Speed-sintering and the mechanical properties of 3-5 mol% Y2O3-stabilized zirconias

Should the load-to-fracture test for CAD/CAM monolithic molar crowns be standardized and how? A systematic review and finite element analysis

The load-to-fracture test is widely used to evaluate crowns made of new CAD/CAM materials, even though its validity in predicting clinical performances is often questioned. Despite its limitations, the test is useful in assessing the load-bearing capacity of crowns subjected to accidental overloads and setting up step-stress regimes for fatigue testing. This study combined a systematic review (SR) and finite element analysis (FEA) to assess whether the test should be standardized and how. The SR evaluated load-to-fracture studies of monolithic CAD/CAM molar crowns published in Q1 and Q2 journals. Findings from 85 studies highlighted the lack of standardization in test methods, particularly regarding loading head type and die material. This variability led to a wide dispersion of fracture load results, limiting the utility of the load-to-fracture test. The FEA evaluated the influence of loading head type and die material on tensile stress distribution in lithium disilicate (LD) and polymer-infiltrated ceramic network (PICN) crowns. Eight in vitro conditions were simulated, varying the loading head (4 mm and 10 mm spheres, inverse V-shaped device, opposing teeth) and die material (stiff, E = 207 GPa; non-stiff, E = 13 GPa). The FEA confirmed that the stress distribution and peak tensile stress in LD and PICN crowns depend significantly on these factors as well as the crown material properties, with the peak stress variation from LD to PICN ranging from -4 % to 237 %. Using larger-diameter spheres with a die material approximating dentin in stiffness resulted in stress distributions more representative of clinical conditions.

Fatigue strength of bilayer yttria-stabilized zirconia after low-temperature degradation

This study examined the impact of interfacial interactions on bilayer yttria-stabilized zirconia (YSZ) used in dental restorations. In-house bilayer structures of 3YSZ and 5YSZ composition underwent hydrothermal degradation to compare the properties of control and low-temperature degradation (LTD) treated groups. Biaxial flexural strength via piston-on-three-balls, staircase fatigue strength over 106 cycles at 15 Hz, phase characterization and quantification through XRD and Rietveld refinement, and fractography were conducted. Weibull analysis was employed to determine the Weibull modulus and characteristic strength. Results demonstrated an enhancement in the mechanical performance of 3YSZ composition after LTD treatment, whereas the mechanical properties of 5YSZ remained largely unaffected post-degradation. Fractographic analysis revealed that failure originated at the surface tensile location across all specimen groups. These findings offer insights into the mechanical behavior of bilayer zirconia structures and reinforce the significance of hydrothermal treatment in enhancing their performance, particularly in the case of 3Y compositions.

Microwave versus conventional sintering on mechanical properties of 3Y-TZP - A systematic review

This systematic review evaluates the mechanical properties of yttria-stabilized zirconia (3Y-TZP) processed by microwave sintering compared to conventional sintering methods. Zirconia, known for its excellent strength and esthetics, has seen advancements in sintering techniques to enhance its properties. Conventional sintering, while effective, is time-consuming and less energy-efficient. In contrast, microwave sintering, introduced in 1999, offers rapid heating and improved control over temperature and shrinkage. This review, adhering to PRISMA guidelines, included studies from 2000 to 2023 that compared the effects of both sintering methods on relative density, flexural strength, Young's modulus and hardness of zirconia. Results indicate that microwave sintering improves hardness and reduces processing time, whereas conventional sintering provides higher relative density, flexural strength, and Young's modulus. The findings suggest that the choice of sintering technique should align with specific material property requirements, with each method offering distinct advantages for zirconia applications.

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.

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.

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.