STRUCTURE, STRENGTH, AND OXIDATION RESISTANCE OF ULTRAHIGH-TEMPERATURE ZrB₂–SiC–WC CERAMICS

Abstract

The hot pressing method was employed to produce dense ultrahigh-temperature ZrB₂–15 vol.% SiC–5 vol.% WC ceramics. Hot pressing at 2050 °С and 30 MPa with a holding time of 15 min formed refractory (Zr, W)C and WB phases. The hot pressing process was peculiar in that the (Zr, W)B₂ solid solution formed at the zirconium boride grain boundaries. Annealing in vacuum at 1600 °С 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 °С. The high strength at 1800 °С 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 °С 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 + Me_xO_y­ and base material depleted of boron and silicon. At an oxidation temperature of 1600 °С 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 + Me_xO_y­ 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 °С with a holding time of 50 h and 50% strength after oxidation at 1600 °С for 2 h, which was higher than from the base ZrB₂–15 vol.% SiC–5 vol.% ceramics.