ULTRAHIGH-TEMPERATURE HfB2-BASED CERAMICS: STRUCTURE, HIGH-TEMPERATURE STRENGTH, AND OXIDATION RESISTANCE

Ultrahigh-temperature hafnium diboride ceramics with additions of 15 vol. % MoSi2 or 15 vol. % SiC or a combined addition of 15 vol.% SiC and 5 vol.% WC were produced by hot pressing in the range 1800-2000 degrees C. The density of the produced composite ceramics was >98%. The components interacted in the hot pressing process to form new high-temperature phases (WB, MoB). The graine size of all structural elements did not exceed 5 mu m. The maximum bending strength was reached by the HfB2-15 vol.% SiC-5 vol.% WC samples: 587 +/- 25 MPa at room temperature and 535 +/- 18 MPa at a test temperature of 1800 degrees C, being associated with transcrystalline fracture of the ceramics. A three-layer oxide film formed: the upper layer was borosilicate glass with a HISiO4 interlayer, the middle layer was HfO2 with B2O3-SiO2 inclusions, and the lower layer consisted of hafnium oxide and inclusions of other oxides. The total thickness of the oxide film was similar to 50 mu m for the material oxidized at 1600 degrees C for 5 h and similar to 150 mu m at 1500 degrees C for 50 h. The highest oxidation resistance was acquired by the HfB2-15 vol. % MoSi2 composite, where the oxidation rate did not exceed similar to 1 mg/cm(2) h because a dense and homogeneous HISiO4 layer developed on the surface. However, the most corrosion-resistant zirconium diboride composite, ZrB2-15 vol. % MoSi2, showed an oxidation rate of similar to 2 mg/cm(2) h. This high oxidation resistance of the hafnium diboride ceramics is explained by slower oxygen diffusion in HfO2 and HISiO4 than in ZrO2 and ZrSiO4, which is confirmed by mathematical modeling of the oxidation process.