On the use of TiO₂ in Ti(C,N)-WC/Mo₂C-(Ta,Nb)C-Co/Ni cermets

Abstract

In this study, Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets with varying [Mo]/([W] + [Mo]) mole ratios and constant [W]+[Mo] content were prepared by using TiO2 and carbon as substitutes for a part of Ti(C,N). In situ carbothermal reaction took place with subsequent liquid phase sintering. The thermal behavior and outgassing behavior were conducted using differential thermal analysis/thermal gravity analysis (DTA-TG) and massspectrometric evolving gas analysis (MS-EGA). The microstructure was investigated by scanning electron microscopy (SEM). After being sintered in N2 atmosphere, cermets without TiO2 additions generally showed rimless microstructures with black grains independently distributed from other phases, whereas the cermets with TiO2 addition showed complete core-rim microstructures. The average grain size generally decreased with increasing Mo content, and increased with increasing TiO2 substitution. Moreover, the average grain size increased with decreased N2 pressure from 50 mbar to 10 mbar at any [Mo]/([Mo] + [W]) ratio. The most interesting result was for the Mo-free cermets with 5 mol% TiO2 and 10 mol% TiO2 addition sintered in 50 mbar N2: both the hardness and fracture toughness increased compared with the cermets without TiO2 addition. The HV-KIC behavior of the majority of the cermets was superior to the industrial grades.