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Boron carbide powders were synthesized from elemental powders and studied using X‐ray diffraction (XRD) and UV–visible diffuse reflectance, Raman, and diffuse reflectance IR spectroscopies. Following reaction at 1400°C for 6 h, synthesized powders exhibited possible faulting as suggested by XRD patterns. B3C, B4.3C, and B5C powders contained graphitic carbon whereas the boron carbides with higher B/C ratios contained no residual carbon, suggesting that the carbon rich phase boundary is likely temperature dependent. Analysis by Raman and IR spectroscopy suggested that Raman spectra are influenced by excitation frequency due to resonance. We suggest that measurement of boron carbides with resonant Raman lifts the selection rules to allow measurement of Raman silent modes that are present in the IR spectra. Optical reflectance of the boron carbide powders revealed that the B/C ratio governed the indirect and direct optical band gaps of the faulted powders. B3C and B4.3C powders were light gray in spite of the presence of the carbon, whereas B5C, B6.5C, B10C, and B12C were gray, green, brown, and dark brown, respectively. Increasing carbon content increased the optical indirect band gap from 1.3 eV for B12C to 3.2 eV for B3C, causing the observed color changes. [ABSTRACT FROM AUTHOR]

Three phase boride and carbide ceramics were found to have remarkably high hardness values. Six different compositions were produced by hot pressing ternary mixtures of Group IVB transition metal diborides, SiC, and B 4 C. Vickers' hardness at 9.8 N was ~31 GPa for a ceramic containing 70 vol% TiB 2 , 15 vol% SiC, and 15 vol% B 4 C, increasing to ~33 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values for the ceramics containing ZrB 2 and HfB 2 were ~30% and 20% lower than the corresponding TiB 2 containing ceramics, respectively. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the previously reported ceramic containing equal volume fractions of TiB 2 , SiC and B 4 C was ~54 GPa, the highest of the ceramics in the present study and higher than the hardness values reported for so-called "superhard" ceramics at comparable indentation loads. The previously reported ceramic containing 70 vol% TiB 2 , 15 vol% SiC, and 15 vol% B 4 C also displayed the highest flexural strength of ~1.3 GPa and fracture toughness of 5.7 MPa·m1/2, decreasing to ~0.9 GPa and 4.5 MPa·m1/2 for a ceramic containing equal volume fractions of the constituents. [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]

The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B 4 C led to a decrease in the ZrB 2 grain size from 22 µm for nominally pure ZrB 2 to 5.4 µm for ZrB 2 containing 15 vol% B 4 C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB 2 to 80 W/m·K for ZrB 2 containing 15 vol% B 4 C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB 2 , the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB 2 containing 15 vol% B 4 C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB 2 and less than 6 W/m·K when 15 vol% B 4 C was added. [ABSTRACT FROM AUTHOR]

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]

Zirconium diboride ceramics were densified by hot pressing and spark plasma sintering with heating rates varying from 5 to 300℃/min. Slower heating rates produced larger grains due to the longer times spent at temperatures between 1500 and 1900℃, which is the temperature range in which ZrB2 grains coarsen. Heating rates above 50℃/min resulted in rapid densification, but this led to the retention of up to 3.3 vol.% of ZrO2 particles in the ceramics. After densification, changes to the microstructure were evaluated to interpret the effects of heating rate on thermal and mechanical properties. The flexure strength of ceramics processed by hot pressing up to 80℃/min was proportional to the inverse square root of the maximum grain size based on the Griffith criteria. Conversely, densification by spark plasma sintering, which had heating rates of up to 300℃/min, resulted in microcracks that decreased the elastic modulus from >500 GPa for pristine specimens to <485 GPa for microcracked materials. The use of heating rates >20℃/min also reduced the thermal conductivity due to the presence of retained ZrO2, but improved the strength by reducing the maximum grain size. [ABSTRACT FROM AUTHOR]

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB 2. The resulting (Zr,Ti)B 2 ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB 2 resulted in grain size decreasing from 22 μm for nominally pure ZrB 2 to 7 μm for ZrB 2 –50 vol.% TiB 2. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB 2 to 58 W/m⋅K for ZrB 2 –50 vol.% TiB 2. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB 2 at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB 2. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB 2 decreasing to 57 W/m⋅K when 50 vol.% TiB 2 was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB 2. Additions of ≥25 vol.% TiB 2 reduced the phonon contribution to nearly zero for all temperatures. [ABSTRACT FROM AUTHOR]

Densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB2) ceramics modified with a complex Zr/Si/O‐based additive were studied. ZrB2 ceramics with 5–20 vol.% additions of Zr/Si/O‐based additive were densified to >95% relative density at temperatures as low as 1400°C by hot‐pressing. Improved densification behavior of ZrB2 was observed with increasing additive content. The most effective additive amount for densification was 20 vol.%, hot‐pressed at 1400°C (∼98% relative density). Microstructural analysis revealed up to 7 vol.% of residual second phases in the final ceramics. Improved densification behavior was attributed to ductility of the silicide phase, liquid phase formation at the hot‐pressing temperatures, silicon wetting of ZrB2 particles, and reactions of surface oxides. Room temperature strength ranged from 390 to 750 MPa and elastic modulus ranged from 440 to 490 GPa. Vickers hardness ranged from 15 to 16 GPa, and indentation fracture toughness was between 4.0 and 4.3 MPa·m1/2. The most effective additive amount was 7.5 vol.%, which resulted in high relative density after hot‐pressing at 1600°C and the best combination of mechanical properties. [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]

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]

Triplex particulate composites composed of boride and carbide ceramics were found to have high strength, hardness, and fracture toughness values. Two compositions consisting of 70:15:15 and 1:1:1 volume ratios of TiB2, SiC, and B4C were produced from commercially available powders by hot-pressing. The 70:15:15 ceramic exhibited a strength of ~1.3 GPa, while the 1:1:1 ceramic had a strength of ~0.9 GPa. These strengths are comparable to super-strong Y2O3-PSZ and β-SiAlON based composites. The Vickers' hardness values of these ceramics were ~32 GPa for indent loads of 9.8 N. Hardness increased as indentation load decreased. The 1:1:1 ceramic had a hardness of ~53 GPa at an indentation load of 0.49 N, higher than values reported for so-called "super-hard" ceramics, and comparable to c-BN. [ABSTRACT FROM AUTHOR]

A ceramic particulate composite composed of oxide, and carbide ceramics was found to have high strength, hardness, and fracture toughness values. A composition consisting of Al 2 O 3 with 15 vol% SiC and 15 vol% B 4 C additions was produced by hot-pressing at 1650 °C for 30 min, with full density reached after ~5 min at temperature. Both WB and WB 2 were observed, with the W source presumably an impurity from WC milling media, and Al 18 B 4 O 33 was also detected following densification. Strength was ~880 MPa, which is greater than values reported for comparable composites of Al 2 O 3 containing 30 vol% SiC or B 4 C. Vickers hardness was ~21 GPa, and fracture toughness was ~4.5 MPa m ½ , comparable to values reported for the binary mixtures. The calculated critical flaw size of the material was similar to the size of the SiC/B 4 C clusters and microcracking at grain boundaries. The latter resulting from thermal expansion mismatch between the Al 2 O 3 matrix and SiC/B 4 C reinforcing phases. [ABSTRACT FROM AUTHOR]

The processing, microstructure, and mechanical properties of zirconium diboride-boron carbide (ZrB 2 -B 4 C) ceramics were characterized. Ceramics containing nominally 5, 10, 20, 30, and 40 vol% B 4 C were hot-pressed to full density at 1900 °C. The ZrB 2 grain size decreased from 4 to 2 µm and B 4 C inclusion size increased from 3 to 5 µm for B 4 C additions of 5 and 40 vol% B 4 C, respectively. Elastic modulus decreased from 525 to 515 GPa and Vickers hardness increased from 15 to 21 GPa as the B 4 C content increased from 5 to 40 vol%, respectively, following trends predicted using linear rules of mixtures. Flexure strength and fracture toughness both increased with increasing B 4 C content. Fracture toughness increased from 4.1 MPa m ½ at 5 vol% B 4 C to 5.3 MPa m ½ at 40 vol% B 4 C additions. Flexure strength was 450 MPa with a 5 vol% B 4 C addition, increasing to 590 MPa for a 40 vol% addition. The critical flaw size was calculated to be ~30 µm for all compositions, and analysis of the fracture surfaces indicated that strength was controlled by edge flaws generated by machining induced sub-surface damage. Increasing amounts of B 4 C added to ZrB 2 led to increasing hardness due to the higher hardness of B 4 C compared to ZrB 2 and increased crack deflection. Additions of B 4 C also lead to increases in fracture toughness due to increased crack deflection and intergranular fracture. [ABSTRACT FROM AUTHOR]

Zirconium diboride ceramics produced using commercial ZrB 2 powders, and milled with zirconium diboride grinding media, were fabricated by hot-pressing at temperatures of 2100–2200 °C with hold times of 30–120 min. This ZrB 2 exhibits no additional impurities typically introduced by milling with grinding media of differing composition. Microstructure analysis revealed grain sizes ranging from ~25 to ~50 µm along with ~3 vol% porosity. Flexure strength ranged from 335 to 400 MPa, elastic modulus from 490 to 510 GPa, fracture toughness from 2.7 to 3.2 MPa m ½ , and hardness from 13.0 to 14.4 GPa. Strength limiting flaws were identified as surface grain pullout induced by machining. Elastic modulus and hardness were found to increase with decreasing porosity. Compared to the fine grained ceramics typically reported, large grain zirconium diboride ceramics exhibit higher than expected room temperature strengths. [ABSTRACT FROM AUTHOR]

The mechanical properties of dense, hot-pressed ZrB2-30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot-pressed or after hot-pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot-pressed ZrB2-SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot-pressed ZrB2-SiC. Neither ZrB2 grain size nor SiC cluster size changed for these heat-treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB2-based ceramics. [ABSTRACT FROM AUTHOR]

The mechanical properties of a ZrB2-10 vol% ZrC ceramic were measured up to 2300°C in an argon atmosphere. Dense billets of ZrB2-9.5 vol% ZrC-0.1 vol% C were produced by hot-pressing at 1900°C. The ZrB2 grain size was 4.9 μm and ZrC cluster size was 1.8 μm. Flexure strength was 695 MPa at ambient, decreasing to 300 MPa at 1600°C, increasing to 345 MPa at 1800°C and 2000°C, and then decreasing to 290 MPa at 2200°C and 2300°C. Fracture toughness was 4.8 MPa·m½ at room temperature, decreasing to 3.4 MPa·m½ at 1400°C, increasing to 4.5 MPa·m½ at 1800°C, and decreasing to 3.6 MPa·m½ at 2300°C. Elastic modulus calculated from the crosshead displacement was estimated to be 505 GPa at ambient, relatively unchanging to 1200°C, then decreasing linearly to 385 GPa at 1600°C, more slowly to 345 GPa at 2000°C, and then more rapidly to 260 GPa at 2300°C. Surface flaws resulting from machining damage were the critical flaw up to 1400°C. Above 1400°C, plasticity reduced the stress at the crack tip and the surface flaws experienced subcritical crack growth. Above 2000°C, microvoid coalescence ahead of the crack tip caused failure. [ABSTRACT FROM AUTHOR]

The mechanical properties of hot pressed zirconium diboride–silicon carbide–boron carbide (ZrB 2 –SiC–B 4 C) ceramics were characterized from room temperature up to 2200 °C in an argon atmosphere. The average ZrB 2 grain size was 3.0 μm. The SiC particles segregated into clusters, and the largest clusters were >30 μm in diameter. The room temperature flexural strength was 700 MPa, decreasing to 540 MPa at 1800 °C and to 260 MPa at 2200 °C. The strength was controlled by the SiC cluster size up to 1800 °C. At higher temperatures, strength was controlled by formation of liquid phases, and precipitation of large BN and B–O–C–N inclusions. The mechanical behavior of these materials changes at ∼1800 °C, meaning that extrapolation of properties from lower temperatures is not accurate. Mechanical behavior in the ultra-high temperature regime was dominated by impurities and changes in microstructure. Therefore, the use of higher purity materials could lead to significant improvements in ultra-high temperature strength. [ABSTRACT FROM AUTHOR]

The mechanical properties of zirconium diboride–silicon carbide (ZrB2–SiC) ceramics were characterized from room temperature up to 1600°C in air. ZrB2 containing nominally 30vol% SiC was hot pressed to full density at 1950°C using B4C as a sintering aid. After hot pressing, the composition was determined to be 68.5vol% ZrB2, 29.5vol% SiC, and 2.0vol% B4C using image analysis. The average ZrB2 grain size was 1.9μm. The average SiC particles size was 1.2μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680MPa and strength increased to 750MPa at 800°C. Strength decreased to ∼360MPa at 1500°C and 1600°C. The elastic modulus at room temperature was 510GPa. Modulus decreased nearly linearly with temperature to 210GPa at 1500°C, with a more rapid decrease to 110GPa at 1600°C. The fracture toughness was 3.6MPa·m? at room temperature, increased to 4.8MPa·m? at 800°C, and then decreased linearly to 3.3MPa·m? at 1600°C. The strength was controlled by the SiC cluster size up to 1000°C, and oxidation damage above 1200°C. [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]

Particulate composites composed of boride and carbide ceramics were found to have remarkably high hardness values. Two different compositions were produced by hot-pressing commercially available ZrB2, SiC, and B4C powders. Using a Vickers' indentation load of 9.8 N, measured hardness was ~20 GPa for a ceramic containing 70 vol% ZrB2, 15 vol% SiC, and 15 vol% B4C, but increased to ~29 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the ceramic containing equal volume fractions of the three constituents was ~43 GPa, which was higher than the hardness values reported for so-called ''superhard'' ceramics at comparable indentation loads. [ABSTRACT FROM AUTHOR]