"Alumina" Surface Modification of Silicon Nitride for Colloidal Processing

Two different methods are used to coat silicon nitride particles with an alumina precursor to make Si₃N₄ behave like Al₂O₃ in aqueous slurries. The first method involves the precipitation of an aluminum hydroxycarbonate from dissolved Al(NO₃)₃ during the decomposition of urea. In the second method, dry silicon nitride powder is reacted with aluminum tri- sec -butoxide in hexane at room temperature. Both methods produce a coated powder in which the electrophoretic and rheological properties of aqueous slurries mimic those of alumina. When salt is added to slurries consisting of coated Si₃N₄ powder, all rheological evidence suggests the presence of a short-range repulsive potential that produces a weakly attractive particle network similar to that previously reported for Al₂O₃ powder. Although electrophoretic and rheological data showed that the coated powder behaved like Al₂O₃, consolidation data indicated that slurries of coated powder with added salt did not pack to high density. In addition, these bodies were not plastic as found for bodies consolidated from dispersed and salt-added Al₂O₃ slurries.     

Effect of Inclusions on Densification: I, Microstructural Development in an Al2O3 Matrix Containing a High Volume Fraction of ZrO2 Inclusions

The microstructural changes produced by large (38 to 53 μ m), single-crystal ZrO₂ inclusions (0, 0.09, 0.30 volume fractions, based on solid volume) within an Al₂O₃ powder matrix were detailed as a function of constrained densification. Composite powder compacts were produced by pressure filtration for conditions where the Al₂O₃ slurry was either flocced or dispersed. For both conditions, the ZrO₂ inclusions constrained densification. Microstructural observations for all composites revealed (1) the presence of cracks with large opening displacements between inclusions and (2) large density variations within the matrix. The cracks were most frequent at high volume fraction of inclusions in composites produced from flocced slurries and apparently originated during specimen preparation. Their large opening displacment was a result of matrix densification. Fewer cracks were observed in composites produced from dispersed slurries. Instead, these microstructures were dominated by large variations in matrix density, viz., dense regions surrounding low-density regions, not consitent with the initial packing density of the matrix powder. The denser regions were formed early in the densification schedule. The lower-density regions eventually developed into regions containing large, elongated voids as the Al₂O₃ matrix grains became larger with heat-treatment time. This pore enlargement process was shown to result from the disappearance of necks between originally sintered grains and appeared similar to the thermodynamic instability observed in thin films and constrained fibers.     

Colloidal Processing and Ionic Conductivity of Fine-Grained Cupric-Oxide-Doped Tetragonal Zirconia

CuO-doped tetragonal ZrO₂ (3-mol%-Y₂O₃-doped tetragonal zirconia, 3Y-TZ) green bodies were consolidated from zirconia slurries with Cu²⁺ by a pressure filtration method. The slurries were prepared by dispersing 3Y-TZ powder in a solution of [NH₄OH + NH₃NO₃] = 0.1 M at pH 11 and adding an appropriate amount of Cu(NO₃)·3H₂O solution. Green bodies with narrow pore-size distribution were obtained after cold isostatically pressing the pressure-filtrated bodies. Small amounts of CuO-doped samples were densified fully at 1200°C. The size of a grain of 0.16-mol%-CuO-doped 3Y-TZ sintered at 1200°C was 84 nm. Bulk and grain-boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO-doped 3Y-TZ was almost equal to the undoped one, but the grain-boundary conductivity decreased with CuO addition.     

Influence of Interparticle Forces on the Rheological Behavior of Pressure-Consolidated Alumina Particle Slurries

The stress relaxation behavior of alumina powder compacts, consolidated from aqueous slurries by pressure filtration, is reported. The interparticle forces were controlled prior to consolidation by changing the pH from 3 to 9 and by adding different amounts of salt (0.1 to 2.0 M NH₄Cl) to slurries maintained at pH 4. Disk-shaped bodies were rapidly compiessed to an axial strain of 2%, and the resulting stress relaxation data were monitored as a function of time. For bodies formed from dispersed slurries (pH ≤ 4 without added salt), the stress relaxation behavior consisted of an irreproducible mixture of plastic and elastic response. The initial stress and the stress retained afterlong relaxation periods were highest for bodies formed with flocced slurries (pH 9). For bodies formed with coagulated slurries (pH 4 with added salt), the initial stress increased with the addition of 0.1 to 0.5M salt, but the bodies behaved plastically, with more than 90% of the initial stress relaxing within a short period. These results are consistent with a shortrange, repulsive interparticle force that lowers the attractive force between particles. They also suggest that interparticle forces in consolidated bodies can be controlled in a way that should prove useful in preventing damage that occurs during processing and reshaping Operations.     

Pressure Sensitivity for Particle Packing of Aqueous Al2O3 Slurries vs Interparticle Potential

The relation between relative density and applied network pressure has been determined for aqueous Al₂O₃ slurries prepared with different interparticle potentials and consolidated by centrifugation. Attractive interparticle potentials were obtained by either changing the pH to the isoelectric point or adding an excess electrolyte. A range of centrifugal speeds produced consolidation pressures between 10^–3 and 10 MPa. At lower speeds, the density gradient was determined with an X-ray absorption technique. The maximum packing density (0.62 ± 0.02) was achieved for both dispersed and coagulated slurries at network pressures >0.5 MPa. At lower pressures, thepacking density of these slurries was pressure-dependent, where the coagulated (salt added) slurries had a relative density between that of the dispersed and flocced slurries. Slurries flocculated by adjusting the pH to the isoelectric point never reached the highest packing density at the largest pressure.     

Consolidation of Alumina—Zirconia Mixtures by a Colloidal Process

The relationship between the dispersion of colloidal powder particles in Al₂O₃–ZrO₂ suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al₂O₃ powder and ZrO₂ sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al₂O₃ and ZrO₂ layers were obtained from well-dispersed suspensions with pH values below about 4 and salt concentration of 0.0527 M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al₂O₃ and ZrO₂ agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO₂ agglomerates were distributed in an Al₂O₃ matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single-component suspensions. The ZrO₂ particles formed three-dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation.     

Uniformity of Al2O3-ZrO2 Composites by Colloidal Filtration

Dispersion states of aqueous composite Al₂O₃/ZrO₂ colloidal suspensions were studied by measuring particle size distribution as a function of pH. Mutual dispersion was achieved at pH values of 2.0 to 3.5. Consolidated composites formed by colloidal filtration reflected the uniformity of the colloidal state. The mean flexural strength (896 MPa) of the sintered compacts was 1.6 times that of bodies consolidated by isostatic pressing .     

Plastic-to-Brittle Transition of Saturated, Alumina Powder Compacts

Alumina slurries, in which different particle pair potentials were produced by adjusting ρH and salt concentration, were used to form cylindrical, consolidated bodies by pressure filtration, at applied pressures between 0.25 and 100 MPa. The mechanical properties of these bodies were investigated by uniaxial compressive loading at a specific rate. Saturated bodies formed from flocced slurries (ρH 9) were plastic at low relative densities (<0.54, formed at filtration pressures < 40 MPa) but were brittle (fractured prior to flow) at higher relative densities. Bodies formed from the dispersed slurries (ρH 4) were always brittle. When initially stressed, plastic bodies consolidated from the coagulated slurries (ρH 4, 5, and 6 with additions of NH₄C1) produced a stress-strain behavior characterized by a peak stress, followed by a much lower flow stress. The peak stress reduced to the flow stress upon several reloading cycles. The peak stress observed during the initial loading rapidly increased with consolidation pressure. These bodies exhibited a transition from plastic to brittle behavior at large consolidation pressures (∼65 MPa), with little change in relative density. It was reasoned that the plastic-to-brittle transition occurred for bodies formed from coagulated slurries when a sufficient fraction of the particles were pushed into their primary minimum to form a touching particle network. The reduction of the peak stress to the flow stress was reasoned to occur once the touching network was broken apart to reestablish the weakly attractive, but nontouching, network that existed in the slurry state. This can only occur when the fraction of particles in the touching network is less than that necessary for fracture. It was also noted that the flow stress for certain bodies formed from the coagulated slurry had a nearly identical flow stress as measured for a commercial, throwing clay.     

Development of Short-Range Repulsive Potentials in Aqueous, Silicon Nitride Slurries

The properties of aqueous, dispersed, silicon nitride slurries, with an isoelectric point of pH 5.5, can be changed with additions of NH₄CI. At pH 10 the effect of adding NH₄Cl is similar to that suggested by DLVO theory; namely, for concentrations .0.5 M , the viscosity vs shear rate behavior, the elastic modulus, and the relative packing density are identical to those for slurries prepared at the isoelectric point. On the other hand, the effect of salt on dispersed slurries prepared at pH 2 differs from the behavior implied by classic DLVO theory; i.e., measurement of the same properties showed that the attractive interparticle potential was much weaker relative to that produced at the isoelectric point. As previously reported for alumina slurries, the results suggest that a short-range, repulsive interparticle potential is developed in salt-added slurries prepared at pH 2 which prevents attractive particles in the slurry from touching and aids particle packing. The same short-range potential apparently is not developed with salt additions at pH 10. The difference between silicon nitride and alumina slurries is apparent when the slurries are consolidated. Bodies consolidated from any silicon nitride slurry are elastic (i.e., they fracture before they flow) unlike salt-added alumina slur-ries, which are plastic.     

Centrifugal Consolidation of Al2O3 and AI2O3/ZrO2 Composite Slurries vs Interparticle Potentials: Particle Packing and Mass Segregation

Dispersed ceramic slurries with high concentrations of indifferent electrolyte have been shown to effectively eliminate mass segregation effects in composite materials. Furthermore, high packing efficiencies have been achieved with these systems. It has been suggested that the addition of high concentrations of indifferent electrolytes to an otherwise dispersed ceramic slurry modifies the interaction potential so that the short-range adhesive attraction is diminished by additional repulsive forces. This lower attractive force is still high enough to cause coagulation which raises the viscosity, but small enough to allow easy particle rearrangement during filtration or centrifugation. This rearrangement leads to high packing densities. The effectiveness of this behavior on the improvement of ceramic powder processing of composites is demonstrated with Al₂O₃ and ZrO₂.     

Fabrication of Stable Al2O3 Slurries and Dense Green Bodies Using Wet Jet Milling

A wet jet milling process was used as a novel method to prepare Al₂O₃ slurries. The wet jet-milled slurries showed very low viscosity compared with the ball-milled slurries. Moreover, the viscosity of the wet jet-milled slurries was constant for long times, whereas that of the ball-milled slurries increased rapidly with time. Al₂O₃ particles after wet jet milling retained initial surface conditions, although Al₂O₃ particles after ball milling yielded more OH groups on the surface. Casting rate was sensitive to the solid content and preparation method of slurry. The relative density of the green bodies prepared from the wet jet-milled slurries was about 65% or more and was independent of the slurry solid content. On the other hand, the relative density of the green bodies prepared from the ball-milled slurries increased with increasing solid content and was higher than 60% at the solid content of 50% by volume. Linear shrinkage of the sintered bodies prepared from the wet jet-milled slurries was very low and independent of the solid content of the slurry whereas that of the sintered bodies prepared from the ball-milled slurries increased with decreasing solid content.     

Processing-Related Fracture Origins: III, Differential Sintering of ZrO2 Agglomerates in Al2O3/ZrO2 Composite

Large, hard ZrO₂ agglomerates remained in an Al₂O₃/ZrO₂ composite suspension after inefficient ball-milling. The ZrO₂ agglomerates shrank away from the consolidated Al₂O₃/ZrO₂ powder matrix during sintering, producing crack-like voids which were responsible for strength degradation.     

Phase Relations in the System ZrO2-Y2O3 at Low Y2O3 Contents

Subsolidus phase relations in the low-Y₂O₃ portion of the system ZrO_2-Y₂O₃ were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y₂O₃ is soluble in monoclinic ZrO₂, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y₂O₃ below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y₂O₃ from ∼500° to ∼1600°C. At higher Y₂O₃ compositions, cubic ZrO₂ solid solution occurs.     

Thermally Induced Reversible Coagulation in Ceramic Powder-Polymer Liquid Suspensions

It was observed that slurries of oxide powders in oxidized polybutene fluids can be caused to change reversibly between fluid, nearly Newtonian behavior and plastic behavior by modest changes in temperature. This phenomenon was believed to result from changes in the dispersion vs association among the particles. The rheological effects of temperature, polymer oxidation, and particle size were observed for 30 vol% slurries of TiO₂, Al₂O₃, and ZrO₂ powders in polybutene fluids. Elasticity (in oscillation) and low-shear-rate viscosity (in steady shear) were observed to increase with increasing temperature for TiO₂ and Al₂O₃ particles in oxidized polybutene fluids. This behavior was attributed to the creation of interparticle structures. The attainment of this structure on heating was observed to be inhibited by increased oxidation of the polymer and increased particle size. It was concluded that the adsorption of oxidized molecules from the polymer liquid, along with the high viscosity of the bulk polymer, resulted in suspensions that were metastable against coagulation. Increased temperature resulted in lower viscosities of the liquid, allowing coagulation on a short time scale. The presence of the adsorbed polymer, however, prevented intimate contact among the particles so that the coagulated structure was easily destroyed upon subsequent cooling and shearing.     

Aqueous Colloidal Processing of ZTA Composites

Two different zirconia-alumina composites, ZTA-30 (70 wt% Al₂O₃+30 wt% ZrO₂) and ZTA-60 (40 wt% Al₂O₃+60 wt% ZrO₂), with potential for orthopedic applications, were processed in aqueous media and consolidated by slip casting (SC), hydrolysis-assisted solidification (HAS), and gelcasting (GC) from suspensions containing 50 vol% solids loading. For comparison purposes, the same ceramic compositions were also consolidated by die pressing of freeze-dried granules (FG). In the HAS process, 5 wt% of Al₂O₃ in the precursor mixture was replaced by equivalent amounts of AlN to promote the consolidation of the suspensions. Ceramics consolidated via GC exhibited higher green (three-point bend) strengths (∼17 MPa) than those consolidated by other techniques. Further, these ceramics also exhibited superior fracture toughness and flexural strength properties after sintering for 1 h at 1600°C in comparison with those consolidated by other techniques, including conventional die pressing (FG).     

Tensile Ductility in Zirconia-Dispersed Alumina at High Temperatures

High-temperature plastic flow in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) has been examined at temperatures between 1400° and 1500°C. Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) exhibits much higher flow stress and smaller tensile elongation below about 1450°C than 0.1 wt% MgO-doped single-phase Al₂O₃. The suppression of grain growth with ZrO₂ dispersion into Al₂O₃ is not effective for improving the tensile ductility. The limited ductility in Al₂O₃-10 wt% ZrO₂ (2.5 mol% Y₂O₃) is associated with the increment of flow stress caused by ZrO₂. The ZrO₂ dispersion or segregation in Al₂O₃/Al₂O₃ boundaries suppresses the grain boundary sliding and hence results in the increased flow stress at high temperatures.     

Fabrication, Microstructure, and Mechanical Properties of Cr2O3/ZrO2(2.5Y) Composite Ceramics in the Cr2O3-Rich Region

Intimate mixtures of Cr₂O₃/ZrO₂(2.5Y) in the Cr₂O₃-rich region are produced at low temperatures from amorphous materials prepared by the hydrazine method. Spark plasma sintering (SPS) has been performed for 10 min at 1300°C and 30 MPa. Composite ceramics with homogeneously dispersed fine ZrO₂ (0.2 µm) give 99.8% of theoretical densities. Their mechanical properties are examined in connection with increased ZrO₂ content. A high fracture toughness of 9.3 MPam^1/2 and an excellent bending strength of 1290 MPa are achieved in the composite ceramics containing 50 mol% ZrO₂.     

Constrained Densification of Alumina/Zirconia Hybrid Laminates, I: Experimental Observations of Processing Defects

Various forms of damage were observed in pressure-less-sintered Al₂O₃/ZrO₂symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO₂ layers, Al₂O₃ edge-effect cracks parallel to the layers, delamination in the Al₂O₃layers, and debonding between the Al₂O₃and ZrO₂layers. Based on detailed microscopic observations, the defects were attributed to sintering rate and thermal expansion mismatch between the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sintering or acted as preexisting flaws for thermal expansion mismatch cracks. Consequently, the extent of cracking could be reduced or even eliminated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooling rates or by adding Al₂O₃in the ZrO₂layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric laminates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al₂O₃and ZrO₂and were consistent with the dilatometric data that were obtained for the component layers.     

High-Strain-Rate Superplasticity in Y2O3-Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High-strain-rate superplasticity is attained in a 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP) dispersed with 30 vol% MgAl₂O₄ spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10⁻²– 3.3 × 10⁻¹ s⁻¹. The flow behavior and the microstructure of this material indicate that the MgAl₂O₄ dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO₂ matrix and from stress relaxation due to the dispersed MgAl₂O₄ grains.     

Phase Equilibrium and Martensitic Transformation in Lanthana-Doped Zirconia

The system ZrO₂-La₂O₃ has been studied in the 0 to 15 mol% La₂O₃ range using X-ray diffraction and thermal analysis. Two kinds of diagrams were elaborated: First is a phase equilibrium diagram where the eutectoid decomposition of ZrO_{2, ss} (T) → ZrO_{2, ss} (M) + Py _ss occurs at 1100°C and 0.75 mol% La₂O₃ (Py is the pyrochlore compound Zr₂La₂O₇). The maximum solubility of La₂O₃ in ZrO_{2, ss} (M) is 0.5 mol% at room temperature. Second, a nonequilibrium diagram is determined showing the correlations between grain size, La₂O₃ content, and the martensitic transformation temperature start ( Ms ).