NbC-based cermets: influence of secondary carbide addition and metal binder

The influence of Al metal and WC, Cr3C2, VC, TiC and Mo2C secondary carbide additions on the microstructure and mechanical properties of Fe, Co and Ni bonded NbC cermets was investigated. Powder mixtures were fully densified by pressureless liquid-phase sintering for one hour at 1420 °C in vacuum. Microstructural and compositional analysis, as well as the elemental distribution in the dense cermets, was performed by electron probe microanalysis (EPMA) to assess the effect of the additions on the NbC grain growth, NbC morphology and metal binder composition, allowing correlation of the microstructure with the fracture toughness and hardness of the cermets. The substitution of WC with 40 wt.%NbC in WC-12 wt.%Co cemented carbides resulted in a significant improvement of hardness when adding 0.9-3.6 wt.%VC or Cr3C2 as WC grain growth inhibitors. Compared to pure NbC-Ni, a limited NbC grain growth and homogeneous grain size distribution were obtained upon adding secondary phase carbides or Al metal. WC and Mo2C s gnificantly enhanced the hardness and toughness of NbC-Ni cermets. The Vickers hardness (HV 10) of 976 kg/mm2 and indentation toughness of 9.2 MPa m1/2 of a NbC-15 vol.%Ni could be increased to 1357 kg/mm2 and 10.1 MPa m1/2 respectively upon adding both 8.5 vol.%WC and 8.5 vol.%Mo2C. The addition of 25-50 at.%Al in Fe binder also increased the hardness of NbC-Ni/Fe cermets, but with decreased toughness. The maximum hardness of the investigated cermets was obtained for a 40 wt.%NbC substituted WC-12 wt.%Co cemented carbide, exhibiting a Vickers hardness of 1697 kg/mm2 and toughness of 7.8 MPa m1/2, whereas the WC and Mo2C added NbC-Ni cermets exhibited the optimum combination of hardness and fracture toughness. (AU) Copyright © 2018 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved