Effect of surface treatments on the biaxial flexural strength, phase transformation, and surface roughness of bilayered porcelain/zirconia dental ceramics

Current speed sintering and high-speed sintering protocols compromise the translucency but not strength of yttria-stabilized zirconia

OBJECTIVES

To investigate the impacts of speed and high-speed sintering on the densification, microstructure, phase composition, translucency, and flexural strength of yttria-stabilized zirconia (YSZ).

METHODS

A total of 162 disc-shaped specimens (n = 18) were cold-isostatically pressed from 3YSZ (Zpex), 4YSZ (Zpex 4), and 5YSZ (Zpex Smile) powders (Tosoh Corporation) and sintered according to the following protocols: conventional (control, ∼12 h), speed (∼28 min for 3YSZ; ∼60 min for 4YSZ and 5YSZ), and high-speed (∼18 min) sintering. Dimensions of zirconia specimens after sintering and polishing (1-μm diamond grit finish) were Ø13.75 × 1 mm. Density, microstructure, phase content, translucency parameter, and biaxial flexural strength were evaluated using Archimedes', SEM, XRD, spectrophotometric, and piston-on-3-ball methods, respectively. Data were analyzed with either one-way ANOVA and Tukey's test or Kruskal-Wallis with Dunn's test (α = 0.05).

RESULTS

For all YSZ compositions, conventional sintering yielded the highest density followed by speed then high-speed sintering. All sintering protocols resulted in similar strength values; however, speed and high-speed sintering protocols afforded significantly lower translucency relative to conventional sintering. XRD analysis revealed similar spectra for YSZs sintered by various protocols. The speed sintered specimens had the smallest grain size whereas the high-speed sintered 5YSZ possessed the largest grain size among all groups. SEM examination of all YSZ compositions revealed that the average pore size was an order of magnitude smaller than the average grain size.

SIGNIFICANCE

Speed and high-speed sintering of YSZs yield similar strength but diminished density and translucency relative to their conventionally sintered counterparts.

Fracture strength of porcelain veneer on surface-treated zirconia

In this study, we investigated the effects of surface treatment on the fracture strength of porcelain-veneered zirconia. Highly translucent 4 mol% yttria-stabilized zirconia disks (KATANA HT, Kuraray Noritake Dental) were divided into three surface-treatment groups: 1)as-sintered, 2) alumina sandblasted, and 3) ground. Crystallographic and surface-roughness analyses were conducted for each group. Veneering ceramics (Cerabien ZR, Kuraray Noritake Dental) were applied to the zirconia surfaces. The fracture strengths of the porcelain-veneered zirconia disks were measured using biaxial flexural-strength tests. Crystallographic analysis revealed that grinding and sandblasting increased the fractions of the monoclinic and rhombohedral zirconia phases. The ground specimens had a higher surface roughness than the sandblasted specimens. Weibull analysis showed no significant differences in biaxial flexural strength among the three groups. The results suggest that these surface treatments do not affect the fracture strength of porcelain-veneered zirconia.

The influence of core-build up materials on biaxial flexural strength of monolithic strength-gradient zirconia; an in-vitro study

BACKGROUND

Since their introduction, there has been limited research regarding the mechanical properties of novel strength-gradient monolithic zirconia. In addition to that, studies evaluating the effect of different core-build materials on the strength of indirect restorations are scarce. Therefore, the aim of this study was to investigate the effect of using different core build-up materials on biaxial flexural strength of a new monolithic multilayered zirconia material.

METHODS

Forty zirconia discs were fabricated from IPS e.max ZirCAD Prime (Ivoclar Vivadent AG, Schaan, Liechtenstein) and divided into 2 groups (n = 20). Forty composite discs were prepared from Tetric N-Ceram (Ivoclar Vivadent AG, Schaan, Liechtenstein) and MultiCore Flow (Ivoclar Vivadent AG, Schaan, Liechtenstein). The zirconia discs were adhesively cemented to the 2 types of composite forming 2 groups (Zirconia-Tetric N-Ceram and Zirconia-MultiCore Flow). Biaxial flexural strength was determined using a piston-on-3-ball test. The data were statistically analyzed with an independent t-test for significant differences (p = 0.05).

RESULTS

Tetric N-Ceram had significantly higher strength than MultiCore Flow (p < 0.001) but no statistically significant differences were found in strength values between Zirconia-Tetric N-Ceram and Zirconia-MultiCore Flow bilayered samples (p = 0.27).

CONCLUSIONS

It was concluded that although the tested composite core materials significantly differ in their biaxial flexural strength values, they had no influence on the biaxial flexural strength of the overlying zirconia.

Flexural Strength Properties of Five Different Monolithic Computer-Aided Design/Computer-Aided Manufacturing Ceramic Materials: An In Vitro Study

Objective

The purpose of this in vitro study is to compare the flexural strength and Weibull modulus of 5 different monolithic computer-aided design/computer-aided manufacturing (CAD/CAM) ceramics.

Methods

A total of 50 specimens were fabricated, 10 from each of the following materials: lithium disilicate-based ceramic (IPS e.max CAD), zirconia -reinforced lithium-silicate ceramic (Vita Suprinity), leucite-based glass ceramic (IPS Empress CAD), and two zirconia-based ceramics (Zenostar and CopraSmile). The specimens were 4 mm wide, 2 mm thick, and 16 mm long. Flexural strength test was executed using a universal testing machine (Model 5980, Instron Industrial Products, Norwood, MA, USA). The two-parameter Weibull distribution function was used to analyze the variability of flexural strength values. Statistical analysis was performed on SPSS Version 23 (IBM Corp., Armonk, NY, USA) using one-way analysis of variance (ANOVA) and post-hoc Tukey’s test.

Results

Suprinity had the highest Weibull modulus value, while Empress CAD displayed the lowest value. One-way ANOVA showed significant difference in the flexural strength between the different materials tested (p<0.05). Post-hoc analysis revealed significant differences among all the test groups in terms of flexural strength. Zenostar presented the highest mean flexural strength value (1033.90 MPa), while Empress CAD had the lowest value.

Conclusion

High-translucency zirconia had superior flexural properties than translucent zirconia, lithium disilicate ceramics, zirconia-reinforced lithium silicate ceramics, and leucite-based glass ceramics.

Layer characteristics in strength-gradient multilayered yttria-stabilized zirconia

OBJECTIVES

To investigate crystallography, translucency, phase content, microstructure and flexural strength of two commercial strength-gradient multilayered dental zirconia grades.

METHODS

Two zirconia grades, i.e., KATANA Zirconia YML (Kuraray Noritake; referred to as "YML"; composed of four layers: enamel, body 1-3) and IPS e.max ZirCAD Prime (Ivoclar Vivadent; referred to as "Prime"; composed of three layers: enamel, transition, body) were investigated. Fully sintered square-shaped zirconia specimens from each layer were prepared. Microstructure, chemical composition, translucency parameter and zirconia-phase composition of each layer were characterized. Four-point and biaxial flexural strength of each layer was measured using fully sintered bar- and square-shaped specimens. Square-shaped samples were used to measure strength across the layers.

RESULTS

For both multilayer zirconia grades, the 'enamel' layer contains a higher amount of c-ZrO₂, which resulted in higher translucency but lower flexural strength than the 'body' layers. The characteristic 4-point flexural strength of the YML 'body 2' (923 MPa) and 'body 3' (911 MPa) layers, and of the Prime 'body' (989 MPa) layer were comparable and higher than for the YML 'enamel' (634 MPa), Prime 'transition' (693 MPa) and 'enamel' (535 MPa) layers. The biaxial strength of specimens sectioned across the layers was in-between that of the 'enamel' and 'body' layers for both YML and Prime, implying the interfaces did not form a weak link.

SIGNIFICANCE

The difference in yttria content affects the phase composition and mechanical properties of each layer of the multi-layer zirconia. The strength-gradient approach allowed to integrate monoliths with irreconcilable properties.

Effect of Different Cleaning Methods on Shear Bond Strength of Resin Cement to Contaminated Zirconia

The aim of this study was to evaluate the effect of different cleaning methods on the shear bond strength (SBS) of resin cement to contaminated zirconia specimens. Eighty rectangular-shaped specimens (2 × 5 × 10 mm) were fabricated from Zirconia blocks (IPS e.max ZirCAD) and randomly divided into 8 groups (n = 10). Group A (control) was not exposed to contaminants. The following tests specimens were contaminated with saliva and silicone indicating paste. Group B was coated with ceramic primer, then subjected to contamination. Groups C, D, E, F, G, and H were contaminated; cleaned with water rinse, Ivoclean, air particle abrasion, hydrofluoric acid, KATANA^TM Cleaner and ZirClean^TM, respectively, and then coated with ceramic primer and bonded to dual cure resin cement cylinders. All the specimens were subjected to artificial aging and surviving specimens were subjected to the SBS test. For statistical analysis, ANOVA and multiple comparison methods at the 0.05 significance level were used. There was no statistically significant difference among Ivoclean (21.48 ± 2.90 MPa), air particle abrasion (21.92 ± 2.85 MPa), and the control group (24.68 ± 5.46). The application of ceramic primer before contamination did not preserve the SBS of resin cement to zirconia. Cleaning the contaminated zirconia surface with hydrofluoric acid (15.03 ± 3.63) or KATANA^TM Cleaner (17.27 ± 7.63) did not restore SBS to the uncontaminated state, but it was significantly higher than simply rinsing with water (12.46 ± 5.17) or the use of ZirClean^TM (11.59 ± 5.53). The bond strength of resin cement to zirconia was influenced by cleaning methods.

A Comparative Study on Simulated Chairside Grinding and Polishing of Monolithic Zirconia

This study evaluated the effects of different simulated chairside grinding and polishing protocols on the physical and mechanical properties of surface roughness, hardness, and flexural strength of monolithic zirconia. Sintered monolithic zirconia specimens (15 mm × 3 mm × 3 mm) were abraded using three different burs: diamond bur, modified diamond bur (zirconia specified), and tungsten carbide bur, along with a group of unprepared specimens that served as a control group. The study was divided into two phases, Phase 1 and Phase 2. Surface roughness, surface hardness, and flexural strength were assessed before and after the grinding procedure to determine the ‘best test group’ in Phase 1. The best abrasive agent was selected for Phase 2 of the study. The specimens in Phase 2 underwent grinding with the best abrasive agent selected. Following the grinding, the specimens were then polished using commercially available diamond polishing paste, a porcelain polishing kit, and an indigenously developed low-temperature sintered zirconia slurry. The physical and mechanical properties were again assessed. Results were analyzed using one-way ANOVA test. Specimens were observed under scanning electron microscopy (SEM) and X-ray diffraction (XRD) for their microstructure and crystalline phases, respectively. Grinding with diamond burs did not weaken zirconia (p > 0.05) but produced rougher surfaces than the control group (p < 0.05). Tungsten carbide burs did not significantly roughen the zirconia surface. However, specimens ground by tungsten carbide burs had a significantly reduced mean flexural strength (p < 0.05) and SEM revealed fine surface cracks. Phase transformation was not detected by XRD. Polishing with commercially available polishing agents, however, restored the surface roughness levels to the control group. Dental monolithic zirconia ground with tungsten carbide burs had a significantly reduced flexural strength and a smooth but defective surface. However, grinding with diamond burs roughened the zirconia surface. These defects may be reduced by polishing with commercially available polishing agents. The use of tungsten carbide burs for grinding dental zirconia should not be advocated. Grinding with diamond abrasives does not weaken zirconia but requires further polishing with commercially available polishing agents.

Influence of high-speed sintering protocols on translucency, mechanical properties, microstructure, crystallography, and low-temperature degradation of highly translucent zirconia

OBJECTIVES

Impacts of high-speed sintering on the optical and mechanical properties, microstructure, crystallography, and low-temperature degradation of commercial yttria-partially stabilized zirconia (Y-PSZ) were investigated.

METHODS

Five commercial Y-PSZ products (KATANA HT, KATANA STML, KATANA UTML, Zpex 4, and Zpex Smile) were investigated. Specimens were sintered following speed-sintering (~90 min) and conventional-sintering protocols (~7 h), and a group of KATANA STML was super-speed-sintered (18 min). Dimensions of the zirconia specimens after sintering were 14.5 mm (diameter) and 1.2 mm (thickness). Translucency was assessed using a colorimeter. Biaxial flexural strength was measured using a universal testing machine, followed by Weibull analysis. Scanning electron microscopy was used for microstructure assessments. X-ray diffraction was used to analyze the crystallography before and after hydrothermal aging. Low-temperature degradation (LTD) tests were performed at 134 °C under 2-3 bar water vapor in an autoclave.

RESULTS

The translucency and flexural strength were not affected significantly by the sintering programs (p > 0.05). The conventionally sintered KATANA STML and speed-sintered Zpex 4 presented the highest and lowest Weibull modulus, respectively. The conventionally-sintered Y-PSZ had a larger average grain size and smaller fraction of fine grains than those of the speed-sintered specimens. The fractographic analysis of the speed- and conventionally sintered Y-PSZ yielded comparable results. The speed-sintered Y-PSZ exhibited a lower c-ZrO₂ content than that of conventionally-sintered Y-PSZ, except for KATANA HT and KATANA STML. LTD tests indicated that some of the speed and conventionally-sintered Y-PSZ exhibited similar monoclinic volume fractions.

SIGNIFICANCE

Speed-sintering programs are acceptable for Y-PSZ zirconia.

On the behaviour of zirconia-based dental materials: A review

Zirconia-based dental materials are extensively used in clinical practice due to their tooth-like appearance, biofunctionality, biocompatibility, and affordability. However, premature clinical failures of veneering porcelains raise a concern about their integrity. Extensive studies have been performed over a decade to resolve this issue, but it is challenging to reference all information effectively. A single source identifying the significance of potential parameters on material performance has not previously been available. An evidence-based meta-narrative review technique was used to review the characteristic parameters that can affect the overall behaviour of zirconia-based materials. Keywords were chosen to assess manuscripts based on scientific coherence with this paper's research objective. Online keyword searches were carried out on ScienceDirect, PubMed, and SAGE databases for relevant published manuscripts from year 1985-2020.261 out of 3170 identified manuscripts were included. A total of 10 parameters were identified and classified into the material, manufacturing, and geometric aspects. The effect of every parameter was reviewed on the performance of the material. A discrepancy in findings was observed and is attributed to the fact that there is no standard methodology. This review acts as a single source that summarizes various parameters' contribution to zirconia-based dental materials' performance. This review facilitates manufacturing improvements by accounting for every parameter's effect on overall performance.

Survival and complications of monolithic ceramic for tooth-supported fixed dental prostheses: A systematic review and meta-analysis

STATEMENT OF PROBLEM

Although recent studies have reported the success of implant-supported monolithic restorations, consensus on the use of monolithic ceramic restorations is lacking.

PURPOSE

The purpose of this systematic review and meta-analysis was to evaluate the survival and biological and technical complication rates of monolithic single crowns and fixed partial dentures (FPDs).

MATERIAL AND METHODS

An electronic search was conducted by 2 independent authors on the PubMed/MEDLINE, Scopus, and Cochrane Library databases. The Newcastle-Ottawa scale and Cochrane risk of bias tool were used to assess the quality and risk of bias of the included studies. Meta-analysis was performed by using the R software program.

RESULTS

The search identified 763 articles, 18 of which met the eligibility criteria. A total of 15 studies evaluated monolithic ceramic single crowns, and 4 studies evaluated FPDs. The studies included 1061 monolithic single crowns (524 lithium disilicate, 461 zirconia, and 76 polymer-infiltrated ceramic network [PICN]) and 104 FPDs (36 lithium disilicate and 68 zirconia). Meta-analysis of single-arm studies indicated the proportion of survival, biological, and technical complication rates of 1% (95% confidence interval [CI]: 0% to 3%), 1% (CI: 0% to 4%), and 2% (CI: 1% to 4%), respectively, for single crowns, independent of ceramic material, and 3% (CI: 0% to 34%), 5% (CI: 1% to 21%), and 5% (CI: 1% to 21%) for FPDs, respectively. Only 5 studies performed a direct comparison between monolithic and veneered ceramic restorations, and no significant difference was observed in terms of survival (risk ratio [RR]: 0.68; CI: 0.25-1.91; P=.96), biological (RR: 0.69; CI: 0.31-1.53; P=.35), and technical complication rates (RR: 0.87; CI: 0.40-1.88; P=.29).

CONCLUSIONS

The use of monolithic ceramic can be considered a favorable treatment for tooth-supported single crowns and FPDs, with high survival and low complication rates. However, further randomized controlled trials are needed to reassess these clinical performances, mainly by comparing them with the performance of veneered restorations.

Impact of sandblasting on the flexural strength of highly translucent zirconia

The objective of this study was to assess the influence of alumina sandblasting on the flexural strength of the latest generation of highly translucent yttria partially stabilized dental zirconia (Y-PSZ). Fully-sintered zirconia disk-shaped specimens (14.5-mm diameter; 1.2-mm thickness) of four Y-PSZ zirconia grades (KATANA HT, KATANA STML, KATANA UTML, all Kuraray Noritake; and Zpex Smile, Tosoh) were sandblasted at 0.2 MPa with 50-μm alumina (Al₂O₃) sand (Kulzer) or left as-sintered (control). For each zirconia grade, the yttria (Y₂O₃) content was determined using X-ray fluorescence (XRF). Surface roughness was assessed using 3D confocal laser microscopy. Micro-Raman spectroscopy (μ-Raman) and X-ray diffraction (XRD) were used to assess potentially induced residual stresses. Biaxial flexural strength (n = 20) was statistically compared by Weibull analysis. Focused ion beam - scanning electron microscopy (FIB/SEM) was used to observe the subsurface microstructure. Fracture surfaces after biaxial flexural strength testing were observed by SEM. KATANA UTML had the highest Y₂O₃ content (6 mol%), followed by KATANA STML and Zpex Smile (5 mol%), and KATANA HT (4 mol%). Al₂O₃-sandblasting significantly increased surface roughness of KATANA UTML and Zpex Smile. μRaman and XRD revealed the presence of residual compressive stress on all Al₂O₃-sandblasted surfaces. FIB/SEM revealed several sub-surface microcracks in the sandblasted specimens. Weibull analysis revealed that Al₂O₃-sandblasting increased the characteristic strength of KATANA HT, KATANA STML, whereas it decreased the strength of KATANA UTML. The strength enhancement after Al₂O₃-sandblasting of KATANA HT was the highest, followed by KATANA STML. For Zpex Smile, the influence was statistically insignificant. The impact of Al₂O₃-sandblasting on the Weibull modulus was controversial. The strength of zirconia after Al₂O₃-sandblasting is determined by the balance between microcrack formation (decreased strength) and surface compressive stress build-up (increased strength).

Effect of sandblasting on the surface roughness and residual stress of 3Y-TZP (zirconia)

A defined surface roughness is important for the adhesive bonding of veneering ceramic to zirconia frameworks. Thus, we investigated the effects that sandblasting has upon the surface roughness and residual stress of zirconia. So as to achieve well-defined blasting parameters and also generate highly reproducible results; a custom-made, automated sandblasting machine was constructed (based on a 3D-printer construction kit). Thirty-two samples (measuring 30 mm × 35 mm × 0.2 mm) were cut from densely sintered 3Y-TZP blanks and then ground with a 45 µm grinding disc. After undergoing regeneration firing, the samples were sandblasted with varying working parameters: blasting angle (45°, 60° and 90°); blasting pressure (2 bar, 4 bar and 6 bar); and working distance (0.5 cm, 1.0 cm and 1.5 cm). The sandblasting path was chosen so as to create samples with an even surface roughness. Changes in blasting pressure resulted in the greatest changes in the surface roughness, the highest surface roughness values were achieved with sandblastings perpendicular to the sample surface. Furthermore, a correlation was found between the mean surface roughness and the residual stress.

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.

The adverse effects of tungsten carbide grinding on the strength of dental zirconia

OBJECTIVES

This study investigated the effects of tungsten-carbide grinding on the surface characteristics and mechanical strength of dental 3 mol% yttria-stabilized zirconia (3Y-TZP).

METHODS

Two types of tungsten-carbide burs (TC), 6-blade (TC₁) and 8-blade (TC₂) were used to grind 3Y-TZP, in a dental air-turbine handpiece with water-cooling and were also subjected to air-particle abrasion (APA): TC₁ + APA and TC₂ + APA; and rubber polishing (RP): TC₁ + RP and TC₂ + RP; one group received only rubber-polishing RP. The control group received no treatment. Surface characterization was examined by surface roughness (R_a) and atomic force microscopy. Specimens were also observed with scanning electron microscopy (SEM) and X-ray-diffraction (XRD) for microstructure and crystalline phases. A piston-on-three-balls biaxial-flexural strength (BFS) test was performed with 15 samples-per-group and the broken specimen were observed under SEM to investigate the fracture origin pattern. One-way ANOVA, Kruskal-Wallis test and Weibull analysis were performed at α = 0.05.

RESULTS

Groups TC₁ and TC₂ had the lowest mean BFS (p < 0.05) with up to 74 % reduction in strength. APA and RP both significantly increased the mean BFS after tungsten-carbide grinding but was still less than the control (p < 0.05). Compared to the control, the mean BFS was significantly reduced for all groups except for the RP group (p < 0.05). APA and rubber-polishing following TC₂ grinding had significant higher mean BFS than those following TC₁ grinding respectively (p < 0.05). SEM revealed distinct micro-cracks after tungsten-carbide grinding.

SIGNIFICANCE

Tungsten-carbide burs (6- and 8-blade) are not recommended for zirconia grinding due to the significant reduction of biaxial-flexural strength and observed micro-structural surface and subsurface damage.

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.

Effect of extrinsic pigmentation and surface treatments on biaxial flexure strength after cyclic loading of a translucent ZrO2 ceramic

OBJECTIVE

To evaluate the influence of extrinsic pigmentation on the biaxial flexural strength and surface topographic of translucent Y-TZP (InCoris TZI - Sirona - USA) subjected to several surface treatments.

METHODS

Sintered zirconia discs-shaped specimens (n=120) (ø:12mm; thickness:1.2mm; ISO 6872) were prepared and divided (n=15) according to various factors: "extrinsic pigmentation" (n: without; p: with) and "surface treatments" (C: control - as sintered; A: abraded with silica-coated alumina particles (30μm); G: glazed with a thin film of low-fusing porcelain glaze; GH: glazed and etched with 10% hydrofluoridric acid for 60s. Mechanical cycling (1.2×106 cycles, 200N, 3.8Hz) and flexural strength test (1mm/min - 1000kg cell) were performed. Two-way ANOVA and Tukey's were used for statistical test (α=0.05). Weibull analysis was used to evaluate the strength reliability. Samples were analyzed via (1) an optical profilometer to determine the surface roughness (Ra); (2) an X-ray diffraction (XRD) to evaluate phase transformations; and (3) a SEM equipped with an energy dispersive X-ray spectroscopy (EDX) to elucidate morphological properties and chemical compositions.

RESULTS

Regardless of the surface treatment (p=0.5459) (Cn: 560.16MPa; Gn: 573.36MPa; An: 643.51MPa; GHn: 542.94MPa; Cp: 628.04MPa; Gp: 641.90MPa; Ap: 554.47MPa; GHp :602.84MPa) and extrinsic pigmentation (p=0.1280) there was no difference in the flexural strength among the experimental groups. According to the XRD analysis, phase transformations occurred in the An group (t→m) and in Ap group (t→c). Surface roughness was affected by surface treatments (An - p=0.001) and extrinsic pigmentation (Gp - p=0.001).

SIGNIFICANCE

The biaxial flexural strength of the tested samples was not affected neither by surface treatments nor by pigmentation, although it can cause phase transformation and promote surface roughness.

Surface characterization of monolithic zirconia submitted to different surface treatments applying optical interferometry and raman spectrometry

This study evaluated roughness parameters and phase transformation of monolithic zirconia ceramics after various mechanical and laser thermal treatments. Fully sintered monolithic zirconia cylinder specimens were divided to five groups, according to the applied surface treatment: CL: control, GB: grit-blasted with glass particles. AL50: grit-blasted with 50 μm alumina particles, AL90: grit-blasted with 90 μm dyed-alumina particles and FEML: subjected to femto laser thermal treatment. Six roughness parameters (Sa, Sq, Sz, Sci, Svi and Sdr) were measured by optical profilometry. Phase transformation in zirconia was determined by micro-Raman spectroscopy. The highest roughness values were recorded in AL90 and FEML groups, followed by AL50. AL90 presented statistically higher monoclinic phase content compared to all other groups. Control and GB groups presented similar roughness without phase transformation. Laser thermal treatment causes minimal destruction of the zirconia surface, and can be suggested as an alternative to other roughening treratments, for enhancing the adhesive potential to dentin.

Effect of thickness and surface modifications on flexural strength of monolithic zirconia

STATEMENT OF PROBLEM

A recommended minimum thickness for monolithic zirconia restorations has not been reported. Assessing a proper thickness that has the necessary load-bearing capacity but also conserves dental hard tissues is essential.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of thickness and surface modifications on monolithic zirconia after simulated masticatory stresses.

MATERIAL AND METHODS

Monolithic zirconia disks (10 mm in diameter) were fabricated with 1.3 mm and 0.8 mm thicknesses. For each thickness, 21 disks were fabricated. The specimens of each group were further divided into 3 subgroups (n=7) according to the surface treatments applied: untreated (control), airborne-particle abrasion with 50-μm Al₂O₃ particles at a pressure of 400 kPa at 10 mm, and grinding with a diamond rotary instrument followed by polishing. The biaxial flexure strength was determined by using a piston-on-3-balls technique in a universal testing machine. Flexural loading was applied with a 1.4-mm diameter steel cylinder, centered on the disk, at a crosshead speed of 0.5 mm/min until fracture occurred. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed. The data were statistically analyzed with 2-way ANOVA, Tamhane T2, 1-way ANOVA, and Student t tests (α=.05).

RESULTS

The 1.3-mm specimens had significantly higher flexural strength than the 0.8-mm specimens (P<.05). Airborne-particle abrasion significantly increased the flexural strength (P<.05). Grinding and polishing did not affect the flexural strength of the specimens (P>.05).

CONCLUSIONS

The mean flexural strength of 0.8-mm and 1.3-mm thick monolithic zirconia was greater than reported masticatory forces. Airborne-particle abrasion increased the flexural strength of monolithic zirconia. Grinding did not affect flexural strength if subsequently polished.

Characterization of a Diamond Ground Y-TZP and Reversion of the Tetragonal to Monoclinic Transformation

PURPOSE

To characterize the surface of an yttria-stabilized zirconia (Y-TZP) ceramic after diamond grinding in terms of its crystalline phase, morphology, mean roughness (Ra), and wettability as well as to determine a thermal treatment to reverse the resulting tetragonal to monoclinic (t-m) transformation.

METHODS AND MATERIALS

Y-TZP specimens were distributed into different groups according to the actions (or no action) of grinding and irrigation. Grinding was accomplished using a diamond stone at a low speed. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy, goniometry, and profilometry. In situ high-temperature XRD was used to determine an annealing temperature to reverse the t-m transformation. Ra was submitted to the Kruskal-Wallis test, followed by the Dunn test (α=0.05). The volume fraction of the monoclinic phase and contact angle were submitted to one-way analysis of variance, followed by the Tukey test (α=0.05).

RESULTS

Monoclinic zirconia was observed on the surface of samples after dry and wet grinding with a diamond stone. The volume fraction of the monoclinic phase was smaller on the dry ground samples (3.6%±0.3%) than on the wet ground samples (5.6%±0.3%). High-temperature XRD showed reversion of the t-m phase transformation, which started at 700°C and completed at 800°C in a conventional oven.

CONCLUSIONS

Grinding with a diamond stone partially transformed the crystalline phase on the surface of a Y-TZP ceramic from tetragonal to monoclinic zirconia while simultaneously increasing the surface roughness and wettability. The t-m transformation could be reversed by heat treatment at 800°C or 900°C for 60 minutes or 1000°C for 30 minutes.

Effect of simulated chairside grinding procedures using commercially available abrasive agents on the surface properties of zirconia

Aim:

The aim of the present study was to assess the change in physical properties (surface roughness, surface hardness and phase transformation) after surface grinding of zirconia by using three commercially available abrasives.

Materials and Methods:

Thirty sintered zirconia specimens were prepared and divided into three groups namely Group M (grinded using Mani Dia diamond bur standard grit), Group T (grinded using Tri Hawk diamond bur coarse grit) and Group P (grinded using Predator carbide bur). A customised assembly was used to follow a standardised protocol for surface grinding. The surface roughness, surface hardness and phase transformation was recorded before and after the grinding procedure.

Statistical Analysis Used:

ANOVA and Bonferroni post hoc test were used to assess the values obtained after the testing the surface roughness and surface hardness.

Results:

The results of the present study revealed the average values of change in surface roughness as Group M (0.44 μm) and Group T (1.235 μm) and Group P (-0.88 μm). The average values of change in surface hardness were Group T (19.578 HV), Group M (46.722 HV) and Group P (36.429 HV). The change in surface hardness was not statistically significant. There was no phase transformation seen after the grinding procedure.

Clinical Significance:

Carbide burs along with copious water irrigation when used to grind zirconia intra-orally produces has a polishing effect, minimal change in hardness & no phase transformation. The present study advocates the use of carbides for chair-side grinding of zirconia.

Cleaning Methods for Zirconia Following Salivary Contamination

PURPOSE

To determine the best method of cleaning saliva-contaminated zirconia in preparation for resin bonding.

MATERIAL AND METHODS

Flat blocks of zirconia were particle abraded with aluminum oxide and divided into eight groups (n = 10). Groups 1 to 4 were first treated with one coat of an MDP primer (Z-prime Plus) and groups 5 to 8 were left untreated. A thin coat of fresh whole saliva was painted on all specimens (except the control, group 1) and left in contact for 3 minutes. The specimens were then cleaned by rinsing with water for 20 seconds (groups 2 and 5), or cleaned with 35% phosphoric acid for 20 seconds and rinsed (groups 3 and 6), or cleaned with a zirconia cleaning solution (Ivoclean) for 20 seconds and rinsed (groups 4 and 7), or abraded with aluminum oxide particles and rinsed (group 5). Following the cleaning procedure, groups 5 to 8 were treated with one coat of Z-prime Plus. A 2.5 mm diameter tube was filled with composite (Z100), affixed to the zirconia surface, and light-cured. Specimens were stored in water (37°C for 24 hours) followed by thermocycling (5°C to 55°C for 10,000 cycles). Shear bond strength testing was performed at a crosshead speed of 1 mm/min. Bond strength values were compared to the control using a Dunnett's test (alpha = 0.05). Contact angles of the zirconia specimens with and without Z-prime Plus were measured with water using a goniometer.

RESULTS

Groups 2, 5, and 8 were not significantly different from the control. Application of Z-prime made the zirconia surface more hydrophobic.

CONCLUSIONS

If salivary contamination occurs after MDP application, rinsing off the saliva with water will preserve bond strength. If salivary contamination occurs prior to MDP application, particle abrasion or cleaning with Ivoclean will preserve bond strength.