Institute of Materials Science of Khabarovsk Science Centre of the Far Eastern Branch of Russian Academy of Sciences, Khabarovsk, Russia:
M. I. Dvornik, Senior Researcher, e-mail: Maxxxx80@mail.ru

E. A. Mikhailenko, Researcher

This work presents the studies on production of functionally graded hard alloy with a fracture toughness distribution (13.1–18.9 MPa√m) and an increased surface hardness (1680 HV) by sintering a sample, consisting of a medium-grained WC – 15Co layer and an ultrafine-grained layer WC – 8Co – 0.4VC – 0.4Cr₃C₂, at 1390 ^оC. The redistribution of toughness occurs as a result of the molten cobalt phase migration from the medium-grained to the ultrafine-grained layer. An increase in the concentration of cobalt in the ultrafine-grained layer from 8 to 14–18% increases its toughness to 13.1–18.7 MPa√m. Reducing the concentration of cobalt in the medium-grained layer from 15 to 7% reduces its toughness to 16.1–18.9 MPa√m. Inhibitors, migrating to the surface of the ultrafine-grained layer together with cobalt, limit the growth of grains of tungsten carbide. As a result, the hardness of the ultrafine-grained surface layer increases to 1680 HV. Near the interface, the low concentration of grain growth inhibitors in the ultrafine-grained layer and a relatively high cobalt concentration (11%) lead to an increased toughness of this area to the maximum values (18.7–18.9 MPa√m). The proposed method enables reducing the consumption of expensive ultrafine-grained tungsten carbide powder and improving the performance characteristics of the resulting hard alloy. The comparison of the obtained results with the theoretical model showed that the amount of cobalt, migrating from the medium-grained to the ultrafine-grained layer, is lower than the one, predicted by the model.

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