Thermal shock of ß-alumina with zirconia additions

Sodium-alumina electrolyte tubes were prepared from spray dried precursor powders with additions of 2, 5, 10 and 15 wt % ZrO₂ made by the sodium zirconate route. The thermal shock behaviour of ring segments cut from the tubes was examined by quenching into water at 0° C. At the 2% ZrO₂ level the dispersed ZrO₂ particle size was low, <0.5m, and the tetragonal phase was retained. For higher weight fractions, particle coarsening during the-alumina sintering schedule was extensive and the large particles transformed to monoclinic on cooling. At the 15% ZrO₂ level the-alumina had a slightly reduced strength and high resistance to thermal shock and to thermal shock damage, quenching into water from 800° C effecting only a 43% reduction in strength. These observations are consistent with the effects of microcracking in the vicinity of second phase ZrO₂ particles.     

Effects of heteroflocculation of powders on mechanical properties of zirconia alumina composites

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (>50 vol% solids) had ZrO₂ content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabilized, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the non-transformable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to 99% theoretical density, the elastic properties, flexure strength and fracture toughness were examined at room temperature. Dynamic moduli of elasticity of fully deagglomerated compositions did not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made from submicron powders and with low content of dispersed phase (less than 10 to 20 vol %), the strength increased with increasing ZrO₂ content to a maximum of 1 GPa, irrespective of the degree of stabilization of t-ZrO₂. With increasing content of the dispersed phase (> 20 vol%), heteroflocculation of powder mixtures during wet-processing led to the formation of ZrO₂ grain clusters of increasing size. Residual tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO₂ phase transformation in final composites with transformable t-ZrO₂, but also led to lateral microcracking of ZrO₂/Al₂O₃ interfaces. This enhanced fracture toughness, but at larger ZrO₂ contents the flexure strength always decreased due to intensive microcracking, both radial and lateral. The important microstructural aspects of strengthening and toughening mechanisms in ZTA composites are related in discussion to the effects of heteroflocculation of powder mixtures during wet-processing.     

Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (tm) occurring in ZrO₂ precipitates in a Ca-PSZ alloy and particles in Al₂O₃-ZrO₂ composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al₂O₃-ZrO₂ alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.     

Polycrystalline t′-ZrO2(Ln2O3) formed by displacive transformations

ZrO₂ (3 mol% Y₂O₃) tetragonal and t-ceramics (displacively formed ceramics) were compared with ZrO₂ ceramics (3 mol% Ln₂O₃, where Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Er, or Yb) processed in an identical manner. Sintering at 1500 °C for 2 h produced mainly tetragonal polytypes for the dopants with smaller ionic radii than Dy(i.e., Er, Y and Yb) but when ZrO₂ was reaction sintered with dopants with larger ionic radii excessive monoclinic phase transformation and associated microcracking resulted. High-temperature annealing in the cubic stability regime and rapid cooling through the tetragonal stability regime was used to fabricate t-composites of ZrO₂ doped with Y, Yb, Er or Dy. Room-temperature fracture toughness and strength values are explained on the basis of a ferroelastic-cubic-to-tetragonal transformation. The domain structure was viewed by transmission optical microscopy (TOM) or transmission electron microscopy (TEM).     

Effect of ZrO2 inclusions on fracture properties of MgCr204

The effects of unstabilized ZrO₂ inclusions on the strength, fracture surface energy and thermal-shock resistance of MgCr₂O₄ have been evaluated. The fracture surface energy for MgCr₂O₄-ZrO₂ composites was observed to depend on the agglomerate particle size distribution, and volume fraction of the ZrO₂ inclusions. Large, nonuniformly distributed ZrO₂ inclusions tended to produce a relatively small increase in the fracture surface energy of MgCr₂O₄. The fracture surface energy increased with increasing ZrO₂ content to a maximum value of 24.5J m^–2 at 16.5 vol % ZrO₂, and decreased as the ZrO₂ content increased further. It is proposed that this four-fold increase in fracture surface energy results from the absorption of energy due to microcrack formation in the MgCr₂O₄ matrix, which results primarily from the tensile stresses due to the tetragonal monoclinic phase transformation of ZrO₂ and the associated volume expansion. The improvement in mechanical properties, specifically the four-fold increase in fracture surface energy, resulted in a substantial increase in thermal-shock resistance of MgCr₂O₄-ZrO₂ composites as indicated by the results of thermal-shock experiments.     

Spherulites and phase separation in plasma-dissociated zircon

Plasma-dissociated zircon consists of a mixture of monoclinic and tetragonal ZrO₂ in SiO₂ glass. The proportion of tetragonal ZrO₂ increases with decreasing initial zircon particle size and increasing cooling rate; zircon sprayed onto a cold substrate consists entirely of tetragonal ZrO₂ in glass. Tetragonal ZrO₂ is nucleated at large undercooling during cooling of molten zircon particles and grows in a spherulitic manner because of a low-diffusivity boundary layer at the growing crystal-liquid interface. At smaller particle sizes and higher cooling rates the thermal history of liquid droplets is such that an alternative process of phase separation into ZrO₂-rich and SiO₂-rich liquids becomes possible in the liquid remaining between spherulites. The particle size distribution of the ZrO₂ crystallites which form in this way shifts towards smaller particle sizes with increasing cooling rate and those smaller than 20 nm diameter do not transform to the equilibrium monoclinic form on cooling to room temperature because of a surface/ matrix restraint effect.     

Zirconium oxides formed by hydrolytic condensation of alkoxides and parameters that affect their morphology

When zirconium oxides are formed via hydrolytic condensation of zirconium alkoxides, the particle size and morphology of the resultant zirconia is strongly affected by certain parameters during the condensation. These parameters include: the type of alkyl group in the alkoxide, water/alkoxide ratio, molecular separation of species, and the reaction temperature. The particle size and the morphology in turn affect the sintering behaviour and crystalline transformation of ZrO₂, In this work the parameters that affect the formation of ZrO₂ from zirconium alkoxides are investigated. It has been shown that the alkyl groups and molecular separations during the hydrolytic polycondensation have particular significance in the modification of monoclinic tetragonal transformation of the resultant ZrO₂. Tetragonal phase can also be stabilized by copolymerization of ZrO₂ with SiO₂ producing tough ceramic materials.     

The mechanical and electrical properties of ZrO2-Naβ″-Al2O3 composites

ZrO₂-Na-Al₂O₃ composites were prepared by a conventional method using two different powder routes and different milling liquids. The retained tetragonal-phase ZrO₂ was 85 to 90% for composites with 2.4 to 15 vol% ZrO₂. The fracture toughness (K _lc) and strength increased with increasing ZrO₂ content. At 20 vol % ZrO₂,K _lc and bend strength were 4.35 M Pa m^1/2 and 390 MPa, respectively. Stress-induced transformation toughening is the predominant toughening mechanism. Dispersion toughening also contributes to the increase ofK _lc. Surface strengthening was found to be an effective strengthening method for low ZrO₂ levels. The critical tetragonal ZrO₂ grain size was found to increase from 0.86 to 1.02gmm as the ZrO₂ content increased from 2.5 to 15 vol %. A detailed study of the ionic conductivity of the 15 vol % ZrO₂ dispersed sample was conducted by an a.c. technique between –124° C and 300° C. The bulk and total conductivities were calculated via complex-plane analysis. The total (grain and grain-boundary) ionic specific resistivity was 9 cm at 300° C. The activation enthalpies of the bulk and total conductivity processes were 0.30 and 0.32 eV, respectively.     

Transformation toughening and grain size control inβ″-Al2O3/ZrO2 composites

In order to fabricate transformation-toughened-Al₂O₃ and optimize its mechanical and electrical properties it was found to be necessary to carefully control the particle size distribution of the starting powders and their mixing. The ionic resistivity of the composites depended primarily on the volume fraction of ZrO₂. Additions between 10 and 20 vol% produced materials with ionic resistivities (300° C) between 6 and 10 cm and eliminated exaggerated grain growth of the-Al₂O₃. Comparison of-Al₂O₃ composites containing either tetragonal (t-) ZrO₂ or cubic (c-) ZrO₂ with the single phase material showed that the major strengthening mechanism is the reduction in critical flaw size. This occurred by the elimination of the flaw population associated with abnormally large grains. For maximum increases in fracture toughness and strength, however, the use of t-ZrO₂ (transformation toughening) as a second phase is preferred.     

Processing parameter effects on the reaction bonding of aluminum oxide process*

The reaction bonding of aluminum oxide (RBAO) process was analyzed from the milling of the precursor mixtures to the sintering of the reaction bonded -alumina as a function of precursor powder composition, Al particle size, temperature, and heating rate. The RBAO process involves both solid-gas (T < T_M,AI) and liquid-gas (T > T_M,AI) oxidation of Al + -Al₂O₃ powder compacts. It has been demonstrated that maximum Al content of the precursor powder is limited to 60 vol %. In addition, it was observed that the initial Al particle size affects the oxidation behavior significantly and hence final properties of -Al₂O₃ compacts. Therefore, the initial Al particle size is very critical for the RBAO process. The critical Al particle size (i.e., the largest Al particle size can be used to obtain dense ceramic materials by the RBAO process) was determined as 1.5 m. It has been demonstrated that heating rate can be used to improve the final microstructures of RBAO ceramics. Although there is no large (>4 vol %) amount of ZrO₂ addition, alpha aluminum oxide ceramics with 97% TD have been produced by optimizing the processing parameters such as fine (<1.5 m) Al particles and slower heating rate during the liquid-gas oxidation.     

Effect of hydrolysis time and type of catalyst on the stability of tetragonal zirconia-silica composites synthesized from alkoxides

ZrO₂-transformation-toughened glass-ceramics have been prepared by the sol-gel process from either colloidal gels (CG) or polymeric gels from alkoxides (PG), using, in the latest method, a different catalyst (acid, basic and without catalyst), and different compositions in the ZrO₂·SiO₂ system (free ZrO₂ included). The different methods of synthesis of tetragonal zirconia in SiO₂ matrix were compared in order to obtain thermal stability, and a high amount of stabilized tetragonal zirconia. The effect of catalyst, hydrolysis time, chemical composition, critical particle size in ZrO₂(t)ZrO₂(m) transformation, the doping with vanadium on the ZrO₂(t) stability in SiO₂ matrix and Vickers microhardness of samples have also been studied.     

Toughened glass-ceramics in the ZrO2-Al2O3-SiO2 system prepared by the sol-gel process

Monolithic glass-ceramics containing Al₂O₃ or TiO₂ were prepared in the ZrO₂-SiO₂ system by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by heat treatment at 900–1200 °C and its crystal growth was increased by adding TiO₂ or Al₂O₃. Further heating at higher temperature resulted in the precipitation of zircon and monoclinic ZrO₂ which was transformed from tetragonal ZrO₂. The addition of Al₂O₃ had less effect on both the tetragonal-to-monoclinic ZrO₂ transformation and the precipitation of zircon. The fracture toughness increased as the size of tetragonal ZrO₂ particles increased and then decreased with the appearance of monoclinic ZrO₂ or zircon. The fracture toughness of the glass-ceramics was measured in the glass-forming regions of the ZrO₂-Al₂O₃-SiO₂ system. The fracture toughness was sensitively dependent on both Al₂O₃ and ZrO₂ content, of which the highest value achieved was 9 MPa m^1/2 for the 50ZrO₂·10Al₂O₃·40SiO₂ composition.     

Preparation of monodispersed Y-doped ZrO2 powders

3 mol% Y-doped ZrO₂ powders prepared by the controlled hydrolysis of metal alkoxides were monodispersed and grown to 0.5m after 5 h ageing. The as-prepared powder was amorphous and hydrate but transformed into a tetragonal single phase by heating. Furthermore, the Y₂O₃ concentration of each particle was almost the same in all particles. The synthesis conditions such as ageing time, ageing temperature and water concentration greatly affected the particle morphology. The refluxing of the alkoxide solution was particularly necessary to prepare the monodispersed particles. On the basis of the variation of size distribution with ageing time, the mechanism of particle growth was discussed.     

Preparation of ZrO2 coated graphite powders

Graphite powders were coated with ZrO₂ by the controlled hydrolysis of a zirconium oxychloride aqueous solution. Thehydrolysis process was carried out with temperature control because of the low wettability of ZrO₂ to the surface of thegraphite. PVA was added to the solution for the enhancement of Zr ion adsorption. The surface of the graphite particles coatedwith ZrO₂ was observed by TEM. There are two types of ZrO₂ particles; (a) primary particles a few nm in size and (b) secondary particles with 0.1 m size were obtained. The data of oxidation weight loss and surface potentialshow that the graphite surface was successfully modified by the forced hydrolysis of the zirconium chloride aqueous solution.     

The zeta-potential measurement for concentrated aqueous suspension by improved electrophoretic mass transport apparatus — application to Al2O3, ZrO3 and SiC suspensions

The zeta-potentials for Al₂O₃, ZrO₂ and SiC particles in concentrated suspensions were determined by an improved mass transport apparatus in order to obtain reliable and reproducible data on particle dispersion in the slip casting process. The values of isoelectric point for each particle were as follows: pH 8 for Al₂O₃, 5 for monoclinic ZrO₂, 6.5 for tetragonal ZrO₂, 4 for -SiC and 5.5 for -SiC. It was suggested that the values of zetapotentials for those particles were affected by the chemical and physical state of the particle surfaces, i.e. crystal structure, comtamination and hydration, etc. The apparatus used in this experiment will be also available for use with suspensions containing surfactants in practical slip casting.     

X-Ray diffraction study of tensile deformation in a bulk polycrystalline Ag-30 at. % Cd alloy

X-ray diffraction techniques have been used to study plastic deformation in a polycrystalline Ag-30 at.% Cd alloy under tensile load. The positions and shapes of all (hkl) reflections were recorded using a parafocusing arrangement up to a maximum true strain of 0.265. The effects on the peak displacements caused by stacking faults and by macroscopic strains normal to the surface were distinguished. The longitudinal true stress in the surface layer evaluated by least square analysis was smaller than the macroscopic flow stress by an approximately constant amount over the whole range of strain (in accord with previous observations of a stress gradient near a free surface); the apparent rate of work hardening in the surface was equal to that for the specimen as a whole. The stacking fault probability was approximately a linear function of strain and attained a maximum value of 7×10^–3.Fourier analyses were performed on the profiles of (111) — (222) and (200) — (400) pairs of reflections. The effective particle sizes D _e(111) and D _e(100) and the estimated true domain size D decreased approximately inversely with increasing strain, tending to limiting values at high strains of 220, 150 and 300Å respectively. Similarly, the microscopic strains [_L ²_hkl ^*]^1/2 tended to limiting values at high mechanical strains. The twin fault concentration was found to be negligibly small. The particle size and microstrain parameters were compared with values for cold-worked filings of the alloy. A plot of against _L=50Å ²₁₁₁ ^* for the solid specimens and for the filings was linear and yielded a stacking fault energy for Ag-30 at. % Cd of 6.1 ergs/cm².This work was supported by the Office of Naval Research     

Dispersion of ZrO2 particles in aqueous suspensions by ammonium polyacrylate

The effects of poly anionic-electrolyte (ammonium polyacrylate, PAA) as a dispersant on two kinds of ZrO₂ (monoclinic and yttria-doped tetragonal zirconia) aqueous suspensions were examined by the measurements of-potential and viscosity, the sedimentation test and the determination of the wet point and flow point of the powders. Additions above 2.5 wt% PAA to zirconia gave a negative high-potential above –30 mV, and then –45 and –30 mV were obtained for monoclinic and tetragonal zirconia above 5 wt% PAA, respectively. A high negative-potential above –30 mV was retained with 5 wt% PAA for a change in pH over a wider range (pH 6 to 10 for monoclinic ZrO₂, 7 to 9 for tetragonal ZrO₂) in comparison to that of ZrO₂ without dispersant. The increase of the-potential resulted in a decrease in the viscosity. The evaluation of dispersion by the sedimentation test was correlated well with the value of-potential and the viscosity of the suspensions. The presence of native positive charge of monoclinic and tetragonal zirconia powders required an excess amount of PAA to attain dispersion of the suspension. There was a small difference in the least amount of PAA required to attain good dispersion between monoclinic and tetragonal ZrO₂. The difference was also indicated by changes of the flow point on PAA addition. Addition of 0.1% PAA to monoclinic ZrO₂ and 0.25 wt% to tetragonal ZrO₂ gave a maximum value of the flow point, whereas the positive-potential fell to zero. Measurement of the flow point was a simple and useful technique for rapid evaluation of a required amount of dispersant for ZrO₂ suspensions.     

Effect of calcination on characteristics and sintering behaviour of Al2O3-ZrO2 composite powders

The effect of calcination on the characteristics and sintering behaviour of zirconia-toughened alumina (ZTA) composite powders has been investigated. TiO₂ was selected as an additive to promote the sinterability of ZTA powders. The starting materials were Al₂O₃ powder, Zr(OC₃H₇)₄ and Ti(OC₃H₇)₄, and homogeneous ZTA powder containing Zr-O-Ti bonding was prepared. Calcination affected the tetragonalmonoclinic phase transformation temperature of ZrO₂ crystallizing from the gels. Calcination improved the densification rate of ZTA powder compact during sintering, which was attributed to the optimal ZrO₂ particle size and distribution on the surface of alumina. A ZTA specimen with high bulk density and high tetragonal ZrO₂ content was obtained under the conditions of 850°C/1 h calcination and 1500°C/1 h sintering.     

Microstructural development of a ZrO2 Naβ″-Al2O3 composite

Microstructural development in Na-Al₂O₃ containing 15 vol% ZrO₂ particles is described. Intergranular ZrO₂ particles inhibit abnormal grain growth of the Na-Al₂O₃. The growth of the Na-Al₂O₃ grains and the intergranular ZrO₂ followed a cubic time law. Direct particle coalescence consisting of encounter and spheroidizing processes was found to be the basic growth mechanism for the intergranular ZrO₂     

Mechanochemical effects for some Al2O3 powders of dry grinding

Three kinds of Al₂O₃ powders, i.e. two kinds of low-soda Al₂O₃ with average particle sizes of 3.9 and 0.6 m and an electrofused Al₂O₃ with an average particle size of 21.8 m, were ground for up to 300 h in a dry vibration ball mill. Variations in particle-size distribution, specific surface area, crystallite size, lattice strain, effective temperature factor and lattice constant were examined against milling time. The mechanism of grinding was found to differ between low-soda Al₂O₃ and electrofused Al₂O₃. The mechanochemical effects on these Al₂O₃ powders occurred in the order decrease of crystallite size increase of effective temperature factor increase of lattice strain. The length of the a-axis was clearly increased by a prolonged grinding. The difference in the ground state of three specimens was attributed to differences in the physical state of particles originating from the preparation methods, and also to particle size.