Effect of initial WC particle size on grain growth behavior and gradient structure formation of bilayer functionally graded cemented carbides

Abstract

In this study, functionally graded WC-TiC-Co cemented carbides with a unique bilayer microstructure have been successfully synthesized by high pressure nitriding sintering. The effect of initial WC particle size on the final gradient microstructure has been investigated systematically. The results show that a face-centered-cubic (fcc) phase-riched surface layer and an intermediate layer (cobalt-rich and coarse- WC grain) are formed during the nitriding sintering. With the increasing of initial WC particle size, the carbide grain size of the surface layer, the intermediate layer, and the inner region increases after sintering. Compared with the inner region, the intermediate layer shows significantly coarser WC grains. The thickness of the bilayer increases with the increment of initial WC particle size, which indicates that there is a constraining effect of ultrafine WC grains on the gradient microstructure formation. The formation mechanism of the bilayer functionally graded cemented carbides containing WC powder with different particle sizes is investigated by analyzing the diffusion path of Ti, N, Co and W, phase composition and grain growth during nitriding sintering.