Fracture of composite alumina/yttria-stabilized zirconia joints

Four-point bend tests have been performed on samples consisting of yttria-stabilized zirconia containing 0–80% alumina joined by plastic deformation to the same or different composition. The fracture strength of joints between the same composition was equal to the strength of the monolithic material. Fracture of joints made between different compositions occurred at the position of maximum tensile residual stress, as determined by finite-element analysis, not at the interface. Measured strengths were in accord with fracture mechanics and the calculated residual stresses.     

High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.     

Synthesis of alumina and zirconia fibers

The results of the investigations of conversions occurring during synthesis of aluminum oxide and zirconium dioxide porous fibres with alloying additions in the temperature range from 20 to 1300°C are presentated. The analysis of the experimental results enabled to reveal features typical for pyrolitic conversion and sintering of aluminum oxide — and zirconium dioxide — base ceramic fibres.     

Fracture behaviour of alumina ceramics by biaxial ball-on-3-ball test

The fracture behaviour of alumina ceramics was studied using a biaxial ball-on-3-ball test. The polished surfaces of the alumina specimens were indented at positions 0, 1, 2 and 3 mm distant from the center of the specimen along a path (A), passing through the middle point between two supporting balls from the center of the specimen, and also along a path (B), passing through the top point of a supporting ball from the center of the specimen. The fracture strength of the indented specimens was measured using the biaxial ball-on-3-ball test. The fracture strength increased with increasing distance of the indented position from the center of the specimen. The fracture strength of the specimen indented along a path (B) was higher than that of the specimen indented along a path (A). It was also found that the fracture was brought about by the tangential stress rather than the radial stress when the indentations were made at points 1 and 2 mm distant from the center of the specimen. The results were in good agreement with the results of finite element analysis.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.     

Synthesis of zirconia–alumina and alumina–zirconia core–shell particles via a heterocoagulation mechanism

This study describes the synthesis of core–shell particles, consisting of a ZrO₂ or Al₂O₃ submicron nucleus coated by a nanolayer of Al₂O₃ or ZrO₂, respectively. The oxide layers around the cores are deposited via a heterocoagulation route, based on the attraction of oppositely charged core and shell particles.TEM micrographs clearly show a homogeneous Al₂O₃ shell (originating from boehmite or γ-Al₂O₃ particles) around the ZrO₂ cores and in the other case, a ZrO₂ layer (originating from hydrothermally prepared ZrO₂) around the submicron Al₂O₃ cores. From PCS measurements, it can also be deduced that the cores are enwrapped by a shell and it is calculated that the thickness of the Al₂O₃ shell is about 30–35 nm and the ZrO₂ layer is approximately 80 nm. The coated powders are additionally characterized by XRD.     

Influence of surfactant addition sequence on the suspension properties and electrophoretic deposition behaviour of alumina and zirconia

A well-known polyelectrolyte salt, ammonium polymethacrylate (Darvan-C) is used to stabilise ethanol-based Al₂O₃ and Ce-ZrO₂ suspensions with butylamine addition. The sequence in which n-butylamine and Darvan-C are added to the suspension greatly affects the properties of the wet deposit obtained by electrophoretic deposition. To investigate this effect, electrical conductivity of the suspension and the shear rate dependence of its viscosity are investigated. When n-butylamine is added first, the equilibrium in the suspension is almost immediately reached and a plastically deformable wet deposit is obtained over a large n-butylamine/Darvan-C ratio. The suspension has a shear-thinning viscosity and the deformable deposit is characterised by a high solvent content, which allows the rearrangement of particles during drying. When Darvan-C is added before the n-butylamine, the wet deposit is smooth and rigid. The suspension has a lower viscosity and a near-Newtonian behaviour is observed. A similar behaviour is observed for Al₂O₃ and Ce-ZrO₂ suspensions. The green density of the dried deposits is not influenced by the addition sequence and higher green densities are obtained for Al₂O₃ when compared to Ce-ZrO₂.     

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.     

Stabilization of zirconia lamellae in rapidly solidified alumina–zirconia eutectic composites

In-situ fabrication of ceramic eutectic composites by rapid solidification of eutectic drops is a cheap and quick method compared to directional solidification or to multi-step fabrication methods of fiber reinforced/layered materials for high temperature use. Binary eutectic composites with a homogeneous periodic microstructure have been obtained by directional solidification of eutectic melts for many years, but typical solidification velocities used in directional solidification are limited to the range of cm/hour or, more recently, up to 15 mm/min. The present study aims to determine the effects of faster solidification rates on the structure of the alumina–zirconia binary composites obtained at higher growth rates by rapid solidification from eutectic melts in air or vacuum. A binary composite with zirconia stabilized in the high-temperature tetragonal form is presented. The stabilization of the tetragonal phase has not been observed before in bulk crystalline pellets of binary Al₂O₃–ZrO₂ eutectic composites.     

Particle-stabilized ultra-low density zirconia toughened alumina foams

Zirconia toughened alumina (ZTA) is one of the leading engineering ceramics; it is used in a wide range of components and products in applications for which high strength, high toughness, and high temperature stability are needed. The particle-stabilized direct foaming method has lately become a subject of particular interest. Nevertheless only a few studies on combining ZTA ceramics and particle-stabilized direct foaming have been reported. Therefore, in this study, ultra-low density ZTA foams having single strut wall thickness, cell size ranging from 80 μm to 200 μm, and above 90% porosity were successfully fabricated via the particle-stabilized direct foaming method. Valeric acid was used as particle surface modifier to render the particles partially hydrophobic, which stabilized the air/water interface of the ZTA foams. The sintered foams maintained compressive strength up to 8 MPa with porosity of 90%.     

Nacre-like alumina composites reinforced by zirconia particles

Nacre-like alumina is a class of bio-inspired ceramic composite manufactured by field-assisted sintering of green bodies made primarily of alumina platelets with an anisotropic microstructure. Here we investigate the addition of zirconia particles to enhance the mechanical properties of the composite. The resulting structure is a nacre-like anisotropic structure which features deflection and reinforcement during crack propagation. Monoclinic zirconia has no impact on the mechanical properties of the composite while tetragonal zirconia improves its fracture resistance properties. Both types of zirconia seem to slow down grain growth during sintering. The addition of zirconia stabilised in the tetragonal phase is thus a good option to obtain a composite with a fine microstructure and higher mechanical properties than a standard nacre-like alumina, with a flexural strength of 626 ± 39 MPa and a crack initiation toughness of 6.1 ± 0.6 MPa.m^0.5.     

CSL grain boundary distribution in alumina and zirconia ceramics

The distributions of general and coincidence site lattice (CSL) grain boundaries (GBs) in texture-free alumina and zirconia ceramics sintered at two different temperatures were investigated based on electron backscatter diffraction (EBSD) measurements. Results were compared with the distributions obtained from random 2D spatial models and with calculated random distributions reported in the literature. All alumina samples independent on sintering temperature show the same characteristic deviations of the measured general GB distributions from the random model. No such features can be seen in zirconia. The total fractions of CSL GBs in alumina and zirconia samples are clearly larger, for both sintering temperatures, than those observed in the random simulations. A general GB prominence factor, similar to the twin prominence factor for fcc metals, was defined to simplify the representation of the CSL GB content in zirconia. The observed deviations from the random model show no dependence on sintering temperature nor on lattice geometry. In alumina, however, the change in the CSL GB character distribution with sintering temperature seems to be crystallographically controlled, i.e. directly dependent on the orientation of the CSL misorientation axis.     

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.     

Effect of magnesium oxide on sol–gel spun alumina and alumina–zirconia fibres

Alumina fibres and alumina–zirconia fibres were prepared by sol–gel process. The starting materials used for the preparation of alumina and zirconia sol were aluminium-tri-isopropoxide and zirconium oxychloride respectively. Alumina sol and zirconia sol were mixed in definite proportions, so that the final composition contains 10 wt.% ZrO₂. Alumina and alumina–zirconia fibres were prepared from the alumina sol and mixed sol respectively. MgO was introduced in the fibres by adding MgNO₃ into the sol. Sintered alumina fibre has α-Al₂O₃ phase and alumina–zirconia fibre has α-Al₂O₃ and t-ZrO₂ phases. The phase transition to α-Al₂O₃ takes place at higher temperature in the alumina fibre with MgO, whereas the effect is less in alumina–zirconia fibre. The addition of MgO reduces the grain size and increases the tensile strength.     

Processing and properties of tape-cast alumina/zirconia laminates composites

In the present work, laminar ceramic structures formed by layers of alumina and partially stabilized zirconia were fabricated by water-based tape casting. Rheological, physical and mechanical properties of slurries and laminates were evaluated. The laminates consisted of stacked alumina and zirconia green tapes produced by thermopressing. Pyrolysis was carried out at 450 °C and sintering at 1500 °C. The alumina/zirconia laminates were studied for a better understanding of the formation behavior and crack propagation at the laminate interface. The flexural strength values of laminates depend on the stress state on their surface. The laminates with the highest amount of zirconia layers presented low strength values (6.7 MPa), while the laminates with more alumina layers had a higher strength level (57.7 MPa). This is because these laminates have alumina layers on the surface which are in a state of residual compressive stress.     

Deformation behaviour of iron-doped alumina

Compressive creep tests in air were carried out on 1 cat.% Fe-doped alumina at a temperature T=1400 °C. Iron doping affected the plastic deformation by different ways in relation with Fe²⁺ cations population. Fe²⁺ cations sped up the deformation rates. FeAl₂O₄ spinel precipitates were identified and they were found (i) to interact with alumina grain boundaries (ii) to limit the grain growth within a range of strain. The Fe²⁺ cations underwent oxidation and this resulted in the dissolution of the some precipitates and in the decrease of deformation rates. It was suggested that deformation sped up this evolution through mass transport and that time was not a dominating parameter.     

Electrochemical behaviour of thin films of Co/Al layered double hydroxide prepared by electrodeposition

Thin films of controlled thickness of Co/Al layered double hydroxide with different Co and Al percentages have been electrosynthesized by the cathodic reduction of a 0.03 M Co and Al nitrate solution. The electrochemical behaviour of the films deposited on Pt or ITO electrodes has been deeply studied in 0.1 M NaOH solution. Our findings demonstrate that as soon as a potential is applied in the anodic direction, the films undergo an irreversible change of phase.     

Sodalite zeolite as an alternative all-ceramic infiltrating material for alumina and zirconia toughened alumina frameworks

The purpose of this study was to determine the effects of synthesized sodalite zeolite infiltration achieved by a direct in-situ hydrothermal reaction followed by sintering process on the flexural strength and hardness of alumina and zirconia-toughened alumina (ZTA) frameworks. Ceramic core materials were prepared as disk-shaped specimens with 16 mm diameter and 1.2±0.2 mm thickness. The case-study group was synthesized sodalite zeolite-infiltrated alumina (IA-SOD) and synthesized sodalite zeolite-infiltrated ZTA (IZ-SOD); and the control group was glass-infiltrated alumina (IA-glass) and glass-infiltrated ZTA (IZ-glass). The biaxial flexural strength (piston-on-three-balls test) and Vickers microhardness were compared among groups (n=10 specimens in each group). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to investigate the structural characteristics of specimens at the fracture and cross-sectional surfaces. For both IA-SOD and IZ-SOD, the biaxial flexural strength exceeded the required value of 100–150 MPa as specified by ISO 6872(2015), indicating their potential as all-ceramic core materials. The flexural strengths and Vickers microhardness of IZ-SOD were respectively 324.7 MPa and 1162 VHN, while these values were measured 233.6 MPa and 1013 VHN for IA-SOD. The mechanical properties and microstructure of core materials have been advocated as crucial to the clinical performance of all-ceramic dental restorations. This investigation provides data regarding the flexural strength, hardness and microstructure of partially sintered alumina and ZTA frameworks with synthesized sodalite zeolite infiltration.     

Extrusion of alumina fibers using zirconia sol as binder

Zirconia sol was used as an extrusion aid for the preparation of alumina fibers. The amount of zirconia sol required for getting an extrudable slurry has been optimised. The only phase present in the dried and sintered fibers is -Al₂O₃. Sintering studies showed that a dense microstructure is formed at 1600 °C. SEM micrographs revealed intergranular fracture to be the predominant fracture mode. Tensile strength is the highest for fibers sintered at 1600 °C.