Dense ZrB2 Research Articles

Reactive hot pressing of zirconium diboride

The reaction of zirconium and boron was investigated as a potential route to form dense monolithic zirconium diboride (ZrB 2) ceramics. Attrition milling of the precursors produced nanosized (less than 100 nm) zirconium metal particles that reacted with boron to form ZrB 2 with an average particle size of less than 100 nm at temperatures as low as 600 °C. Scanning electron microscopy of ZrB 2 compacts heated to 1450 °C and 1650 °C showed average particle sizes of 0.6 μm and 1.0 μm, respectively, suggesting that the fine particle size was maintained during densification. Ceramics with a relative density of ∼99% were produced by hot pressing at 2100 °C. Dense ZrB 2 produced by the reactive hot pressing process had mechanical properties that were comparable to ceramics produced by conventional processes. The four-point flexure strength of ZrB 2 produced in this study was 434 MPa.

Densification, microstructure and mechanical properties of ZrB 2–SiCw ceramic composites

ZrB 2–SiCw composites were prepared through hot-pressing at a low temperature of 1800 °C, and Al 2O 3 plus Y 2O 3 were added as sintering aids. Analysis revealed that additives may react with impurities (i.e. surface oxygen impurities and residual metallic impurities) to form a transient liquid phase, thus promote the sintering and densification of ZrB 2–SiCw composites. The content of additives was found to have a significant influence on the sinterability, microstructure and mechanical properties of ZrB 2–SiCw composites. ZrB 2–SiCw composite prepared with a small amount of additives (3 vol.%) provided the optimal combination of microstructure (relative density of 98.3%) and excellent properties, including flexural strength of 783 MPa and fracture toughness of 6.7 MPa m 1/2. With further addition of additives, SiC whiskers were inclined to gather together and be enveloped by excessive liquids to form core-rim-like structures, which lead to little decrease in mechanical properties.

Pressureless sintering mechanism and microstructure of ZrB 2–SiC ceramics doped with boron

A pressureless densification mechanism and microstructure development in ZrB2–20 vol.% SiC ceramics with boron additive were investigated. Results showed that the boron addition removed oxygen impurity and improved the densification process. By adding 1 wt.% boron, fully dense ZrB2–SiC ceramics with a fine-grained microstructure was obtained. However, a higher boron addition would lead to fast grain growth of both SiC and ZrB2 grains and a deteriorated densification behavior by forming a liquid phase. The mechanical properties of the sintered ceramics with 1 wt.% boron addition were also evaluated.

Microstructure and densification of ZrB 2–SiC composites prepared by spark plasma sintering

ZrB 2–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB 2–SiC composites were investigated. Fully dense ZrB 2–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB 2 grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB 2–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB 2–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB 2–SiC composites containing 50 mass% SiC independent of sintering temperatures.

Simultaneous synthesis and densification of α-Zr(N)/ZrB 2 composites by self-propagating high-temperature combustion under high nitrogen pressure

Simultaneous synthesis and densification of α-Zr(N)/ZrB 2 composites from a 85 mol% Zr/15 mol% B mixed-powder compacts have been achieved by self-propagating high-temperature under a nitrogen pressure of 10 MPa. Composites consist of fine and short rodlike ZrB 2 grains (0.1 μm ϕ–0.5 μm l) dispersed into α-Zr(N) matrix (3 μm). Dense composite materials (96.5% of theoretical) exhibit excellent mechanical properties, in which their bending strength and H v are 560 MPa and 6.5 GPa, respectively. This bending strength is much superior to those (205 and 480 MPa) of dense equi-axial α-Zr(N) (10 μm) and dense ZrB 2 (6 μm). Fine and rodlike ZrB 2 grains greatly enhanced their mechanical properties.

Spark plasma sintering of UHTC powders obtained by self-propagating high-temperature synthesis

Fully dense ZrB2–SiC and HfB2–SiC ultra-high-temperature ceramics (UHTCs) composites are fabricated by first synthesizing via self-propagating high-temperature synthesis (SHS) the composite powders from B4C, Si, and Zr or Hf reactants, and subsequently consolidating the product by spark plasma sintering (SPS) without the addition of any sintering aid. It was found that the SHS technique leads to the complete conversion of reactants to the desired products and the SPS allows for the full consolidation (>99.5% relative density) under the optimal operating conditions of 1800 °C/20 min/20 MPa and 1800 °C/30 min/20 MPa, for the cases of ZrB2–SiC and HfB2–SiC, respectively. Based on the results reported in this work, it can be stated that the combination of SHS and SPS methods represents a particularly rapid and convenient preparation route (lower sintering temperature and processing time) for UHTCs as compared to the techniques available in the literature for the fabrication of analogous products.

Spark Plasma Sintering of Zirconium Diborides

The densification behavior and grain growth of single‐phase ZrB2 ceramics by a spark plasma sintering (SPS) process were systematically investigated. The sintering was conducted at temperatures ranging from 1800° and 1950°C in vacuum under a pressure of 50 MPa. The heating rate and the soaking time varied from 50° to 430°C/min and from 0 to 20 min, respectively. The microstructure was observed by field emission scanning electron microscopy. A near‐fully dense ZrB2 ceramic can be obtained at a lower temperature, within a much shorter time, than those used in conventional hot‐pressing process. The density of the sintered ceramics increases with increase in sintering temperature, heating rate, and holding time. On the other hand, microstructural examination shows a strong dependence of grain growth on sintering temperature, heating rate, and holding time. The fully dense ZrB2 ceramic with fine grain and homogeneous structure could be achieved by the processing conditions as follows: sintering temperature of 1900°C, holding time of 3 min, and heating rate of 200°–300°C/min.

Mechanical behavior of two-step hot-pressed ZrB 2-based composites with ZrSi 2

In this study, fully dense ZrB 2-based composites containing ZrSi 2 were sintered using a two-step hot pressing process. The elastic moduli, fracture toughness and flexural strength of the hot-pressed composites were determined. The effects of ZrSi 2 content on densification behavior and properties of the composites were assessed. The results indicated that the ZrSi 2 improved the sinterability of ZrB 2 powders. Fully dense ZrB 2-based composites with ZrSi 2 were obtained at 1550 °C for 20–40 vol.% ZrSi 2-conatining ZrB 2 powders. The microstructure of the resulting composites was fine and homogeneous. The elastic moduli, fracture toughness and flexural strength of the obtained composites depended on ZrSi 2 content. The shear, Young's, and bulk moduli decreased with ZrSi 2 content. The range of fracture toughness values was measured to be 3.8–4.8 MPa m 1/2. The flexural strength, which was 556 MPa, was almost the constant for ZrSi 2 content of 30 vol.% or less. For 40 vol.% ZrSi 2, however, the strength lowered significantly to 382 MPa.

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 ZrB2with 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 ZrB2with 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 ZrB2was not possible without the removal of oxygen‐based impurities on the particle surfaces by a chemical reaction.

Pressureless densification of ZrB 2–SiC composites with vanadium carbide

Adding a small amount of vanadium carbide as sintering aids, nearly fully dense ZrB2–SiC composites were obtained by pressureless sintering methods at 2000–2100 °C. Thermodynamic calculations and experimental results proved that VC is better at removing the surface oxide impurities of ZrB2 than WC, and played a very important role during the densification process. The mechanical properties of the sintered ceramics were also reported.

In situ synthesis mechanism and characterization of ZrB 2–ZrC–SiC ultra high-temperature ceramics

The synthesis route, microstructure and properties of ZrB 2–ZrC–SiC composites prepared from a mixture of Zr–B 4C–Si powders by in situ reactive synthesis were investigated. The reactive path and synthesized mechanism of ZrB 2–ZrC–SiC composite were studied through series of pressureless heat treatments ranging from 800 °C to 1700 °C in argon. The in situ ZrB 2–ZrC–SiC composites were fabricated under different synthesis processing. The one with 88.4% relative density performed poorly in mechanical properties due to the occurring of self-propagating high-temperature synthesis (SHS). The fully dense ZrB 2–ZrC–SiC composite was obtained under the optimized synthesis processing without SHS reactions. Its Vickers hardness, flexural strength and fracture toughness were 20.22 ± 0.56 GPa, 526 ± 9 MPa and 6.70 ± 0.20 MPa m 1/2, respectively.

Enhanced densification and mechanical properties of ZrB 2–SiC processed by a preceramic polymer coating route

Dense ZrB2–SiC ultra-high temperature ceramics were prepared by a novel pressureless sintering process. The relative density of sintered ceramics increased from 62.5% for ZrB2 to 96.7% by coating the starting ZrB2 powder with polycarbosilane, which was converted to C and SiC by pyrolysis. The mechanism of enhanced densification and the mechanical properties of the sintered ZrB2–SiC ceramics were investigated.

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.

Elastic properties of spark plasma sintered (SPSed) ZrB 2–ZrC–SiC composites

This study investigates the elastic moduli of ZrB 2–ZrC–SiC (ZZS) composites with or without AlN and Si 3N 4 additives consolidated by spark plasma sintering. The effect of the additives on the properties is assessed. The soundwave velocities in the longitudinal and transverse modes were measured. The elastic moduli were calculated from the sound velocities and densities. The results indicate that the elastic moduli of ZrB 2–ZrC–SiC materials depend on additives and porosity. This dependence is described by a linear relation between the elastic moduli and the longitudinal soundwave velocity. The obtained elastic moduli of the fully dense ZrB 2–ZrC–SiC materials with AlN and Si 3N 4 additives are the following: E = 459 GPa, G = 196 GPa, ν = 0.17 for ZZS, E = 418 GPa, G = 178 GPa, ν = 0.18 for ZZS + AlN, and E = 462 GPa, G = 197 GPa, ν = 0.17 for ZZS + Si 3N 4.

Evolution of structure during the oxidation of zirconium diboride–silicon carbide in air up to 1500 °C

The structures that developed as dense ZrB 2–SiC ceramics were heated to 1500 °C in air were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. The oxidation behavior was also studied using thermal gravimetric analysis (TGA). Below 1200 °C, a protective B 2O 3-rich scale was observed on the surface. At 1200 °C and above, the B 2O 3 evaporated and the SiO 2-rich scale that formed was stable up to at least 1500 °C. Beneath the surface, layers that were rich in zirconium oxide, and from which the silicon carbide had been partially depleted, were observed. The observations were consistent with the oxidation sequence recorded by thermal gravimetric analysis.

Beneficial effects of an ultra-fine α-SiC incorporation on the sinterability and mechanical properties of ZrB2

A fully dense ZrB2 ceramic containing 10 vol. % ultra-fine α-SiC particulate was successfully hot pressed at 1900 °C for 20 min and 40–50 MPa of applied pressure. Faceted ZrB2 grains (average size ≈3 μm) and SiC particles dispersed regularly characterized the base material. No extra secondary phases were found. The introduction of the ultra-fine α-SiC particulate was recognized as the key factor that enabled both the control of the diboride grain growth and the achievement of full density. The mechanical properties offered an interesting combination of data: 4.8±0.2 MPa\(\surd{m}\)fracture toughness, 507±4 GPa Young’s modulus, 0.12 Poisson’sratio, and 835±35 MPa flexural strength at room temperature. The flexural strength measured at 1500 °C (in air) provided values of 300±35 MPa. The incorporated ultra-fine α-SiC particulate was fundamental, sinterability apart, to enhancing the strength and oxidation resistance of ZrB2. The latter property was tested at 1450 °C for 20 h in flowing dry air. In such oxidizing conditions, the formation of a thin external borosilicate glassy coating supplied partial protection for the faces of the material exposed to the hot environment. The oxidation attack penetrated into the material’s bulk and created a 200-μm-thick zirconia scale. The SiC particulate included in the oxide scale, lost by active oxidation, left carbon-based inclusions in the formerly occupied sites.

Electrochemical Behavior of ZrB2 in Aqueous Solutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Improved Thermal Diffusivity Method Applied to TiB2, ZrB2, and HfB2 from 200°–1300°C

The thermal diffusivities of dense polycrystalline TiB2, ZrB2, and HfB2 were measured by a laser flash method over the temperature range 200°–1300°C. An investigation of the effects of radial heat flow was performed and a special sample shape was used to comply with the assumption of one-dimensional heat flow. The values of the diffusivities were found to be described by the empirical equations αTiB2=0.1290+[39.26/T(∘K)]cm2/secαZrB2=0.1424+[81.17/T(∘K)]αHfB2=0.1436+[35.97/T(∘K)].Values of thermal conductivity computed from the diffusivity data were found to be described by λTiB2=0.7867+[10.30/T(∘K)]W/cm degλZrB2=0.8235+[108.2/T(∘K)]λHfB2=0.7157+[34.24/T(∘K)].