Tape Casting of Lanthanum Chromite

The effects of process additives, ball milling, and solids loading were evaluated for tape casting suspensions of glycinenitrate-synthesized La_0.7Ca_0.31CrO₃ powder. An optimized formulation was obtained based on rheological characterization, electrokinetic sonic amplitude measurements, qualitative examination of green tapes, and the sintered microstructure. The tape casting formulation incorporated 66:34 methyl ethyl ketone/ethyl alcohol solvent, an aliphatic phosphate ester dispersant, and 80 wt% (35 vol%) solids. The best binder/plasticizer system was 12 wt% (15 vol%) poly(isobutyl methacrylate) and 5 wt% (6.3 vol%) benzyl butyl phthalate plasticizer (binder:plasticizer = 2.3). Cast tapes were sintered at 1300°C for 2 h, producing a bulk density of 96.2% theoretical, with linear shrinkage of 22% and an approximate grain size of 1.3 μm.     

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB₂) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB₂ with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB₂ with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB₂ was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.     

Dispersion Behavior of ZrB2 Powder in Aqueous Solution

Dispersion conditions of ZrB₂ powder in water were investigated using poly(ethyleneimine) (PEI) as a dispersant. Pulverization of ZrB₂ powder to submicrometer size was difficult and a substantial amount of large particles remained after an intensive planatery milling for 72 h. The isoelectric point (IEP) of ZrB₂ powder was measured to be pH 5.8 by electrophoresis, which shifted to pH 6.2 after milling. The application of PEI changed the IEP of the boride slurry to ∼pH 11. Well-dispersed aqueous ZrB₂ slurries with a high solid loading (up to 45 vol%) were fabricated at pH 6.5–7.5 by the application of 1.5 wt% PEI.     

Oriented Structure and Crystallography of Directionally Solidified LaB6—ZrB2 Eutectic

Directional solidification of LaB₆—ZrB₂, by use of an electron beam heating technique, yielded oriented ZrB₂ fibers in a LaB₆ matrix. The average diameter of the ZrB₂ fibers was ∼0.2–1.2 µm, with fiber lengths up to 100 µm. Primary platelike LaB₆ dendrites formed upon the solidification of an ingot with a composition of LaB₆—18 wt% ZrB₂. LaB₆ was the first phase to nucleate when eutectic growth occurred, and ZrB₂ showed nonfaceted growth. For the ingot solidified with planar growth the orientation relations of the phases were as follows: growth direction, [001]_LaB6∥[00.1]_ZrB2; interfacial plane, (11.0)_LaB6∥(11.0)_ZrB2.     

Pressureless Sintering of High-Density ZrB2–SiC Ceramic Composites

Ultra-high-temperature ceramic composites of ZrB₂ 20 wt%SiC were pressureless sintered under an argon atmosphere. The starting ZrB₂ powder was synthesized via the sol–gel method with a small crystallite size and a large specific surface area. Dry-pressed compacts using 4 wt% Mo as a sintering aid can be pressureless sintered to ∼97.7% theoretical density at 2250°C for 2 h. Vickers hardness and fracture toughness of the sintered ceramic composites were 14.82±0.25 GPa and 5.39±0.13 MPa·m^1/2, respectively. In addition to the good sinterability of the ZrB₂ powders, X-ray diffraction and scanning electron microscopy results showed that Mo formed a solid solution with ZrB₂, which was believed to be beneficial for the densification process.     

Colloidal Processing of Glass–Ceramics for Laminated Object Manufacturing

Green tapes of Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) parent glass were produced by aqueous tape casting as the starting material for the laminated object manufacturing (LOM) process. The rheological behavior of the powder suspensions in aqueous media, as well as the mechanical properties of the cast tapes, was evaluated. According to ξ potential measurements, the LZSA glass powder particles showed acid surface characteristics and an IEP of around 4 when in aqueous media. The critical volume fraction of solids was about 72 wt% (27 vol%), which hindered the processability of more concentrated slurries. The glass particles also showed an anisometric profile, which contributed to an increase in the interactions between particles during flow. Therefore, the suspensions could not be processed at high solids loadings. Aqueous-based glass suspensions were also characterized by shear thickening after the addition of dispersants. Three slurry compositions were formulated, suitable green tapes were cast, and tapes were successfully laminated by LOM to a gear wheel geometry. A higher tensile strength of the green tapes corresponded to a higher tensile strength of the laminates. Thermal treatment was then applied to the laminates: pyrolysis at 525°C, sintering at 700°C for 1 h, and crystallization at 850°C for 30 min. A 20% volumetric shrinkage was observed, but no surface flaws or inhomogeneous areas were detected. The sintered part maintained the curved edges and internal profile after heat treatment.     

High-Strength Zirconium Diboride-Based Ceramics

Zirconium diboride (ZrB₂) and ZrB₂ ceramics containing 10, 20, and 30 vol% SiC particulates were prepared from commercially available powders by hot pressing. Four-point bend strength, fracture toughness, elastic modulus, and hardness were measured. Modulus and hardness did not vary significantly with SiC content. In contrast, strength and toughness increased as SiC content increased. Strength increased from 565 MPa for ZrB₂ to >1000 MPa for samples containing 20 or 30 vol% SiC. The increase in strength was attributed to a decrease in grain size and the presence of WC.     

Dispersion of Zirconium Diboride in an Aqueous, High-Solids Paste

The dispersion of ZrB₂ particles was investigated. In aqueous systems, the surface of ZrB₂ consists of a thin layer of ZrO₂ that controls the surface chemistry and surface charge. Measurements showed that the ZrB₂ had an isoelectric point of pH=4.7 and a maximum ζ potential of −50 mV at pH=9. The addition of a dispersant, either an ionic ammonium polyacrylate or a nonionic alkoxylated polyether, increased the ζ potential of ZrB₂ by as much as 60 mV to −110 mV. Viscosity measurements were used to optimize dispersant concentrations. High-solids loading (∼45 vol.% ZrB₂) aqueous pastes were prepared with two different dispersants. The pastes had viscosities of 40–50 Pa·s which was acceptable for extrusion, and were used to fabricate three-dimensional components from ZrB₂.     

Effect of Fe and Cr Addition on the Sintering Behavior of ZrB2 Produced by Self-Propagating High-Temperature Synthesis

An investigation of the sintering behavior of ZrB₂ powder with Fe and Cr (0 to 20 wt%) addition was conducted. It was observed that Fe addition helps to enhance the density of ZrB₂ only up to 10 wt%. Further addition of Fe degrades the sintering by segregation of Fe-rich phases. Formation of a eutectic phase containing a Fe:Zr ratio of 92.57:7.43 was also found in Fe-added samples. The addition of Cr to a ZrB₂ matrix was found to result in swelling of the samples, leading to several cracks.     

Rheological Property and Stress Development during Drying of Tape-Cast Ceramic Layers

Rheological property and stress development of tape-cast ceramic layers derived from nonaqueous alumina (A1₂O₃)-poly(vinyl butyral) (PVB) suspensions were observed during drying. Casting suspensions exhibited strong shear-thinning behavior, with a low shear Newtonian plateau apparent viscosity >10² Pa^.s. The apparent suspension viscosity displayed a power-law dependence on the A1₂O₃ volume fraction during the initial stage of drying (<30% solvent loss). Stress development, measured by a cantilever deflection method, and parallel weight loss measurements were performed during the drying of tape-cast layers and pure binder coatings. Maximum drying stresses (σ_max) of 1.37-0.77 MPa were observed for plasticized tapes cast at gap heights of 150-400 μm. In contrast, nonplasticized tapes of similar thickness displayed a more gradual stress increase, with σ_max values approximately an order of magnitude higher than their plasticized counterparts. The stress histories of the corresponding binder coatings were quite similar to the tape-cast layers, albeit slightly lower σ _max values were observed. Stresses decayed beyond σ_max with a logarithmic time dependence to an almost constant value of 0.2-0.4 MPa for the plasticized tapes. Based on these observations, process methodologies have been offered to minimize stress development and retention in tape-cast ceramic layers     

Transmission Electron Microscopy Characterization of Hot-Pressed ZrB2 with MoSi2 Additive

Microstructure of the hot-pressed ZrB₂ with MoSi₂ additive was investigated by transmission electron microscopy (TEM). The effect of MoSi₂ addition on the microstructure of the ceramic was assessed. For the pure ZrB₂, the microstructure consisted of the equiaxed ZrB₂ grains and a few elongated ZrB₂ grains. For the ZrB₂ with MoSi₂ additive, the microstructure consisted almost entirely of equiaxed ZrB₂ grains. A few dislocations were present in the ZrB₂ grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB₂ grain boundaries in the ZrB₂ with MoSi₂ additive.     

Fabrication and Characterization of ZrB2-Based Ceramic Using Synthesized ZrB2–LaB6 Powder

ZrB₂–LaB₆ powder was obtained by reactive synthesis using ZrO₂, La₂O₃, B₄C, and carbon powders. Then ZrB₂–20 vol% SiC–10 vol% LaB₆ (ZSL) ceramics were prepared from commercially available SiC and the synthesized ZrB₂–LaB₆ powder via hot pressing at 2000°C. The phase composition, microstructure, and mechanical properties were characterized. Results showed that both LaB₆ and SiC were uniformly distributed in the ZrB₂ matrix. The hardness and bending strength of ZSL were 17.06±0.52 GPa and 505.8±17.9 MPa, respectively. Fracture toughness was 5.7±0.39 MPa·m^1/2, which is significantly higher than that reported for ZrB₂–20 vol% SiC ceramics, due to enhanced crack deflection and crack bridging near SiC particles.     

Pressureless Sintering of Zirconium Diboride Using Boron Carbide and Carbon Additions

The synergistic roles of boron carbide and carbon additions in the enhanced densification of zirconium diboride (ZrB₂) by pressureless sintering have been studied. ZrB₂ was sintered to >99% relative density at 1900°C. The combination of 2 wt% boron carbide and 1 wt% carbon promoted densification by removing surface oxide impurities (ZrO₂ and B₂O₃) and inhibiting grain growth. Four-point bending strength (473±43 MPa), Vickers' microhardness (19.6±0.4 GPa), fracture toughness (3.5±0.6 MPa·m^1/2), and Young's modulus (507 GPa) were measured. Thermal gravimetry showed that the combination of additives did not have an adverse effect on the oxidation behavior.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Pressureless Sintering of Zirconium Diboride

Zirconium diboride (ZrB₂) ceramics were sintered to a relative density of ∼98% without applied external pressure. Densification studies were performed in the temperature range of 1900°–2150°C. Examination of bulk density as a function of temperature revealed that shrinkage started at ∼2100°C, with significant densification occurring at only 2150°C. At 2150°C, isothermal holds were used to determine the effect of time on relative density and microstructure. For a hold time of 540 min at 2150°C, ZrB₂ pellets reached an average density of 6.02±0.04 g/cm³ (98% of theoretical) with an average grain size of 9.0±5.6 μm. Four-point bend strength, elastic modulus, and Vickers' hardness were measured for sintered ZrB₂ and compared with values reported for hot-pressed materials. Vickers' hardness of sintered ZrB₂ was 14.5±2.6 GPa, which was significantly lower when compared with 23 GPa for hot-pressed ZrB₂. Strength and elastic modulus of the ZrB₂ were 444±30 MPa and 454 GPa, which were comparable with values reported for hot-pressed ZrB₂. The ability to densify ZrB₂ ceramics without hot pressing should enable near-net shape processing, which would significantly reduce the cost of fabricating ZrB₂ components compared with conventional hot pressing and machining.     

New Route to Synthesize Ultra-Fine Zirconium Diboride Powders Using Inorganic–Organic Hybrid Precursors

Ultra-fine zirconium diboride (ZrB₂) powders have been synthesized using inorganic–organic hybrid precursors of zirconium oxychloride (ZrOCl₂·8H₂O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. The reactions were substantially completed at a relatively low temperature (∼1500°C). The synthesized powders had a smaller average crystallite size (<200 nm), a larger specific surface area (∼32 m²/g), and a lower oxygen content (<1.0 wt%), which were superior to some commercially available ZrB₂ powders. The thermodynamic change in the ZrO₂–B₂O₃–C system was mainly studied by thermogravimetric and differential thermal analysis. The crystallite size and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.     

Reaction Processes and Characterization of ZrB2 Powder Prepared by Boro/Carbothermal Reduction of ZrO2 in Vacuum

The present work was concentrated mainly on the reaction processes of boro/carbothermal reduction (BCTR) of ZrO₂ with B₄C and carbon in vacuum, and characterization of morphology and sinterability of the obtained ZrB₂ powder. Combining the thermodynamic calculations, X-ray diffraction results, and the trend of furnace pressure with temperature during synthesis, a detailed explanation of the reaction processes of BCTR was developed. Most of the ZrB₂ particles obtained at 1650°C presented a nearly spherical morphology, whereas those synthesized at 1750°C showed a nearly columnar morphology with an increased size. Compared with the powder synthesized at 1750°C as well as the commercially additive-free powder used in the reported work, the ZrB₂ powder synthesized at 1650°C showed a better sinterability due to its smaller particle size and lower oxygen content.     

Processing and Mechanical Properties of Zirconium Diboride-Based Ceramics Prepared by Spark Plasma Sintering

Zirconium diboride (ZrB₂) reinforced by nano-SiC whiskers has been prepared by spark plasma sintering (SPS). Of most interest is the densification of ZrB₂–SiCw composites accomplished by SPS at a temperature as low as 1550°C. The relative density of ZrB₂–SiCw composites could reach to 97% with an average grain size of 2–3 μm. Both flexural strength and fracture toughness of the composites were improved with increasing amount of SiCw. Flexural strengths ranged from 416 MPa for monolithic ZrB₂ to over 545 MPa for ZrB₂–15 vol% SiCw composites. Similarly, fracture toughness also increased from 5.46 MPa·m^1/2 to more than 6.81 MPa·m^1/2 in the same composition range. The relative density of ZrB₂–SiCw composites could be further improved to near 100% by adding some sintering aids such as AlN and Si₃N₄; however, the effects of different sintering additives on the mechanical properties of the composites were different.     

Preparation of Titanium Nitride/Alumina Laminate Composites

A preparation route for TiN/Al₂O₃ laminate composites has been described. A water-based process using Al₂O₃ and TiN slurries with solids contents of 40 and 35 vol%, respectively, was used to make TiN and Al₂O₃ tapes. The removal of the binder was monitored by weight-loss measurements in a thermogravimetry unit. Bodies composed of Al₂O₃ and TiN tapes were densified at temperatures of 1400° and 1500°C using the Spark Plasma Sintering^® (SPS) technique. Densities of >98% of the theoretical densities were approached. Crack-free and almost fully densified TiN/Al₂O₃ compacts were prepared by heating the burned-out green bodies to the final sintering temperature (1500°C) at a rate of 100°C/min, and with a holding time of 5–10 min, under a pressure of 75 MPa. The microstructures of the obtained compacts were studied using scanning electron microscopy. Grain sizes in the sintered Al₂O₃ and TiN compacts were similar to those of the precursor powders. Hardness and indentation fracture toughness were measured at room temperature, and the monolithic compacts as well as the laminate composites exhibited anisotropic mechanical behavior; i.e., the cracks propagated much more easily in a direction parallel to the laminas than perpendicular to them.     

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).