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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]

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 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]

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]

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]

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 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]