Biaxial Flexure Research Articles

Fracture Resistance of Lithıum Disilicate, Indirect Resin Composite and Zirconıa by Using Dual Cure Resin Cements

Objective: The aim of the study was to examine the fracture resistance of lithium disilicate, indirect resin composite and zirconia by using dual cure resin cements.Methods: Three groups of 180 samples (n= 60) of E-max, zirconia and indirect resin composite materials (10mm diameter and 1 mm thickness). Discs were fabricated and cemented with three dual curing resin cements. Aging treatment was then applied to the discs by using thermal cycle machine (at 5°C to 55°C/dwell time: 20s), 10000 cycles for 168 hours’ 7 days. Fracture tests were performed to the sample discs using piston on three balls test to determine the biaxial flexure strength of the 180 discs of the three materials. The results were analysed by using one-way analysis of variance (ANOVA) and t-test.Results: Statistically significant difference was found between control groups (before cementation and thermal cycle) and both group B (after cementation before thermal cycle) and group C (after cementation and thermal cycle) in all materials (P<0.05). Comparing Zirconia, Gradia and E-max all control groups showed statistically significant difference and Zirconia was showed greater flexural resistance against other materials. In addition, all materials also showed statistically significant difference in Variolink/Multilink cemented Group B and C. In Nexus cemented Group B and C statistically significant difference was found only Zirconia material. Similar to control group results, Zirconia material was showed greater flexural resistance values with both cements in Group B and C.Conclusion: There is a difference between flexural strength of the three materials, Zirconia has a better flexural strength when compared to lithium disilicate and indirect resin composite.

Effect of different surface treatments on the biaxial flexure strength, Weibull characteristics, roughness, and surface topography of bonded CAD/CAM silica-based ceramics

ObjectiveTo investigate the influence of different surface treatments on biaxial flexure strength, roughness, and surface topography of lithium silicate/disilicate-based ceramics. Methods225 discs (∅: 12 mm; 1.2 mm – ISO 6872) were made from three ceramics: IPS e.max CAD (LD – Ivoclar Vivadent), Suprinity (LSS - Vita) and Celtra Duo (LSC - Dentsply). The samples were randomly divided into 5 groups (n = 15): no treatment (C); 10% hydrofluoric acid + silane (HF); sandblasting Al2O3 + silane (SB); silicatization + silane (SC); and self-etching ceramic primer (SEP). After surface treatment, a resin cement layer was applied to the disc surface (RelyX U200, 3M ESPE), mechanical cycled (1.2 × 106 cycles, 50 N, 3.8 Hz) and submitted to biaxial flexural strength test (1 mm/min, 1000 Kgf). Roughness, EDS and SEM were also performed. Data were analyzed by one-way ANOVA, Tukey test (5%) and Weibull. ResultsANOVA revealed that the “surface treatment” factor was significant for all ceramics (p < 0.05). The groups LD-HF (289.30 ± 40) LD-SEP (298. 87 ± 53.29), LSC-HF (195.51 ± 42.12), LSS-HF (269.58 ± 27.07) and LSS-SEP (207.45 ± 28.63) presented significantly higher biaxial flexure strength than respective control groups, except for the LSC-SEP (165.41 ± 33.86), which was statistically similar to the control. The Weibull modulus was significantly higher for the LD-SB, LSC-SC groups. Additionally, the LD-SB, LSC-SC and LSS-HF groups showed higher roughness compared to the other treatments. SignificanceHF etching followed by silanization and self-etching ceramic primer are the most suitable surface treatments for lithium silicate/disilicate-based glass-ceramics.

Design and analysis of a bi-axial centralized butterfly flexure hinge for fast steering mirrors

A bi-axial centralized butterfly flexure hinge for a fast steering mirror (FSM) was presented to adapt highly stabile accuracy of beam-pointing control performance in space laser communication. According to the requirements of two-dimensional reciprocating movements and high bandwidth provided for the FSM, the solid model of the bi-axial centralized butterfly flexure hinge was designed. By applying Castigliano’s displacement theorem, the numerical model was simplified and deduced. Furthermore, to quantify the numerical model, natural frequencies of the finite-element analysis (FEA) and experiments were carried out the results of which were compared with the analytic solutions. The experimental results show that the in-plane natural frequencies are 66.37 and 112.2 Hz, respectively. The comparison shows that the errors between numerical analytic and experimentation are 3.0% and 1.4%, respectively, and errors between FEA and experimentation are 1.7% and 2.4%, respectively. It is proven that the bi-axial centralized butterfly flexure hinge we built is an appropriate structure as a two-axis guide mechanism in two-dimensions for an FSM system with a large bandwidth of 225 Hz.

Standardized Weibull statistics of ceramic strength

In order to estimate Weibull parameters in the Weibull statistical fracture theory as truly material properties independent of specimen geometry and loading mode, first the Weibull statistical fracture theory is transformed into the ordinary Weibull distribution function under certain approximation. Then the standardized format of ordinary Weibull distribution is introduced to enable Weibull modulus as the single parameter for estimation via the maximum likelihood method. The method of using standardized Weibull distribution for strength data synchronization and Weibull modulus estimation is validated by analyzing extensive strength data sets measured from uniaxial flexure, biaxial flexure and their combination, and from smooth and notched specimens. The technical path to estimate the scale parameter and threshold strength as material properties in the Weibull statistical fracture theory and effect of sample size on the estimation accuracy are also discussed.

Femtosecond Ti: Sa ultra short-pulse laser irradiation effects on the properties and morphology of the zirconia surface after ageing

Femtosecond pulses from a Ti:Sapphire laser were used to irradiate specimens of yttria-stabilised (35% mol) tetragonal zirconia (Y-TZP) with the purpose of studying the effects of the irradiations on their surface properties and morphology after ageing. Zirconia disks were divided into eight groups (n = 32) according to their surface treatment and subsequent ageing: Control: no treatment; sandblasting: Al2O3 sandblasting 50 μm; and ultrashort laser pulses irradiation with 25 μJ pulses, considering two different scanning steps based on the width between two grooves. These groups were duplicated and submitted to ageing. The surfaces were analysed using scanning electron microscopy (SEM), and X-ray diffraction. A finite element analysis, a biaxial flexure test, as well as fractographic and Weibull analyses, were performed. The strengths of the disks were statistically different for the treatment factor, and the principal stresses seemed to be concentrated at the centre of the specimens, as predicted by the computer simulations. Ageing decreased the strengths for all groups and increased the Weibull modulus for the laser group with the 40 μm-width between two grooves. The sandblasting group presented the highest monoclinic phase peak. Although the most significant strength was found within the sandblasting group, the phase transformation was favourable to the laser groups. The Weibull modulus was higher for the laser group with the 60 μm-width between two grooves, confirming the highest homogeneity of its failure distribution. Regardless of the surface treatment, strength was decreased with ageing in all groups. The femtosecond Ti:Sa ultra-short pulse laser irradiation can be suggested as an alternative to the gold standard sandblasting in long-term Y-TZP zirconia rehabilitations, such as crowns and veneers.

Influence of zirconia surface treatments of a bilayer restorative assembly on the fatigue performance.

This study evaluated the influence of different surface treatments of zirconia used to enhance bonding with veneering porcelain, and thermocycling on the resistance to porcelain cracking and delamination during fatigue test. Bilayer ceramic discs were made from zirconia blocks (IPS e.max Zircad MO, Ivoclar Vivadent - 0.7 mm thickness) and randomized into 8 groups (n= 15) according to two factors: 'zirconia surface treatment' (Control; Grinding - diamond bur; Air-abrasion - aluminum oxide particles; and Liner - application of a ceramic liner [IPS e.max Zirliner, Ivoclar Vivadent]); and 'thermocycling' (presence - 12,000 thermal cycles; 5-55ºC; or absence). The discs were veneered with porcelain (IPS e.max Ceram, Ivoclar Vivadent - 0.7 mm; totaling 1.4 mm thickness) according to ISO 6872:2015 for biaxial flexure strength testing. Fatigue tests (step-stress approach; 20 to 100 MPa; step of 10 MPa; 10,000 cycles per step; 10 Hz frequency) were run, followed by the data analysis (Kaplan-Meier and Mantel-Cox post-hoc tests). Analysis of roughness, topography, crystallographic phase arranges and fractography were also executed. The surface treatment and thermocycling did not influence the porcelain crack nor delamination resistance. When only comparing the surface treatments for crack resistance outcome, the liner application depicted the worst fatigue performance in comparison to grinding and air-abrasion, while all groups were similar for delamination. Neither the surface treatment of the zirconia nor the thermocycling influences the porcelain crack resistance or the resistance to delamination of the bilayer porcelain-veneered zirconia specimens.

Strength-limiting damage and its mitigation in CAD-CAM zirconia-reinforced lithium-silicate ceramics machined in a fully crystallized state

ObjectivesThe objective was to explore how clinically relevant machining process and heat treatment influence damage accumulation and strength degradation in lithium silicate-based glass ceramics machined in the fully crystallized state. MethodsA commercial zirconia-reinforced lithium silicate (ZLS) glass ceramic with a fully developed microstructure (Celtra® Duo) was studied. Disk-shaped specimens (nominal 10 mm diameter and 1 mm thickness) were fabricated either using a CAD-CAM process, creating a clinically relevant dental restoration surface, or were sectioned from water-jet cut cylindrical blocks with their critical surfaces consistently polished. Bi-axial flexure strength (BFS) was determined in a ball-on-ring configuration, and fractographic analysis was performed on failed specimens. XRD, AFM and SEM measurements were conducted before and after heat treatment. For each sample group, BFS was correlated with surface roughness. A two-way ANOVA and post-hoc Tukey tests were used to determine differences in BFS between machining and heat treatment groups (ɑ = 0.05). ResultsA two-way ANOVA demonstrated that BFS was influenced by fabrication route (p < 0.01) with CAD-CAM specimens exhibiting significantly lower mean BFS. A factorial interaction was observed between heat treatment and machining route (p < 0.01), where a significant strengthening effect of post-manufacture heat treatment was noted for CAD-CAM specimens but not sectioned and polished samples. CAD-CAM specimens exhibited sub-surface lateral cracks alongside radial cracks near fracture origin which were not observed for polished specimens. BFS did not correlate with surface roughness for polished specimens, and no change in microstructure was detectable by XRD following heat treatment. SignificanceThe mechanical properties of the ZLS ceramic material studied were highly sensitive to the initial surface defect integral associated with manufacturing route and order of operations. CAD-CAM manufacturing procedures result in significant strength-limiting damage which is likely to influence restoration performance; however, this can be partially mitigated by post-machining heat treatment.

Effect of Different Beaching Protocols on Biaxial Flexural Strength and Micro Hardness of Two Ceramic Materials

Purpose: The aim of this study was to evaluate the effect of different bleaching protocols on biaxial flexural strength and micro hardness of two ceramic materials. Material and Methods: A total of 112 discs, 56 discs of each material, were prepared from Celtra® Duo and VITA ENAMIC® with 10mm diameter and 1.2mm thickness. Each material was divided into 4 groups (n=14) according to the bleaching protocol done: Group 1 Control group “no bleaching”, Group 2 home bleaching “Carbamide Peroxide 22%”, Group 3 home bleaching “Hydrogen Peroxide 14%” and Group 4 In-office bleaching using diode Laser. For each material, half the number of discs in each group (n=7) were subjected to Biaxial Flexural Test using Universal Testing Machine Instron-3345 together with Instron BlueHill® universal software, while the other half was subjected to Micro-Hardness Test using Tukon™ 1102 Wilson® Hardness Tester. Data was represented by mean and standard deviation (SD) values. Two-way ANOVA was used to study the effect of different tested variables and their interaction on biaxial flexural strength (Mpa) and micro-hardness, The significance level was set at p ≤0.05 within all tests. Statistical analysis was performed with IBM® SPSS® Statistics Version 26 for Windows. Results: For Biaxial Flexural Strength and Micro-hardness; Celtra duo samples had a significantly higher value than samples made with Vita Enamic for all bleaching groups. Conclusion: Bleaching resulted in a significant diminution for both biaxial flexural strength and surface micro-hardness of Celtra Duo and Vita Enamic.

Effect of Multiple Firing Cycles on the Physical Properties of Three Pressable Lithium Silicate Glass Ceramics

Statement of the problem To achieve esthetic perfection with dental ceramics, ceramic dental restorations may need to undergo multiple firing cycles for characterization. The behavior of lithium silicate ceramics when exposed to those firing cycles is not well understood, the way it might affect the optical and mechanical properties of restorations. Purpose The aim of this in-vitro study was to investigate the effect of multiple firing cycles on translucency and biaxial flexure strength Of 3 different types of pressable lithium silicate ceramics. Materials and methods A total of 72 disc shape specimens (10mm diameter and 1mm thickness) were fabricated by heat pressing of LT A1 shaded ingots. 24 specimens of Celtra press and the same number of IPS e.max press and Livento press lithium silicates materials. Each material group were subdivided in to three subgroups (8 samples per group) according to the number of firing cycles. One, three, and five firing cycles’ protocols were constructed, to simulate the firing protocols of the three common laboratory fabrication techniques (staining, cutback, and layering) of glass ceramic restorations. Translucency parameters and biaxial flexural strength were evaluated for specimens to detect the effect of multiple firing cycles. Results All tested materials showed statistically significant decrease in translucency parameters values with multiple firing cycles. No significant effect was detected on the biaxial flexural strength values after repeated firing. Conclusion Translucency of tested materials is negatively affected by multiple firing cycles. Multiple firings have no effect on the biaxial flexural strength of lithium silicate ceramics.

Effect of titania addition and sintering temperature on the microstructure, optical, mechanical and biological properties of the Y-TZP/TiO2 composite

ObjectiveThe aims of this study were: 1) to evaluate the effect of sintering temperature on microstructure, density and flexural strength of a 3Y-TZP/TiO2 composite containing 12.5 wt% of TiO2 compared to 3Y-TZP specimens (control); 2) to compare 3Y-TZP with the experimental 3Y-TZP/TiO2 composite, both sintered at 1400 °C, with respect to the following parameters: optical properties, characteristic strength, Weibull modulus, fatigue behavior, induction of osteoblasts proliferation and differentiation (mineralization nodules formation). MethodsThe 3Y-TZP and 3Y-TZP/TiO2 powders were uniaxially pressed and sintered at 1200 °C, 1300 °C, 1400 °C or 1500 °C for one hour in a furnace. The microstructural analysis consisted of X-ray diffraction and scanning electron microscopy. The density was measured by the Archimedes' principle and the flexural strength was obtained by the biaxial flexure test. The optical properties were measured using a spectrophotometer operating in the visible light wavelength range. The step-stress accelerated life testing was performed by the pneumatic mechanical cycler and the biological behavior achieved by using osteoblast-like cells (Osteo-1 cell line). ResultsTetragonal zirconia was identified in all groups and cubic zirconia was identified only at 3Y-TZP group. The addition of TiO2 decreased the values of density and flexural strength of the composite 3Y-TZP/TiO2 in relation to 3Y-TZP regardless of the sintering temperature. The color difference between the two materials was not significant regarding L*a*b* parameters. The composite showed higher probability of failure, and induced higher proliferation and differentiation than control. SignificanceThe composite developed have good aesthetic and biologics properties. However, its microstructure and mechanical properties need to be improved for future dental implant applications.

The number of specimens in a furnace affects the biaxial flexural strength of veneered zirconia specimens after sintering

This study evaluated the effect of the number of specimens in a furnace during the firing of porcelain-zirconia (Y-TZP) discs on their biaxial flexural strength. Eighty bilayer disc-shaped specimens (∅: 15mm; thickness: 1.2 mm, ISO 6872) of zirconia (Vita In-Ceram YZ Cubes/Vita) and porcelain (Vita VM9) ceramics, were prepared and divided into four groups (n = 20) according to the number of specimens in a furnace for each porcelain sintering: G1- one disc/20 sintering cycles.; G2 – two discs/10 sintering cycles.; G5-five discs/4 sintering cycles; and G10 – ten discs/2 sintering cycles. Then, the specimens were submitted to biaxial flexure testing (BFS) (1 mm/min, water). The data (MPa) were analyzed with Kruskal-Wallis test followed by the Dunn's post hoc analysis at a 95% level of confidence (α = 0.05). Kruskal-Wallis showed that the number of samples in the furnace affected flexural strength (p < 0.0001). The highest BFS was observed for the G5 (132.3 MPa)A, which was significantly higher than G1 (53.6 MPa)B. No significant difference was observed for the Weibull moduli and characteristic strength among the groups. In terms of flexural strength, approximately five specimens at a time can be placed in a furnace for sintering.

Crack growth behavior of a biomedical polymer-ceramic interpenetrating scaffolds composite in the subcritical regimen

We subjected a commercial dental composite formed by interconnecting polymer-ceramic scaffolds to extensive quasi-static, static and cyclic experiments under biaxial flexure. By this we meant to obtain static and cyclic subcritical crack growth exponents that, based on established relationships describing the degradation of frictional bridging mechanisms, challenging the notion of a suggested R-curve behavior. By exploring the fracture statistics of specimens with increasing effective volumes and effective areas, we demonstrated the presence of a bi-modal defect size distribution in disaccord with the Weibull behavior across length scales. Lifetime distributions seemed to follow the strength distributions, and were used to derive crack growth velocity diagrams for combined levels of applied stress. Ultimately, the claim of an R-curve behavior could not be supported based on the absence of any significant cyclic fatigue effect, i.e. bridging degradation.

Effect of Low-Concentration Hydrofluoric Acid Etching on Shear Bond Strength and Biaxial Flexural Strength after Thermocycling.

This study evaluated the shear bond strength (SBS) and biaxial flexural strength (BFS) of resin cements according to the surface treatment method using low-temperature hot etching with hydrofluoric acid (HF) on a yttrium-stabilized tetragonal zirconia (Y-TZP) surface; 96 discs and 72 cubes for BFS and SBS tests for Y-TZP were randomly divided into four groups of BFS and three groups of SBS. Specimens were subjected to the following surface treatments: (1) no treatment (C), (2) air abrasion with 50 μm Al2O3 particles (A), (3) hot etching with HF at 100 °C for 10 min (E), and (4) air abrasion + hot etching (AE). After treatments, the specimens were coated with primer, and resin cement was applied with molds. The specimens were evaluated for roughness (Ra) via scanning electron microscopy and x-ray diffraction, and the data were analyzed by an analysis of variance (ANOVA) and Kruskal–Wallis tests. Group E produced significantly higher SBS compared to group A and AE before and after thermocycling. The BFSs of all groups showed no significant differences before thermocycling; however, after thermocycling, C and E treatment groups were significantly higher compared to group A and AE. All groups showed phase transformation. Group E was observed lower monoclinic phase transformation compared to other groups.

Denture Base Composites: Effect of Surface Modified Nano- and Micro-Particulates on Mechanical Properties of Polymethyl Methacrylate.

The most commonly used denture base material, polymethyl methacrylate, lacks ideal mechanical properties, which are reflected in its relatively high failure rate. Several methods have been explored to reinforce the material and reduce the cost of denture repair and replacement. In this study, various surface modified filler particles at different concentrations were dispersed in conventional and high-impact denture base materials and tested for their improvement in mechanical properties. Inorganic filler particles were coated with different silane coupling agents using an ultrasonic device. The particulates were dispersed in the resin and the composites polymerised through an innovative dual-cure technique. Charpy impact test, single-edge notch three-point bend fracture toughness test and Biaxial Flexural Strength (BFS) were performed on the specimens. The results showed that mechanical properties of the denture base resin can be improved by incorporating filler particles; however, the surface characteristics, quantity and level of dispersion of the particles play critical role in the mechanical behaviour of the composites. The results of this study are a promising step towards developing more fracture-resistant denture base materials.

Effect of glazing application side and mechanical cycling on the biaxial flexural strength and Weibull characteristics of a Y-TZP ceramic

Glaze application on monolithic zirconia (Y-TZP) can be a practical approach to improve the mechanical properties of this material. Objective Our study evaluated the effect of glazing side and mechanical cycling on the biaxial flexure strength (BFS) of a Y-TZP.Methodology Eighty sintered Y-TZP discs (Ø:12 mm; thickness: 1.2 mm - ISO 6872) were produced and randomly assigned into eight groups (n=10), according to the factors “glazing side” (control – no glazing; GT – glaze on tensile side; GC – glaze on compression side; GTC – glaze on both sides) and “mechanical aging” (non-aged and aged, A – mechanical cycling: 1.2×106, 84 N, 3 Hz, under water at 37°C). Specimens were subjected to BFS test (1 mm/min; 1,000 Kgf load cell) and fractured surfaces were analyzed by stereomicroscopy and SEM. Hsueh’s rigorous solutions were used to estimate the stress at failure of glazed specimens. Two-way ANOVA, Tukey’s test (5%), and Weibull analysis were performed.Results The “glazing side”, “mechanical aging” and the interaction of the factors were significant (p<0.05). Groups GC (1157.9±146.9 MPa), GT (1156.1±195.3 MPa), GTC (986.0±187.4 MPa) and GTC-A (1131.9±128.9 MPa) presented higher BFS than control groups (Tukey, 5%). Hsueh’s rigorous solutions showed that the maximum tensile stress was presented in the bottom of zirconia layer, at the zirconia/glaze interface. Weibull characteristic strength (σo) of the GC was higher than all groups (p<0.05), except to GT, GTC-A and GTC, which were similar among them. The fractography showed initiation of failures from zirconia the tensile side regardless of the side of glaze application and fatigue.Conclusion Glazing zirconia applied on both tensile and compression sides improves the flexural strength of Y-TZP, regardless the mechanical aging.

Biaxial flexural failure of woven composite plates investigated by the ring on ring bending test

Abstract The ring on ring (ROR) bending test is an ASTM standard (C1499) for characterizing the equibiaxial flexural strength of ceramics. This simple test protocol induces biaxial flexure by applying an out of plane uniaxial load via a small loading ring, to a plate or disc shaped specimen, simply supported on a larger ring. The main objective of this paper is to apply this test method to analyze the biaxial flexural failure of woven fabric composite plates. Ring on ring bending tests are conducted on square shaped plate specimens of epoxy/carbon plain woven composites, until failure. Experiments reveal interesting similarities and differences when compared to ceramics. First it is seen that while for ceramics the bottom surface fractures under biaxial tension, for woven composites the top surface fractures under biaxial compression. The failure pattern however, is similar to ceramics, and consists of several radial kink bands which initiate near the center and propagate outwards towards the specimen edges. Subsequent numerical stress analysis of the test reveals that the stress state in the region of the plate encompassed by the loading ring is almost exactly equibiaxial. This is primarily because the test method induces pure bending and because the two in-plane principal moduli are equal to each other, and much larger than all the other moduli. This implies near axisymmetry of the stress state despite material orthotropy. This aspect is seen to enable a simple determination of the biaxial flexural strength using the in-situ measured principal strains. It also implies that this biaxial flexural strength could be a good approximation of the equibiaxial compressive strength for the woven composite, directly measuring which is usually a challenging task. Conditions for the suitability of this test method are explored via further numerical modeling.

Influence of Aging on Biaxial Flexural Strength and Hardness of Translucent 3Y-TZP.

The purpose of this research was to evaluate the influence of aging and surface treatment on surface roughness, biaxial flexural strength (BFS), and Vickers hardness (VHN) of translucent dental zirconia. Half of 80 disc-shaped zirconia specimens (1.2 mm thickness and 12 mm diameter) were aged (group A) in an autoclave for 20 h (134 °C and 0.2 MPa) and the other half were not aged (group N). Specimens were subjected to: no surface treatment (SIN), particle air-abrasion with 50 µm alumina particles at 1 bar (0.1 MPa) and 2.5 bar (0.25 MPa), or polishing down to 1 µm (POL). Specimens were analyzed using X-ray diffraction, laser scanning microscope, BFS, and VHN tests. Three groups (N-SIN, N-POL, and A-POL) showed almost no monoclinic phase. While other groups showed monoclinic phase ratios ranging from 7.5 vol. % ± 2.4 vol. % (N-0.1 MPa) to 41.5 vol. % ± 0.3 vol. % (A-0.1 MPa). Aging and particle air-abrasion increased significantly the BFS, ranging from 720 ± 37 MPa (N-SIN) to 1153 ± 92 MPa (N-0.1 MPa). The hardness was not influenced significantly by aging. A certain amount of monoclinic phase at the surface strengthens the high translucent dental zirconia, while hardness and roughness are not influenced. The pressure of particle air-abrasion showed no influence on the evaluated properties.

Effect of extrinsic pigmentation and surface treatments on biaxial flexure strength after cyclic loading of a translucent ZrO2 ceramic

ObjectiveTo evaluate the influence of extrinsic pigmentation on the biaxial flexural strength and surface topographic of translucent Y-TZP (InCoris TZI – Sirona – USA) subjected to several surface treatments. MethodsSintered zirconia discs-shaped specimens (n=120) (ø:12mm; thickness:1.2mm; ISO 6872) were prepared and divided (n=15) according to various factors: “extrinsic pigmentation” (n: without; p: with) and “surface treatments” (C: control — as sintered; A: abraded with silica-coated alumina particles (30μm); G: glazed with a thin film of low-fusing porcelain glaze; GH: glazed and etched with 10% hydrofluoridric acid for 60s. Mechanical cycling (1.2×106 cycles, 200N, 3.8Hz) and flexural strength test (1mm/min — 1000kg cell) were performed. Two-way ANOVA and Tukey’s were used for statistical test (α=0.05). Weibull analysis was used to evaluate the strength reliability. Samples were analyzed via (1) an optical profilometer to determine the surface roughness (Ra); (2) an X-ray diffraction (XRD) to evaluate phase transformations; and (3) a SEM equipped with an energy dispersive X-ray spectroscopy (EDX) to elucidate morphological properties and chemical compositions. ResultsRegardless of the surface treatment (p=0.5459) (Cn: 560.16MPa; Gn: 573.36MPa; An: 643.51MPa; GHn: 542.94MPa; Cp: 628.04MPa; Gp: 641.90MPa; Ap: 554.47MPa; GHp :602.84MPa) and extrinsic pigmentation (p=0.1280) there was no difference in the flexural strength among the experimental groups. According to the XRD analysis, phase transformations occurred in the An group (t→m) and in Ap group (t→c). Surface roughness was affected by surface treatments (An — p=0.001) and extrinsic pigmentation (Gp — p=0.001). SignificanceThe biaxial flexural strength of the tested samples was not affected neither by surface treatments nor by pigmentation, although it can cause phase transformation and promote surface roughness.

Comparison between Biaxial Flexural Strength and Microstructure of Polished and Glaze-Fired Specimens of Zirconia Lithium Silicate Glass Ceramic

Introduction: There has been a growing interest in glass ceramic systems with good esthetics, high fracture resistance, bonding durability, and simplified fabrication techniques using CAD/CAM. The aim of this study was to compare flexural strength in and and fired specimens of zirconium lithium silicate (ZLS) CAD/CAM blocks Celtra Duo. Methods: A total of 14 specimens of Celtra Duo (Dentsply Sirona, Germany) were designated and equally divided into two groups, including group I (n=7; as polished) and group II (n=7; as glazed and fired) according to the manufacturers’ instructions. Then biaxial flexural strength was tested according to ISO 6872 using a universal testing machine and piston-on-three-ball method after polishing, glazing, and firing. The fractured surfaces and microstructure were observed by scanning electron microscope (Jeol/GSM5300). Data were then statistically analyzed using SPSS software (version 20.0). Results: There were statistically significant differences in biaxial flexural strength between a polished and glaze-fired specimen of ZLS glass ceramic discs as biaxial flexural strength was lower in polished specimens (mean±standard deviation: 158.7±21.67 MPa), while the glazed specimens showed higher biaxial flexural strength (mean±standard deviation: 261.5±31.89 MPa). Conclusion: Glazing and firing of Celtra Duo specimens increased biaxial flexural strength significantly in comparison to polishing with no significant change in the microstructure.

Effect of sintering parameters on phase evolution and strength of dental lithium silicate glass-ceramics

ObjectiveWith the establishment of CAD/CAM technology, competing lithium silicate based formulations have been introduced for clinical use, but little is known about their phase composition. Here we investigate a commercially available SiO2-Al2O3-K2O-Li2O-P2O5-ZrO2 system to evaluate the crystal phase evolution during the second heat treatment by changing the main crystallization parameters. MethodsWith a focus on the final stage of crystallization, we characterized the dimensional changes in the crystallographic structure of the residual Li2SiO3 and the lithium orthophosphate (Li3PO4) phases with variations in crystallization parameters, i.e. time, temperature and cooling rate over the range of the glass transition temperature Tg. The phase fractions (crystalline and glass) and the sizes of coherent scattering domains (CSDs) were resolved by means of quantitative X-Ray Diffraction using Rietveld refinement combined with an external standard method (G-factor). Biaxial flexure testing was conducted to evaluate the influence of crystallization parameters on the characteristic strength and natural defect distribution. ResultsAn increase in crystallization temperature from 840 to 880°C resulted in a significant reduction of the Li2Si2O5 content, which indicated a reversion of the Li2SiO3 to Li2Si2O5 phase transformation. Reduction to 800°C had no significant effect. Furthermore, the CSD sizes of Li2SiO3 and Li3PO4 continuously increased with increasing temperature, which was accompanied by an increase in strength parameters. Reducing the cooling rate over the range of Tg resulted in an increased strength at low failure probabilities. SignificanceThese findings help to establish recommendations for adjustment of the crystallization protocol, which has potential to increase the clinical reliability of the material investigated.