In situ formation of La containing dispersed phase on the sinter skin of La2O3 and Cr3C2 unitedly doped WC–Co cemented carbide

The as-sintered sinter skin of WC–11 wt% Co–0.71 wt% Cr₃C₂–0.06 wt% La₂O₃ cemented carbide was observed and analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. It was unexpectedly discovered that lanthanum which was added in the form of oxide could migrate directionally from the alloy to the sinter skin during the sintering process. As a result, La₂O₂S and LaCoO₃ phases were formed in situ on the sinter skin. It is significant that both phases are characterized with layered-structure related self-lubricating function and high melting point. Noteworthily, La₂O₂S grains preferentially grew up along a group of parallel faces with orientation index 0 0 1] in the WC + β matrix. It was revealed that the alloy had a diffusion-resulted dual structure (i.e., core and skin structure), i.e., WC + β + η three-phase structure in the inner part and WC + β + La₂O₂S + LaCoO₃ four-phase structure on the sinter skin. Cr₃C₂ with carbon-donation ability and the impurity elements from the sintering atmosphere with much bigger electronegative parameters than lanthanum are thought to be the driving force behind the directional migration.