Microstructure and mechanical properties at room and elevated temperatures of reactively hot pressed TiB2-TiC-SiC composite ceramic tool materials

TiB2-based composite ceramic tool materials with different amounts of TiC and SiC were fabricated via a reactive hot pressing process. The mechanical properties at room temperature and flexural strength at 800-1300 °C were tested in ambient air. The composition and microstructure before and after the high-temperature strength tests were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectrometer (EDS). The flexural strength increment/degradation mechanisms at elevated temperatures were investigated. In-situ synthesized TiC improved the sinterability and mechanical properties of the materials at both room and elevated temperatures. Comparing with TTS (TiB2-15.9 wt%TiC-10.6 wt%SiC) and TS (TiB2-22.4 wt%SiC), TTS3 (TiB2-8.1 wt%TiC-16.4 wt%SiC) had the optimum room temperature mechanical properties, i.e., flexural strength of 862 MPa, fracture toughness of 6.4 MPa m1/2, hardness of 22.8 GPa, and relative density of 99.3%. The improved mechanical properties were ascribed to the fine grain size. The flexural strength of the TTS composite at 800 °C was higher than that at room temperature. The improvement of the flexural strength was attributed to the healing of preexisting flaws and the relief of residual stress. Substantial strength degradation took place when the temperature exceeded 1000 °C, due to softening of the grain boundaries, surface oxidation and elastic modulus degradation.