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Oxidation of ZrB2 + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE-TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C-1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C-1600°C. The mass gain increased from ~5 to 45 g/m2 and parabolic rate constants increased from 0.037 to 2.2 g2/m4·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass-ceramic surface layer as rate limiting. [ABSTRACT FROM AUTHOR]

An original catalytic method has been proposed to synthesize carbon nanotubes (CNTs)-ZrB2-ZrO2 heterostructures using ZrB2 polymeric precursor. The pyrolysized gases from the ZrB2 polymeric precursor are identified to be the carbon sources for CNTs growth. A parametric study is conducted to control the CNTs growth by optimizing parameters such as synthesis temperature and catalyst content. Observations show that the in situ grown CNTs are homogeneously dispersed in the powders, and the structure and the amount of CNTs are significantly dependent on the synthesis parameters. There are two kinds of grown CNTs existed in the produced hybrid heterostructures: (i) the kinking structured CNTs that are disordered and incomplete graphitization; (ii) the improved and graphitized CNTs. The ZrB2 polymeric precursor during thermal pyrolysis provides capable of supplying substantial carbon source for CNTs nucleation and growth by homogeneous vapor-liquid-solid reactions. [ABSTRACT FROM AUTHOR]

The formation of a porous SiC-depleted region in ZrB2-SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C-1800°C) and times (5 min-100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO2 + C/borosilicate oxidation product layer sequence was observed above the ZrB2-SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB2 matrix below the ZrO2 and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO2 ( T < 1627°C) and SiC depletion in ZrB2 ( T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas-phase diffusion-controlled process. [ABSTRACT FROM AUTHOR]

The oxidation of ZrB2 nanoparticles was observed at high temperature of 1500°C under low oxygen partial pressure of 5 × 10−2 Pa by an environmental transmission electron microscope. The results demonstrate that the oxidation starts on the surface of ZrB2 nanoparticles with decomposition of ZrB2 into ZrO2 and B2 O3. The nucleation and growth of ZrO2 on the surface of ZrB2 proceed with B2 O3 being evaporated. [ABSTRACT FROM AUTHOR]

The oxidation kinetics of ZrB2-30 vol% SiC were analyzed statistically with the goal of understanding the underlying mechanisms for observed variability. A box furnace was used to oxidize specimens for times between 30 s and 100 h at temperatures of 1300°C-1550°C in air. The specimens were characterized to determine weight change, scale thickness, and scale composition to quantify the oxidation behavior. Weight gain measurements of different specimens after 100 min of exposure showed differences of up to 2 mg/cm2 for the same testing conditions where the average weight gain was 2.54 mg/cm2. Variation of 30%-80% was observed in the average thickness of each layer of the oxide within a single specimen. Viscous glass flow was ruled out as a potential mechanism. Glass bubble formation was proposed as the main cause for oxidation kinetics variability. [ABSTRACT FROM AUTHOR]

Influence of boron on densification and microstructure of zirconium diboride ( ZrB2) ceramics using pressureless sintering has been studied. The ZrB2 powders were synthesized by borothermal reduction of ZrO2 with three different ZrO2/ B ratios to produce powders nominally phase pure or that contained excess boron. The as-synthesized powders were then densified by heating to temperatures from 1800°C to 2200°C. The incorporation of 1.2 wt% excess boron increased relative density from ~54.3% to ~96.1% by reacting with and removing oxide impurities ( ZrO2 and B2 O3) after sintering at 2000°C, whereas it also led to an exaggerated grain growth from 1-3 to 20-80 μm presumably due to high grain-boundary mobility. The faster grain growth resulted in the formation of intragranular pores and prevented the ZrB2 ceramics from reaching full density. By the incorporation of 3 wt% excess boron, a fully dense ZrB2 ceramics with relative density of ~99.7% and grain size of 5-10 μm was obtained at 2000°C. The full densification was attributed to the synergistic effects of boron through oxide impurities removal and grain- growth inhibition. [ABSTRACT FROM AUTHOR]

Aero-thermodynamic tests have been carried out in an arc-jet supersonic plasma wind tunnel using a very sharp wedge made of ultra-high temperature ceramic ( UHTC) in the ZrB2-SiC system. The comparison with a lower thermal conductivity ceramic material (Si3N4-MoSi2) with the same sharp shape, pointed out at the performance advantages of the UHTC material. When subjected to heat fluxes in the order of 7 MW/m2, the surface temperature of the UHTC wedge increased up to 2450°C near the leading edge. The present study demonstrated that the high thermally conductive UHTC survived such extreme conditions by re-distributing heat over colder regions downstream of the sharp tip. As a consequence, radiative equilibrium temperatures in the range 1400°C-1650°C were established over 85% of the exposed surface. On the other hand, the less thermally conductive Si3N4-MoSi2 material failed to withstand the same heat flux and underwent partial melting with significant mass loss. The post-test microstructural observations of the UHTC wedge proved to be a fundamental source of information which was input into a Computational Fluid Dynamics ( CFD) code and by a thermal simulation software to simulate the experimental tests and correlate the in situ observations of the material evolution during testing. [ABSTRACT FROM AUTHOR]

ZrB2-SiC-TaC ceramics with different content of TaC were prepared by hot-pressing at 1800°C in vacuum environment. The oxidation behavior of these ceramics was studied in the temperature range of 1200°C-1500°C in air. It was found that low concentration of TaC (10 vol%) deteriorated the oxidation resistance of ZrB2- SiC, while high concentration of TaC (30 vol%) significantly improved the oxidation resistance of the ceramics. Reoxidation experiments indicated that the metallic species (tantalum and/or silicon) diffusing out of the materials into the oxides was initially involved in the controlling process. [ABSTRACT FROM AUTHOR]

Plate-like ZrB2 grains were synthesized at 1550°C by in situ solid/liquid reaction using Zr and B powders mixed with transition metal ( Mo, Nb, Ti, or W) and Si powder. The preferred growth direction of plate grains was along a- or b-axis depending on the initial content of transition metal and silicon in the mixtures. The synthesis mechanism of plate-like grain was possibly related to the catalysis of in situ formed silicides. [ABSTRACT FROM AUTHOR]

Addition of LaB6 is adopted to improve the oxidation resistance of ZrC at 1500°C. Mixed powder of ZrC-25 vol% LaB6 is reactively hot pressed at 1900°C for 30 min under vacuum with an applied pressure of 25 MPa. The LaB6 reacts with ZrC to form ZrB2 and a layered La-containing phase. ZrB2 improves the oxidation resistance of ZrC in static air. The La-containing phase is beneficial to increasing the relative density of oxide scale during oxidation and in enhancing the oxide scale stability during exposure to thermal cycles. [ABSTRACT FROM AUTHOR]