Borides, especially transition metal borides, have a high hardness, high melting point and relatively high electrical conductivity among hard materials. Hence, borides are promising candidates for wear resistant applications and have been intensively studied. However, borides show a poor sinterability, extreme brittleness and strong reactivity with metals, making it difficult to prepare boride based cermets from borides and metals. Reaction boronizing sintering is a novel strategy to form a ternary boride coexisting with a metal matrix in the cermets during liquid phase sintering. This new sintering technique has successfully developed ternary boride based cermets with excellent mechanical properties, such as Mo2FeB2, Mo2NiB2 and WcoB based cermets, and has been applied in wear resistant applications such as injection molding machine parts, can forming tools, and dies for the extrusion of copper. In particular, Mo2FeB2 based cermets have attracted much attention because of cheap raw material, simple preparation method and superior properties.
Mo2FeB2 based cermets consisted of the Mo2FeB2 hard phase and Fe based binder phase. Previous work revealed that the mechanical properties of Mo2FeB2 based cermets could be improved by introducing Cr and Ni additions. Further studies showed that Mn and V additions decreased the grain size and remarkably increased TRS. In this case, carbon is generally added to cermets as a sintering aid, and little attention has been paid on the effect of carbon addition on the microstructure and mechanical properties of Mo2FeB2 based cermets. However, since the cermets consist of a Fe based binder phase, it is clear that the properties of the binder phase are also, at least to some extent, influenced by the content of carbon addition.
The carbon addition significantly decreased the grain size. A stable Fe/C cluster may be the explanation of the grain refinement at a higher carbon contents. The grain size was almost kept invariant when saturation concentration of the carbon addition in Fe based binder phase was reached at sintering temperature.An increasing carbon addition increased the dissolution of Mo element in the binder phase. In addition, the binder phase changed from the ferrite to martensite with increasing carbon content. The cermets without carbon addition exhibited the highest TRS and fracture toughness, and the maximum hardness was found for the cermets with 0.5 wt.% carbon addition.
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