A KINETIC MODEL FOR COBALT GRADIENT FORMATION DURING LIQUID PHASE SINTERING OF FUNCTIONALLY GRADED WC-Co

摘要

Functionally graded WC-Co composites offer solutions to the tradeoff between wear resistance and fracture toughness in WC-Co composites. Although liquid phase sintering is the most economically viable method for producing WC-Co parts, the main challenge in fabricating a functionally graded WC-Co composite using liquid phase sintering is that the cobalt content homogenizes across the layers. The homogenization process results in a sintered part with uniform cobalt content. However, a cobalt gradient can be established after sintering by introducing an initial gradient in carbon within a WC-Co bi layer prior to sintering. In this paper, a kinetic model is used to describe the graded microstructure as a function of sintering time and temperature, as well as other factors including the volume fraction of Co_3W_3C (η) phase, liquid migration pressure and carbon content. The model predicts a decreasing rate of change of the thickness of the WC-Co product layer (X) due to the phase reaction between carbon and η phase with the volume fraction of η phase (f_η) in the sample and sintering time (t). Good agreement is observed between the model and experimental results.