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

The feasibility of using MoSi2 additions (up to 10 wt%) to improve the mechanical properties (hardness, fracture toughness, flexural strength), while enhancing the densification behaviour of TiB2, was assessed. Sharp instrumented indentation response (2 N load) of a range of hot pressed TiB2-MoSi2 compositions was evaluated. The Vickers indentation data obtained at macro load (up to 100 N) were also analysed. Experimental results revealed that the addition of only 2.5 wt% MoSi2 to TiB2 resulted in the attainment of a high sinter density ( > 99% of theoretical), after hot pressing at 1700 C. The dense TiB2-2.5 wt% MoSi2 composite composition possessed a good combination of mechanical properties, including high hardness (Hv5 about 30 GPa) and moderate fracture toughness (about 6 MPa.m1/2). The four-point flexural strength values varied in the range 370-400 MPa, with the maximum value for 2.5 wt% MoSi2 containing composite. An increase in the additive (MoSi2) content above 5 wt% degraded the mechanical properties. This was attributed to the presence of secondary phases (Ti5Si3, Mo5Si3) and its effect has been critically analysed. The mechanical response data, recorded using depth sensing instrumented Vickers indenter, were analysed in the light of the elastic/plastic work done, and the elastic modulus (E) values, as computed from the initial slopes of unloading curves, varied in the range 460-500 GPa. Additionally, the analysis of macro Vickers hardness measurements revealed 'indentation-size effect', which has been discussed in terms of mechanical response at the indented zone, and in correlation with the material properties.