The Effect of Sintering Temperature on the Phase Composition, Microstructure, and Mechanical Properties of Yttria-Stabilized Zirconia

Effects of Induction Plasma Spheroidization on Properties of Yttria-Stabilized Zirconia Powders for Thermal Barrier Coating Applications

In this study, the induction plasma spheroidization (IPS) technique was adopted to improve the microstructure and properties of the traditional agglomerated ZrO2-7wt%Y2O3 (YSZ) powders used in thermal barrier coating (TBC) applications. Compared with agglomerated YSZ powders, IPS-treated powder has a more desirable microstructure, and the overall performance of the spray powders for TBC preparation is significantly improved. Specifically, IPS-treated powder has a dense, solid, defect-free, and chemically uniform microstructure, and its apparent density, flowability, and powder strength are significantly improved, which is believed to substantially enhance the coating performance when prepared with this IPS-treated powder.

A Comparison of Failure Loads for Polycrystalline Zirconia Ceramics with Varying Amounts of Yttria, Glass-Ceramics and Polymers in Two Different Test Conditions

It is unclear how zirconia dental crowns with different yttria compositions will perform clinically, and how they will compare with crowns made of glass-ceramics and polymers. The present objective was to determine failure loads of crowns and discs made of glass ceramics or polymers as compared to yttria-partially stabilized zirconia (Y-PSZ) crowns and discs with varying yttria concentrations. Crowns of zirconia (Cercon XT, Katana UTML, BruxZir Anterior), glass ceramic (Celtra press, IPS e.max press, Lisi press), and polymeric materials (Trilor, Juvora, Pekkton) were fabricated and cemented to epoxy abutments. The total number of specimens was 135 for crowns and 135 for discs (n = 15 specimens per material type and design). A universal testing machine was used to perform compressive loading of crowns/discs to failure with a steel piston along the longitudinal axis of the abutments. Energy dispersive spectroscopy (EDS) was used to identify the yttria concentration for each zirconia brand. The data were analyzed using generalized linear models and regression analyses. The results revealed significant differences (p < 0.05) in mean failure loads for different crown materials: Trilor (6811 ± 960 N) > Juvora (5215 ± 151 N) > Cercon (4260 ± 520 N) = BruxZir (4186 ± 269 N) = e.max (3981 ± 384 N) > Katana (3195 ± 350 N) = Lisi (3173 ± 234 N) = Pekkton (3105 ± 398 N) > Celtra (2696 ± 393 N). The general linear model revealed significant differences (p < 0.05) in mean failure loads when comparing the different materials for the discs, i.e., Trilor (5456 ± 1748 N) > Juvora (4274 ± 869 N) > Pekkton (3771 ± 294 N) > Katana (2859 ± 527 N) > Cercon (2319 ± 342 N) = BuxZir (2250 ± 515 N) = e.max (2303 ± 721 N) = Lisi (2333 ± 535 N) > Celtra (1965 ± 659 N). EDS showed that the zirconia materials contained yttria at different concentrations (BruxZir = 5Y-PSZ, Cercon = 4Y-PSZ, Katana = 3Y-PSZ). The yttria concentration had a significant effect on the failure load of the Katana (3Y-PSZ) crowns, which revealed lower failure loads than the Cercon (4Y-PSZ) and BruxZir (5Y-PSZ) crowns, whose failure loads were comparable or higher than e.max glass ceramic. The failure load of the trilayer disc specimens did not correlate with the failure load of the respective crown specimens for the zirconia, glass-ceramic and polymeric materials.

Preparation and cutting performance study of YSZ-toughened PcBN superhard tools

Titanium alloy, as a recognized difficult-to-cut material, places higher demands on the performance of cutting tools. Compared with the mainstream cemented carbide tools, PcBN tools have a higher life and better machining performance. In this paper, a new type of cubic boron nitride superhard tool was prepared by introducing Y₂O₃-stabilized ZrO₂ (YSZ) under high temperature and high pressure (1500 °C, 5.5 GPa), and the effect of the variation of YSZ addition on the mechanical properties of the tool was systematically analyzed, and the cutting performance of the tool was also analyzed by cutting TC4. It was found that a small amount of YSZ addition, which generated a sub-stable t-ZrO₂ phase during the sintering process, could improve the mechanical properties of the tool and increase its cutting life. When YSZ was added at 5 wt%, the flexural strength and fracture toughness of the composites reached the maximum values of 637.77 MPa and 7.18 MPa m^1/2, while the cutting life of the tools reached the maximum value of 2615.81 m. And when YSZ was added at 2.5 wt%, the hardness of the material reached the maximum value of 43.62 GPa.

Porous 8YSZ Ceramics Prepared with Alkali Halide Sacrificial Additives

8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) ceramics were prepared with KCl and LiF additions to obtain porous specimens with high skeletal density. Thermogravimetric and differential thermal analyses (TG/DTA) were carried out on 8YSZ and on 8YSZ mixed to 5 wt.% KCl or 5 wt.% LiF as sacrificial pore formers that were thermally removed during sintering. The melting and evaporation of the alkali halides were evaluated by differential thermal analysis. Dilatometric analysis was also carried out following the same TG/DTA temperature profile with results suggesting rearrangement of the 8YSZ particles during LiF and KCl melting. The dilatometric data of 8YSZ green pellets mixed to KCl or LiF exhibited an initial expansion up to the melting of the alkali halide, followed by shrinkage due to sintering evolution with grain growth and pore elimination. The time that the alkali halide molten phase was kept during sintering was found to be an important parameter for obtaining 8YSZ-sintered specimens with specific pore content; bulk density and open porosity could then be tuned by controlling the time the alkali halide remained liquid during sintering. Scanning electron microscopy images of the pellet fracture surfaces showed pores that contributed to increasing the electrical resistivity as evaluated by impedance spectroscopy analysis.

Effect of Two-Step Sintering on the Mechanical and Electrical Properties of 5YSZ and 8YSZ Ceramics

Yttria-stabilized zirconia (YSZ) has been widely used in structural and functional ceramics because of its excellent physicochemical properties. In this paper, the density, average gain size, phase structure, and mechanical and electrical properties of conventionally sintered (CS) and two-step sintered (TSS) 5YSZ and 8YSZ are investigated in detail. As the grain size of YSZ ceramics became smaller, dense YSZ materials with a submicron grain size and low sintering temperature were optimized in terms of their mechanical and electrical properties. 5YSZ and 8YSZ in the TSS process significantly improved the plasticity, toughness, and electrical conductivity of the samples and significantly suppressed the rapid grain growth. The experimental results showed that the hardness of the samples was mainly affected by the volume density, that the maximum fracture toughness of 5YSZ increased from 3.514 MPa·m^1/2 to 4.034 MPa·m^1/2 in the TSS process, an increase of 14.8%, and that the maximum fracture toughness of 8YSZ increased from 1.491 MPa·m^1/2 to 2.126 MPa·m^1/2, an increase of 42.58%. The maximum total conductivity of the 5YSZ and 8YSZ samples under 680 °C increased from 3.52 × 10^-3 S/cm and 6.09 × 10^-3 S/cm to 4.52 × 10^-3 S/cm and 7.87 × 10^-3 S/cm, an increase of 28.41% and 29.22%, respectively.

Qualitative SEM analysis of fracture surfaces for dental ceramics and polymers broken by flexural strength testing and crown compression

PURPOSE

To perform qualitative analysis using scanning electron microscopy (SEM) of fracture surfaces for ceramic and polymeric dental materials broken via standardized flexural and crunch-the-crown (CTC) tests.

MATERIALS AND METHODS

Zirconia, glass-ceramic, and polymeric (Trilor; TRI, Juvora; JUV, Pekkton; PEK) materials were loaded using crowns for CTC tests, discs (zirconia and glass-ceramics) for piston-on-3 ball tests, bars (polymer) for 3-point bend tests, and bars (zirconia, glass-ceramics) for 4-point bend tests. SEM was used to characterize the fracture surfaces and identify fracture surface features (e.g., origin, mist, hackle, and the direction of crack propagation [DCP]). Electron dispersive spectroscopy was used to identify the local chemistry.

RESULTS

Fracture surface features were found to be less visually apparent for glass-ceramics than zirconia. For zirconia bars, fractures originated roughly midway between the corner and center for processing defects related to sintering. Fractures originated at the bottom corners of glass-ceramic bars (void or surface flaw) and PEK bars (surface flaw). TRI bar failures exposed glassy fibers. Fracture features were generally less discernable for discs compared to bars for zirconia and glass-ceramics. Ceramic crowns fractured into 2 to 3 pieces, with fractures originating at the occlusal surface and clear evidence for the DCP. Failures of TRI and JUV specimens (bars and crowns) were less catastrophic than for the ceramics, with exposed fibers (TRI) and surface cracks (JUV). PEK crown and bar fractures presented dimple (ductile) features formed due to microvoid coalescence followed by brittle crack propagation.

CONCLUSIONS

The critical flaws responsible for failure initiation were a function of material composition and test configuration. Fractographic analysis can reveal problems associated with the manufacturing of materials, their handling, grinding and finishing/polishing procedures, the structural design and choice of material, and the quality of the final laboratory-delivered restoration.

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.

The Effect of Yttria Content on Microstructure, Strength, and Fracture Behavior of Yttria-Stabilized Zirconia

Yttria-stabilized zirconia (YSZ) is well-known as a material with perfect mechanical, thermal, and electrical properties. It is used for manufacturing various high-temperature components for aerospace and energy generation, as well as wear- and corrosion-resistant devices in medicine. This work investigated the effect of a Y2O3 addition to ZrO2 on the microstructure and mechanical properties of YSZ ceramics produced by one sintering schedule. ZrO2 ceramics doped with 3, 4, 5, 6, 7, and 8 mol% Y2O3 (designated 3YSZ through to 8YSZ) were prepared by using conventional sintering at 1550 °C for 2 h in argon. The effect of yttria content was analyzed with respect to grain size, morphology of the microstructural features, phase composition, parameters of fracture surface, and flexural strength. The 7YSZ ceramics sintered at 1550 °C for 2 h showed the highest level of flexural strength due to the formation of the fine-grained microstructure containing mainly the monoclinic and tetragonal zirconia phases. The fracture micromechanism in the studied YSZ ceramics is discussed.

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.