Grain growth inhibition in ultrafine hardmetals

Ultrafine and nanoscaled hardmetals show mechanical properties like hardness and bending strength which lie way above conventional fine or submicron grained hardmetals. To achieve such fine microstructures very fine WC starting powders as well as grain growth inhibitors such as Cr3C2 or VC are needed. To study the grain growth inhibition in ultrafine hardmetals investigations on samples made from nearly nanoscaled WC and Co starting powders with and without the addition of Cr3C2 were done. For studying the dissolution behaviour of Cr3C2 and the evolution of density, magnetic properties and lattice parameters of WC during sintering, interrupted sintering experiments were carried out. Thermal analysis techniques including TG-MS and DSC were used, to link the observed changes to expected reactions. The results show that grain growth inhibitors greatly influence the sintering behaviour already way below the eutectic melting of WC-Co. Especially dissolution of Cr3C2 and homogenous distribution of Cr within the samples already starts below 800 °C with the reduction of W-surface oxides and the creation and spreading of Cr-oxides. The findings are relevant for optimising sintering regime, composition (amount of grain growth inhibitors) as well as the microstructure and mechanical properties.