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.     

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.     

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.     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Solution-Based Synthesis of Submicrometer ZrB2 and ZrB2–TaB2

Zirconium diboride and a zirconium diboride/tantalum diboride mixture were synthesized by solution-based processing. Zirconium n -propoxide was refluxed with 2,4-pentanedione to form zirconium diketonate. This compound hydrolyzed in a controllable fashion to form a zirconia precursor. Boria and carbon precursors were formed via solution additions of phenol–formaldehyde and boric acid, respectively. Tantalum oxide precursors were formed similarly as zirconia precursors, in which tantalum ethoxide was used. Solutions were concentrated, dried, pyrolyzed (800°–1100°C, 2 h, flowing argon), and exposed to carbothermal reduction heat treatments (1150°–1800°C, 2 h, flowing argon). Spherical particles of 200–600 nm for pure ZrB₂ and ZrB₂–TaB₂ mixtures were formed.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel Powder

MgAl₂O₄ (MA) spinel powder was synthesized by heating an equimolar composition of MgO and Al₂O₃ in LiCl, KCl, or NaCl. The synthesis temperature can be decreased from >1300°C (required by the conventional solid–solid reaction process) to ∼1100°C in LiCl, or to ∼1150°C in KCl or NaCl. The molten salt synthesized MA powder was pseudomorphic and retained, to a large extent, the size and morphology of the original Al₂O₃ raw material, indicating that a "template formation mechanism" plays an important role in the synthesis process.     

Microwave Dielectric Properties of Low Firing Temperature Bi2W2O9 Ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi₂W₂O₉ ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi₂W₂O₉ powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi₂W₂O₉ to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna 2₁, with lattice parameters a =5.4401(8), b =5.4191(8), c =23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi₂WO₆ appears as a secondary phase. The measured relative permittivity of Bi₂W₂O₉ ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi₂W₂O₉ corrected for porosity was calculated as 41.3. Bi₂W₂O₉ ceramics fired up to 875°C exhibit moderate quality factors, Q × f _r, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi₂W₂O₉ ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.     

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.     

Nanometer-Sized ZrO2 Particles Prepared by a Sol–Emulsion–Gel Method

ZrO₂ powder was prepared by a sol–emulsion–gel method at temperatures below 140°C from ZrO(NO₃)₂· n H₂O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600°C. The metastable tetragonal powder (600°C) was comprised of ultrafine 4- to 6-nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100°C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100°C and with a grain size of 0.25 μ.     

Effect of ZrO2 Inclusions on the Sinterability of Al2O3

The sinterability of Al₂O₃/ZrO₂ composite powder compacts containing 2 and 10 vol% ZrO₂ was compared to the sinterability of their single-phase constituents through constant-heating-rate experiments. The ZrO₂ inclusion phase delayed the initiation of bulk shrinkage and the temperature of maximum strain rate by 100°C. The ZrO₂ inclusion phase also significantly inhibited grain growth. These results, discussed with regard to the thermodynamics of pore disappearance, suggest that phenomena inhibiting grain growth may also inhibit densification.     

Properties of a Pressureless-Sintered ZrB2–MoSi2 Ceramic Composite

A ZrB₂-based composite was fully densified by pressureless sintering at 1850°C with addition of 20 vol% MoSi₂. The microstructure was very fine, with mean dimensions of ZrB₂ grains around 2.5 μm. The four-point flexural strength in air was in excess of 500 MPa up to 1500°C.     

Synthesis and Characterization of MgAl2O4 Spinel Precursor from a Heterogeneous Alkoxide Solution Containing Fine MgO Powder

A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl₂O₄ spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg₄Al₂(OH)₁₄· 3H₂O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al₂O₃ at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C.     

Dense Al2O3/ZrO2 Particulate Composites by Free Sintering of Coated Powders

Composite powders, prepared by coating coarse ZrO₂ particles with fine Al₂O₃ powder using a chemical precipitation technique, were compacted and sintered freely at a constant heating rate of 4°C/min to ∼1600°C. Composites containing up to ∼30 vol% inclusions were sintered to nearly full density under the same conditions used for the unreinforced matrix. Furthermore, the sintering kinetics were not influenced significantly by the inclusion volume fraction. The sinterability of the composites formed from the coated powders was significantly better than that for similar composites formed from mechanically mixed powders. The present data provide a further demonstration that the use of coated powders may have widespread applicability for the fabrication, by free sintering, of dense ceramic particulate composites.     

Influence of Nd2O3 Doping on the Reaction Process and Sintering Behavior of BaCeO3 Ceramics

The influence of Nd₂O₃ doping on the reaction process and sintering behavior of BaCeO₃ is investigated. Formation of BaCeO₃ is initiated at 800°C and completed at 1000°C. When Nd₂O₃ is added to the starting materials, the formation of BaCe_1–xNd_xO_3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe_1-xNd_xO_3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd₂O₃ content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO₃ is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO₃ with Nd₂O₃ markedly enhances its sinterability. The enhancement of the sinterability of Nd₂O₃-doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering.     

Coprecipitation and Hydrothermal Synthesis of Ultrafine 5.5 mol% CeO2-2 mol% YO1.5ZrO2 Powders

Ultrafine 5.5 mol% CeO₂—2 mol% YO_1.5ZrO₂ powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m²/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.     

Effects of Phase Change and Oxygen Permeability in Oxide Scales on Oxidation Kinetics of ZrB2 and HfB2

A wide range of experimental data on the oxidation of ZrB₂ and HfB₂ as a function of temperature (800°–2500°C) is interpreted using a mechanistic model that relaxes two significant assumptions made in prior work. First, inclusion of the effect of volume change associated with monoclinic to tetragonal phase change of the MeO₂ phases is found to rationalize the observations by several investigators of abrupt changes in weight gain, recession, and oxygen consumed, as the temperature is raised through the transformation temperatures for ZrO₂ and HfO₂. Second, the inclusion of oxygen permeability in ZrO₂ is found to rationalize the enhancement in oxidation behavior at very high temperatures (>1800°C) of ZrB₂, while the effect of oxygen permeability in HfO₂ is negligible. Based on these considerations, the significant advantage of HfB₂ over ZrB₂ is credited to the higher transformation temperature and lower oxygen permeability of HfO₂ compared with ZrO₂.     

Synthesis of Transparent Nd-doped HfO2-Y2O3 Ceramics Using HIP

A Nd-doped HfO₂-Y₂O₃ ceramic having excellent transmittance was synthesized by HIPing, using high-purity powders (>99.99 wt%) of Y₂O₃, Nd₂O₃, and HfO₂. The mixed powder compacts of these powders were sintered at 1650°C for 1 h under vacuum and HIPed at 1700°C for 3 h under 196 MPa of Ar. The specimen after HIPing consisted of uniform grains measuring about 30 μm and having pore-free structure. The optical transmittance of 1 at.% Nd-doped 2.6 mol% HfO₂-Y₂O₃ ceramics ranging between visible and infrared wavelength was almost equivalent or superior to that of a Nd:Y₂O₃ single crystal grown by the Verneuil method.     

Laser Sintering of ZrB2

Preliminary results about laser sintering of zirconium diboride (ZrB₂), a good ultra high-temperature ceramics candidate, are presented. In order to evaluate a suitable sintering method, single pulsed laser and a concentric dual laser system were carried out. Two different ZrB₂ powders having ∼15 and ∼2 μm particle size were assessed for sintering. Scanning electron microscopy images showed that the concentric dual laser sintered layer using ∼2 μm particle size of ZrB₂ had relatively smooth surface morphology. X-ray diffraction results confirmed that the sintered layer mainly retained the crystalline phases as the starting powder. In addition, the rapid cooling rate of laser sintering enabled the formation of needle-like nanostructures at the sintered surface.     

Oxidation Resistance of Fully Dense ZrB2 with SiC, TaB2, and TaSi2 Additives

Specimens of ZrB₂ containing various concentrations of B₄C, SiC, TaB₂, and TaSi₂ were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation resistances were studied by scanning thermogravimetry over the range 1150°–1550°C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content. Tantalum additions to ZrB₂–B₄C–SiC in the form of TaB₂ and/or TaSi₂ increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi₂ proved to be a more effective additive than TaB₂. Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This interior layer was less porous in tantalum-containing compositions.