Microstructural evolution and formation mechanism of liquid exudation induced surface metal capping on Ti(C, N)-based cermets

The surface microstructure of advanced tool materials is critical, as it directly affects subsequent processing or the performance in use as original condition. In this work, the microstructural evolution and formation mechanism of metal capping on the surface of Ti(C, N)-based cermets, during liquid phase sintering were investigated. Surface metal capping on Ti(C, N)-based cermets is formed during the cooling stage of liquid phase sintering, driven by the contraction pressure of ceramic particle skeleton in the middle stage, in which an adequate amount of liquid phase and significant densification are vital prerequisites. The formation of a step-like textured metal capping is attributed to volume contraction during solidification of liquid phase on the sample surface, in which directional solidification caused by carbon gradient is necessary in this process. Transport behavior of molten metal associated with metal capping on Ti(C, N)-based cermets is divided into four stages in order as infiltration, extrusion, aggregation, and evaporation. The energy of crack propagation is greatly dissipated by plastic deformation of metal capping, thereby greatly improving the fracture toughness of original surface of the sample.