In situ synthesis mechanism of double core/rim structure Ti(C,N)-based cermets prepared by reactive hot pressing

Abstract

In situ Ti(C,N)-based cermets with white core/gray rim and black core/gray rim structures were synthesized from a Co–Ti–C–TiN–W system. In addition, the reaction mechanism was explored. Results showed that the allotropic transformation of εCo into αCo initially occurred. Then, αCo–Ti solid state reaction to fabricate CoTi2 took place. Subsequently, Co–Ti liquid was formed by a peritectic reaction of CoTi2. With the spreading of Co–Ti liquid, C atoms dissolved into Co–Ti melt, resulting in the presence of Co–Ti–C ternary liquid. In the melt, C atoms reacted with Ti atoms to yield TiC. With increasing temperature, TiC and TiN particles combined to produce Ti(C,N). Continuous heating induced the dissolution of W atoms and a part of Ti(C,N) into Co–Ti–C melt. This contributed to the appearance of Co–Ti–C–N–W liquid. Once W, Ti, C and N atoms in liquid became saturated, they were precipitated. When the precipitation appeared on the surface of undissolved Ti(C,N), large Ti(C,N) particles with black core/gray rim structure were obtained. As they directly nucleated and grew from the melt, fine Ti(C,N) particulates with white core/gray rim structure were prepared. Ti(C,N)-based cermets can be produced at a low sintering temperature of 1200 °C. With an increased sintering temperature, the relative density and homogeneity of cermets were gradually improved, and more Ti(C,N) particles with core/rim structure were obtained. These effects in turn led to the increase in hardness, flexure strength and fracture toughness of cermets. Results from this work can provide a valuable reference for the formation of white core/gray rim Ti(C,N) particles, which remains controversial in traditional powder metallurgy.