Sustainable recycling of WC–Co grinding sludge: Compositional control, phase stability and mechanical performance of cemented carbides

This study investigated the recycling of WC–Co cemented carbide grinding sludge using conventional powder metallurgy, assessing residue additions from 0% to 100% in WC-30Co mixtures. The sludge was washed, dried, disaggregated, and sieved, then mixed with virgin powders, uniaxially pressed at 75 MPa, and vacuum sintered at 1370 °C. Characterization of the pure sludge (C100) by EDS confirmed a cemented carbide alloy with a high-binder phase content (Co Ni), validating the choice of the WC-30Co matrix as a reference and the equivalence of the residue to the commercial grade G6 (30 wt% binder). The results indicate that increasing the sludge content elevated sintering mass loss (from 1.49% to 16.81%) and porosity (from 1.30% to 9.32%), while promoting anomalous WC grain growth and the formation of η phases (Co₃W₉C₄, Co₃W₁₀C₃,₄) due to decarburization during sintering. Despite the increased microstructural heterogeneity, Vickers hardness remained within the industrially relevant range of 800–1000 HV60, and fracture toughness (K IC ) decreased gradually with increasing sludge content, remaining consistent with high-binder WC–Co grades. Residual stresses were predominantly compressive for all compositions, with significantly higher magnitudes on ground and polished surfaces. Tribological tests under dry reciprocating sliding conditions revealed a nearly constant coefficient of friction (0.30–0.35), while wear mass loss decreased with increasing recycled content, indicating enhanced wear resistance. The results demonstrate the technical feasibility of complete sludge recycling, although controlling the C/W ratio by adding carbon black to inhibit embrittling phases is recommended to ensure balanced mechanical, tribological, and structural performance of recycled WC–Co alloys.