Pressureless sintering of ultra-low-binder WC-Ni Hardmetals via bimodal powder packing and colloidal processing

Abstract

Achieving high densification in cobalt-free WC-based hardmetals with ultra-low binder contents remains a major challenge for conventional pressureless sintering. In the present work, this challenge is addressed by systematically interrelating colloidal processing, bimodal WC-Ni controlled packing efficiency, and pressureless vacuum sintering. Slurries viscosity, green density of compacts as well as dilatometric response, final density, microstructure, and hardness were analysed as a function of the nanosized WC (n-WC) fraction in compositions containing 1 and 5 vol-% of Ni. The suspensions exhibited an exponential increase in viscosity with increasing n-WC content, whereas the highest green densities were obtained when the total submicron powder fraction (n-WC + Ni) was in the 25–30 vol-%, thereby defining a narrow packing optimum. Dilatometric analysis showed that n-WC accelerates sintering kinetics in all compositions but also revealed a marked dependence of the densification behaviour on Ni content. In WC-5Ni, the packing optimum coincides with the highest final density, whereas in WC-1Ni highest final density corresponds with a full n-WC composition. Relative densities of 97.6% and 98.2% were achieved for the optimised WC-1Ni and WC-5Ni compositions, respectively, with corresponding maximum hardness values of 16.4 ± 0.8 and 15.5 ± 0.4 GPa. STEM-EDS identified Ni at grain boundaries and triple points, while XRD and HR-TEM supported the formation of an interfacial ternary η-carbide (Ni₂W₄C) between Ni and adjacent WC grains. The results establish a processing-based route for the design of dense WC-Ni hardmetals with very low metallic binder contents using conventional pressureless sintering.