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Oxidation behavior of additively manufactured ZrB2–SiC in air and in CO2 is reported in the temperature range of 700–1000 °C. Observed scale morphologies in air and in CO2 were similar, featuring an outer borosilicate layer and an inner porous zirconia layer containing partially oxidized silicon carbide particles and remnant borosilicate products. Oxide scale thicknesses and parabolic scaling constants in air were approximately twice those observed in CO2 across all studied temperatures. Activation energies for oxidation of 140 ± 20 kJ/mol in air and 110 ± 20 kJ/mol in CO2 were determined, indicating similar diffusion processes that appear to be rate-limiting. The formation of protective scales across wide temperature ranges both in air and in CO2 makes additively manufactured ZrB2–SiC an attractive candidate for high-temperature industrial process applications featuring varied oxidants such as heat exchangers. [ABSTRACT FROM AUTHOR]

Abstract: A silicon carbide‐based ceramic, containing 50 vol% SiC, 35 vol% ZrB2, and 15 vol% ZrC was plasma arc welded to produce continuous fusion joints with varying penetration depth. The parent material was preheated to 1450°C and arc welding was successfully implemented for joining of the parent material. A current of 138 A, plasma flow rate of ~1 L/min or ~0.5 L/min, and welding speed of ~8 cm/min were utilized for repeated joining, with full penetration fusion zones along the entire length of the joints. Solidification was determined to occur through the crystallization of β‐SiC (3C), then the simultaneous solidification of SiC and ZrB2, and lastly through the simultaneous solidification of SiC, ZrB2, and ZrC through a ternary eutectic reaction. The ternary eutectic composition was determined to be 35.3 ± 2.2 vol% SiC, 39.3 ± 3.8 vol% ZrB2, and 25.4 ± 3.0 vol% ZrC. A dual fusion zone microstructure was always observed due to convective melt pool mixing. The SiC content at the edge of the fusion zone was 57 vol%, while SiC content at the center of the fusion zone was 42 vol% although the overall SiC content was still nominally 50 vol% throughout the entire fusion zone. [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 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 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]

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. [ABSTRACT FROM AUTHOR]

The article focuses on a study that evaluated internal stresses produced during cooling after hot pressing zirconium diboride-silicon carbide (ZrB2-SiC) ceramics using commercial finite element modeling software. Researchers changed the size and shape of the SiC inclusions to determine their effect on the residual stresses. They also modeled alternate SiC particle shapes designed to limit residual tensile stresses to minimize residual thermal stresses.

Structure–property relations were examined for ZrB2 containing 30 volume percent SiC particulates. Two grades of ZrB2 with initial particle sizes of 2 and 6 μm were used. Billets of ZrB2–SiC were produced by hot pressing at 1850, 1950 or 2050 °C for 45 min. In addition, the material prepared from ZrB2 with an initial particle size of 2 μm was hot pressed at 2050 °C for 90 and 180 min. Microstructures and mechanical properties were characterized to determine the effects of the initial particle size, hot pressing time, and hot pressing temperature on the final grain size and morphology. The average grain size of the ZrB2 phase ranged from 2.2 to 4.7 μm. Similarly, the average grain size of the SiC phase ranged from 1.2 to 2.7 μm. Hardness and modulus of elasticity were not affected by the processing conditions with average values of 22 and 505 GPa, respectively. However, flexural strength decreased as grain size increased from a maximum of ∼1050 MPa for the finest grain sizes to ∼700 MPa for the largest grain sizes. Analysis suggested that the strength of ZrB2–SiC was limited by the size of the SiC inclusions in the ZrB2 matrix. [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]

Abstract: Dense ZrB2 containing 15vol.% SiC and 15vol.% graphite was exposed to flowing air at 1500°C. A layered scale structure developed that consisted of (1) a uniform SiO2-rich layer on the surface, (2) a layer of ZrO2 and SiO2, (3) a layer of ZrO2 (4) a partially oxidized layer composed of porous ZrB2, ZrO2, and graphite, and (5) unaffected ZrB2–SiC–C. A thermodynamic model based on volatility diagrams and consistent with the experimental observations was constructed to explain the development of the layered structure. Oxidation behavior was consistent with passive oxidation and formation of a protective surface layer. Analysis indicated that it may not be possible to form a protective surface layer without actively oxidizing SiC and producing a porous partially oxidized layer between the outer protective layer and the underlying unoxidized material. [Copyright &y& Elsevier]

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. [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]

Abstract: The influence of silicon carbide (SiC) particle size on the microstructure and mechanical properties of zirconium diboride–silicon carbide (ZrB2–SiC) ceramics was investigated. ZrB2-based ceramics containing 30vol.% SiC particles were prepared from four different α-SiC precursor powders with average particle sizes ranging from 0.45 to 10μm. Examination of the dense ceramics showed that smaller starting SiC particle sizes led to improved densification, finer grain sizes, and higher strength. For example, ceramics prepared from SiC with the particle size of 10μm had a strength of 389MPa, but the strength increased to 909MPa for ceramics prepared from SiC with a starting particle size of 0.45μm. Analysis indicates that SiC particle size controls the strength of ZrB2–SiC. [Copyright &y& Elsevier]