Yttria-stabilized Tetragonal Zirconia Research Articles

Effect of Hydrothermal, Chemical, and Mechanical Degradation on Flexural Strength and Phase Transformation of Ground, Glazed, and Polished Zirconia

Objectives: Evaluation of the effect of grinding on flexural strength of zirconia after low temperature degradation (LTD) and pH-cycling. Materials and Methods: Sixty-four bar-shaped specimens of yttria-stabilized tetragonal polycrystalline zirconia were milled, sintered, wet-polished, and divided into 8 groups (N=8). The four control groups were not aged while artificial aging was performed in the 4 experimental groups in three steps including LTD in steam for 40h, pH-cycling, and tooth brushing for artificial aging. All groups underwent surface preparations as follows: standard polishing without surface treatment (Sp), grinding with a blue-yellow band diamond instrument (Gr); grinding with a diamond rotary instrument (DRI) and then over-glazing (Gl); grinding with a DRI followed by two-step intraoral polishing (Po); standard polishing and aging (Sp-Ag); grinding and aging (Gr-Ag), grinding, over-glazing and aging (Gl-Ag); and grinding, polishing and aging (Po-Ag). Monoclinic content was assessed in one specimen of each group by X-ray diffraction (XRD). The 3-point flexural strength test was performed in a universal testing machine. The results were analyzed with two-way ANOVA and Tukey’s test (α=0.05). Results: Mean flexural strength (Mpa) was significantly higher in groups Gr and Po compared to group Sp (both, P<0.0001) and group Gl (both, P<0.0001). In XRD analyses, the highest monoclinic phase before aging was observed in group Gr (12.6%), and after aging in group Gr-Ag (51.2%). Conclusion: Grinding and polishing increased the flexural strength, while glazing did not exhibit any significant effect on this parameter. Furthermore, aging did not adversely affect flexural strength.

Experimental investigation of some photon interaction parameters of popular restorative dental materials with a non-destructive technique

Dental restorative materials are widely used to restore esthetics and function in prosthetic treatments. In this paper, reflection coefficients and effective atomic numbers of some restorative materials (Polyetheretherketone (PEEK), feldspathic porcelain (veneering porcelain on cobalt–chromium alloy as metal framework), lithium disilicate glass-ceramic, zircon core (veneering porcelain on yttria-stabilized tetragonal zirconia polycrystal), monolithic zirconia, and zirconia-reinforced lithium silicate glass ceramic) were measured by using 59.54 keV energy gamma rays emitted from an Am-241 radioactive source. The scattering peaks of the restorative materials were detected using an HPGe detector. The gamma radiation absorption parameters of these materials (MAC, LAC, MFP, and HVL) were also investigated using a ULEGe detector for 59.54 keV photons. It is observed that the largest MAC value is Monolithic zirconia. The material with the highest reflection parameter was found to be PEEK. Of the dental restorative materials investigated, PEEK has the lowest effective atomic number value of 21.650 and Monolithic zirconia has the highest effective atomic number value of 37.841. Effective atomic numbers can be used in non-destructive analysis and medical imaging, as is well known. In addition, the calibration curve obtained can be used in the qualitative analysis of different restorative and implant materials.

Flexural Strength of Different Monolithic Computer-Assisted Design and Computer-Assisted Manufacturing Ceramic Materials Upon Accelerated Aging.

The durability of ceramic is crucial, which is probably influenced by aging. This study evaluated the effect of aging on flexural strength of different ceramics. One-hundred twenty ceramic discs (Ø 12 mm, 1.5 mm thickness) were prepared from zirconia-reinforced lithium silicate (ZLS, C), lithium disilicate (LS2, E), precolored yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP, Ip), and customized color Y-TZP (Ic). Samples were randomly divided into two groups for accelerated aging (A) between 5 and 55°C water baths, 30-second immersing time each, for 10,000 cycles, and nonaged group (N), serving as control. Biaxial flexural strength (σ) was evaluated utilizing the piston-on-three-balls at 0.5 mm/min speed. Analysis of variance and Tukey comparisons were determined for significant differences (α = 0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic strength (σo). Microstructures were evaluated with scanning electron microscopy and X-ray diffraction (XRD). The highest σ and σo were seen for IcN, followed by IcA, IpN, IpA, EN, CA, CN, and EA, respectively. CN showed the highest m, while EA showed the lowest m. Significant differences of σ for each ceramic were indicated (p < 0.05). Aging caused a significant difference in σ (p < 0.05). XRD showed t→m phase transformation of Ip and Ic after aging. Aging affected strength of ceramics. Comparable strength between LS2 and ZLS was evidenced, but both were less strength than Y-TZP either aging or non-aging. Comparable strength between precolored Y-TZP and customized color Y-TZP was indicated. Better resisting aging deterioration of Y-TZP than LS2 and ZLS is suggested for fabrication restorative reconstruction.

Influence of Different Surface Treatments on the Bond Strength of Yttria-Stabilized Tetragonal Zirconia Ceramic.

Objective: This in vitro study evaluates the shear bond strength (SBS) of yttria-stabilized tetragonal zirconia (Y-TZP) and resin cement after different surface treatments. Materials and methods: Forty-eight ceramic cubes were divided into four groups (n = 12): G1 (control) sandblasting with Al2O3; G2-sandblasting with silica-coated Al2O3 (Rocatec); G3-Rocatec + CO2 laser; and G4-CO2 laser + Rocatec. A metallic primer was applied to the pretreated ceramic. A rubber ring was adapted on the central area, and then, the resin cement was inserted into the matrix and photoactivated. The samples were evaluated regarding surface roughness (Ra), SBS, failure type, and qualitatively with scanning electron microscopy (SEM). The data were analyzed by one-way analysis of variance followed by Tukey's test (p < 0.05). Results: The mean values of Ra (μm) were as follows: G1-4.52a, G2-4.24a,b, G3-4.10a,b, and G4-2.90b and the mean values of SBS (MPa) were as follows: G1-7.84a , G2-4.41b , G3-4.61b and G4-6.14a,b. SEM analyses showed superficial irregularities for all groups, being more prominent for G1. The presence of silica deposits was observed for G2, G3, and G4, but in the last two groups there were some linear areas, promoted by the fusion of silica, due to the thermomechanical action of the CO2 laser. Conclusions: The surface treatment with CO2 laser + Rocatec, using one MDP-based cement, can be an alternative protocol for the adhesion cementation of Y-TZP ceramic since it was as effective as the conventional pretreatment with aluminum oxide sandblasting.

Defects in the grain interiors of 3 mol% yttria-stabilized tetragonal zirconia polycrystal ceramics with 0.25 wt% alumina

Defects in the grain interiors of 3 mol% yttria-stabilized tetragonal zirconia polycrystal ceramics with 0.25 wt% alumina

Thermal conductivity prediction of Al[formula omitted]O[formula omitted]-doped tetragonal YSZ coatings using deep learning

Thermal barrier coatings, including Al2O3-doped tetragonal yttria-stabilized zirconia (t-YSZ) coatings are vital in diverse applications. Thermal conductivity is a key property, but prediction often requires some complex experiments. In this study, a novel dual-structure feature extraction coupled with a multi-scale attention fusion network (RCFNet) is introduced for accurate thermal conductivity prediction using electron backscattered diffraction (EBSD) microstructure images. The method requires minimal data, and the result shows high accuracy with a Mean Square Error (MSE) of 0.003 W(m⋅k)−1 and an R-squared (R2) value of 0.81. Furthermore, the model reveals specific correlations between microstructural characteristics and thermal conductivity, facilitating rapid structure selection for specific thermal conductivity.

Topographical and crystalline change on surface by sandblasting improve flexural and shear bond strength of niobia-modified yttria-stabilized tetragonal zirconia polycrystal.

This study aimed to investigate the effects of sandblasting on the physical properties and bond strength of two types of translucent zirconia: niobium-oxide-containing yttria-stabilized tetragonal zirconia polycrystals ((Y, Nb)-TZP) and 5 mol% yttria-partially stabilized zirconia (5Y-PSZ). Fully sintered disc specimens were either sandblasted with 125 µm alumina particles or left as-sintered. Surface roughness, crystal phase compositions, and surface morphology were explored. Biaxial flexural strength (n=10) and shear bond strength (SBS) (n=12) were evaluated, including thermocycling conditions. Results indicated a decrease in flexural strength of 5Y-PSZ from 601 to 303 MPa upon sandblasting, while (Y, Nb)-TZP improved from 458 to 544 MPa. Both materials significantly increased SBS after sandblasting (p<0.001). After thermocycling, (Y, Nb)-TZP maintained superior SBS (14.3 MPa) compared to 5Y-PSZ (11.3 MPa) (p<0.001). The study concludes that (Y, Nb)-TZP is preferable for sandblasting applications, particularly for achieving durable bonding without compromising flexural strength.

Shear bond strength, finite element analysis, flexural strength and accuracy analysis of additively manufactured bio-inspired 3Y-TZP for dental applications

Weak shear bond strength (SBS) has always been a challenge for clinical bonding of 3mol% yttria-stabilized tetragonal zirconia (3Y-TZP). With the development of additive manufacturing technology, vat photopolymerization (VPP) is an alternative that allows zirconia to form precise complex structures and achieve bionic structure molding based on the natural advantages of different organisms in relation to biological structure, microstructure, and mechanical behavior. As report goes, the toe structure of tree frog can effectively achieve friction enhancement on humid and flooded surfaces. In this paper, we present a new type of 3Y-TZP material loaded with tree-frog bionic structures via VPP additive manufacturing to improve SBS between resin cement and 3Y-TZP for clinical requirements. And flexural strength, finite element analysis and accuracy analysis of bio-inspired 3Y-TZP is carried out to verify its feasibility of clinical application. The SBS of the bio-inspired group with 250 μm width and 250 μm height reached a maximum of 22.7 ± 1.3 MPa before hydrothermal aging, the flexural strength of the bio-inspired group with 250 μm width and 150 μm height decreased the least, reaching 825 ± 30 MPa, and the accuracy analysis of the central-incisor veneer restoration loaded with the bionic structures showed that the average deviation value was 0.033 ± 0.036 mm. This study has found that these bionic structures can effectively improve the SBS of the interface between zirconia and resin cement, while flexural strength and accuracy can also meet requirements of clinical dental application.

The Effect of Bioglass Coating on Microshear Bond Strength of Resin Cement to Zirconia.

Objectives: Durable bonding to zirconia is a challenging issue in dentistry. This study aimed to assess the effect of bioglass coating of zirconia on the microshear bond strength of resin cement to zirconia and to study the effect of thermocycling on this bond. Materials and Methods: This in-vitro experimental study was conducted on 60 yttria-stabilized tetragonal zirconia blocks in six groups (N=10) based on surface pretreatment and thermocycling. Surface pretreatments included no treatment control, alumina particle abrasion, and bioglass-coating of zirconia. Resin bonding was performed with Panavia F2.0 cements. Then, half of the specimens underwent a 24-hour incubation in 37°C water, while the other half were subjected to thermocycling (12000 cycles, 5-55°C, 60s for each batch) following the same incubation period. Subsequently, the microshear bond strength of the specimens was measured. Additionally, one block from each group was subjected to scanning electron microscopy and X-ray diffraction. The data were analyzed using Kruskal-Wallis and Mann-Whitney U tests. Results: There was a significant difference between the bond strength values of different groups (P<0.001). Alumina particle abrasion and bioglass coating equally increased the bond strength compared to the untreated control group (P<0.001). Thermocycling caused significant decreases in bond strength in all the groups (P<0.001); however, the bond strength value of the thermocycled bioglass-coated group was significantly higher than that reported for the thermocycled alumina particle abraded group (P=0.015). Conclusion: Despite the decrease in the bond strength values after thermocycling, the long-term efficacy of the bioglass coating of zirconia was promising.

Femtosecond Laser Irradiation to Zirconia Prior to Calcium Phosphate Coating Enhances Osteointegration of Zirconia in Rabbits.

Calcium phosphate (CaP) coating of zirconia and zirconia-based implants is challenging, due to their chemical instability and susceptibility to thermal and mechanical impacts. A 3 mol% yttrium-stabilized tetragonal zirconia polycrystal was subjected to femtosecond laser (FsL) irradiation to form micro- and submicron surface architectures, prior to CaP coating using pulsed laser deposition (PLD) and low-temperature solution processing. Untreated zirconia, CaP-coated zirconia, and FsL-irradiated and CaP-coated zirconia were implanted in proximal tibial metaphyses of male Japanese white rabbits for four weeks. Radiographical analysis, push-out test, alizarin red staining, and histomorphometric analysis demonstrated a much improved bone-bonding ability of FsL-irradiated and CaP-coated zirconia over CaP-coated zirconia without FsL irradiation and untreated zirconia. The failure strength of the FsL-irradiated and CaP-coated zirconia in the push-out test was 6.2-13.1-times higher than that of the CaP-coated zirconia without FsL irradiation and untreated zirconia. Moreover, the adhesion strength between the bone and FsL-irradiated and CaP-coated zirconia was as high as that inducing host bone fracture in the push-out tests. The increased bone-bonding ability was attributed to the micro-/submicron surface architectures that enhanced osteoblastic differentiation and mechanical interlocking, leading to improved osteointegration. FsL irradiation followed by CaP coating could be useful for improving the osteointegration of cement-less zirconia-based joints and zirconia dental implants.

Effect of zirconia surface conditioning before glazing on the wear of opposing enamel: an in vitro study.

This in vitro study aimed to evaluate the wear of natural teeth opposing 3mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) with different surface conditions. Sixty 3Y-TZP specimens were randomly assigned to six groups (n = 10), differing in surface condition. In three groups, the samples underwent glazing-with the glaze applied to roughened (i.e., 106-µm-grit diamond-finished), as-sintered, and polished zirconia. The three remaining groups consisted of unglazed specimens: solely polished samples and diamond-finished samples (106-µm-grit and 46-µm-grit) without further conditioning. Two-body wear was evaluated at extracted, non-carious molars (n = 60), which served as antagonists in chewing simulation (10,000 masticatory cycles, 49N load). As a control, natural teeth with intact enamel surfaces were tested against natural molars (n = 10). All samples were 3D-scanned before and after the chewing simulation (7 Series, Straumann). Volume loss was calculated (Inspect Software, GOM), and statistically analyzed (SPSS Statistics 24, IBM). Volume loss of the natural antagonists decreased in the following order: 106-µm-grit diamond-finished zirconia (4.6 ± 2.5 mm3), glazed 106-µm-grit diamond-finished zirconia (3.8 ± 1.1 mm3), glazed as-sintered zirconia (3.5 ± 0.9 mm3), 46-µm-grit diamond-finished zirconia (1.7 ± 0.6 mm3), control (1.6 ± 0.7 mm3), glazed polished zirconia (1.4 ± 0.5 mm3), and solely polishing (0.4 ± 0.2 mm3). Even when polishing the surfaces before glazing, volume loss was not mitigated to the same extent as after polishing alone. The zirconia surface condition beneath the glazing influences antagonist wear. Although polishing before glazing resulted in acceptable levels of antagonist wear, this approach did not yield as favorable results as polishing alone. For operators favoring glazing, pre-polishing the zirconia surface could be advantageous to reduce wear.

Analysis of soft tissue integration-supportive cell functions in gingival fibroblasts cultured on 3D printed biomaterials for oral implant-supported prostheses.

To date, it is unknown whether 3D printed fixed oral implant-supported prostheses can achieve comparable soft tissue integration (STI) to clinically established subtractively manufactured counterparts. STI is mediated among others by gingival fibroblasts (GFs) and is modulated by biomaterial surface characteristics. Therefore, the aim of the present work was to investigate the GF response of a 3D printed methacrylate photopolymer and a hybrid ceramic-filled methacrylate photopolymer for fixed implant-supported prostheses in the sense of supporting an STI. Subtractively manufactured samples made from methacrylate polymer and hybrid ceramic were evaluated for comparison and samples from yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), comprising well documented biocompatibility, served as control. Surface topography was analyzed by scanning electron microscopy and interferometry, elemental composition by energy-dispersive x-ray spectroscopy, and wettability by contact angle measurement. The response of GFs obtained from five donors was examined in terms of membrane integrity, adhesion, morphogenesis, metabolic activity, and proliferation behavior by a lactate-dehydrogenase assay, fluorescent staining, a resazurin-based assay, and DNA quantification. The results revealed all surfaces were smooth and hydrophilic. GF adhesion, metabolic activity and proliferation were impaired by 3D printed biomaterials compared to subtractively manufactured comparison surfaces and the 3Y-TZP control, whereas membrane integrity was comparable. Within the limits of the present investigation, it was concluded that subtractively manufactured surfaces are superior compared to 3D printed surfaces to support STI. For the development of biologically optimized 3D printable biomaterials, consecutive studies will focus on the improvement of cytocompatibility and the synthesis of STI-relevant extracellular matrix constituents.

Comparative analysis of flexural strength of 3D printed and milled 4Y-TZP and 3Y-TZP zirconia

Statement of problemThe mechanical properties of 3 dimensionally (3D) printed zirconia have been reported to be comparable with those of milled zirconia, except for the flexural strength. However, most previous studies tested 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), making it necessary to study 3D printed zirconia with 4 mol% yttria content (4Y-TZP). PurposeThe purpose of this in vitro study was to compare the flexural strength of 3D printed 4Y-TZP with 3Y-TZP materials and milled 4Y-TZP. Material and methodsA total of 80 disk specimens (Ø15×1.5 mm) were fabricated and divided into 4 groups (n=20) using the fabrication method and yttria content: milled 3Y-TZP (Katana HT; Kuraray Noritake), 3D printed 3Y-TZP (TZ-3Y-E; Tosoh), milled 4Y-TZP (Katana STML; Kuraray Noritake), and 3D printed 4Y-TZP (3DMAT; Genoss). The biaxial flexural strength was determined with a piston-on-3-ball test (n=15). The flexural strength of each specimen was measured, and the Weibull modulus (m) and characteristic strength (σ0) were estimated from the fracture load distribution. Two intact and fractured specimens were examined with scanning electron microscopy (SEM). The crystalline phase of the specimens in each group was identified through X-ray diffraction (XRD) analysis (n=5). A 1-way ANOVA was used to compare the flexural strength among different groups. Subsequently, pairwise comparisons were conducted with the Tukey post hoc method (α=.05). ResultsThe flexural strength of 3D printed 4Y-TZP was significantly higher than that of milled 4Y-TZP (P<.001). In contrast, the flexural strength of 3D printed 3Y-TZP was significantly lower than that of milled 3Y-TZP (P<.001). X-ray diffraction (XRD) analysis revealed that the tetragonal phase was the dominant phase in all groups, with the identification of some cubic phase peaks. ConclusionsThree dimensionally printed 4Y-TZP showed significantly higher flexural strength than milled 4Y-TZP and exhibited a clinically acceptable flexural strength exceeding 800 MPa.

Activating continuous dislocation pinning enhanced toughness of nanocomposite coating through specially oriented semicoherent heterointerface lattice distortion

Ionic crystal ceramics are known to be susceptible to brittle cracks along the intergranular due to the strong ionic bond forces. Accordingly, an outstanding toughened coating consisting of yttria-stabilized tetragonal zirconia (YSTZ) and MgO, with a reinforced heterointerface, has been successfully achieved through the plasma electrolytic oxidation process. The results demonstrate that the enhanced Zr/Y–O bond strength improves the interfacial strength of the ionic crystal, while the lattice distortion activates continuous dislocation pinning at the (101) YSTZ//(111) MgO semicoherent heterointerface effectively hinders crack propagation of the coating. With the YSTZ content approaching 65 %, the toughness (KIC) value surpasses that of PEO coating by 42 %. These findings not only elucidate the scientific foundation for the improved toughness of the YSTZ/MgO nanocoating but also provide valuable insights into design strategies for improving ceramic toughness.

Load-bearing capacity of 3D-printed incisor partial-coverage crowns made from zirconia and composite.

This study investigated the fracture resistance of 0.5-mm-thick restorations for minimally invasive therapy. Anterior partial-coverage crowns composed of three-dimensional (3D)-printed 3-mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP; Lithacon 3Y210, Lithoz) and 3D-printed composite (Varseo Smile Crown plus, Bego) were compared with a control group made from milled 3Y-TZP (Cercon ht, DentsplySirona). Three groups each with 27 restorations were produced. For milled 3Y-TZP partial-coverage crowns, drill compensation was needed so the milling bur could access the inner surface at the incisal edge. Restoration fit was verified by cross-sectioning 12 specimens in each group. The remaining 15 restorations were sandblasted (Al2O3, 0.1 MPa) and adhesively cemented (Panavia SA, Kuraray) onto CoCr teeth. Static load-to-failure tests were performed. The load was induced on the incisal edge. The forces needed to fracture the specimens were analyzed using the Welch analysis of variance and post hoc Dunnet-T3 tests. The Weibull parameters were also calculated. Drill compensation increased cement thickness at the loading area by 200 µm in milled 3Y-TZP restorations compared with the 3D-printed partial-coverage crowns. Fracture resistance was the highest in 3D-printed 3Y-TZP restorations (1570±661N) followed by milled 3Y-TZP (886±164N) and 3D-printed composite partial-coverage crowns (570±233 N). Milled 3Y-TZP was associated with a substantially higher Weibull modulus (m=6) than the 3D-printed materials (m=2), suggesting greater reliability. Fracture resistance increased with tighter fit, demonstrating the benefit of the geometric freedom associated with 3D-printing. Future research should focus on making 3D-printed 3Y-TZP more reliable to increase its safety in clinical use.

Rapid processes for the production of nanocrystal yttria-stabilized tetragonal zirconia polycrystalline ceramics: ultrasonic spray pyrolysis synthesis and high-frequency induction sintering

In this study, nanocrystalline 3 mol % yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic was produced by sintering with a high-frequency induction heating (HFIH) system of granular powders obtained by ultrasonic spray pyrolysis (USP) at 600 °C. The granular nano-sized powders (10-30 nm) were micron in size (average size: 700 nm), spherical in shape and amorphous. The influences of the HFIH sintering temperature (1400-1600 °C), applied current time (60-300 sec.) and the mechanical pressure (10 MPa and 20 MPa) on the final density and grain size of the products were investigated. The amorphous granular Y-TZP powders compressed with the HFIH system allow very rapid condensation in the tetragonal phase at high density and avoid grain growth. High density (relative density over 95) nanocrystalline Y-TZP ceramic with ∼70 nm size could be obtained from the simultaneous application of 20 MPa pressure and an induced current within 300 sec. of sintering time at 1500 °C. In this condition, the sample’s maximum hardness and fracture toughness values were reached at 14.9 GPa and 3.8 MPa·m1/2, respectively. Y-TZP powders produced in nano-micro structure with the USP system were sintered with the HFIH system and rapid production was achieved by preventing grain growth.

In Vitro Investigation of Shear Bond Strength of Titanium Alloy Bonded to Monolithic Zirconia Prepared Via Different Surface Roughening Methods Using Different Cements

Objective: To evaluate the shear bond strength (SBS) of yttrium-stabilized tetragonal zirconia polycrystals bonded to titanium alloys via different surface treatment methods using four different cements.&#x0D; Methods: Eighty titanium and monolithic zirconia discs were prepared with computer-aided design/manufacturing (CAD/CAM) technology. All titanium discs and 40 of monolithic zirconia discs were polished by using silicon carbide paper and sandblasted with 50 μm aluminum oxide (Al2O3). Tribochemical silica coating was applied to remaining 40 monolithic zirconia discs. The monolithic zirconia discs were divided into eight groups after surface treatment (n=10). Titanium discs were cemented using conventional glass ionomer cement (GIC), resin-modified GIC, self- adhesive resin cement, and dual-cure resin cement. The SBS test was performed using a universal testing machine. The failure patterns were examined by using a scanning electron microscope (SEM). Data were statistically analyzed with one-way analysis of variance (ANOVA), two-way ANOVA and Tukey’s test (α

Effect of firing time and wall thickness on the biaxial flexural strength of 3D-printed zirconia

ObjectivesTo evaluate the effect of accelerated firing on 3D-printed zirconia. MethodsTo check if formulae provided by ISO 6872 can be extended to thin samples, finite element analyses were carried out in advance of fabricating 3-mol% yttria-stabilized tetragonal zirconia polycrystal discs by milling and by 3D-printing. Four groups (n = 38 each) of 3D-printed specimens were produced with two nominal thicknesses (0.6 mm and 1.2 mm) and two firing strategies (long: 51 h, accelerated: 14.5 h). In the milled group (thickness 1.2 mm, n = 30), a standard firing program (9.8 h) was selected. Biaxial flexural strength tests were applied and mean strength, characteristic strength, and Weibull modulus were calculated for each group. Differences were analyzed using Welch ANOVA and Dunnett-T3 post-hoc tests. ResultsMaximum tensile stresses occurring during biaxial strength testing can be calculated according to ISO 6872 for thin samples with b > 0.3 mm. Variability of measured strengths values was smaller for milled zirconia compared with 3D-printed zirconia. The 1.2-mm-thick 3D-printed samples had significantly decreased strength after accelerated firing than after long firing. However, for the 0.6-mm-thick samples, comparable mean biaxial strength values of about 1000 MPa were measured for both firing protocols. SignificanceAt the moment, long fabrication time for zirconia restorations is a major drawback of 3D-printing when compared with milling technology. This investigation showed that the strength of 0.6-mm-thick zirconia discs fabricated by 3D-printing was not impaired by accelerated firing. Thus, overnight firing of thin-walled 3D-printed zirconia restorations could be possible.

Minimally processed recycled yttria-stabilized tetragonal zirconia for dental applications: Effect of sintering temperature on glass infiltration

This study aimed to develop a recycling process for the remnants of milled 3Y-TZP and enhance their properties using glass infiltration. 3Y-TZP powder was gathered from the vacuum system of CAD–CAM milling equipment, calcined and sieved (x < 75 μm). One hundred twenty discs were fabricated and pre-sintered at 1000 °C/h. These specimens were then divided into four groups, categorized by glass infiltration (non-infiltrated [Zr] or glass-infiltrated [Zr-G]) and sintering temperature (1450 °C [Zr-1450] or 1550 °C [Zr-1550]/2h). After sintering, the specimens were characterized by X-Ray Diffraction (XRD), relative density measurement, and scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS). The biaxial flexural strength test was performed according to the ISO 6872 and followed by fractographic analysis. Subsequent results were analyzed using Weibull statistics. Relative density values of the sintered specimens from Zr-1450 and Zr-1550 groups were 86.7 ± 1.5% and 92.2 ± 1.7%, respectively. Particle size distribution revealed particles within the range of 0.1–100 μm. XRD analysis highlighted the presence of the ZrO2-tetragonal in both the Zr-1450 and Zr-1550 groups. Glass infiltration, however, led to the formation of the ZrO2-monoclinic of 9.84% (Zr-1450-G) and 18.34% (Zr-1550-G). SEM micrographs demonstrated similar microstructural characteristics for Zr-1450 and Zr-1550, whereas the glass-infiltrated groups exhibited comparable infiltration patterns. The highest characteristic strength was observed in the glass-infiltrated groups. Fractographic analyses suggested that fracture origins were related to defects on the tensile side, which propagated to the compression side of the samples. Both the sintering temperature and glass infiltration significantly influenced the mechanical properties of the 3Y-TZP recycled.

Mechanical and chemical surface treatment enhances bond strength between zirconia and orthodontic brackets: an in vitro study

ObjectivesTo assess the shear bond strength (SBS) between metal orthodontic brackets and zirconia after receiving different mechanical and chemical surface treatments, and different types of resin adhesive. The failure mode of each treatment protocol was also evaluated.Materials and methodsThe present in vitro experimental study consisted of six surface treatment protocols with two different resin adhesives. One-hundred and forty-four rectangular-shaped 3 mol% yttrium-stabilized tetragonal zirconia polycrystal blocks were milled, sintered, and embedded in acrylic resin. They were randomly divided into three mechanical (none, air abrasion, and bur grinding) and two chemical surface treatment conditions (no primer and Z-primer). The specimens were divided into two groups according to the resin adhesive received: self-cured (RelyX U200) and light-cured adhesives (Transbond XT). The SBS between the metal bracket and zirconia was tested using a universal testing machine (1-mm/min crosshead speed), and the failure mode was evaluated. Differences in SBS and failure mode were analyzed using Welch ANOVA followed by post-hoc comparison and Fisher’s Exact test, respectively.ResultsBur grinding produced the highest SBS, followed by air abrasion. Z-primer application typically provided a higher SBS regardless of resin adhesive used (p < 0.001). Without primer application, RelyX U200 provided a higher SBS than Transbond XT (p < 0.001). After grinding, using Z-primer and RelyX U200 resulted in a higher SBS than no primer and using Transbond XT (p < 0.001). Adhesive failure at the zirconia–adhesive interface occurred only when Transbond XT was applied without bur grinding, and when using Transbond XT after grinding, but no Z-primer application.ConclusionBur grinding combined with applying an MDP-containing primer and resin adhesive enhances the SBS between zirconia and metal orthodontic brackets.