Fabrication of WC-Co cemented carbides with gradient distribution of WC grain size and Co composition by lamination pressing and microwave sintering

Abstract

WC-Co composite powders with different particle sizes and Co contents were prepared by ball milling WC and Co powder mixtures for different durations. Functionally graded WC-Co cemented carbides with both Co content and WC grain size gradient were prepared by lamination pressing different WC-xCo (x = 10, 15, 20) powder mixtures and microwave sintering the layered compacts. The WC-xCo powder mixtures were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), and the phase composition and microstructure of the functionally graded cemented carbides (FGCCs) were investigated by XRD, and FE-SEM coupled with energy dispersive spectroscopy (EDS). Mechanical behaviors of the layered WC-Co materials were measured and compared with those of WC-Co cemented carbides with single composition. The results showed that increasing the milling time from 6 to 24 h, results in the decrease of the particle size of WC-Co composite powders from 0.31 to 0.11 µm. After lamination pressing and microwave sintering, the WC-Co samples show nearly complete densification with a relative density higher than 99.7% and no ƞ-phase was detected in the FGCCs. The Co content and WC grain size in FGCCs decrease from the core to the surface. Homogenization of Co has hardly occurred and no cracks have formed between the layers in the sintered samples. In the inner layer, the mean WC grain size is 529 nm, while in the outer layer it is only 274 nm. Because of the difference in Co content and WC grain size, FGCCs have a Rockwell hardness of 90.75 HRA at the surface, which decreases to 86.75 HRA in the core. However the fracture toughness increases from 11.53 at the surface to 18.12 MPa m−1/2 in the core. The present results show that FGCCs with high outer layer hardness and high inner layer toughness were successfully prepared.