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.     

Effect of sintering regimes and thickness on optical properties of zirconia ceramics for dental applications

In dentistry, monolithic zirconia restorations have been preferred over all-ceramic restorations in recent years. Translucency is an aesthetic demand in dental restorations, and it can be identified with translucency parameter (TP). Zirconia thickness, Y₂O₃ content, and sintering conditions are important parameters that influence the translucency of the restorations. It is crucial to investigate monolithic zirconia ceramics under different sintering regimes and reveal the critical parameters for dental restorations. The aim of this study was to determine the optical and microstructural behaviors of monolithic zirconia ceramics containing different amounts of yttria depending on various sintering regimes and thicknesses. Therefore, a conventional zirconia CopranZri (CZI) and two monolithic zirconia materials, CopraSupreme (CSP), CopraSmile (CSM) were used. Slow, normal, speed, and translucency sintering regimes with thicknesses of 0.7, 1.0, 1.3 mm were selected. TP of the specimens was calculated, and statistical analyses were performed by using one-way ANOVA and Tukey-Kramer post hoc tests. Characterization of the specimens was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. The results showed that the effect of different sintering programs is more critical for CSP and CSM in terms of translucency variations and translucency program led to the most grain growth.     

Fractography of self-glazed zirconia with improved reliability

The fractography of a new grade of zirconia ceramics, known as self-glazed zirconia, was investigated. The as-sintered intact top surface was made with superior smoothness that mimicked the optical appearances of the natural teeth enamel. The beneath surface opposite to this was made hierarchically rough with microscopic pits of the size up to 60 μm together with grain-level roughness of about 2 μm. The three-point bending test of the samples made with the hierarchically rough surface being tensile one demonstrated an average bending strength of 1120 ± 70 MPa and a Weibull modulus of as high as 18 ascribed to the improved structural homogeneity. Surface topography was found the main origins of crack initiation leading to fracture. The observed unusually predominant transgranular fracture mode of submicron-sized grains disclosed a possible toughening mechanism of disassembling of mesocrystalline grains that differs significantly from the commonly quoted phase transformation toughening of this category of ceramics.     

Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic

The effect of conventional sintering from 1300 to 1550 °C on the properties of 1 mol% ceria-doped scandia stabilized zirconia was investigated. In addition, the influence of rapid sintering via microwave technique at low temperature regimes of 1300 °C and 1350 °C for 15 min on the properties of this zirconia was evaluated. It was found that both sintering methods yielded highly dense samples with minimum relative density of 97.5%. Phase analysis by X-ray diffraction revealed the presences of only cubic phase in all sintered samples. All sintered pellets possessed high Vickers hardness (13–14.6 GPa) and fracture toughness (~3 MPam^1/2). Microstructural examination by using the scanning electron microscope revealed that the grain size varied from 2.9 to 9.8 µm for the conventional-sintered samples. In comparison, the grain size of the microwave-sintered zirconia was maintained below 2 µm. Electrochemical Impedance Spectroscopy study showed that both the bulk and grain boundary resistivity of the zirconia decreases with increasing test temperature regardless of sintering methods. However, the grain boundary resistivity of the microwave-sintered samples was higher than the conventional-sintered ceramic at 600 °C and reduced significantly at 800 °C thus resulting in the enhancement of electrical conduction.     

等离子喷涂氧化锆涂层的显微结构分析

利用透射电镜(TEM)、扫描电镜(SEM)和X射线衍射仪(XRD)等分析技术,表征了等离子喷涂氧化锆涂层的显微结构.结果表明:等离子喷涂氧化锆涂层是由典型的柱状晶粒组成的层状结构;柱状晶粒晶型发育完整,晶粒之间具有清楚晶界;涂层表面存在明显的完全熔融区和未熔融区;涂层中分布有一定的大气孔.涂层的主晶相是四方氧化锆,没有单斜氧化锆相存在;涂层中裂纹的扩展是穿晶断裂和沿晶断裂共存.     

Effect of hot-etching treatment on shear bond strength of zirconia to resin cement

ABSTRACT

The purpose of this study was to evaluate the effect of hot-etching surface treatment on the shear bond strength between zirconia ceramics and resin cement. Ceramic cylinders were divided randomly into 10 groups (n?=?10) according to different surface treatments (blank control; airborne particle abrasion; hot-etching for 10?min; hot-etching for 30?min; hot-etching for 60?min) and whether or not performed thermal cycling fatigue test. Flat enamel surfaces, were prepared from human permanent incisors and were bonded to the zirconia discs. All specimens were subjected to shear bond strength test by a universal testing machine. All data were statistically analysed using one-way analysis of variance and multiple comparison least significant difference tests (α?=?0.05). Hot-etching for 60?min treatment produced higher bond strengths than the other treatment. Surface treatment of zirconia with a hot-etching solution might enhance surface roughness and bond strength between zirconia and resin cement.     

Low–thermal–conductivity and high–toughness CeO2–Gd2O3 co–stabilized zirconia ceramic for potential thermal barrier coating applications

Yttria partially stabilized zirconia (～4.0?mol% Y₂O₃–ZrO₂, 4YSZ) has been widely employed as thermal barrier coatings (TBCs) to protect the high–temperature components of gas–turbine engines. The phase stability problem existing in the conventional 4YSZ has limited it to application below 1200?°C. Here we report an excellent zirconia system co–doped with 16?mol% CeO₂ and 4?mol% Gd₂O₃ (16Ce–4Gd) presenting nontransformable feature up to 1500?°C, in which no detrimental monoclinic (m) ZrO₂ phase formed on partitioning. It also exhibits a high fracture toughness of ～46?J m^?2 and shows high sintering resistance. Besides, the thermal conductivity and thermal expansion coefficient of 16Ce–4Gd are more competent for TBCs applications as compared to the 4YSZ. The combination of properties suggests that the 16Ce–4Gd system could be of potential use as a thermal barrier coating at 1500?°C.     

Microstructure and Bending Strength in the Ternary (Mg,Ca)-Partially-Stabilized Zirconia

Microstructure and bending strength of commercial (MgO,CaO)-partially-stabilized zirconia subjected to heat treatments in the temperature range 900° to 1550°C were characterized. The as-received optimally aged material contained small ellipsoid-shaped tetragonal ( t ) precipitates dispersed homogeneously in the cubic ( c ) grains. Annealing at lower temperatures caused isothermal martensitic transformation to the monoclinic ( m ) phase. Annealing at higher temperatures caused formation of chevronlike and irregularshaped cluster precipitates, due to stress-induced coarsening and precipitate impingement. The loss of bending strength was related to the stored strain energy contributed by each type of precipitate morphology, in addition to decrease in the volume fraction of the t phase.     

A study of compressive strength between zirconia framework and veneering ceramic as a function of thermal expansion coefficient using Shell–Nielsen test method

This study investigated the adhesion between zirconia framework and four veneering ceramic (VC) materials with varying coefficients of thermal expansions (CTE). Zirconia rods (N?=?40) (ICE Zirkon) (diameter: 4 mm, height: 20 mm) were milled and sintered. After firing, the zirconia rods were air-abraded and cleaned. They were randomly assigned to receive four VCs (n?=?10/group), namely (a) Vita VM9 (VZ; 9–9.2?×?10^?6? K^?1), (b) Cerabien ZR (CZ; 9.1?×?10^?6 K^?1), (c) Matchmaker ZR (MM; 9.4?×?10^?6?K^?1), and (d) Ice Zirconia Ceramic (IZ; 9.6?×?10^?6?K^?1). The VCs were then fired onto zirconia rods (height: 2 mm, thickness: 2 mm) circumferentially and were thermocycled for 6000 times (5/55 °C, dwell time: 30?s). Specimens were loaded from the top of the zirconia rods (0.5 mm/min) in a universal testing machine until debonding. Shell–Nielsen bond strength values were calculated (MPa). Failure types were evaluated under SEM. The data were statistically analyzed (one-way ANOVA, Tukey’s; α?=?0.05). Weibull distribution values including the Weibull modulus (m) (0.05) was calculated. The highest mean bond strength (MPa) was obtained for CZ (42.08?±?4.08), followed by VZ (41.77?±?4.92), MM (40.7?±?3.64), and IZ (40.05?±?5.78). While mean bond strength for VZ, MM, and IZ were not significantly different (p?>?0.05), CZ was significantly higher than that of IZ (p?<?0.05). The lowest shape value was for VZ (m?=?16.94) and the highest for MM (m?=?20.16). Mainly, adhesive failures followed by mixed failures were observed. VCs with a greater mismatch of CTE with the zirconia framework exhibited similar Shell–Nielsen bond strength to those with fewer mismatches. CTE mismatch did not affect the results of CZ (9.1?×?10^?6 K^?1) and IZ (9.6?×?10^?6 K^?1).     

Ceria–zirconia mixed oxides: Synthetic methods and applications

The primary objective of this review was to illustrate the significance of ceria–zirconia (CZ) mixed oxides as catalysts and catalyst supports as employed for a wide variety of catalytic applications both in the liquid and gaseous phases. In particular, we were interested in bringing together the recent literature pertaining to these mixed oxides with catalysis perspective. The most prominent application of CZ mixed oxides is in three-way catalysis (TWC) as oxygen storage and release material for several years by replacing cerium dioxide as it shows better efficiency and a high thermal stability. Doping with zirconium oxide, as it is alone a non-reducible oxide, makes the CZ mixed oxide a highly reactive, thermally stable, and more reducible with elevated oxygen storage capacity (OSC) that are important for TWC applications. Apart from the TWC use, the CZ mixed oxides have a huge number of applications, as a direct component or a support, ranging from water–gas shift reaction, reforming of hydrocarbons, dehydration of alcohols, CO₂ utilization, catalytic combustion of pollutants, fine chemicals production, photocatalysis, and so on. All these applications are mainly dependent on three parameters of the mixed oxides, namely, OSC or redox nature, acid–base properties, and crystalline phases. Besides, most of the applications are influenced by the physical properties such as specific surface area, pore volume, pore diameter, crystallite size, and so on. In this review, many details pertaining to the synthesis of these mixed oxides by various conventional and non-conventional methods, their characterization by several techniques, and their application for various reactions of energy and environmental significance, as reported in the literature, are assessed.     

Dynamic wear characteristics and fracture strength of high-translucent monolithic zirconia crowns

The dental ceramic restorations inevitably undergo a wear damage. The dynamic wear behavior and fracture strength after wear fatigue of the high-translucent zirconia crowns have rarely been studied. In the present study, the zirconia crowns were fabricated using a high-translucent zirconia material and subsequently cemented to the PMMA abutments with a resin cement. The specimens were fixed in a chewing simulator and dynamically loaded for 3.6 million fatigue cycles (a maximal load of 350 N, 1.7 Hz). The wear volume and wear rate at the designated checkpoints between 0–36 × 10⁵ cycles with an interval of 3 × 10⁵ cycles were measured. The fracture strength of the zirconia specimens before wear fatigue as well as after 9, 24 and 36 × 10⁵ cycles was analyzed. The morphology of the wear surface was observed by SEM. The results showed that the wear volume increased with the wear cycles. In the initial stage (0–9 × 10⁵ cycles), the wear rate remained at a significantly high level (about 0.289–0.349 mm³/10⁵ cycles). Subsequently, it decreased rapidly and stabilized at a relatively low level (about 0.052–0.081 mm³/10⁵ cycles). As the number of fatigue cycles was increased, the fractured strength decreased significantly, except after 9 × 10⁵cycles. The worn surface after wear were flattened, and only a small number of scattered cracks and small punctate defects were observed. The high-translucent zirconia crowns exhibited dynamic wear characteristics, and the fracture strength decreased significantly.     

Effect of high-speed sintering on the microstructure,mechanical properties and ageing resistance of stereolithographic additive-manufactured zirconia

Digital light processing (DLP) demonstrates significant application potential in the fine printing of dental zirconia. However, its complicated print process and post-treatments, such as sintering, are time consuming and sensitive to technical details. Therefore, the feasibility of using a high-speed sintering (HS) method for the fabrication of DLP-based zirconia was investigated in this study. Zirconia samples fabricated using DLP and conventional subtractive manufacturing techniques were all sintered following the different protocols: HS and conventional sintering (CS). Then, the density, Vickers hardness, fracture toughness, surface micro-topography, phase assemblage, and ageing resistance were assessed. The results showed that samples fabricated using the HS technique presented moderate Vickers hardness, fracture toughness, and ageing sensitivity in comparison with the other groups of specimens; moreover, they exhibited moderate initial cubic phase content and average grain size. Conversely, specimens sintered using the CS protocol with a peak temperature of 1580 °C showed high ageing sensitivity and unbalanced mechanical properties. The DLP- and SM-fabricated specimens showed similar trends for the studied properties. Overall, sintering parameters can significantly affect the macro- and micro-properties of DLP specimens, and the proposed HS method showed potential for producing DLP-based zirconia that is acceptable for clinical applications.     

Investigation of the structure and mechanical properties of a novel dental graded glass/zirconia ceramic

Constructing graded structure is a promising solution to reduce the occurrence of cracking and delamination of bilayered zirconia prosthesis. In this work, a novel graded glass/zirconia ceramic was developed by utilizing the interdiffusion between dense zirconia and a novel lithium disilicate glass in the SiO₂-Li₂O-Al₂O₃ system. Results demonstrated that a graded glass/zirconia structure with a depth of about 300 µm was constructed, which exhibited obviously gradient characteristics in microstructure, glass content and mechanical properties. The hardness (H) and elastic modulus (EM) values at the surface reduced significantly (64% for H and 79% for EM), and increased gradually with depth of graded layer. When the graded layer was subjected to loading force, the biaxial flexural strength increased. The mechanism of the evolution of graded structure and change of strength were also elucidated in detail. This study provides a promising strategy to improve the interface stability of bilayered zirconia restoration.     

Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction

The deformation of 9.4 mol% magnesia-partially-stabilized zirconia under compressive loads up to 1225 MPa was studied using mechanical testing with in situ neutron diffraction. The material shows obvious plastic deformation at applied stresses in excess of an estimated critical stress of 925 ± 20 MPa. Most of the accumulated strain occurred by transient room-temperature creep. Plastic deformation was associated with considerable stress-induced tetragonal-to-monoclinic transformation. The volume change calculated from the strain gauges correlates well with the amount of t → m transformation observed. Unlike previous studies of Ce-TZP and Y-TZP, ferroelasticity was not observed, nor was the t → o transformation observed. Minor microstructural changes were noted, including an increase in the root mean square internal strain of 0.05%, commensurate with an increase in internal stress of ∼100 MPa. It would appear that transformation selectivity was exercised with the transformation occurring first in tetragonal crystallites favorably oriented to the applied stress. The stress-induced monoclinic phase therefore exhibits a strong preferred orientation. Comparison is made with the other commercially interesting zirconia ceramics, Ce-TZP and Y-TZP, which have been studied using the same techniques.     

Crystallization and Phase Transformation Process in Zirconia: An in situ High-Temperature X-ray Diffraction Study

Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500°C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO₂ phase, while the slowly precipitated precursor produced the tetragonal ZrO₂ phase. The crystallization and phase transformations were followed by in situ high-temperature X-ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450°C. A tetragonal-to-monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275°C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal-to-monoclinic transformation for the rapidly precipitated ZrO₂ sample. The crystallite size for the t -ZrO₂ of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t → m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal-to-monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.     

Fatigue of Yttria-Stabilized Zirconia: I, Fatigue Damage, Fracture Origins, and Lifetime Prediction

Uniaxial tension–compression fatigue behavior of 3-mol%-yttria-stabilized tetragonal zirconia polycrystals was investigated. Hysteresis in the stress–plastic strain curve featured cumulative plastic strain and weakened elastic stiffness. Fracture statistics in terms of cycle-to-failure depends strongly on the maximum stress and less on the stress amplitude. Preexisting processing flaws were identified as the fracture origins in all cases. We suggest that microcracking is the dominant mechanism of fatigue damage, that nucleation of fatigue crack is usually not necessary, and that fatigue lifetime is primarily controlled by crack propagation, which is most sensitive to the maximum stress.     

Low-Temperature Aging of t'-Zirconia: The Role of Microstructure on Phase Stability

Polycrystalline, tetragonal ( t ') zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3-mol%-yttria-doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As-polished surfaces of polycrystalline t ' materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ceramics of grain size >0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y-TZP but decreased that of t ' materials. Even then, the t ' materials exhibited better resistance to degradation than the Y-TZP ceramics. Excellent resistance of the t ' materials to low-temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic domain structure of these materials.     

Combined Novel Bonding Method of Resin to Zirconia Ceramic in Dentistry: A Pilot Study

Zirconia is a promising metal-free framework material that can be used to construct all-ceramic resin-bonded restorations in modern minimally invasive dentistry. The lack of a durable bond to zirconia is the major limitation against its widespread use. A technique to promote adhesion to the zirconia surface has thus been actively sought in dental materials research. Selective infiltration etching (SIE) has emerged as a method of conditioning that creates a highly retentive zirconia surface. This in vitro pilot study tested a novel adhesion procedure in which two newly engineered silane-based zirconia primers were combined with the SIE method. Zirconia discs were SIE-surface-treated, coated with one of the 2 zirconia primers, and bonded to composite resin discs. Primer activation (hydrolysis) was monitored by Fourier transform infrared (FTIR) spectroscopy. The bilayered specimens were sectioned into microbars and subjected to the microtensile bond strength test. As-sintered zirconia discs served as controls. Surface analysis of zirconia specimens was carried out using photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Zirconia specimens that had been treated with both SIE and primers had a significantly higher (ANOVA) zirconia resin bond strength (40.6 MPa, SD 5.8 MPa) than control specimens (2.6 MPa, SD 3.0 MPa; p < 0.05, F = 13.8, ANOVA). Controls also exhibited spontaneous failure during sectioning. Additionally, the interfacial failure rate was lower for the specimens subjected to the new combined surface treatment than for controls. The novel combined method of surface treatment method might open new opportunities for enhanced adhesion of resin-bonded zirconia restorations.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.     

Tough and damage-tolerant monolithic zirconia ceramics with transformation-induced plasticity by grain-boundary segregation

Conventional ceria-stabilized tetragonal zirconia (Ce-TZP) with modest flexural strength has rarely been used as compared to yttria-stabilized zirconia, even though it has excellent hydrothermal stability and high toughness. Ce-TZP-based composites were recently developed, being tough and remarkably in combination with transformation-induced plasticity. However, distinct from the widely applied composite approach to improve the strength of Ce-TZP, in this study, a simpler and easily tailorable method was proposed by doping aliovalent oxides that are able to segregate at the zirconia-grain boundaries. 0.2–1 mol% divalent oxides with different cation size (Mg²⁺, Ca²⁺, Ba²⁺ and Sr²⁺) were selected to dope 10 mol% ceria-stabilized zirconia. CaO and MgO dopants were able to enter the tetragonal Ce-TZP lattice and showed a grain-boundary segregation effect, thereby tailoring the microstructure and transformation behavior. At a higher dopant concentration of 1 mol% MgO or CaO, the ceramics were strong but brittle with a typical elastic linear fracture behavior, whereas at a low dopant concentration of 0.1–0.2 mol% CaO or 0.2–0.4 mol% MgO doping, the ceramics deformed in-elastically to a certain degree without changing the Young’s modulus. At the transition between both fracture behaviors, the best combination of toughness (>10 MPa m^1/2), biaxial strength (≥1200 MPa), reliability (Weibull modulus up to 30) and damage-tolerance was obtained. In-situ fracture testing revealed that transformation in such tough and deformable zirconia ceramics took place well before crack initiation with a large transformation zone of ～100 µm ahead of the crack tip having been formed before failure.