Synthesis of bulk tetragonal zirconia without stabilizer: The role of precursor nanopowders

The densification of undoped zirconia nanopowder was performed using the High-Pressure Field Assisted Sintering technique, with the aim of preparing bulk size-stabilized tetragonal zirconia. The role played by the structural and microstructural properties of the starting powders on the characteristics of the sintered materials was investigated by exploring several different synthetic routes. Nanopowders prepared by solvothermal methods were proven to give the most satisfactory results thanks to their microstructure, particularly characterized by uniform spherical microaggregates. Using solvothermally synthesized powder as a starting material, nearly fully dense (97% relative density) samples of undoped size-stabilized tetragonal zirconia were obtained with a sintering cycle of 5 min under a uniaxial pressure of 700 MPa, at sintering temperatures as low as 900 °C.     

Processing of alumina-coated tetragonal zirconia materials and their response to sliding wear

Alumina-coated tetragonal zirconia stabilised with 3 mol% of Y₂O₃ (YTZP) specimens (30 mm × 30 mm × 6 mm) have been obtained by dipping of pre-sintered YTZP compacts in alumina suspensions and subsequent sintering. The coated specimens present hardness values and a wear resistance similar to those of reference dense alumina specimens and significantly higher than those of the YTZP substrates.     

Microorganism adhesion inhibited by silver doped Yttria-stabilized zirconia ceramics

The aim of this study was to incorporate silver nanoparticles into yttria-stabilized zirconia (YSZ) ceramic for eliminating microorganism adhesion on dental restoration graft. Y₂O₃ (3% mol) partially stabilized ZrO₂ powder with particle size around 80 nm was employed to fabricate tetragonal phase dominated YSZ ceramic block. Silver nanoparticles were efficiently loaded at pH 9.5 by NaBH₄ reducing of AgNO₃. The biocompatibility of silver incorporated YSZ was evaluated by MTT assay. Antimicrobial activities were quantitatively determined by optical density measurement and qualitatively analysed by scanning electro microscope. Inductively coupled plasma-mass spectrometry (ICP-MS) revealed that YSZ block containing 0.0047 wt% nanosilver, which is safe to mammalian cell, can inhibit the growth of Escherichia coli, Streptococcus mitis and Candida albicans. The pristine YSZ disc had no effect on bacterial growth. This study suggests that silver nanoparticles incorporated into YSZ blocks possess a broad spectrum antimicrobial effect and may help prevent biofilm formation on their surfaces to improve implant survival rate.     

Strengthening of Vickers indented 3Y-TZP by hydrothermal ageing

The effect of hydrothermal ageing on indentation cracks has been determined in 3Y-TZP by measuring the flexure strength of indented specimens before and after ageing. A substantial increase in strength was observed after ageing, in contrast to the well known decrease in strength in smooth specimens with only natural flaws. The increase in strength with ageing also occurs if the indentation residual stresses are previously removed by annealing. Observations around the crack tip show the formation of a highly microcracked zone during vapour exposure. Fractographic and micro-Raman analysis observations show that the profile of the cracks is marked on the fracture surface by this zone which is intergranular with a crumbled appearance and in which transformation has taken place. The increase in strength is discussed in terms of crack tip blunting induced by the multiaxial stresses that develop in front of the crack under bending.     

Thermal expansion of zirconia–zirconium titanate materials obtained by slip casting of mixtures of Y-TZP–TiO2

The objective of this work was to establish the optimum conditions to get zirconia materials with different proportions of zirconium titanate by reaction sintering of ZrO₂ stabilized with 3 mol% of Y₂O₃ (Y-TZP) and TiO₂. The green bodies were fabricated from stable colloidal suspensions of the powders by slip casting in plaster moulds.The rheological characterization of the suspensions allowed the establishment of the optimum green processing conditions. Reaction sintering was performed at 1500 °C during 2 h, and the obtained materials have been characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy with energy dispersive X-ray microanalysis (FE-SEM-EDX). Under these conditions, zirconium titanate materials with tetragonal zirconia could not be obtained because a solid solution of cubic zirconia with titania and yttria is formed at 1500 °C. The thermal expansion of the materials was determined by differential dilatometry from room temperature up to 850 °C, demonstrating that the incorporation of zirconium titanate reduces the thermal expansion of zirconia.     

Preparation and mechanical characterization of dense and porous zirconia produced by gel casting with gelatin as a gelling agent

A modified gel casting procedure based on a natural gelatin for food industry and commercial polyethylene spheres as pore formers was successfully exploited to produce dense and porous ceramic bodies made of yttria stabilized tetragonal zirconia polycrystal (Y-TZP). Vickers and Knoop microhardness, elastic modulus and fracture toughness measurements on dense samples obtained by experimental investigation closely matched results found in the literature for similar materials. However, after a careful analysis of obtained results, no indentation size effect and a lower scattering of experimental data from low load indentations were observed, in comparison with literature.     

Relationship between fracture toughness determined by surface crack in flexure and fracture resistance measured by indentation fracture for silicon nitride ceramics with various microstructures

The reliability of the Vickers indentation fracture (IF) method for various types of silicon nitride (Si₃N₄) ceramics was assessed by comparing the fracture resistance, K_R obtained from the IF test with the fracture toughness, K_Ic from the surface crack in flexure (SCF) technique in the same crack depth region. The K_R of a fine-grained and equiaxed Si₃N₄ matched with the K_Ic from the SCF test when Miyoshi's equation was used, while the K_Ic of a bearing-grade Si₃N₄ was found to lie between K_R values calculated with Niihara's equation (higher side) and Miyoshi's equations (lower side). In the case of coarse Si₃N₄ with elongated grains, the K_R determined using Niihara's equation gave the best fit with K_Ic. The inconsistent outcomes were explained by the probable mechanisms, indicating that the K_R from the IF test cannot be correlated directly with the K_Ic unless the effective crack length for the IF test was clarified.     

Toughening alumina with silver and zirconia inclusions

Both silver and zirconia inclusions are added into an alumina matrix, the strength and toughness of the composites are determined. The toughening agents prohibit the grain growth of the matrix, the strength of alumina is, therefore, enhanced. The addition of two toughening agents also enhances the toughness of alumina. The presence of Ag inclusions raises the transformation ability of ZrO₂; however, the toughness increase of the Al₂O₃–ZrO₂–Ag composites is slightly lower than the sum of the toughness increase of Al₂O₃–ZrO₂ and of Al₂O₃–Ag composites. The present study demonstrates that the toughening effects contributed by a transformation toughening agent and a ductile toughening agent can interact with each other; nevertheless, such interaction depends strongly on the microstructure of the composites.     

Effect of the volume ratio of zirconia and alumina on the mechanical properties of fibrous zirconia/alumina bi-phase composites prepared by co-extrusion

Fibrous zirconia/alumina composites with different composition were fabricated by piston co-extrusion. After a 3rd extrusion step and sintering at 1600 °C, crack-free composites with a fibre width of 50 μm were obtained for all compositions. The effect of the volume ratio of secondary phase on the mechanical properties was investigated. The Young's modulus of the composites decreased linearly with increasing the zirconia content. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPa m^1/2 was attained in the 3rd co-extruded 47/53 vol% zirconia/alumina composite. The improvement in toughness was attributed to both “stress-induced” transformation of zirconia and a crack deflection mechanism due to thermal expansion mismatch between the two phases. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.     

Influence of erbia or europia doping on crystal structure and microstructure of ceria–zirconia (CZ) solid solutions

In this study we have prepared three zirconia–ceria compositions, namely 12, 50 and 80 mole% CeO₂. Along with each pure ceria–zirconia composition we have prepared two parallel erbia- or europia-doped materials in which 0.5 mole% of each of the two starting oxides is replaced by 1 mole erbia or europia.     

Texturing of 3Y-TZP zirconia by electrophoretic deposition in a high magnetic field of 17.4 T

Ceramics can be textured during colloidal processing by aligning the suspended particles in a strong magnetic field and retaining this alignment in the green body. Attempts to align tetragonal zirconia particles however were not successful, not even in 12 T magnetic fields. In the current work, monoclinic zirconia was successfully aligned with its (1 0 0) plane perpendicular to the magnetic field direction by electrophoretic deposition (EPD) in a 17.4 T field. Moreover, textured tetragonal zirconia was developed by reactive sintering of undoped pure monoclinic zirconia and co-precipitated 8 mol% yttria-stabilized zirconia. The sintered tetragonal zirconia inherited the alignment of the monoclinic zirconia particle precursor and aligned with its (0 0 1) plane perpendicular to the magnetic field direction. The toughness of the (0 0 1)-oriented 3Y-TZP along the [0 0 1] direction of the textured zirconia was 65% higher than normal to it and 48% higher than the randomly oriented material.     

Fabrication of yttria-stabilized-zirconia coatings using electrophoretic deposition: Effects of agglomerate size distribution on particle packing

Yttria-stabilized-zirconia (YSZ) particles with various size distributions have been electrophoretically deposited (EPD) on Fecralloy substrate to investigate the particle size effect on EPD coatings. The deposition rates, as-deposited particle packing densities, green densities and sintered (for 2 h at 1250 °C in air) coating hardnesses are dependent on particle size. The particle packing arrangement in EPD coatings can be affected by further electric field densification (EFD) of the as-deposited coating in which the wet EPD coating is immersed in pure solvent (acetylacetone) with the application of a constant electric field. The effect of EFD was found to be most effective on small particles (<0.5 μm) when they are co-deposited with large particles (>1 μm). The improvements are reflected in increased mechanical hardness of sintered coatings.     

Modification of the catalytic properties of sulfated zirconia by addition of metal promoters

We have studied the effects of promoting sulfated zirconia catalysts by addition of transition metals. We have found that the addition of Ni causes an activity enhancement comparable to that caused by the addition of Fe and Mn. The TPD/DRIFTS results show that the catalytic activity does not correlate with either Brønsted or Lewis acidity. The high isomerization activity of these catalysts is explained in terms of a bifunctional mechanism in which the metal promoters are responsible for an enhancement in the surface concentration of olefins rather than an enhanced acidity.     

Acoustic characterization and microstructure of high zirconia electrofused refractories

A new line of electrofused refractory materials with a very high content of zirconia (HZ) has been developed to satisfy the needs of new generation manufacturing glass furnaces. Such materials are subjected to severe operating conditions (temperature and corrosion) during their manufacturing and service life. These HZ materials required very high temperature casting and a suitable annealing process to prevent defects and cracks during manufacturing. Therefore, a research program has been launched to build numerical tools able to predict the thermo-mechanical behaviour of these materials during one of the most critical phases of manufacturing: the controlled cooling of the refractory blocks after melting and casting. The efficient development of such a tool requires the knowledge of thermo-mechanical properties of these materials with temperature, in conditions close to that occurring during processing.In the framework of this approach, the present paper deals with the characterization of elastic properties of two HZ materials, using two mechanical testing devices i.e. a pulse echography technique and a tensile test device. An innovative acoustic emission device is also used to help in identification of microdamage occurrence. The goal of this study is to investigate the microstructure organisation of materials at very fine scale (<100 μm) in order to correlate the obtained results with the macroscopic properties of the material. Characterisations are also performed at intermediate temperature to establish correlations with the manufacturing process.     

Flash grain welding in yttria stabilized zirconia

Self-standing samples made of isostatically pressed powders of ZrO₂:8 mol% Y₂O₃ were submitted to AC signals of 60 and 1000 Hz. At temperatures of around 900 °C, a current density exceeding approximately 100 mA/cm² starts an avalanche process with a fast increasing current under constant voltage. It lasts about 60 s and it is preceded by an induction period of the order of 30 s. After that, the material is sintered as confirmed by impedance diagrams. Relative densities of 94% could be obtained.     

Fracture behaviour of Mg-PSZ ceramics: Comparative estimates

The mechanical behaviour of different Mg-PSZ ceramics is studied. Results of their edge fracture (EF) and single edge V-notch beam (SEVNB) tests are discussed. These inelastic ceramics exhibit nonlinear relations between the fracture load and the distance from the extreme point on the chip scar to the specimen edge. They also possess nonlinearly rising R-lines. It is established that the data points plotted in the EF base diagram fall below the baseline (lower barrier to the onset of fracture). By projecting these data points onto the baseline, fracture toughness values close to those of the matrix are determined. The limitations of conventional procedures for evaluating the mechanical behaviour of these ceramics were found out. It has been demonstrated that the EF test method can be quite adequate for enhancing the reliability of comparative fracture resistance estimates.     

Influence of the urea content on the YSZ hydrothermal synthesis under dilute conditions and its role as dispersant agent in the post-reaction medium

The effect of the urea content, under mild and dilute conditions, on the extension of the YSZ hydrothermal reaction, on the crystalline zirconia phases obtained and, on its primary particle size has been studied. The key role of the urea as a dispersant agent for the synthesized YSZ nanoparticles has also been investigated in terms of particle size distribution and zeta potential. The latest study has been performed in the post-reaction medium. Pure nanocrystalline YSZ is obtained when a basic pH (10) is achieved during the synthesis. In addition, the urea is protonated at 5 < pH < 7 in the post-reaction medium and it allows its specific adsorption on the surface of the particles by electrostatic and steric mechanisms, keeping a stable suspension. In those conditions, the measured average particle size is 20 nm and the agglomeration factor (F_ag) is 2. However, by HR-TEM particles with a size even less than 5 nm are observed.     

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO₂ and CO gases using a porous cell of Ru-8 mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800 °C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO₂ gas supplied at 50 ml/min was decomposed to solid carbon and O₂ gas through the cell at the electric field strengths of 0.9–1.0 V/cm. The outlet gas at a flow rate of 3 ml/min included 61–63% CO₂ and 37–39% O₂ at 700–800 °C and the outlet gas at a flow rate of 50 ml/min included 73–96% (average 85%) CO₂ and 4–27% (average 15%) O₂ at 800 °C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O₂ and CO₂ gases at 800 °C. The fraction of outlet gas at a flow rate of 50 ml/min during the CO decomposition at 800 °C for 5 h was 11–36% CO, 59–81% O₂ and 2–9% CO₂. The detailed decomposition mechanisms of CO₂ and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O²⁻ ions or O₂ gas.     

Pseudoplastic deformation pits on polished ceramics due to cavitation erosion

In a previous study, pseudoplastic deformation pits created by cavitation exposure were reported in silicon nitride and zirconia. In this research, further comparison of the size and number of pits between several silicon nitride and zirconia materials is carried out. The pits are larger and much more numerous in silicon nitride than in zirconia although silicon nitride is harder than zirconia. An explanation of this phenomenon is given. Also, in the previous study it was reported that apparently a partially stabilized zirconia with yttria oxide developed a delay in the phase transformation from tetragonal to monoclinic after being exposed to cavitation. In this research, further experiments related with this phase transformation delay are carried out. Also, the phase transformation is verified with X-ray diffraction analysis. It is concluded that the “activation” of the partial stabilized zirconia happens regardless of the oxide used to stabilize it.     

Surface modification of 3Y-TZP with cerium oxide

Surface modification with cerium oxide of tetragonal zirconia polycrystals stabilized with 3 mol.% yttria (3Y-TZP) was carried out in order to improve the resistance to low temperature degradation. Specimens were coated with pressed CeO₂ powder and then annealed at 1400 °C and 1500 °C for periods of up to 10 h. Similar treatments were performed in specimens coated with a sub-micron CeO₂ layer by means of magnetron sputtering. Cerium penetration in the surface modified specimens is about 10 μm into the bulk and the grain size increases in the surface layer affected by cerium diffusion. The indentation bulk fracture toughness and Vickers hardness are not affected by the surface modification treatments. Berkovich nanoindentation was performed to observe the contact hardness and elastic modulus at the surface, showing no significant difference after surface modification. Surface modification with ceria induces a large increase in the resistance to hydrothermal ageing without impairing mechanical properties.