B. Ries

A novel sintering technique for fabrication of functionally gradient WC–Co cemented carbides

Functionally graded or functionally gradient WC–Co cemented carbides with Co and/or hardness gradients can potentially have great practical importance. In this article is described a novel sintering technique for fabrication of functionally gradient WC–Co cemented carbides. This technique includes (1) employing green carbide bodies with low (or high) carbon contents within the two-phase region of the W–Co–C phase diagram; (2) their pre-sintering in the solid state to obtain a certain green density and consequently gas permeability; (3) selective carburisation (or decarburisation) of their surface layer in a carburising (or decarburising) gas atmosphere; and (4) final liquid-phase sintering at tailored sintering conditions to obtain a Co drift (also known as ‘Co migration’) either from the surface towards the core or from the core towards the surface. The kinetics of Co drift between couples of model alloys with very similar WC mean grain sizes but different carbon contents were examined. The microstructure, hardness, Co contents, residual stresses and wear-resistance of the gradient cemented carbides with low-Co surface layers obtained by the selective surface carburisation of carbide green bodies with the original low carbon content were examined. Their surface layers were found to contain significantly less Co than the core resulting in a higher hardness of the surface layer. The surface layer is also characterised by high residual compressive stresses in both the carbide phase and binder phase, which results in an improved combination of hardness and fracture toughness. The microstructure, hardness and Co contents of gradient cemented carbide comprising high-Co surface layers obtained by selective surface decarburisation of carbide bodies with the original high carbon content were also examined. The surface layer of the gradient cemented carbide contains noticeably more Co than the core which is beneficial when using this functionally gradient carbide as a substrate for polycrystalline diamond coatings.

Novel hardmetal with nano-strengthened binder

The novel nano-structured hardmetal with nano-strengthened binder was developed. The microstructure of modern alloy consists of the rounded grains of tungsten carbide and the cobalt-based binder, which is reinforced by nanoscaled particles of the θ phase (Co2W4C). The microhardness of binder for the novel hardmetal exceeds significantly the values of this parameter for usual hardmetals due to reinforcement by nanoscaled particles. The simultaneous combination of microstructural features and nanograin reinforced binder leads to the fact that the novel alloy is characterized by large value of service durability, which is substantially higher than that in usual alloys with similar grain size and identical cobalt content.