Two SiC‐containing metal diborides materials, classified in the ultra‐high‐temperature ceramics (UHTCs) group, were fabricated by hot‐pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450°C for 1200 min, had mass gains lower than 5 mg/cm2. Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20°C of bath temperature). Experimental data showed that the (ZrB2+HfB2)–SiC and the ZrB2–SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475° and 385°C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB2+HfB2)–SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm × 2.5 mm × 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196° and 218°C for ZrB2+HfB2–SiC and ZrB2–SiC, respectively) as the specimen size increases.
The emissivity and the catalytic efficiency related to atomic oxygen recombination were investigated experimentally in the range 1000-2000 K for ZrB 2 and ZrB 2 -HfB 2 -based ceramics. In order to evaluate the effect of the machining method, two series of samples, one prepared by electrical discharge machining and the other machined by diamond-loaded tools, were tested. High emissivity (about 0.7 at 1700 K) and low recombination coefficients (on average 0.08 at 1800 K) were found for all the materials. The experimental data showed an effect of the surface machining on the catalytic behavior only on the ZrB 2 -based composite; conversely, small variations were found in the recombination coefficients of ZrB 2 -HfB 2 -based samples for the different machining processes. The surface finish affected the emissivity at lower temperatures in both compositions, with the effect becoming negligible at temperatures above 1500 K.
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