Formation mechanism of in situ continuous gradient in the near surface of WC-6Co cemented carbide tool materials via spark plasma sintering

Abstract

A novel WC-6Co cemented carbide tool materials was prepared by spark plasma sintering (SPS), regulating sintering pressure and leveraging high-temperature liquid-phase diffusion to control the concentration distribution of the metallic phase to form a near-surface continuous gradient structure. The results indicate that Co with lower melting point spontaneously migrates from the surface to the subsurface under high-temperature liquid-phase diffusion mechanisms in response to pressure variations, thereby in situ forming a continuous gradient structure with varying metallic concentrations near the tool surface. The reduced metallic content at the tool surface resulted in enhanced surface hardness (16.07 ± 0.27 GPa) and the formation of residual compressive stress. A Co-rich region was observed in the subsurface, effectively suppressing microcrack formation. At the optimal sintering temperature of 1200 °C, the fracture toughness of the subsurface and flexural strength of the material reached 23.20 ± 0.37 MPa·m^1/2 and 2094.64 ± 83 MPa, respectively, representing improvements of 18.61 % and 7.78 % compared to homogeneous counterparts. Dry machining tests on isostatically pressed Al₂O₃ green ceramics demonstrated that the developed WC-6Co tool achieved a 17.8 % reduction in cutting force, a 14.9 % decrease in cutting temperature, and a 10.3 % extension in tool life relative to conventionally sintered tools.