The influence of grain growth inhibitors on the microstructure and properties of submicron, ultrafine and nano-structured hardmetals - A review

The rapid coarsening of ultrafine and nanocrystalline WC powders during consolidation has challenged the production of bulk nano-structured hardmetals for decades. In response, it has become common practice to alloy WC-Co composites with carbides of transition metals in order to inhibit the WC grain growth during sintering. This review discusses various WC grain growth modes, details of grain growth inhibition and reports on the influences of grain growth inhibitors on the microstructure and properties of hardmetals found in current literature. The effectiveness of grain growth inhibitors to suppress WC grain growth during liquid-phase sintering is compared, and their limitations, with regards to sintering of nano-crystalline materials, are described. The effectiveness of grain growth inhibitors at under-eutectic temperatures, in the absence of a binder phase and in alternative binder systems, is analyzed. Currently recognized mechanisms of grain growth inhibition, on the basis of segregating thin films of carbide phases at WC/Co grain boundaries, are summarized and analyzed. The influence of grain growth inhibitors on the mechanical properties of WC-Co hardmetals are evaluated and graphically presented.