Correlation between phase evolution, mechanical properties and instrumented indentation response of TiB2-based ceramics

Abstract

In densifying non-oxide high temperature ceramics, like titanium di-boride (TiB2), the type and amount of sinter-aid critically determine the microstructural developments and consequently the mechanical properties. In this perspective, the present work is carried out to assess the feasibility whether MoSi2 addition (up to 10 wt.%) can potentially improve the mechanical properties (hardness, fracture toughness, flexural strength), while enhancing the densification behaviour of TiB2. In order to answer such an important issue and to understand the mechanisms contributing to the variations in mechanical properties, we have evaluated sharp instrumented indentation response (2 N load) of a range of hot pressed TiB2–MoSi2 compositions. In addition, the Vickers indentation data obtained at macro load (up to 100 N) are also analyzed. Our experimental results reveal that the addition of only 2.5 wt.% MoSi2 to TiB2 results in the attainment of high sinter density (>99% ρth), after hot pressing at 1700 °C. The dense TiB2–2.5 wt.% MoSi2 composite composition has been found to possess a good combination of mechanical properties, including high hardness (Hv5∼30 GPa) and moderate fracture toughness (∼6 MPa m1/2). Furthermore, the four-point flexural strength values vary in the range of 370–400 MPa, with the maximum value for 2.5 wt.% MoSi2 containing composite. Interestingly, an increase in the additive (MoSi2) content above 5 wt.% degrades the mechanical properties. The degradation in mechanical properties has been attributed to the presence of secondary phases (Ti5Si3, Mo5Si3) and its effect has been critically analyzed. The mechanical response data, recorded using depth sensing instrumented Vickers indenter has been analyzed in the light of the elastic/plastic work done, and the elastic modulus (E) values, as computed from the initial slopes of unloading curves, have been found to vary in the range of 460–500 GPa. Additionally, the analysis of macro Vickers hardness measurements reveal ‘indentation-size effect’, which has been discussed in terms of mechanical response at the indented zone, and in correlation with the material properties.