Effect of C content on microstructure and mechanical properties of Cr-based hard composites obtained by different sintering methods

Nanocrystalline Cr-based-WC powders with different C contents (5.59, 6.03, and 6.69 wt% C) were sintered by four different sintering methods, including liquid phase sintering (LPS), hot pressing (HP), field-assisted hot pressing (FAHP), and spark plasma sintering (SPS). The densification, average WC grain size, phase composition, microstructure, and mechanical properties of Cr-based-WC hardmetals were dependent on the C contents and the applied sintering methods. The spark plasma sintering technique was superior to the other processing methods, giving fully dense samples (99.7%) at a lower sintering temperature and a shorter sintering time. The average WC grain sizes of samples with 5.59 wt% C sintered by LPS, HP, FAHP, and SPS were 0.9 μm, 1.0 μm, 0.7 μm, and 0.1 μm, respectively, which demonstrated that Cr-based binder had a strong inhibitory effect on the growth of WC grains. Brittle carbides (M₂₃C₆/M₆C) were formed and further grew in LPSed and HPed samples, while Cr₂O₃ phase was detected in all FAHPed and SPSed samples. C element could react with the oxides and remove them from the samples by forming gaseous species such as CO and CO₂ in LPSed and HPed samples sintered for hours. On the contrary, Cr₂O₃ phase was formed due to the strong affinity between Cr and O during FAHP and SPS sintering process, where the densification was finished in minutes. Moreover, the C content played a key role in determining the microstructure and phase formation. Generally, M₆C, M₂₃C₆, Cr₇C₃, and graphite phases were subsequently induced with the increase of C content from 5.59 to 6.69 wt%, which was examined by the thermodynamic simulation of W–C–Cr–Fe–O system. The mechanical properties, including hardness, fracture toughness, transverse rupture strength, and compressive strength, of Cr-based-WC hardmetals were fully investigated. All sintered Cr-based-WC hardmetals showed an obvious advantage in hardness, considering that the WC volume fraction (only 70%) was much lower than that (≥90%) of commercial Co-based WC hardmetals. Especially, SPS-1 sample had achieved amazing compressive mechanical properties (hardness of 2219 HV30, fracture toughness of 8.2 MPa m^−1/2, transverse rupture strength of 531 MPa, and compressive strength of 3296 MPa), which were superior to nanosized Co-based-WC hardmetals. The microstructural evolution mechanisms during LPS and SPS were further inferred.