Effects of Y2O3 addition on microstructures and mechanical properties of WC–Co functionally graded cemented carbides

Functionally graded cemented carbides (FGCCs) have an excellent combination of high hardness and high toughness, and the mechanical properties are strongly influenced by the graded structures. In this work, the effects of the addition of Y2O3 on the microstructures and mechanical properties of FGCCs were studied. The FGCCs were prepared by pre-sintering and carburizing of carbon-deficient WC–Co cemented carbides. It was found that the addition of Y2O3 does not change much the microstructures, but refines the grain size of WC in the gradient layer during the pre-sintering and the carburization processes. Moreover, the thickness of the gradient layer in the Y2O3-added FGCCs is almost doubled compared with that in the Y2O3-free FGCCs. Chemical analyses indicated that Y is mainly distributed in the Co phase. Therefore, Y has a strong effect on suppressing the abnormal grain growth of WC grains by influencing the dissolution behaviors of alloying elements in liquid Co during the pre-sintering and the carburizing processes. The increased thickness of the graded layer is proposed to be due to the refinement of microstructures and the increased diffusion channels for carbon through the Co phase. The FGCCs with the addition of Y2O3 exhibit a much higher transverse rupture strength than the Y-free FGCCs due to thicker gradient layer, finer microstructures and solution strengthening of Y in the Co phase.