Strength, Fracture Mode, and Thermal Stress Resistance of HfB2 and ZrB2

Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 1800°C and 120,000 psi. Bend strengths were measured on the fully dense materials from 25° to 1400° C in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700° and 1000°C and vary from 39,000 to 68,000 psi for HfB₂ and 30,000 to 56,000 psi for ZrB₂. The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds.     

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.     

Thermophysical Properties of ZrB2 and ZrB2–SiC Ceramics

Thermophysical properties were investigated for zirconium diboride (ZrB₂) and ZrB₂–30 vol% silicon carbide (SiC) ceramics. Thermal conductivities were calculated from measured thermal diffusivities, heat capacities, and densities. The thermal conductivity of ZrB₂ increased from 56 W (m K)⁻¹ at room temperature to 67 W (m K)⁻¹ at 1675 K, whereas the thermal conductivity of ZrB₂–SiC decreased from 62 to 56 W (m K)⁻¹ over the same temperature range. Electron and phonon contributions to thermal conductivity were determined using electrical resistivity measurements and were used, along with grain size models, to explain the observed trends. The results are compared with previously reported thermal conductivities for ZrB₂ and ZrB₂–SiC.     

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.     

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.     

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.     

Thermal Shock Resistance and Fracture Behavior of ZrB2–Based Fibrous Monolith Ceramics

The thermal shock resistance and fracture behavior of zirconium diboride (ZrB₂)-based fibrous monoliths (FM) were studied. FMs containing cells of ZrB₂–30 vol% SiC with cell boundaries composed of graphite–15 vol% ZrB₂ were hot pressed at 1900°C. The average flexure strength of the FMs was 375 MPa, less than half of the strength of hot-pressed ZrB₂–30 vol% SiC. Flexure specimens failed noncatastrophically and retained 50%–85% of their original strength after the first fracture event. A critical thermal shock temperature (Δ T _c) of 1400°C was measured by water quench thermal shock testing, a 250% improvement over the previously reported Δ T _c values for ZrB₂ and ZrB₂–30 vol% SiC of similar dimensions (4 mm × 3 mm × 45 mm). The flexure strength was maintained with Δ T _c values of 1350°C and below. As Δ T _c increased, the stiffness of the flexure specimen decreased linearly. The lower stiffness and improvement in thermal shock resistance is attributed to crack propagation in the cell boundary and crack deflection around the load-bearing cells. The critical thermal shock was attributed to the fracture of the ZrB₂–30% SiC cell material.     

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.     

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.     

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB₂–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m²/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB₂+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B₄C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB₂ matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.     

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.     

Preparation and Microstructure of a ZrB2–SiC Composite Fabricated by the Spark Plasma Sintering–Reactive Synthesis (SPS–RS) Method

A mixture of Zr, B₄C, and Si powders was adopted to synthesize a ZrB₂–SiC composite using the spark plasma sintering–reactive synthesis (SPS–RS) method. SPS treatments were carried out in the temperature range of 1350°–1500°C under a varying pressure of 20–65 MPa with a 3-min holding time. A dense (∼98.5%) ZrB₂–SiC composite was successfully fabricated at 1450°C for 3 min under 30 MPa. The microstructure of the composite was investigated. The in situ formed ZrB₂ and SiC phases dispersed homogeneously on the whole. The grain size of ZrB₂ and SiC was <5 and 1 μm, respectively. A number of in situ formed ultrafine SiC particles were observed entrapped in the ZrB₂ grains.     

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.     

Flexural Creep Deformation of ZrB2/SiC Ceramics in Oxidizing Atmosphere

Flexural creep of ZrB₂/0–50 vol% SiC ceramics was characterized in oxidizing atmosphere as a function of temperature (1200°–1500°C), stress (30–180 MPa), and SiC particle size (2 and 10 μm). Creep behavior showed strong dependence on SiC content and particle size, temperature and stress. The rate of creep increased with increasing SiC content, temperature, and stress and with decreasing SiC particle size, especially, at temperatures above 1300°C. The activation energy of creep showed linear dependence on the SiC content increasing from about 130 to 511 kJ/mol for ceramics containing 0 and 50 vol% 2-μm SiC, respectively. The stress exponent was about 2 for ZrB₂ containing 50 vol% SiC regardless of SiC particle size, which is an indication that the leading mechanism of creep for this composition is sliding of grain boundaries. Compared with that, the stress exponent is about 1 for ZrB₂ containing 0–25vol% SiC, which is an indication that diffusional creep has a significant contribution to the mechanism of creep for these compositions. Cracking and grain shifting were observed on the tensile side of the samples containing 25 and 50 vol% SiC. Cracks propagate through the SiC phase confirming the assumption that grain-boundary sliding of the SiC grains is the controlling creep mechanism in the ceramics containing 50 vol% SiC. The presence of stress, both compressive and tensile, in the samples enhanced oxidation.     

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.     

Oxidation of ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.     

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.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

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.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.