Effects of sintering temperature on the microstructure and mechanical properties of double-hard-phase TiB2–TiC cermets

Abstract

The mechanical properties and microstructure of double-hard-phase TiB₂–25-wt.%-TiC–20-wt.%-CoNi cermets prepared at different temperatures were investigated by performing scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results showed that the transverse rupture strength, indentation fracture toughness, hardness, and relative density of the cermets reached the maximum values of 1654 MPa, 11.06 MPa m^1/2, 90.1 HRA, and 99.42 %, respectively, when the sintering temperature was 1462 °C. Solid- and liquid-phase sintering occurred at temperatures below and above 1354.5 °C, respectively. The cermets were composed of TiB₂, TiC, TiB, and CoNi phases when sintered at 1440 °C and below, but the TiB phase disappeared when sintered at 1462 °C and above. Both TiB₂ core–(Ti, Co, Ni)B rim and TiC core–(Ti, Co, Ni)C rim structures appeared in cermets sintered at 1440 °C and above. In addition, a few TiC core–(Ti, W)C inner–(Ti, W, Co, Ni)C outer double rim structures were formed. A coherent interface was found between the (Ti, Co, Ni)B rim and TiB₂ core. A thin CoNi amorphous metallic layer existed at the interface between the (Ti, Co, Ni)C rim and TiC core. In addition, amorphous CoNi was observed in the interfacial region between the CoNi binder and (Ti, Co, Ni)C rim phase and inside the CoNi binder phase. These multi-interface and core–rim structures resulted in excellent strength and toughness of the cermets.