Effect of the Interparticle Pair Potential on the Rheological Behavior of Zirconia Powders: I, Electrostatic Double Layer Approach

Aqueous slurries containing 20 vol% ZrO₂ powder doped with 3 mol% Y₂O₃ were prepared by first dispersing the powder at pH 11, then adding 0.1 M to 1.0 M tetramethylammonium chloride (TMACl), or 1.0 M of TPACl, CsCl, and LiCl to produce different, weakly attractive particle networks. The particle pair potentials in the slurries were investigated by viscosity versus shear rate measurements. Slurries exhibited increasing viscosities (at a given shear rate) with increasing salt concentration and decreasing (unhydrated) counterion size. The viscosities for these weakly attractive networks were intermediate to dispersed (pH 11 without added salt) and flocced (isoelectric point, pH 7.5) slurries. Cylindrical bodies were consolidated from these slurries by pressure filtration at different applied pressures. The bodies consolidated from slurries formulated with TMACl had the highest packing densities relative to those consolidated from a flocculated slurry, but the relative densities were much lower than those achieved from bodies consolidated from a dispersed slurry. The plastic or brittle nature of these bodies was determined in uniaxial compression. Powder compacts consolidated from flocced slurries and slurries coagulated with 1 M TMACl, CsCl, and LiCl showed plastic behavior for filtration pressures ≤7.5 MPa. Results for ZrO₂ will be compared with those previously obtained for Al₂O₃, which produces plastic, consolidated bodies over a much broader range of slurry conditions.     

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 .     

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.     

Effect of Agglomeration on Mechanical Properties of Porous Zirconia Fabricated by Partial Sintering

Porous ZrO₂ ceramics were fabricated by compacting a fine ZrO₂ powder, followed by pressureless sintering. Two unidirectional pressures of 30 and 75 MPa were used to prepare the green compacts. The strength and the fracture toughness of porous ZrO₂ specimens sintered from the compacts prepared by 75 MPa were substantially higher than those by 30 MPa, especially for the specimens with low porosity. However, the corresponding Young's moduli were identical. This caused the strain to failure of these porous bodies to increase significantly with increasing compaction pressure. Microstructural analyses showed that a number of voids and small flaws existed in the green compacts prepared by the lower pressure, due to the agglomeration of fine ZrO₂ grains. It was revealed that the ZrO₂ agglomeration resulted in a localized nonuniform shrinkage and degraded the mechanical properties of porous ZrO₂ ceramics.     

Powder Processing for the Fabrication of Si3N4 Ceramics: I, Influence of Spray-Dried Granule Strength on Pore Size Distribution in Green Compacts

The effect of spray-dried granule strength on the micro-structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si₃N₄ powder made with ultrafine A1₂O₃ and Y₂O₃ powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray-drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.     

Biomorphic Silicon Nitride Ceramics with Fibrous Morphology Prepared by Sol Infiltration and Reduction–Nitridation

Biomorphic silicon nitride (Si₃N₄) ceramics with fibrous morphology were fabricated by combining sol–gel infiltration with carbothermal reduction nitridation from wood precursor. Y₂O₃-incorporated silica sol was used as the infiltrated solution to promote the formation of fibrous Si₃N₄ grain at 1600°C under high nitrogen pressure (0.6 MPa). The influence of sintering conditions (additive and temperature) on the phase composition and microstructure of sintering bodies was analyzed, and the reaction mechanism is discussed.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Microstructural Development, Densification, and Hot Pressing of Celsian Ceramics from Ion-Exchanged Zeolite Precursors

Dense monoclinic celsian ceramics (melting point of 1760°C) have been fabricated utilizing zeolite precursors. A sodium zeolite (Na₈₆Al₈₆Si₁₀₆O₃₈₄H₂O) was ion-exchanged in aqueous solutions to replace Na with Ba ions. The ion-exchanged powders were then heat-treated to effect the collapse of the zeolite structure and formation of an amorphous phase at 627°C, followed by crystallization of the celsian ceramic at 990°C. Inducing viscous flow from a thermal soak above the glass transition temperature was necessary to form a dense body from cold-pressed powders. Hot pressing at a pressure >5 MPa and above the crystallization temperature resulted in densities >90% of theoretical and eliminated the necessity of adding seed particles to form monoclinic celsian. To fabricate shaped bodies, the amorphous phase was molded at a temperature just above the glass transition temperature and then crystallized to monoclinic celsian at 1050°C. This processing technique demonstrates the potential of using zeolites as precursors for the low-temperature fabrication of shaped refractory parts.     

Synthesis of Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet from Amorphous Powders

Amorphous powders of Al₂O₃—37.5 mol% Y₂O₃ (yttrium aluminum garnet (YAG)) were prepared by coprecipitation, decomposed at 800°C, and hot-pressed uniaxally at low temperature (600°C) and a moderate pressure (750 MPa). Optimum conditions yielded microstructures with only 2% porosity and partial crystallization of YAG. Further processing using high quasi-hydrostatic pressure (1 GPa) at 1000°C enabled the production of fully crystallized YAG with >96% relative density and a nanocrystalline grain size of ∼70 nm.     

Hot Isostatic Pressing of Chromium Nitrides (Cr2N and CrN) Prepared by Self-Propagating High-Temperature Synthesis

Cr₂N, CrN, and their mixtures (with desired fractions) have been prepared via self-propagating high-temperature synthesis (SHS) under a controlled nitrogen pressure, followed by hot isostatic pressing at 1300°C and 196 MPa under an atmosphere of argon gas. The combustion temperature increased as the nitrogen pressure increased. Single-phase Cr₂N was formed at 1040°C under a pressure of 0.18 MPa, and single-phase CrN was formed at 1730°C under a pressure of 2 MPa. The mechanical properties of the dense nitride ceramics (99.2% of the theoretical density) have been examined; the Vickers hardness (11.2 GPa for CrN and 14.5 GPa for Cr₂N) increased linearly as the fraction of Cr₂N increased, whereas the fracture toughness (∼4.7 MPa·m^1/2) and bending strength (∼355 MPa) are constant, regardless of the fraction of Cr₂N/CrN.     

Synthesis of Zirconium Titanate-Based Materials by Colloidal Filtration and Reaction Sintering

Zirconium titanate exhibits crystallographic anisotropy in thermal expansion, which makes it a suitable candidate for low thermal expansion materials. In this work, zirconium titanate has been synthesized by reaction sintering the green bodies, which have been obtained by colloidal filtration of concentrated suspensions of yttria-tetragonal zirconia polycrystals (Y–TZP) and titania. Powders were mixed in a 50/50 mol% ratio (ZT50) to obtain pure zirconium titanate. Rheological characterization of the suspensions has allowed the establishment of optimum green processing conditions. Sintering has been performed at 1400°C for 2 h, and the obtained materials have been characterized by X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray microanalysis. The ZT50 material has Zr₅Ti₇O₂₄ as the major phase, although Y₂ ((Zr_0.3Ti_0.7)₂O₇) and unreacted Y–TZP can still be detected.     

Rheology and Particle Packing of Chem- and Phys-Adsorbed, Alkylated Silicon Nitride Powders

Three alcohols with extended carbon chain lengths between ∼1 and 2 nm were chem-adsorbed on a Si₃N₄ powder by reacting with hydroxyl surface groups at temperatures 200°C. Slurry rheology, particle packing density, and body rheology were determined for toluene and dodecane slurries formed with these chem-adsorbed powders. These same properties were determined for slurries where the alcohol was simply added, but not reacted with the powder (phys-adsorbed powders). The viscosities of chem-adsorbed slurries are shear-thinning with longer chains producing lower viscosities at a given shear rate. The relative density of powder compacts produced by pressure filtration (10 MPa) was high (∼0.60) for octadecanol and dodecanol-reacted powders, and lower (∼0.50) for the octanol-reacted powder. When a sufficient amount > 10 times that required for chem-adsorption) of the same alcohol was simply added to the unreacted Si₃N₄ slurry system, the phys adsorbed slurries exhibited similar rheological behavior as the chem-dsorbed slurries, but, unlike chem-adsorbed slurries, their packing densty was lower, and their slurries were destabilized by water vapor. Stress relaxation experiments showed that bodies formed with the octadecanol chem-adsorbed powders were plastic after consolidation, whereas phys-dsorbed bodies were brittle (fractured before flow). All evidence suggests that the short-chained alkyl groups are steric "stabilizers" at small interparticle distances and thus prevent the particles from making surface-surface contact in common organic liquids; i.e., they produce a short-range interparticle, repulsive potential. Chem-adsorbed molecules, but not phys-adsorbed molecules, persist during particle packing and in moist environments.     

Coagulation Method of Aqueous Concentrated Alumina Suspensions by Thermal Decomposition of Hydroxyaluminum Diacetate

Consolidation of aqueous concentrated suspensions was used to shape alumina green bodies because it enabled us to obtain complex-shape components with accurate sizes. A high state of alumina particle dispersion was achieved by using (HO)₂C₆H₂(SO₃Na)₂ (Tiron), which allowed us to obtain stable alumina suspensions at pH 9 with a powder concentration higher than 60 vol%. The addition to the suspension of hydroxyaluminum diacetate, (CH₃CO₂)₂AlOH, which decomposed as the temperature increased, permitted us to coagulate an alumina suspension dispersed with Tiron efficiently. Adsorption measurements, electrokinetic mobility, and the rheological behavior of the suspensions provided useful methods to characterize each processing stage. Dense green bodies with sufficient cohesion could be demolded and dried, demonstrating that the dispersant and the flocculant agent chosen permit one to optimize the direct coagulation casting processing of alumina components.     

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.     

Compaction and Pressureless Sintering of Zirconia Nanoparticles

Four nanometer-sized zirconia powders stabilized by 3 mol% Y₂O₃ were used for the preparation of dense bulk ceramics. Ceramic green bodies were prepared by cold isostatic pressing at pressures of 300–1000 MPa. The size of the pores in ceramic green bodies and their evolution during sintering were correlated with the characteristics of individual nanopowders and with the sintering behavior of powder compacts. Only homogeneous green bodies with pores of <10 nm could be sintered into dense bodies (>99% t.d.) at a sufficiently low temperature to keep the grain sizes in the range <100 nm. Powders with uniform particles 10 nm in size yielded green bodies of required microstructure. These nanoparticle compacts were sintered without pressure to give bodies (diameter 20 mm, thickness 4 mm) with a relative density higher than 99% and a grain size of about 85 nm (as determined by the linear intercept method).     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

Colloidal Processing and Mechanical Properties of Whisker-Reinforced Mullite Matrix Composites

Aqueous colloidal suspensions in the two systems of CVD-processed ultrafine mullite powder (<0.1 μm), -Si₃N₄ whisker and -mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze-dried porous green compacts were hot-pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N₂ atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si₃N₄ whisker increased the density, flexural strength, and fracture toughness of the hot-pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.     

In Situ Processing and Properties of SiC/MoSi2 Nanocomposites

A novel concept for in situ processing of SiC/MoSi₂ nanocomposites has been developed that combines the pyrolysis of MoSi₂ particles coated with polycarbosilane and subsequent densification by hot pressing. After densification, a uniform dispersion of SiC particles is obtained in the MoSi₂ matrix. The strength at both room and elevated temperature is dramatically improved by the processing protocol employed. The average room-temperature flexural strength measured for the SiC/MoSi₂ nanocomposite was 760 versus 150 MPa for unreinforced MoSi₂. The average 1250°C flexural strength measured for the SiC/MoSi₂ nanocomposite was 606 versus 77 MPa for unreinforced MoSi₂.     

Strength of Green Ceramics with Low Binder Content

Acrylic-based polymers are common binders that impart high green strength (>2 MPa) at low concentrations (<5.0 vol%). Strength at low binder concentrations may be determined by chemical bonding at the ceramic–polymer interface. We have studied the binding mechanisms as a function of ceramic surface chemistry using a cross-linkable binder, which is based on a soluble poly(acrylic acid) (PAA, MW = 60 000) and glycerol. The cross-linked PAA binder system has been integrated into a solid freeform fabrication process, which provides a means of fabricating very reproducible green bodies, including SiO₂, TiO₂, Al₂O₃, multicomponent oxides, and non-oxides, with uniform density and composition. The ceramic parts contain only 2.5 vol% binder (solids basis), which increases the strength of the ceramic systems by at least a factor of 8 while the strength of Al₂O₃ components increases by a factor of ∼24 (0.3 to 7.6 MPa). Addition of the binder improves the toughness of the ceramic bodies by an order of magnitude with SiO₂ representing the largest relative increase (2.8 × 10⁻³ to 4.4 × 10⁻² MPa·m^1/2). The mechanical properties are dictated by two binding mechanisms: binder adsorption and mechanical interlocking. High green strengths result from adsorption of the binder onto the ceramic surface whereas toughness is enhanced by poor adhesion of the binder to the ceramic surface.     

High-Temperature Strength of Liquid-Phase-Sintered SiC with AlN and Re2O3 (RE = Y, Yb)

Using AlN and RE₂O₃ (RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot-pressing and subsequent annealing under pressure. The ceramics had a self-reinforced microstructure consisting of elongated α-SiC grains and a grain-boundary glassy phase. High-temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er₂O₃. SiC ceramics with AlN and Y₂O₃ showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er₂O₃ the best strength (∼550 MPa) at 1600°C.