Increase of surface hardness and substrate toughness due to migration of grain growth inhibitors in the ultrafine-grained layer during sintering of functionally graded cemented carbides

This work is devoted to increasing the hardness of the surface and toughness of the substrate of the functionally graded hard alloy obtained from sintering the medium-grained (WC-15Co) and ultrafine-grained (WC-8Co-0.4VC-0.4Cr3C2) layers. The migration of the liquid phase from the medium-grained layer to the ultrafine-grained layer during sintering leads to the displacement of grain growth inhibitors (VC + Cr3C2) dissolved in cobalt to the surface of the ultrafine-grained layer. Local slowing of grain growth increases the hardness of the surface of the ultrafine-grained layer up to 1680 HV. An increase in the local concentration of cobalt near the interface in the medium-grained layer increases the toughness to 18.9 MPa√m. Studies on cobalt and grain growth inhibitors concentrations, average grain diameters, contiguity, hardness and toughness over the depth of samples sintered at 1350 °C, 1370 °C, 1390 °C, and 1410 °C were carried out. The obtained results were compared with theoretical estimates based on the Gurland equations.