Pressureless sintering of TiB^sub 2^ with low concentration of Co binder to achieve enhanced mechanical properties

This paper studies the pressureless sintering of TiB2 based materials with a low concentration of 3 wt% Co binder to result in enhanced mechanical properties. Utilizing ultra-fine TiB2 powders obtained from a special carbon coated precursors method, TiB2-3 wt% Co composite can be densified to a relative density of ~98.6% at a temperature of 1500 °C without external pressure, which is over ~200 °C lower than literature reported temperatures. The Co binder partially reacts with TiB2 and converts into Co2B and Ti-B-Co with good wettings with TiB2. Due to the low sintering temperature, the microstructure is fine with a grain size of ~1.75 ± 0.16 µm. The sample also combines superior mechanical properties including Vickers hardness of ~28.4 ± 0.6 GPa, elastic modulus of ~519.6 ± 17.2 GPa, indentation fracture toughness of ~7.0 ± 0.4 MPa√m, and flexural strength of ~638.3 ± 34.9 MPa. The variation of the Co content to 1, 10, or 20 wt% either leads to low relative density (~91.5% for 1 wt% Co even at a temperature of 1600 °C) or deteriorated mechanical properties (the combination of hardness and fracture toughness, for samples containing 10 and 20 wt% Co) due to the formation of significant amount of brittle Co2B. Correlations between mechanical properties and microstructure are further discussed.