Some fundamental aspects concerning processing of Ti(C,N)-based cermets

Abstract

The various events occurring during processing of Ti(C,N)-Mo2C-Ni cermets have been investigated. The results show that during heating to the liquid phase sintering temperature, Mo2C helps keep the hard phase clean of oxides and the system of residual oxygen via a series of dissolution, solute repartitioning and mixed rim formation reactions. It also helps moderate the content of Ti in the Ni binder phase, thus preventing the formation of intermetallic phases and ensuring good wetting and spreading of the liquid binder phase during subsequent sintering stage. Even with good wetting, our analysis and experimental results show that there exists a critical radius ratio such that the liquid binder phase will not spread when the radius ratio of the liquid binder sphere to the hard phase particles is smaller than the critical value. Our analysis, together with experimental results, further show that rim growth during liquid phase sintering is dominated by solute precipitation during cooling at low sintering temperatures (i.e. 1400-1480A?C) and by Ostwald ripening at sufficiently high sintering temperatures (i.e. larger than 1560A?C). An activation energy of 34 kJ/mol was obtained for the dissolution of the hard phase in Ti(C0.7,N0.3)-Mo2C-Ni system and the contact angles between the liquid Ni binder and Ti(C0.7,N0.3) grains with and without the addition of Mo2C were determined to be about 10A? and 65A?, respectively.