Effect of Bimodal WC Particle Size Distribution on the Mechanical Properties of WC–Mo2C–Co Cemented Carbides

In this study, the influence of bimodal WC particle size design on the microstructure and mechanical properties of WC–27 wt.% Mo₂C–10 wt.% Co cemented carbides was systematically investigated. Bimodal hard-phase designs were realized by combining ultrafine WC (300 nm) and coarse WC (1.8 μm) at various ratios, followed by consolidation via spark plasma sintering (SPS). During sintering, Mo₂C preferentially dissolved into the Co-rich liquid phase due to its higher solubility than WC, forming a Co–Mo–C liquid. During sintering progresses, ultrafine WC selectively dissolved owing to its high interfacial energy, gradually transforming the liquid composition into a Co–Mo–W–C system. Owing to the short holding time and rapid cooling rate of SPS, the η-phase (M₆C) formed during sintering remained metastable. Meanwhile, selective dissolution–reprecipitation resulted in the formation of Mo₂C-based core–rim structures with W enrichment in the rim region as (Mo, W)₂C. As the fraction of ultrafine WC increased, the hardness increased from 1769 to 1997 kgf/mm2, whereas the fracture toughness exhibited an insignificant difference from 6.56 to 6.65 MPa•m¹ᐟ². Fracture behavior analysis revealed that crack deflection and crack bridging occurred at the Mo₂C core–rim interfaces, effectively suppressing straight crack propagation. These results demonstrate that the introduction of ultrafine WC plays a dominant role in enhancing mechanical performance, and that bimodal WC design combined with Mo₂C addition is a highly effective strategy for developing high-performance cemented carbides for machining