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

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 epsilon Co into alpha Co initially occurred. Then, alpha Co-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 degrees 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. (C) 2022 The Authors. Published by Elsevier B.V.