The WC-Co/Fe–Ni interface: Effect of holding time on the microstructure, grain size and grain growth mechanism

Abstract

The sinter-bonding of Fe-36 wt.% Ni powder to WC-Co, compacted under a vacuum, was performed at 1300 °C, for 2, 8 and 16 h. Three different holding times were studied during sample preparation. The results indicated that the compacted Fe-36 wt.% Ni/WC-Co sinter-bonded powders yielded a consolidated interface comprised primarily of hexagonal α-WC, cubic Fe0.64Ni0.36 and the presence of the Co3W3 complex, whose content increased as a function of holding time. Prolonged holding times promote bonding of the WC-Co/Fe–Ni component. However, excessive holding times result in significant contractions and failure by cracking. Furthermore, with increased holding time, grain sizes increased from the original size of 80 nm–390, 460, and 480 nm, when subjected to sintering temperatures of 1300 °C, for 2, 8 and 16 h, respectively. Grain growth occurs through the WC/Co/WC interfaces, not only in the liquid phase, but also in the solid state, at lower temperatures. Actual sintering temperatures are significantly lower than the melting point temperatures of cobalt or tungsten carbide because of the formation of intergranular films (complexion) at the WC/Co interface. Additionally, two abnormal grain-types were formed, exhibiting tungsten-rich characteristics that were complexed at the WC/Co interface, in addition to extreme abnormal grain growth (AGG), in the absence of cobalt films, located between the WC/WC grain boundaries. At the WC/Co interface, nanosized grains contribute to the AGG owing to the extremely large surface areas and short diffusion distances. The concentration gradient, together with extreme stress imbalance, at the WC-Co/Fe–Ni interface, results in particles to rotate, repack, and rearrange, which leads to the observed abnormal grains.