A new TiB2 + CrSi2 composite - Densification, characterization and oxidation studies

A new composite of TiB2 with CrSi2 has been prepared with excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM with EDAX. Mechanical and physical properties were evaluated. Extensive oxidation studies were also carried out. A near theoretical density (99.9% TD) was obtained with a small addition of 2.5 wt.% CrSi2 by hot pressing at 1700 degrees C under a pressure of 28 MPa for 1 h. The microstructure of the composite revealed three distinct phases, (a) dark grey matrix of TiB2, (b) black phase - rich in Si and (c) white phase - Cr laden TiB2. Hardness and fracture toughness were measured as 29 +/- 2 GPa and 5.97 +/- 0.61 MPa m(1/2), respectively. Crack branching, deflection and bridging mechanisms were responsible for the higher fracture toughness. With increase in CrSi2 content, density, hardness and fracture toughness values of the composite decreased. Thermo gravimetric studies revealed the start of oxidation of the composite at 600 degrees C in O-2 atmosphere. Isothermal oxidation of these composites showed better oxidation resistance by formation of a protective oxide layer. TiO2, Cr2O3 and SiO2 phases were identified on the oxidized surface. Effects of CrSi2 content, temperature and duration of oxidation on the oxide layer formation are reported. Activation energy of the composite was calculated as similar to 110 kJ/mol using Arrhenius equation. Diffusion controlled mechanism of oxidation was observed in all the composites. (C) 2010 Elsevier Ltd. All rights reserved.