Superhard boride layer deposition on a carbide-cobalt hard alloy

Abstract

The interaction of a carbide-cobalt hard alloy (K 10, International Standards Organization) with powdery B 4C and the product Borozar-HM and the effect of this interaction on the structure and hardness of the diffusion layer obtained are investigated. The phase composition of the diffusion layers is shown to depend on the type of the powders and on the temperature and duration of the boronizing process. As a result of thermal treatment with Borozar-HM, the phase CoWB is formed in the diffusion layer whereas in the case of B 4C the boron-richer CoW 2B 2 and CoB phases prevail. The Co-W-B phase diagram, the concentration of dissolved tungsten in the binder and the difference in mass transfer of boron from the two boride powders are used to determine the phase composition of the layer. The homogeneous diffusion layer with a small number of defects, formed during the interaction of the alloy with Borozar-HM, was studied by electron microscope analysis. It is shown that the difference in phase composition of the diffusion layers obtained with the two boron compounds affects the layer hardness. The thermochemical treatment of the alloy with Borozar-HM leads to the appearance of superhard surface layers exceeding in hardness those obtained with B 4C. The maximum Vickers hardness measured IIV 01 = 23.4 GPa is attributed to the single-phase diffusion layer of CoWB obtained.