.

OBJECTIVE: The objective of this study was to 1) compare the stress corrosion coefficient (n) of a Y-TZP obtained by two fatigue tests: cyclic and dynamic and 2) evaluate the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and slow crack growth. METHODS: A total of 145 Y-TZP specimens were produced in accordance with the manufacturer's instructions. These specimens, measuring 4.0 × 3.0 × 25.0 mm, were used for dynamic (n = 70) and cyclic fatigue tests (n = 75). The specimens were obtained from CAD/CAM blocks, sectioned, and sintered in a furnace at 1530 °C with a heating rate of 25 °C/min. They were tested in their "as-sintered" form without any additional surface treatment. The fatigue tests were conducted using a four-point bending to obtain the slow crack growth parameters (n). The cyclic fatigue test was also conducted in two frequencies (2 and 10 Hz), using stress levels between 350 and 600 MPa. Data from these tests were analyzed using ASTM C 1368-00 formulas and Weibull statistics. Scanning electron microscope (SEM) was used for fracture surface analysis to identify the origin of the fracture. Critical defect size was measured and used, along with flexural strength values, to estimate fracture toughness. Dynamic fatigue test data were used to obtain subcritical crack growth (SCG) parameters and perform Weibull statistical analysis. The cyclic fatigue data were used in the General Log-linear Model equation using the ALTA PRO software. Data were analyzed using one-way ANOVA followed by Tukey post-hoc tests and Student's t-test at a significance level of p ≤ 0.05. RESULTS: In the dynamic fatigue test, the values obtained for σfo and n were 667 and 54, respectively. This parameter indicates how the strength of the material diminishes over time due to internal cracks. The Weibull parameters obtained from the same test results were m = 7.9, σ0 = 968, 9 and σ5% = 767, which indicates the reliability of the material. The Weibull parameters obtained by cyclic fatigue were statistically similar for the two frequencies used, the m* was 0.17 (2 Hz) and 0.21 (10 Hz); characteristic lifetimes (η) were 1.93 × 106 and 40,768, respectively. The n values obtained by cyclic fatigue were 48 and 40 at frequencies of 2 and 10 Hz, respectively. There was no effect of the frequency, the stress level or the interaction of the two in the Y-TZP lifetime, when analysed by General Log Linear Model. SIGNIFICANCE: the n values obtained by cyclic and dynamic fatigue tests showed no statistically significant difference and the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and subcritical growth were not observed in the tested specimens.

Influence of piston material on the fatigue behavior of a glass-ceramic.

STATEMENT OF PROBLEM: The lack of standardization regarding the loading piston material used in fatigue tests could limit the interpretation of study findings. PURPOSE: The purpose of this in vitro study was to evaluate the effect of the piston material on the fatigue behavior of a lithium disilicate glass-ceramic. MATERIAL AND METHODS: Plate-shaped, 1.2-mm-thick, lithium disilicate glass-ceramic specimens were cemented onto a dentin analog substrate with resin cement. The specimens were divided into 4 groups according to the piston material used in the fatigue test (n=30): metal, glass fiber-reinforced epoxy resin, ceramic, and human tooth. The fatigue test was performed in a mechanical cycling machine by using the boundary technique at 2 Hz in distilled water at 37 °C. The fatigue data were analyzed by using the Weibull distribution and a lifetime-inverse power law relationship. Failures were evaluated with fractography and transillumination. RESULTS: The Weibull modulus (ß) was similar among groups. The exponent of crack growth (n) was significantly greater for glass fiber-reinforced epoxy resin and tooth groups than for metal and ceramic; therefore, the probability of failure (Pf) of glass-ceramic specimens loaded by resin and tooth pistons depended more on load amplitude. Specimens tested with tooth showed the highest value of K (characteristic lifetime), which is an indication of greater survival. Radial crack was the only failure mode observed for all experimental groups. CONCLUSIONS: The piston material influenced the fatigue survival of the lithium disilicate glass-ceramic. The glass fiber-reinforced epoxy resin piston closely simulated the fatigue behavior induced by the human tooth on the evaluated glass-ceramic.

Mechanical behavior and adhesive potential of glass fiber-reinforced resin-based composites for use as dentin analogues.

PURPOSE: To characterize experimental glass fiber-reinforced resin-based composites (GFIR-isophthalic; and GFOR-orthophthalic), evaluating their mechanical behavior and adhesive potential to ceramic in comparison to human dentin and a traditional glass fiber-reinforced epoxy resin (G10). METHODS: Density (ρ), elastic modulus (E), and Poisson's ratio (ν) were evaluated using 2 mm thick specimens from GFIR, GFOR, human dentin and G10. Biaxial flexural strength (δf), Knoop hardness and surface topography under scanning electron microscopy (SEM) were assessed for GFIR and GFOR specimens. G10 was also tested for δf. For the adhesive potential, ceramic specimens (n=10) bonded to GFIR, GFOR or human dentin were tested for microtensile bond strength (MTBS). Disc-shaped ceramics were cemented onto dentin, GFIR, GFOR and G10 (n=15) and loaded to failure. Data were statistically evaluated using Weibull, ANOVA, and Tukey's test (α=0.05). RESULTS: The experimental resins (GFIR and GFOR) showed similar values of HK (53.1 and 52.7 GPa), (ν (0.44 and 0.43) and δf (41.2 MPa and 40.7 MPa). Considering the human dentin values for ρ and E, the corresponding values obtained from GFIR, GFOR and G10 were different, with GFOR values being closer to dentin than GFIR and G10. G10 had statistically greater(δf than GFIR and GFOR. Mean bond strength of ceramic to GFIR, GFOR and dentin were statistically similar. The fracture load of resin-cemented ceramic was influenced by substrate. CLINICAL SIGNIFICANCE: The experimental materials (GFIR and GFOR) showed similar adhesion characteristics to human dentin, however GFOR showed a better potential to be used as a dentin analogue.

Step-stress analysis for predicting dental ceramic reliability.

OBJECTIVE: To test the hypothesis that step-stress analysis is effective to predict the reliability of an alumina-based dental ceramic (VITA In-Ceram AL blocks) subjected to a mechanical aging test. METHODS: Bar-shaped ceramic specimens were fabricated, polished to 1µm finish and divided into 3 groups (n=10): (1) step-stress accelerating test; (2) flexural strength-control; (3) flexural strength-mechanical aging. Specimens from group 1 were tested in an electromagnetic actuator (MTS Evolution) using a three-point flexure fixture (frequency: 2Hz; R=0.1) in 37°C water bath. Each specimen was subjected to an individual stress profile, and the number of cycles to failure was recorded. A cumulative damage model with an inverse power law lifetime-stress relation and Weibull lifetime distribution were used to fit the fatigue data. The data were used to predict the stress level and number of cycles for mechanical aging (group 3). Groups 2 and 3 were tested for three-point flexural strength (σ) in a universal testing machine with 1.0MPa/s stress rate, in 37°C water. Data were statistically analyzed using Mann-Whitney Rank Sum test. RESULTS: Step-stress data analysis showed that the profile most likely to weaken the specimens without causing fracture during aging (95% CI: 0-14% failures) was: 80MPa stress amplitude and 10(5) cycles. The median σ values (MPa) for groups 2 (493±54) and 3 (423±103) were statistically different (p=0.009). SIGNIFICANCE: The aging profile determined by step-stress analysis was effective to reduce alumina ceramic strength as predicted by the reliability estimate, confirming the study hypothesis.

Three-dimensional finite element modelling of all-ceramic restorations based on micro-CT.

OBJECTIVES: To describe and apply a method of modelling dental crowns and three-unit fixed partial dentures (FPD) for finite element analyses (FEA) from 3D images obtained using a micro-CT scanner. METHODS: A crown and a three-unit fixed partial denture (FPD) made of a ceramic framework (Y-TZP) and veneered with porcelain (VM9) were scanned using an X-ray micro-CT scanner with a pixel size of 6.97 µm. Slice images from both structures were generated at each 0.034 mm and processed by an interactive image control system (Mimics). Different masks of abutments, framework and veneer were extracted using thresholding and region growing tools based on X-ray image brightness and contrast. 3D objects of each model were incorporated into non-manifold assembly and meshed simultaneously. Volume meshes were exported to the FEA software (ABAQUS), and the load-generated stress distribution was analyzed. RESULTS: FEA models showed great shape resemblance with the structures. The use of non-manifold assembly ensured matching surfaces and coinciding nodes between different structural parts. For the crown model, tensile stresses were concentrated in the internal surface of the core, near to the applied load. For the FPD model, the highest tensile stresses were located in the framework, on the cervical area of connectors and pontic. CONCLUSIONS: Valid 3D models of dental crown and FPD can be generated by combining micro-CT scanning and Mimics software, emphasizing its importance as design tool in dental research. CLINICAL SIGNIFICANCE: The 3D FEA method described in this work is an important tool to predict the stress distribution, assisting on structural design of dental restorations.

Flexural strength and failure modes of layered ceramic structures.

OBJECTIVE: To evaluate the effect of the specimen design on the flexural strength (σ(f)) and failure mode of ceramic structures, testing the hypothesis that the ceramic material under tension controls the mechanical performance of the structure. METHODS: Three ceramics used as framework materials for fixed partial dentures (YZ--Vita In-Ceram YZ; IZ--Vita In-Ceram Zirconia; AL--Vita In-Ceram AL) and two veneering porcelains (VM7 and VM9) were studied. Bar-shaped specimens were produced in three different designs (n=10): monolithic, two layers (porcelain-framework) and three layers (TRI) (porcelain-framework-porcelain). Specimens were tested for three-point flexural strength at 1MPa/s in 37°C artificial saliva. For bi-layered design, the specimens were tested in both conditions: with porcelain (PT) or framework ceramic (FT) layer under tension. Fracture surfaces were analyzed using stereomicroscope and scanning electron microscopy (SEM). Young's modulus (E) and Poisson's ratio (ν) were determined using ultrasonic pulse-echo method. Results were statistically analyzed by Kruskal-Wallis and Student-Newman-Keuls tests. RESULTS: Except for VM7 and VM9, significant differences were observed for E values among the materials. YZ showed the highest ν value followed by IZ and AL. YZ presented the highest σ(f). There was no statistical difference in the σ(f) value between IZ and IZ-FT and between AL and AL-FT. σ(f) values for YZ-PT, IZ-PT, IZ-TRI, AL-PT, AL-TRI were similar to the results obtained for VM7 and VM9. Two types of fracture mode were identified: total and partial failure. SIGNIFICANCE: The mechanical performance of the specimens was determined by the material under tension during testing, confirming the study hypothesis.

Adaptation of all-ceramic fixed partial dentures.

OBJECTIVES: To measure the marginal and internal fit of three-unit fixed partial dentures (FPDs) using the micro-CT technique, testing the null hypothesis that there is no difference in the adaptation between the ceramic systems studied. METHODS: Stainless steel models of prepared abutments were fabricated to design the FPDs. Ten FPDs were produced from each framework ceramic (YZ - Vita In-Ceram YZ and IZ - Vita In-Ceram Zirconia) using CEREC inLab according to the manufacturer instructions. All FPDs were veneered using the recommended porcelain. Each FPD was seated on the original model and scanned using micro-CT. Files were processed using NRecon and CTAn software. Adobe Photoshop and Image J software were used to analyze the cross-sections images. Five measuring locations were used as follows: MG - marginal gap; CA - chamfer area; AW - axial wall; AOT - axio-occlusal transition area; OA - occlusal area. The horizontal marginal discrepancy (HMD) was evaluated in another set of images. Results were statistically analyzed using ANOVA and Tukey tests (α=0.05). RESULTS: The mean values for MG, CA, AW, OA and HMD were significantly different for all tested groups (p<0.05). IZ exhibited greater mean values than YZ for all measuring locations except for AW and AOT. OA showed the greatest mean gap values for both ceramic systems. MG and AW mean gap values were low for both systems. SIGNIFICANCE: The ceramic systems evaluated showed different levels of marginal and internal fit, rejecting the study hypothesis. Yet, both ceramic systems showed clinically acceptable marginal and internal fit.

Effect of the microstructure on the lifetime of dental ceramics.

OBJECTIVES: To evaluate the effect of the microstructure on the Weibull and slow crack growth (SCG) parameters and on the lifetime of three ceramics used as framework materials for fixed partial dentures (FPDs) (YZ - Vita In-Ceram YZ; IZ - Vita In-Ceram Zirconia; AL - Vita In-Ceram AL) and of two veneering porcelains (VM7 and VM9). METHODS: Bar-shaped specimens were fabricated according to the manufacturer's instructions. Specimens were tested in three-point flexure in 37°C artificial saliva. Weibull analysis (n=30) and a constant stress-rate test (n=10) were used to determine the Weibull modulus (m) and SCG coefficient (n), respectively. Microstructural and fractographic analyzes were performed using SEM. ANOVA and Tukey's test (α=0.05) were used to statistically analyze data obtained with both microstructural and fractographic analyzes. RESULTS: YZ and AL presented high crystalline content and low porosity (0.1-0.2%). YZ had the highest characteristic strength (σ(0)) value (911MPa) followed by AL (488MPa) and IZ (423MPa). Lower σ(0) values were observed for the porcelains (68-75MPa). Except for IZ and VM7, m values were similar among the ceramic materials. Higher n values were found for YZ (76) and AL (72), followed by IZ (54) and the veneering materials (36-44). Lifetime predictions showed that YZ was the material with the best mechanical performance. The size of the critical flaw was similar among the framework materials (34-48µm) and among the porcelains (75-86µm). SIGNIFICANCE: The microstructure influenced the mechanical and SCG behavior of the studied materials and, consequently, the lifetime predictions.

Characterization of glass-infiltrated alumina-based ceramics.

OBJECTIVE: Characterize the microstructure, composition, and important properties of glass-infiltrated alumina-based ceramics similar to the In-Ceram system. METHODS: Materials used were: IA, In-Ceram Alumina (Vita); IAE, IA electrophoretically deposited (Vita); AEM, IA using a vacuum driven method (Vita); VC, Vitro-Ceram (Angelus); TC, Turkom-Cera (Turkom-Ceramic); CC, Ceramcap (Foto-Ceram); and AG, Alglass (EDG). Ceramic specimens were fabricated following manufacturers' instructions and ISO6872 standard and polished successively through 1 microm alumina abrasive. Semi-quantitative and qualitative analyses were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and stereology (Vv). The elastic modulus (E) and Poisson's ratio (nu) were determined using time-of-flight data measured in an ultrasonic pulser/receiver and the density (rho) was determined using a helium pycnometer. Vicker's indentation was used to calculate hardness (H). Bar specimens (25 mm x 4 mm x 1.2 mm) were loaded in three-point bending to fracture using a universal testing machine with cross-head speed of 1mm/min. Flexural strength (sigma(3P)) was calculated and statistically analyzed using ANOVA, Tukey (alpha = 0.05) and Weibull (m = modulus, sigma(0) = characteristic strength). RESULTS: SEM and EDS analyses revealed similar microstructure for all-ceramics, except for a lead-based matrix in CC and a zirconia phase in VC. TC, AG and CC showed significantly lower mean sigma(3P) values than the other ceramics (p< or =0.05). AEM showed the greatest m (16). CONCLUSION: Despite few differences in microstructure and composition, the IA, IAE, AEM and VC ceramics have similar properties. SIGNIFICANCE: The glass-infiltrated alumina-based ceramics from different manufacturers presented distinct characteristics. It is necessary to characterize new commercially available materials to understand their properties.