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 mu m, 1.0 mu m, 0.7 mu m, and 0.1 mu m, 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 ther-modynamic 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 (& GE;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.