Transformation Of Zirconia Research Articles

Low energy Ar+ ion irradiation effects in monoclinic zirconia

Argon ion beam induced phase transformation (monoclinic to tetragonal) and bubble formation in monoclinic zirconia (ZrO2) has been studied using grazing incidence X-ray diffraction, electron microscopy, Raman scattering and photoluminescence spectroscopy. Monoclinic zirconia samples were irradiated with low energy (120 keV) Ar+ ions at temperatures 300 K and 143 K. The as-sintered and Ar+ ion irradiated zirconia samples were then irradiated using an excimer laser of wavelength 248 nm (energy 5 eV) to study the defect rich material (after ion irradiation) behavior at extreme condition (where ionization process is dominant). Zirconia was found to undergo a partial phase transformation from monoclinic to tetragonal (≈20%; ion fluence 2 × 1017 ions/cm2) upon Ar+ ion irradiation at 143 K. Further, Ar bubbles were formed and the size of the bubble is found to increase with the ion fluence, irrespective of sample temperature during ion irradiation. The oxygen vacancies (produced during ion irradiation) lead to strain field in the sample and this strain field is the prime cause for phase transformation. The rate of phase transformation was found to be faster in case of ion irradiation at 143 K. Further it was observed that Ar+ ion irradiation lead to creation of defects and laser irradiation on as-sintered zirconia leads to annihilation of defects. However, when laser irradiation was carried out on ion irradiated zirconia, there is a competition between production and annihilation of defects where the production of defects by the Ar+ ion irradiation is a dominant one. Further, the present work confirms the role of O vacancy in the monoclinic to tetragonal phase transformation and the results are consistent with the earlier report (Simeone et al., 2000).

Effect of sintering parameters on the mechanical properties of monolithic zirconia.

Background. Zirconia restorations with high mechanical properties are the current treatment options for fixed restorations with advantages of high biocompatibility and low pulp irritation. Although the effect of sintering time and temperature on the optical and mechanical properties of zirconia core material were investigated, the effect of these parameters on the translucent monolithic zirconia is still uncertain. This study aimed to evaluate the effect of the changes in sintering temperature and holding time on the mechanical and structural properties of monolithic zirconia. Methods. Totally, 340 self-colored (A2) zirconia specimens from two different monolithic zirconia groups (n=170) were prepared, measuring 15.5×12.5×1.2 mm. Then, 17 subgroups (n=10), including the control groups, were sintered according to sintering parameters. XRD analysis was used to determine phase transformations. The surface roughness of the specimens was evaluated using profilometry, and the flexural strength of the specimens was evaluated by the three-point bending test. The data were analyzed using three-way ANOVA and post hoc multiple comparison test with Bonferroni correction (a=0.05) at a significance level of 0.05. Independent-samples t-test was used to compare the subgroups between the control groups (P˂0.05). Results. No tetragonal-to-monoclinic phase transformation was observed in the groups. Changes in the sintering parameters did not significantly affect the surface roughness and flexural strength of monolithic zirconia. Surface roughness values for all the subgroups were above the clinically critical limit. Conclusion. According to the results of this study, changes in the sintering parameters did not affect the surface phase transformation, surface roughness, and flexural strength of monolithic zirconia.

An Atomistic-Informed Phase-Field Model for Non-Isothermal Phase Transformation and Plasticity in Yttria-Stabilized Tetragonal Zirconia

An Atomistic-Informed Phase-Field Model for Non-Isothermal Phase Transformation and Plasticity in Yttria-Stabilized Tetragonal Zirconia

The effects of surface grinding and polishing on the phase transformation and flexural strength of zirconia.

PURPOSEThe purpose of this in vitro study was to evaluate the effect of surface grinding and polishing procedures using high speed zirconia diamond burs with different grit sizes on the phase transformation and flexural strength of zirconia.MATERIALS AND METHODSForty disc shape specimens (15 × 1.25 mm) with a cylindrical projection in the center of each disc (1 × 3 mm) were fabricated with 3Y-TZP (Prettau, Zirkonzahn, Italy). The specimens were divided into 4 groups (n=10) according to the grinding and polishing procedures: Control group - grinding (coarse-grit diamond bur), Group 1 - grinding (coarse-grit diamond bur) + polishing, Group 2 - grinding (fine-grit diamond bur) + polishing, and Group 3 - grinding (fine grit diamond bur). Each specimen was analyzed by 3D-OM, XRD analysis, and biaxial flexural strength test.RESULTSBased on the surface morphology by 3D-OM images, polished specimens showed smoother surface and lower roughness value (Ra). In the result of XRD analysis, partial phase transformation from tetragonal to monoclinic zirconia occurred in all groups. Control group, ground with a coarse grit diamond bur, showed more t→m phase transformation and lower flexural strength than Groups 1 and 2 significantly.CONCLUSIONThe flexural strength in all specimens after grinding and polishing showed over 500 MPa, and those were clinically acceptable. However, grinding with a coarse grit diamond bur without polishing induced the phase transformation and low strength. Therefore, surface polishing is required for the occlusal adjustment using a high speed zirconia diamond bur to reduce the phase transformation and to prevent the decrease of flexural strength of zirconia.

Influence of different veneering techniques and thermal tempering on flexural strength of ceramic veneered yttria partially stabilized tetragonal zirconia polycrystalline restoration.

BackgroundDifferent technique for ceramic veneering and thermal tempering process are expected to be a reason for alteration in strength of ceramic veneered zirconia. This study evaluates the effect of different veneering technique and varied thermal tempering process on flexural strength of ceramic veneered zirconia.Material and MethodsCeramic veneered zirconia bars (25 mm length, 4 mm width, 0.7&1.0mm of zirconia & ceramic thickness) were prepared from zirconia block (e.max® ZirCAD), sintered at 1500°C for 4 hours, and veneered with ceramics with different techniques including CAD-fused using e.max CAD® (C), Pressed-on using e.max® Zirpress (P), and layering using e.max® ceram (L), with different tempering process through fast (F), medium (M), and slow (L) cooling (n=15). The specimens were determined for flexural strength on a universal testing machine. ANOVA and Bonferroni’s multiple comparisons were used to determine for significant difference (α=0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristics strength (σc). The interfaces were microscopically examined. The phase transformation of zirconia was determined using X ray diffraction.ResultsThe mean±sd (MPa), m, σc of flexural strength were 922.06±83.45, 12.78, 958.32 for CF, 924.26±74.64, 14.28, 959.62 for CM, 930.25±92.42, 11.83, 970.83 for CS, 518.29±59.97, 10.11, 542.97 for PF, 516.50±67.51, 8.75, 539.17 for PM, and 520.51±42.38, 14.59, 544.51 for PS, 604.36±64.09, 11.28, 630.67 for LF, 583.81±56.95, 11.67, 609.81 for LM, 547.33±52.23, 12.19, 569.36 for LS. The flexural strength was significantly affected by veneering technique (p<0.05). No significant effect on flexural strength upon tempering process was evidenced (P >0.05). Phase transformation from t→m related with veneering and tempering procedure.ConclusionsStrength of ceramic veneered zirconia associated with different veneering techniques, but not directly related with tempering process. CAD-on ceramic veneering zirconia is benefit for enhancing the strength of ceramic bilayer and was recommended as a method for ceramic veneering zirconia. Key words:CAD-CAM, cooling process, flexural strength, thermal tempering, zirconia.

Study on the fracture behavior of the planar-type solid oxide fuel cells

In this work, the commercial SUS430 ferritic stainless steel was used as the current collector to study the fracture behaviors of the planar electrolyte-supported and anode-supported solid oxide fuel cells with the size of 10 cm × 10 cm during the thermal cycle process. The anode-supported cells (ASCs) were fabricated by tape-casting, spraying, screen-printing and co-sintering. The electrolyte-supported cells (ESCs) using fully-stabilized zirconia as material were prepared by tape-casting, multilayer lamination, screen-printing and co-sintering. The output power densities at 0.7 V (P0.7V) of the anode-supported and electrolyte-supported cells could reach 0.60 W cm−2 and 0.43 W cm−2 at 850 °C, respectively. During the thermal cycle testing, the ESC sample was cracked, while the ASC sample had no obvious changes. The thermal expansion coefficient (TEC) of the different samples was studied, and the two cells were characterized by the XRD after the thermal cycle testing. In addition, the fracture behaviors of the two cells were discussed. The results indicated that the ASC sample had the good stability during the thermal cycle testing. The excellent anti-cracking performance of the ASC was attributable to two aspects. Firstly, the TEC of the anode support is more close to that of the SUS430 material; secondly, the phase transformation of zirconia from monoclinic phase to tetragonal phase induced by stress is favorable for contraction of the anode support layer during the cooling process.

Al2O3-ZrO2 nanocomposites coating on aluminum alloy by plasma electrolytic-electrophoretic hybrid process

Alumina-zirconia nanostructured layers were coated on an aluminum alloy (AA7075) by the plasma electrolytic (PE) technique in a direct current galvanostatic mode at 0.1–0.4 A/cm2 current density. The coatings were formed in an electrolyte containing monoclinic nano-ZrO2 powder as a zirconia source. The microstructure of the produced coatings was studied by transmission electron microscopy (TEM), scanning electron microscopy and X-ray diffraction to develop an understanding of the growth mechanism. The investigation of the coating process was complemented with voltage-time response measurements and in-situ optical spectroscopy observations. The results showed formation of various alumina-zirconia composite microstructures as a function of the current density during processing. At the higher current density, the composite layer consists of high temperature phases (tetragonal zirconia and α-alumina) in addition to monoclinic zirconia. High current density introduced larger amounts of zirconia to the coated layer due to the high energy applied to the nanoparticles in the electrolyte. TEM analysis showed formation of four sub-layers across the coating. The coating-substrate interface (sub-layer 1) contained higher amounts of alumina (both α and γ) while the amount of zirconia nanoparticles increased by moving toward the outer surface. In sub-layer 2, formation of tetragonal zirconia was observed resulting from a phase transformation of monoclinic to tetragonal zirconia. In sub-layer 3, the existence of untransformed monoclinic zirconia hints to inadequate energy for the phase transformation due to lower temperatures. At the top region of the coating, sub-layer 4, a shallow amorphous layer was formed due to quenching from the direct contact with the electrolyte. The PE process was found to be responsible for the monoclinic to tetragonal zirconia transformation, while the electrophoretic process facilitated the deposition of the original monoclinic zirconia from the electrolyte. The results showed that the coating mechanism involves a hybrid PE-electrophoretic process.

Enamel wear and aging of translucent zirconias: In vitro and clinical studies

Statement of problemZirconia is a widely used restorative material. However, phase transformation on clinical application of zirconia has not yet been studied. PurposeThe purpose of this study was to evaluate the wear, surface roughness, and aging associated with polished translucent zirconia in both in vitro and clinical experiments. Material and methodsIn vitro experiments were performed with Rainbow and Katana zirconia blocks and natural tooth enamel as the control. They were subjected to 100 000 loading cycles with a maxillary premolar antagonist. All specimens were analyzed for wear, and the zirconia specimens were evaluated for surface roughness and monoclinic phase (m-phase) transformation by X-ray diffractometry before and after cyclic loading. The clinical study included participants who required single-crown implant-supported restorations replacing the first or second molar. The participants received Rainbow or Katana zirconia prostheses (n=15, each). For wear analysis, impressions of each prosthesis, antagonist, and adjacent tooth were made at 1 week and 6 months after crown delivery. The occlusal relationship of the crowns in maximum intercuspation was evaluated by using the T-Scan 8 occlusal diagnostic system. The degree of transformation of zirconia to the m-phase was measured by using X-ray diffractometry of the crowns after 6 months of use. ResultsZirconia induced significantly greater enamel wear than the natural tooth control. Katana specimens exhibited significantly greater wear and surface roughness than the Rainbow specimens. The degrees of antagonistic wear and zirconia phase transformation in the clinical experiment were significantly greater than those in the in vitro experiment. The Katana groups showed significantly higher m-phase levels than the Rainbow groups. ConclusionsPhase transformation of zirconia occurs within 6 months of clinical use, and the wear and degrees of phase transformation varied according to the zirconia product used.

Selective laser melting of yttria-stabilized zirconia

The mechanism of phase transformation and microstructure of yttria-stabilized zirconia (YSZ), prepared by selective laser melting (SLM) at a laser wavelength of 1 μm, has been investigated. Also, the effect of different working gases and heat treatment has been studied. Experimental results present that processing parameters can be optimized and determined by Liner Energy Density. X-ray diffraction analysis confirmed the absence of monoclinic phase under argon flow during SLM, with gray-color appearance due to the lack of oxygen, which impedes the transformation from tetragonal to monoclinic phase partially. Later, the color becomes white during heat treatment because samples are exposed to the air with sufficient amount of the oxygen and the diffraction peaks of monoclinic phase increased. When SLM and heat treatment was carried out in the air, the YSZ samples appeared in white-color, which indicates the presence of both monoclinic and tetragonal phase. The morphology and microstructure showed there occurred bonding phenomenons among particles, only surface partially melted between particles with weak bonding strength. The top surface morphology of air-prepared sample showed cracks, un-melting particles and cave-like pores because of uneven distribution of temperature variations. This paper also made a preliminary research and provided a feasibility method to melt zirconia ceramics completely, in which more energy input by multi-scanning in the air working gas.

Wettability and Durability of Si-O Coatings on Zirconia Substrate by RF-Magnetron Plasma Sputtering

The wettability and durability of Si-O coatings prepared on zirconia substrate using radiofrequency magnetron sputtering (rf-sputtering) was studied. XRD analysis revealed no phase transformation of zirconia before/after rf-sputtering process. XPS spectroscopy showed that as-deposited films with a SiO2 configuration was formed. EDX analysis indicated that the Si/Zr ratio was high and when magnetron rf-sputtering was performed using a plasma gas Ar+5% O2, which may be the optimum condition of rf-sputtering to form a sustainable hydrophilic layer on zirconia substrate. The wear testing using pin on disc wear apparatus was performed. The wettability and durability of Si-O coatings fabricated by magnetron radiofrequency magnetron sputtering (rf-sputtering) on zirconia substrate was studied. A plasma gas Ar+5% O2 may be the optimum condition of rf-sputtering to form a sustainable hydrophilic layer on zirconia substrate

Structural, optical tuning, and mechanical behavior of zirconia toughened alumina through europium substitutions.

The structural and optical features of zirconia toughened alumina (ZTA) due to the assorted range of Eu3+ substitutions are demonstrated. The characterization studies affirm the pivotal role of Eu3+ on the improved structural stability of ZTA and associated tetragonal zirconia (t-ZrO2 ) → cubic zirconia (c-ZrO2 ) transformation. Eu3+ prefers accommodation at the lattice sites of ZrO2 and their gradual accumulation induces t- → c-ZrO2 transition. Beyond the substitution limit, Eu3+ reacts with Al2 O3 to form EuAlO3 . Optical studies validate typical Eu3+ emissions, and further, the emission spectrum also predicts the symmetry of Eu3+ coordination at the ZrO2 lattice. Uniform distribution of ZrO2 and Al2 O3 grains throughout the microstructures are evident from the morphological analysis. Further, the influence of Eu3+ on the enhanced mechanical stability of ZTA is ensured from indentation technique. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1170-1179, 2019.

β-TCP coatings on zirconia bioceramics: The importance of heating temperature on the bond strength and the substrate/coating interface

β-tricalcium phosphate (β-TCP) coatings were synthesized on tetragonal zirconia (Y-TZP) discs by heating the apatite coating between 800 °C and 1200 °C. The study results suggest that heating temperature has a strong influence on the coating bond strength and microstructure of the substrate/coating interface. The β-TCP coatings fired at 800 °C and 900 °C exhibited excellent tensile bond strength (∼50 MPa) while heating at 1100 °C and 1200 °C led to decreased bond strength (∼30 MPa) as the result of substantial structural and microstructural changes: diffusion of Y3+ from the zirconia substrate in the coating resulting in partial crystal transformation (t-m) of zirconia, formation of surface uplifts and nanoporosity in zirconia, as well as generation of large residual thermal stresses leading to microcracking of the β-TCP coatings. However, these structural changes did not have any measurable effect on the flexural strength of the bulk zirconia substrates.

Ion beam induced phase transformation and krypton bubble formation in monoclinic zirconium oxide

Low energy krypton ion beam induced phase transformation and bubble formation in monoclinic zirconia (ZrO2) has been studied using electron microscopy, Raman scattering, grazing incidence X-ray diffraction and photoluminescence spectroscopy. The zirconia samples were synthesized by the thermal decomposition method and 60 keV Kr+ ion irradiation was carried out at 300 K and 143 K. The as-sintered ZrO2 particles were found to be monoclinic in structure, however, upon 60 keV Kr+ ion irradiation, a fraction of the sample (∼4.9% (300 K) and ∼8.3% (143 K) for the ion fluence of 1 × 1017 ions/cm2) was transformed from monoclinic to tetragonal phase along with the formation of krypton bubbles. The size of the bubbles is found to increase with the ion fluence, irrespective of the sample temperature during ion irradiation. The phase transformation from monoclinic to tetragonal structure was mainly due to radiation damage process where the driving force is strain field associated with the O-vacancies. The monoclinic to tetragonal transformation rate is faster when the ion irradiation was carried out at 143 K and this is attributed to the immobility of defects and production of large number of oxygen vacancies. The krypton bubble formation might be hindering the monoclinic to tetragonal phase transformation rate.

Effect of Surface Modification on In-Depth Transformations and Flexural Strength of Zirconia Ceramics.

Chairside surface adjustments of zirconia dental restorations enhance the toughening stress-induced tetragonal-to-monoclinic phase transformation and domain reorientation by ferro-elastic domain switching (FDS), but also trigger subsurface damage, which could compromise long-term clinical performance. The purpose of this study was to assess the depth of phase transformation, associated FDS, and flexural strength of dental zirconia (BruxZir HT 2.0), after chairside surface treatments. Square specimens were sectioned from CAD/CAM blocks and sintered according to manufacturer's recommendations (n = 30). They were left as-sintered (AS; control), air abraded with fine (AAF) or coarse (AAC) alumina particles, ground (G) or ground and polished (GP). Roughness was measured by profilometry. Crystalline phases were investigated by grazing incidence X-ray diffraction (GIXRD) (n = 3). GIXRD data were fit using semi-log regression protocols to assess transformation depth and extent of FDS. The mean biaxial flexural strength was measured according to ISO 6872. Subsurface damage was assessed from SEM images using a bonded polished interface configuration. Flaw distribution was assessed by Weibull analysis. Results were analyzed by Kruskal-Wallis with Tukey's adjustment for multiple comparisons (p < 0.05). Air-abraded and ground groups exhibited higher mean surface roughness than control. AAF group exhibited the highest flexural strength (1662.6 ± 202.6 MPa) with flaw size (5.9 ± 1.8 μm) smaller than transformation (14.5 ± 1.2 μm) or FDS depth (19.3 ± 1.1 μm), followed by GP group (1567.2 ± 209.7 MPa) with smallest FDS depth (9.3 ± 2.0 μm) and flaw size (2.6 ± 1.8 μm), but without m-phase. AAC group (1371.4 ± 147.6 MPa) had the largest flaw size (40.3 ± 20.3 μm), transformation depth (47.2 ± 3.0 μm) and FDS depth (41.2 ± 2.2 μm). G group (1357.0 ± 196.7 MPa) had the smallest transformation depth (8.6 ± 1.5 μm), and mean FDS depth (19.8 ± 3.7 μm) and flaw size (18.6 ± 3.1 μm). AAC and AAF exhibited the highest Weibull modulus (11.2 ± 0.4 and 9.8 ± 0.3 μm, respectively). Variations in mean biaxial flexural strength were explained by the balance between the depth of toughening mechanisms (phase transformation and FDS) and subsurface damage. AAF and GP groups were the most efficient surface adjustments in promoting the highest mean biaxial flexural strength.

A novel way to fabricate fibrous mullite ceramic using sol-gel vacuum impregnation

Inspired by bird's nest structure, fibrous mullite ceramic was fabricated by vacuum impregnation with mullite fibers as raw material and zirconia sol-gel as inorganic binder. The effect of impregnation times on the properties of the fibrous mullite ceramic, such as porosity, microstructure, compressive strength and room-temperature thermal conductivity were investigated. The results showed that low density (0.45–0.66 g/cm3), relative high compressive strength (0.62–3.34 MPa) and low thermal conductivity (0.037–0.125 W/mk) were exhibited for the sample. The toughness of fibrous mullite ceramic was enhanced due to the micro-creaks caused by transformation of zirconia. From the experimental results, it is suggested that it was an optimal method which have the potential use in high-temperature thermal insulation materials to produce fibrous mullite ceramics.

Effect of the application of surface treatments before and after sintering on the flexural strength, phase transformation and surface topography of zirconia

ObjectivesTo evaluate the effects of airborne-particle abrasion (APA) and Er,Cr:YSGG laser irradiation on 4-point-flexural strength, phase transformation and morphologic changes of zirconia ceramics treated at pre-sintered or post-sintered stage. MethodsThree hundred and forty-two bar shaped zirconia specimens were milled with different sizes according to the flexural strength test (n = 10), X-ray diffraction (XRD) (n = 4) and field emission scanning electron microscope (FE-SEM) (n = 4) analyses. For each test protocol, specimens were divided into 4 main groups whether the surface treatments applied before or after sintering and whether the specimens received heat treatment or not as pre-sintered, post-sintered no-heat and post-sintered heat-treated groups, and a group was served as control. Main groups were further divided into 6 equal subgroups according to surface treatment method applied (2 W-, 3 W-, 4 W-, 5 W-, 6 W-laser irradiations and APA). Surface treatments were applied to pre-sintered groups before sintering and to post-sintered groups after sintering. Post-sintered heat-treated groups were subjected to veneer ceramic firing simulation after surface treatments. Flexural strength and flexural modulus values were statistically analysed and monoclinic phase content was calculated. Weibull analysis was used to evaluate strength reliability and fractographic analysis was conducted. ResultsHighest flexural strength values were detected at post-sintered no-heat APA and 4W-laser groups (P < 0.05). Pre-sintered groups showed statistically lower flexural strength values. Heat treatment decreased the strength of the specimens. Monoclinic phase content was only detected at post-sintered no-heat groups and the highest amount was detected at APA group. Rougher surfaces and deeper irregularities were detected at FE-SEM images pre-sintered groups. ConclusionsApplication of surface treatments at pre-sintered stage may be detrimental for zirconia ceramics in terms of flexural strength. Clinical significanceTreating the surface of zirconia ceramic before sintering process is not recommended due to significant decrease in flexural strength values. 2 W–4 W Er,Cr:YSGG laser irradiations can be regarded as alternative surface treatment methods when zirconia restoration would be subjected to veneer ceramic firing procedures.

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

PURPOSEThe purpose of this study was to evaluate and compare three polishing systems on the surface roughness and phase transformation of monolithic zirconia.MATERIALS AND METHODS100 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.RESULTSAll 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.CONCLUSIONZirconia polishing systems created a smoother surface on zirconia than the porcelain polishing system. Phase transformation did not occur during the polishing procedure.

Role of coefficient of thermal expansion on bond strength of ceramic veneered yttrium-stabilized zirconia.

BackgroundIncompatible coefficient of thermal expansion (CTE) is supposed to be a reason for chipping of ceramic veneered zirconia. This study evaluates the effect of veneering ceramic at varied CTE on bond strength to zirconia.Material and MethodsZirconia disks (Z, Ø 10 mm, 1.0 mm thickness) were prepared from Y-TZP (Cercon®) and sintered at 1350°C for 6 hours. All zirconia disks were veneered with ceramics ((Ø 7.0 mm, 1.5 mm thickness) with varied CTE including VITADur® alpha (VDα), VITAVM®7 (VM7), VITAVM®9 (VM9), Cercon® ceramkiss (CCK), IPSe.max® ceram (IeC), and IPS dSIGN® (IdS) (n=15). The specimens were thermo-cycled (5-55 °C, 500 cycles) prior to determine the shear bond strength on a universal testing machine. The veneering ceramic and zirconia rods (Ø 4 mm, 30 mm length) were prepared for CTE evaluation. ANOVA and Tukey’s multiple comparisons were used to determine the statistically significant difference (α=0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristics strength (σo) of the shear bond. The interfaces were microscopically examined. The phase transformation of zirconia was determined using X ray diffraction.Results The mean±sd (MPa), m, and σo of bond strength were 20.45±2.32, 9.25, and 21.53 for Z-VDα, 19.47±4.53, 4.66, and 20.31 for Z-VM7, 21.05±3.96, 5.61, and 21.88 for Z-IeC, 25.85±2.74, 9.93, and 27.15 for Z-VM9, 25.82±4.39, 6.27, and 27.06 for Z-CCK, and 2.96±0.73, 4.11, and 3.28 for Z-IdS. The CTE (×10-6/°C) were 10.80, 7.83, 7.87, 9.86, 9.93, 10.03, and 12.95 for Z, VDα, VM7, IeC, VM9, CCK, and IdS. The bond strength was significantly affected by the CTE difference (p<0.05). The t→m phase transformation related with the CTE difference.ConclusionsThe CTE’s differences induced stress that affected the bond strength. CTE’s compatibility of veneering ceramic to zirconia is crucial for enhancing the bond strength. The CTE difference approximately 0.77-0.87×10-6/°C was recommended. Key words:Bond strength, coefficient of thermal expansion, zirconia.

Structural investigations on the tetragonal to cubic phase transformations in zirconia induced by progressive yttrium additions

Yttria stabilized zirconia (YSZ) is inevitable in many industrial applications. Yttrium additions in ZrO2 system induces either tetragonal (t-ZrO2) or cubic (c-ZrO2) phase stabilization that predominantly depends on the yttrium content. Here, the structural changes in ZrO2 system upon a wide range of yttrium additions are investigated. Powder synthesis is carried out through citrate-nitrate assisted sol-gel technique and analytical techniques involving XRD, Raman spectra and structural analysis through Rietveld refinement were utilized to investigate the structural changes in ZrO2 upon progressive yttrium additions. The critical limit of yttrium content to retain unique t-ZrO2, mixtures of t-ZrO2 and c-ZrO2 and discrete c-ZrO2 is determined.

PREPARATION OF Bi2O3-DOPED NiO/YSZ ANODE MATERIALS FOR SOFCs

The NiO/YSZ anode materials of SOFCs with Bi2O3 as sintering aids were successfully prepared by dry-pressing process. The effects of Bi2O3 content on the sintering properties of NiO/YSZ anode materials were studied, including the effects on the density, open porosity, weight loss, phase composition, microstructure, mechanical strength and electrical conductivity. In particular, the weight loss, relative density and bending strength of the sintered NiO/YSZ ceramics increased with the increase of Bi2O3 content while the open porosity decreased gradually. The density of the NiO–YSZ ceramics with 6[Formula: see text]wt.% Bi2O3 sintered at 1250[Formula: see text]C could reach that of the ceramics without Bi2O3 sintered at 1400[Formula: see text]C. For Bi2O3 concentration higher than 6[Formula: see text]wt.%, phase transformation of zirconia from monoclinic to cubic structure occurs, causing volume change and microcracks in the anode materials. Therefore, the optimum adding amount of Bi2O3 is 6[Formula: see text]wt.%, which can reduce the sintering temperature to 1250[Formula: see text]C. The ceramics with 6[Formula: see text]wt.% Bi2O3 have relatively high bending strength and low sintering temperature. This is of great significance for improving the performance of the anode and cutting the cost.