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Abstract: Non-isothermal, isothermal and cyclic oxidation behavior of hot pressed ZrB2–20 (vol.%) SiC (ZS) and HfB2–20 SiC (HS) composites have been compared. Studies involving heating in thermogravimetric analyzer have shown sharp mass increases at 740 and 1180°C for ZS, and mass gain till 1100°C followed by loss for HS. Isothermal oxidation tests for 1, 24 and 100h durations at 1200 or 1300°C have shown formation of partially and completely stable oxide scales after ∼24h exposure for ZS and HS, respectively. X-ray diffraction, scanning electron microscopy and energy or wavelength dispersive spectroscopy has confirmed presence of ZrO2 or HfO2 in oxide scales of ZS or HS, respectively, besides B2O3–SiO2. Degradation appears more severe in isothermally oxidized ZS due to phase transformations in ZrO2; and is worse in HS on cyclic oxidation at 1300°C with air cooling, because of higher thermal residual stresses in its oxide scale. [ABSTRACT FROM AUTHOR]

Abstract: Electrical resistivities, thermal conductivities and thermal expansion coefficients of hot-pressed ZrB2–SiC, ZrB2–SiC–Si3N4, ZrB2–ZrC–SiC–Si3N4 and HfB2–SiC composites have been evaluated. Effects of Si3N4 and ZrC additions on electrical and thermophysical properties of ZrB2–SiC composite have been investigated. Further, properties of ZrB2–SiC and HfB2–SiC composites have been compared. Electrical resistivities (at 25°C), thermal conductivities (between 25 and 1300°C) and thermal expansion coefficients (over 25–1000°C) have been determined by four-probe method, laser flash method and thermo-mechanical analyzer, respectively. Experimental results have shown reasonable agreement with theoretical predictions. Electrical resistivities of ZrB2-based composites are lower than that of HfB2–SiC composite. Thermal conductivity of ZrB2 increases with addition of SiC, while it decreases on ZrC addition, which is explained considering relative contributions of electrons and phonons to thermal transport. As expected, thermal expansion coefficient of each composite is reduced by SiC additions in 25–200°C range, while it exceeds theoretical values at higher temperatures. [Copyright &y& Elsevier]