Alumina Zirconia Ceramics Research Articles

One-step synthesis and low-temperature rapid sintering of alumina-zirconia ceramics by flash sintering combined with phase transformation

High-performance alumina-zirconia ceramics are generally sintered above 1500 °C and consume lots of energy; rapid sintering at low temperature is an ideal technique due to its economy and low-energy consumption. For the first time, flash sintering combined with phase transformation-assisted sintering techniques was reported in this work. The alumina-zirconia amorphous precursor powder was used to replace the final product powder for ceramic processing, and the one-step synthesis and rapid sintering densification of alumina-zirconia ceramics at low temperature of 849 °C for 60 s were realized. The effects of electric field on the low-temperature sintering of amorphous powders are summarized into three aspects. Firstly, the electric field reduces the viscosity of the amorphous powder and promotes liquid phase sintering. Secondly, the electric field promotes the phase transition of the amorphous powder from metastable to stable phase, and accelerates the atomic migration. Thirdly, Joule heating and defect generation in the flash sintering promotes more effective material transport. Overall, this work reduces the energy for calcination of amorphous powders, reduces the sintering temperature of ceramics, well hinders the grain coarsening and shows excellent mechanical properties.

Features of calcium hexaaluminate formation in alumina-zirconia ceramics

Alumina-zirconia composites containing calcium hexaaluminate in the amount from 0 to 15 wt.% were investigated. The materials were obtained by water dispersion, granulation, axial pressing, and free sintering. Density and open porosity were determined by the hydrostatic weighing method. Phase analysis was performed using synchrotron radiation. Structural investigations were conducted using scanning and transmission electron microscopy. Vickers hardness was determined at a load of 10 kg. Fracture toughness was determined by the indentation method. With increasing CaAl12O19 content in the composites, the relative density decreased from 98.5% to 91.8%, and the open porosity increased from 0.2 to 1.4%. The lattice parameters of t-ZrO2 crystal lattice did not change up to 12 wt.% CaAl12O19, and the degree of tetragonality was 1.435. The degree of tetragonality decreased for the material with 15 wt.% CaAl12O19 and reached 1.420. The lattice parameters of CaAl12O19 decreased with increasing content. Platelet size increased with increasing calcium hexaaluminate content. For the materials containing up to 9 wt.% CaAl12O19, the average length of the platelets was 2 μm, the width was 0.4 μm, and the aspect ratio was 5. For the material with maximum calcium hexaaluminate content, the average length of the platelets was 4.2 μm, the width was 0.6 μm, and the aspect ratio was 7. With increasing CaAl12O19 content, the hardness decreased from 1700±25 to 1390±30 Hv, and the critical stress intensity factor increased by 34% to 6.7±0.3 MPa·m1/2.

Multiscale modelling of the thermoelastic properties of alumina-zirconia ceramics for 3D printing

Additive manufacturing appears to facilitate the accurate manufacturing of alumina-zirconia technical ceramics. Nevertheless, the fine tuning of the manufacturing of these components by 3D printing requires an analysis of the parameters that influence their final thermoelastic properties. In this context, this work presents the application of (finite element-based) numerical procedures that aim at the prediction of the effective thermoelastic properties of 3D-printed alumina-zirconia ceramics. The numerical modelling considers three different scales: micro-, meso- and macroscale. The microscale corresponds to the microstructural level of, sintered at 1500 °C, slip-casted samples with different compositions of alumina-zirconia. On the other hand, the macroscale corresponds to the macrostructural level of porous lattice of 3D-printed ceramics, being defined at the mesoscale level by a periodic unit cell. Thus, an initial microstructural analysis (at microscale level) provides the influence of the alumina/zirconia ratio on the (macroscopically homogeneous and isotropic) material thermoelastic properties, which together with the definition of the geometry of a periodic unit cell (at mesoscale level), provides, by a second analysis (at both the meso- and macroscale levels), the coupled influence of material and geometry of the macrostructural lattice on the structural (macroscopically heterogeneous and anisotropic) thermoelastic properties. Moreover, experimental thermoelastic properties of the sintered slip-casted specimens were obtained for several alumina/zirconia ratios and analysed together with microstructure patterns. Prediction of the microstructural effective thermoelastic properties was also made using micromechanics and composite theory (analytical) models. All the numerical, experimental and analytical results for the microstructural level are presented and compared. Numerical results for the meso- and macrostructural levels are also presented.

Drilling of alumina and zirconia ceramics using diamond-coated carbide drill

The drilling characteristics and availability of a diamond-coated carbide drill for drilling holes in ceramics were investigated and verified. An alumina and yttria-stabilized zirconia ceramics were used as workpiece materials. The drilling characteristics were evaluated based on the cutting-edge behavior, thrust force, integrity of the inner surface of the drilled hole, and chip morphologies. Alumina drilling had appropriate drilling conditions under which the diamond coating did not flake; under these conditions, extremely long tool life was obtained. Even when the diamond coating flaked off, the thrust force was reduced owing to the sharp cutting edges on the remaining coating ridgelines on the flank face. This flaking occurred around the corner edge region and expanded toward the thinning and chisel edge region with an increase in the number of holes drilled. In addition, the diamond flaking on the rake face occurred earlier when the feed rate of the drill was increased. In drilling alumina, the surface roughness of the inner surface deteriorated as the feed rate increased but improved as the diamond coating flaked off. Whereas in drill zirconia, a considerably better inner surface was obtained, regardless of the flaking of the diamond coating and drilling conditions. In both ceramics, thinning the diamond coating from 20 to 10 μm resulted in premature flaking of the coating. The drill life was reached when the flaking region reached the chisel edge from the corner side. The results of this investigation will assist with the drilling of holes in brittle ceramics using a diamond-coated carbide drill.

Facile method for preparing lamellar structured alumina-zirconia ceramics with high toughness

This study describes a facile and effective method for fabricating alumina-zirconia composites with innovative lamellar structure as well as outstanding mechanical strength/toughness. To this end, the particle-stabilized foam method has been utilized to synthesize alumina ceramic foams built by single-layer pore wall firstly, providing the basic lamellar structure for the subsequently mixing with zirconia particles and hot-press sintering to achieve robust alumina-zirconia ceramics. The unique lamellar microstructure is composed of alumina sheets and zirconia grain layers alternated consistently, resulting in superior fracture toughness of 9.38 MPa m1/2, high flexural strength of 1.01 GPa and Vickers hardness of 17.04 GPa at the relative density of 99.63%. This approach opens up new possibilities for creating alumina-zirconia ceramics with exceptional mechanical performance.

A novel method for fabricating brick-mortar structured alumina-zirconia ceramics with high toughness

The present work reports a novel and simple approach to prepare alumina-zirconia composites with superior toughness. Alumina microspheres were innovatively used as the raw materials, followed by coating zirconia and hot-pressing sintering to fabricate alumina-zirconia ceramics. The resultant ceramics are given a unique brick-mortar microstructure, in which the zirconia “mortar” layers continuously distribute around the alumina “brick” matrix, leading to outstanding fracture toughness of 7.34 MPa·m1/2 and high strength of 635.84 MPa when prepared with zirconia contents of 10 wt%. The major explanation could be ascribed to that crack tips in sintered samples tend to propagate along the zirconia “mortar” layer, accompanied by deflection and branching, which effectively improve the fracture toughness of composites. The uniformity and integrity of the brick-mortar structure could be well tuned by varying the amount of zirconia. This method has reference significance for the preparation of high toughness alumina-based multiphase ceramics.

Influence of strontium hexaaluminate percentage on the structure and properties of alumina-zirconia ceramics

In this paper, alumina-zirconia ceramics with different contents of strontium hexaaluminate are investigated. Materials were obtained by pressing granulated powder and following free sintering. An XRD analysis was carried out. The structure was studied using a scanning electron microscope. Vickers hardness was determined at a load of 10 kg. Fracture toughness was evaluated using the indentation method. The change of composites structure at different temperatures was analyzed. The growth of SrAl12O19 platelet length proceeds predominantly up to 1400 °C. The platelet length range from 0.30 μm to 1 μm. At a temperature of 1500 °C the platelets thickness increases. In materials sintered at 1520 °C for 5 h with an increase in the SrAl12O19 content, the relative density decreases, and the porosity of the materials increases. Increasing the strontium hexaaluminate content in sintered materials decreases the alumina grain size. With strontium hexaaluminate content of 15 wt %, the average grain size of the alumina component decreases by 19% compared to unaided alumina ceramics (from 0.74 ± 0.06 to 0.60 ± 0.04 μm). With increasing SrAl12O19 content, the length of the platelets increased one and a half times from 0.80 μm to 1.20 μm. The width of the platelets remained essentially unchanged. Formation of strontium hexaaluminate platelets in materials and increase of its content contribute to increasing of fracture toughness of alumina-zirconia ceramics. The maximum level of fracture toughness is recorded at 15 wt % strontium hexaaluminate and is 7.7 ± 0.3 MPa*m1/2. The hardness of the composite with 15 wt % SrAl12O19 is 18% lower than alumina zirconia ceramics and is ∼1400 HV. Mechanisms for increasing the fracture toughness of alumina-zirconia ceramics depend on the strontium hexaaluminate content. At hexaaluminate strontium content up to 6 wt %, bending, platelet fracture, and bridging mechanisms are observed. At 9 and 12 wt % SrAl12O19 deflection mechanisms, crack branching and platelets fracture were detected. At 15 wt % SrAl12O19 the crack deflection is observed.

Grain growth during capsule-free hot isostatic pressing

This paper provides a new insight to the capsule free hot isostatic pressing (HIP) of alumina and yttria-stabilized zirconia ceramics. The first part of the paper focuses on the pre-sintering stage, namely the impact of conventional (2, 5, 10 °C/min) and rapid heating rate (100 °C/min) on pore closure and grain growth. It is shown that the rapid heating neither changes the pore closure behavior nor restricts the grain growth compared to conventional pre-sintering. The second part investigates the influence of temperature, dwell time, and applied pressure on the microstructure development during HIP. Our findings indicate the existence of a kinetic window, providing the optimal HIP conditions (1200–1250 °C, 200 MPa, 1–4 h for investigated powders) for production of almost fully dense compacts with only minimal grain growth. Our results also show that the applied pressure (up to 200 MPa) does not facilitate the grain boundary mobility of the investigated materials even at a higher temperature (1300 °C) at which the grain growth normally occurs. The potential of optimized capsule-free HIP was demonstrated by preparation of a transparent c-YSZ sample by a combination of rapid pre-sintering and optimized post-HIP.

Hardness and Indentation Fracture Toughness of Slip Cast Alumina and Alumina-Zirconia Ceramics.

Alumina (Al2O3) and zirconia (ZrO2) have good overall properties and thus are widely used oxide technical ceramics. The biggest drawback of Al2O3 is its low fracture toughness. In contrast, ZrO2 is relatively tough, but is also much more expensive. In order to improve the alumina toughness, composite ceramics are being developed. Slip casting technology has economic advantages over the conventional hot isostatic pressure technology, but problems may arise when preparing stable highly-concentrated suspensions (slip) for filling the mold. The purpose of this study is to prepare aqueous suspensions using 70 wt. % α-Al2O3, with 0, 1, 5 and 10 wt. % of added t-ZrO2. Suspensions were electrosterically stabilized using the ammonium salt of polymethylacrylic acid, an alkali-free anionic polyelectrolyte dispersant. Also, magnesium oxide in form of magnesium aluminate spinel (MgAl2O4) was used to inhibit the abnormal alumina grain growth during the sintering process. Minimum viscosities were used as stability estimators, where an increase in ZrO2 content required adding more dispersant. After sintering, the Vickers indentation test was used to determine the hardness and the indentation fracture toughness from the measurement of the crack length. Also, the brittleness index (Bi, μm−1/2) was calculated from values of Vickers hardness and the Vickers indentation fracture toughness. It was found that with increasing ZrO2 content the fracture toughness increased, while the hardness as well as the brittleness index decreased. Zirconia loading reduces the crystallite sizes of alumina, as confirmed by the X-ray diffraction analysis. SEM/EDS analysis showed that ZrO2 grains are distributed in the Al2O3 matrix, forming some agglomerates of ZrO2 and some pores, with ZrO2 having a smaller grain size than Al2O3.

Wire-electrical discharge machinable alumina zirconia niobium carbide composites – Influence of NbC content

Alumina zirconia (ZTA) ceramics can be made electric discharge machinable by addition of a percolating network of an electrically conductive phase. In this study the influence of NbC content on the mechanical and electrical properties as well as the ED-machinability of ZTA-NbC ceramics containing 17 vol.% zirconia and 24–32 vol.% NbC were investigated. Samples were hot pressed from mixed and milled starting powders. Surface morphology and surface roughness of wire electrical discharge machined surfaces were studied by SEM and perthometry. Cutting speed was determined to benchmark the ED-machinability.Rising NbC contents progressively impede sinterability. Maximum strength, Young’s modulus and hardness were found at intermediate NbC contents. Conductivity evidently rises with NbC content, the cutting performance showed an adverse tendency. The surface quality of the materials was improved by increasing the content of conductive phase. Additional trimming operations can reduce the mean roughness of machined surfaces to 1μm.

Effects of SiC, SiO2 and CNTs nanoadditives on the properties of porous alumina-zirconia ceramics produced by a hybrid freeze casting-space holder method

Highly porous alumina-zirconia ceramics were produced by adding space-holder materials during freeze casting. To increase the strength of porous ceramics, different amounts of nanoadditives (silicon carbide-SiC, silica-SiO2, and multi-wall carbon nanotubes-CNTs) were added. Space-holder materials were removed by preheating, and solid samples were produced by sintering. Up to 68% porosity was achieved when 40% space-holder was added to the solid load of slurry. Wall thicknesses between pores were more uniform and thinner when nanoadditives were added. Compressive tests revealed that SiC nanoparticles increased the strength more than other nanoadditives, and this was attributed to formation of an alumina-SiC phase and a uniform distribution of SiC nanoparticles. Results indicated that by including 20% space-holder materials and 15% SiC nanoparticles, the density decreases by 33.8% while maintaining a compressive strength of 132.5MPa and porosity of 43.4%. Relatively low thermal conductivities, less than 3.5W/K-m, were measured for samples with SiC nanoparticles.

A failure scenario of ceramic laminates with strong interfaces

Over the last years many researchers have put a lot of effort into designing layered structures combining different materials in order to improve low fracture toughness and mechanical reliability of ceramics. It has been proven that an effective way is to create layered ceramics with strongly bonded interfaces. Significant internal residual stresses are developed within the composite layers after the cooling process from the sintering temperature, due to the different coefficients of thermal expansion of individual composite constituents. Residual stresses can significantly change the crack behaviour. Suitable choice of material of layers and ratio between layer thicknesses can lead to higher value of the so-called apparent fracture toughness, i.e. higher resistance of the ceramic laminate to the crack propagation.The paper deals with a description of the specific crack behaviour in the layered alumina–zirconia ceramics. Attention is devoted to the differences in the stress field description in the vicinity of the crack front. Two-dimensional and three-dimensional numerical models are developed for this purpose. The main aim is to clarify crack behaviour in the compressive layer and provide computational tools for estimation of crack behaviour in the field of strong residual stresses. The crack propagation is investigated on the basis of linear elastic fracture mechanics. Fracture parameters are computed numerically and by routines of authors. The sharp change of the crack propagation direction is estimated using the Sih’s criterion based on the strain energy density factor, and conditions for crack bifurcation are determined. Estimated crack behaviour is qualitatively in a good agreement with experimental observations.

Energy aspects and workpiece surface characteristics in ultrasonic-assisted cylindrical grinding of alumina–zirconia ceramics

Ultrasonic assisted grinding is a novel method for improving the grinding process of difficult-to-cut materials. In the present research a novel setup has been designed and manufactured for utilizing ultrasonic vibrations in external cylindrical grinding. The designed ultrasonic head vibrates a rotating workpiece in axial direction. An alumina–zirconia ceramic (AZ90) has been selected as the workpiece material. Energy aspects and workpiece surface characteristics of ultrasonic assisted cylindrical grinding (UACG) and conventional cylindrical grinding (CG) processes have been analytically modeled and corresponding grinding experiments have been performed. The combined kinematics of the cylindrical plunge grinding process and axial ultrasonic vibrations provide a unique surface treatment conditions, which leads to reduced peak heights and increased valley depths of the surface topography. The main axial vibration mode provides the overlap of the adjacent cutting traces and consequently smoothens the surface topography in cylindrical plunge grinding. It has been analytically and experimentally shown that, applying ultrasonic vibrations, grinding energy can be reduced up to more than 35% depending on the process parameters. The surface characteristics of the ground workpieces have been investigated in terms of four surface roughness parameters and the roundness error.

Influence of nanopowders and pore–forming additives on sintering of alumma–zircorna ceramics

This work reviews the influence of nanopowders and pore forming additives on sintering and on properties of fine–pored alumina–zirconia ceramics. The optimum ratio of additives for ceramics' sintering, as well as the content of additives to obtain microporosity have been determined.

Investigation of the Properties of Alumina-Zirconia Ceramics

The article focuses on the investigation of the properties of alumina-zirconia ceramics possessing high mechanical characteristics and good conductivity at high temperatures. Measurement results of the dielectric dissipation factor, dielectric constant, electric conductivity when using direct and alternating current for the ceramics samples of 80%(ZrO2-3%Y2O3)-20% Al2O3 composition are presented in the paper. Measurements were conducted simultaneously in the electrostatic field in vacuum while heating the samples to the temperatures ranging from 300 to 1700K. Investigations showed that alumina-zirconia ceramics at high temperatures obtains ferroelectric properties not typical of these structures.

Influence of Technological Factors on Structure and Properties of Alumina-Zirconia Ceramics

The article is devoted to the investigation of alumina-zirconia ceramics properties depending on its production technology. Analysis of the technological factors that allow acquiring fine-grained structure as well as high physical and mechanical properties of alumina-zirconia ceramics is presented in the paper. It is shown that when using magnetic pulse compaction due to the pulse impact and adiabaticity of the process the most compact structure is formed. It is possible to obtain pressings with the density of up to 68% of the theoretical value. The obtained compact structure secures high physical, mechanical, and operating characteristics of the finished product.

Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods

Porous alumina–zirconia ceramics were obtained by infiltrating porous alumina ceramics, which were prepared by tert-butyl alcohol (TBA)-based gel-casting method. Back scattering images of the fracture surface and energy dispersive spectroscopy were performed to obtain composition profiles on the fracture surface and across sections of the sintered composites. The porosity, pore size distribution and compressive strength were also investigated. The results show that the content of zirconia can be adjusted effectively by infiltration times and it decreases with increasing distance from the surface of the samples. The porosity and compressive strength can also be controlled by the infiltration times. With increases of the infiltration times from 1 to 3cycles, the open porosity decreases slightly from 62.43% to 56.62%, while the compressive strength of the porous alumina–zirconia ceramics increases from 13.57±1.21 to 26.87±2.01MPa, indicating that the porous ceramics with high porosity and high strength can be prepared by TBA-based gel-casting method combined with the infiltration process.

Consolidation of nanoparticle suspensions by centrifugation in non-porous moulds

Alumina and zirconia nanosuspensions with a mean particle size of 100nm and 15nm, respectively, were consolidated by centrifugal compaction in non-porous moulds. The nanosuspensions consolidated by high-speed centrifugation were deposited irregularly, resulting in a powder deposit with density profile. The homogeneity of the powder deposits was described and homogeneous and well packed deposit regions were identified. Plate-like bodies were prepared from the homogeneous regions of the deposit. The advantage of regular and dense nanoparticle packing by centrifugal compaction was demonstrated by fabricating transparent alumina and tetragonal zirconia ceramics. The transparent alumina had an in-line transmission of 55% in the visible light at a thickness of 0.8mm. The transparent tetragonal zirconia reached a dense microstructure with an average grain size of 65nm and an in-line transmission of 25% at a thickness of 0.5mm.

Comparison of different zirconia powders for slip casting of alumina–zirconia ceramics

Two commercial 3 mol.-% yttria partially stabilised zirconia powders, 0·3 wt-%Al2O3 doped (Al doped Y-PSZ) and without Al2O3 (Y-PSZ), were used to produce alumina (Al2O3)–zirconia (ZrO2) slip cast composites. The effect of the substitution of Al2O3 and/or Y-PSZ by Al doped Y-PSZ on the rheological properties of concentrated aqueous Al2O3–ZrO2 slips was studied. In addition, the density of green cast samples was studied and related to the degree of slip dispersion. The viscosity and yield stress values of Al2O3 Al doped Y-PSZ suspensions were higher than those of Al2O3 Y-PSZ ones for all the compositions and solid loading studied. This behaviour could be explained by a larger effective size of the Al doped Y-PSZ particles, which resulted in a higher effective solid volume fraction and a lower amount of free liquid available for flow. The substitution of Y-PSZ by Al doped Y-PSZ in the Al2O3–ZrO2 mixtures resulted in a less dense packing of cast samples.

Microstructure Evolution in Sintering of Alumina-Zirconia Ceramics Simulated by a Modified Phase Field Method

The densification and grain growth during sintering of alumina-zirconia (Al2O3ZrO2) ceramics were simulated using a modified phase field method, which considered simultaneously a density field, a composition filed and orientation fields. The results indicate that the model can capture the main microstructure features in the different stages of sintering. A higher relative green density leads to a higher final density and a larger final grain size in the sintered ceramics. A higher volume fraction of the ZrO2 phase results in a lower relative density and a smaller final grain size.