Low-temperature degradation of a Y-TZP ceramic after surface treatments

Investigation of Spark Plasma Sintering on Microstructure-Properties of Zirconia Reinforced Fluormica Glass for Dental Restorations

BACKGROUND

Conventional sintering methods of dental ceramics have limitations of high temperature and slow cooling rates with requirements of additional heat treatment for crystallization. Spark plasma sintering (SPS) is an emerging technique that has the potential to process dental restorations with dense microstructures and tailor-made clinically relevant properties with optimized processing parameters. This study explored the potential of the SPS of zirconia-reinforced fluormica glass (FM) for dental restorative materials.

METHODS

FM glass frit was obtained through the melt-quench technique (44.5 SiO2-16.7 Al2O3-9.5 K2O-14.5 MgO-8.5 B2O3-6.3 F (wt.%)). The glass frit was ball-milled with 20 wt.% of 3 mol% yttria-stabilized zirconia (FMZ) for enhanced fracture toughness. The mixtures were SPS sintered at a pressure of 50 MPa and a heating rate of 100 °C/min for 5 min with an increase in temperature from 650-750 °C-850 °C-950 °C. Phase analysis was carried out using XRD and microstructural characterization with SEM. Micro-hardness, nano-indentation, porosity, density, indentation fracture toughness, and genotoxicity were assessed.

CONCLUSIONS

The increase in the SPS temperature of FMZ influenced its microstructure and resulted in reduced porosity, improved density, and optimal mechanical properties with the absence of genotoxicity on human gingival fibroblast cells.

Mechanical properties of additively manufactured zirconia with alumina air abrasion surface treatment

This study aimed to evaluate the mechanical properties of zirconia fabricated using additive manufacturing technology and compare them to those of zirconia fabricated using subtractive manufacturing technology. Sixty disc-shaped specimens were fabricated for the additive (n = 30) and subtractive manufacturing groups (n = 30), and each group was divided into two subgroups according to their air-abrasion surface treatment: control (n = 15) and air-abrasion groups (n = 15). Mechanical properties including the flexural strength (FS), Vickers hardness, and surface roughness were determined, and the values were analyzed by one-way ANOVA and Tukey's post hoc test (α = 0.05). X-ray diffraction and scanning electron microscopy were used for phase analysis and surface topography evaluation, respectively. The SMA group exhibited the highest FS (1144.97 ± 168.1 MPa), followed by the SMC (944.58 ± 141.38 MPa), AMA (905.02 ± 111.38 MPa), and AMC groups (763.55 ± 68.69 MPa). The Weibull distribution showed the highest scale value (1213.55 MPa) in the SMA group, with the highest shape value in the AMA group (11.69). A monoclinic peak was not detected in both the AMC and SMC groups, but after air abrasion, the monoclinic phase content ([Formula: see text]) reached 9% in the AMA group, exceeding that in the SMA group (7%). The AM groups exhibited statistically lower FS values than those of the SM groups under the same surface treatment (p < 0.05). Air-abrasion surface treatment increased the monoclinic phase content and FS (p < 0.05) in both the additive and subtractive groups, while it increased the surface roughness (p < 0.05) only in the additive group and did not affect the Vickers hardness in either group. For zirconia manufactured using additive technology, the mechanical properties are comparable to those of zirconia manufactured using subtractive technology.

A brief review on fatigue test of ceramic and some related matters in Dentistry

The characteristics of dental ceramics have been extensively studied over the years to provide highly qualified materials for use in prosthetic restorations. The ability to adhere to dental substrates, outstanding aesthetics (translucency, color, and substrate masking ability) and improved mechanical properties provide these materials with optical features and high strength to withstand masticatory stimuli. Different classifications are adopted, and it is generally considered that glass-ceramics have better optical characteristics due to the high glass content, and polycrystalline ceramics have superior strength favored by their densified and organized crystals, hampering crack growth. This knowledge was largely built-up during years of scientific research through different testing methodologies, but mainly employing static loads. It is important to not only take into account the intensity of loads that these materials will be exposed to, but also the effect of the intermittence of cyclic load application leading to mechanical fatigue and the influence of factors related to the crack origin and their propagation under this condition. Furthermore, the bonding surface of ceramic restorations requires surface treatments that improve the bond strength to luting agents; however, these treatments require caution because of their potential to produce defects and affect the structural behavior. Moreover, ceramic restorations often require internal adjustments for proper seating or external adjustments for fitting the occlusal contact with the antagonist. In this sense, finishing/polishing protocols may alter the defect population, as luting agents may also interact by filling in the superficial defects on the restoration intaglio surface. Thus, the balance among all these factors will define the performance of a restorative setup, as well as the posterior exposure to the humid environment and the masticatory stimuli (cyclical loading), which may favor developing slow and subcritical growth of cracks in ceramic materials and the degradation of the bond interface. Therefore, it is essential that the concepts which explain the fatigue mechanism are understood, as well as the crack propagation and failure patterns of restorative ceramic materials.

The Effect of Polishing, Glazing, and Aging on Optical Characteristics of Multi-Layered Dental Zirconia with Different Degrees of Translucency

(1) Background: Considering that the appearance of a dental material is an important factor that contributes to the success of prosthetic restorations. The purpose of this study is to investigate the optical properties and color changes among the layers of three commercial zirconias, to compare the aspect of the polished and glazed surfaces before and after aging and to evaluate the effects of hydrothermal degradation on their aesthetics. (2) Methods: Forty-eight plate-shaped samples were sectioned from presintered blocks of each multilayer translucent zirconia with different Yttrium content: Ceramill Zolid fx ML (5 mol%) = CeZ, STML (4 mol%) = STM, IPS e.maxZirCAD CEREC/in Lab MT Multi (4 mol% + 5 mol%) = IPZ. The samples were sintered according to the recommendation of each manufacturer, and half (24) of them were polished and the other half (24) glazed on one of the surfaces. Each type was equally divided into one control and one aging group, and, for each material, this resulted in four groups (n = 12): polished-control, polished-autoclaved glazed-control, and glazed-autoclaved. The artificial aging was carried out with an autoclave and distilled water at 134 °C, 0.2 MPa for 1 h, and for optical parameters (TP, CR, OP) and color change (ΔE*) measurements on a black and white background in a CIE L*a*b* color system, a spectrophotometer was used. The specimens were evaluated in incisal, cervical, and medium areas on polished and glazed samples before and after the aging stage. Statistical analysis was achieved with a two-way ANOVA test, the unpaired t-test, and the paired t-test. (3) Results: Before and after aging, the mean TP values for polished samples were higher than the glazed ones. After aging, the mean TP values increased for all groups (except polished CeZ), and significant differences were reported for polished STM, IPZ. After LTD, the opalescence registered an increase for almost all groups (except polished CeZ, polished and glazed IPZ-medium area), and significant differences were reported for almost all groups (except STM-incisal, IPZ-cervical, medium areas). The levels of color change were between extremely slight to perceivable. (4) Conclusions: Optical properties of the selected multilayer zirconia were influenced by polishing and glazing as surface treatment and affected by artificial aging (CeZ the least); perceivable color changes for polished STM, IPZ were detected.

Aging resistance of infiltrated monolithic zirconia compared to noninfiltrated monolithic zirconia: A systematic review of in vitro studies

Aim

The aim of the study is to systematically assess the impact of low-temperature degradation (LTD) simulation in an autoclave on mechanical and microstructural properties of infiltrated monolithic zirconia compared to the noninfiltrated zirconia.

Settings and Design

Systematic review following Preferred Reporting Items for Systematic Reviews and Meta-Analysis 2020 guidelines.

Materials and Methods

An electronic search was done within these databases: PubMed, Scopus, and Web of Science, Science Direct, Embase, Wiley, Google Scholar for articles published between 2000 and March 2021. Search results that met eligibility criteria were categorized into two groups based on properties assessed of infiltrated monolithic zirconia exposed to LTD (also called aging simulation) - (a) mechanical (flexural strength and fracture toughness) and (b) microstructural properties (phase transformation rate and m content).

Statistical Analysis Used

Qualitative analysis.

Results

The search identified 272 preliminary results. After discarding duplicates, and screening of titles, abstracts, and full texts, 10 articles finally met inclusion criteria. Data were collected on author's details and their countries, journal and year of publication, type and percentage of infiltration, aging protocol (duration and temperature), mechanical, and microstructural properties. All the included studies invariably revealed better aging resistance without a change in mechanical properties for infiltrated monolithic zirconia as compared to noninfiltrated species.

Conclusion

Infiltration within monolithic zirconia can reduce degradation and simultaneously maintain their mechanical properties by preventing water entry into grain contours. The final m content was less for infiltrated Zirconium, indicating a lesser phase transformation and better aging resistance.

Other Information

Systematic review protocol registered at PROSPERO CRD42021248153.

Current scenario on adhesion to zirconia; surface pretreatments and resin cements: A systematic review

Several methods have been proposed to increase bonding of zirconia with resin. However, we are still to find the Holy Grail. A systematic literature review was performed through PubMed on international literature from January 2000 to May 2021 with relevant Medical Subject Headings terms. 56 articles were found to be relevant. Of all the different methods proposed, mechanochemical pretreatment of zirconia surface with alumina oxide and use of 10-methacryloyloxydecyl dihydrogen phosphate were found to be most effective as per majority of studies. New methods that require further research also surfaced.

Effects of Different Air Particle Abrasion Protocols on the Biaxial Flexural Strength and Fractography of High/Ultra-Translucent Zirconia

In this study, the biaxial flexural strength (BFS) and fractography of high/ultra-translucent monolithic zirconia ceramics subjected to different mechanical surface pretreatments were evaluated. A total of 108 disc-shaped samples (12 mm diameter, 1.2 mm thickness) of three zirconia materials (5Y-ZP KATANA Zirconia UTML (ML), 3Y-TZP DD Bio ZX2 (DB), and 5Y-ZP DD cube X2 (DC)) were used. The BFS was investigated after subjecting the samples to surface treatment using air abrasion particles of two types (aluminum oxide or glass microbeads). The data were analyzed using two-way analysis of variance, followed by Scheffe's post hoc test for multiple comparisons. The mean ± standard deviation BFS for DB was highest after treatment with 50 µm Al₂O₃ (1626.05 ± 31.9 MPa), with lower values being observed following treatment with 50 µm glass microbeads (1399.53 ± 24.2 MPa) and in the control sample (1198.51 ± 21.1 MPa). The mean ± standard deviation (SD) BFSs for DC and ML were the highest in the control groups. Surface air abrasion with 50 µm Al₂O₃ particles and 2 bar pressure is recommended for 3Y-TZP translucent zirconia, while no abrasion of 5Y-ZP translucent zirconia ceramic.

Effect of different zirconia surface pretreatments on the flexural strength of veneered Y-TZP ceramic before and after in vitro aging

PURPOSE

The investigation of zirconia core surface pretreatments on the flexural strength of bilayered zirconia ceramics before and after artificial accelerating aging.

METHODS

Ninety bar-shaped specimens were manufactured from Yttria Stabilized Tetragonal Zirconia Polycrystal (Y-TZP) and divided in three groups depending on zirconia surface pretreatment before veneering: layering with liner, pretreatment with silane-containing gas flame (SGF) with the Silano-Pen device and alumina air-abrasion. Half of the veneered specimens in each group (n=15) underwent artificial accelerating aging. A 4-point bending test was performed to determine flexural strength. Three specimens from each group were further analyzed using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) before veneering and after fracture (aged and non-aged subgroups).

RESULTS

Alumina air-abrasion was correlated to increased phase transformation from tetragonal to monoclinic zirconia phase. Qualitative analysis revealed that with the majority of the specimens pretreated with the silane-containing gas flame, areas of the veneering material remained firmly attached to the zirconia core after flexural strength testing. There was no statistically significant difference on the flexural strength among the groups before or after aging. Artificial accelerating aging resulted in statistically significant higher flexural strength of the specimens after aging.

CONCLUSION

SGF pretreatment can be an acceptable and feasible alternative method before the veneering of Y-TZP zirconia as it presented slightly higher bond strength compared with alumina air-abrasion which was associated with higher tetragonal to monoclinic (t→m) phase transformation. Accelerating aging leads to an increase of the mechanical properties under in vitro conditions.

Surface and bulk properties of zirconia as a function of composition and aging

Yttria-stabilized zirconia (Y-SZ) materials with different levels of translucency have been used for indirect dental restorations. Y-SZ composition and microstructure are modified to improve translucency, and it is not clear how these materials respond to aging. This study evaluated the effect of hydrothermal aging (HA) performed in an autoclave on the properties of four dental Y-SZ materials with different compositions. Sintered bar-shaped specimens (14 x 4 x 2 mm) were prepared from four different zirconia-based materials (n = 40): low translucency 3 mol % Y-SZ (3Y-LT; Ceramill ZI, Amann Girrbach); high translucency 4 mol % Y-SZ (4Y-HT; Ceramill Zolid); and two high translucency 5 mol % Y-SZ (5Y-HT - Lava Esthetic, 3M; 5Y-SHT - Ceramill Zolid FX). Fully sintered specimens were exposed to HA for different times (control - 0 h, 5 h, 10 h, or 15 h at 134 °C, 2 bar pressure) and characterized for surface roughness, flexural strength (three-point bending), hardness and elastic modulus (nanoindentation), surface wettability (sessile drop technique) and crystalline content (x-ray diffraction, XRD). Data was analyzed by two-way ANOVA and Tukey HSD (p < 0.05). Zirconia composition significantly affected roughness (p = 0.016). Zirconia*aging interaction affected flexural strength (p = 0.012), surface wettability (p < 0.001), and hardness (p = 0.002). Zirconia composition (p = 0.011) and aging (p = 0.001) affected elastic modulus, while the interaction effect was not significant (p = 0.94). HA affects zirconia-based materials in different degrees. For 3Y-LT and 4Y-HT, surface and bulk properties were affected by aging to a similar extent. However, surface and bulk properties may change during clinical use as a result of prolonged degradation of Y-SZ.

Mechanical performance of a hybrid zirconia developed through hydrothermal treatment and Room-Temperature Atomic Layer Deposition (RT-ALD)

OBJECTIVE

A silica-based nanofilm has been successfully deposited via Room-Temperature Atomic Layer Deposition (RT-ALD) on the surface of a glass. The purpose of this study was to evaluate the mechanical performance of a hybrid interface created between yttria-stabilized zirconia (Y-PSZ) transformed layer and silica-based nanofilm via RT-ALD.

MATERIAL AND METHODS

Fully-sintered Y-PSZ (14 × 4.0 × 1.5 mm) specimens in different translucencies (MO, MT, LT; IPS e.max Zircad, Ivoclar Vivadent) were distributed in 5 groups: control (C - no treatment); hydrothermal treatment (HT- 15h, 134°C, 2 bar); alumina blasting (B - 50 μm Al₂O₃); RT-ALD silica deposition (S); HT followed by silica deposition (HTS). RT-ALD cycles consisted of the sequential exposure of specimens to tetramethoxysilane orthosilicate (TMOS - 60s) and ammonium hydroxide (NH₄OH - 10 min) vapors in 40 cycles. Mechanical performance was analyzed by flexural strength (FS) (n = 10) and fatigue failure load (staircase method; n = 20) tests. Surface hardness (H) and Young's modulus (YM) were analyzed by nanoindentation. For surface chemical and topographical characterization, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were performed. Data from surface H, YM, FS, and fatigue limit (FL) were analyzed by two-way analysis of variance (ANOVA).

RESULTS

The interaction between material and treatment had a significant effect on FS (p < 0.001). The FS values ranged from 436.23 MPa to 856.65 MPa. HT resulted in the highest FS (856.65 MPa) for LT and the lowest FS (436.23 MPa) for MO zirconia. For all materials, S and B treatments resulted in similar FS values (p > 0.410). S did not affect FL when compared to the C group (p > 0.277) for any material investigated. HTS resulted in higher FL than S for LT and MO materials (p < 0.001). Surface hardness and modulus were similar between control and S-treated specimens for all materials analyzed. XPS analysis showed homogeneous silica content after 20 and 40 RT-ALD cycles, and SEM did not show significant changes in surface morphology between C and S-treated specimens.

CONCLUSION

RT-ALD resulted in effective silica deposition without any deleterious effect on zirconia-based materials mechanical properties. Alumina blasting promoted higher alteration on surface topography. HT prior to S resulted in superior FL (for MO and MT) and flexural strength (MO) for some of the materials investigated.

Survival Rate and Deformation of External Hexagon Implants with One-Piece Zirconia Crowns

This study aimed to evaluate the survival rates of several external hexagon implants directly connected to zirconia crowns after thermomechanical fatigue. The deformation of the hexagons and the integrity of zirconia crowns were also evaluated. A monolithic zirconia crown (Y-TZP) and four different external hexagon dental implants (n = 10, N = 40) were mounted together and embedded in polyurethane. The specimens were subjected to thermomechanical cycling for 2.5 × 106 cycles, at 3.0 Hz frequency, at 200 N loading. The interface of the implant/zirconia crown system, zirconia crowns integrity before and after cycling, and the implant hexagon surface were evaluated under stereomicroscopy and SEM. A nanohardness analysis was performed to verify the hardness of zirconia and implants. Statistical analysis was performed using the Kaplan-Meier test, Multi-Sample Survival Tests, Logrank Test, (p = 0.05). The data did not show significant differences in the survival rates of different implant groups. However, some crowns presented fractures (16.67%) and the external hexagon region of the implants presented plastic deformations (100%). During chewing simulation, the interface between titanium implant and zirconia abutment can promote plastic deformation in the metal and surface defects in the ceramic. In addition, the types of interface defects can be affected by the external hexagon design.

Biaxial flexural strength and phase transformation characteristics of dental monolithic zirconia ceramics with different sintering durations: An in vitro study

STATEMENT OF PROBLEM

Zirconia is a polymorphic metastable material which can react through a phase transformation from tetragonal to monoclinic when exposed to mechanical, physical, or chemical stimuli.

PURPOSE

The purpose of this in vitro study was to investigate the fracture strength and phase structure of different high-translucency zirconia ceramics depending on the changes in sintering duration and thermocycling.

MATERIAL AND METHODS

Two monolithic zirconia ceramics, Katana (KAT) and NexxZr (NEX), were used to prepare disk-shaped specimens (n=66). The sintering temperature was 1500 °C, and 3 different sintering durations were tested: 1 hour, 2 hours (recommended by the manufacturer), and 3 hours. Thermocycling was applied to half the specimens. Fracture strength was calculated, and the specimens were analyzed with an X-ray diffractometer (XRD) to determine the level of the phase transformation. The normal distribution of the data was analyzed by using the Shapiro-Wilk test. Two-way ANOVA was used to compare multiple groups (α=.05). The Fisher least significant difference test was applied to identify significant differences in fracture strength. The paired-specimen t test was applied to perform intragroup comparisons.

RESULTS

Sintering duration significantly affected the fracture strength of KAT (P=.007). For nonthermocycled specimens, the fracture strength of NEX was significantly higher than that of KAT (P<.001). Thermocycling had a significant effect on fracture strength depending on sintering duration and zirconia ceramic interaction (P=.046).

CONCLUSIONS

The sintering duration only affected the KAT zirconia, and the fracture strength of KAT decreased when sintered for 3 hours. Thermocycling decreased the fracture strength of both zirconias, except when the sintering duration was 2 hours for NEX. The fracture strength was higher for NEX than for KAT. Tetragonal-monoclinic phase transformation was not found for either zirconia according to the XRD analysis.

Impact of changes in sintering temperatures on characteristics of 4YSZ and 5YSZ

OBJECTIVES

The aim of this study was to evaluate the effect of different sintering temperatures on biaxial flexural strength (BFS), dynamic loading, surface hardness, color reproduction, translucency, surface roughness and microstructure of zirconia with 4 mol% yttria (4YSZ) compared to zirconia with 5 mol% yttria (5YSZ).

METHODS

Zirconia discs with 12 mm diameter and 1.2 mm thickness were prepared and divided into three groups (n = 53) according to different sintering temperatures (1400 °C, 1500 °C and 1600 °C). Each group was divided into five subgroups (n = 10) according to the dynamic loading procedure (none, 50%, 65%, 75% and 80%) conducted before the quasi-static BFS test and another subgroup (n = 3) used for X-ray-diffraction (XRD) microstructure analysis. BFS test and dynamic loading were performed with a piston-on-three-ball test. The surface hardness was evaluated according to Vickers. Color reproduction and translucency were measured with a spectrophotometer. A 3D laser scanning microscope was used to determine the surface roughness. Grain size measurements were performed using SEM.

RESULTS

A significant increase in biaxial flexural strength was observed while the sintering temperature decreased. 4YSZ had significantly higher results in biaxial flexural strength than 5YSZ. A decrease in sintering temperature resulted in a significant increase in Vickers hardness. Furthermore, 4YSZ showed significantly better color reproduction with increasing sintering temperature. At higher temperatures (1500 °C and 1600 °C), 4YSZ showed better color reproduction than 5YSZ. Compared to 4YSZ, specimens of 5YSZ exhibited significant higher translucency. Using XRD, a distorted tetragonal phase was detected in addition to regular tetragonal and cubic phases in specimens without any stress and at a low sintering temperature. The grain sizes of both materials increased with an increase in sintering temperature.

CONCLUSION

The sintering temperature has significant effects on the microstructure and thus on the mechanical and optical properties of the evaluated zirconia.

Effect of Acid Mixtures on Surface Properties and Biaxial Flexural Strength of As-Sintered and Air-Abraded Zirconia

The aim of this work was to evaluate the effects of application time of an acid mixture solution on the surface roughness, phase transformation, and biaxial flexural strength of 3Y-TZP after sintering or air abrasion. For the biaxial flexural strength measurement, 220 3Y-TZP disk-shaped specimens were prepared after as-sintering or air abrasion. The etching solution comprised a mixture of hydrofluoric acid, sulfuric acid, hydrogen peroxide, methyl alcohol, and purified water. The samples were divided into 11 subgroups according to the etching times (Control, 1, 2, 3, 5, 8, 10, 12, 15, 20, and 30 min). The results showed that acid treatment on both as-sintered and air-abraded 3Y-TZP surfaces increased the surface roughness. However, it had no significant effects on the monoclinic phase or flexural strength of as-sintered zirconia. The monoclinic phase and flexural strength of air-abraded zirconia increased sharply after air abrasion; however, they gradually decreased after acid treatment, to a similar level to the case of the untreated surface. Surface treatment with acid mixture increased the roughness, but the lack of increase of monoclinic phase is thought to be because the loose monoclinic particles remaining on the surface were removed through the etching process.

Intraoral low-temperature degradation of monolithic zirconia dental prostheses: Results of a prospective clinical study with ex vivo monitoring

OBJECTIVE

To investigate the intraoral development and kinetics of low-temperature degradation (LTD) in second-generation 3 mol.% yttria-doped tetragonal zirconia polycrystal (3Y-TZP) monolithic prostheses, as well as the influence of masticatory mechanical stress and glaze layer on it.

METHODS

A total of 101 posterior tooth elements were included in a prospective clinical study, which included ex vivo LTD monitoring (at baseline, 6 months, 1 year, and 2 years) using Raman spectroscopy (n = 2640 monoclinic phase measurement points per evaluation time) and SEM. Four types of areas (1-2 mm² surface, 6 on molars, and 4 on premolars) were analyzed on each element surface: occlusal, axial, glazed, or unglazed. Raman depth mapping and high-resolution SEM were performed on the selected samples.

RESULTS

LTD developed in 3Y-TZP monolithic restorations 6 months after intraoral placement and progressed with time. After two years, the tetragonal-to-monoclinic transformation was non-uniform, with the presence of localized clusters of transformed grains. In axial areas, the grain aspect was typical of the classical nucleation-growth process reported for LTD, which progresses from the surface to a depth of several tens of microns. However, in occlusal areas, tribological stress generated surface crushing and grain pull-out from the clusters, which induced an underestimation of the aging process when the evaluation was limited to monoclinic phase quantification. Glazing cannot be considered a protection against LTD.

SIGNIFICANCE

If LTD occurs in dental prostheses in the same way as in orthopedic prostheses, its clinical impact is unknown and needs to be further studied.

Microstructural, mechanical, and optical characterization of an experimental aging-resistant zirconia-toughened alumina (ZTA) composite

OBJECTIVE

To evaluate the effect of aging on the microstructural, mechanical, and optical properties of an experimental zirconia-toughened alumina composite with 80%Al₂O₃ and 20%ZrO₂ (ZTA Zpex) compared to a translucent zirconia (Zpex) and Alumina.

METHODS

Disc-shaped specimens were obtained by uniaxial and isostatic pressing the synthesized powders (n = 70/material). After sintering and polishing, half of the specimens underwent aging (20 h, 134 °C, 2.2 bar). Crystalline content and microstructure were evaluated using X-ray diffraction and scanning electron microscopy, respectively. Specimens underwent biaxial flexural strength testing to determine the characteristic stress, Weibull modulus, and reliability. Translucency parameter (TP) and Contrast ratio (CR) were calculated to characterize optical properties.

RESULTS

ZTA Zpex demonstrated a compact surface with a uniform dispersion of zirconia particles within the alumina matrix, and typical alumina and zirconia crystalline content. ZTA Zpex and alumina exhibited higher CR and lower TP than Zpex. ZTA Zpex and Zpex showed significantly higher characteristic stress relative to alumina. While aging did not affect optical and mechanical properties of ZTA Zpex and alumina, Zpex demonstrated a significant increase in translucency, as well as a in characteristic stress. Alumina reliability was significantly lower than others at 300 MPa, ZTA Zpex and Zpex reliability decreased at 800 MPa, except for aged Zpex.

SIGNIFICANCE

While aging did not affect the mechanical nor the optical properties of ZTA Zpex and alumina, it did alter both properties of Zpex. The results encourage further investigations to engineer ZTA as a framework material for long span fixed dental prostheses specially where darkened substrates, such as titanium implant abutments or endodontically treated teeth, demand masking.

Impact of multiple firings and resin cement type on shear bond strength between zirconia and resin cements

PURPOSE

The aim of this study was to evaluate the effect of multiple firings on the bond strength between yttria-tetragonal zirconia polycrystals (Y-TZP) and 2 types of resin cements.

MATERIALS AND METHODS

Sixty 3Y-TZP specimens (LAVA Frame Multi) were divided into 3 groups depending on the following firing procedures: (1) 2-firing cycles, (2) 5-firing cycles, (3) 10-firing cycles. Two samples from each group were investigated by using SEM to determine the morphological changes. All specimens were treated with 125 µm airborne-particle abrasion and the surface roughness of each specimen was measured. The specimens from each firing group were then further divided into 2 subgroups (n = 9) to apply 2 types of resin cement (MDP-free resin cement: RelyX Unicem-RU, and MDP containing resin cement: Panavia F 2.0-PA). The shear bond strength (SBS) test was performed and failure types of all the debonded specimens were classified by using a stereomicroscope as adhesive, cohesive, and mixed. The statistical analysis of surface roughness and SBS data were performed by using 1-way ANOVA and 2-way ANOVA followed by Tukey-HSD tests (α=.05). Failure modes were calculated as a percentage for each group.

RESULTS

The bond strength of RU and PA to the specimens obtained with 2 firings were not statistically different from each other (P=.1). However, the SBS values of PA were found to be significantly higher than RU for the specimens obtained with 5 and 10 firing cycles (P=.001 and P=.02, respectively). Surface roughness analysis revealed no statistical difference between groups (P=.2). The SEM analysis of samples fired 5- and 10- times showed irregularities and boundary loss in zirconia grains, and empty spaces between zirconia grains.

CONCLUSION

The bond strength of PA cement was higher than that of RU to the zirconia subjected to repeated firings (5 and 10 firing cycles). When zirconia is subjected to multiple firings, using MDP-containing resin cement can be recommended.

Effect of different grinding protocols on surface characteristics and fatigue behavior of yttria-stabilized zirconia polycrystalline: An in vitro study

STATEMENT OF PROBLEM

Zirconia frameworks milled by computer-aided design and computer-aided manufacturing (CAD-CAM) often require clinical adjustments. In addition, zirconia prefabricated abutments can also require customization to achieve an adequate emergence profile. However, the influence of grinding adjustment on the surface characteristics and mechanical behavior of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and the best grinding protocol is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of different grinding protocols on the surface characteristics, phase transformation, and mechanical behavior of Y-TZP for frameworks and implant abutments.

MATERIAL AND METHODS

Bar-shaped specimens were fabricated according to ISO 6872-2016 and divided into 3 groups: GC (control, untreated), GA (grinding and finishing with medium and fine diamond rotary instruments using high-speed handpiece under constant water cooling), and GB (grinding and finishing with coarse and medium diamond rotary instruments, respectively, using slow-speed handpiece without water cooling). After specimen grinding, the topography and surface roughness were evaluated by using a laser confocal microscope, the Young modulus was measured by the impulse excitation technique, and crystallographic phase transformation was analyzed by X-ray diffraction. Specimens were then submitted to step-stress accelerated life testing (n=18). The surface roughness and Young modulus results were analyzed by 1-way ANOVA and the Tukey honestly significant difference test (α=.05). The data of step-stress accelerated life testing were analyzed by the survival probability considering the number of cycles and force until fracture.

RESULTS

Statistically significant differences were found among groups considering surface roughness (GA>GB>GC) (P<.05) and Young modulus (GB>GA=GC) (P=.003). X-ray diffraction showed that grinding leads to phase transformation, GC showed only tetragonal phase, while GA and GB showed tetragonal and monoclinic phases. No statistically significant difference (P<.05) was found among groups submitted to the same loading profile when the survival probability was compared, but significant difference was found between the light and moderate loading (P=.002) and light and severe loading (P=.014) of GB when different loading profiles in each group were compared.

CONCLUSIONS

Although grinding protocols affected surface characteristics and promoted phase transformation, the mechanical behavior of Y-TZP was not impaired. Therefore, both the grinding protocols tested can be safely used based on the evaluated properties.

Grinding the intaglio surface of yttria partially- and fully-stabilized zirconia polycrystals restorations: Effect on their fatigue behavior

This study evaluated the effects of diamond bur grinding the intaglio surface of second (yttria partially-stabilized zirconia polycrystals, PSZ) and third-generation zirconia (fully-stabilized zirconia polycrystals, FSZ) adhesively cemented to dentin analogue substrate on the fatigue failure load, cycle number until failure, surface micromorphology and phase transformation. Disc-shaped specimens were produced from second (Katana ML-HT, Kuraray) and third-generation zirconia (Katana STML, Kuraray) and randomly allocated (n = 15) into two groups according to the intaglio surface treatment: Control - Ctrl (without grinding); Grinding - Gr (grinding at the center of the intaglio surface). The ceramic discs were adhesively cemented (Multilink Automix System) onto dentin analogue discs. Fatigue tests were executed by the step-stress method. The obtained data were analyzed by Kaplan Meier and Mantel-Cox tests. In addition, surface topography, roughness, phase transformation and fractography analyses were performed. SEM analysis showed that grinding increased the surface roughness and introduced defects in zirconia from both generations. Grinding increased the fatigue failure load, number of cycles to failure and survival rates of the second-generation zirconia statistically (control: 1373.33 N < grinding: 1600 N), while these same outcomes were reduced by grinding for the third-generation zirconia significantly (control: 766.67 N > grinding: 620 N). Thus, clinical adjustments with diamond burs damage the fatigue behavior of adhesively cemented third-generation zirconia.

Effects of low-temperature degradation on the surface roughness of yttria-stabilized tetragonal zirconia polycrystal ceramics: A systematic review and meta-analysis

STATEMENT OF PROBLEM

A wide range of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics have been applied for dental restorations. However, whether hydrothermal aging affects the surface roughness of Y-TZP is unknown.

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate the effects of low-temperature degradation (LTD) on the surface roughness of Y-TZP ceramics.

MATERIAL AND METHODS

This report follows the Preferred Reporting Items for Systematic Reviews and Qualitative Analyses statement. The literature search was conducted with Medline through the PubMed, Web of Science, and Cochrane Library with no publication year limits. The screening and quality assessment were performed by 2 independent reviewers. The studies comparing the surface roughness of Y-TZP ceramics after LTD by using steam autoclave aging were included in the meta-analysis. Meta-analyses were conducted with a random-effects model (α=.05) by using Review Manager Software (Cochrane Collaboration).

RESULTS

Of the 203 potentially relevant studies, 32 full texts were assessed for eligibility. A total of 17 articles were included in the systematic review, and 15 were included in the qualitative analyses. The results showed no significant difference in the arithmetic average height (R_a) values between the nonaged and aged Y-TZP (P=.670; mean difference=0.01; 95% confidence interval=-0.03 to 0.05). Subgroup analyses revealed that the aging duration (P=.003) and specimen preparation (P=.010) contributed significantly to the changes in the surface roughness of the Y-TZP ceramics.

CONCLUSIONS

Although LTD was found to have no significant effects on the surface roughness of Y-TZP ceramics, the effects of LTD depended on the duration of the steam autoclave process and the specimen preparation.

Biological and physicochemical implications of the aging process on titanium and zirconia implant material surfaces

STATEMENT OF PROBLEM

Changes in physicochemical properties because of implant material aging and natural deterioration in the oral environment can facilitate microbial colonization and disturb the soft-tissue seal between the implant surfaces.

PURPOSE

The purpose of this in vitro study was to investigate the effect of aging time on the physicochemical profile of titanium (Ti) and zirconia (ZrO₂) implant materials. Further microbiology and cell analyses were used to provide insights into the physicochemical implications of biological behavior.

MATERIAL AND METHODS

Disk-shaped specimens of Ti and ZrO₂ were submitted to roughness, morphology, and surface free energy (SFE) analyses before nonaging (NA) and after the aging process (A). To simulate natural aging, disks were subjected to low-temperature degradation (LTD) by using an autoclave at 134 ºC and 0.2 MPa pressure for 20 hours. The biological activities of the Ti and ZrO₂ surfaces were determined by analyzing Candida albicans (C. albicans) biofilms and human gingival fibroblast (HGF) cell proliferation. For the microbiology assays, a variance analysis method (ANOVA) was used with the Tukey post hoc test. For the evaluation of cellular proliferation, the Kruskal-Wallis test followed by Dunn multiple comparisons were used.

RESULTS

Ti nonaging (TNA) and ZrO₂ nonaging (ZNA) disks displayed hydrophilic and lipophilic properties, and this effect was sustained after the aging process. Low-temperature degradation resulted in a modest change in intermolecular interaction, with 1.06-fold for TA and 1.10-fold for ZA. No difference in biofilm formation was observed between NA and A disks of the same material. After 48 hours, the viability of the attached HGF cells was very similar to that in the NA and A groups, regardless of the tested material.

CONCLUSION

The changes in the physicochemical properties of Ti and ZrO₂ induced by the aging process do not interfere with C. albicans biofilm formation and HGF cell attachment, even after long-term exposure.

Effect of finishing/polishing techniques and low temperature degradation on the surface topography, phase transformation and flexural strength of ultra-translucent ZrO2 ceramic

OBJECTIVE

To investigate the effect of different surface finishing and polishing regimes and low temperature degradation on flexural strength, phase transformation and surface topography of ultra-translucent ZrO₂ ceramic.

METHODS

300 (n=15/group) of conventional zirconia (Z: Ice Zirkon Transluzent) and ultra-translucent zirconia (UT: Prettau Anterior) bar-specimens were made and divided according to the "Finishing/Polishing" - (C - Control, B - diamond rubber polishers, P - adjusting with burs, PB - adjusting with burs+diamond polishers, PG - adjusting with burs+glaze), "Low temperature Degradation (LTD)" (with or without a treatment at 127°C, 1.7bar/24h). Then, a 3-point mini flexural test was performed in a universal testing machine (1mm/min, 500kgf load cell). SEM, EDS, XDR, AFM, optical profilometry and Weibull analysis were performed. Data were analyzed by 3-way ANOVA and Tukey's post-test (5%).

RESULTS

Groups ZPB_D (1670±253MPa), ZB_D (1664±217MPa), and ZB (1655±3678MPa) showed significantly higher flexural strength than the UTPG group (372±56MPa). The Weibull modulus was significantly higher for the ZP_D group compared to the UB, UC_D, UP_D and UPB_D, while UTB, UTC_D and UTP_D had the lowest value. Monoclinic phases were observed only in the conventional zirconia groups and were more evident after LTD. Diamond rubber polishers presented less roughness for both zirconias.

SIGNIFICANCE

The use of diamond rubber polishers is the most suitable finishing/polishing method for zirconia ceramic restorations and that final glazing reduces the fracture resistance of these materials.

Influence of Aging on Biaxial Flexural Strength and Hardness of Translucent 3Y-TZP

The purpose of this research was to evaluate the influence of aging and surface treatment on surface roughness, biaxial flexural strength (BFS), and Vickers hardness (VHN) of translucent dental zirconia. Half of 80 disc-shaped zirconia specimens (1.2 mm thickness and 12 mm diameter) were aged (group A) in an autoclave for 20 h (134 °C and 0.2 MPa) and the other half were not aged (group N). Specimens were subjected to: no surface treatment (SIN), particle air-abrasion with 50 µm alumina particles at 1 bar (0.1 MPa) and 2.5 bar (0.25 MPa), or polishing down to 1 µm (POL). Specimens were analyzed using X-ray diffraction, laser scanning microscope, BFS, and VHN tests. Three groups (N-SIN, N-POL, and A-POL) showed almost no monoclinic phase. While other groups showed monoclinic phase ratios ranging from 7.5 vol. % ± 2.4 vol. % (N-0.1 MPa) to 41.5 vol. % ± 0.3 vol. % (A-0.1 MPa). Aging and particle air-abrasion increased significantly the BFS, ranging from 720 ± 37 MPa (N-SIN) to 1153 ± 92 MPa (N-0.1 MPa). The hardness was not influenced significantly by aging. A certain amount of monoclinic phase at the surface strengthens the high translucent dental zirconia, while hardness and roughness are not influenced. The pressure of particle air-abrasion showed no influence on the evaluated properties.

Impact of different pretreatments and aging procedures on the flexural strength and phase structure of zirconia ceramics

OBJECTIVE

To test the impact of zirconia pretreatment and aging on flexural strength and phase structure.

METHODS

For flexural strength measurements, 180 3Y-TZP_0.25 specimens were fabricated and pretreated: (i) air-abraded (105-μm alumina, 0.25MPa), (ii) air-abraded (50-μm alumina, 0.25MPa), (iii) air-abraded (30-μm silica-coated alumina, 0.28MPa) (iv) non-pretreated. Each pretreated group (n=15) was aged: (a) hydrothermal (134°C, 0.23MPa, 2h) (b) in a mastication simulator (1,200,000×, 5/55°C) and (c) not aged. The fractured specimens were stored dry for 5 years (23°C) for analysis of phase transformation. Additionally, specimens were fabricated from 3Y-TZP_0.25 (n=12) and 3Y-TZP_0.05 (n=8), pretreated (i, ii, iii, iv), and hydrothermally aged. Each air-abrasion method was alternated using 0.05, 0.25 and 0.4MPa pressure. The phase transformation was examined by Raman spectroscopy and surface topography by scanning electron microscope. Data were analyzed using univariate ANOVA with the Scheffé post hoc test and partial-eta-squared (ƞ_p²) (α=0.05).

RESULTS

The highest impact on flexural strength was exerted by the pretreatment (η_P²=0.261, p<0.001), followed by interactions between pretreatment and aging (η_P²=0.077, p=0.033). Non-pretreated and non-aged specimens showed the lowest monoclinic percentage. Hydrothermal aging and 5 years of storage at room temperature increased the monolithic percentage of 3Y-TZP_0.25. The highest phase transformation was observed in groups air-abraded with 105-μm alumina particles. Increasing pressure during the air-abrading process increased the content of the monoclinic phase in zirconia surfaces.

SIGNIFICANCE

Air-abrasion with 30-μm silica-coated alumina powder can be recommended for pretreatment of 3Y-TZP_0.25 and 3Y-TZP_0.05. For air-abrasion using alumina powder lower pressure should be used.

Effect of different combinations of surface treatment on adhesion of resin composite to zirconia

Purpose: The purpose of this laboratory study was to evaluate the effect of different surface treatment combinations on resin zirconia bonding. Materials and methods: One hundred and five pre-sintered zirconia quadrangles were prepared out of zirconia blocks, polished, then sintered and divided into five groups (n=21). Group I (control): samples were untreated, group II: grit-blasting with 50 µm alumina particles, group III: grit-blasting with 100 µm alumina particles, group IV: Er,Cr:YSGG laser, and group V: selective infiltration etching technique. Microstructural analysis was performed using scanning electron microscopy, atomic force microscopy, a diffractometer, and a profilometer. Cylinders of composite resin were luted with Panavia resin composite cementand Clearfil ceramic primer. Shear bond strength (SBS) was determined using a universal testing machine. Results: SBS results were analyzed using one-way ANOVA followed by Tukey post hoc tests for multiple comparisons. The level of significance was set to 0.05. SBS values of the studied groups II, III, IV, and V were 16.2±1.8 MPa, 15.7±3.7 MPa, 14.8±3.4 MPa, and 16.8±3.0 MPa, respectively. All values were significantly higher than the control group (10.48±1.80 MPa), but without a significant difference between them. Group III exhibited the roughest surface, and Group I had a more significantly reduced surface roughness value than any other group. Group III presented the highest significant increase of tetragonal to monoclinic phase transformation (13%). Conclusion: The use of grit-blasting with greater particles size enhanced SBS with resin composite cement, but induced a higher amount of monoclinic phase transformation. The use of primer based on adhesive monomer with the resin cement is required to enhance the bonding efficiency. The use of laser enhanced the surface roughness and the bonding ability to zirconia.

Does airborne-particle abrasion before, rather than after, zirconia sintering lead to higher mechanical strength even under aging challenge?

STATEMENT OF PROBLEM

Information concerning the effect of airborne-particle abrasion before zirconia sintering on its strength after aging is lacking.

PURPOSE

The purpose of this in vitro study was to determine the influence of airborne-particle abrasion moment (before or after zirconia sintering), particle size, and aging on the mechanical strength of a yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic.

MATERIAL AND METHODS

Four hundred presintered zirconia disks were allocated to nonabraded (control) and abraded groups with 50 μm or 120 μm Al₂O₃ particles before (50/BS or 120/BS) or after sintering (50/AS or 120/AS). The disks were aged by storing them in distilled water for 24 hours at 37 °C (24 h) (control); mechanical cycling (MC) (1×10⁶ cycles; 2 Hz; 100 N); hydrothermal aging (HA) (134 °C; 0.2 MPa; 20 hours); or by both methods (MC+HA), totaling 20 groups (n=20). The specimens were subjected to biaxial flexural strength (BFS) testing. Fractographic analysis was performed to identify the fracture origin. Tetragonal to monoclinic transformation was determined by X-ray diffraction. The BFS data (MPa) were analyzed by 2-way ANOVA and the Games-Howell post hoc test (α=.05). Weibull statistics was also applied.

RESULTS

The BFS and characteristic strength (σ0) of the 120/AS groups were significantly higher compared with the other abraded groups and statistically similar or higher compared with the respective nonabraded groups. In contrast, the 120/BS/24 h and all the 50/BS groups presented the lowest values. In all groups, the initial defect had its origin on the tensile stress side. Neither airborne-particle abrasion nor aging (except for the 50/AS/MC+HA) reduced the zirconia reliability compared with the respective controls. Abrading the zirconia in its presintered stage or after sintering increased the monoclinic phase content, and the sintering process eliminated this phase.

CONCLUSIONS

Airborne-particle abrasion before zirconia sintering yields an inferior performance compared with that after sintering under aging challenges. The BFS and σ0 were strongly determined by the particle size and airborne-particle abrasion moment, while aging mainly acted in the nonabraded zirconia or when this material was abraded before sintering.

Improvement of mechanical properties of Y-TZP by thermal annealing with monoclinic zirconia nanoparticle coating

OBJECTIVE

To assess whether a thermal annealing with a monoclinic zirconia (mZrO₂) nanoparticle coating can improve the reliability of sandblasted yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) and maintain its mechanical strength.

METHODS

Commercially available Y-TZP (Lava Frame, 3M Dental Products) disks were sintered and surface-treated as follows: AS (as sintered, with no treatment); SB (sandblasting); SB-TA (sandblasting followed by thermal annealing at 1000 °C); and SB-mZr-TA (sandblasting followed by thermal annealing at 1000 °C with the mZrO₂ nanoparticle coating). The mZrO₂ nanoparticles of 21 nm in size were prepared by a hydrothermal method, and coated onto Y-TZP sintered disks as a 5 g/L ethanol dispersion. Biaxial flexural strength (S) was measured using the piston-on-three-ball test, and reliability was evaluated by the Weibull modulus (m).

RESULTS

Biaxial flexural tests showed a significant increase in the strength of Group SB (S_SB = 1445 ± 191 MPa) compared with Group AS (S_AS = 1071 ± 112 MPa). The thermal annealing improved the reliabilities of the sandblasted Y-TZP (m_SB-TA = 20.14 and m_SB-mZr-TA = 21.33), as compared with Group SB (m_SB = 7.77). However, the conventional thermal annealing without the mZrO₂ coating caused a significant decrease in the strength of sandblasted Y-TZP (S_SB-TA = 1273 ± 65 MPa). Importantly, the mZrO₂ coating prevented the decrease in the strength caused by conventional thermal annealing (S_SB-mZr-TA = 1379 ± 65 MPa).

SIGNIFICANCE

The thermal annealing with the mZrO₂ nanoparticle coating can improve the reliability of sandblasted Y-TZP and maintain its mechanical strength, which would otherwise be decreased by the conventional annealing process.

Effect of hydrothermal aging on the optical properties of precolored dental monolithic zirconia ceramics

STATEMENT OF PROBLEM

The long-term color stability of precolored monolithic zirconia has not been thoroughly investigated.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of hydrothermal aging on the optical properties, phase transformation, and surface topography of precolored monolithic zirconia ceramics.

MATERIAL AND METHODS

Precolored monolithic zirconia specimens (17.0×17.0×1.5 mm, n=50) and lithium disilicate glass-ceramic specimens (16.0×16.0×1.5 mm, n=50) were artificially aged in an autoclave at 134°C under 0.2 MPa for 0, 1, 3, 5, or 10 hours (n=10). CIELab color parameters were obtained from spectral measurements. The translucency parameter (TP) and CIEDE2000 color differences (ΔE₀₀) were calculated. The microstructural and surface properties were analyzed by X-ray diffraction (XRD), atomic force microscope (AFM), and scanning electron microscope (SEM). Data were analyzed with 2-way ANOVA and pairwise comparison (α=.05).

RESULTS

Significant interactions were found between aging time and ceramic material on L*, a*, b*, and TP (P<.001) as follows: b* partial eta squared [η_p²]=0.689; L* η_p²=0.186; a* η_p²=0.176; and TP η_p²=0.137. The b* values significantly decreased after aging for zirconia (P<.001), whereas TP increased after aging for zirconia (P<.014) except at 10 hours (P=.389) and for lithium disilicate (P<.001). The ΔE₀₀ values relative to baseline ranged from 2.03 to 2.52 for aged zirconia and from 0.07 to 0.23 for aged lithium disilicate. XRD analysis revealed that hydrothermal aging promoted an increase in m-phase contents. AFM and SEM demonstrated surface alterations after aging.

CONCLUSIONS

Optical properties and microstructures of precolored monolithic zirconia ceramics were affected by hydrothermal aging, and translucency increased slightly with aging time.

Y-TZP surface behavior under two different milling systems and three different accelerated aging protocols

BACKGROUND

To evaluate the influence of aging and milling system on zirconia surface roughness (SR) and phase transformation.

METHODS

Eighty crowns were divided in two groups according to yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic milling system (CAD-Neodent or MAD-Zirkonzahn). The crowns were submitted to different aging protocols (N.=10): 1) no aging; 2) autoclaving; 3) pH cycling; or 4) thermocycling. Next, the samples were submitted to an optical profilometry analysis to determine differences in the SR (µm). An X-ray diffraction analysis (XRD) was performed to quantify the percentage of monoclinic phase transformation. SR data was analyzed by two-way ANOVA (α=0.5%).

RESULTS

Aging protocol (P=0.42) and milling system (P=0.08) had no influence on the SR. However, the number of monoclinic phases was influenced by the autoclaving and pH cycling.

CONCLUSIONS

The surface roughness of zirconia-based crowns was not influenced by low temperature degradation or milling system. Regarding the phase transformation, autoclaving and pH-cycling aging presented a monoclinic phase increase when compared to the control group and thermocycled groups.

Fatigue behavior and surface characterization of a Y-TZP after laboratory grinding and regeneration firing

This study evaluated the effect of grinding and regeneration firing on the flexural fatigue limit and surface characterization of Lava™ Y-TZP ceramic. Forty bar-shaped specimens with 20 × 4.0 × 1.2 mm constituted the as-sintered group (AS = control group), and 80 specimens with 20 × 4.0 × 1.5 mm were ground with cylindrical laboratory stone under water-cooling (WG) or in a dry condition (G) to reach 1.2 mm in thickness. Half of specimens were submitted to regeneration firing (1000 °C, 30 min), forming the groups AS/R, WG/R and G/R. Fatigue limit (500,000 cycles, 10 Hz) was determined by staircase method in a 4-point flexural fixture. Data were analyzed by 2-way ANOVA and Tukey HSD tests (α = 0.05). The surface topography (n = 3) and fracture area (n = 3) were evaluated by SEM. Samples were also analyzed by Rietveld refinement from X-ray diffraction data. ANOVA revealed significant differences (P < .001) for grinding protocol, regeneration firing and their interaction. In the groups not submitted to regeneration firing, the mean flexural fatigue limit of WG was higher (P < .05) than that of G and AS, with no statistical difference between each other (P > .05). After regeneration firing the inequality WG>AS>G (P < .05) was observed. The regeneration firing increased the fatigue limit of AS group and decreased those of G and WG groups (P < .05). Grinding protocols created evident grooves on zirconia surface. Failures initiated on tensile side of all specimens. The percentages (wt%) of monoclinic phase before cyclic loading were: AS (7.4), AS/R (6.5), G (2.8), G/R (0.0), WG (4.4), WG/R (0.0); and after cyclic loading: AS (8.6), AS/R (1.2), G (2.4), G/R (5.7), WG (6.3), WG/R (0.0). Wet grinding did not compromise the fatigue limit of zirconia, increasing its mechanical strength. Regeneration firing reduced the fatigue limit of ground samples, despite reducing the amount of monoclinic phase in all experimental conditions.

Evaluation of the Effect of Different Types of Abrasive Surface Treatment before and after Zirconia Sintering on Its Structural Composition and Bond Strength with Resin Cement

This study evaluated the effect of air abrasion before and after sintering with different particle type, shape, and size on the surface morphology, monoclinic phase transformation, and bond strength between resin cement and zirconia surface using primer containing silane and MDP. Airborne particle abrasion (APA) was performed on zirconia before and after sintering with different particle shape and size (50 μm Al₂O₃ and 25 μm silica powder). 120 square shaped presintered zirconia samples (Amann Girrbach) were prepared (3 mm height × 10 mm width × 10 mm length) and polished with grit papers #800, 1000, 1200, 1500, and 2000. Samples were divided into 6 groups according to surface treatment-group A: (control) no surface treatment; group B: APA 50 μm Al₂O₃ before sintering (BS); group C: APA 50 μm Al₂O₃ after sintering (AS); group D: APA25 μm silica powder (BS); group E: APA25 μm silica powder (AS) at a pressure of 3.5 bar; and group F: APA 25 μm silica powder (AS) at a pressure of 4 bar. Samples were analyzed using XRD, AFM, and SEM. The samples were submitted to shear bond strength (SBS) test. A dual cure resin cement (RelyX Ultimate) and primer (Scotchbond Universal) were used. Data were analyzed with ANOVA and Tukey test (α ≥ 0.05). APA in group B significantly increased the surface roughness when compared to all other groups. A significant monoclinic phase transformation (t-m) value was observed in groups C and F and a reverse transformation occurred in presintered groups. The SBS value of group A was 11.58 ± 1.43 and the highest significant shear bond strength value was for groups B (15.86 ± 1.92) and C (17.59 ± 2.21 MPa) with no significant difference between them. Conclusions. The use of APA 50 μm Al₂O₃ before sintering and the application of primer containing MDP seem to be valuable methods for durable bonding with zirconia. The use of APA 50 μm Al₂O₃ after sintering induced the highest (t-m) phase transformation.

Mechanical reliability, fatigue strength and survival analysis of new polycrystalline translucent zirconia ceramics for monolithic restorations

This study characterized the mechanical properties (static and under fatigue), the crystalline microstructure (monoclinic - m, tetragonal - t and cubic - c phase contents) and the surface topography of three yttrium-stabilized zirconia (YSZ) materials with different translucent properties, before and after aging in an autoclave (low temperature degradation). Disc-shaped specimens were produced from second generation (Katana ML/HT - high-translucent) and third generations (Katana STML - super-translucent and UTML - ultra-translucent) YSZ ceramics (Kuraray Noritake Dental Inc.), following ISO 6872-2015 guidelines for biaxial flexural strength testing (final dimensions: 15 mm in diameter and 1.2 ± 0.2 mm in thickness), and then subjected to the respective tests and analyses. ML was mainly composed of tetragonal crystals, while STML and UTML presented cubic content. Aging increased the monoclinic content for ML and did not affect STML and UTML. Topographical analysis highlights different grain sizes on the ceramic surface (UTML > STML > ML) and aging had no effect on this outcome. Weibull analysis showed the highest characteristic strength for ML both before and after aging, and statistically similar Weibull moduli for all groups. ML material also obtained the highest survival rates (ML > STML > UTML) for both fatigue strength and number of cycles to failure. All fractures originated from surface defects on the tensile side. Third generation zirconia (Katana STML and UTML) are fully stabilized materials (with tetragonal and cubic crystals), being totally inert to the autoclave aging, and presented lower mechanical properties than the second-generation zirconia (Katana ML - metastable zirconia).

The effect of various polishing systems on surface roughness and phase transformation of monolithic zirconia

PURPOSE

The purpose of this study was to evaluate and compare three polishing systems on the surface roughness and phase transformation of monolithic zirconia.

MATERIALS AND METHODS

100 disk shaped specimens (10 mm diameter, 3 mm thickness) were fabricated from monolithic zirconia blocks. 20 specimens were left as a control group and remaining specimens were grinded by diamond bur to simulate the occlusal adjustments. Grinded specimens were randomly divided into 4 groups: group G (no polishing), group M (Meisinger, zirconia polishing kit), group E (EVE Diacera, zirconia polishing kit), and group P (EVE Diapol, porcelain polishing kit). Surface roughness was measured with profilometer and surface topography was observed with SEM. XRD analysis was performed to investigate the phase transformation. Statistical analysis was performed with one-way ANOVA and Tukey's post hoc tests at a significance level of P=.05.

RESULTS

All polishing groups showed a smoother surface than group G. Among 3 polishing systems, group M and group E exhibited a smoother surface than the group P. However, no significant differences were observed between group M and group E (P>.05). Grinding and polishing did not cause phase transformations in zirconia specimens.

CONCLUSION

Zirconia polishing systems created a smoother surface on zirconia than the porcelain polishing system. Phase transformation did not occur during the polishing procedure.

Effect of Air Abrasion on the Number of Particles Embedded in Zironia

Background: Determination of the number of abrasive particles embedded in the zirconia surface after variable parameters of treatment. Methods: One hundred thirty cylindrical disks made from zirconia were divided into 7 groups (n = 10): one control and six test groups treated by air abrasion using Al₂O₃ or SiC with grain sizes: 60, 110, 250 μm with a working pressure of 0.2 or 0.35 MPa. The SEM images were observed in BSE and BSE 3D. The chemical composition was determined by means of X-ray microanalysis with EDS. The surface was determined by quantitative metallography methods. Surfaces (%) depending on the particle type were compared using the Mann-Whitney test, depending on the pressure were compared using the Mann-Whitney test, and depending on the grain size were compared using the Kruskal-Wallis test as well as the Jonckheere-Terpstra trend test as well as the Dunn post-hoc testA probability. Value of p < 0.05 was deemed significant, while a p-value of p < 0.01 was regarded as highly significant. Results: After blasting aluminium and silicon particles were embedded in zirconia surface. When blasted with Al₂O₃, the average amount of embedded grain was higher, while in the case of SiC. Highly significant differences were observed in the surface share of the abrasive depending on the grain size. At a pressure of 0.20 MPa the quantity of embedded abrasive amounted to 6.63, and at the pressure of 0.35 MPa rose to 7.17. Most particles of abrasive material became embedded when sandblasting with grain size 60 μm grain. No significant difference was observed in the surface share of the abrasive depending on the pressure. Conclusion: The quantity of embedded abrasive depends on its type and grain size, and the pressure applied.

Zirconia toughened mica glass ceramics for dental restorations

OBJECTIVE

The objective of the present study is to understand the role of yttria stabilized zirconia (YSZ) in achieving the desired spectrum of clinically relevant mechanical properties (hardness, elastic modulus, fracture toughness and brittleness index) and chemical solubility of mica glass ceramics.

METHODS

The glass-zirconia mixtures with varying amounts of YSZ (0, 5, 10, 15 and 20wt.%) were ball milled, compacted and sintered to obtain pellets of glass ceramic-YSZ composites. Phase analysis was carried out using X-ray diffraction and microstructural characterization with SEM revealed the crystal morphology of the composites. Mechanical properties such as Vickers hardness, elastic modulus, indentation fracture toughness and chemical solubility were assessed.

RESULTS

Phase analysis of sintered pellets of glass ceramic-YSZ composites revealed the characteristic peaks of fluorophlogopite (FPP) and tetragonal zirconia. Microstructural investigation showed plate and lath-like interlocking mica crystals with embedded zirconia. Vickers hardness of 9.2GPa, elastic modulus of 125GPa, indentation toughness of 3.6MPa·m^1/2, and chemical solubility of 30μg/cm² (well below the permissible limit) were recorded with mica glass ceramics containing 20wt.% YSZ.

SIGNIFICANCE

An increase in hardness and toughness of the glass ceramic-YSZ composites with no compromise on their brittleness index and chemical solubility has been observed. Such spectrum of properties can be utilised for developing a machinable ceramic for low stress bearing inlays, onlays and veneers.

Structural and Morphological Evaluation of Presintered Zirconia following Different Surface Treatments

AIM

The aim of this study was to evaluate the effect of different surface treatments on roughness, grain size, and phase transformation of presintered zirconia.

MATERIALS AND METHODS

Surface treatments included airborne particle abrasion (APA) before and after sintering with different particles shape, size, and pressure (50 μm Al₂O₃, 50 μm glass beads, and ceramic powder). Thirty-five square-shaped presin-tered yttrium-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic slabs (Zenostar ZR bridge, Wieland) were prepared (4 mm height × 10 mm width × 10 mm length) and polished with silicon carbide grit papers #800, 1000, 1200, 1500, and 2000 to ensure identical initial roughness. Specimens were divided into five groups according to surface treatment: group I (control): no surface treatment; group II: APA 50 μm Al₂O₃ after sintering; group III: APA 50 μm Al₂O₃ particles before sintering; group IV: APA 50 μm glass bead particles before sintering; and group V: APA ceramic powder before sintering. Specimens were analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) analyses, and tested for shear bond strength (SBS). Data were statistically analyzed using one-way analysis of variance (ANOVA) followed by post hoc tests for multiple comparisons Tukey's test (a > 0.05).

RESULTS

Air abrasion before sintering significantly increased the surface roughness when compared with groups I and III. The highest tetragonal to monoclinic (t-m) phase transformation (0.07%) was observed in group III, and a reverse transformation was observed in presintered groups (0.01%). Regarding bond strength, there was a significant difference between APA procedures pre- and postsintering.

CONCLUSION

Air abrasion before sintering is a valuable method for increasing surface roughness and SBS. The abrasive particles' size and type used before sintering had a little effect on phase transformation.

CLINICAL SIGNIFICANCE

Air abrasion before sintering could be supposed to be an alternative surface treatment method to air abrasion after sintering.

Mechanical behavior of yttria-stabilized tetragonal zirconia polycrystal: Effects of different aging regimens

This study aimed to characterize and compare the effect of different aging regimens on surface characteristic (topography and roughness), structural stability (phase transformation) and mechanical performance (Weibull analysis) of a Y-TZP ceramic. Discs (15 × 1.2 mm; VITA In-Ceram YZ) were prepared according to ISO 6872-2015 for biaxial flexural strength testing and randomly assigned into five groups (n = 30): as-sintered, no aging treatment (CTRL); 20 h in autoclave at 134°C, 2 bar pressure (AUT); intermittent mechanical loading at 20 Hz/106 load pulses (MechLoad); AUT followed by MechLoad (AUT+MechLoad); and storage in distilled water at 37°C, for 1 year (STO). The following analyses were performed: roughness (n = 30), surface topography (n = 2), phase transformation (n = 2) and biaxial flexure strength (n = 30). Phase transformation (increase of m-phase content) was shown to be a spontaneous, unavoidable and time-dependent process, occurring even under ambient conditions (dry storage after 1 year = 6.0% increase), and is considerably accelerated in the presence of moisture (STO = 17.6%; AUT= 63.1%; and AUT+MechLoad = 59.9%). For roughness parameters, only Ra was affected by aging, and the highest values were observed for AUT+MechLoad (0.25 ± 0.07 µm). For Weibull analysis, structural reliability (Weibull moduli) and characteristic strength were not impaired after aging, and some aging conditions led to increased values (highest weibull moduli in AUT, and highest characteristic strength in STO). Phase transformation proves to be a time-dependent spontaneous mechanism that is accelerated in the presence of different stimuli. However, none of the aging regimens had a negative effect on the characteristic strength and structural reliability of Y-TZP ceramic.

Grinding With Diamond Burs and Hydrothermal Aging of a Y-TZP Material: Effect on the Material Surface Characteristics and Bacterial Adhesion

The aim of this study was to evaluate the effect of grinding with diamond burs and low-temperature aging on the material surface characteristics and bacteria adhesion on a yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) surface. Y-TZP specimens were made from presintered blocks, sintered as recommended by the manufacturer, and assigned into six groups according to two factors—grinding (three levels: as sintered, grinding with extra-fine diamond bur [25-μm grit], and grinding with coarse diamond bur [181-μm grit]) and hydrothermal aging—to promote low-temperature degradation (two levels: presence/absence). Phase transformation (X-ray diffractometer), surface roughness, micromorphological patterns (atomic force microscopy), and contact angle (goniometer) were analyzed. Bacterial adhesion (colony-forming units [CFU]/biofilm) was quantified using an in vitro polymicrobial biofilm model. Both the surface treatment and hydrothermal aging promoted an increase in m-phase content. Roughness values increased as a function of increasing bur grit sizes. Grinding with a coarse diamond bur resulted in significantly lower values of contact angle (p&lt;0.05) when compared with the extra-fine and control groups, while there were no differences (p&lt;0.05) after hydrothermal aging simulation. The CFU/biofilm results showed that neither the surface treatment nor hydrothermal aging simulation significantly affected the bacteria adherence (p&gt;0.05). Grinding with diamond burs and hydrothermal aging modified the Y-TZP surface properties; however, these properties had no effect on the amount of bacteria adhesion on the material surface.