Structure, Strength, and Oxidation Resistance of Ultrahigh-Temperature ZrB2–SiC–WC Ceramics

Abstract

The hot pressing process was employed to produce dense ultrahigh-temperature ZrB2–15 vol.% SiC–5 vol.% WC ceramics. Refractory (Zr, W)C and WB phases emerged in hot pressing at 2050°C and 30 MPa with a holding time of 15 min. The hot pressing process was peculiar in that a (Zr, W)B2 solid solution formed at the zirconium boride grain boundaries. Annealing in vacuum at 1600°C decreased the amount of oxygen in the ceramics from 0.3 to 0.1 wt.% through interaction of B2O3, SiO2, and ZrO2 with tungsten carbide. The ZrB2–15 vol.% SiC–5 vol.% WC ceramics had 505 ± 60 MPa strength at room temperature and 802 ± 94 MPa strength at 1800°C. The high strength at 1800°C was reached through transcrystalline fracture of zirconium boride grains. High-temperature oxidation resulted in scale consisting of three layers: borosilicate glass as the upper layer, zirconium oxide with other oxide phases (WO3 and SiO2) as the middle layer, and the base material depleted of boron and silicon as the lower layer. At an oxidation temperature of 1500°C and a holding time of 50 h, the scale was 85 μm thick, including a SiO2–B2O3 layer 64 μm thick and a layer of ZrO2 + SiO2 + MexOy and base material depleted of boron and silicon. At an oxidation temperature of 1600°C and a holding time of 2 h, the scale was 84 μm thick, including a SiO2–B2O3 layer 10 μm thick and a layer of ZrO2 + SiO2 + MexOy and base material depleted of boron and silicon. The dense scale developed on the material allowed 70% of its initial strength to be retained after oxidation at 1500°C with a holding time of 50 h and 50% strength after oxidation at 1600°C for 2 h, which was higher than for the base ZrB2–15 vol.% SiC–5 vol.% ceramics.