Faceting/Roughening of WC/Binder Interfaces in Cemented Carbides: A Review.

Hardmetals (or cemented carbides) were invented a hundred years ago and became one of the most important materials in engineering. The unique conjunction of fracture toughness, abrasion resistance and hardness makes WC-Co cemented carbides irreplaceable for numerous applications. As a rule, the WC crystallites in the sintered WC-Co hardmetals are perfectly faceted and possess a truncated trigonal prism shape. However, the so-called faceting-roughening phase transition can force the flat (faceted) surfaces or interfaces to become curved. In this review, we analyze how different factors can influence the (faceted) shape of WC crystallites in the cemented carbides. Among these factors are the modification of fabrication parameters of usual WC-Co cemented carbides; alloying of conventional cobalt binder using various metals; alloying of cobalt binder using nitrides, borides, carbides, silicides, oxides; and substitution of cobalt with other binders, including high entropy alloys (HEAs). The faceting-roughening phase transition of WC/binder interfaces and its influence on the properties of cemented carbides is also discussed. In particular, the increase in the hardness and fracture toughness of cemented carbides correlates with transition of WC crystallites from a faceted to a rounded shape.

Interfaces between Model Co-W-C Alloys with Various Carbon Contents and Tungsten Carbide.

Interfaces between alloys simulating binders in WC-Co cemented carbides and tungsten carbide were examined on the micro-, nano-, and atomic-scale. The precipitation of fine WC grains and η-phase occurs at the interface of the alloy with the low carbon content. The precipitation of such grains almost does not occur in the alloy with the medium-low carbon content and does not take place in the alloy with the high carbon content. The formation of Co nanoparticles in the binder alloy with the medium-low carbon content was established. Interfaces in the alloy with the medium-low carbon content characterized by complete wetting with respect to WC and with the high carbon content characterized by incomplete wetting were examined at an atomic scale. The absence of any additional phases or carbon segregations at both of the interfaces was established. Thus, the phenomenon of incomplete wetting of WC by liquid binders with high carbon contents is presumably related to special features of the Co-based binder alloys oversaturated with carbon at sintering temperatures.