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Zirconium diboride ceramics with and without carbon and boron carbide nanopowder additives were prepared by ball milling with ZrB 2 grinding media and pressureless sintering. Additions of up to 1 wt% nano-B 4 C and 0.5 wt% C were made to the ZrB 2 powder. The materials were then sintered between 1800 and 2300 °C for between 90 and 360 min in an Ar/10H 2 atmosphere. After sintering at 2200 °C for 90 min, densities ranged from 88.3 to 90.7% for the ZrB 2 with 0–1.0% nano-B 4 C addition. Carbon additions of 0.5 wt% and nano-B 4 C additions from 0 to 1.0 wt% resulted in densities ranging from 90.9 to 91.9% after sintering at 2100 °C for 90 min. Grain size ranged from 16.6 to 21.7 μm for ZrB 2 with nano-B 4 C content increasing from 0 to 1.0 wt%, sintered at 2200 °C. For the ZrB 2 with 0.5 wt% C, increasing the nano-B 4 C content from 0 to 1.0 wt% resulted in a decrease in grain size from 25.4 to 18.5 μm. The densities achieved in this study were lower than previous pressureless sintering studies of ZrB 2 that used WC-6Co grinding media, presumably due to the absence of WC and Co that can also act as sintering aids. [ABSTRACT FROM AUTHOR]

The mechanical response was studied for dense laminates containing layers of ZrB2 (~145 µm) and graphite—10 vol% ZrB2 (~20 µm). Individual layers were formulated by mixing starting powders with thermoplastic polymers and pressing into sheets. Laminates were produced by stacking and warm pressing the sheets, debinding, and hot pressing at 2050°C, 32 MPa, in Ar. The laminates were fractured at temperatures up to 2000°C in Ar. Laminates exhibited room temperature flexure strength of 260 MPa, increasing to 300 MPa at 1600°C, and then decreasing to 160 MPa at 2000°C. Inelastic work of fracture was 0.6 kJ/m2 at room temperature, reached a maximum of 1.3 kJ/m2 at 1400°C, and reverted to linear elastic failure at 2000°C. During fracture, cracks were deflected at the interfaces between the strong ZrB2 layers and the relatively weak C‐ZrB2 layers, which led to an increased inelastic work of fracture by more than an order of magnitude compared to conventional ZrB2 ceramics. This study demonstrated that laminate architectures are a promising approach for improving the damage tolerance of ZrB2‐based ceramics at elevated temperatures. [ABSTRACT FROM AUTHOR]

The mechanical properties of a nominally phase pure ZrB2 ceramic were measured up to 2300°C in an argon atmosphere. ZrB2 was hot pressed at 2000°C utilizing borothermally synthesized powder from high purity ZrO2 and B raw materials. The relative density of the ceramics was about 95% with an average ZrB2 grain size of 8.8 µm. The room temperature flexural strength was 447 MPa, with strength decreasing to 196 MPa at 1800°C, and then increasing to 360 MPa at 2300°C. The strength up to 1800°C was likely controlled by a combination of effects: surface damage from oxidation of the specimens, stress relaxation, and decreases in the elastic modulus. The strength above 1800°C was controlled by flaws in the range consistent with sizes of the maximum ZrB2 grain size and the largest pores. Fracture toughness was 2.3 MPa·m1/2 at room temperature, increasing to 3.1 MPa·m1/2 at 2200°C. The use of higher purity starting materials improved the mechanical behavior in the ultra‐high temperature regime compared to previous studies. [ABSTRACT FROM AUTHOR]

Solute segregation was examined in zirconium diboride and zirconium‐tantalum diboride solid solution ceramics that were produced by reactive hot pressing. Microstructural analysis demonstrated that the ZrB2 and (Zr,Ta)B2 ceramics reached nearly full relative density and were nominally phase‐pure. X‐ray diffraction was consistent with full incorporation of Ta into solid solution within the ZrB2 structure, and energy‐dispersive spectroscopy demonstrated that tantalum was well distributed throughout the bulk of the Ta‐doped specimens. The weak characteristic X‐rays for B led to inaccurate results for total atom concentrations in boride ceramics by energy‐dispersive spectroscopy. Atom probe tomography was used to analyze the amount and spatial distribution of Ta species. No obvious Ta segregation was observed in grains or grain boundaries. However, nitrogen strongly segregated to a grain boundary. This study demonstrated that atom probe tomography is an accurate method for characterizing the amount and spatial distribution of metallic and nonmetallic species in ZrB2 ceramics. [ABSTRACT FROM AUTHOR]

A novel electrical testing assembly was designed, fabricated, and validated to allow elevated temperature electrical resistivity measurements using the van der Pauw method up to 2173 K. The assembly consisted of h‐BN insulators, ZrB2‐based fixture components, and W signal wires. The assembly was installed in a refractory metal element furnace for temperature control and an oxygen gettering system was utilized to control the oxygen activity of the process gas. The system was validated using high‐purity Mo as a reference standard, demonstrating good agreement with accepted electrical resistivity values for Mo. The system was utilized to measure the electrical resistivity of a ZrB2 ceramic that was produced from commercially available ZrB2 powder using C and ZrH2 as sintering aids and densified by hot‐pressing at 2423 K. The resulting ZrB2 ceramic was ~96% dense with ~0.2 wt.% ZrO2 secondary phase and a grain size of ~40 µm. Electrical resistivity of ZrB2 was measured from room temperature to 2133 K. The electrical resistivity of the ZrB2 exhibited distinct linear behavior with respect to temperature, increasing from 7.3 µΩ‐cm at 298 K to 35.7 µΩ‐cm at 1223 K (~0.031 µΩ‐cm/K), and then further increasing to 76.0 µΩ‐cm at 2133 K (~0.044 µΩ‐cm/K). To the knowledge of the authors, these measurements were performed at the highest temperatures ever reported using this method. [ABSTRACT FROM AUTHOR]

ZrB2 ceramics were prepared by in‐situ reaction hot pressing of ZrH2 and B. Additions of carbon and excess boron were used to react with and remove the residual oxygen present in the starting powders. Additions of tungsten were utilized to make a ZrB2‐4 mol%W ceramic, while a change in the B/C ratio was used to produce a ZrB2‐10 vol% ZrC ceramic. All three compositions reached near full density. The baseline ZrB2 and ZrB2–ZrC composition contained a residual oxide phase and ZrC inclusions, while the W‐doped composition contained residual carbon and a phase that contained tungsten and boron. All three compositions exhibited similar values for flexure strength (~520 MPa), Vickers hardness (~15 GPa), and elastic modulus (~500 to 540 GPa). Fracture toughness was about 2.6 MPa m1/2 for the W‐doped ZrB2 compared to about 3.8 MPa m½ for the ZrB2 and ZrB2–ZrC ceramics. This decrease in fracture toughness was accompanied by an observed absence of crack deflection in the W‐doped ZrB2 compared with the other compositions. The study demonstrated that reaction‐hot‐pressing can be used to fabricate ZrB2 based ceramics containing solid solution additives or second phases with comparable mechanical properties. [ABSTRACT FROM AUTHOR]

Metallic Nb-coatings were deposited on top of ZrB 2 by means of magnetron sputtering to improve its oxidation resistance. High temperature oxidation tests have revealed that the metallic Nb-coatings lead to the formation of a dense solid and protective reaction zone in addition to a more stable B 2 O 3 liquid solution at the surface. Compared to baseline ZrB 2 , a reduction in the oxidation kinetics, as well as the thickness of the porous zirconia layer by 71%, has been achieved with the help of Nb-coatings. A liquid phase sintering by molten Nb 2 O 5 mechanism was responsible for the formation of the dense reaction zone under the boria layer, resulting in the improved oxidation resistance. Furthermore, the experiments revealed the need for an annealing process of Nb-coated specimens, in vacuum, to improve the adhesion of the coatings with ZrB 2 and to avoid spallation during oxidation. [ABSTRACT FROM AUTHOR]

The strength of zirconium diboride ( ZrB2) ceramics was measured up to 2300°C, which are the first reported measurements above 1500°C since 1970. ZrB2 ceramics were prepared from commercially available powder by hot pressing. A mechanical testing apparatus capable of testing material in the ultra-high temperature regime with atmosphere control was built, evaluated, and used. Four-point bend strength was measured as a function of temperature up to 1600°C in air and between 1500°C and 2300°C in argon. Strength between room temperature and 1200°C was ~390 MPa, decreasing to a minimum of ~170 MPa between 1400°C and 1500°C, with strength increasing to ~220 MPa between 1600°C and 2300°C. [ABSTRACT FROM AUTHOR]

This paper presents a study wherein the Laser Sintering (LS) process is used to perform additive manufacturing of zirconium diboride (ZrB2) parts. Experiments were conducted to determine values of LS process parameters (laser power, scan speed, energy density, line spacing, and layer thickness) that can be used to improve building of ZrB2 parts. A sacrificial plate with a proper number of layers was first constructed in order to reduce thermal gradients in building the main part. The sacrificial plate was found to eliminate cracks that would otherwise occur in the bottom of the main part. The fabricated green parts went through post-processing steps, including binder burnout and sintering at appropriate heat treatment schedules, to remove the binder and sinter the ZrB2 particles. The test bars after sintering had an average relative density of 87% and an average flexural strength of 250 MPa. [ABSTRACT FROM PUBLISHER]

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 Tc) of 1400°C was measured by water quench thermal shock testing, a 250% improvement over the previously reported Δ Tc values for ZrB2 and ZrB2–30 vol% SiC of similar dimensions (4 mm × 3 mm × 45 mm). The flexure strength was maintained with Δ Tc values of 1350°C and below. As Δ Tc 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. [ABSTRACT FROM AUTHOR]

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·m1/2), 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. [ABSTRACT FROM AUTHOR]

Zirconium diboride–silicon carbide ceramics with relative densities in excess of 95% were produced by reactive hot pressing (RHP) at temperatures as low as 1650°C. The ZrB2 matrix was formed by reacting elemental zirconium and boron. Attrition milling of the starting powders produced nanosized (<100 nm) Zr particulates that reacted with B below 600°C. The reaction resulted in the formation of nanoscale ZrB2 crystallites that could be densified more than 250°C below the temperatures required for conventional ZrB2 powder. Because of the low-temperature densification, the resulting ZrB2 grain sizes were as small as 0.5±0.30 μm for specimens densified at 1650°C and 1.5±1.2 μm for specimens densified at 1800°C. Vickers hardness, elastic modulus, and flexure strength of fully dense materials produced by RHP were 27, 510, and 800 MPa, respectively. [ABSTRACT FROM AUTHOR]

The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO2 was used to produce an oxygen partial pressure of ∼10−10 Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO2 produced a non-protective oxide surface scale. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ZrB 2 was mixed with 0.5 wt% carbon and up to 10 vol% ZrC and densified by hot-pressing at 2000 °C. All compositions were > 99.8% dense following hot-pressing. The dense ceramics contained 1–1.5 vol% less ZrC than the nominal ZrC addition and had between 0.5 and 1 vol% residual carbon. Grain sizes for the ZrB 2 phase decreased from 10.1 µm for 2.5 vol% ZrC to 4.2 µm for 10 vol% ZrC, while the ZrC cluster size increased from 1.3 µm to 2.2 µm over the same composition range. Elastic modulus was ~505 GPa and toughness was ~2.6 MPa·m½ for all compositions. Vickers hardness increased from 14.1 to 15.3 GPa as ZrC increased from 2.5 to 10 vol%. Flexure strength increased from 395 MPa for 2.5 vol% ZrC to 615 MPa for 10 vol% ZrC. Griffith-type analysis suggests ZrB 2 grain pullout from machining as the strength limiting flaw for all compositions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Carbon-coated zirconium diboride powders were sintered to near theoretical density at temperatures as low as 1900°C without an external pressure. Relative density increased from ∼70% for uncoated ZrB2 to >99% for ZrB2 coated with at least 1.0wt% carbon. Thermodynamic analysis indicated that carbon reacted with and removed oxide impurities (ZrO2 and B2O3) that were present on the particle surfaces, which promoted densification by minimizing grain coarsening. The effects of initial particle size and carbon content on densification, microstructure, and properties were investigated. [Copyright &y& Elsevier]

Abstract: The structures that developed as dense ZrB2–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 B2O3-rich scale was observed on the surface. At 1200°C and above, the B2O3 evaporated and the SiO2-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. [Copyright &y& Elsevier]