The effect of speed sintering on the optical properties and microstructure of multi-layered cubic zirconia.

PURPOSE: Speed sintering was introduced to fabricate time-efficient chairside restorations, however, its influence on the microstructure and optical properties of zirconia is still unclear. This study investigated the influence of speed sintering on the microstructure and optical properties of ultra-translucent multi-layered cubic zirconia. MATERIALS AND METHODS: Ultra-translucent cubic zirconia (Katana™ UTML; Kuraray Noritake Dental Inc., Aichi, Japan) was cut into a total of 80 specimens, twenty within each layer of the blank. The specimens were divided into 2 groups: speed and conventional sintering. The translucency parameter, opalescence, chromaticity, and color difference were recorded using a spectrophotometer. Scanning electron microscope images were used for crystallographic analysis. One-way ANOVA and Tukey's post-hoc test (p < 0.05) were used for data analysis. RESULTS: Speed sintering significantly reduced the translucency and opalescence compared to conventional sintering excluding the opalescence of the dentin layer. Chromaticity significantly decreased with speed sintering in less chromatic layers and significantly increased in more chromatic layers. Mean color change ranged between 0.65 and 1.25 across different layers. Mean crystal size decreased with speed sintering. CONCLUSIONS: With speed sintering, translucency, and opalescence decrease while chromaticity increases in the more chromatic layers. Additionally, no clinically perceptible color change was found compared to conventional sintering.

Impact of the material and sintering protocol, layer thickness, and thermomechanical aging on the two-body wear and fracture load of 4Y-TZP crowns.

OBJECTIVES: The aim of this study is to investigate the influence of the material and corresponding sintering protocol, layer thickness, and aging on the two-body wear (2BW) and fracture load (FL) of 4Y-TZP crowns. MATERIALS AND METHODS: Multi-layer 4Y-TZP crowns in three thicknesses (0.5 mm/1.0 mm/1.5 mm) were sintered by high-speed (Zolid RS) or conventional (Zolid Gen-X) sintering. 2BW of ceramic and enamel antagonist after aging (1,200,000 mechanical-, 6000 thermal-cycles) was determined by 3D-scanning before and after aging and subsequent matching to determine volume and height loss (6 subgroups, n = 16/subgroup). FL was examined initially and after aging (12 subgroups, n = 16/subgroup). Fractographic analyses were performed using light-microscope imaging. Global univariate analysis of variance, one-way ANOVA, linear regression, Spearman's correlation, Kolgomorov-Smirnov, Mann-Whitney U, and t test were computed (alpha = 0.05). Weibull moduli were determined. Fracture types were analyzed using Ciba Geigy table. RESULTS: Material/sintering protocol did not influence 2BW (crowns: p = 0.908, antagonists: p = 0.059). High-speed sintered Zolid RS presented similar (p = 0.325-0.633) or reduced (p < 0.001-0.047) FL as Zolid Gen-X. Both 4Y-TZPs showed an increased FL with an increasing thickness (0.5(797.3-1429 N) < 1.0(2087-2634 N) < 1.5(2683-3715 N)mm; p < 0.001). For most groups, aging negatively impacted FL (p < 0.001-0.002). Five 0.5 mm specimens fractured, four showed cracks during and after aging. CONCLUSIONS: High-speed sintered crowns with a minimum thickness of 1.0 mm showed sufficient mechanical properties to withstand masticatory forces, even after a simulated aging period of 5 years. CLINICAL RELEVANCE: Despite the manufacturer indicating a thickness of 0.5 mm to be suitable for single crowns, a minimum thickness of 1.0 mm should be used to ensure long-term satisfactory results.

Speed sintering translucent zirconia for chairside one-visit dental restorations: Optical, mechanical, and wear characteristics.

The fabrication of zirconia dental restorations is a time-consuming process due to traditional slow sintering schemes; zirconia (Y-TZP) produced by these conventional routes are predominantly opaque. Novel speed sintering protocols have been developed to meet the demand for time and cost effective chairside CAD/CAM-produced restorations, as well as to control ceramic microstructures for better translucency. Although the speed sintering protocols have already been used to densify dental Y-TZP, the wear properties of these restorations remain elusive. Fast heating and cooling rates, as well as shorter sintering dwell times are known to affect the microstructure and properties of zirconia. Thus, we hypothesize that speed sintered zirconia dental restorations possess distinct wear and physical characteristics relative to their conventionally sintered counterparts. Glazed monolithic molar crowns of translucent Y-TZP (inCoris TZI, Sirona) were fabricated using three distinct sintering profiles: Super-speed (SS, 1580 °C, dwell time 10 min), Speed (S, 1510 °C, dwell time 25 min), and Long-term (LT, 1510 °C, dwell time 120 min). Microstructural, optical and wear properties were investigated. Crowns that were super-speed sintered possessed higher translucency. Areas of mild and severe wear were observed on the zirconia surface in all groups. Micropits in the wear crater were less frequent for the LT group. Groups S and SS exhibited more surface pits, which caused a scratched steatite surface that is associated with a greater volume loss. Tetragonal to monoclinic phase transformation, resulting from the sliding wear process, was present in all three groups. Although all test groups had withstood thermo-mechanical challenges, the presence of hairline cracks emanating from the occlusal wear facets and extending deep into the restoration indicates their susceptibility to fatigue sliding contact fracture.

Impact of the sintering parameters on the grain size, crystal phases, translucency, biaxial flexural strength, and fracture load of zirconia materials.

OBJECTIVES: To investigate the influence of the zirconia and sintering parameters on the optical and mechanical properties. METHODS: Three zirconia materials (3/4Y-TZP, 4Y-TZP, 3Y-TZP) were high-speed (HSS), speed (SS) or conventionally (CS) sintered. Disc-shaped specimens nested in 4 vertical layers of the blank were examined for grain size (GS), crystal phases (c/t'/t/m-phase), translucency (T), and biaxial flexural strength. Fracture load (FL) of three-unit fixed dental prostheses was determined initially and after thermomechanical aging. Fracture types were classified, and data statistically analyzed. RESULTS: 4Y-TZP showed a higher amount of c + t'-phase and lower amount of t-phase, and higher optical and lower mechanical properties than 3Y-TZP. In all materials, T declined from Layer 1 to 4. 3/4Y-TZP showed the highest FL, followed by 3Y-TZP, while 4Y-TZP showed the lowest. In 4Y-TZP, the sintering parameters exercised a direct impact on GS and T, while mechanical properties were largely unaffected. The sintering parameters showed a varying influence on 3Y-TZP. Thermomechanical aging resulted in comparable or higher FL. CONCLUSION: 3/4Y-TZP presenting the highest FL underscores the principle of using strength-gradient multi-layer blanks to profit from high optical properties in the incisal area, while ensuring high mechanical properties in the lower areas subject to tensile forces. With all groups exceeding maximum bite forces, the examined three-unit FDPs showed promising long-term mechanical properties.

In Vitro Evaluation of Speed Sintering and Glazing Effects on the Flexural Strength and Microstructure of Highly Translucent Multilayered 5 mol% Yttria-Stabilized Zirconia.

This study aimed to investigate the impact of speed sintering and glazing on the flexural strength and microstructure of multilayered 5 mol% yttria-stabilized (5Y-) zirconia, which remains unknown. Bar-shaped specimens (N = 600) were fabricated from 5Y-zirconia (FX; Ceramill Zolid FX ML, ST; Katana STML) by cutting, polishing, sintering (conventional and speed sintering), and then glazing. A flexural strength test (n = 30/group), field emission scanning electron microscopy (FE-SEM) observation (n = 2/group), and an X-ray diffraction (XRD) study with Rietveld refinement (n = 1/group) were performed. The flexural strength was analyzed using three-way ANOVA and a post hoc Scheffé test. The grain size was analyzed using the Kruskal-Wallis H test and Bonferroni-Dunn post hoc test. Flexural strength slightly decreased in the nonglazed FX after speed sintering (p < 0.05). Glazing with and without glazing paste did not affect flexural strength at both sintering speeds (p > 0.05). Speed sintering and glazing minimally changed the Weibull modulus and phase fraction, and did not affect grain size (p > 0.05). ST had a larger grain size and lower tetragonal phase content than FX and had a lower flexural strength than FX in most groups (p < 0.05). Overall, the multilayered 5Y-zirconia is considered suitable for dental application using speed sintering and glazing.

Effect of speed sintering process on the microstructure, flexural strength and translucency of zirconia.

Objectives: To determine the impact of the speed sintering program on the microstructure, flexural strength and translucency of zirconia in comparison with those of the conventional sintering program. Materials and methods: rectangular shape specimens (12.5 × 15.5 × 1.2 mm) were prepared from four commercial pre-sintered zirconia ceramics (KATANA HTML, KATANA STML, InCoris TZI and InCoris ZI) that were sintered with conventional and speed sintering programs according to the manufacturer's instructions. The phase composition of the sintered specimens was determined by X-ray diffraction (XRD). The grain size was evaluated using scanning electron microscopy (SEM), while the three-point flexural strength was assessed based on the ISO 6872: 2015 standard. Translucency was assessed using a spectrophotometer. The data were analyzed using independent t tests (α = 0.05) and one-way ANOVA. Results: The XRD patterns were similar for all the groups, indicating that there was no phase transformation. SEM revealed that the average grain size was lower than 1 µm. The grain size, flexural strength and translucency results showed increasing trends when speed sintering is compared with the conventional one but the differences were not significant (P > 0.05). Conclusions: The results of this research indicate that the speed sintering program had no significant impact on the microstructure, flexural strength and translucency of the examined zirconia, a speed-sintering program can process the ceramic material within a short time with slightly increase their flexural strength and translucency Therefore, a speed-sintering program is appropriate for zirconia (Y-TZP).

Effect of low-temperature degradation and sintering protocols on the color of monolithic zirconia crowns with different yttria contents.

This study investigated the effects of low-temperature degradation (LTD) on the L*, a*, and b* values of highly translucent zirconia crowns. Four types of zirconia disks with different yttria contents (IPS e.max ZirCAD LT, IPS e.max ZirCAD MT, IPS e.max ZirCAD MT Multi, IPS e.max ZirCAD Prime, Ivoclar) and two shades (A2 and BL) were used. A crown was manufactured using four types of zirconia and LTD treated. Color measurements were performed, and the color difference (ΔE00) before and after LTD was calculated. The microstructure was determined through X-ray fluorescence and X-ray diffractometry. Highly translucent zirconia crowns showed greater changes in the a* and b* values than in the L* value after LTD, regardless of the shade. The Multi2 crowns exhibited a discernible color change due to the LTD treatment. The X-ray fluorescence results did not reveal any apparent change in the microstructure between sintering programs for all zirconia specimens.

Effect of speed sintering and aging on the translucency of high-translucent zirconia.

OBJECTIVES: This work aimed to evaluate the effect of speed sintering and low-temperature degradation on the translucency of high-translucent zirconia. METHODS: The ST and TT specimens were randomly divided into two groups depending on the sintering process: conventional sintering and speed sintering. The sintered specimens were divided into three subgroups according to the aging time: aged for 0, 5, and 20 h. Chromatic parameters (L*, a*, and b* values) were measured by Shade Eye NCC computer colorimeter in a dark environment under black and white background, and the translucency parameter (TP) was used to evaluate the translucency of zirconia. RESULTS: Speed sintering may decrease the TP of ST and increase the TP of TT. As for the effect of low-temperature degradation on the translucency of zirconia, the TP of ST decreased with the extension of aging time, and no significant difference was found in rapid sintering ST. Although the TP of TT decreased, no statistical difference was observed. CONCLUSIONS: Speed sintering may decrease the translucency of high-strength zirconia and increase the translucency of high-translucent zirconia. Low-temperature degradation had no effect on the translucency of high-translucent zirconia. Speed sintering can be recommended for high-translucent zirconia in terms of translucency.

Current speed sintering and high-speed sintering protocols compromise the translucency but not strength of yttria-stabilized zirconia.

OBJECTIVES: To investigate the impacts of speed and high-speed sintering on the densification, microstructure, phase composition, translucency, and flexural strength of yttria-stabilized zirconia (YSZ). METHODS: A total of 162 disc-shaped specimens (n = 18) were cold-isostatically pressed from 3YSZ (Zpex), 4YSZ (Zpex 4), and 5YSZ (Zpex Smile) powders (Tosoh Corporation) and sintered according to the following protocols: conventional (control, ∼12 h), speed (∼28 min for 3YSZ; ∼60 min for 4YSZ and 5YSZ), and high-speed (∼18 min) sintering. Dimensions of zirconia specimens after sintering and polishing (1-µm diamond grit finish) were Ø13.75 × 1 mm. Density, microstructure, phase content, translucency parameter, and biaxial flexural strength were evaluated using Archimedes', SEM, XRD, spectrophotometric, and piston-on-3-ball methods, respectively. Data were analyzed with either one-way ANOVA and Tukey's test or Kruskal-Wallis with Dunn's test (α = 0.05). RESULTS: For all YSZ compositions, conventional sintering yielded the highest density followed by speed then high-speed sintering. All sintering protocols resulted in similar strength values; however, speed and high-speed sintering protocols afforded significantly lower translucency relative to conventional sintering. XRD analysis revealed similar spectra for YSZs sintered by various protocols. The speed sintered specimens had the smallest grain size whereas the high-speed sintered 5YSZ possessed the largest grain size among all groups. SEM examination of all YSZ compositions revealed that the average pore size was an order of magnitude smaller than the average grain size. SIGNIFICANCE: Speed and high-speed sintering of YSZs yield similar strength but diminished density and translucency relative to their conventionally sintered counterparts.

Effect of Ga2O3 Dopant on High Speed Sintered 5 mol% Y2O3 Stabilized Dental Zirconia.

The high-speed sintering of zirconia has become essential for ceramic dental prosthesis treatment in a single visit. Previous studies have shown that 5 mol% yttria-stabilized zirconia (5Y zirconia), with the exception of some types, loses strength and translucency with high-speed sintering. In this study, 0.15-0.92 wt% Ga2O3, which is expected to promote the sintering of zirconia, was added to improve the properties of 5Y zirconia high-speed sintered bodies, and the effect of its addition was evaluated. The specimens were characterized by density and translucency measurements, a three-point bending test, X-ray diffraction (XRD), scanning electron microscopy (SEM), and shrinkage measurement. The addition of Ga2O3 improved both translucency and flexural strength of 5Y zirconia high-speed sintered bodies. XRD and SEM observations revealed that this improvement in properties was due to the change in the crystal phase composition and the decrease in the amount and size of pores due to the addition of Ga2O3. Shrinkage measurements also revealed that the addition of Ga2O3 changed the sintering behavior of 5Y zirconia, suggesting that this change led to a reduction in porosity. From the above results, it was concluded that Ga2O3 addition is effective in improving the properties of 5Y zirconia high-speed sintered bodies.

Effect of High-Speed Sintering on the Optical Properties, Microstructure, and Phase Distribution of Multilayered Zirconia Stabilized with 5 mol% Yttria.

As dental 5 mol% yttria-stabilized (5Y-) zirconia demand high esthetics, it is necessary to clarify how the optical properties are affected by high-speed sintering, which is not yet fully understood. Our study aimed to investigate the effect of high-speed sintering on the translucency and opalescence parameters (TP and OP, respectively), as well as their related microstructure and phase distribution, using two types of multilayered 5Y-zirconia. Multilayered 5Y-zirconia (Cercon xt ML, Lava Esthetic) were cut layer-by-layer, followed by conventional and high-speed sintering. The TP and OP values were subsequently obtained using a spectrophotometer, and field emission scanning electron microscopy images were used to analyze the average grain size. The phase fractions were analyzed using X-ray diffraction. Regardless of the zirconia type, the TP was slightly lowered by high-speed sintering in all the layers except the dentin layer (DL) for Lava Esthetic (p < 0.05). The OP decreased by high-speed sintering in the DL for Cercon xt ML and in all the layers for Lava Esthetic (p < 0.05). The decrease in translucency after high-speed sintering was attributed to a decrease in the yttria-rich t'-phase with low tetragonality, along with an increase in the yttria-lean t-phase with high tetragonality.

Transient thermal stresses developed during speed sintering of 3 mol% yttria-stabilized tetragonal zirconia polycrystals.

OBJECTIVES: To investigate transient thermal stresses that developed in 3Y-TZP green compacts during speed sintering. METHODS: A total of 312 disc-shaped green compacts (Ø17.1 ×1, 1.5, 2, 2.5, 3 mm) were cold-isostatically pressed from 3Y-TZP powder (Zpex, Tosoh Corp.) for speed sintering studies as well as compositional analysis and biaxial flexural strength measurements (both at room temperature and following heating at 90 °C/min to 500 °C). Flexural strength was determined using the piston-on-3-ball method. Phase assemblies were analyzed using the X-ray diffraction method. Effects of heating/cooling rates on transient stresses were investigated by conducting definitive sintering studies to determine the threshold for fracture. Finite element analysis (FEA) was used to validate the experimental findings using measured thermomechanical properties. RESULTS: The bulk and relative density of the green compact were 2.95 ± 0.03 g/cm3 and 48.52% ± 0.45%. The flexural strength was drastically decreased from 10.3 ± 0.4 MPa to 1.09 ± 0.07 MPa following heating at 90 °C/min to 500 °C. The monoclinic and tetragonal contents were 54.9% and 45.1%, respectively. The threshold for fracture was located at 500 °C during the first heating stage with a 90 °C/min heating rate in specimens of 2.5 mm thickness or greater. No fractures occurred in the second heating stage and cooling phase. The FEA estimated that the principal transient tensile stress was ∼1.14 MPa at 500 °C during the heating phase, which exceeded the corresponding flexural strength (1.09 ± 0.07 MPa). SIGNIFICANCE: Advanced FEA methods are an accurate and efficient tool to analyze the history of transient stresses during sintering of ceramic dental restorations.

Effect of sintering time on microstructure and optical properties of yttria-partially stabilized monolithic zirconia.

OBJECTIVES: To evaluate the impact of speed sintering on the microstructure and optical properties of 3 and 5 mol% yttria-partially stabilized monolithic zirconias. METHODS: 120 plate-shaped zirconia specimens (10x10x 0.4 mm) were prepared from three commercial 5 mol% yttria-partially stabilized zirconia blocks (5Y-PSZs); Katana UTML (Kuraray Noritake), Cercon xtML (Dentsply Sirona), and Zolid FX white (Amann Girrbach), and two commercial 3 mol% yttria-partially stabilized zirconia blocks (3Y-PSZs); Lava Plus (3 M ESPE) and InCoris (Sirona, GmbH). Specimens were either conventional sintered (CS) using a 7-hour program or speed sintered (SS) using a quick 90-minute program. The microstructure was inspected with a scanning electron microscope (SEM), and phase fractions were detected using x-ray diffraction analysis (XRD). Translucency (TP00), and contrast ratio (CR) were obtained using a spectrophotometer (VITA Easyshade V). Color difference (ΔE00) between both sintering processes was calculated with the CIEDE2000 formula. ΔE00 up to 1.8 was set as the acceptability threshold. Data were analyzed using two-way ANOVA, Krusakll-Wallis, and Mann-Whitney U tests. (n = 12, α = .05). RESULTS: Grain size was significantly decreased after SS for all tested materials (P < .0001). The average grain sizes of 5Y-PSZs were significantly larger than those of 3Y-PSZs. The atomic structure, microstructure, and transparency of CS and SS were all affected by the amount of yttria, the size of the crystals, and tetragonality. SS significantly reduced TP00 (F = 7135.95, P < 0.0001) and increased CR (F = 453.21, P < 0.0001). The CS Katana presented the highest TP00 and lowest CR value. ΔE00 between the CS and SS groups were clinically acceptable except for Lava, which had values above the set threshold (1.89). SIGNIFICANCE: SS altered the grain size and internal structure of the tested materials, which was reflected in translucency.

Evaluation of the Effect of High-Speed Sintering and Specimen Thickness on the Properties of 5 mol% Yttria-Stabilized Dental Zirconia Sintered Bodies.

High-speed sintering of zirconia has become essential to single-visit dental prosthetic treatments. This important prosthetic dentistry technique demands a translucent material tougher than porcelain. Previous studies on high-speed sintered zirconia did not take heat and material thickness into consideration. We evaluated pre-sintered specimen thickness and the effect of high-speed sintering on the properties of 5 mol% Y2O3-stabilized zirconia (5Y zirconia). High-speed sintered bodies of 5Y zirconia were evaluated by density measurements, translucency measurements, three-point flexural and fracture toughness tests, X-ray diffraction (XRD), and scanning electron microscopy (SEM). High-speed sintering reduced the translucency and mechanical properties of 5Y zirconia. XRD and SEM observation results clarified that these reductions were due to the change in crystal phase composition and to the increase in residual pores, respectively, both resulting from high-speed sintering. Moreover, in high-speed sintering, as the thickness of the specimen increased, the number and size of internal pores increased, and the translucency and strength decreased. The threshold value for avoiding a reduction in translucency and mechanical properties was found to lie at ~4.4 mm. From the above results, it was concluded that 5Y zirconia is not suitable for high-speed sintering applications.

Evaluation of Polyethylene Terephthalate Powder in High Speed Sintering.

Laser Sintering (LS) was the first Powder Bed Fusion (PBF) method for polymers and it is now quite an established process for rapid prototyping and even for the production of functional parts. High Speed Sintering (HSS) is a variant of PBF which was later developed and it has the potential to be more scalable than LS. Most of the work for HSS and LS has been conducted with polyamide-12 (PA 12). This work reports the first effort to use polyethylene terephthalate (PET) in HSS. Well defined, simple and complex parts could be printed without any build failures. However, limitations were induced by current HSS machines which led to some curvature (warpage) in tensile bars after manufacturing. The reason for this was that all currently available machines for HSS are built for polymers such as polyamide 12, which means their maximum bed temperature is limited to 190 °C. This corresponds to the lower limit of processability of PET in PBF processes. The slightly curved tensile bars were straightened by heating them to 230 °C with a weight on top, and afterwards the mechanical properties were measured. The tensile modulus was similar to what was obtained with PET via LS but the strength and elongation-at-break (EAB) was lower. Microscopy showed that the reason for the lower strength and EAB was the incomplete melting of particles. This arose from the temperature limitation of the current generation of HSS machines. The porosity was established as 2.23% by helium pycnometry which is the same as for LS. The results of the thermal analysis indicated that the PET parts manufactured with HSS were semi-crystalline like the PET parts manufactured via LS.

Comparative analysis on intaglio surface trueness, wear volume loss of antagonist, and fracture resistance of full-contour monolithic zirconia crown for single-visit dentistry under simulated mastication.

PURPOSE: This analysis aimed to evaluate the intaglio surface trueness, antagonist's wear volume loss, and fracture resistance of full-contour crowns of (Y, Nb)-stabilized fully-sintered zirconia (FSZ), 4 mol% or 5 mol% yttria-stabilized partially sintered zirconia (4YZ or 5YZ) with high-speed sintering. MATERIALS AND METHODS: A total of 42 zirconia crowns were separated into three groups: FSZ, 4YZ, and 5YZ (n = 14). The intaglio surface trueness of the crowns was evaluated at the inner surface, occlusal, margin, and axial areas and reported as root-mean-square, positive and negative average deviation. Half of the specimens were aged for 120,000 cycles in the chewing simulator, and the wear volume loss of antagonist was measured. Before and after chewing, the fracture load was measured for each group. The trueness values were analyzed with Welch's ANOVA, and the wear volume loss with the Kruskal-Wallis tests. Effect of the zirconia type and aging on fracture resistance of crowns was tested using two-way ANOVA. RESULTS: The intaglio surface trueness measured at four different areas of the crown was less than 50 µm, regardless of the type of zirconia. No significant P in wear volume loss of antagonists were detected among the groups (P > .05). Both the type of zirconia and aging showed statistically significant effects on fracture resistance (P < .05). CONCLUSION: The full-contour crowns of FSZ as well as 4YZ or 5YZ with high-speed sintering were clinically acceptable, in terms of intaglio surface trueness, antagonist's wear volume loss, and fracture resistance after simulated mastication.

Comparison of Regular and Speed Sintering on Low-Temperature Degradation and Fatigue Resistance of Translucent Zirconia Crowns for Implants: An In Vitro Study.

BACKGROUND: Although there are a few studies which compare fast and slow sintering in normal zirconia crowns, it is essential to compare the cracks and load-bearing capacity in zirconia screw-retained implant crowns between regular and speed sintering protocols. This research aimed to compare the surface structure, cracks, and load-bearing capacity in zirconia screw-retained implant crowns between regular sintering (RS) and speed sintering (SS) protocol with and without cyclic loading (fatigue). METHODS: A total of 60 screw-retained crowns were fabricated from zirconia (Katana STML Block) by the CAD/CAM system. Then, 30 crowns were subjected to the RS protocol and 30 crowns were subjected to the SS protocol. Cyclic loading was done in half zirconia crowns (15 crowns in each group) using a chewing simulator CS-4.8/CS-4.4 at room temperature. The loading force was applied on the middle of the crowns by a metal stylus underwater at room temperature with a chewing simulator at an axial 50 N load for 240,000 cycles and lateral movement at 2 mm. Scanning electron microscopy was done to study the surface of the crowns and the cracks in the crowns of the regular and speed sintering protocols, with and without fatigue. RESULTS: For the speed sintering group, the surface looks more uniform, and the crack lines are present at a short distance compared to regular sintering. The sintering protocol with a larger Weibull module and durability increases the reliability. It showed that the Speed group showed the maximum fracture load, followed by the regular, speed fatigue, and regular fatigue groups. The fracture load in various groups showed significant differences. CONCLUSIONS: It was found that the speed group showed the maximum fracture load followed by the regular, speed fatigue, and regular fatigue. The crack lines ran from occlusal to bottoms (gingiva) and the arrest lines were perpendicular to the crack propagations.

Influence of high-speed sintering protocols on translucency, mechanical properties, microstructure, crystallography, and low-temperature degradation of highly translucent zirconia.

OBJECTIVES: Impacts of high-speed sintering on the optical and mechanical properties, microstructure, crystallography, and low-temperature degradation of commercial yttria-partially stabilized zirconia (Y-PSZ) were investigated. METHODS: Five commercial Y-PSZ products (KATANA HT, KATANA STML, KATANA UTML, Zpex 4, and Zpex Smile) were investigated. Specimens were sintered following speed-sintering (~90 min) and conventional-sintering protocols (~7 h), and a group of KATANA STML was super-speed-sintered (18 min). Dimensions of the zirconia specimens after sintering were 14.5 mm (diameter) and 1.2 mm (thickness). Translucency was assessed using a colorimeter. Biaxial flexural strength was measured using a universal testing machine, followed by Weibull analysis. Scanning electron microscopy was used for microstructure assessments. X-ray diffraction was used to analyze the crystallography before and after hydrothermal aging. Low-temperature degradation (LTD) tests were performed at 134 °C under 2-3 bar water vapor in an autoclave. RESULTS: The translucency and flexural strength were not affected significantly by the sintering programs (p > 0.05). The conventionally sintered KATANA STML and speed-sintered Zpex 4 presented the highest and lowest Weibull modulus, respectively. The conventionally-sintered Y-PSZ had a larger average grain size and smaller fraction of fine grains than those of the speed-sintered specimens. The fractographic analysis of the speed- and conventionally sintered Y-PSZ yielded comparable results. The speed-sintered Y-PSZ exhibited a lower c-ZrO2 content than that of conventionally-sintered Y-PSZ, except for KATANA HT and KATANA STML. LTD tests indicated that some of the speed and conventionally-sintered Y-PSZ exhibited similar monoclinic volume fractions. SIGNIFICANCE: Speed-sintering programs are acceptable for Y-PSZ zirconia.

Conventional, Speed Sintering and High-Speed Sintering of Zirconia: A Systematic Review of the Current Status of Applications in Dentistry with a Focus on Precision, Mechanical and Optical Parameters.

The aim of this systematic review was to provide an overview of the technical and clinical outcomes of conventional, speed sintering and high-speed sintering protocols of zirconia in the dental field. Data on precision, mechanical and optical parameters were evaluated and related to the clinical performance of zirconia ceramic. The PICOS search strategy was applied using MEDLINE to search for in vitro and in vivo studies using MeSH Terms by two reviewers. Of 66 potentially relevant studies, 5 full text articles were selected and 10 were further retrieved through a manual search. All 15 studies included in the systematic review were in vitro studies. Mechanical, precision and optical properties (marginal and internal fit, fracture strength and modulus, wear, translucency and opalescence, aging resistance/hydrothermal aging) were evaluated regarding 3-, 4- and 5-YTZP zirconia material and conventional, high- and high-speed sintering protocols. Mechanical and precision results were similar or better when speed or high-speed sintering methods were used for 3-, 4- and 5-YTZP zirconia. Translucency is usually reduced when 3 Y-TZP is used with speed sintering methods. All types of zirconia using the sintering procedures performed mechanically better compared to lithium disilicate glass ceramics but glass ceramics showed better results regarding translucency.

In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns.

OBJECTIVES: The purpose of this study was to evaluate the fit, fracture load and aging resistance of the monolithic zirconia tooth-borne crowns with conventional and high-speed sintering. METHODS: The Y-TZP block was machined and sintered with conventional and high-speed sintering furnace. The marginal and internal gap between the crown and abutment was measured using a microscope and a fit checking material. A total of 28 crowns were further divided into an undegraded and a degraded group. An accelerated aging test was carried out on the degraded group. The crown was cemented and a fracture resistance was tested. X-ray diffraction was used to evaluate the crystalline structure. The data were analyzed with Student's t-test, and a one-way ANOVA and Tukey's multiple comparison test. RESULTS: There was no significant difference in mean marginal gap between the two groups. The mean internal gap was significantly greater in the speed sintering than in the conventional sintering (P <0.001). The mean fracture load of the conventional sintering crowns was not significantly different from that of speed sintering crowns after aging. The occurrence of monoclinic crystals of degraded crown was significantly higher than that of undegraded crown both in the conventional (P <0.001) and speed-sintering group (P <0.001). CONCLUSIONS: It was concluded that the monolithic zirconia crowns produced by high-speed sintering showed no significant difference in the marginal gap and the fracture load after aging compared to conventional sintering. Therefore, the high-speed sintering seems a valid method of producing tooth-borne monolithic zirconia crowns.