Yttria Tetragonal Zirconia Polycrystalline Research Articles

Influence Of Low-Temperature Degradation On Phase Transformation And Biaxial Flexural Strength On Different High-Translucent 4Y-PSZ, 5Y-PSZ, 6Y-PSZ Monolithic Zirconia

Objective: This study aimed to investigate the effect of low-temperature degradation (LTD) in phase transformation and biaxial flexural strength of high-translucent yttria partially stabilized zirconia (Y-PSZ) and yttria tetragonal zirconia polycrystalline (3-YTZP).
 Methods: A total of 120 new high-translucent 3-YTZP (NMS) and Y – PSZ (KST, KUT, NQ3MS) zirconia disc specimens were manufactured according to ISO 6872 for biaxial flexural strength (14 mm., 1.2 ± 0.02 mm). The specimens from each type of material were divided into 3 subgroups (n:30) according to the LTD in an autoclave at 134 C0 at 2 bar (n:10) (at 5, 20 hour (h)). Specimens without LTD served as the control. Data of the monoclinic phase changes (Xm) and flexural strength were analyzed using two-way ANOVA followed by post hoc MannWhitney U test. Weibull statistics were used to analyze strength reliability.
 Results: LTD increased the monoclinic content significantly for NMS and slightly for the KST group. A monoclinic phase was not detected for KUT and NQ3MS groups. The biaxial flexural strength of the NMS group was affected significantly and decreased with an increase in the 20 h aging. For flexural strength values, there was no significant difference in aging times for each of the KST, KUT, and NQ3MS groups. Weibull analysis showed the highest characteristic strength for NMS (1412.9), KST (750.1), NQ3MS(790.5) and KUT (615.2) groups. The Weibull modulus (m) increased in the NMS, KUT, and NQ3MS groups compared with the control group and decreased in the KST group.
 Conclusion: LTD caused a significant decrease in the biaxial flexural strength results of the NMS group but did not significantly affect the KST, KUT, and NQ3MS groups’ values.

Influence of graphene-based nanostructures processing routes and aspect ratio in flexural strength and fracture mechanisms of 3Y-TZP-matrix composites

In this work, the influence of the aspect ratio of graphene-based nanostructures (GBNs) and content on the mechanical properties of 3 mol% yttria tetragonal zirconia polycrystalline 3Y-TZP matrix composites was analysed. The influence of the dispersion method and sintering parameters on the flexural strength and elastic modulus of the composites was studied, and the fracture surfaces were characterized to determine the fracture mechanisms that occur. The results showed that small amounts of exfoliated graphene nanoplatelets, with reduced lateral size, and few layer graphene, up to 1.0 and 2.5 vol%, respectively, slightly increase the 3Y-TZP flexural strength. This has been attributed to the tortuosity of the crack propagation pathways and strengthening mechanisms. Higher contents cause a decrease in flexural strength and stiffness because of overlapped GBNs, which can promote the crack propagation. The pull-out of GBN was more significant in composites with non-exfoliated graphene nanoplatelets, where no increase on the flexural or biaxial strength was measured.

Single-retainer all-ceramic resin-bonded fixed dental prostheses: Long-term outcomes in the esthetic zone.

To present an update on the concept of cantilevered single-retainer all-ceramic resin-bonded fixed dental prostheses (RBFDPs) first presented 25 years ago in the Journal of Esthetic Dentistry. The initially presented case of the concept was followed clinically over 26 years and is presented along with two additional clinical long-term cases using varying methods to obtain an esthetic and hygienic ovate pontic design. Veneered alumina and zirconia ceramic (3mol% yttria-tetragonal zirconia polycrystalline ceramic; 3Y-TZP) was used and bonded with a phosphate monomer containing luting resin after 50 μm alumina particle air-abrasion at 0.25 MPa pressure. The restorations replacing incisors did not debond and soft tissues in the pontic area were maintained over 26 years. Cantilevered single-retainer all-ceramic RBFDPs today made from veneered 3Y-TZP zirconia ceramic can be considered a standard of care for the replacement of single incisors and provide an excellent esthetic outcome with a long-term preservation of soft tissues in the pontic area. Bonding nonretentive oxides ceramics such as alumina and zirconia ceramic with phosphate monomer containing luting resins after alumina particle air-abrasion is durable over decades. This proves that bonding to zirconia ceramic is not of any problem when adequate methods are used. Single-retainer zirconia ceramic RBFDPs maintain soft tissues in the edentulous area of single missing incisors and often deem implants unessential for this indication.

Y-TZP Physicochemical Properties Conditioned with ZrO2 and SiO2 Nanofilms and Bond Strength to Dual Resin Cement.

Commercial Yttria-tetragonal zirconia polycrystalline (Y-TZP) was subjected to surface treatments, and the bond strength of dual resin cement to Y-TZP and failure modes were evaluated. Disks (12 mm × 2 mm), cylinders (7 mm × 3.3 mm), and bars (25 mm × 5 mm × 2 mm) were milled from Y-TZP CAD-CAM blocks, divided into seven groups, and subjected to different surface treatments; silicatization was used as control. On the basis of the literature, this study evaluated modifications with films containing SiO2 nanoparticles and silane; SiO2+ZrO2-SiO2 (50%) and ZrO2 (50%) nanoparticles, SiO2+ZrO2/Silane-SiO2 (50%) and ZrO2 (50%) nanoparticles, and silane. Specimens were analyzed by wettability (n = 3), surface free energy (n = 3), X-ray diffraction (n = 1), Fourier transform infrared spectroscopy (FTIR) (n = 1), roughness (n = 5), shear bond test (n = 10), and dynamic modulus (n = 3). Specimens treated with hydrofluoric acid-HF 40% presented significantly higher contact angle and lowest surface free energy (p < 0.05). The SiO2/Silane presented crystalline SiO2 on the surface. The surface roughness was significantly higher for groups treated with nanofilms (p < 0.05). Shear bond strength was significantly higher for silicatization, HF 40%/silicatization, SiO2/Silane, and SiO2+ZrO2/Silane groups. The proposed treatments with nanofilms had potentially good results without prejudice to the physicochemical characteristics of zirconia. Generally, groups that underwent silica surface deposition and silanization had better bond strength (p < 0.005).

Ultrashort-pulse laser as a surface treatment for bonding between zirconia and resin cement

ObjectivesTo evaluate ultrashort-pulse laser (UPL) as a surface treatment for improved bond strength to Yttria-tetragonal zirconia polycrystalline (Y-TZP). MethodsFully-sintered Y-TZP samples received either no treatment (CTL), or were treated by alumina blasting (ALB), tribochemical silica coating (SIL), or one of two UPL patterns: multiple pulses laser surface dots with 2.5μm spacing (8mJ, 10kHz)(LSD); or single pulse laser surface lines with 2.5μm spacing (4mJ, 6.7kHz)(LSL). Surface roughness, wettability (contact angle), and quantification of crystalline phases were evaluated for each group (n=3/group). Y-TZP treated slabs were cemented to resin composite slabs using silane and 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing adhesive. Beams from the Y-TZP/resin blocks were microtensile tested (n=5/group) after 48h water incubation (37°C) with or without subsequent thermocycling (5–55°C, 5000 cycles). ResultsAll surface treatments increased surface roughness values versus control (P<0.001). Contact angles were lowest for SIL (6.57±2.37°) and highest for control (50.97±6.30°). LSL and LSD were the only treatments that did not increase the relative monoclinic phase. All surface treatments significantly increased microtensile bond strengths (μTBS) compared with the control group (P<0.001), with highest values for UPL (LSD: 35.40±4.53MPa>LSL: 31.84±8.46MPa>SIL: 19.95±3.99MPa=ALB: 19.51±2.55MPa>CTL: 14.51±2.23MPa). Thermocycling significantly reduced bond strength for all treatments in a surface treatment-dependent manner. SignificanceThe ability of UPL to alter Y-TZP surface morphology, increase wettability and μTBS without increasing the monoclinic content suggests its potential to improve bonding to the underlying resin cement and tooth without compromising the strength of the restoration.

Graphene nanoplatelets for electrically conductive 3YTZP composites densified by pressureless sintering

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 2.5, 5 and 10 vol% graphene nanoplatelets (GNP) were pressureless sintered in argon atmosphere between 1350 and 1450 °C. The effects of the GNP content and the sintering temperature on the densification, microstructure and electrical properties of the composites were investigated. An isotropic distribution of GNP surrounding ceramic regions was exhibited regardless the GNP content and sintering temperature used. Electrical conductivity values comparable to the ones of fully dense composites prepared by more complex techniques were obtained, even though full densification was not achieved. While the composite with 5 vol% GNP exhibited electrical anisotropy with a semiconductor-type behaviour, the composite with 10 vol% GNP showed an electrically isotropic metallic-type behaviour.

Optimizing the homogenization technique for graphene nanoplatelet/yttria tetragonal zirconia composites: Influence on the microstructure and the electrical conductivity

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composite powders with 10 vol% nominal content of graphene nanoplatelets (GNPs) were prepared using four different homogenization routines: dispersion of the powder mixture by ultrasonication in isopropyl alcohol, homogenization in a high-energy planetary ball mill in wet or dry conditions after ultrasonication, and milling of the powders in a high-energy planetary ball mill in dry conditions. A significant effect of the homogenization routine on the powders particle size distribution was revealed by laser granulometry and Raman spectroscopy. Highly densified composites were obtained after spark plasma sintering (SPS) and remarkable differences on the GNP size, shape and distribution throughout the ceramic matrix and also in the electrical conductivity were observed in the four different composites. The composite with the best performance in terms of electrical conductivity was the one prepared after planetary ball milling of the powders in dry conditions as a consequence of the reduced dimensions of the GNPs and their excellent distribution throughout the ceramic matrix.

Electrical conduction mechanisms in graphene nanoplatelet/yttria tetragonal zirconia composites

Yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composites with 5, 10 and 20 vol% graphene nanoplatelets (GNPs) were prepared by spark plasma sintering (SPS) and their electrical conductivity as a function of temperature was characterized. The composites exhibit anisotropic microstructures so the electrical conductivity studies were carried out in two directions: perpendicular (σ⊥) and parallel (σ||) to the SPS pressing axis. The composites with 5 and 10 GNP vol% showed high electrical anisotropy, whereas the composite with 20 GNP vol% exhibited nearly isotropic electrical behavior. σ⊥ shows metallic-type behavior in the composites with 10 and 20 vol% GNP revealing that charge transport takes place through defect-free GNPs. For the composite with 5 vol% GNP the observed semiconductor-type behavior was explained by a two dimensional variable range hopping mechanism. σ|| shows metallic-type conductivity in the composite with 20 GNP vol% and positive dσ||/dT slope in the composites with 5 and 10 GNP vol%.

Shear bond strength of resin adhesive to laser treated surface of yttria tetragonal zirconia polycrystalline

Purpose: This study evaluated the effects of surface treatment of zirconia with erbium-doped yttrium aluminum garnet (Er-YAG) laser on shear bond strength to resin cement. Materials and Methods: Thirty disc-shaped zirconia specimens (10 mm in diameter and 2 mm in thickness) were fabricated, randomly assigned to 3 groups (n = 10) and treated surface with: Gr. N) no treated surface, Gr. S) sandblasted with 50 micron alumina and Gr. L) treated with an Er-YAG laser. Zirconia surface was then applied with metal/zirconia primer prior to be cemented to composite cylinder (5 mm in diameter and 4 mm in thickness) with resin cement. Shear bond strength test was performed at a crosshead speed of 0.5 mm/min. Data were analyzed by ANOVA and Tukey HSD tests. Results: The means and standard deviations of shear bond strength (MPa) were: Gr. N) 10.79 ± 1.03, Gr. S) 14.15 ± 3.07, Gr. L) 14.11 ± 1.98. The treatment of zirconia surface were significantly affected to shear bond of resin adhesive to zirconia (p 0.05). Conclusion: Pretreatment of zirconia by Er-YAG laser improved bond strength to resin cement. The study suggested that Y-TZP ceramics should be treated surface with Er-YAG laser before cementation. Keywords: Zirconia; Resin Cement; Er-YAG Laser; Shear Bond Strength

Milling properties of low temperature sintered zirconia blocks for dental use

To investigate the milling properties of different yttria-tetragonal zirconia polycrystalline (Y-TZP) block materials by applying a dental computer numerical control (CNC) milling center. Low temperature sintering zirconia block denoted by KMUZ (experimental) with two commercial zirconia blocks for T block made in Taiwan and a G block made in Germany were compared for the milling properties. Seventy-two specimens were milled using the same CNC milling center, and properties were evaluated by measuring the weight loss (g), milling time (s), margin integrity (%) and broken diameter (μm). The crystalline phases contents were identified by X-ray diffraction and the microstructures of the sintering specimens were observed by scanning electron microscopy and transmission electron microscopy. The mean milling time of G and KMUZ were significantly shorter than T (P<0.05). The KMUZ samples exhibited the least weight loss among the three kinds of blocks (P<0.05). The percentages of marginal integrity after milling were high in G and KMUZ but low in T (P<0.05). The mean broken diameters were from 90μm to 120μm. The phase transformation of t-ZrO2 (KMUZ: 7.4%, G: 5.9%, T: 3.2%) to m-ZrO2 when facing the milling pressure in ZrO2 blocks was observed by XRD. The result of TEM microstructure of KMUZ revealed that Y and Si were soluble in grain boundaries. The results show that the milling properties of KMUZ were better than one commercial T and near the G. The hindered grain growth, as a result of the Y3+ content in the grain boundaries, also plays a role in promoting the abnormal grain growth of 3Y-TZP.

Relationships between structural and electrical properties in mixed conductors duplex materials in the ZrO2-Y2O3-TiO2ternary system

Duplex materials constituted by 0.50 mol titania-doped yttria-stabilized zirconia (YSZ) and 0.50 mol titania-doped yttria-tetragonal zirconia polycrystalline (YTZP) solid solutions in the ZrO${}_{2}$-Y${}_{2}$O${}_{3}$-TiO${}_{2}$ ternary system can be obtained using different processing strategies. In this study, different amounts of TiO${}_{2}$ dopant and different sintering times have been used for the preparation of the duplex materials: Doping YSZ with 10 mol% of TiO${}_{2}$ and sintering in air for 8 h (10Ti8h) and doping YSZ with 15 mol% of TiO${}_{2}$ and sintering in air for 2 h (15Ti2h) are both successful routes to obtain duplex materials. If we compare the field emission scanning electron microscopy-energy dispersive x-ray analyses of each cubic and each tetragonal solid solution of the two duplex materials, we conclude that the composition of each phase is different from each other, as was expected. The total ionic conductivity of both duplex samples is strongly reduced with respect to that of YSZ, and this reduction increases with the Ti content. In addition, the activation energy for ionic migration in 10Ti8h and 15Ti2h is lower than that for YSZ but higher than that for YTZP, as expected in terms of the relative amount of both YSZ and YTZP fractions in the materials. X-ray absorption spectroscopy (XAS) results, both x-ray absorption near-edge structure and extended x-ray absorption fine structure, show that Ti is sixfold coordinated in both the cubic and tetragonal phases of the duplex materials, departing from the eightfold coordination expected if a simple substitution at the Zr sites would take place. The XAS results also point out that the ability of Ti to trap oxygen vacancies in the cubic phase increases as Ti content does, in agreement with the electrical conductivity behavior, which cannot be accounted for in terms of a dilution effect.

Confocal Raman microscopic analysis of the zirconia/feldspathic ceramic interface

ObjectivesEsthetic demands and biocompatibility have prompted the development of all-ceramic dental crowns. Yttria tetragonal zirconia polycrystalline (Y-TZP) framework material has the best mechanical properties compared to other all-ceramic systems, but the interface is the weakest component of core veneered restorations. Confocal Raman microscopy possibilities are used to ensure the understanding of the zirconia-feldspathic ceramic relationship, which is not well known. MethodsBilayered zirconia (Vita In-Ceram® YZ) veneer (Vita VM®9) blocks were manufactured. Raman analyses were performed using two protocols: (1) single spectra, line scans and images on a sectioned and polished specimen and (2) in depth line scans on unprepared specimen. Single spectra, images and line scans provide information about the crystalline phases, their distribution and the existence of a possible diffusion at the Y-TZP/VM®9 interface, respectively. The elemental distribution of zirconium (Zr) and silicon (Si) around this interface were investigated using energy dispersive X-ray spectroscopy (EDS). ResultsRaman single spectra embodied a unique spectrum (crystalline) on Y-TZP and two spectra (crystalline and amorphous) on VM®9; these spectra were clearly distinguished. Raman line scans showed a series of transition spectra across the interface from VM®9 to Y-TZP. It emphasized an interdiffusion zone, which was estimated at a maximum of 2 microns, found on 2d Raman images and confirmed by EDS. The elemental distribution with EDS showed a mutual diffusion of Zr and Si and was mainly dominated by Si diffusion in Y-TZP. SignificanceConfocal Raman microscopy highlights an interdiffusion zone at the zirconia-feldspathic ceramic interface. The elemental transition layer is estimated and is supported by EDS analysis as a coupling technique.

Mixed conductivity, structural and microstructural characterization of titania-doped yttria tetragonal zirconia polycrystalline/titania-doped yttria stabilized zirconia composite anode matrices

Taking advantage of the fact that TiO 2 additions to 8YSZ cause not only the formation of a titania-doped YSZ solid solution but also a titania-doped YTZP solid solution, composite materials based on both solutions were prepared by solid state reaction. In particular, additions of 15 mol% of TiO 2 give rise to composite materials constituted by 0.51 mol fraction titania-doped yttria tetragonal zirconia polycrystalline and 0.49 mol fraction titania-doped yttria stabilized zirconia (0.51TiYTZP/0.49TiYSZ). Furthermore, Y 2(Ti 1− y Zr y ) 2O 7 pyrochlore is present as an impurity phase with y close to 1, according to FT-Raman results. Lower and higher additions of titania than that of 15 mol%, i.e., x=0, 5, 10, 20, 25 and 30 mol% were considered to study the evolution of 8YSZ phase as a function of the TiO 2 content. Furthermore, zirconium titanate phase (ZrTiO 4) is detected when the titania content is equal or higher than 20 mol% and this phase admits Y 2O 3 in solid solution according to FE-SEM-EDX. The 0.51TiYTZP/0.49TiYSZ duplex material was selected in this study to establish the mechanism of its electronic conduction under low oxygen partial pressures. In the pO 2 range from 0.21 to 10 −7.5 atm. the conductivity is predominantly ionic and constant over the range and its value is 0.01 S/cm. The ionic plus electronic conductivity is 0.02 S/cm at 1000 °C and 10 −12.3 atm. Furthermore, the onset of electronic conductivity under reducing conditions exhibits a −1/4 pO 2 dependence. Therefore, it is concluded that the n-type electronic conduction in the duplex material can be due to a small polaron-hopping between Ti 3+ and Ti 4+.

On the construction of wear maps for Y-TZP dental ceramics in aqueous environments: pH, exposure time and impact angle effects

The object of this work was to evaluate the particulate erosion performance of Y-TZP (Yttria Tetragonal Zirconia Polycrystalline), used in the construction of artificial teeth, at various pH values, impact angles and exposure times. The results showed that the highest degradation recorded was in acidic environments. However, the incubation time to high wastage was also affected by impact angle and exposure time. Wear maps were constructed, based on the results, identifying wastage regimes for the material as a function of the exposure conditions. In addition, a wear map was generated based on the observed phase transformations for the material.

SEM observations of porcelain Y-TZP interface

Objectives The metastability of the tetragonal phase of yttria tetragonal zirconia polycrystalline (Y-TZP) ceramics is a cause for concern in dental crown and bridge applications. One specific problematic area is the nature of the interface between the veneering porcelain and the Y-TZP framework and whether the associated preparation procedures and reactions result in a reduction of the stability of the zirconia. Methods To investigate this aspect, high-resolution SEM observations were made of polished and etched (HF content gel) cross-sections of the interface area. Dry and moist veneering porcelain powders were built up on the zirconia base. Results In some instances the zirconia grains at the interface appear to show multiplicity of subgrain faceting whereas in other instances they do not. The latter indicate destabilisation of the tetragonal phase occurs and in addition that the porcelain veneering material wets and some dissolution of the Y-TZP occurs. Significance These results and their relevance to the long-term stability of the interface adhesion to the veneering porcelain as well as possible tetragonal to monoclinic crystal transformations at the interface are discussed.

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al 2O 3 in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.

Mixed (Oxygen Ion and n‐Type) Conductivity and Structural Characterization of Titania‐Doped Stabilized Tetragonal Zirconia

By using X‐ray diffraction lattice parameter measurements and Raman spectroscopy studies, both the solid solubility limit of titania in yttria tetragonal zirconia polycrystalline solid solutions and the tetragonal solid solution field in the system have been established. Valence state, site symmetry, and changes in local structures of Ti ions in Y‐TZP with 5 and 10 mol % are studied for the first time using EXAFS (extended X‐ray absorption fine structure), XANES (X‐ray absorption near edge structure), and X‐ray photoelectron spectroscopy. The total electrical conductivity in air of the tetragonal solid solution decreases with increasing titania content. XANES results show that as the dissolves into the tetragonal zirconia Y‐TZP matrix, a displacement of ions from the center of symmetry seem to take place which leads to a nonrandom substitution of ions on lattice sites. Ti‐O bond distances derived from EXAFS results indicate that ion can be in a square‐pyramidal arrangement, i.e., fivefold oxygen‐coordinated. As a consequence, two kind of cation‐oxygen vacancy associations ( and ) with different diffusion dynamics are created. This results in a decrease of the global concentration of moving oxygen vacancies and therefore, a decrease of ionic conductivity. Electronic conductivity, n‐type, only appeared at oxygen partial pressure lower than atm and above 800°C in Y‐TZP containing 10 mol % titania. Such an n‐type electronic conduction was attributed to a hopping of electrons between and cations by a small polaron hopping mechanism. © 1999 The Electrochemical Society. All rights reserved.

Y-TZP sliding reciprocatively against various AION ceramics

The friction and wear characteristics of various yttria tetragonal zirconia polycrystalline (Y-TZP)-aluminium boxynitride (AION) sliding systems were examined. These systems consisted of a Y-TZP sphere reciprocating against three different translucent AION plates. The fully dense AIONs used contained 67.5, 73.0 and 77.5 mol% Al2O3 and had a grain size of 28 to 56 μm. The tests were performed at a constant track length, two loads, two frequencies and two different time intervals at room temperature. The humidity was controlled by flushing with dry nitrogen. The total vertical displacement and the friction coefficient were measured continuously and sampled. The worn surfaces were observed microscopically. It was concluded that there is no difference in wear behaviour between the three AION types. It was also concluded that the amount of wear increases approximately linearly with load at a frequency of 1 Hz. The tests at 4 Hz show a stochastic transition between relatively mild wear and severe wear. The observations and data were used to derive a wear mechanism.

Formation of monoclinic zirconia at the anodic face of tetragonal zirconia polycrystalline solid electrolytes

The stability of yttria tetragonal zirconia polycrystalline (Y-TZP) materials with and without the addition of alumina has been investigated during charge flow in solid electrolyte cells. A considerable amount of monoclinic zirconia is formed (up to 50–60%) on the anodic side of the solid electrolyte discs during current flow. The thickness of the surface layer in which maximum transformation takes place was determined to be 3–4.5 μm. On the cathodic side, the amount of monoclinic zirconia detected was relatively small (< 5%). The amount of monoclinic formed on the anodic side varied with the microstructure of the ceramic and was considerably less in materials free of pores and with uniform grain size distribution. Relaxation experiments indicate that the tetragonal to monoclinic zirconia phase transformation is related to the oxygen evolution reaction and is not due to oxygenion transport within the solid electrolyte. The observed behaviour has been explained in terms of the creation of space charge layers at the electrode/electrolyte interface leading to the saturation of vacancies by oxygen ions and instability of the tetragonal phase in the surface region on the anodic side of the solid electrolyte.

Preferred orientation of the transformed monoclinic phase in fracture surfaces of Y-TZP ceramics

X-ray diffraction (XRD) has been used in a study of yttria-tetragonal zirconia polycrystalline (Y-TZP) ceramics to assess the extent of transformation to the monoclonic phase of fracture surfaces. Comparison of powder diffraction patterns and those obtained from fracture surfaces indicated that there are changes in the relative intensities of the (111) m and (11 1) m reflections and also shifts in the angles at which the reflections appear. From the experimental data, lattice parameters for the monoclinic phase are recalculated, using four reflections, by an iterative technique. Using the new calculated lattice parameters the position of a fifth reflection is predicted accurately. The integrated intensity of the (11 1) m reflection is increased compared with the (111) m reflection. The ratio of the integrated intensities is expected to be about 1·5, but is found to be as high as 7·5. This can give rise to an error in the predicted monoclinic phase content; a correction is proposed. The preferred orientation is considered to arise because of the importance of the shear component of the martensitic transformation. The importance of this finding for an understanding of the fracture process is discussed.