Microstructure,composition, and flexural strength of different layers within zirconia materials with strength gradient

ObjectivesThe aim of this study was to compare composition, microstructure, and mechanical strength of current multilayer zirconia blanks.MethodsBar shaped specimens were made from several layers of multilayer zirconia blanks (Cercon ht ML, Dentsply Sirona, US; Katana Zirconia YML, Kuraray, J;SHOFU Disk ZR Lucent Supra, Shofu, J; priti multidisc ZrO₂ Multi Translucent, Pritidenta, D; IPS e.max ZirCAD Prime, Ivoclar Vivadent, FL). Flexural strength was determined in a three-point bending test on extra-thin bars. X-ray diffraction (XRD) with Rietveld refinement was used to assess crystal structure and scanning electron microscopy (SEM) imaging to visualize the microstructure of each material and layer.ResultsMean flexural strength varied between 467.5 ± 97.5 MPa (top layer, IPS e.max ZirCAD Prime) and 898.0 ± 188.5 MPa (bottom layer, Cercon ht ML) with significant (p ≤ 0.055) differences between the individual layers. XRD indicated 5Y-TZP for enamel-layers, 3Y-TZP for dentine-layers, individual mixtures of 3Y-TZP, 4Y-TZP, or 5 Y-TZP for intermediate layers. SEM analysis showed grain sizes between approx. 0.15 and 4 µm. Grain size tended to decrease from top to bottom layers.SignificanceThe investigated blanks differ predominantly in the intermediate layers. In addition to dimensioning of restorations, the milling position in the blanks must also be taken into account when using multilayer zirconia as restorative material.     

Novel speed sintered zirconia by microwave technology

ObjectiveContinuous efforts have been made to hasten the zirconia densification process without compromising properties. This study evaluated the long-term structural durability of microwave speed-sintered zirconia (MWZ) relative to a conventionally sintered zirconia (CZ).MethodsAs-machined dental 3Y-TZP discs (Ø12 × 1.2 mm) were speed sintered at 1450 °C for 15 min using an industrial microwave oven, while conventional sintering was conducted in a standard dental furnace at 1530 °C for 2 h. Both were followed by natural cooling. The total sintering time was 105 min for MWZ and 600 min for CZ. Groups were compared regarding density, grain size, phase composition, and fracture resistance. Structural durability was investigated employing two fatigue protocols, step-stress and dynamic fatigue.ResultsCompared to CZ, MWZ exhibited a slightly lower density (MWZ = 5.98 g/cm³, CZ = 6.03 g/cm³), but significantly smaller grain sizes (MWZ = 0.53 ± 0.09 μm, CZ = 0.89 ± 0.10 μm), lower cubic-zirconia contents (MWZ = 15.3%, CZ = 22.7%), and poorer translucency properties (TP) (MWZ = 13 ± 1, CZ = 29 ± 0.8). However, the two materials showed similar flexural strength (MWZ = 978 ± 112 MPa, CZ = 1044 ± 161 MPa). Additionally, step-stress testing failed to capture the fatigue effect in 3Y-TZP, whereas dynamic fatigue revealed structural degradation due to moisture-assisted slow-crack-growth (SCG). Finally, MWZ possessed a slightly higher Weibull modulus (MWZ = 7.9, CZ = 6.7) but similar resistance to SCG (MWZ = 27.5, CZ = 24.1) relative to CZ.SignificanceDental 3Y-TZP with similar structural durability can be fabricated six-times faster by microwave than conventional sintering.     

Comparison of properties and cost efficiency of zirconia processed by DIW printing,casting and CAD/CAM-milling

ObjectivesThe aim of this study was to evaluate the mechanical properties and cost efficiency of direct ink writing (DIW) printing of two different zirconia inks compared to casting and subtractive manufacturing.MethodsZirconia disks were manufactured by DIW printing and the casting process and divided into six subgroups (n = 20) according to sintering temperatures (1350 °C, 1450 °C and 1550 °C) and two different ink compositions (Ink 1, Ink 2). A CAD/CAM-milled high strength zirconia (3Y-TZP) was added as reference group. The biaxial flexural strength (BFS) was measured using the piston-on-three-balls test. X-ray-diffraction (XRD) was used for microstructural analysis. The cost efficiency was compared for DIW printing and subtractive manufacturing by calculation of the manufacturing costs of one dental crown.ResultsUsing XRD, monoclinic and tetragonal phases were detected for Ink 1, for all other groups no monoclinic phase was detected. The CAD/CAM-milled ceramic showed a significantly higher BFS than all other groups. The BFS of Ink 2 was significantly higher than the BFS of Ink 1. At a sintering temperature of 1550 °C the mean BFS of the printed Ink 2 was 822 ± 174 MPa. The BFS of the cast materials did not show a significantly higher BFS than the corresponding printed group for any tested parameter-set. The manufacturing costs of DIW printed crowns are lower than the manufacturing costs of CAD/CAM-milled crowns.ConclusionDIW has a high potential to replace subtractive processes for dental applications, as it shows promising mechanical properties for appropriate ink compositions and facilitates a highly cost effective production.     

Biaxial flexural strength of zirconia: A round robin test with 12 laboratories

ObjectiveThe aim of this interlaboratory round robin test was to prove the robustness of the DIN EN ISO 6872:2019 and to identify the influence of processing and testing variations.MethodsEach of the 12 laboratories participated (A–L) received 60 (n = 720) assigned zirconia specimens. All participants seperated the specimens from the blanks, sintered them, polished half of all specimens and performed the biaxial flexural test (DIN EN ISO 6872:2019). The surface roughness was determined by using tactile measuring device. Fractographic examination was performed under scanning-electron-microscopy (SEM). Data was analysed using Kolmogorov–Smirnov-, Kruskal–Wallis-, Mann–Whitney-U-test and two-parametric Weibull statistic (p < 0.05).ResultsThe results for both preparation methods (as-fired and polished) showed significant differences for some participants. The values for as-fired groups ranged between 513 (I) and 659 (E) MPa. H showed higher Weibull modulus than C, E and I. Within polished groups flexural strengths values from 465 (L) to 1212 (E) MPa were observed, with a tendency to clustered groups A, I, J, L (465–689 MPa) and remaining groups (877–1212 MPa). E presented the highest and H the lowest Weibull modulus. Within A and J, no impact of the preparation method on flexural strength values was observed. Within L, as-fired specimens showed higher flexural strength than polished ones. The flexural strength increase did only associate to a certain extent with measured surface roughness. Fractography showed defect populations depending on polishing techniques, associated to the strength level, especially for polished groups. Reduced strength is related to machining defects, regardless of the surface state.SignificanceDIN EN ISO 6872:2019 can be seen as guidance to biaxial flexural strength testing but additional effort is necessary to ensure interlaboratory comparability. Calibrated furnaces and reliable sintering conditions are mandatory requirements together with detailed specifications on finishing or polishing procedures. Biaxial flexural testing is really a matter of understanding specimen preparation, alignment and mechanical testing by itself.DIN EN ISO 6872:2019 should further recommend reporting of mean surface roughness along with any biaxial flexural strength data. Fractography is a mandatory tool in interpretation and understanding of strength data.     

Thermo and mechanical cycling and veneering method do not influence Y-TZP core/veneer interface bond strength

ObjectivesThe purpose of this study was to evaluate the influence of thermal and mechanical cycling and veneering technique on the shear bond strength of Y-TZP (yttrium oxide partially stabilized tetragonal zirconia polycrystal) core–veneer interfaces.Materials and methodsCylindrical Y-TZP specimens were veneered either by layering (n = 20) or by pressing technique (n = 20). A metal ceramic group (CoCr) was used as control (n = 20). Ten specimens for each group were thermal and mechanical cycled and then all samples were subjected to shear bond strength in a universal testing machine with a 0.5 mm/min crosshead speed. Mean shear bond strength (MPa) was analysed with a 2-way analysis of variance and Tukey's test (p < 0.05). Failure mode was determined using stereomicroscopy and scanning electron microscopy (SEM).ResultsThermal and mechanical cycling had no influence on the shear bond strength for all groups. The CoCr group presented the highest bond strength value (p < 0.05) (34.72 ± 7.05 MPa). There was no significant difference between Y-TZP veneered by layering (22.46 ± 2.08 MPa) or pressing (23.58 ± 2.1 MPa) technique. Failure modes were predominantly adhesive for CoCr group, and cohesive within veneer for Y-TZP groups.ConclusionsThermal and mechanical cycling, as well as the veneering technique does not affect Y-TZP core–veneer bond strength.Clinical significanceDifferent methods of veneering Y-TZP restorations would not influence the clinical performance of the core/veneer interfaces.     

Effect of sintering process on microstructure, 4-point flexural strength,and grain size of yttria-stabilized tetragonal zirconia polycrystal for use in monolithic dental restorations

Statement of problemModifications have been made in the microstructure and sintering parameters of monolithic zirconia to improve esthetics qualities and avoid chipping of 2-layer restorations. However, how these modifications affect the physical and mechanical properties of zirconia is unclear.PurposeThe purpose of this in vitro study was to compare the influence of different sintering parameters on the microstructure, 4-point flexural strength, density, and grain size of 2 commercially available zirconia advocated for the fabrication of monolithic dental prostheses and 1 commercially available zirconia for use as a core material.Material and methodsThree presintered blocks (Ceramill Zolid, Prettau, and IPS e.max ZirCAD) were used. Specimens were cut and sintered as per the manufacturer's recommendation or as per a modified heating protocol. Ceramill Zolid (Z1450) was sintered at 1450 °C, Prettau (P1600) was sintered at 1600 °C, and IPS e.max ZirCAD (I1530) was sintered at 1530 °C by following the manufacturer's heating protocol. Groups Ceramill Zolid Z1530 and Z1600 were sintered at temperatures higher than the manufacturer's recommendation. Specimens from each group (n=13) were analyzed with X-ray diffraction (XRD), density calculus, average grain size measurement, and 4-point flexural tests. Data were compared by using ANOVA (α=.05).ResultsXRD analysis showed the presence of a tetragonal metastable phase in all groups before and after sintering. No significant differences were found in relative density values for the 3 Ceramill groups (5.98 g/cm³). Groups I1530 (6.03 g/cm³) and P1600 (6.03 g/cm³) had similar density results greater than 6.00 g/cm³. The average grain size of all groups remained lower than 1 μm. P1600 had the highest grain size (0.817 μm), and Z1450 presented the lowest grain size (0.465 μm). P1600 showed the most homogeneous flexural strength and the highest Weibull modulus (m=8.22). Z1530 presented the lowest Weibull modulus (m=4.58). IPS e.max ZirCAD (I1530) had the highest flexural strength (1057.41 ±150.54 MPa), and Ceramill Zolid Z1450 had the lowest (621.01 ±138.08 MPa).ConclusionsThe flexural strength, microstructure, crystal phase, and grain size of the analyzed zirconia varied as per the sintering processing. The IPS e.max ZirCAD had the highest flexural strength (1057.41 ±150.54 MPa), followed by Prettau zirconia (864.18 ±118.21), with a statistically significant difference (P<.05). The Ceramill Zolid zirconia presented the lowest flexural strength, as well as the lowest reliability for all sintering parameters used (Z1450: 621.01 ±138.08 MPa and m=5.42; Z1530: 713.10 ±175.44 MPa and m=4.58; Z1600: 630.15 ±112.08 MPa and m=6.87). At a lower heating rate (8 °C/min), the final density increased and excessive grain growth in group Z1530 was prevented.     

Impact of varying step-stress protocols on the fatigue behavior of 3Y-TZP, 4Y-TZP and 5Y-TZP ceramic

ObjectiveTo test the impact of three varying step-stress protocols on the fatigue behavior of two 3Y-TZP, one 4Y-TZP and one 5Y-TZP zirconia materials.MethodsEight specimens per zirconia material (N = 32) were selected for static testing to determine the start load for dynamic tests (30% of the mean value of static fracture load). 45 specimens per material (N = 180) were used for dynamic load tests using three step-stress protocols: 1. 50 N/5000 cycles; 2. 5% of static load/5000 cycles, and 3. 10 N/1000 cycles. Following materials were tested: 3Y-TZP_{(<0.25 Al2O3)} (O: opaque) 3Y-TZP_{(<0.05 Al2O3)} (T: translucent), 4Y-TZP_{(<0.01 Al2O3)} (ET: extra translucent) and 5Y-TZP_{(<0.01 Al2O3)} (HT: high translucent). The specimens (4 ± 0.02 × 3 ± 0.02 × 45 mm) were placed centrally on the support rolls and the load was applied perpendicularly over the 4 mm specimen side (～4-point flexural strength according to the DIN 6872:2019). Data was analyzed with Kolmogorov–Smirnov-test, t-test, one-way ANOVA with post-hoc Scheffé-test, Chi-square-test, Kaplan–Meier with Log-Rank-test and two-parametric Weibull analysis (p < 0.05).ResultsThe step-stress protocols showed no impact on the fracture load or Weibull modulus within one zirconia material. However, the zirconia materials T, ET and HT showed differences in cycle number to fracture between the step-stress protocols (T: 3 > 2 > 1; ET: 2 > 3 > 1; HT: 2, 3 > 1) with lowest cycle number to fracture for protocol 1. Within one step-stress protocol, the cycle number to fracture varied according to the zirconia material as follows: 1: T, O ≥ O, ET > HT; 2: ET > O, T, HT; 3: O, T, ET > HT. Cracking started at the tensile side of the specimens at all times. All specimens showed typical compression curls (single or double). Fragmentation patterns were similar for all materials with a lot of crack branching and fragmentation due to secondary cracks indicating high energy fractures.SignificanceDynamic fatigue tests seem to provide important information on the long-term stability of zirconia materials. Zirconia materials with higher opacity seem to be more robust towards varying step-stress protocols than translucent zirconia materials. Regarding expenditure of time, a step-stress protocol with a load increase of 50 N every 5000 cycles seems favorable to gain information on the long-term stability of zirconia materials.     

Flexural strength and Weibull characteristics of stereolithography additive manufactured versus milled zirconia

Statement of problemZirconia restorations can be processed by using stereolithography additive manufacturing (AM) technologies. However, whether additive manufactured zirconia could achieve flexural strength values comparable with those of milled zirconia is unclear.PurposeThe purpose of this in vitro study was to compare the flexural strength and Weibull characteristics of milled and additive manufactured zirconia.Material and methodsA total of 40 zirconia bars (25×4×1.2 mm) were obtained by using 2 manufacturing procedures, subtractive (CNC group) (IPS e.max ZirCAD; Ivoclar Vivadent AG) and additive manufacturing (AM group) (3DMix ZrO₂; 3DCeram) technologies and assigned to 2 subgroups according to accelerating artificial aging procedures (mastication simulation): nonaged and aged (n=10). Flexural strength was measured in all specimens by using 3-point bend tests according to ISO/CD 6872.2 with a universal testing machine (Instron Model 8501; Instron Corp). Two-parameter Weibull distribution values, including the Weibull modulus, scale (m), and shape (0) were calculated. Flexural strength values were analyzed by using 2-way ANOVA and Student t statistical tests (α=.05).ResultsThe manufacturing procedure (P<.001), the mastication simulating aging procedure (P<.001), and the interaction between them (P<.001) significantly affected flexural strength values. The CNC group exhibited statistically higher flexural strength values than those in the AM group when the specimens were tested before performing an aging procedure (P<.001) and after mastication simulation (P<.001). Moreover, mastication simulation produced a significant reduction in flexural strength for both the CNC group (P<.039) and the AM group (P<.001).ConclusionsThe manufacturing process reported a significant effect on the flexural strength of the zirconia material tested. Mastication simulation as a means of accelerating artificial aging resulted in the significantly decreased flexural strength values of milled and additively manufactured zirconia material, with the Weibull moduli being significantly higher for the milled groups versus the milled specimens.     

Impact of hydrothermal aging on the light transmittance and flexural strength of colored yttria-stabilized zirconia materials of different formulations

Statement of problemYttria-stabilized tetragonal zirconia polycrystal (Y-TZP) materials of different formulations (3Y-TZP, 4Y-TZP, and 5Y-TZP) can be colored by using color liquids. However, practically and clinically relevant factors such as modifications of sintering protocols and hydrothermal aging might affect the extent of light transmittance and flexural strength of zirconia materials of different formulations; studies on these outcomes, however, are lacking.PurposeThe purpose of this in vitro study was to test the impact of hydrothermal aging on the light transmittance and flexural strength of colored zirconia materials compared with a lithium-disilicate (LiSi₂) ceramic.Material and methodsA total of 210 specimens were prepared from 3Y-TZP_0.25 (n=30), 3Y-TZP_0.05 (n=30), 5Y-TZP (n=30), 4Y-TZP (n=60), _pre4Y-TZP (preshaded, n=30), and LiSi₂ (n=30). All specimens, except for _pre4Y-TZP and LiSi₂, were manually colored, predried, and either conventionally sintered at 1450 °C (3Y-TZP_0.25, 3Y-TZP_0.05, 5Y-TZP, and half of 4Y-TZP) or high-speed sintered at 1580 °C (other half of 4Y-TZP and _pre4Y-TZP). Light transmittance was measured initially and after 2, 5, 10, 20, 40, and 160 hours of hydrothermal aging (134 °C, 0.2 MPa). Biaxial flexural strength was tested initially and after 160 hours of hydrothermal aging (n=15). The Kolmogorov-Smirnov test, multivariate analysis, and 1-way ANOVA with the Tukey HSD post hoc test, the t test, and linear mixed models were calculated (α=.05).ResultsLiSi₂ showed the highest translucency, followed by 5Y-TZP, 4Y-TZP, _pre4Y-TZP_speed, 3Y-TZP_0.05, and 3Y-TZP_0.25. 4Y-TZP_speed was most opaque and matte. The decrease in translucency related to aging hours was higher for LiS₂ and conventional sintered zirconia materials than for 4Y-TZP_speed and _pre4Y-TZP_speed. Initially, 3Y-TZP_0.25 had the highest flexural strength, followed by 3Y-TZP_0.05, 4Y-TZP, and _pre4Y-TZP_speed. _pre4Y-TZP_speed was comparable with 4Y-TZP_speed but significantly higher than 5Y-TZP. LiSi₂ had the lowest biaxial flexural strength. Hydrothermal aging increased biaxial flexural strength for 3Y-TZP_0.25 and 3Y-TZP_0.05 (P<.001) but decreased it for 5Y-TZP (P=.005) and _pre4Y-TZP_speed (P<.001). After aging, 4Y-TZP_speed showed comparable values of flexural strength with 4Y-TZP (P=.06) and higher values than _pre4Y-TZP_speed after aging (P=.019).ConclusionsManually colored, conventionally sintered 4Y-TZP was resistant to hydrothermal aging regarding flexural strength. High-speed sintering inhibited color development for manually colored 4Y-TZP but did not affect the resistance to hydrothermal aging. The findings were reversed for industrially preshaded 4Y-TZP.     

Compatibility of machined Ce-TZP/Al2O3 nanocomposite and a veneering ceramic

OBJECTIVES: Due to its high fracture toughness ceria-stabilized ZrO(2)/Al(2)O(3) nanocomposite (Ce-TZP/A) could be superior to Y-TZP in clinical use. However, the compatibility to veneering ceramics is not yet investigated and therefore subject of this study. METHODS: Fracture strength and crazing resistance of veneered Ce-TZP/A frameworks (Nanozir, Matsushita Electric Works) were investigated in comparison to sintered and hipped Y-TZP (Hint-ELs). Frameworks were machined (HiCut, Hint-ELs), sintered where applicable (hiTherm, Hint-ELs), and veneered in a standardized procedure (Cerabien ZR, Noritake). Fracture strength of the crowns was assessed in a shear test (n=10). Biaxial flexural strength of the core materials was measured according to ISO 6872 (n=10). To assess the thermal compatibility between framework and veneer the coefficient of thermal expansion (CTE) of all materials was determined (n=3) and a crazing test carried out (n=12). RESULTS: Fracture strength was equal for crowns with Ce-TZP/A (866.6+/-132.1N) and sintered Y-TZP (904.5+/-168.2N), while crowns with hipped Y-TZP were significantly stronger (1380.6+/-253.2N) (Student's t-test, p<0.05). These values correlated with the biaxial flexural strength of the three core materials (Ce-TZP/A: 1238.0+/-200.0MPa, sintered Y-TZP: 1181.8+/-232.5MPa and hipped Y-TZP: 1521.8+/-163.8MPa). Failure of both hipped and sintered Y-TZP crowns occurred by chipping of the veneer or total fracture, while the Ce-TZP/A crowns predominantly failed by chipping of the veneer. In the crazing test both sintered and hipped Y-TZP performed slightly better than Ce-TZP/A. CTE's were as follows: Ce-TZP/A: 10.3microm/mK, sintered Y-TZP: 10.7microm/mK, hipped Y-TZP: 10.9microm/mK, and Cerabien ZR: 9.9microm/mK. SIGNIFICANCE: The CTE of the veneering ceramic has to be adjusted to Ce-TZP/A frameworks.     

Graded structures for damage resistant and aesthetic all-ceramic restorations

ObjectivesClinical studies revealed several performance deficiencies with alumina- and zirconia-based all-ceramic restorations: fracture; poor aesthetic properties of ceramic cores (particularly zirconia cores); and difficulty in achieving a strong ceramic–resin-based cement bond. We aim to address these issues by developing a functionally graded glass/zirconia/glass (G/Z/G) structure with improved damage resistance, aesthetics, and cementation properties.MethodsUsing a glass powder composition developed in our laboratory and a commercial fine zirconia powder, we have successfully fabricated functionally graded G/Z/G structures. The microstructures of G/Z/G were examined utilizing a scanning electron microscopy (SEM). The crystalline phases present in G/Z/G were identified by X-ray diffraction (XRD). Young's modulus and hardness of G/Z/G were derived from nanoindentations. Critical loads for cementation radial fracture in G/Z/G plates (20 mm × 20 mm, 1.5 or 0.4 mm thick) bonded to polycarbonate substrates were determined by loading with a spherical indenter. Parallel studies were conducted on homogeneous yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) controls.ResultsThe G/Z/G structure consists of an outer surface aesthetic glass layer, a graded glass–Y-TZP layer, and a dense Y-TZP interior. The Young's modulus and hardness increase from surface to interior following power-law relations. For G/Z/G plates of 1.5 and 0.4 mm thick, critical loads for cementation radial fracture were 1990 ± 107 N (mean ± S.D., n = 6) and 227 ± 20 N (mean ± S.D., n = 6), respectively, which were ～30 and 50% higher than those for their monolithic Y-TZP counterparts (1388 ± 90 N for 1.5 mm and 113 ± 10 N for 0.4 mm thick; mean ± S.D., n = 6 for each thickness). A 1-sample t-test revealed significant difference (p < 0.001) in critical loads for radial fracture of G/Z/G and homogeneous Y-TZP for both specimen thicknesses.SignificanceOur results indicate that functionally graded G/Z/G structures exhibit improved damage resistance, aesthetics, and potentially cementation properties compared to homogeneous Y-TZP.     

牙科氧化锆陶瓷的低温时效性能研究

目的 研究时效对5种牙科氧化锆陶瓷结构稳定性的影响.方法 5种氧化锆预烧结块(A组:TZ-3YS;B组:Vita In-Ceram YZ;C组:Ivoclar;D组:Cercon Smart;E组:Kavo)各切取15 mm×15 mm×1.5 mm氧化锆试件18片,常压致密烧结.时效处理如下:置于高温高压消毒炉中进行循环热处理,条件是134℃,0.2 MPa压力,维持时间为0h、1h、2h、3h、4 h、5h.X射线衍射分析(X-ray diffraction,XRD)试件的晶相结构,计算试件表面单斜相(m相)的相对含量.原子力显微镜观察A组和D组时效1、4 h的抛光试件.A组氧化锆再制备25 mm×4 mm×1.2 mm的试件30根,均分为未退火组(时效前)、退火组(时效前)和时效组(时效后),测试试件的三点弯曲强度.结果 除D组试件外,A、B、C、E组试件的m相含量均随时效时间的延长而增加,A组试件的m相含量最高,E组次之,B组和C组相近.XRD未检测到D组试件表面有m相,但使用原子力显微镜可观察到D组试件有m相晶核的成核和生长.时效前后A组试件的三点弯曲强度未衰减,未退火组试件三点弯曲强度[(1301±169)MPa]显著高于退火组[(1120±185)MPa]和时效组[(1158±111)MPa],差异有统计学意义(P＜0.05);而退火组与时效组间差异无统计学意义(P＞0.05).结论 牙科氧化锆陶瓷的低温时效效应具有时间依赖性,但时效效应并未使TZ-3YS的三点弯曲强度降低.     

Improvement of mechanical properties of Y-TZP by thermal annealing with monoclinic zirconia nanoparticle coating

ObjectiveTo assess whether a thermal annealing with a monoclinic zirconia (mZrO₂) nanoparticle coating can improve the reliability of sandblasted yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) and maintain its mechanical strength.MethodsCommercially available Y-TZP (Lava Frame, 3M Dental Products) disks were sintered and surface-treated as follows: AS (as sintered, with no treatment); SB (sandblasting); SB-TA (sandblasting followed by thermal annealing at 1000 °C); and SB-mZr-TA (sandblasting followed by thermal annealing at 1000 °C with the mZrO₂ nanoparticle coating). The mZrO₂ nanoparticles of 21 nm in size were prepared by a hydrothermal method, and coated onto Y-TZP sintered disks as a 5 g/L ethanol dispersion. Biaxial flexural strength (S) was measured using the piston-on-three-ball test, and reliability was evaluated by the Weibull modulus (m).ResultsBiaxial flexural tests showed a significant increase in the strength of Group SB (S_SB = 1445 ± 191 MPa) compared with Group AS (S_AS = 1071 ± 112 MPa). The thermal annealing improved the reliabilities of the sandblasted Y-TZP (m_SB-TA = 20.14 and m_SB-mZr-TA = 21.33), as compared with Group SB (m_SB = 7.77). However, the conventional thermal annealing without the mZrO₂ coating caused a significant decrease in the strength of sandblasted Y-TZP (S_SB-TA = 1273 ± 65 MPa). Importantly, the mZrO₂ coating prevented the decrease in the strength caused by conventional thermal annealing (S_SB-mZr-TA = 1379 ± 65 MPa).SignificanceThe thermal annealing with the mZrO₂ nanoparticle coating can improve the reliability of sandblasted Y-TZP and maintain its mechanical strength, which would otherwise be decreased by the conventional annealing process.     

Effect of grinding and polishing on roughness and strength of zirconia

Statement of problem

The clinical applications of high-translucency monolithic zirconia restorations have increased. Chairside and laboratory adjustments of these restorations are inevitable, which may lead to increased roughness and reduced strength. The influence of grinding and polishing on high-translucency zirconia has not been investigated.

Effect of high-speed sintering on the flexural strength of hydrothermal and thermo-mechanically aged zirconia materials

ObjectiveTo investigate the influence of high-speed and conventional sintering on the flexural strength (FS) of three zirconia materials initial and after artificial aging.MethodsMilled zirconia specimens (3Y-TZP: ZI and Zolid; 4Y-TZP: Zolid HT+; Amann Girrbach AG; N = 288, n = 96/group) were sintered in a high-speed sintering protocol (final temperature 1580 °C, n = 48/subgroup) or a conventional sintering protocol (control group, final temperature 1450 °C, n = 48/subgroup). FS was tested initially and after artificial aging (10 h in an autoclave or 1,200,000 chewing cycles; n = 16/subgroup). Univariate ANOVAs, post-hoc Scheffé, partial eta-squared, Kolmogorov–Smirnov-, Kruskal–Wallis- and Mann–Whitney-U-test were performed (p < 0.05).ResultsZI showed the highest and HT+ the lowest FS, regardless of the sintering protocols and aging regimens (p < 0.001). High-speed sintered HT+ showed higher initial FS than the control group (p < 0.001). ZI (p < 0.001–0.004) and Zolid (p < 0.001–0.007) showed higher FS after thermo-mechanical aging. High-speed sintered HT+ showed higher FS in the initial stage (p < 0.001). The Weibull modulus of the three thermo-mechanically aged materials was negatively influenced by high-speed sintering.SignificanceAs shorter sintering times represent a cost and time efficient alternative, high-speed sintering is a valid alternative to conventional sintering protocols.     

Effect of silica coating and laser treatment on the flexural strength,surface characteristics,and bond strength of a dental zirconia

This study investigated the effect of irradiation with an erbium‐doped yttrium aluminium garnet (Er:YAG) laser and coating with silica on the surface characteristics, bond strength, and flexural strength of dental zirconia. Three hundred and forty‐three standard zirconia specimens were created, and 49 were assigned to each of seven surface treatment groups: (i) no treatment; Er:YAG laser (80 mJ/2 Hz) with pulse widths of 50 μs (ii), 100 μs (iii), 300 μs (iv), or 600 μs (v); or tribochemical silica coating at the partially sintered stage (vi) or after sintering was complete (vii). All specimens were sintered after the surface treatments, except for the group in which specimens were sintered before treatment. The study outcomes were roughness, surface loss, microshear bond strength (μSBS), and biaxial flexural strength (BFS). Mean roughness and surface loss values were significantly higher in specimens from irradiated groups than in those from silica‐coated groups. Regarding μSBS, after aging, specimens from all experimental groups presented very low and similar μSBS values, irrespective of the surface treatment. Silica coating after sintering yielded the highest BFS (1149.5 ± 167.6 MPa), while coating partially sintered specimens with silica resulted a BFS (826.9 ± 60.9 MPa) similar to that of the untreated control group (794.9 ± 101.7 MPa). Laser treatments, irrespective of pulse width used, significantly decreased the BFS. In the group treated with laser at 300 μs pulse width, specimens exhibited the lowest BFS value (514.1 ± 71.5 MPa). Adhesion to zirconia was not stable after aging, regardless of the surface treatment implemented.     

A new technique for incorporation of TiO2 nanotubes on a pre-sintered Y-TZP and its effect on bond strength as compared to conventional air-borne particle abrasion and silicatization TiO2 nanotubes application on pre-sintered Y-TZP

ObjectiveThis study evaluated the microshear bond strength of a resin cement to Y-TZP after different methods of TiO₂ nanotubes (nTiO₂) incorporation on pre-sintered Y-TZP surfaces.MethodsnTiO₂ were synthesized and incorporated on Y-TZP slices as follows (n = 15): 1) nTiO₂ mixed with isopropyl alcohol/manual application (MAl); 2) nTiO₂ mixed with acetone/manual application (MAc); 3) nTiO₂ mixed with isopropyl alcohol/high-pressure vacuum application (HPVAl); 4) nTiO₂ mixed with acetone/high-pressure vacuum application (HPVAc). As controls, surfaces were sandblasted with Al₂O₃ (OX) or Rocatec silicatization (ROC). All ceramics were sintered after nTiO₂ incorporation. Surface treatments of OX and ROC were made after sintering. Surfaces were characterized by confocal laser microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Resin composite cylinders (1.40 mm diameter and 1 mm height) were cemented with a resin cement, stored in water at 37 °C for 24 h and thermocycled for 10 000 cycles before microshear bond strength evaluation. Data were analyzed with one-way ANOVA and Games-Howell (α = 0.05), and fracture analysis was performed using a stereomicroscope.ResultsEDS confirmed the presence of TiO₂ on treated Y-TZP. The confocal analysis showed higher roughness for HPVAc and OX. There were significant differences between surface treatments (p < 0.001). HPVAl (22.96 ± 10.3), OX (34.16 ± 7.9) and ROC (27.71 ± 9.4) showed higher microshear bond strengths and were statistically similar (p > 0.05). MAC showed intermediary values, and HPVAc and MAl presented decreased bond strength, with a high percentage of premature debonding.ConclusionHigh-pressure vacuum application of nTiO₂ mixed with isopropyl alcohol was able to produce bond strength values compared to conventional air abrasion and Rocatec silicatization.SignificanceThe infiltration of TiO₂ nanostructures on the pre-sintered Y-TZP is an interesting approach that can improve bond strength without the need of sandblasting methods.     

牙科着色氧化锆陶瓷饰瓷前后的力学性能及断裂模式分析

目的:本研究对牙科氧化锆陶瓷着色及构筑饰面瓷前后的力学性能及断裂模式进行分析.方法:球磨混合一定组分的着色剂(氧化铁及氧化铈)和3Y-TZP(钇稳定氧化锆)粉体制备着色氧化锆陶瓷.根据ISO6872牙科陶瓷标准,制备着色和未着色氧化锆陶瓷的三点弯曲强度样品和构筑饰面瓷后的双层瓷样品,核心瓷与饰面瓷的厚度比为1:1.对所获得样品进行三点弯曲强度、断裂韧性和维氏硬度测试.采用X射线衍射(XRD)分析物相结构,扫描电镜(SEM)观察样品的断裂模式.结果:白色氧化锆陶瓷与着色氧化锆陶瓷的三点弯曲强度分别为1301 MPa和1203:MPa,统计学结果显示二者间没有明显差异,构筑饰面瓷后的三点弯曲强度分别为817 MPa和803 MPa,统计学结果显示二者间没有明显差异,但构筑饰面瓷后比单一材料样品强度降低,二者间有显著差异.在所有的双层瓷样品中,断裂模式的应力位移曲线显示为单峰断裂模式,断裂起始于核心瓷的张应力面中心区域同时发生饰面瓷的破碎和瓷层剥脱.结论:对氧化锆粉体的适当着色并没有明显影响材料的机械性能,着色和未着色氧化锆材料与饰面瓷具有良好的物理匹配性.     

Bond strength between temporary 3D printable resin and conventional resin composite: influence of cleaning methods and air-abrasion parameters

Objectives

The influence of different cleaning methods, air-abrasion parameters, and aging on shear bond strength (SBS) and tensile bond strength (TBS) of 3D resin luted to composite resin.

Materials and methods

Nine hundred resin substrates were 3D printed (D20II, Rapid Shape) and cleaned with either isopropanol (ISO), butyldiglycol-based solution (BUT), or centrifugation (CEN). After 24-h storage in 37 °C water, specimens were air-abraded (mean particle size 50 µm; n?=?60) with either alumina at 0.1 MPa (AL0.1) or 0.4 MPa (AL0.4) and glass pearls at 0.1 MPa (GP0.1) and 0.4 MPa (GP0.4) or conditioned with visio.link (control) and luted with PanaviaV5. Initially (24 h, 37 °C water storage) or after aging (10,000 thermal cycles), SBS and TBS were measured, and fracture types were examined. Surface free energy (SFE) and roughness (Ra) were determined after air-abrasion. Kolmogorov–Smirnov, Kruskal–Wallis H, Mann–Whitney U, chi-square, and partial eta-squared were computed.

Results

SBS measurements presented higher values than TBS (p?<?0.001–0.033). Within the pretreatment groups, CEN showed the highest SBS and TBS values compared to cleaning with ISO or BUT (p?<?0.001–0.040). Pretreatment with GP0.1 displayed the lowest bond strength values (p?<?0.001–0.049), and mostly adhesive fractures occurred. The highest Ra values (p?<?0.001) were observed for AL0.4 pretreatment.

Conclusions

Pretreatment with AL0.4 and the control group mainly presented the highest bond strength values. Thermocycling had a positive effect on the bond strength.

Clinical relevance

According to this study, 3D-printed restorations should be pretreated with AL0.4 or with visio.link before adhesive luting, regardless of their cleaning.

     

Mechanical behavior and microstructural characterization of different zirconia polycrystals in different thicknesses

PURPOSETo characterize the microstructure of three yttria partially stabilized zirconia ceramics and to compare their hardness, indentation fracture resistance (IFR), biaxial flexural strength (BFS), and fatigue flexural strength.MATERIALS AND METHODSDisc-shaped specimens were obtained from 3Y-TZP (Vita YZ HT), 4Y-PSZ (Vita YZ ST) and 5Y-PSZ (Vita YZ XT), following the ISO 6872/2015 guidelines for BFS testing (final dimensions of 12 mm in diameter, 0.7 and 1.2 ± 0.1 mm in thicknesses). Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed, and mechanical properties were assessed by Vickers hardness, IFR, quasi-static BFS and fatigue tests.RESULTSAll ceramics showed similar chemical compositions, but mainly differed in the amount of yttria, which was higher as the amount of cubic phase in the diffractogram (5Y-PSZ > 4Y-PSZ > 3Y-TZP). The 4Y- and 5Y-PSZ specimens showed surface defects under SEM, while 3Y-TZP exhibited greater grain uniformity on the surface. 5Y-PSZ and 3Y-TZP presented the highest hardness values, while 3Y-TZP was higher than 4Y- and 5Y-PSZ with regard to the IFR. The 5Y-PSZ specimen (0.7 and 1.2 mm) showed the worst mechanical performance (fatigue BFS and cycles until failure), while 3Y-TZP and 4Y-PSZ presented statistically similar values, higher than 5Y-PSZ for both thicknesses (0.7 and 1.2 mm). Moreover, 3Y-TZP showed the highest (1.2 mm group) and the lowest (0.7 mm group) degradation percentage, and 5Y-PSZ had higher strength degradation than 4Y-PSZ group.CONCLUSIONDespite the microstructural differences, 4Y-PSZ and 3Y-TZP had similar fatigue behavior regardless of thickness. 5Y-PSZ had the lowest mechanical performance.