Investigations on synthesis of ZrB 2 and development of new composites with HfB 2 and TiSi 2

Abstract

This paper presents the results of experimental investigations carried out on the synthesis of pure ZrB 2 by boron carbide reduction of ZrO 2 and densification with the addition of HfB 2 and TiSi 2. Process parameters and charge composition were optimized to obtain pure ZrB 2 powder. Monolithic ZrB 2 was hot pressed to full density and characterized. Effects of HfB 2 and TiSi 2 addition on densification and properties of ZrB 2 composites were studied. Four compositions namely monolithic ZrB 2, ZrB 2 + 10% TiSi 2, ZrB 2 + 10% TiSi 2 + 10% HfB 2 and ZrB 2 + 10% TiSi 2 + 20% HfB 2 were prepared by hot pressing. Near theoretical density (99.8%) was obtained in the case of monolithic ZrB 2 by hot pressing at 1850 °C and 35 MPa. Addition of 10 wt.% TiSi 2 resulted in an equally high density of 98.9% at a lower temperature (1650 °C) and pressure (20 MPa). Similar densities were obtained for ZrB 2 + HfB 2 mixtures also with TiSi 2 under similar conditions. The hardness of monolithic ZrB 2 was measured as 23.95 GPa which decreased to 19.45 GPa on addition of 10% TiSi 2. With the addition of 10% HfB 2 to this composition, the hardness increased to 23.08 GPa, close to that of monolithic ZrB 2. Increase of HfB 2 content to 20% did not change the hardness value. Fracture toughness of monolithic sample was measured as 3.31 MPa m 1/2, which increased to 6.36 MPa m 1/2 on addition of 10% TiSi 2. With 10% HfB 2 addition the value of K IC was measured as 6.44 MPa m 1/2, which further improved to 6.59 MPa m 1/2 with higher addition of HfB 2 (20%). Fracture surface of the dense bodies was examined by scanning electron microscope. Intergranular fracture was found to be a predominant mode in all the samples. Crack propagation in composites has shown considerable deflection indicating high fracture toughness. An oxidation study of ZrB 2 composites was carried out at 900 °C in air for 64 h. Specific weight gain vs time plot was obtained and the oxidized surface was examined by XRD and SEM. ZrB 2 composites have shown a much better resistance to oxidation as compared to monolithic ZrB 2. A protective glassy layer was seen on the oxidized surfaces of the composites.